Wear of Polycrystalline Boron Nitride Tool During the Friction Stir Welding of Steel

Wear of Polycrystalline Boron Nitride Tool During the Friction Stir Welding of Steel The wear issue of a polycrystalline boron nitride (PCBN) tools during the friction stir welding of two grades of steel, DH36 and EH46, was studied. Two welding traverse and tool rotational speeds were used when welding the DH36 steel. A low tool speed (200RPM, 100 mm/min) and a high tool speed (550RPM, 400 mm/min) were denoted by W and W , 1D 2D respectively. Nine welding conditions were applied to the welding of EH46 steel plate including seven plunge/dwell trials (W –W ) and two steady-state trials (W and W ). SEM–EDS and XRD tests were applied in order to reveal the 1E 7E 8E 9E boronitride (BN) particles inside the welded joints, and the percentage (%) of BN was calculated according to the standard quantitative metallographic technique. The findings showed that tool wear increases when the tool rotational speed increases as a result of binder softening which is a function of the peak temperature (exceeds 1250 C) at the tool/workpiece interface. When considering the EH46 steel trials, it was found that an increase in the tool traverse speed in friction stir welding caused a significant tool wear with 4.4% of BN in the top of the stirred zone of W compared to 1.1% 9E volume fraction of BN in W which was attributed to the higher thermomechanical action on the PCBN tool surface. Tool 8E wear was also found to increase with an increase in tool plunge depth as a result of the higher contact between the surface of friction stir welding tool and the workpiece. Keywords Friction stir welding  PCBN tool  Wear  DH36 and EH46 steel Introduction complex designs at relatively low costs. Welding high melting alloys requires a FSW tool material with higher Friction stir welding (FSW) is a solid-state welding tech- thermal and mechanical properties compared to other tools nique invented at The Welding Institute (TWI) in 1991 [1]. used for low melting point materials. Polycrystalline boron This solid-state process was successfully developed to nitride (PCBN) is the most widely used super-abrasive enable welding of aluminum alloys such as the 2XXX and ceramic FSW tool. Table 1 [2] shows the mechanical and 7XXX series which, at that time, were considered un- thermal properties of a PCBN tool compared to tungsten weldable using standard fusion welding processes [1]. carbide (WC) and H13 FSW tools. The microhardness of However, for welding high melting alloys such as steel, the this tool HV (2600–3500) shows that the PCBN tool is the process is still limited because of the high cost of the FSW second hardest tool after diamond. It also has a low coef- tool. For welding low melting alloys such as aluminum, the ficient of friction which in turn helps in producing smooth tool is usually made from a suitable steel grade such as weld surfaces [3]. However, a low coefficient of friction in alloy H13 which can be used to produce tools with a FSW tool necessitates an increase in tool rotational speed to produce the required heat for welding [4]. The PCBN tool represents an alternative to refractory tools such as & M. Almoussawi tungsten-based materials which showed severe wear, inj.mun@atu.edu.iq; b1045691@my.shu.ac.uk especially during FSW of steel [5]. The PCBN tool usually 1 contains cubic boron nitride (CBN) particles in an alu- Al-Furat Al-Awsat Technical University, Kufa, Iraq minum nitride (AlN) binder [6]. This combination between MERI, Sheffield Hallam University, S1 1WB Sheffield, UK the CBN and the binder is designed to increase the strength Coventry University, Coventry, UK 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 253 Table 1 The physical properties Property Units PCBN Tungsten carbide 4340 Steel of PCBN compared with some other materials [2] Coefficient of friction … 0.10–0.15 0.2 0.78 -6 Coefficient of thermal expansion 10 /C 4.6–4.9 4.9–5.1 11.2–14.3 Thermal conductivity W/mK 100–250 95 48 Compressive strength N/mm 2700–3500 6200 690 10 psi 391–507 899 100 Fracture toughness MPa m 3.5–6.7 11 100 Hardness Knoop kg/mm 2700–3200 … 278 Vickers kg/mm 2600–3500 1300–1600 280 Tensile strength N/mm … 1100 620 10 psi … 160 89.9 Transverse N/mm 500–800 2200 … Rupture strength 10 psi 72–115 319 … of the tool. Despite the high strength and thermal proper- workpiece. The PCBN tool is also attached to a shank ties, the PCBN tool has low fracture toughness and is also made of WC which acts as a holder and attaches the tool to susceptible to wear problems, especially during the plunge the PowerStir FSW machine. Both parts of the tool, period of the weld due to the higher temperature generated including the PCBN material and WC shank, are fastened which can cause a softening of the binder [3]. The plunge together by a collar. The different parts of the PCBN tool period is also associated with a high plunge force which geometry are listed in Table 2 which is according to the can encourage BN particles to detach from the tool and workpiece thickness (6 and 14 mm). Welding a 6-mm- stick into the workpiece. The development in metal-com- thick specimen is usually carried out using a PCBN tool posite material has encouraged manufacturers such as with 5.5-mm probe length, while for 14-mm-thick speci- MegaStir to produce new grades of PCBN tools with a men a probe length of 12 mm is usually used. The Pow- longer service life. This development was a step forward in erStir FSW machine includes a cooling system applied to order to commercialize the FSW of high melting alloys. the tool shank in order to reduce the temperature resulting from the friction stir welding of steel which is readily For example, a FSW tool made from Q70 (70%PCBN, 30%WRe) has a higher toughness compared to the previous transmitted due to the high thermal conductivity of the one which included an AlN binder [6]. The melting point PCBN tool. Argon shielding is usually applied during the of a Q70 tool PCBN material was determined to exceed FSW process mainly to extend the tool life and also to 3000 C, and the microstructure and tool image are shown prevent oxidation of the welded joint. A thermocouple is in Fig. 1a and b, respectively [6]. The Q70 tool has been usually attached to the tool shank and is located behind the used extensively by TWI to join many steel grades PCBN tool. This telemetry system is employed to monitor including 316L stainless steel, 304 stainless steel, and tool temperature during the FSW process. The FSW tool DH36 and EH46 steel grades which are under considera- manufacturer recommends that the temperature of the tool tion in this work. PCBN-WRe Q70 tool as shown in Fig. 1b measured by the telemetry thermocouples system should be consists of a shoulder and probe surrounded by a collar of a kept in the range of 800–900 C in order to protect the tool Ni–Cr alloy which works as an insulator for the tool from from excessive wear/breakage issues [5]. Other tool the environment, so the heat generated during the FSW is materials and their properties which have been used for almost distributed between the PCBN tool and the high melting alloys are usually made of refractory Fig. 1 (a) An SEM image of the PCBN microstructure of a cross section PCBN through a PCBN Q70 (70% cBN, 30%WRe) [6] FSW tool, Collar (b) PCBN tool image WRe binder Shank ab 123 254 Metallography, Microstructure, and Analysis (2018) 7:252–267 Table 2 PCBN tool geometry provided by TWI Tool PCBN shoulder PCBN probe Collar Shank PCBN tool geometry for welding 23.7 mm diameter with spiral 5.5 mm length, 10 mm base 23.9 mm inner 23.9 mm diameter 6 mm thickness convex shape diameter diameter 80.35 mm length Spiral tapered with 20 thread 37 mm outer per inch (TPI) diameter 24 mm length PCBN tool geometry for welding 38 mm mm diameter with 12 mm length, 20 mm base 38.1 mm inner 38.1 mm diameter 10–15 mm thickness spiral convex shape diameter diameter 100 mm length Spiral tapered with 20 thread 52 mm outer per inch (TPI) diameter 30 mm length materials based on tungsten such as WC, W-Re, W-Co and the ability to react with Ti-forming TiB which in turn can can be found in the literature [3, 7]. This paper will focus cause grain refinement and a hardness increase [10]. PCBN only on the PCBN tool wear as this is the tool which has tool breakage is more likely to occur during the FSW been employed to produce the samples under study. process due to the lower fracture toughness of a PCBN tool Wear resistance in hybrid PCBN FSW tools is consid- compared to a WC tool, Table 1. Unsuitable welding ered better than other refractory materials such as tungsten- parameters such as improper plunging or extracting and based tools. W-25%Re FSW tool life of 4 m was reported low welding temperatures were determined as the main in welding steel grades and titanium [7]. However, wear factors for tool breakage [5]. issues still exist in PCBN tools when welding high melting Previous work on PCBN tool wear during the friction alloys especially the high strength alloys such as the EH46 stir welding of steel did not investigate the possibility of steel grade. An experimental study of a PCBN tool has W-Re binder softening as a result of high temperature been carried out by Rai et al. [3] and showed that the main which may reach the local melting of steel alloys during factors which can cause wear are abrasion and diffusion. the process, especially when the tool rotational speed Softening and recrystallization of the binder after reaching reaches specific limits. Also limited work has been carried a welding temperature of 1000 C is also found to be one out to investigate the effect of tool traverse speed on the reason for reducing the tools resistance to wear [3]. wear rate and the consequences on the physical properties Ramalingam and Jacobson [8] reported a decrease in of the welded joints. The current work focuses on PCBN Knoop hardness (HK) of W-25Re from HK 675 (HV 638) tool wear which occurs during the FSW process of two to HK 500 (HV 478) when carrying out heating from room steel grades, DH36 and EH46. The FSW tool wear has temperature to 1225 C. The hardness was found to been studied by calculating the volume fraction of BN decrease dramatically to HK 300 (HV 290) when the (originally from the PCBN FSW tool) existed in the ther- temperature increases to 1450 C. Hooper et al. [9] sug- momechanical affected zone (TMAZ) microstructure. The gested that the higher thermal conductivity of cBN aims are to identify the causes of PCBN tool wear and to (100–250 W/m K) can result in defects in the microstruc- highlight the different suitable welding parameters that can ture when the temperature exceeds 1200 K, while with a increase the tool longevity. comparison with cBN-TiC, they found that a protective layer (mainly TiC) is formed on the latter and is associated with the higher temperature as a result of lower thermal Experimental Method conductivity of cBN-TiC. PCBN tool wear can also change the properties of the material being welded. For example Chemical Composition of Parent Metal tool wear can affect negatively a stainless steel welded and Welding Conditions joint as boron particles can react with Cr to form borides which in turn can result in a reduction in corrosion resis- Tables 3 and 4 show the as-received chemical composition tance [3]. When welding titanium plates, boron can (wt.%) of the 6-mm-thick DH36 and 14-mm-thick EH46 improve the mechanical properties of the joint because of steel plates. The welded samples have been produced at Table 3 Chemical composition C Si Mn P S AlN Nb V Ti Cu CrNiMo of as-received DH36 steel grade (wt.%) 0.16 0.15 1.2 0.01 0.005 0.043 0.02 0.02 0.002 0.001 0.029 0.015 0.014 0.002 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 255 Table 4 Chemical composition of as-received EH46 steel grade (wt.%) C Si Mn P S Al N Nb V Ti 0.20 0.55 1.7 0.03 0.03 0.015 0.02 0.03 0.1 0.02 TM TWI/Yorkshire using their PowerStir FSW machine, Scanning Electron Microscopy (SEM) and Energy- employing a Q70 PCBN FSW tool. The welding conditions Dispersive X-ray Spectroscopy (EDS) Examination used for the production of samples of DH36 steel (W and 1D W ) are shown in Table 5. These were chosen to examine SEM and EDS were carried out on polished (until 1 lm) 2D the effect of tool rotational and traverse speeds on tool and etched (2% nital) FSW samples using the FEI Nova wear. In order to examine tool traverse speeds above Nano SEM. The examination included the surface of the 200 mm/min, it is necessary to simultaneously increase the stirred zone (SZ) and thermomechanical affected zone tool rotational speed. The welding conditions for the pro- (TMAZ) which are the regions which experienced both duction of samples of EH46 steel (W –W ) which thermal and mechanical effects during the FSW process 1E 7E examined the effect of rotational speed and plunge depth under steady-state conditions and at the plunge period. The on tool wear are shown in Table 6. The welding process SEM produced high-quality and high-resolution images of parameters used to examine the effect of tool traverse microconstituents using the secondary electron (SE) speed on tool wear (W and W ) for EH46 steel steady imaging mode with an accelerating voltage of between 10 8E 9E state are shown in Table 7. and 20 kV. The working distance (WD) used was 5 mm, but in some cases, it was altered (decreased or increased) to enhance the contrast at higher magnification. EDS–SEM examination was mainly used to detect the BN particle Table 5 Welding conditions for FSW DH36 steel samples W and W 1D 2D Weld Tool rotational Traverse Rotational/traverse Average Average tool Axial force Longitudinal Heat input xtorque no. speed ðxÞ RPM speed speeds (rev/mm) spindle torque torque N m (average) force (average) (V) mm/min Nm KN KN W 200 100 2 278 105 57.55 12.8 210 1D W 550 400 1.375 163 62 47 12 64.625 2D Table 6 The welding conditions for FSW EH46 steel to examine the effect of rotational speed and actual plunge depth on tool wear (W –W ) 1E 7E Weld Tool rotational speed x Max. axial (plunge) Max. longitudinal Max. torque Plunge depth (Z) mm Dwell time (t) sec at trial no. (RPM) at dwell period force (F )KN force (F )KN (M) N m from FSW machine (dwell) period z x W 200 157 17 498 13 6 1E W 200 127 17 471 13 8 2E W 120 116 21 598 13 7 3E W 120 126 20 549 13 6 4E W 120 115 17 532 13 7 5E W 120 105 18 583 13 7 6E W 120 119 20 548 13 7 7E Table 7 Welding conditions for FSW EH46 steel during steady-state welding (W and W ) 8E 9E Weld Tool Traverse Rotational/traverse Average Average tool Axial force Longitudinal Heat input xtorque no. rotational speed mm/ speeds spindle torque torque N m (average) KN force (average) speed RPM min Nm KN W 150 50 3 300 114 66 13 342 8E W 150 100 1.5 450 171 72 14 256.5 9E Examining the effect of traverse speed on tool wear 123 256 Metallography, Microstructure, and Analysis (2018) 7:252–267 inside the TMAZ. Spot analysis (point and ID) has been employed to elementally identify small second-phase par- ticles in the scanned SEM images. Additionally, EDS mapping was used to analyze the whole scanned SEM area. X-ray Diffraction X-ray diffraction (Cu X-ray tube) using the Empyrean Philips XRD has been carried out on the FSW samples for the following reasons: • To reveal and characterize the as-received sample phases in order to allow for the detection of any phase changes in the steel following FSWing. • To detect other additional phases, elements, and/or boronitride (BN) particles which may appear in the welded joints. Infinite Focus Microscopy (IFM) Fig. 3 SEM image of the top of SZ of DH36 W shows the 2D existence of BN The infinite focus microscopy (IFM) (Alicona) has been the SZ of sample W and W were scanned at a maximum 8E 9E employed to create accurate optical light microscopy magnification of 10,0009. The area fraction of BN particles images of the welded joint. The IFM is a device based on and TiN precipitates has been measured manually using the optical 3-dimensional measurements which has the ability square grid method. The number of intersections of the grid to vary the focus in order to obtain a 3D vertical scanned falling in the BN particles is counted and compared with the image of the surface. The scanned area of interest can be total number of points laid down [11]. transferred into a 3D image by the aid of Lyceum software; thus, the surface area can be calculated accurately. Result Calculating the Percentage (%) of BN in the Welded Joints Effect of Tool Rotation and Traverse Speed on Tool Wear in FSW of DH36 Steel: Samples W 1D The % of BN particles originating from the PCBN FSW tool and W 2D in the TMAZ of the welded joints were calculated using the SEM. A 1 mm area between the shoulder and the probe side The existence of BN particles originating from the PCBN (W –W ) and a square mm (1 mm ) from the middle top of 1E 7E FSW tool was investigated in the FSW joints of grade Fig. 2 Ten-micrometer BN particles in SZ of FSW DH36 joint (W 550RPM, 400 mm/min) 2D 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 257 Fig. 4 XRD scan of FSW of DH36 steel (a)W (200RPM, 100 mm/min) microstructure consists of BCC ferrite after the FSW process. (b)W 1D 2D (550RPM, 400 mm/min), microstructure is mainly ferritic BCC phase; BN peaks are also present (Co-X-ray tube) Fig. 5 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 1E magnification (region under tool shoulder) and (b) high magnification 123 258 Metallography, Microstructure, and Analysis (2018) 7:252–267 DH36 steel which had been produced at high tool speeds in Effect of Tool Rotational Speed and Actual order to attempt to assess tool wear. SEM–EDS examina- Plunge Depth on Tool Wear in FSW EH46 Steel: tion of FSW joint W (DH36, 550RPM, 400 mm/min) in Samples W –W 2D 1E 7E Figs. 2 and 3 shows different sizes of BN particles in the SZ. Figure 4a and b shows the XRD scan results from The presence of BN particles has been investigated in the samples W and W , respectively. Note that peaks of microstructure of FSW samples of EH46 weld joints during 1D 2D ferrite are shown in both welds and BN peaks are seen in the plunge/dwell period in the shoulder-probe region. The sample W . The low tool speed welded joint of DH36 shoulder-probe region is believed to experience the highest 2D W (200RPM, 100 mm/min) did not show a significant material flowing due to the thermomechanical combination 1D presence of BN particles in the microstructure. of both the shoulder and the probe tool parts. Figures 5, 6, Fig. 6 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 2E magnification (region under tool shoulder) and (b) high magnification Fig. 7 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 3E magnification (region under tool shoulder) and (b) high magnification 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 259 Fig. 8 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 4E magnification (region under tool shoulder) and (b) high magnification Fig. 9 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 5E magnification (region under tool shoulder) and (b) high magnification 7, 8, 9, 10, and 11 are SEM images (low and high mag- Tool Wear in FSW EH46 Steel for Samples W 8E nification) of samples W –W (plunge/dwell cases of and W FSW Under Steady-State Conditions 1E 7E 9E EH46 grade steel), respectively, which show different sizes and volume fraction of BN particles. Figure 12a and b PCBN tool wear in samples W and W (EH46) steady 8E 9E shows the BN particles at the probe side bottom (region-2 state has also studied by revealing the BN particles in the bottom) of samples W and W (EH46 plunge/dwell microstructure in order to understand the effect of tool 2E 6E period), respectively. Figure 13 shows the XRD scans rotational/traverse speeds on the wear issue. Figure 14 is a taken of samples W –W ; peaks of ferrite and BN can be high magnified SEM image showing a 13-lm BN particle 1E 7E recognized. Table 8 shows the IFM measurements of found in the SZ of W ; the binder of W-Re is also shown 8E plunge depth and areas of affected zones of samples W – (the darker phase). Figures 15 and 16 show the SEM–EDS 1E W (EH46 steel). Table 9 shows the calculated percentage scanning (point and ID technique) of BN of FSW W at 7E 8E (%) area fraction of BN particles in a 1 mm scanned the top surface of the SZ and at the probe end, respectively. microstructure of the shoulder-probe region [11]. Figures 17 and 18 show the SEM–EDS of BN in the SZ of 123 260 Metallography, Microstructure, and Analysis (2018) 7:252–267 Fig. 10 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 6E magnification (region under tool shoulder) and (b) high magnification Fig. 11 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 7E magnification (region under tool shoulder) and (b) high magnification FSW W at the top surface of the SZ and at the probe end, 9E Discussion respectively. SEM images of the top surface of SZ and in the probe Tool Wear in FSW DH36 at High Tool Rotational/ end of W and W are shown in Figs. 19, 20, 21, and 22, 8E 9E Traverse Speeds respectively. Figure 23 is an SEM image with high mag- nification of W at the probe end which shows BN par- 9E SEM–EDS of FSW DH36 joint W (550RPM, 400 mm/ 2D ticles. Table 10 shows the calculated percentage (%) [11] min) in Figs. 2 and 3 shows different sizes of BN particles of BN in a 1 mm scanned microstructure at the middle top in the SZ. Tool wear is expected to be a result of the parent of the SZ and at the probe end of samples W and W . material resistance to the thermomechanical process that is 8E 9E Figure 24 shows the XRD of samples W and W , 8E 9E friction stir welding. Although the tool torque for sample respectively. W (Table 5) was higher than that of W , it did not show 1D 2D 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 261 Fig. 12 EH46 at plunge/dwell period, probe side bottom (region-2 bottom), (a)W and (b)W 2E 6E Fig. 13 XRD scan of FSW of grade EH46 steel (plunge cases from W –W at region 1 1E 7E under the shoulder). The microstructure is mainly ferrite phase. BN peaks are presented Table 8 The welding conditions and resulting IFM measurements of the plunge depth and areas of the various affected zones for samples W – 1E W EH46 steel 7E Weld Tool rotational speed Max. Max. Max. Plunge Time (t) sec at Total TMAZ (Region 3) (Region 4) no. x (RPM) at plunge traverse torque depth (dwelling) (region 1 ? 2) IHAZ OHAZ 2 2 2 maximum plunge force (F ) force (F ) (M)Nm (Z)mm plunge period area mm (IFM) mm mm z x depth KN KN (IFM) (IFM) (IFM) W 200 157 17 498 11.05 6 47.46 64.7 82 1E W 200 127 17 471 11.43 8 67.5 78.5 102 2E W 120 116 21 598 11.56 7 58 69.6 112.6 3E W 120 126 20 549 11.47 6 66 64.25 118 4E W 120 115 17 532 11.47 7 55 93.5 120.4 5E W 120 105 18 583 11.78 7 68 99.5 143.8 6E W 120 119 20 548 11.57 7 57.2 91 120 7E 123 262 Metallography, Microstructure, and Analysis (2018) 7:252–267 Table 9 BN percentage (%) in EH46 plunge period at shoulder/probe W accompanied with higher-temperature generation can 2D side region, the scanned area is 1 mm exacerbate the tool wear, leading to a disastrous damage of the FSW tool. The top surface of the SZ as shown in Fig. 3 Weld no. W W W W W W W 1E 2E 3E 4E 5E 6E 7E showed the highest presence of BN particles. These results BN% 1.4 2.8 0.65 0.7 0.9 3.3 1.2 are in agreement with the published work [12] in which it was found from modeling of the FSW of DH36 that the maximum temperature can approach the melting point of DH36 steel (1450 C) when applying tool speeds of 550RPM/400 mm/min. W-25Re hardness was found in previous work to reduce by about 50% when the temper- ature increases from room temperature to 1450 C[8]. Tool Wear in FSW EH46 W1–W7 Plunge/Dwell Cases Figures 5, 6, 7, 8, 9, 10, and 11 are SEM images (low and high magnification) of samples W –W plunge/dwell 1E 7E cases, respectively. The images are taken from the shoul- der-probe region and show the different sizes and amounts of BN particles present. BN particles sizes were detected between 0.5 and 13 lm, and the percentage (%) of BN was calculated [11] and is reported in Tables 9 and 10. Depending on the welding conditions and the calculated results of TMAZ size and plunge depth mentioned in Table 8, the calculated percentage (%) of BN particles Fig. 14 Large BN particle in the top of SZ of FSW EH46 (W ) 8E have varied as shown in Table 9. Sample W showed the 6E maximum percentage (%) of BN in the shoulder-probe significant evidence of BN particles in the microstructure region as a result of maximum plunge depth and TMAZ [as revealed by X-ray diffraction, Fig. 4a and b]. The W-Re size. W ,W ,W , and W have shown the lowest 3E 4E 5E 7E binder softening [8] as a result of temperature increase values of BN particles which can be attributed to the low coming from the increase in tool rotational speed is most tool rotational speed and also low plunge depth. W also 2E likely to be the reason for the higher % of BN and therefore showed a higher % of BN in the shoulder-probe region the higher PCBN tool wear in sample W rather than in 2D compared to W which may be a result of the higher 1E W . Also the higher traverse speed of the FSW tool in 1D plunge depth despite the fact that the same tool rotational Fig. 15 BN particles with different sizes at the top of SZ of FSW EH46 (W ) 8E 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 263 Fig. 16 0.5-lm BN particle at the probe end of FSW EH46 (W ) 8E Fig. 17 Top surface of the SZ (steady state) showing evidence of different sizes of BN particle (EH46 W ) 9E speeds were applied. The %BN in sample W was also peaks, but no phase change in the microstructure or no 2E higher than W ,W ,W , and W which have almost recrystallization of the cubic BN has occurred. 3E 4E 5E 7E the same plunge depth as W . This finding can be 2E attributed to the higher tool rotational speed of W which Tool Wear in FSW EH46 W and W FSW Steady 2E 8E 9E in turn can cause an increase in the temperature at the State tool/workpiece contact region, and thus, greater softening of the W-Re binder can be expected. Figure 12a and b is Figure 14 is an SEM image with high magnification which SEM images of the probe side bottom of sample W and shows a 13-lm BN particle in the SZ of FSW of W joint; 2E 8E W , respectively, which show significant % of BN parti- the binder of W-Re was also detected by SEM–EDS and is 6E cles, particularly in W and less in W . The higher also shown. Softening of the binder accompanied by 6E 2E plunge depth in W may be the reason for this increase in mechanical action (tool rotational/traverse speeds) may be 6E tool wear. The XRD result (Fig. 13) shows ferrite and BN the reason for the separation of the BN particles from the 123 264 Metallography, Microstructure, and Analysis (2018) 7:252–267 Fig. 18 Evidence of BN particle in SZ of the probe end region of FSW EH46 (W ) 9E Fig. 19 Top center of SZ of EH46 W (steady state), showing evidence of BN particles, (a) low magnification and (b) high magnification 8E (etched in 2% nital) PCBN FSW tool which is then followed by those released of the top surface of SZ and in the probe end of samples particles becoming attached and entrapped in the SZ W and W are shown in Figs. 20, 21, and 22, respec- 8E 9E microstructure of the workpiece during the FSW process. tively. Figure 21a shows a significant amount of BN par- Figures 15 and 16 show the SEM–EDS scanning (spot ticles at the top surface of the SZ of sample W until 9E analysis) of BN in a FSW of sample EH46 W at the top 250 lm depth, whereas Fig. 21b is a higher-magnification 8E surface of the SZ and at the probe end, respectively. Fig- SEM image of the SZ which shows BN particles sur- ures 17 and 18 show the SEM–EDS of BN in the SZ of rounded by W-Re binder (the darker color). The middle of FSW EH46 sample W at the top surface of the SZ and at the SZ of samples W and W did not show a significant 9E 8E 9E the probe end, respectively. More BN particles were shown presence of BN as shown in Figs. 20a and 22a, respec- in sample W at the top and bottom of the SZ rather than tively, whereas BN at the probe end of both welds is clearly 9E in sample W which is the result of increasing the tool evident in Figs. 20b and 22b. The significant existence of 8E traverse speed from 50 to 100 mm/min. More SEM images BN in the top and bottom of the SZ but less in the middle 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 265 Fig. 20 EH46 W (steady state). (a) The middle of SZ (no BN particles), microstructure is mainly acicular ferrite, (b) probe end SZ (BN 8E particles are present), microstructure is mainly granular ferrite and some short plated cementite Fig. 21 Top middle center of SZ of EH46 W (steady state) showing evidence of BN particles, (a) low magnification (un-etched) and (b) high 9E magnification (etched) of the SZ can be attributed to the fact that tool edge (top shoulder periphery and probe end. This can be seen in and bottom) is the most vulnerable locations with regard to Fig. 23 where BN particles are found at the probe end of wear as a result of experiencing higher temperatures or sample W as a result of higher traverse speed. Table 10 9E higher shear stress. Al-Moussawi et al. [12] showed by shows the calculated percentage (%) of BN in a 1 mm at simulation that the tool shoulder periphery has experienced the middle top of the SZ and at the probe end of samples the maximum peak temperature on the advancing-trailing W and W . Wear of the FSW tool at the tool shoulder 8E 9E side and the maximum shear stress was on the leading- periphery in W is about 3 times that in sample W , 9E 8E retreating side. They also showed that at higher traverse whereas, at the probe end it is approximately double. This speeds, the maximum value of shear stress was at the finding is supported by the XRD analysis shown in Fig. 24 123 266 Metallography, Microstructure, and Analysis (2018) 7:252–267 Fig. 22 EH46 W (steady state). (a) The middle of SZ (no BN particles), microstructure is mainly acicular ferrite, (b) probe end (BN particles 9E are evident), the microstructure is mainly granular ferrite and cementite taken from the middle top of the SZ which shows that the peak associated with BN in sample W is stronger than in 9E sample W . 8E Conclusion From the work carried out the following can be concluded: • PCBN FSW tool wear has been found to increase with an increasing tool rotational speed as a result of W-Re binder softening. The top of the SZ and the weld root regions have showed the maximum presence of BN particles which indicates that the shoulder and probe end are the most affected tool parts for wear as a result of the thermomechanical effect. Fig. 23 High-magnification SEM image of sample W (EH46) • Increasing the plunge depth is associated with an 9E (steady state) at the probe end showing BN particles increase in tool wear as a result of the increase in the surface contact area which in turn raises the temper- ature in the tool/workpiece contact region. Table 10 BN percentage (%) in EH46 steady-state of samples W 8E • Increasing the tool traverse speed has resulted in an and W at the middle top of SZ and at the probe end, the scanned 9E area is 1 mm increase in tool wear especially at the tool shoulder periphery. The increase in the value of shear stress on Weld no. and W top W probe W top W probe 8E 8E 9E 9E the tool surface was the main reason of this wear. region SZ end SZ end • The current study represents a step change in under- BN% 1.1 1.3 4.4 2.8 standing the PCBN tool wear during the FSW process. Tool wear can be reduced by choosing the 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 267 Fig. 24 XRD scan of FSW of EH46 grade (comparison between W and W at the top 8E 9E of SZ, W shows a stronger 9E peak of BN than W ,Cu 8E X-ray-tube) 4. J.M. Seaman, B. Thompson, Challenges of friction stir welding of suitable combination of tool rotational/traverse speeds thick-section steel, in Proceedings of the Twenty-first, 2011, and plunge depth. International Offshore and Polar Engineering Conference, Maui, Hawaii, USA, June 19–24, 2011 5. P.J. Konkol, M.F. Mruczek, Comparison of friction stir weld- Acknowledgments The authors would like to thank the Ministry Of ments and submerged arc weldments in HSLA-65 steel. Suppl. Higher Education, Iraq, for funding this project. Thanks are also due Weld. J. 86, 187–195 (2007) to The Welding Institute (TWI) for providing the friction welded 6. J. Perrett, J. Martin, J. Peterson, R. Steel, S. Packer, Friction stir samples and processing data. welding of industrial steels. in Paper Presented at TMS Annual Meeting 2011. 27 Feb.–3 March 2011, San Diego, CA., USA Open Access This article is distributed under the terms of the Creative 7. C.D. Sorensen, T.W. Nelson, Friction Stir Welding of Ferrous Commons Attribution 4.0 International License (http://creative and Nickel Alloys (Materials Park, ASM International, 2007), commons.org/licenses/by/4.0/), which permits unrestricted use, dis- pp. 111–121 tribution, and reproduction in any medium, provided you give 8. M.L. Ramalingam, D.L. Jacobson, Elevated temperature soften- appropriate credit to the original author(s) and the source, provide a ing of progressively annealed and sintered W-Re alloys. J. Less link to the Creative Commons license, and indicate if changes were Common Metals 123, 153–167 (1986) made. 9. R.M. Hooper, J.I. Shakib, C.A. Brookes, Microstructure and wear of TiC cubic BN tools. Mater. Sci. Eng., A A106, 429–433 (1988) 10. Y. Zhang, Y.S. Sato, H. Kokawa, S.H.C. Park, S. Hirano, Stir References zone microstructure of commercial purity titanium friction stir welded using pcBN tool. Mater. Sci. Eng., A A488(1–2), 25–30 1. R.S. Mishra, Z.Y. Ma, Friction stir welding and processing. Mater (2008) Sci Eng R Rep 50(1–2), 1–78 (2005) 11. F.B. Pickering, The Basis of Quantitative Metallography (Insti- 2. Megastir. Friction stir welding of high melting temperature tute of Metallurgical Technicians, London, 1976) materials (2013). http://megastir.com/products/tools/fsw_tool. 12. M. Al-Moussawi, A.J. Smith, A. Young, S. Cater, M. Faraji, aspxpdf Modelling of friction stir welding of DH36 steel. Int. J. Adv. 3. R. Rai, A. De, H.K.D.H. Bhadeshia, T. DebRoy, Review: friction Manuf. Technol. (2017). https://doi.org/10.1007/s00170-017- stir welding tools. Sci. Technol. Weld. Joining 16(4), 325–342 0147-y (2011) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Metallography, Microstructure, and Analysis Springer Journals

Wear of Polycrystalline Boron Nitride Tool During the Friction Stir Welding of Steel

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Copyright © 2018 by The Author(s)
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Materials Science; Metallic Materials; Characterization and Evaluation of Materials; Structural Materials; Surfaces and Interfaces, Thin Films; Nanotechnology
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2192-9262
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10.1007/s13632-018-0439-0
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Abstract

The wear issue of a polycrystalline boron nitride (PCBN) tools during the friction stir welding of two grades of steel, DH36 and EH46, was studied. Two welding traverse and tool rotational speeds were used when welding the DH36 steel. A low tool speed (200RPM, 100 mm/min) and a high tool speed (550RPM, 400 mm/min) were denoted by W and W , 1D 2D respectively. Nine welding conditions were applied to the welding of EH46 steel plate including seven plunge/dwell trials (W –W ) and two steady-state trials (W and W ). SEM–EDS and XRD tests were applied in order to reveal the 1E 7E 8E 9E boronitride (BN) particles inside the welded joints, and the percentage (%) of BN was calculated according to the standard quantitative metallographic technique. The findings showed that tool wear increases when the tool rotational speed increases as a result of binder softening which is a function of the peak temperature (exceeds 1250 C) at the tool/workpiece interface. When considering the EH46 steel trials, it was found that an increase in the tool traverse speed in friction stir welding caused a significant tool wear with 4.4% of BN in the top of the stirred zone of W compared to 1.1% 9E volume fraction of BN in W which was attributed to the higher thermomechanical action on the PCBN tool surface. Tool 8E wear was also found to increase with an increase in tool plunge depth as a result of the higher contact between the surface of friction stir welding tool and the workpiece. Keywords Friction stir welding  PCBN tool  Wear  DH36 and EH46 steel Introduction complex designs at relatively low costs. Welding high melting alloys requires a FSW tool material with higher Friction stir welding (FSW) is a solid-state welding tech- thermal and mechanical properties compared to other tools nique invented at The Welding Institute (TWI) in 1991 [1]. used for low melting point materials. Polycrystalline boron This solid-state process was successfully developed to nitride (PCBN) is the most widely used super-abrasive enable welding of aluminum alloys such as the 2XXX and ceramic FSW tool. Table 1 [2] shows the mechanical and 7XXX series which, at that time, were considered un- thermal properties of a PCBN tool compared to tungsten weldable using standard fusion welding processes [1]. carbide (WC) and H13 FSW tools. The microhardness of However, for welding high melting alloys such as steel, the this tool HV (2600–3500) shows that the PCBN tool is the process is still limited because of the high cost of the FSW second hardest tool after diamond. It also has a low coef- tool. For welding low melting alloys such as aluminum, the ficient of friction which in turn helps in producing smooth tool is usually made from a suitable steel grade such as weld surfaces [3]. However, a low coefficient of friction in alloy H13 which can be used to produce tools with a FSW tool necessitates an increase in tool rotational speed to produce the required heat for welding [4]. The PCBN tool represents an alternative to refractory tools such as & M. Almoussawi tungsten-based materials which showed severe wear, inj.mun@atu.edu.iq; b1045691@my.shu.ac.uk especially during FSW of steel [5]. The PCBN tool usually 1 contains cubic boron nitride (CBN) particles in an alu- Al-Furat Al-Awsat Technical University, Kufa, Iraq minum nitride (AlN) binder [6]. This combination between MERI, Sheffield Hallam University, S1 1WB Sheffield, UK the CBN and the binder is designed to increase the strength Coventry University, Coventry, UK 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 253 Table 1 The physical properties Property Units PCBN Tungsten carbide 4340 Steel of PCBN compared with some other materials [2] Coefficient of friction … 0.10–0.15 0.2 0.78 -6 Coefficient of thermal expansion 10 /C 4.6–4.9 4.9–5.1 11.2–14.3 Thermal conductivity W/mK 100–250 95 48 Compressive strength N/mm 2700–3500 6200 690 10 psi 391–507 899 100 Fracture toughness MPa m 3.5–6.7 11 100 Hardness Knoop kg/mm 2700–3200 … 278 Vickers kg/mm 2600–3500 1300–1600 280 Tensile strength N/mm … 1100 620 10 psi … 160 89.9 Transverse N/mm 500–800 2200 … Rupture strength 10 psi 72–115 319 … of the tool. Despite the high strength and thermal proper- workpiece. The PCBN tool is also attached to a shank ties, the PCBN tool has low fracture toughness and is also made of WC which acts as a holder and attaches the tool to susceptible to wear problems, especially during the plunge the PowerStir FSW machine. Both parts of the tool, period of the weld due to the higher temperature generated including the PCBN material and WC shank, are fastened which can cause a softening of the binder [3]. The plunge together by a collar. The different parts of the PCBN tool period is also associated with a high plunge force which geometry are listed in Table 2 which is according to the can encourage BN particles to detach from the tool and workpiece thickness (6 and 14 mm). Welding a 6-mm- stick into the workpiece. The development in metal-com- thick specimen is usually carried out using a PCBN tool posite material has encouraged manufacturers such as with 5.5-mm probe length, while for 14-mm-thick speci- MegaStir to produce new grades of PCBN tools with a men a probe length of 12 mm is usually used. The Pow- longer service life. This development was a step forward in erStir FSW machine includes a cooling system applied to order to commercialize the FSW of high melting alloys. the tool shank in order to reduce the temperature resulting from the friction stir welding of steel which is readily For example, a FSW tool made from Q70 (70%PCBN, 30%WRe) has a higher toughness compared to the previous transmitted due to the high thermal conductivity of the one which included an AlN binder [6]. The melting point PCBN tool. Argon shielding is usually applied during the of a Q70 tool PCBN material was determined to exceed FSW process mainly to extend the tool life and also to 3000 C, and the microstructure and tool image are shown prevent oxidation of the welded joint. A thermocouple is in Fig. 1a and b, respectively [6]. The Q70 tool has been usually attached to the tool shank and is located behind the used extensively by TWI to join many steel grades PCBN tool. This telemetry system is employed to monitor including 316L stainless steel, 304 stainless steel, and tool temperature during the FSW process. The FSW tool DH36 and EH46 steel grades which are under considera- manufacturer recommends that the temperature of the tool tion in this work. PCBN-WRe Q70 tool as shown in Fig. 1b measured by the telemetry thermocouples system should be consists of a shoulder and probe surrounded by a collar of a kept in the range of 800–900 C in order to protect the tool Ni–Cr alloy which works as an insulator for the tool from from excessive wear/breakage issues [5]. Other tool the environment, so the heat generated during the FSW is materials and their properties which have been used for almost distributed between the PCBN tool and the high melting alloys are usually made of refractory Fig. 1 (a) An SEM image of the PCBN microstructure of a cross section PCBN through a PCBN Q70 (70% cBN, 30%WRe) [6] FSW tool, Collar (b) PCBN tool image WRe binder Shank ab 123 254 Metallography, Microstructure, and Analysis (2018) 7:252–267 Table 2 PCBN tool geometry provided by TWI Tool PCBN shoulder PCBN probe Collar Shank PCBN tool geometry for welding 23.7 mm diameter with spiral 5.5 mm length, 10 mm base 23.9 mm inner 23.9 mm diameter 6 mm thickness convex shape diameter diameter 80.35 mm length Spiral tapered with 20 thread 37 mm outer per inch (TPI) diameter 24 mm length PCBN tool geometry for welding 38 mm mm diameter with 12 mm length, 20 mm base 38.1 mm inner 38.1 mm diameter 10–15 mm thickness spiral convex shape diameter diameter 100 mm length Spiral tapered with 20 thread 52 mm outer per inch (TPI) diameter 30 mm length materials based on tungsten such as WC, W-Re, W-Co and the ability to react with Ti-forming TiB which in turn can can be found in the literature [3, 7]. This paper will focus cause grain refinement and a hardness increase [10]. PCBN only on the PCBN tool wear as this is the tool which has tool breakage is more likely to occur during the FSW been employed to produce the samples under study. process due to the lower fracture toughness of a PCBN tool Wear resistance in hybrid PCBN FSW tools is consid- compared to a WC tool, Table 1. Unsuitable welding ered better than other refractory materials such as tungsten- parameters such as improper plunging or extracting and based tools. W-25%Re FSW tool life of 4 m was reported low welding temperatures were determined as the main in welding steel grades and titanium [7]. However, wear factors for tool breakage [5]. issues still exist in PCBN tools when welding high melting Previous work on PCBN tool wear during the friction alloys especially the high strength alloys such as the EH46 stir welding of steel did not investigate the possibility of steel grade. An experimental study of a PCBN tool has W-Re binder softening as a result of high temperature been carried out by Rai et al. [3] and showed that the main which may reach the local melting of steel alloys during factors which can cause wear are abrasion and diffusion. the process, especially when the tool rotational speed Softening and recrystallization of the binder after reaching reaches specific limits. Also limited work has been carried a welding temperature of 1000 C is also found to be one out to investigate the effect of tool traverse speed on the reason for reducing the tools resistance to wear [3]. wear rate and the consequences on the physical properties Ramalingam and Jacobson [8] reported a decrease in of the welded joints. The current work focuses on PCBN Knoop hardness (HK) of W-25Re from HK 675 (HV 638) tool wear which occurs during the FSW process of two to HK 500 (HV 478) when carrying out heating from room steel grades, DH36 and EH46. The FSW tool wear has temperature to 1225 C. The hardness was found to been studied by calculating the volume fraction of BN decrease dramatically to HK 300 (HV 290) when the (originally from the PCBN FSW tool) existed in the ther- temperature increases to 1450 C. Hooper et al. [9] sug- momechanical affected zone (TMAZ) microstructure. The gested that the higher thermal conductivity of cBN aims are to identify the causes of PCBN tool wear and to (100–250 W/m K) can result in defects in the microstruc- highlight the different suitable welding parameters that can ture when the temperature exceeds 1200 K, while with a increase the tool longevity. comparison with cBN-TiC, they found that a protective layer (mainly TiC) is formed on the latter and is associated with the higher temperature as a result of lower thermal Experimental Method conductivity of cBN-TiC. PCBN tool wear can also change the properties of the material being welded. For example Chemical Composition of Parent Metal tool wear can affect negatively a stainless steel welded and Welding Conditions joint as boron particles can react with Cr to form borides which in turn can result in a reduction in corrosion resis- Tables 3 and 4 show the as-received chemical composition tance [3]. When welding titanium plates, boron can (wt.%) of the 6-mm-thick DH36 and 14-mm-thick EH46 improve the mechanical properties of the joint because of steel plates. The welded samples have been produced at Table 3 Chemical composition C Si Mn P S AlN Nb V Ti Cu CrNiMo of as-received DH36 steel grade (wt.%) 0.16 0.15 1.2 0.01 0.005 0.043 0.02 0.02 0.002 0.001 0.029 0.015 0.014 0.002 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 255 Table 4 Chemical composition of as-received EH46 steel grade (wt.%) C Si Mn P S Al N Nb V Ti 0.20 0.55 1.7 0.03 0.03 0.015 0.02 0.03 0.1 0.02 TM TWI/Yorkshire using their PowerStir FSW machine, Scanning Electron Microscopy (SEM) and Energy- employing a Q70 PCBN FSW tool. The welding conditions Dispersive X-ray Spectroscopy (EDS) Examination used for the production of samples of DH36 steel (W and 1D W ) are shown in Table 5. These were chosen to examine SEM and EDS were carried out on polished (until 1 lm) 2D the effect of tool rotational and traverse speeds on tool and etched (2% nital) FSW samples using the FEI Nova wear. In order to examine tool traverse speeds above Nano SEM. The examination included the surface of the 200 mm/min, it is necessary to simultaneously increase the stirred zone (SZ) and thermomechanical affected zone tool rotational speed. The welding conditions for the pro- (TMAZ) which are the regions which experienced both duction of samples of EH46 steel (W –W ) which thermal and mechanical effects during the FSW process 1E 7E examined the effect of rotational speed and plunge depth under steady-state conditions and at the plunge period. The on tool wear are shown in Table 6. The welding process SEM produced high-quality and high-resolution images of parameters used to examine the effect of tool traverse microconstituents using the secondary electron (SE) speed on tool wear (W and W ) for EH46 steel steady imaging mode with an accelerating voltage of between 10 8E 9E state are shown in Table 7. and 20 kV. The working distance (WD) used was 5 mm, but in some cases, it was altered (decreased or increased) to enhance the contrast at higher magnification. EDS–SEM examination was mainly used to detect the BN particle Table 5 Welding conditions for FSW DH36 steel samples W and W 1D 2D Weld Tool rotational Traverse Rotational/traverse Average Average tool Axial force Longitudinal Heat input xtorque no. speed ðxÞ RPM speed speeds (rev/mm) spindle torque torque N m (average) force (average) (V) mm/min Nm KN KN W 200 100 2 278 105 57.55 12.8 210 1D W 550 400 1.375 163 62 47 12 64.625 2D Table 6 The welding conditions for FSW EH46 steel to examine the effect of rotational speed and actual plunge depth on tool wear (W –W ) 1E 7E Weld Tool rotational speed x Max. axial (plunge) Max. longitudinal Max. torque Plunge depth (Z) mm Dwell time (t) sec at trial no. (RPM) at dwell period force (F )KN force (F )KN (M) N m from FSW machine (dwell) period z x W 200 157 17 498 13 6 1E W 200 127 17 471 13 8 2E W 120 116 21 598 13 7 3E W 120 126 20 549 13 6 4E W 120 115 17 532 13 7 5E W 120 105 18 583 13 7 6E W 120 119 20 548 13 7 7E Table 7 Welding conditions for FSW EH46 steel during steady-state welding (W and W ) 8E 9E Weld Tool Traverse Rotational/traverse Average Average tool Axial force Longitudinal Heat input xtorque no. rotational speed mm/ speeds spindle torque torque N m (average) KN force (average) speed RPM min Nm KN W 150 50 3 300 114 66 13 342 8E W 150 100 1.5 450 171 72 14 256.5 9E Examining the effect of traverse speed on tool wear 123 256 Metallography, Microstructure, and Analysis (2018) 7:252–267 inside the TMAZ. Spot analysis (point and ID) has been employed to elementally identify small second-phase par- ticles in the scanned SEM images. Additionally, EDS mapping was used to analyze the whole scanned SEM area. X-ray Diffraction X-ray diffraction (Cu X-ray tube) using the Empyrean Philips XRD has been carried out on the FSW samples for the following reasons: • To reveal and characterize the as-received sample phases in order to allow for the detection of any phase changes in the steel following FSWing. • To detect other additional phases, elements, and/or boronitride (BN) particles which may appear in the welded joints. Infinite Focus Microscopy (IFM) Fig. 3 SEM image of the top of SZ of DH36 W shows the 2D existence of BN The infinite focus microscopy (IFM) (Alicona) has been the SZ of sample W and W were scanned at a maximum 8E 9E employed to create accurate optical light microscopy magnification of 10,0009. The area fraction of BN particles images of the welded joint. The IFM is a device based on and TiN precipitates has been measured manually using the optical 3-dimensional measurements which has the ability square grid method. The number of intersections of the grid to vary the focus in order to obtain a 3D vertical scanned falling in the BN particles is counted and compared with the image of the surface. The scanned area of interest can be total number of points laid down [11]. transferred into a 3D image by the aid of Lyceum software; thus, the surface area can be calculated accurately. Result Calculating the Percentage (%) of BN in the Welded Joints Effect of Tool Rotation and Traverse Speed on Tool Wear in FSW of DH36 Steel: Samples W 1D The % of BN particles originating from the PCBN FSW tool and W 2D in the TMAZ of the welded joints were calculated using the SEM. A 1 mm area between the shoulder and the probe side The existence of BN particles originating from the PCBN (W –W ) and a square mm (1 mm ) from the middle top of 1E 7E FSW tool was investigated in the FSW joints of grade Fig. 2 Ten-micrometer BN particles in SZ of FSW DH36 joint (W 550RPM, 400 mm/min) 2D 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 257 Fig. 4 XRD scan of FSW of DH36 steel (a)W (200RPM, 100 mm/min) microstructure consists of BCC ferrite after the FSW process. (b)W 1D 2D (550RPM, 400 mm/min), microstructure is mainly ferritic BCC phase; BN peaks are also present (Co-X-ray tube) Fig. 5 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 1E magnification (region under tool shoulder) and (b) high magnification 123 258 Metallography, Microstructure, and Analysis (2018) 7:252–267 DH36 steel which had been produced at high tool speeds in Effect of Tool Rotational Speed and Actual order to attempt to assess tool wear. SEM–EDS examina- Plunge Depth on Tool Wear in FSW EH46 Steel: tion of FSW joint W (DH36, 550RPM, 400 mm/min) in Samples W –W 2D 1E 7E Figs. 2 and 3 shows different sizes of BN particles in the SZ. Figure 4a and b shows the XRD scan results from The presence of BN particles has been investigated in the samples W and W , respectively. Note that peaks of microstructure of FSW samples of EH46 weld joints during 1D 2D ferrite are shown in both welds and BN peaks are seen in the plunge/dwell period in the shoulder-probe region. The sample W . The low tool speed welded joint of DH36 shoulder-probe region is believed to experience the highest 2D W (200RPM, 100 mm/min) did not show a significant material flowing due to the thermomechanical combination 1D presence of BN particles in the microstructure. of both the shoulder and the probe tool parts. Figures 5, 6, Fig. 6 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 2E magnification (region under tool shoulder) and (b) high magnification Fig. 7 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 3E magnification (region under tool shoulder) and (b) high magnification 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 259 Fig. 8 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 4E magnification (region under tool shoulder) and (b) high magnification Fig. 9 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 5E magnification (region under tool shoulder) and (b) high magnification 7, 8, 9, 10, and 11 are SEM images (low and high mag- Tool Wear in FSW EH46 Steel for Samples W 8E nification) of samples W –W (plunge/dwell cases of and W FSW Under Steady-State Conditions 1E 7E 9E EH46 grade steel), respectively, which show different sizes and volume fraction of BN particles. Figure 12a and b PCBN tool wear in samples W and W (EH46) steady 8E 9E shows the BN particles at the probe side bottom (region-2 state has also studied by revealing the BN particles in the bottom) of samples W and W (EH46 plunge/dwell microstructure in order to understand the effect of tool 2E 6E period), respectively. Figure 13 shows the XRD scans rotational/traverse speeds on the wear issue. Figure 14 is a taken of samples W –W ; peaks of ferrite and BN can be high magnified SEM image showing a 13-lm BN particle 1E 7E recognized. Table 8 shows the IFM measurements of found in the SZ of W ; the binder of W-Re is also shown 8E plunge depth and areas of affected zones of samples W – (the darker phase). Figures 15 and 16 show the SEM–EDS 1E W (EH46 steel). Table 9 shows the calculated percentage scanning (point and ID technique) of BN of FSW W at 7E 8E (%) area fraction of BN particles in a 1 mm scanned the top surface of the SZ and at the probe end, respectively. microstructure of the shoulder-probe region [11]. Figures 17 and 18 show the SEM–EDS of BN in the SZ of 123 260 Metallography, Microstructure, and Analysis (2018) 7:252–267 Fig. 10 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 6E magnification (region under tool shoulder) and (b) high magnification Fig. 11 SEM images of EH46 (W ) at plunge/dwell period showing BN particles (dark spots) with sizes ranging from 0.5 to 13 lm. (a) Low 7E magnification (region under tool shoulder) and (b) high magnification FSW W at the top surface of the SZ and at the probe end, 9E Discussion respectively. SEM images of the top surface of SZ and in the probe Tool Wear in FSW DH36 at High Tool Rotational/ end of W and W are shown in Figs. 19, 20, 21, and 22, 8E 9E Traverse Speeds respectively. Figure 23 is an SEM image with high mag- nification of W at the probe end which shows BN par- 9E SEM–EDS of FSW DH36 joint W (550RPM, 400 mm/ 2D ticles. Table 10 shows the calculated percentage (%) [11] min) in Figs. 2 and 3 shows different sizes of BN particles of BN in a 1 mm scanned microstructure at the middle top in the SZ. Tool wear is expected to be a result of the parent of the SZ and at the probe end of samples W and W . material resistance to the thermomechanical process that is 8E 9E Figure 24 shows the XRD of samples W and W , 8E 9E friction stir welding. Although the tool torque for sample respectively. W (Table 5) was higher than that of W , it did not show 1D 2D 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 261 Fig. 12 EH46 at plunge/dwell period, probe side bottom (region-2 bottom), (a)W and (b)W 2E 6E Fig. 13 XRD scan of FSW of grade EH46 steel (plunge cases from W –W at region 1 1E 7E under the shoulder). The microstructure is mainly ferrite phase. BN peaks are presented Table 8 The welding conditions and resulting IFM measurements of the plunge depth and areas of the various affected zones for samples W – 1E W EH46 steel 7E Weld Tool rotational speed Max. Max. Max. Plunge Time (t) sec at Total TMAZ (Region 3) (Region 4) no. x (RPM) at plunge traverse torque depth (dwelling) (region 1 ? 2) IHAZ OHAZ 2 2 2 maximum plunge force (F ) force (F ) (M)Nm (Z)mm plunge period area mm (IFM) mm mm z x depth KN KN (IFM) (IFM) (IFM) W 200 157 17 498 11.05 6 47.46 64.7 82 1E W 200 127 17 471 11.43 8 67.5 78.5 102 2E W 120 116 21 598 11.56 7 58 69.6 112.6 3E W 120 126 20 549 11.47 6 66 64.25 118 4E W 120 115 17 532 11.47 7 55 93.5 120.4 5E W 120 105 18 583 11.78 7 68 99.5 143.8 6E W 120 119 20 548 11.57 7 57.2 91 120 7E 123 262 Metallography, Microstructure, and Analysis (2018) 7:252–267 Table 9 BN percentage (%) in EH46 plunge period at shoulder/probe W accompanied with higher-temperature generation can 2D side region, the scanned area is 1 mm exacerbate the tool wear, leading to a disastrous damage of the FSW tool. The top surface of the SZ as shown in Fig. 3 Weld no. W W W W W W W 1E 2E 3E 4E 5E 6E 7E showed the highest presence of BN particles. These results BN% 1.4 2.8 0.65 0.7 0.9 3.3 1.2 are in agreement with the published work [12] in which it was found from modeling of the FSW of DH36 that the maximum temperature can approach the melting point of DH36 steel (1450 C) when applying tool speeds of 550RPM/400 mm/min. W-25Re hardness was found in previous work to reduce by about 50% when the temper- ature increases from room temperature to 1450 C[8]. Tool Wear in FSW EH46 W1–W7 Plunge/Dwell Cases Figures 5, 6, 7, 8, 9, 10, and 11 are SEM images (low and high magnification) of samples W –W plunge/dwell 1E 7E cases, respectively. The images are taken from the shoul- der-probe region and show the different sizes and amounts of BN particles present. BN particles sizes were detected between 0.5 and 13 lm, and the percentage (%) of BN was calculated [11] and is reported in Tables 9 and 10. Depending on the welding conditions and the calculated results of TMAZ size and plunge depth mentioned in Table 8, the calculated percentage (%) of BN particles Fig. 14 Large BN particle in the top of SZ of FSW EH46 (W ) 8E have varied as shown in Table 9. Sample W showed the 6E maximum percentage (%) of BN in the shoulder-probe significant evidence of BN particles in the microstructure region as a result of maximum plunge depth and TMAZ [as revealed by X-ray diffraction, Fig. 4a and b]. The W-Re size. W ,W ,W , and W have shown the lowest 3E 4E 5E 7E binder softening [8] as a result of temperature increase values of BN particles which can be attributed to the low coming from the increase in tool rotational speed is most tool rotational speed and also low plunge depth. W also 2E likely to be the reason for the higher % of BN and therefore showed a higher % of BN in the shoulder-probe region the higher PCBN tool wear in sample W rather than in 2D compared to W which may be a result of the higher 1E W . Also the higher traverse speed of the FSW tool in 1D plunge depth despite the fact that the same tool rotational Fig. 15 BN particles with different sizes at the top of SZ of FSW EH46 (W ) 8E 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 263 Fig. 16 0.5-lm BN particle at the probe end of FSW EH46 (W ) 8E Fig. 17 Top surface of the SZ (steady state) showing evidence of different sizes of BN particle (EH46 W ) 9E speeds were applied. The %BN in sample W was also peaks, but no phase change in the microstructure or no 2E higher than W ,W ,W , and W which have almost recrystallization of the cubic BN has occurred. 3E 4E 5E 7E the same plunge depth as W . This finding can be 2E attributed to the higher tool rotational speed of W which Tool Wear in FSW EH46 W and W FSW Steady 2E 8E 9E in turn can cause an increase in the temperature at the State tool/workpiece contact region, and thus, greater softening of the W-Re binder can be expected. Figure 12a and b is Figure 14 is an SEM image with high magnification which SEM images of the probe side bottom of sample W and shows a 13-lm BN particle in the SZ of FSW of W joint; 2E 8E W , respectively, which show significant % of BN parti- the binder of W-Re was also detected by SEM–EDS and is 6E cles, particularly in W and less in W . The higher also shown. Softening of the binder accompanied by 6E 2E plunge depth in W may be the reason for this increase in mechanical action (tool rotational/traverse speeds) may be 6E tool wear. The XRD result (Fig. 13) shows ferrite and BN the reason for the separation of the BN particles from the 123 264 Metallography, Microstructure, and Analysis (2018) 7:252–267 Fig. 18 Evidence of BN particle in SZ of the probe end region of FSW EH46 (W ) 9E Fig. 19 Top center of SZ of EH46 W (steady state), showing evidence of BN particles, (a) low magnification and (b) high magnification 8E (etched in 2% nital) PCBN FSW tool which is then followed by those released of the top surface of SZ and in the probe end of samples particles becoming attached and entrapped in the SZ W and W are shown in Figs. 20, 21, and 22, respec- 8E 9E microstructure of the workpiece during the FSW process. tively. Figure 21a shows a significant amount of BN par- Figures 15 and 16 show the SEM–EDS scanning (spot ticles at the top surface of the SZ of sample W until 9E analysis) of BN in a FSW of sample EH46 W at the top 250 lm depth, whereas Fig. 21b is a higher-magnification 8E surface of the SZ and at the probe end, respectively. Fig- SEM image of the SZ which shows BN particles sur- ures 17 and 18 show the SEM–EDS of BN in the SZ of rounded by W-Re binder (the darker color). The middle of FSW EH46 sample W at the top surface of the SZ and at the SZ of samples W and W did not show a significant 9E 8E 9E the probe end, respectively. More BN particles were shown presence of BN as shown in Figs. 20a and 22a, respec- in sample W at the top and bottom of the SZ rather than tively, whereas BN at the probe end of both welds is clearly 9E in sample W which is the result of increasing the tool evident in Figs. 20b and 22b. The significant existence of 8E traverse speed from 50 to 100 mm/min. More SEM images BN in the top and bottom of the SZ but less in the middle 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 265 Fig. 20 EH46 W (steady state). (a) The middle of SZ (no BN particles), microstructure is mainly acicular ferrite, (b) probe end SZ (BN 8E particles are present), microstructure is mainly granular ferrite and some short plated cementite Fig. 21 Top middle center of SZ of EH46 W (steady state) showing evidence of BN particles, (a) low magnification (un-etched) and (b) high 9E magnification (etched) of the SZ can be attributed to the fact that tool edge (top shoulder periphery and probe end. This can be seen in and bottom) is the most vulnerable locations with regard to Fig. 23 where BN particles are found at the probe end of wear as a result of experiencing higher temperatures or sample W as a result of higher traverse speed. Table 10 9E higher shear stress. Al-Moussawi et al. [12] showed by shows the calculated percentage (%) of BN in a 1 mm at simulation that the tool shoulder periphery has experienced the middle top of the SZ and at the probe end of samples the maximum peak temperature on the advancing-trailing W and W . Wear of the FSW tool at the tool shoulder 8E 9E side and the maximum shear stress was on the leading- periphery in W is about 3 times that in sample W , 9E 8E retreating side. They also showed that at higher traverse whereas, at the probe end it is approximately double. This speeds, the maximum value of shear stress was at the finding is supported by the XRD analysis shown in Fig. 24 123 266 Metallography, Microstructure, and Analysis (2018) 7:252–267 Fig. 22 EH46 W (steady state). (a) The middle of SZ (no BN particles), microstructure is mainly acicular ferrite, (b) probe end (BN particles 9E are evident), the microstructure is mainly granular ferrite and cementite taken from the middle top of the SZ which shows that the peak associated with BN in sample W is stronger than in 9E sample W . 8E Conclusion From the work carried out the following can be concluded: • PCBN FSW tool wear has been found to increase with an increasing tool rotational speed as a result of W-Re binder softening. The top of the SZ and the weld root regions have showed the maximum presence of BN particles which indicates that the shoulder and probe end are the most affected tool parts for wear as a result of the thermomechanical effect. Fig. 23 High-magnification SEM image of sample W (EH46) • Increasing the plunge depth is associated with an 9E (steady state) at the probe end showing BN particles increase in tool wear as a result of the increase in the surface contact area which in turn raises the temper- ature in the tool/workpiece contact region. Table 10 BN percentage (%) in EH46 steady-state of samples W 8E • Increasing the tool traverse speed has resulted in an and W at the middle top of SZ and at the probe end, the scanned 9E area is 1 mm increase in tool wear especially at the tool shoulder periphery. The increase in the value of shear stress on Weld no. and W top W probe W top W probe 8E 8E 9E 9E the tool surface was the main reason of this wear. region SZ end SZ end • The current study represents a step change in under- BN% 1.1 1.3 4.4 2.8 standing the PCBN tool wear during the FSW process. Tool wear can be reduced by choosing the 123 Metallography, Microstructure, and Analysis (2018) 7:252–267 267 Fig. 24 XRD scan of FSW of EH46 grade (comparison between W and W at the top 8E 9E of SZ, W shows a stronger 9E peak of BN than W ,Cu 8E X-ray-tube) 4. J.M. Seaman, B. Thompson, Challenges of friction stir welding of suitable combination of tool rotational/traverse speeds thick-section steel, in Proceedings of the Twenty-first, 2011, and plunge depth. International Offshore and Polar Engineering Conference, Maui, Hawaii, USA, June 19–24, 2011 5. P.J. Konkol, M.F. Mruczek, Comparison of friction stir weld- Acknowledgments The authors would like to thank the Ministry Of ments and submerged arc weldments in HSLA-65 steel. Suppl. Higher Education, Iraq, for funding this project. Thanks are also due Weld. J. 86, 187–195 (2007) to The Welding Institute (TWI) for providing the friction welded 6. J. Perrett, J. Martin, J. Peterson, R. Steel, S. Packer, Friction stir samples and processing data. welding of industrial steels. in Paper Presented at TMS Annual Meeting 2011. 27 Feb.–3 March 2011, San Diego, CA., USA Open Access This article is distributed under the terms of the Creative 7. C.D. Sorensen, T.W. Nelson, Friction Stir Welding of Ferrous Commons Attribution 4.0 International License (http://creative and Nickel Alloys (Materials Park, ASM International, 2007), commons.org/licenses/by/4.0/), which permits unrestricted use, dis- pp. 111–121 tribution, and reproduction in any medium, provided you give 8. M.L. Ramalingam, D.L. Jacobson, Elevated temperature soften- appropriate credit to the original author(s) and the source, provide a ing of progressively annealed and sintered W-Re alloys. J. Less link to the Creative Commons license, and indicate if changes were Common Metals 123, 153–167 (1986) made. 9. R.M. Hooper, J.I. Shakib, C.A. Brookes, Microstructure and wear of TiC cubic BN tools. Mater. Sci. Eng., A A106, 429–433 (1988) 10. Y. Zhang, Y.S. Sato, H. Kokawa, S.H.C. Park, S. Hirano, Stir References zone microstructure of commercial purity titanium friction stir welded using pcBN tool. Mater. Sci. Eng., A A488(1–2), 25–30 1. R.S. Mishra, Z.Y. Ma, Friction stir welding and processing. Mater (2008) Sci Eng R Rep 50(1–2), 1–78 (2005) 11. F.B. Pickering, The Basis of Quantitative Metallography (Insti- 2. Megastir. Friction stir welding of high melting temperature tute of Metallurgical Technicians, London, 1976) materials (2013). http://megastir.com/products/tools/fsw_tool. 12. M. Al-Moussawi, A.J. Smith, A. Young, S. Cater, M. Faraji, aspxpdf Modelling of friction stir welding of DH36 steel. Int. J. Adv. 3. R. Rai, A. De, H.K.D.H. Bhadeshia, T. DebRoy, Review: friction Manuf. Technol. (2017). https://doi.org/10.1007/s00170-017- stir welding tools. Sci. Technol. Weld. Joining 16(4), 325–342 0147-y (2011)

Journal

Metallography, Microstructure, and AnalysisSpringer Journals

Published: May 22, 2018

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