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The development of a high-performance Ni-superalloy additively manufactured heat pipe

The development of a high-performance Ni-superalloy additively manufactured heat pipe Adv. Manuf. https://doi.org/10.1007/s40436-022-00407-z The development of a high‑performance Ni‑superalloy additively manufactured heat pipe 1,2 2 3 4 5 Sheng Li  · Khamis Essa  · James Carr  · States Chiwanga  · Andrew Norton  · Moataz M. Attallah   Received: 9 August 2021 / Revised: 11 February 2022 / Accepted: 21 June 2022 © The Author(s) 2022 Abstract Additively manufacturing (AM) has been used results showed that most of the heat pipes made by LPBF to manufacture fine structures with structured/engineered had better performance than the conventionally manufac- porosity in heat management devices. In this study, laser tured pipes. This study also investigated the influences of powder bed fusion (LPBF) was used to manufacture a high- the process parameters on the porosity volume fraction and performance Ni-superalloy heat pipe, through tailoring the feature size. The results showed that LPBF process could LPBF process parameters to fabricate thin wall and micro- fabricate thin structure due to the change of melt pool con- channel. By using novel laser scanning strategies, wick tact angle. The relationship between process parameters and structure heat pipes with maximised surface-area-to-vol- bead size reported in this study could help design and manu- ume ratio, fine features size around 100 µm, and controlled facture heat pipe with complex fine structure. porosity were successfully fabricated. Microscopy and X-ray microtomography (micro-CT) were used to investigate the 3D structure of the void space within the pipe. Wick test * Moataz M. Attallah m.m.attallah@bham.ac.uk School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, People’s Republic of China School of Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Henry Moseley X-ray Imaging Facility, Henry Royce Institute for Advanced Materials, Department of Materials, University of Manchester, Manchester M13 9PL, UK European Thermodynamics Ltd, 8 Priory Business Park, Wistow Road, Kibworth, Leicestershire LE8 0RX, UK Rolls-Royce plc, P.O. Box 31, Derby DE24 8BJ, UK School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Vol.:(0123456789) 1 3 S. Li et al. Keywords Laser powder bed fusion (LPBF) · Heat pipe · lattices. Nonetheless, the energy density failed to establish Melt pool · Microtomography (micro-CT) direct correlations with the strut size control. Valdez et al. [29] used LPBF to fabricate porous IN718 but failed to find a clear relationship between process parameters and pore 1 Introduction sizes. Recently, Salem et al. [30] investigated the mecha- nisms of defect formation during LPBF of Ti6Al4V lattices, Heat pipes are widely used in mechanical, electrical and highlighting the complexities of the relationship between energy industries as a thermal management device transfer- the process parameters and the defect type or strut size. The ring latent heat through evaporation and condensation [1–4]. single bead method, which uses a single laser line scanned Most current heat pipes have simple porous structure or sim- on the substrate, is an effective way to study the influences ple groove geometry structure, as they are manufactured of laser parameters on bead size (feature size) [31]. powder sintering or extrusion [2, 5–7]. These manufacturing On the other hand, the wick structure with feature size techniques have very limited control on the geometry and below 150 μm has not been reported, exploring the mini- size of pores, hence the performance of porous wick itself mum feature size of LPBFed will help heat pipe researchers has not been significantly improved in the last few decades. to design LPBFed heat pipes. Searle et al. [32] successfully In recent years, the development of additively manufacturing fabricated heat pipes with a n fi e diameter of around 600 μm. (AM), as a method to produce net-shape or near-net-shape Zhang et al. [33] fabricated a heat exchanger with micro- components, has introduced a game changing manufactur- channels around 220 μm size. A novel method, combining ing process to high performance design with more freedom the single bead fabrication and scan strategy design, was [8–10]. Laser powder bed fusion (LPBF, also known as proposed and investigated in this study. selective laser melting), as a widely used AM technique, can This study aims to investigate the limit of thin structure reliably produce pores from a few hundred microns to a few fabrication in LPBF and reveal the relationship among pro- millimetres and complex structures, which are impossible cessing parameters, structure size and wick performance. to manufacture using traditional methods [11–13]. Previ- INCONEL 718 alloy (commonly referred to as IN718) ous studies used LPBF to build lattice (mesh) and porous heat pipes were produced in this study as an example, dem- structures, which attracted significant interest due to their onstrating the AM of a high temperature heat pipe oper- potential applications in shock absorption, thermal manage- ates at 650˚C. By using the single bead method and novel ment and medical implants [14–17]. laser scanning strategy, a series of IN718 heat pipes with So far there are a few studies on LPBFed heat pipes. controlled porous inner surfaces were fabricated. The per- Ameli et  al. [18] studied the permeability of aluminium formance of LPBFed heat pipes was compared with a tra- wick heat pipe made with diamond shape lattice wick by ditionally manufactured heat pipe via wick test and thermal LPBF process and some manufacturing defects were found imaging. The inner structure of LPBFed heat pipes was in the lattice. Ibrahim et al. [19] and Thompson et al. [20] investigated by microscopy and CT scan. fabricated a Ti6Al4V oscillating heat pipe with 1.5 mm internal channel via additively manufacturing. Jafari et al. [21] compared the conventional heat pipes and additively 2 Materials and experimental method manufactured 316L stainless steel heat pipes with strut size around 200 μm. However, these studies focused on wick 2.1 LPBF performance comparing to traditional heat pipes rather than improving the wick performance by exploring the process IN718 gas atomised powder (supplied by LPW Technol- parameters. ogy, Ltd.) with particle sizes between15 µm and 53 µm was It is well known in AM community that the process processed using a Concept Laser M2 cusing LPBF platform parameters for block materials and fine structure are differ - with a 400 W fibre laser. The powder particle size distribu- ent [13, 22]. Making the wick structure as close to the design tion was detemind by SympaTEC laser diffraction particle as possible is essential before studying the performance. size analyser. The D , D and D value were 14.42 μm, 10 50 90 Therefore, alternative approaches must be devised for pro- 28.11 μm and 42.40 μm, respectively. The machine equpied cess optimisation when building fine and thin-walled struc- a Gaussian energy distributed laser beam with a spot size tures for heat pipes. Some studies have fabricated fine lattice between 67 µm and 73 µm and within 80–400 W power structures, however, the influences of the process parameters range. The laser spot size is 67 μm at 80–100 W, and 71–73 on the strut size and morphology were unclear [17, 23–27]. μm when the power is higher than 200 W. Hence we con- ElSayed et al. [28] analysed the influence of the laser heat sider the diameter of laser spot size as a constant value (70 input, expressed within the form of the energy density, on the ± 3 µm) in this study. The nominal layer thickness was fixed porosity, elastic modulus and surface roughness of Ti6Al4V at 30 µm in this study. Regarding the build orientation, the 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe X-axis is the powder re-coating and argon gas flow direc- P E = , (1) tion; the Y-axis is the planar normal to the X-direction on the ν× h × T substrate surface, whilst the Z-axis is the building direction. All samples were produced on a mild steel substrate under E = , (2) a flowing argon atmosphere controlled to lower than 0.01% ν × h O and subsequently removed from the substrate using a GF Agie Charmilles CUT 20 electric discharge machining E = , (3) (EDM) machine. To represent the heat input, energy density concept is where E is the nominal volume energy density, E the v A conventionally used [16, 34], which gives an equivalent heat nominal area energy density, E the nomical linear energy input value based on the process parameters including laser density. power (P), scan speed (v), scan or hatch spacing (h) and Although volumetric energy density (VED) with laser layer thickness (T). This can be presented as beam diameter included is widely used in many studies [35], Fig. 1 Schematic illustrations for the different geometries 1 3 S. Li et al. Table 1 Coupon geometry and parameters fabricated by different strategies, coupon width w, coupon height l, external wall thickness c and coupon diameter Ø −1 Scan strategy Coupon size (w × l × c) or (Ø × l × c) Power/W Scan speed/(mm·s ) Hatch spacing/mm Low energy 5 mm ×10 mm ×1 mm 80–125 1 000–4 200 0.112 5–0.3 Mesh 2.5 mm × 2.5 mm × 0.5 mm 80–300 1 000–3 500 0.15–0.3 Grooved 10 mm × 10 mm × 1 mm (tooth interval 0.15 80–200 1 000–3 500 0.015–0.03 mm) it is not used in this study as the laser beam diameter is (perpendicular to the pipe inner surface). Different standard almost constant in this study. To study the influences of the tessellation language (STL) files were used for the teeth and process parameters on the porosity, coupons with opened the hollow pipe, ensuring that the scanning strategies would top and bottom ends were built. Inside the coupons, different not interfere with one another. The STL files were carefully scanning strategies were used to create and optimise porous positioned on the substrate to ensure a good overlap between structures, which are the low energy, mesh and grooved- the teeth and the external shell. This grooved-structure could strategies (see Fig. 1). The geometry and parameter informa- be used in both horizontal and vertical directions. tion of these coupons are listed in Table 1. The low energy scanning strategy used a raster scanning (simple scanning) 2.2 Characterisation strategy, rotated 90° between the subsequent powder layers, with a large hatch spacing (varying from 0.112 5 mm to 0.3 Sections of the porous structures were extracted for micro- mm) and relatively low power (within 80–125 W range). scopic examination. The cut sections were mechanically Since only one direction is scanned across each layer and ground and diamond-polished to a 0.3 μm oxide solution large hatch spacing is used, the nominal powder layer thick- finish. The samples were then immersed in ethanol and ultra- ness of the most area (apart from the cross points of the sacn sonically cleaned for one hour to remove the loose powder. paths between subsequent layers) in this strategy is doubled To quantify the porosity fraction, pore size and width of (i.e., 60 µm), as shown in Fig. 1. Therefore, this strategy the mesh beads, images were taken using a ZEISS opti- has a significantly lower VED input than other strategies. cal microscope and an HITACHI TM3000 scanning elec- This approach aims to create a highly porous mesh, with the tron microscope (SEM) before subsequent analysis using large hatch space resulting in non-overlapping laser tracks, ImageJ software. A Nikon custom bay micro-CT system was creating a mesh-like structure. Within the mesh strategy, the used to assess the internal 3D structure of the void space laser beam scansfully hatched patterns in each layer to create between the inner and outer walls of the heat pipe. Acquisi- a regular square mesh structure with the designed mesh size. tion parameters can be found in Table 2. A constant scan spacing was used for each direction and Micro-CT was also used to study the influences of ther - layer, assessing scan spacing within the range of 0.15–0.3 mal cycleson structural integrity. The thermal cycles were mm and power ranging 80–300 W. The mesh structure strat- as follows: ramp up to 600 °C at 5 °C/min, then held for 1 h, egy coupons were 2.5 mm × 2.5 mm × 5 mm mesh cubes, followed by 55 min air cooling. Each sample was built ver- surrounded by a 0.5 mm thick wall. The scan spacing for tically and was thermal cycled 50 times before another CT these cubes was fixed at either 0.24 mm or 0.3 mm. Lastly, scan. Each sample, with a length of 130 mm, was scanned the grooved-strategy was used in pipes with a circular cross in two different locations near the top and bottom of the section to create a pipe with internal teeth-like features. shaft with each scan comprising 20 mm of the shaft length. Although this die ff red from the two previous designs, where a uniform porous structure was used to maximise heat losses, Table 2 Acquisition settings used for the micro-CT the dense teeth-like structure was also known to deliver similar performances [36]. The grooved-structure used 0.15 Value mm long and 0.15 mm spaced teeth which were uniformly Accelerating voltage/kV 130 distributed on the internal surface along with the cylinder Tube current/µA 50 height. A round hollow cylindrical pipe with a 1 mm wall Target material Tungsten (W) thickness was fabricated as the external shell. The teeth Filter 2.5 mm copper (Cu) design aimed at maximising the surface area and reducing Reconstructed voxel size/µm 10 the performance difference between vertical direction and Exposure time/s 4 horizontal direction. The scan strategy for the teeth was a Number of projections 3 142 simple non-rotating scan parallel to the long axis of the teeth 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe The micro-CT provides 3D volume renders and non-destruc- the channels within the medium power sample. The samples tive virtual cross sections through the part. In addition, the in Figs. 3a, b had a bead width of 100–150 µm, increasing to cross-sectional area of the void space between the inner and 300 μm within the regions where balling (poor melt wetting) outer walls was segmented using a water shed segmentation is observed [37]. Conversely, the samples built using high algorithm. This allowed for quantification of the void space power (e.g., Figs. 3c, d), had a thinner bead, with a typical which could be used as a metric for determining changes bead width of lower than 200 µm. Comparing the samples non-destructively. built using different E and the same hatch spacing (e.g., Figs. 3a, c), the lower E condition (see Fig. 3c) had a more 2.3 Assessment of the capillary performance defined bead, due to narrower, yet stable, melt pool. The higher E condition (see Fig. 3a), whose wider melt pool The capillary performance was measured by performing in- was due to balling and its instability, led to discontinuities situ thermal measurements. A ThermaCAM SC640 infrared within the melted bead and the smudging of the channels thermal imaging camera was used to observe 18 °C deion- by partially melted powders. Lastly, when comparing the ised water wicking from the bottom of the heat pipe. The samples built using different hatch spacings (e.g., Figs.  3c, heat pipes were cut into two pieces along the vertical section d), the pore sizes become relatively larger, with a mean pore and a thermal camera was used to trace the temperature of size of 0.030 mm (61 µm equivalent diameter) in Fig. 3c the heat pipe, as shown in Fig. 2. Using an acquisition rate of compared with 0.041  mm (71 µm equivalent diameter) in 24 frames per second, the wicking speed (to achieve 18 °C at Fig. 3d. the top of the pipe) was calculated. A copper heat pipe made By plotting E against the area fraction of porosity (see by powder sintering was a reference sample. Fig. 4a), the porosity fraction reduces almost linearly with an increase in E . A similar trend is reported within the literature for LPBF of Ti-alloys and Ni-superalloys [16, 25, 3 Results and discussion 38, 39], showing a consolidation threshold exists when the energy density becomes sufficient to cause full consolidation 3.1 Low energy strategy of the material. The threshold is typically larger than 1.5 J/ mm for Ni-superalloys [40]. As such, it was expected that SEM micrographs, taken from the top of the builds (see the investigated parameters would create a porous structure. Fig.  3), show the morphology of the pores. The samples There is some scatter within the data, with less linearity were manufactured with three levels of power (named as within the E versus porosity relationships, for the samples low, medium and high powers), as shown in Table  1. As built using low power parameters. It is also found that the the power used in low energy strategy was relative lower highest porosity condition is achieved by the medium power than those in other strategy, the porosity was mainly altered parameter, which is caused by the combination of different by changing hatch spacing. The increasing hatch spacing hatch spacings and nonlinear changes of bead width. Bertoli results in more defined channels when using medium power et al. [35] studied the bead width and melt pool depth of (see Figs. 3a, b). A comparison of the higher power to the 316L SS on a single layer deposition condition. The bead medium power conditions shows the higher power leads to width on a laser fabricated surface has not been studied a better pattern definition, compared with more smudging of yet. Therefore, further study on the influence of E on the Fig. 2 Assessment of the capillary wicking action a the test setup with cold deionised water and the thermal camera, and b thermal imaging still shots at the start, midway and end of the test 1 3 S. Li et al. Fig. 3 SEM micrographs for the top surface and horizontal section of the lower energy strategy samples bead width is necessary. Only six samples built using low the process generates thicker beads which reduce the gaps power were successfully fabricated, creating a structurally between neighbouring tracks, creating a denser material. sound build, out of the 32 samples built at this power. The On the lower energy density end, balling generates non-uni- low power was insufficient to fully melt the powder, instead formed porosity, resulting in smudging of holes and irregular it induced the balling effect [ 41], which could lead to dis- mesh structures, while the higher energy density will block continuities within the laser tracks. Valdez et al. [29] also the mesh pores by densifying the material. Similarly, the reported sintered powder and discontinuous bead in LPBFed hatch spacing must be chosen to balance the energy den- IN718 mesh structures. The number of structurally sound sity effects to achieve the target density. Using Fig.  4, the samples increased with increasing laser power. It must be optimum processing window (50% porosity) is when E is emphasised that uniformed porous structures were only pos- within 0.3–0.4 J/mm range, with the process parameters sible through the use of a large hatch spacing, alongside the following this relation lower energy density range to avoid overlap between the 20P 9 neighbouring tracks. h − = , 80 ≤ P ≤ 125, 200 ≤ h × v ≤ 416.67, 13ν 130 Previous analysis using E considered all parameters (4) [29]. However, to deconvolute the impact of the hatch spac- where the laser beam is considered as a constant beam with ing, it is beneficial to look into the relationship between E average beam diameter of 70 ± 3 μm. When comparing the and the hatch spacing, as shown in Fig. 4b. The combina- results of this strategy with the findings of later strategies, tion of E lower than 0.1 J/mm and hatch spacing within it is not going to be straightforward from an energy den- 0.18–0.3 mm range is required to achieve the target of 50% sity viewpoint. Within the lower energy strategies, the same porosity, while keeping the power lower than the high power laser tracks are repeated every two powder layers, meaning level. The energy density affects both the melt flow and the the nominallayer thickness is 60 μm. This also affects the degree of wetting, which may lead to balling by insufficient integrity of the structure, since the melt pool in each track wetting and discontinuities beads when the energy den- sity is low. Meanwhile, if the energy density is too high, 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe Fig. 4 Porosity area fraction versus a the area energy density, b linear energy density, c power and scan speed (with 0.3 mm h) within the lower energy density builds (the black dash line indicates the unstable parameter area) Fig. 5 Mesh strategy samples produced by a low power, E = 0.064 J/mm, E = 0.27 J/mm , b medium power, E = 0.067 J/mm, E = 0.22 J/ L A L A 2 2 mm , and c high power, E = 0.056 J/mm, E = 0.19 J/mm L A must be deep enough to weld the subsequent layers together, had less energy due to the surface tension of the melt. It especially each n and n+2 layers. is known that evaporation and plasma recoil pressure dur- Regarding the build quality, partially melted/sintered ing LPBF, which may force the powders to move towards powder was found on the sample surface. The size of the the laser tracks, depend on the process parameters, causing powder was between 10 µm and 50 µm, the size range of as- a powder spreading known “denudation” [42]. No strong received powder, suggesting that lower energy reduced the metallurgical bonding formed between the melt track and tendency for consolidation at the melt pool edge. Instead, the surrounding powders, allowing for them to be removed powers partially stuck to the edge of the melt pool, which by sandblasting or a similar technology. 1 3 S. Li et al. 3.2 Mesh strategy porosity viewpoint (see Fig. 6b). Due to the higher energy input, the melt pool becomes hotter and wider, pulling more SEM micrographs, taken from the XY section of the mesh powder particles by surface tension and recoil, which leads strategy samples, showed defined square pores at lower pow - to further partial melt and sintering[42]. In general, the ers, with more partially melted powder particles sticking on structure produced by low energy and mesh strategy shows the structure athigher powers (see Fig. 5). Overall, samples the ability of fabricating thin structure around 100 μm in built with higher laser power had more partially melted/sin- width, which is half the size of micro-channel reported in the tered particles attached to the beads, with less defined mesh literature (220–670 μm) [18, 33, 43]. Comparing low energy structures. This result highlights the impact of scanning strategy to mesh strategy with the same parameters (Fig. 4c strategies in controlling the amount of heat. As aforemen- versus Fig. 6b), low energy strategy has higher porosity. tioned, the nominal layer thickness within the low energy By plotting the bead width (D) against E (see Fig. 7), a strategy is equivalent to half the nominal volumetric energy relation between the bead width and E =P/ν was developed within the mesh strategy, should the same laser parameters D = A(P) × ln E + B(P), (5) be used. As such, balling was less prominent within the low power mesh conditions (see Fig. 5a). Conversely, the where P is laser power, A= –0.026P + 58.38 and B=0.079P higher power conditions resulted in more powders sticking + 214.39. The R value of the model is from 0.972 to 0.998, to the melt bead and consuming surrounding powders (see indicating good accuracy, as shown in Fig. 7. This indicates Fig. 5c). The medium power condition produced stable thin that the laser power has a strong influence on melt pool wall around 100 μm, as shown in Fig. 5b. The lower energy size, as the energy input of laser and number of reflections and mesh strategies were able to deposit thin walled struc- increases with increasing laser power when 80 W ≤ P ≤ 300 tures in this study at energy levels reported as not printable W. The high laser power enables more reflections within parameters in other studies [38]. the powder bed, allowing more powder to absorb the laser In the mesh strategy, two scan spacing levels were used, energy and therefore creat a larger melt pool [44]. Dilip et al. while maintaining a generally low E to avoid full or near [25] also reported the melt pool width in LPBF of Ti6Al4V consolidation of the mesh, due to the creation of thicker followed a logarithmic relation to both the laser power and beads when a higher E is used. The parameters covered the scan speed. The influence of the power on the bead width is most power used in LPBF (up to 350 W). Figure 6a shows less significant for lower power parameters (comparing low the influence of the laser power and scan speed on the laser power to high power in Fig. 7). It also shows that the influ- track (bead) width within the mesh. The bead width linearly ence of power is stronger in higher E parameters (low scan increased with the increase in power and decrease inscan speed) comparing to low E parameters. Comparing with speed. At higher powers, the bead width grows slower at the other studies using single layer single bead method [35], higher scan speeds. The bead width grows higher than hatch our multi-layer single bead study shows that the laser power spacing when the E exceeds 0.13 J/mm. Within conditions L has stronger impact on the bead width (increased from 90 sharing the same E , the bead width slightly increases with µm to 120 µm, when power increased from 80 W to 300 W). the increase in laser power, especially when the power higher This could be associated with the increased laser energy at than the high power level. This behaviour also holds from a Fig. 6 Process maps for the mesh structure showing a the influence of the laser power and scan speed on bead width, and b the influence of the laser power and scan speed on porosity 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe Fig. 7 Variation in bead width with the linear energy density in mesh strategy samples and medium power low energy strategy samples higher powers, as well as the reflection and absorptivity of build direction, is affected by over melting and a lack of sup- powder [44, 45]. The higher power provided more energy port, as shown in Fig. 8c. Partially-melted particles between to the laser, allowing more reflections between powder par - the horizontal teeth (see Fig. 8d) have a similar structure to ticles in the powder bed. The low energy strategy samples heat pipe with sintered-grooved composite wick reported by built by medium power (the same power as the green line Li et al. [46], which showed good performance. In general, of mesh strategy) has slightly higher bead width due to the low E benefits the fine structure fabrication, which ensures thicker nomial powder layer comparing to the mesh strategy a reasonable gap between the teeth. samples. The non-uniform morphology among the bottom, mid- dle and top surface could be overcome by manipulating the 3.3 Grooved‑strategy building orientation and using gradient process parameters. The morphology of the top teeth could be improved by tilt- The low energy and mesh studies show thin wall around ing the samples 45° to the building direction. On the other 100 μm width is printable in LPBF. Previous works sug- hand, using a lower energy process parameters for the mid- gested that horizontally-built structures were affected by diameter sidewall and the top area could also reduce the over-melting and a lack of support at the base surface [16]. over-melting and partially melting of the powders. A few grooved structure heat pipes with the 100–150 µm width teeth perpendicularly positioned to the pipe inner wall 3.4 Micro‑CT were fabricated in both vertical and horizontal directions, and the geometrical integrity of this structure is studied. According to the aforementioned results, three samples To reduce the influences of the laser scanning direction on with 50% porosity were fabricated for micro-CT study. The teeth geometry, the laser scan path is controlled, meaning it parameters and strategy for these samples were medium is always parallel to the longitude direction of the teeth. A power low energy strategy sample (see Figs. 3b, f), medium number of optimum parameters were selected base on the power mesh strategy sample (see Fig. 5b), and low power low energy and mesh studies to maintain a 120 μm teeth grooved strategy sample (see Fig. 8), respectively. Sections interval distance at the tips. Among the investigated param- cut from the top and the bottom of the vertically built heat eters, the E = 0.064 J/mm condition resulted in a smaller pipe were analysed using micro-CT. The grooved-structure teeth width (100 µm), with a uniformed 100 µm spacing with 1 mm teeth length and 150 µm spacing is shown in between the teeth. Fig. 9a, where limited powder sintering on the groove sur- The horizontally-built grooved-structures have good geo- face is found. The vertical builds showed a greater uniform- metrical integrity, at both the bottom and the top of the pipe, ity within the teeth size and the distance between them. as shown in Figs. 8a, b. The geometry accuracy of grooved- Nonetheless, a continuous groove channel was built, as strategies was strongly affected by the build direction. The shown in Fig. 9b. length of the top teeth is shorter, due to a lack of support Figures  10 and 11 shows the calculated mean cross- within the first few layers and an over melting from the pipe section of void space and deviation values of the LPBFed wall, but the gap between the teeth remains the correct size. heat pipes base on CT scaned segmentations. It is found The geometry of the side teeth, which is perpendicular to the that the structures are uniform in both simple structure and 1 3 S. Li et al. Fig. 8 SEM micrographs for the horizontally built grooved-structures a the bottom of the tube with 100–150 μm gap between the teeth, b the top of the pipe, c the mid-diameter side of the tube, and d the side view Fig. 9 Micro-CT 3D models showing a the uniformity of the internal surface of the heat pipe and b the continuity of the grove structures throughout the built height grooved-structure samples, while the mesh structure had three strategies due to reducing heat transfer and increasing higher variation (about 11%). The cross-section area in the heat accumulation at higher building height. These prompt grooved-structure has the lowest deviation as the structure the balling effect and partial melt powder. is uniform along the build direction. The cross section of the In addition to the volume renderings and skeletonized mesh structure, however, has the highest deviation among structures that can be generated using the reconstructed 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe 3.5 Bead formation and melt pool stability The melt pool is considered as a liquid cylinder when study- ing its stability. One of the common defects in LPBF is that the unstable melt pool breaks into droplets [24, 38]. The Plateau-Rayleigh instability contributes to this unstable sta- tus when higher scan speeds are applied [38]. Yadroitsev et al. [23] proposed that the stability of liquid was related to the contact angle Φ (between liquid cylinder and substrate), the diameter of molten pool (D), and molten pool length (L). The melt pool is stable regardless of its dimension when contact angle Φ < π/2. Otherwise, the stable condition is πD 2 > . (6) L 3 As illustrated in Fig. 12a, the melt pool on the flat surface Fig. 10 Mean and standard deviations of the cross-sectional area of could have Φ > π/2, due to surface tension and lower power the void space at different heights of medium power low energy sam- inputs. This well represents the reality of the single bead, ple, low power grooved sample, and medium power mesh sample and single layer condition [23]. During the thin wall or lat- tice structures fabrication in LPBF, the top convex surface of slices (for example, Fig.  9), more intuitive 2D visualiza- the previous layer is close to a semi-cylinder, which means tions can be produced. These “virtual cross-sections” present that the contact angle between the melt pool and solid could a plane through the sample and provide a non-destructive be less than π/2 (see Fig. 12b).Therefore, the melt pool is means for assessing the structure of the heat pipe as shown more stable when producing a thin wall structure than a solid in Fig. 11. The samples were scanned at as-fabricated condi- block. Jasper and Anand [47] reported on reducing contact tion (see Figs. 11 a, c), and then scanned after 50 thermal angle between liquid and convex surface, with the reduction cycles (see Figs. 11b, d) to investigate the thermal cycle sta- insurface radius. As such, the melt pool on the convex sur- bility of the structure. There is no obvious change of cross- face, when producing thinner structures, has better stability section area after thermal cycles, which indicates that the than the melt pool on the flat surface. In the axial direction, samples are stable. the melt pool is unstable if the D/L ratio is less than 0.26 (see Eq. (6)). Thus, the higher the scan speed or the lower Fig. 11 Virtual cross-sections through vertical built sample a grooved-structure in as-fabricated condition, b grooved-structure after 50 thermal cycles, c mesh structure as-fabricated condition, and d mesh structure after 50 thermal cycles (the red portion of the image represents the result of the watershed segmentation) 1 3 S. Li et al. Fig. 12 Schematic drawing of the melt pool during LPBF process when producing different structures a for melt pool on substrate or block sam- ple with a relatively flat surface, b for melt pool on thin structures with a curved surface ,and c more solid area (the red colour is the melt pool, with the yellow and grey the solidified beads) the laser power is, the lower the D/L ratio is, meaning the by a high E parameter to create a stable wide bead with melt pool is less stable. In this study, it is assumed the melt cruved surface, which improve the stability of the melt pool. pool diameter (D) is the same as track/bead width, regardless By gradually reducing the energy input of the initial layers of shrinkage and morphology variations during solidifica- could create a better fabricating surface for the fine structure tion. According to Eq. (6), the maximum stable melt pool and improve the melt pool stability. length of printable parameters with lower energy strategies is between 384.6 µm and 576.9 µm. The maximum stable 3.6 Wick test melt pool length of printable parameters with mesh strategy is between 154 μm and 530 μm. The wick performance of the different scanning strategies Furthermore, the geometry of mesh structure, which are and the manufacturing parameters was measured using thin walls perpendicular to each other, also helps stabilise the response time and height of wicking, as per the test the melt pool by providing more contact surfaces. The peri- described in Sect. 3.3. As shown in Fig. 13, a number of odically solid walls could stabilise the liquid via surface ten- the LPBF IN718 heat pipes demonstrated a good wick per- sion and higher cooling rates on solids, shortening the melt formance, compared with the reference Cu heat pipe made pool length, as shown in Fig. 12c. Most conditions in this from pre-sintered powders, as measured using the response study could have melt pool lengths larger than the distance time. The response time was calculated using camera video between the thin walls. Thus, the thin wall mesh structure record from 5 mm wick height to the maximum wick height has a broader processing window than lower energy strate- and fit to first order respond as equation below gies, and the mesh strategy sample has less porosity compar- −t ing the low energy one with the same parameters (Fig. 6b H = H 1 − e , (7) wick wick−max versus Fig. 4c). From the study above, we found that the stable melt pool where H is the wick height, H the maximum wick wick wick-max conditions of the first layer of fine structure (flat surface) height, and τ the respond time. The maximum wick height and the later layers (curved surface) were different. This is of the heat pipes, however, shows a larger deviation, as the one of the reasons that significant difference is often found partially sintered powders or bead discontinuities due to between the optimised processing parameter of fine structure unstable melt pool on the wall. From the test results, the and block structure in LPBF. Hence, we propose the process heat pipes with fabricated by low power grooved-strategy parameter of the first layer of the fine structure should be dif- has the best overall wick performance within the horizon- ferent to the later layers. The first layer should be fabricated tal direction, while the heat pipe built by low power low 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe between solids. The thin wall structure has a border processing range than the block structure due to round top surface. (iv) The micro-CT can be used as a non-destructive evalu- ation method for the structural performance of LPB- Fed heat pipes. The micro-CT results suggested the structure produced by LPBF was stable under thermal cycles. Acknowledgements The authors would like to acknowledge Rolls- Royce plc, Aerospace Technology Institute, and Innovate UK for fund- ing this research through the Advanced Repair Technologies (113015) programme. The CT scans were performed in the University of Man- chester, which was established through EPSRC Grants EP/F007906/1, EP/I02249X/1 and EP/F028431/1. HMXIF is a part of the Henry Royce Fig. 13 Comparison of the response time and wick height of the Institute for Advanced Materials, established through EPSRC Grants LPBF heat pipes, with a Cu heat pipe as a reference EP/R00661X/1, EP/P025498/1 and EP/P025021/1." Open Access This article is licensed under a Creative Commons Attri- energy strategy (Low P Low Erg. V) has the best perfor- bution 4.0 International License, which permits use, sharing, adapta- mance within the vertical direction. The grooved-strategy tion, distribution and reproduction in any medium or format, as long also showed a good performance within the vertical direc- as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes tion, as the response time was the shortest, yet the wicking were made. The images or other third party material in this article are height was slightly lower than the low energy scan strategy. included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will 4 Conclusions and future work need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Heat pipes with designed porosity structures have been suc- cessfully manufactured by adopting three different strate- gies (lower energy, mesh and grooved). The wick test results References show the good performance of LPBFed heat pipes. The main conclusions are as follow. 1. Jouhara H, Chauhan A, Nannou T et al (2017) Heat pipe based systems—advances and applications. 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Valdez M, Kozuch C, Faierson EJ et al (2017) Induced poros- ity in super alloy 718 through the laser additive manufacturing 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe Sheng Li received Ph.D. States Chiwanga received his degree in Materials Science from BSc degree in Aeronautical University of Birmingham, UK, Engineering from University of in 2017. He is currently working Salford, UK and his MSc degree as a research fellow in the School in Fluid Mechanics from Wash- of Metallurgy and Materials, ington State University, USA. University of Birmingham. He is States has worked in senior posi- the author of 24 research publi- tions for well-known multina- cations. His research interests tional engineering companies include additive manufacturing including Schlumberger, of metal and ceramic, processing Solectron and PDD Group before and microstructure of shape starting Cambridge Engineering memory alloys, Ni superalloys, Analysis and Design Ltd. Ti alloys, Al alloys and metal (CEAD) in 2007. He is now the matrix composite. He has director/CFD &Thermofluids worked on the projects supported Consultant in CEAD. by leading companies across the world including Rolls-Royce, Airbus, Honda, BAE Systems, MBDA Missiles, MicroTurbo, Ford, and Euro- Andrew Norton received his pean Space Agency. MSci in Natural Sciences (Mate- rials Science) from University of Khamis Essa is Reader in Cambridge and DPhil (Ph.D.) Advanced Manufacturing and degree in Materials from Univer- Process Modelling at the School sity of Oxford, UK in 2013. He of Engineering, the University of is now work as technologist and Birmingham and the Director of managing the projects gear- the Advanced Manufacturing funded through Aerospace Tech- Group. His research is focused nology Institute &InnovateUK, on advanced manufacturing tech- involving collaborations with a nologies for instance Metal and number of Rolls-Royce Univer- ceramic 3D printing, Hot Iso- sity Technology Centres and static Pressing and Incremental Small/Medium-Sized Enter- Sheet Forming. He has been prises, and utilise a global developing novel manufacturing Research & Development supply routes and applications for aero- chain network. Including the development of novel systems to perform space, defence, automotive and inspection and repair inside aeroengines, emerging technologies (typi- biomedical industries. The scien- cally Technology Readiness Level 6 and below), supporting the move- tific emphasis of his research is on material and process interaction. He ment of on-wing inspection and repair technologies into production has been leading a number of research projects funded by EU, UK through a wide ranging supply chain network. research councils and industry. His research has been supporting major companies across the world including Rolls-Royce, BAE Systems, Moataz M. Attallah holds a MBDA Missiles, MicroTurbo, Ford, European Space Agency and Cat- chair in advanced materials pro- erpillar. Dr Essa has over 80 journal and conference publications in cessing at the School of Metal- addition to two patents and one book. He is guest editor for the micro- lurgy and Materials University mechanics journal and is setting on the editorial board of three other of Birmingham. His research international journals. focuses on developing a metal- lurgical understanding of the James Carr received his material-process interaction in MPhys degree in 2008 and Ph.D. additive manufacturing of metal- degree in Materials Science in lic materials focusing on the pro- 2014 from the University of cess impact on the microstruc- Manchester. James Carr is an ture and structural integrity Applications Specialist for development. His research is MicroCT at Blue Scientific. He conducted through research part- acquired his first degree in Phys- nerships with various companies ics at the University of Manches- in the aerospace, defence, medi- ter in 2008 and then a PhD in cal, space, and nuclear energy sectors. He co-authored over 170 journal Materials Science in 2015. He and conference papers, 3 book chapters, and is a co-inventor on 8 patent has spent his research career spe- applications. cializing in 3D imaging and image processing at the Henry Moseley X-ray Imaging Facility and Diamond Light Source. 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advances in Manufacturing Springer Journals

The development of a high-performance Ni-superalloy additively manufactured heat pipe

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Abstract

Adv. Manuf. https://doi.org/10.1007/s40436-022-00407-z The development of a high‑performance Ni‑superalloy additively manufactured heat pipe 1,2 2 3 4 5 Sheng Li  · Khamis Essa  · James Carr  · States Chiwanga  · Andrew Norton  · Moataz M. Attallah   Received: 9 August 2021 / Revised: 11 February 2022 / Accepted: 21 June 2022 © The Author(s) 2022 Abstract Additively manufacturing (AM) has been used results showed that most of the heat pipes made by LPBF to manufacture fine structures with structured/engineered had better performance than the conventionally manufac- porosity in heat management devices. In this study, laser tured pipes. This study also investigated the influences of powder bed fusion (LPBF) was used to manufacture a high- the process parameters on the porosity volume fraction and performance Ni-superalloy heat pipe, through tailoring the feature size. The results showed that LPBF process could LPBF process parameters to fabricate thin wall and micro- fabricate thin structure due to the change of melt pool con- channel. By using novel laser scanning strategies, wick tact angle. The relationship between process parameters and structure heat pipes with maximised surface-area-to-vol- bead size reported in this study could help design and manu- ume ratio, fine features size around 100 µm, and controlled facture heat pipe with complex fine structure. porosity were successfully fabricated. Microscopy and X-ray microtomography (micro-CT) were used to investigate the 3D structure of the void space within the pipe. Wick test * Moataz M. Attallah m.m.attallah@bham.ac.uk School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, People’s Republic of China School of Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Henry Moseley X-ray Imaging Facility, Henry Royce Institute for Advanced Materials, Department of Materials, University of Manchester, Manchester M13 9PL, UK European Thermodynamics Ltd, 8 Priory Business Park, Wistow Road, Kibworth, Leicestershire LE8 0RX, UK Rolls-Royce plc, P.O. Box 31, Derby DE24 8BJ, UK School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Vol.:(0123456789) 1 3 S. Li et al. Keywords Laser powder bed fusion (LPBF) · Heat pipe · lattices. Nonetheless, the energy density failed to establish Melt pool · Microtomography (micro-CT) direct correlations with the strut size control. Valdez et al. [29] used LPBF to fabricate porous IN718 but failed to find a clear relationship between process parameters and pore 1 Introduction sizes. Recently, Salem et al. [30] investigated the mecha- nisms of defect formation during LPBF of Ti6Al4V lattices, Heat pipes are widely used in mechanical, electrical and highlighting the complexities of the relationship between energy industries as a thermal management device transfer- the process parameters and the defect type or strut size. The ring latent heat through evaporation and condensation [1–4]. single bead method, which uses a single laser line scanned Most current heat pipes have simple porous structure or sim- on the substrate, is an effective way to study the influences ple groove geometry structure, as they are manufactured of laser parameters on bead size (feature size) [31]. powder sintering or extrusion [2, 5–7]. These manufacturing On the other hand, the wick structure with feature size techniques have very limited control on the geometry and below 150 μm has not been reported, exploring the mini- size of pores, hence the performance of porous wick itself mum feature size of LPBFed will help heat pipe researchers has not been significantly improved in the last few decades. to design LPBFed heat pipes. Searle et al. [32] successfully In recent years, the development of additively manufacturing fabricated heat pipes with a n fi e diameter of around 600 μm. (AM), as a method to produce net-shape or near-net-shape Zhang et al. [33] fabricated a heat exchanger with micro- components, has introduced a game changing manufactur- channels around 220 μm size. A novel method, combining ing process to high performance design with more freedom the single bead fabrication and scan strategy design, was [8–10]. Laser powder bed fusion (LPBF, also known as proposed and investigated in this study. selective laser melting), as a widely used AM technique, can This study aims to investigate the limit of thin structure reliably produce pores from a few hundred microns to a few fabrication in LPBF and reveal the relationship among pro- millimetres and complex structures, which are impossible cessing parameters, structure size and wick performance. to manufacture using traditional methods [11–13]. Previ- INCONEL 718 alloy (commonly referred to as IN718) ous studies used LPBF to build lattice (mesh) and porous heat pipes were produced in this study as an example, dem- structures, which attracted significant interest due to their onstrating the AM of a high temperature heat pipe oper- potential applications in shock absorption, thermal manage- ates at 650˚C. By using the single bead method and novel ment and medical implants [14–17]. laser scanning strategy, a series of IN718 heat pipes with So far there are a few studies on LPBFed heat pipes. controlled porous inner surfaces were fabricated. The per- Ameli et  al. [18] studied the permeability of aluminium formance of LPBFed heat pipes was compared with a tra- wick heat pipe made with diamond shape lattice wick by ditionally manufactured heat pipe via wick test and thermal LPBF process and some manufacturing defects were found imaging. The inner structure of LPBFed heat pipes was in the lattice. Ibrahim et al. [19] and Thompson et al. [20] investigated by microscopy and CT scan. fabricated a Ti6Al4V oscillating heat pipe with 1.5 mm internal channel via additively manufacturing. Jafari et al. [21] compared the conventional heat pipes and additively 2 Materials and experimental method manufactured 316L stainless steel heat pipes with strut size around 200 μm. However, these studies focused on wick 2.1 LPBF performance comparing to traditional heat pipes rather than improving the wick performance by exploring the process IN718 gas atomised powder (supplied by LPW Technol- parameters. ogy, Ltd.) with particle sizes between15 µm and 53 µm was It is well known in AM community that the process processed using a Concept Laser M2 cusing LPBF platform parameters for block materials and fine structure are differ - with a 400 W fibre laser. The powder particle size distribu- ent [13, 22]. Making the wick structure as close to the design tion was detemind by SympaTEC laser diffraction particle as possible is essential before studying the performance. size analyser. The D , D and D value were 14.42 μm, 10 50 90 Therefore, alternative approaches must be devised for pro- 28.11 μm and 42.40 μm, respectively. The machine equpied cess optimisation when building fine and thin-walled struc- a Gaussian energy distributed laser beam with a spot size tures for heat pipes. Some studies have fabricated fine lattice between 67 µm and 73 µm and within 80–400 W power structures, however, the influences of the process parameters range. The laser spot size is 67 μm at 80–100 W, and 71–73 on the strut size and morphology were unclear [17, 23–27]. μm when the power is higher than 200 W. Hence we con- ElSayed et al. [28] analysed the influence of the laser heat sider the diameter of laser spot size as a constant value (70 input, expressed within the form of the energy density, on the ± 3 µm) in this study. The nominal layer thickness was fixed porosity, elastic modulus and surface roughness of Ti6Al4V at 30 µm in this study. Regarding the build orientation, the 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe X-axis is the powder re-coating and argon gas flow direc- P E = , (1) tion; the Y-axis is the planar normal to the X-direction on the ν× h × T substrate surface, whilst the Z-axis is the building direction. All samples were produced on a mild steel substrate under E = , (2) a flowing argon atmosphere controlled to lower than 0.01% ν × h O and subsequently removed from the substrate using a GF Agie Charmilles CUT 20 electric discharge machining E = , (3) (EDM) machine. To represent the heat input, energy density concept is where E is the nominal volume energy density, E the v A conventionally used [16, 34], which gives an equivalent heat nominal area energy density, E the nomical linear energy input value based on the process parameters including laser density. power (P), scan speed (v), scan or hatch spacing (h) and Although volumetric energy density (VED) with laser layer thickness (T). This can be presented as beam diameter included is widely used in many studies [35], Fig. 1 Schematic illustrations for the different geometries 1 3 S. Li et al. Table 1 Coupon geometry and parameters fabricated by different strategies, coupon width w, coupon height l, external wall thickness c and coupon diameter Ø −1 Scan strategy Coupon size (w × l × c) or (Ø × l × c) Power/W Scan speed/(mm·s ) Hatch spacing/mm Low energy 5 mm ×10 mm ×1 mm 80–125 1 000–4 200 0.112 5–0.3 Mesh 2.5 mm × 2.5 mm × 0.5 mm 80–300 1 000–3 500 0.15–0.3 Grooved 10 mm × 10 mm × 1 mm (tooth interval 0.15 80–200 1 000–3 500 0.015–0.03 mm) it is not used in this study as the laser beam diameter is (perpendicular to the pipe inner surface). Different standard almost constant in this study. To study the influences of the tessellation language (STL) files were used for the teeth and process parameters on the porosity, coupons with opened the hollow pipe, ensuring that the scanning strategies would top and bottom ends were built. Inside the coupons, different not interfere with one another. The STL files were carefully scanning strategies were used to create and optimise porous positioned on the substrate to ensure a good overlap between structures, which are the low energy, mesh and grooved- the teeth and the external shell. This grooved-structure could strategies (see Fig. 1). The geometry and parameter informa- be used in both horizontal and vertical directions. tion of these coupons are listed in Table 1. The low energy scanning strategy used a raster scanning (simple scanning) 2.2 Characterisation strategy, rotated 90° between the subsequent powder layers, with a large hatch spacing (varying from 0.112 5 mm to 0.3 Sections of the porous structures were extracted for micro- mm) and relatively low power (within 80–125 W range). scopic examination. The cut sections were mechanically Since only one direction is scanned across each layer and ground and diamond-polished to a 0.3 μm oxide solution large hatch spacing is used, the nominal powder layer thick- finish. The samples were then immersed in ethanol and ultra- ness of the most area (apart from the cross points of the sacn sonically cleaned for one hour to remove the loose powder. paths between subsequent layers) in this strategy is doubled To quantify the porosity fraction, pore size and width of (i.e., 60 µm), as shown in Fig. 1. Therefore, this strategy the mesh beads, images were taken using a ZEISS opti- has a significantly lower VED input than other strategies. cal microscope and an HITACHI TM3000 scanning elec- This approach aims to create a highly porous mesh, with the tron microscope (SEM) before subsequent analysis using large hatch space resulting in non-overlapping laser tracks, ImageJ software. A Nikon custom bay micro-CT system was creating a mesh-like structure. Within the mesh strategy, the used to assess the internal 3D structure of the void space laser beam scansfully hatched patterns in each layer to create between the inner and outer walls of the heat pipe. Acquisi- a regular square mesh structure with the designed mesh size. tion parameters can be found in Table 2. A constant scan spacing was used for each direction and Micro-CT was also used to study the influences of ther - layer, assessing scan spacing within the range of 0.15–0.3 mal cycleson structural integrity. The thermal cycles were mm and power ranging 80–300 W. The mesh structure strat- as follows: ramp up to 600 °C at 5 °C/min, then held for 1 h, egy coupons were 2.5 mm × 2.5 mm × 5 mm mesh cubes, followed by 55 min air cooling. Each sample was built ver- surrounded by a 0.5 mm thick wall. The scan spacing for tically and was thermal cycled 50 times before another CT these cubes was fixed at either 0.24 mm or 0.3 mm. Lastly, scan. Each sample, with a length of 130 mm, was scanned the grooved-strategy was used in pipes with a circular cross in two different locations near the top and bottom of the section to create a pipe with internal teeth-like features. shaft with each scan comprising 20 mm of the shaft length. Although this die ff red from the two previous designs, where a uniform porous structure was used to maximise heat losses, Table 2 Acquisition settings used for the micro-CT the dense teeth-like structure was also known to deliver similar performances [36]. The grooved-structure used 0.15 Value mm long and 0.15 mm spaced teeth which were uniformly Accelerating voltage/kV 130 distributed on the internal surface along with the cylinder Tube current/µA 50 height. A round hollow cylindrical pipe with a 1 mm wall Target material Tungsten (W) thickness was fabricated as the external shell. The teeth Filter 2.5 mm copper (Cu) design aimed at maximising the surface area and reducing Reconstructed voxel size/µm 10 the performance difference between vertical direction and Exposure time/s 4 horizontal direction. The scan strategy for the teeth was a Number of projections 3 142 simple non-rotating scan parallel to the long axis of the teeth 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe The micro-CT provides 3D volume renders and non-destruc- the channels within the medium power sample. The samples tive virtual cross sections through the part. In addition, the in Figs. 3a, b had a bead width of 100–150 µm, increasing to cross-sectional area of the void space between the inner and 300 μm within the regions where balling (poor melt wetting) outer walls was segmented using a water shed segmentation is observed [37]. Conversely, the samples built using high algorithm. This allowed for quantification of the void space power (e.g., Figs. 3c, d), had a thinner bead, with a typical which could be used as a metric for determining changes bead width of lower than 200 µm. Comparing the samples non-destructively. built using different E and the same hatch spacing (e.g., Figs. 3a, c), the lower E condition (see Fig. 3c) had a more 2.3 Assessment of the capillary performance defined bead, due to narrower, yet stable, melt pool. The higher E condition (see Fig. 3a), whose wider melt pool The capillary performance was measured by performing in- was due to balling and its instability, led to discontinuities situ thermal measurements. A ThermaCAM SC640 infrared within the melted bead and the smudging of the channels thermal imaging camera was used to observe 18 °C deion- by partially melted powders. Lastly, when comparing the ised water wicking from the bottom of the heat pipe. The samples built using different hatch spacings (e.g., Figs.  3c, heat pipes were cut into two pieces along the vertical section d), the pore sizes become relatively larger, with a mean pore and a thermal camera was used to trace the temperature of size of 0.030 mm (61 µm equivalent diameter) in Fig. 3c the heat pipe, as shown in Fig. 2. Using an acquisition rate of compared with 0.041  mm (71 µm equivalent diameter) in 24 frames per second, the wicking speed (to achieve 18 °C at Fig. 3d. the top of the pipe) was calculated. A copper heat pipe made By plotting E against the area fraction of porosity (see by powder sintering was a reference sample. Fig. 4a), the porosity fraction reduces almost linearly with an increase in E . A similar trend is reported within the literature for LPBF of Ti-alloys and Ni-superalloys [16, 25, 3 Results and discussion 38, 39], showing a consolidation threshold exists when the energy density becomes sufficient to cause full consolidation 3.1 Low energy strategy of the material. The threshold is typically larger than 1.5 J/ mm for Ni-superalloys [40]. As such, it was expected that SEM micrographs, taken from the top of the builds (see the investigated parameters would create a porous structure. Fig.  3), show the morphology of the pores. The samples There is some scatter within the data, with less linearity were manufactured with three levels of power (named as within the E versus porosity relationships, for the samples low, medium and high powers), as shown in Table  1. As built using low power parameters. It is also found that the the power used in low energy strategy was relative lower highest porosity condition is achieved by the medium power than those in other strategy, the porosity was mainly altered parameter, which is caused by the combination of different by changing hatch spacing. The increasing hatch spacing hatch spacings and nonlinear changes of bead width. Bertoli results in more defined channels when using medium power et al. [35] studied the bead width and melt pool depth of (see Figs. 3a, b). A comparison of the higher power to the 316L SS on a single layer deposition condition. The bead medium power conditions shows the higher power leads to width on a laser fabricated surface has not been studied a better pattern definition, compared with more smudging of yet. Therefore, further study on the influence of E on the Fig. 2 Assessment of the capillary wicking action a the test setup with cold deionised water and the thermal camera, and b thermal imaging still shots at the start, midway and end of the test 1 3 S. Li et al. Fig. 3 SEM micrographs for the top surface and horizontal section of the lower energy strategy samples bead width is necessary. Only six samples built using low the process generates thicker beads which reduce the gaps power were successfully fabricated, creating a structurally between neighbouring tracks, creating a denser material. sound build, out of the 32 samples built at this power. The On the lower energy density end, balling generates non-uni- low power was insufficient to fully melt the powder, instead formed porosity, resulting in smudging of holes and irregular it induced the balling effect [ 41], which could lead to dis- mesh structures, while the higher energy density will block continuities within the laser tracks. Valdez et al. [29] also the mesh pores by densifying the material. Similarly, the reported sintered powder and discontinuous bead in LPBFed hatch spacing must be chosen to balance the energy den- IN718 mesh structures. The number of structurally sound sity effects to achieve the target density. Using Fig.  4, the samples increased with increasing laser power. It must be optimum processing window (50% porosity) is when E is emphasised that uniformed porous structures were only pos- within 0.3–0.4 J/mm range, with the process parameters sible through the use of a large hatch spacing, alongside the following this relation lower energy density range to avoid overlap between the 20P 9 neighbouring tracks. h − = , 80 ≤ P ≤ 125, 200 ≤ h × v ≤ 416.67, 13ν 130 Previous analysis using E considered all parameters (4) [29]. However, to deconvolute the impact of the hatch spac- where the laser beam is considered as a constant beam with ing, it is beneficial to look into the relationship between E average beam diameter of 70 ± 3 μm. When comparing the and the hatch spacing, as shown in Fig. 4b. The combina- results of this strategy with the findings of later strategies, tion of E lower than 0.1 J/mm and hatch spacing within it is not going to be straightforward from an energy den- 0.18–0.3 mm range is required to achieve the target of 50% sity viewpoint. Within the lower energy strategies, the same porosity, while keeping the power lower than the high power laser tracks are repeated every two powder layers, meaning level. The energy density affects both the melt flow and the the nominallayer thickness is 60 μm. This also affects the degree of wetting, which may lead to balling by insufficient integrity of the structure, since the melt pool in each track wetting and discontinuities beads when the energy den- sity is low. Meanwhile, if the energy density is too high, 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe Fig. 4 Porosity area fraction versus a the area energy density, b linear energy density, c power and scan speed (with 0.3 mm h) within the lower energy density builds (the black dash line indicates the unstable parameter area) Fig. 5 Mesh strategy samples produced by a low power, E = 0.064 J/mm, E = 0.27 J/mm , b medium power, E = 0.067 J/mm, E = 0.22 J/ L A L A 2 2 mm , and c high power, E = 0.056 J/mm, E = 0.19 J/mm L A must be deep enough to weld the subsequent layers together, had less energy due to the surface tension of the melt. It especially each n and n+2 layers. is known that evaporation and plasma recoil pressure dur- Regarding the build quality, partially melted/sintered ing LPBF, which may force the powders to move towards powder was found on the sample surface. The size of the the laser tracks, depend on the process parameters, causing powder was between 10 µm and 50 µm, the size range of as- a powder spreading known “denudation” [42]. No strong received powder, suggesting that lower energy reduced the metallurgical bonding formed between the melt track and tendency for consolidation at the melt pool edge. Instead, the surrounding powders, allowing for them to be removed powers partially stuck to the edge of the melt pool, which by sandblasting or a similar technology. 1 3 S. Li et al. 3.2 Mesh strategy porosity viewpoint (see Fig. 6b). Due to the higher energy input, the melt pool becomes hotter and wider, pulling more SEM micrographs, taken from the XY section of the mesh powder particles by surface tension and recoil, which leads strategy samples, showed defined square pores at lower pow - to further partial melt and sintering[42]. In general, the ers, with more partially melted powder particles sticking on structure produced by low energy and mesh strategy shows the structure athigher powers (see Fig. 5). Overall, samples the ability of fabricating thin structure around 100 μm in built with higher laser power had more partially melted/sin- width, which is half the size of micro-channel reported in the tered particles attached to the beads, with less defined mesh literature (220–670 μm) [18, 33, 43]. Comparing low energy structures. This result highlights the impact of scanning strategy to mesh strategy with the same parameters (Fig. 4c strategies in controlling the amount of heat. As aforemen- versus Fig. 6b), low energy strategy has higher porosity. tioned, the nominal layer thickness within the low energy By plotting the bead width (D) against E (see Fig. 7), a strategy is equivalent to half the nominal volumetric energy relation between the bead width and E =P/ν was developed within the mesh strategy, should the same laser parameters D = A(P) × ln E + B(P), (5) be used. As such, balling was less prominent within the low power mesh conditions (see Fig. 5a). Conversely, the where P is laser power, A= –0.026P + 58.38 and B=0.079P higher power conditions resulted in more powders sticking + 214.39. The R value of the model is from 0.972 to 0.998, to the melt bead and consuming surrounding powders (see indicating good accuracy, as shown in Fig. 7. This indicates Fig. 5c). The medium power condition produced stable thin that the laser power has a strong influence on melt pool wall around 100 μm, as shown in Fig. 5b. The lower energy size, as the energy input of laser and number of reflections and mesh strategies were able to deposit thin walled struc- increases with increasing laser power when 80 W ≤ P ≤ 300 tures in this study at energy levels reported as not printable W. The high laser power enables more reflections within parameters in other studies [38]. the powder bed, allowing more powder to absorb the laser In the mesh strategy, two scan spacing levels were used, energy and therefore creat a larger melt pool [44]. Dilip et al. while maintaining a generally low E to avoid full or near [25] also reported the melt pool width in LPBF of Ti6Al4V consolidation of the mesh, due to the creation of thicker followed a logarithmic relation to both the laser power and beads when a higher E is used. The parameters covered the scan speed. The influence of the power on the bead width is most power used in LPBF (up to 350 W). Figure 6a shows less significant for lower power parameters (comparing low the influence of the laser power and scan speed on the laser power to high power in Fig. 7). It also shows that the influ- track (bead) width within the mesh. The bead width linearly ence of power is stronger in higher E parameters (low scan increased with the increase in power and decrease inscan speed) comparing to low E parameters. Comparing with speed. At higher powers, the bead width grows slower at the other studies using single layer single bead method [35], higher scan speeds. The bead width grows higher than hatch our multi-layer single bead study shows that the laser power spacing when the E exceeds 0.13 J/mm. Within conditions L has stronger impact on the bead width (increased from 90 sharing the same E , the bead width slightly increases with µm to 120 µm, when power increased from 80 W to 300 W). the increase in laser power, especially when the power higher This could be associated with the increased laser energy at than the high power level. This behaviour also holds from a Fig. 6 Process maps for the mesh structure showing a the influence of the laser power and scan speed on bead width, and b the influence of the laser power and scan speed on porosity 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe Fig. 7 Variation in bead width with the linear energy density in mesh strategy samples and medium power low energy strategy samples higher powers, as well as the reflection and absorptivity of build direction, is affected by over melting and a lack of sup- powder [44, 45]. The higher power provided more energy port, as shown in Fig. 8c. Partially-melted particles between to the laser, allowing more reflections between powder par - the horizontal teeth (see Fig. 8d) have a similar structure to ticles in the powder bed. The low energy strategy samples heat pipe with sintered-grooved composite wick reported by built by medium power (the same power as the green line Li et al. [46], which showed good performance. In general, of mesh strategy) has slightly higher bead width due to the low E benefits the fine structure fabrication, which ensures thicker nomial powder layer comparing to the mesh strategy a reasonable gap between the teeth. samples. The non-uniform morphology among the bottom, mid- dle and top surface could be overcome by manipulating the 3.3 Grooved‑strategy building orientation and using gradient process parameters. The morphology of the top teeth could be improved by tilt- The low energy and mesh studies show thin wall around ing the samples 45° to the building direction. On the other 100 μm width is printable in LPBF. Previous works sug- hand, using a lower energy process parameters for the mid- gested that horizontally-built structures were affected by diameter sidewall and the top area could also reduce the over-melting and a lack of support at the base surface [16]. over-melting and partially melting of the powders. A few grooved structure heat pipes with the 100–150 µm width teeth perpendicularly positioned to the pipe inner wall 3.4 Micro‑CT were fabricated in both vertical and horizontal directions, and the geometrical integrity of this structure is studied. According to the aforementioned results, three samples To reduce the influences of the laser scanning direction on with 50% porosity were fabricated for micro-CT study. The teeth geometry, the laser scan path is controlled, meaning it parameters and strategy for these samples were medium is always parallel to the longitude direction of the teeth. A power low energy strategy sample (see Figs. 3b, f), medium number of optimum parameters were selected base on the power mesh strategy sample (see Fig. 5b), and low power low energy and mesh studies to maintain a 120 μm teeth grooved strategy sample (see Fig. 8), respectively. Sections interval distance at the tips. Among the investigated param- cut from the top and the bottom of the vertically built heat eters, the E = 0.064 J/mm condition resulted in a smaller pipe were analysed using micro-CT. The grooved-structure teeth width (100 µm), with a uniformed 100 µm spacing with 1 mm teeth length and 150 µm spacing is shown in between the teeth. Fig. 9a, where limited powder sintering on the groove sur- The horizontally-built grooved-structures have good geo- face is found. The vertical builds showed a greater uniform- metrical integrity, at both the bottom and the top of the pipe, ity within the teeth size and the distance between them. as shown in Figs. 8a, b. The geometry accuracy of grooved- Nonetheless, a continuous groove channel was built, as strategies was strongly affected by the build direction. The shown in Fig. 9b. length of the top teeth is shorter, due to a lack of support Figures  10 and 11 shows the calculated mean cross- within the first few layers and an over melting from the pipe section of void space and deviation values of the LPBFed wall, but the gap between the teeth remains the correct size. heat pipes base on CT scaned segmentations. It is found The geometry of the side teeth, which is perpendicular to the that the structures are uniform in both simple structure and 1 3 S. Li et al. Fig. 8 SEM micrographs for the horizontally built grooved-structures a the bottom of the tube with 100–150 μm gap between the teeth, b the top of the pipe, c the mid-diameter side of the tube, and d the side view Fig. 9 Micro-CT 3D models showing a the uniformity of the internal surface of the heat pipe and b the continuity of the grove structures throughout the built height grooved-structure samples, while the mesh structure had three strategies due to reducing heat transfer and increasing higher variation (about 11%). The cross-section area in the heat accumulation at higher building height. These prompt grooved-structure has the lowest deviation as the structure the balling effect and partial melt powder. is uniform along the build direction. The cross section of the In addition to the volume renderings and skeletonized mesh structure, however, has the highest deviation among structures that can be generated using the reconstructed 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe 3.5 Bead formation and melt pool stability The melt pool is considered as a liquid cylinder when study- ing its stability. One of the common defects in LPBF is that the unstable melt pool breaks into droplets [24, 38]. The Plateau-Rayleigh instability contributes to this unstable sta- tus when higher scan speeds are applied [38]. Yadroitsev et al. [23] proposed that the stability of liquid was related to the contact angle Φ (between liquid cylinder and substrate), the diameter of molten pool (D), and molten pool length (L). The melt pool is stable regardless of its dimension when contact angle Φ < π/2. Otherwise, the stable condition is πD 2 > . (6) L 3 As illustrated in Fig. 12a, the melt pool on the flat surface Fig. 10 Mean and standard deviations of the cross-sectional area of could have Φ > π/2, due to surface tension and lower power the void space at different heights of medium power low energy sam- inputs. This well represents the reality of the single bead, ple, low power grooved sample, and medium power mesh sample and single layer condition [23]. During the thin wall or lat- tice structures fabrication in LPBF, the top convex surface of slices (for example, Fig.  9), more intuitive 2D visualiza- the previous layer is close to a semi-cylinder, which means tions can be produced. These “virtual cross-sections” present that the contact angle between the melt pool and solid could a plane through the sample and provide a non-destructive be less than π/2 (see Fig. 12b).Therefore, the melt pool is means for assessing the structure of the heat pipe as shown more stable when producing a thin wall structure than a solid in Fig. 11. The samples were scanned at as-fabricated condi- block. Jasper and Anand [47] reported on reducing contact tion (see Figs. 11 a, c), and then scanned after 50 thermal angle between liquid and convex surface, with the reduction cycles (see Figs. 11b, d) to investigate the thermal cycle sta- insurface radius. As such, the melt pool on the convex sur- bility of the structure. There is no obvious change of cross- face, when producing thinner structures, has better stability section area after thermal cycles, which indicates that the than the melt pool on the flat surface. In the axial direction, samples are stable. the melt pool is unstable if the D/L ratio is less than 0.26 (see Eq. (6)). Thus, the higher the scan speed or the lower Fig. 11 Virtual cross-sections through vertical built sample a grooved-structure in as-fabricated condition, b grooved-structure after 50 thermal cycles, c mesh structure as-fabricated condition, and d mesh structure after 50 thermal cycles (the red portion of the image represents the result of the watershed segmentation) 1 3 S. Li et al. Fig. 12 Schematic drawing of the melt pool during LPBF process when producing different structures a for melt pool on substrate or block sam- ple with a relatively flat surface, b for melt pool on thin structures with a curved surface ,and c more solid area (the red colour is the melt pool, with the yellow and grey the solidified beads) the laser power is, the lower the D/L ratio is, meaning the by a high E parameter to create a stable wide bead with melt pool is less stable. In this study, it is assumed the melt cruved surface, which improve the stability of the melt pool. pool diameter (D) is the same as track/bead width, regardless By gradually reducing the energy input of the initial layers of shrinkage and morphology variations during solidifica- could create a better fabricating surface for the fine structure tion. According to Eq. (6), the maximum stable melt pool and improve the melt pool stability. length of printable parameters with lower energy strategies is between 384.6 µm and 576.9 µm. The maximum stable 3.6 Wick test melt pool length of printable parameters with mesh strategy is between 154 μm and 530 μm. The wick performance of the different scanning strategies Furthermore, the geometry of mesh structure, which are and the manufacturing parameters was measured using thin walls perpendicular to each other, also helps stabilise the response time and height of wicking, as per the test the melt pool by providing more contact surfaces. The peri- described in Sect. 3.3. As shown in Fig. 13, a number of odically solid walls could stabilise the liquid via surface ten- the LPBF IN718 heat pipes demonstrated a good wick per- sion and higher cooling rates on solids, shortening the melt formance, compared with the reference Cu heat pipe made pool length, as shown in Fig. 12c. Most conditions in this from pre-sintered powders, as measured using the response study could have melt pool lengths larger than the distance time. The response time was calculated using camera video between the thin walls. Thus, the thin wall mesh structure record from 5 mm wick height to the maximum wick height has a broader processing window than lower energy strate- and fit to first order respond as equation below gies, and the mesh strategy sample has less porosity compar- −t ing the low energy one with the same parameters (Fig. 6b H = H 1 − e , (7) wick wick−max versus Fig. 4c). From the study above, we found that the stable melt pool where H is the wick height, H the maximum wick wick wick-max conditions of the first layer of fine structure (flat surface) height, and τ the respond time. The maximum wick height and the later layers (curved surface) were different. This is of the heat pipes, however, shows a larger deviation, as the one of the reasons that significant difference is often found partially sintered powders or bead discontinuities due to between the optimised processing parameter of fine structure unstable melt pool on the wall. From the test results, the and block structure in LPBF. Hence, we propose the process heat pipes with fabricated by low power grooved-strategy parameter of the first layer of the fine structure should be dif- has the best overall wick performance within the horizon- ferent to the later layers. The first layer should be fabricated tal direction, while the heat pipe built by low power low 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe between solids. The thin wall structure has a border processing range than the block structure due to round top surface. (iv) The micro-CT can be used as a non-destructive evalu- ation method for the structural performance of LPB- Fed heat pipes. The micro-CT results suggested the structure produced by LPBF was stable under thermal cycles. Acknowledgements The authors would like to acknowledge Rolls- Royce plc, Aerospace Technology Institute, and Innovate UK for fund- ing this research through the Advanced Repair Technologies (113015) programme. The CT scans were performed in the University of Man- chester, which was established through EPSRC Grants EP/F007906/1, EP/I02249X/1 and EP/F028431/1. HMXIF is a part of the Henry Royce Fig. 13 Comparison of the response time and wick height of the Institute for Advanced Materials, established through EPSRC Grants LPBF heat pipes, with a Cu heat pipe as a reference EP/R00661X/1, EP/P025498/1 and EP/P025021/1." Open Access This article is licensed under a Creative Commons Attri- energy strategy (Low P Low Erg. V) has the best perfor- bution 4.0 International License, which permits use, sharing, adapta- mance within the vertical direction. The grooved-strategy tion, distribution and reproduction in any medium or format, as long also showed a good performance within the vertical direc- as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes tion, as the response time was the shortest, yet the wicking were made. The images or other third party material in this article are height was slightly lower than the low energy scan strategy. included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will 4 Conclusions and future work need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Heat pipes with designed porosity structures have been suc- cessfully manufactured by adopting three different strate- gies (lower energy, mesh and grooved). The wick test results References show the good performance of LPBFed heat pipes. The main conclusions are as follow. 1. Jouhara H, Chauhan A, Nannou T et al (2017) Heat pipe based systems—advances and applications. 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Valdez M, Kozuch C, Faierson EJ et al (2017) Induced poros- ity in super alloy 718 through the laser additive manufacturing 1 3 The development of a high-performance Ni-superalloy additively manufactured heat pipe Sheng Li received Ph.D. States Chiwanga received his degree in Materials Science from BSc degree in Aeronautical University of Birmingham, UK, Engineering from University of in 2017. He is currently working Salford, UK and his MSc degree as a research fellow in the School in Fluid Mechanics from Wash- of Metallurgy and Materials, ington State University, USA. University of Birmingham. He is States has worked in senior posi- the author of 24 research publi- tions for well-known multina- cations. His research interests tional engineering companies include additive manufacturing including Schlumberger, of metal and ceramic, processing Solectron and PDD Group before and microstructure of shape starting Cambridge Engineering memory alloys, Ni superalloys, Analysis and Design Ltd. Ti alloys, Al alloys and metal (CEAD) in 2007. He is now the matrix composite. He has director/CFD &Thermofluids worked on the projects supported Consultant in CEAD. by leading companies across the world including Rolls-Royce, Airbus, Honda, BAE Systems, MBDA Missiles, MicroTurbo, Ford, and Euro- Andrew Norton received his pean Space Agency. MSci in Natural Sciences (Mate- rials Science) from University of Khamis Essa is Reader in Cambridge and DPhil (Ph.D.) Advanced Manufacturing and degree in Materials from Univer- Process Modelling at the School sity of Oxford, UK in 2013. He of Engineering, the University of is now work as technologist and Birmingham and the Director of managing the projects gear- the Advanced Manufacturing funded through Aerospace Tech- Group. His research is focused nology Institute &InnovateUK, on advanced manufacturing tech- involving collaborations with a nologies for instance Metal and number of Rolls-Royce Univer- ceramic 3D printing, Hot Iso- sity Technology Centres and static Pressing and Incremental Small/Medium-Sized Enter- Sheet Forming. He has been prises, and utilise a global developing novel manufacturing Research & Development supply routes and applications for aero- chain network. Including the development of novel systems to perform space, defence, automotive and inspection and repair inside aeroengines, emerging technologies (typi- biomedical industries. The scien- cally Technology Readiness Level 6 and below), supporting the move- tific emphasis of his research is on material and process interaction. He ment of on-wing inspection and repair technologies into production has been leading a number of research projects funded by EU, UK through a wide ranging supply chain network. research councils and industry. His research has been supporting major companies across the world including Rolls-Royce, BAE Systems, Moataz M. Attallah holds a MBDA Missiles, MicroTurbo, Ford, European Space Agency and Cat- chair in advanced materials pro- erpillar. Dr Essa has over 80 journal and conference publications in cessing at the School of Metal- addition to two patents and one book. He is guest editor for the micro- lurgy and Materials University mechanics journal and is setting on the editorial board of three other of Birmingham. His research international journals. focuses on developing a metal- lurgical understanding of the James Carr received his material-process interaction in MPhys degree in 2008 and Ph.D. additive manufacturing of metal- degree in Materials Science in lic materials focusing on the pro- 2014 from the University of cess impact on the microstruc- Manchester. James Carr is an ture and structural integrity Applications Specialist for development. His research is MicroCT at Blue Scientific. He conducted through research part- acquired his first degree in Phys- nerships with various companies ics at the University of Manches- in the aerospace, defence, medi- ter in 2008 and then a PhD in cal, space, and nuclear energy sectors. He co-authored over 170 journal Materials Science in 2015. He and conference papers, 3 book chapters, and is a co-inventor on 8 patent has spent his research career spe- applications. cializing in 3D imaging and image processing at the Henry Moseley X-ray Imaging Facility and Diamond Light Source. 1 3

Journal

Advances in ManufacturingSpringer Journals

Published: Aug 1, 2022

Keywords: Laser powder bed fusion (LPBF); Heat pipe; Melt pool; Microtomography (micro-CT)

References