Soldering of Mg Joints Using Zn-Al Solders

Soldering of Mg Joints Using Zn-Al Solders TOMASZ GANCARZ, KATARZYNA BERENT, WOJCIECH SKUZA, and KATARZYNA JANIK Magnesium has applications in the automotive and aerospace industries that can significantly contribute to greater fuel economy and environmental conservation. The Mg alloys used in the automotive industry could reduce mass by up to 70 pct, providing energy savings. However, alongside the advantages there are limitations and technological barriers to use Mg alloys. One of the advantages concerns phenomena occurring at the interface when joining materials investigated in this study, in regard to the effect of temperature and soldering time for pure Mg joints. Eutectic Zn-Al and Zn-Al alloys with 0.05 (wt pct) Li and 0.2 (wt pct) Na were used in the soldering process. The process was performed for 3, 5, and 8 minutes of contact, at temperatures of 425 C, 450 C, 475 C, and 500 C. Selected, solidified solder-substrate couples were cross-sectioned, and their interfacial microstructures were investigated by scanning electron microscopy. The experiment was designed to demonstrate the effect of time, temperature, and the addition of Li and Na on the kinetics of the dissolving Mg substrate. The addition of Li and Na to eutectic Zn-Al caused to improve mechanical properties. Higher temperatures led to reduced joint strength, which is caused by increased interfacial reaction. https://doi.org/10.1007/s11661-018-4617-0 The Author(s) 2018 [3,6] I. INTRODUCTION techniques such as resistant spot welding, reactive [7] [8] [9] brazing, laser welding, ultrasonic-assisted soldering, CHANGES to the electrical engine, which can bring and so on. In order to eliminate the negative effect of about a reduction in its mass, are of significant interest to Mg–Al IMCs on the strength of the joints, an interlayer [1,2] the automotive industry. Al and Mg alloys are increas- [3] intended to block the formation of brittle IMC was used. [3] ingly used to reduce structural mass. Taking into [3] [10] [11] Zn, Sn-Zn alloys, Mg-Zn-Al, and Mg-In-Zn were account the lightest applications, the density of structural used to join Al with Mg alloys. 3 3 [4] materials (q =2.7 gcm , q =1.7 gcm ) is Al Mg In view of the high oxidation and corrosion of Mg very important. The aerospace industry’s requirements alloys, an addition characterized by high electrode [9] for lightweight materials to operate under increasingly potential compared to Mg ( 2.37 V) is made to the demanding conditions calls for reduced mass and solder. In this case, the layers of chromium and Teflon [12] improved mechanical properties. Compared to the Al protected the Mg against corrosion. Mg alloys sol- alloys, the Mg alloys display better physical and mechan- dered at high temperature displayed worse mechanical ical properties, such as high strength-to-weight ratio, high properties, so the joining process temperature should be damping capacity, and a high recycling potential. These lower than 450 C. For lower temperature soldering, flux facts could have a decisive effect on the application of Mg was used to increase wettability, which also protected the alloys in the automotive, electronics, and aerospace alloy surface from the formation the stable magnesium [11] industries. However, similar atomic mass and melting oxide. The application of an Zn interlayer when temperature in the Al-Mg system formed brittle inter- joining Al/Mg alloys using resistant spot welding caused metallic compounds (IMCs) such as Al Mg and 3 2 a reduction in welding current compared with traditional [5] Al Mg in the solidified metal. Furthermore, joining [3] 12 17 welding. Al dissolving into the fusion zone caused the the Al with Mg alloys is difficult, but possible using several increased formation of solid particles, which improves [3] the quality of the joints. Sn-Zn used at low tempera- tures for soldering Al/Mg joints caused the amount of Zn to rise by up to 30 pct, increasing shear strength. In TOMASZ GANCARZ, WOJCIECH SKUZA, and KATARZYNA addition, the dispersive distribution of the Al-Sn-Zn JANIK are with the Institute of Metallurgy and Materials Science, Polish solid solution within the solder reduced the brittleness of Academy of Sciences, 30-059 Krakow, Poland. Contact e-mail: the joint, thus greatly improving the mechanical prop- t.gancarz@imim.pl KATARZYNA BERENT is with the AGH erties. The conducted study, joining AZ31B alloy using University of Science and Technology, Academic Centre for Materials and Nanotechnology, 30-059 Krakow, Poland. Zn, caused the formation of an IMC from the Zn-Mg Manuscript submitted May 24, 2017. system at the interface. However, a higher cooling rate Article published online April 18, 2018 2684—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A was beneficial for increasing the tensile shear strength of microstructure and IMCs occurring at the interface. [9] joints, as it caused the formation of equiaxed dendrites For all samples, three measurements were made at and refined eutectic structure. different areas to improve statistics and to check the To show the influence at the interface during solder- homogeneity of the joints. Mechanical tests were con- ing, research should be carried out for pure Mg. This ducted with an INSTRON 6025 testing machine mod- study demonstrates the effect on the microstructure of ernized by Zwick/Roell. Testing conditions were in joints and mechanical properties of joining pure Mg accordance with ASTM A 264-03, with a strain rate of using eutectic Zn-Al and Zn-Al alloys with additions of 0.00025 (1/s) at room temperature. The mechanical tests Li and Na. were performed for each of the joints with the eutectic Zn-Al alloys, and with Na and Li additions, and three successful measurements were taken. After mechanical testing, the real area of the joints (the area of soldering II. EXPERIMENTAL between Mg substrates) was measured using CorelDraw [13] [13] Cast alloys of eutectic Zn-Al, Zn-Al0.05Li, and with the GetArea module. The shear strength is calcu- [14] Zn-Al0.2Na were used in this study, and the solder lated using the obtained force from mechanical tests [15,16] was tested while soldering Cu substrate. The cast divided by real area of the joints. The samples for the alloys were rolled to a thickness of 1 mm and cut into tests were taken after soldering, where the flow solder 8 9 10 mm pieces. The base material used for soldering from the substrate was removed. The microhardness of was pure Mg (99.9 pct) with dimensions of the formed phases in the soldering region was measured 8 9 4 9 25 mm. The Mg substrate and solder pieces by means of microhardness tests carried out at a load of were cleaned using emery paper and acetone before the 0.5 (N) and over a time of 15 (seconds). soldering process, to remove the oxide from the surface. Soldering the Mg joints was carried out using the sessile [17] drop method, with protective gas Ar (5N), for times III. RESULTS of 3, 5, and 8 minutes of contact and at temperatures of 425 C, 450 C, 475 C, and 500 C. The samples were A. Microstructure Observation moved from the cold zone to the hot zone of the furnace, where a type K thermocouple was touching the bottom All the Mg joints were obtained under a suitable pro- tective gas without flux. However, an experiment using of the holder and the melting process was observed by flux and without protective gas was also performed. CCD camera. After a predetermined time, the sample Fluxes such as ALU12, QJ201, F380Mg, and F390Mg was moved to the cold zone, then removed and placed caused the joints’ mechanical properties to worsen, and on a stone table. A special holder (the same as was used even led to the joints themselves breaking. Taking this in Reference 18) was employed during the process, to into account, a protective atmosphere was used instead keep together the Mg substrate. After the soldering of flux, and this resulted in stable joints. The microstruc- process, the overflow of solder was gently removed in ture is presented in Figure 1, for eutectic Zn-Al and Mg order to preserve the dimensions of the samples. Four joints after soldering for 3, 5, and 8 minutes of contact, samples were prepared for each temperature and time, at a temperature of 425 C. The solder dissolved Mg three for testing of mechanical properties and one for substrates during the soldering process, and formed at microstructure observation. The specimens for cross section were mounted in resin, than grand and polished. the interface an interfacial Mg-Zn layer, which was Microstructural and elemental analyses were performed confirmed by the EDS analysis presented in Table I. using scanning electron microscopy (FEI Quanta 3D With increasing soldering time, the Mg-Zn layer grew. FEG-SEM) coupled with energy dispersive X-ray spec- Furthermore, particles of Mg with a small amount of Zn trometry (EDS), in order to study the interfacial dissolved inside the filler solder. Fig. 1—Microstructure after soldering process for eutectic Zn-Al and Mg joints for (a)3,(b) 5, and (c) 8 min of contact, at a temperature of 425 C. The numbers in the figure denote the points of EDSs presented in Table I. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2685 The amount of Al is on the same level for all of an MgZn phase by a eutectoid reaction at 325 C: [19] measured points, at around 2 to 3 (wt pct). However, Mg Zn fi a  Mg + MgZn. However, in our case, 7 3 the Zn in the Mg-Zn particles increased slightly from the Mg was dissolving and supplied to the melt solder, Mg substrate (~ 3 wt pct) to inside the solder which is observed in the microstructure (Figure 1). With (~ 8 wt pct), which correlates with Mg dissolving in increasing time, a rising number of Mg particles is the solder. The Mg particles in all soldered joints were observed, with the highest being for 8 min. A similar observed. Most importantly, the matrix of the solder microstructure for all eutectic Zn-Al and Zn-Al with Li changed from eutectic Zn-Al to an Mg + MgZn and Na alloys was observed. However, under the same eutectoid structure. This reaction is correlated with the conditions, the highest amount of dissolved Mg in the high amount of dissolved Mg, which, according to the interface, and the greatest number of Mg particles for [5] Mg-Zn phase diagram, formed the lamellar a  Na content, was observed for Zn-Al0.05Li compared to Mg + MgZn eutectoid structure distributed on the eutectic Zn-Al (see Figure 2). In Figure 2, (a) Zn-Al, (b) boundary of the black a-Mg solid solution in the Zn-Al0.05Li, and (c) Zn-Al0.2Na joints after soldering soldering region. EDS analysis (Table I—gray area) at 450 C and 8 minutes are presented. shows that, with increasing time, the amount of Mg in Figure 3 shows the Mg joints soldered by eutectic these eutectoid areas also increases. According to the Zn-Al after 3 minutes and at temperatures of 450 C, [5] Mg-Zn phase diagram, the Mg Zn phase is formed by 475 C, and 500 C. With increasing temperature, the 7 3 a eutectic reaction at 340 C: L fi a  Mg + Mg Zn . Al-Mg interlayers obtained greater thickness and a 7 3 Further reduction of temperature causes the formation higher number of Mg inclusions are observed. [20] According to the Mg-Li phase diagrams, Li con- tent up to 5 (wt pct) dissolved in the solid solution. For [21] Na content (in the whole range) as shown in Mg-Na, Table I. EDS Analysis of Marked Points in Fig. 1 of the liquid occurring across almost the entire Mg range Soldering Joints with Increasing Time of 3, 5, and 8 Min at a Temperature of 425 C of Mg-Na alloys after 97.8 C probably accelerates the dissolving process of Mg substrates. The greatest Weight Percent dissolution with soldering time for Li and Na-containing alloys compared to eutectic Zn-Al is presented in MgK AlK ZnK Figure 4. 1 91.1 2.1 6.8 The measured thickness of the ‘‘interlayer’’ is depen- 2 90.5 2.1 7.4 dent on the time. With increasing temperature and time, 3 90.8 2.1 7.1 the thickness of the interlayer also increased. This effect 4 49.4 3.2 47.4 caused the Mg substrate to dissolve by the grain 5 49.5 3.0 47.5 boundary, and all grains to move to the solder, which 6 90.0 2.0 8.0 is shown for Na content in eutectic Zn-Al in Figure 2(c). 7 91.3 2.1 6.6 The data presented in Figure 4 confirm that, with time 8 90.9 2.4 6.7 and temperature of soldering, the effect of dissolving Mg 9 90.9 2.2 6.9 substrates is highest for Na, then for Li additions, and 10 90.5 2.2 7.3 lowest for eutectic Zn-Al. 11 50.2 3.4 46.4 12 50.1 3.6 46.3 The microstructure for 425 C is presented in 13 91.3 2.3 6.4 Figure 1(a). With increasing temperature, the amount 14 92.0 2.3 5.7 of dissolved Mg in the solder also increases. At the 15 90.4 2.4 7.2 highest temperature (500 C), dissolution of the Mg 16 54.0 3.6 42.4 substrate by the grain boundary is observed after 17 91.2 2.4 6.4 3 minutes, to the same degree as after 8 minutes at a 18 51.6 3.8 44.6 lower temperature, of 450 C. For all alloys, eutectic 19 90.5 2.5 7.0 Zn-Al, and Zn-Al with Li and Na additions, the Fig. 2—Microstructure after soldering process for (a) Zn-Al, (b) Zn-Al0.05Li, (c) Zn-Al0.2Na, respectively, at 450 C for 8 min. The numbers in the figure denote the points of EDSs presented in Table II. 2686—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A observed microstructures are similar. However, the during applied force. Such a solution ensuring the thickness of the interlayer at the interface increases with correct shear strength of joints was applied. additions of Li and Na compared to eutectic Zn-Al, as To determine the shear strength of joints, the device shown in Figure 4. For the highest soldering tempera- presented in Reference 22 and Figure 5 was used, along ture, of 500 C, the dissolution of all Mg grains is with samples of different diameters. However, especially observed for all alloys, which caused increasing con- for the high temperature of 500 C, the Mg pads are sumption of the Mg substrate, an increase in the dissolved by the solder during the soldering process, thickness of the interlayer, and a resulting increase in causing the area of interface reaction to move and the thickness of the area of the joints. expand. The obtained results for the shear strengths of joints with eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na, after soldering times of 3, 5, and 8 minutes B. Mechanical Properties and for temperatures of 425 C, 450 C, 475 C, and At first, tensile tests were used to determine the shear 500 C are presented in Figure 6. [9,10,19] strength of joints. However, during our tensile Zn-Al0.2Na showed the highest shear strength value testing the sample was twisted, causing the Mg substrate (50.7 MPa), followed by Zn-Al0.05Li (32.2 MPa) and to break close to the soldering zone, which increased the eutectic Zn-Al (28.2 MPa). For all solders, the highest errors in calculating the shear strength of the joints. As a values of shear strength of joints for temperatures of result, a different method of tensile testing was proposed 425 C, 450 C, and 475 C were obtained for all times. in order to obtain the correct shear strength of joints. As These values are all similar and fall within error limits. described in Reference 22, the soldered pad is sheared For a soldering temperature of 500 C, shear strength reduces significantly, by as much as 50 pct, in all cases. Such behavior could be caused by the increasing Table II. EDS Analysis of Marked Points in Fig. 2 of Table III. EDS Analysis of Marked Points in Fig. 4 of Soldering Mg Joints at a Temperature of 450 C and a Time Soldering Joints with Increasing Temperatures of 450 C, of 8 Min, by Eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na 475 C, and 500 C and a Time of 3 Min Weight Percent Weight Percent MgK AlK ZnK MgK AlK ZnK 20 91.5 1.9 6.6 37 96.0 0.2 3.8 21 90.9 1.9 7.2 38 95.2 0.2 4.6 22 90.4 1.9 7.7 39 49.3 0.9 49.8 23 49.7 3.0 47.3 40 49.9 0.9 49.2 24 49.4 3.0 47.6 41 94.4 0.3 5.3 25 92.7 1.8 5.5 42 95.5 0.6 3.9 26 92.8 1.7 5.5 43 95.6 0.6 3.8 27 92.3 1.7 6.0 44 95.7 0.5 3.8 28 92.7 1.7 5.6 45 57.2 1.6 41.2 29 49.1 3.1 47.8 46 56.0 1.6 42.4 30 48.7 3.0 48.3 47 96.5 0.5 3.0 31 92.9 1.8 5.3 48 95.5 0.6 3.9 32 92.6 1.9 5.5 49 96.5 0.5 3.0 33 92.2 1.8 6.0 50 96.4 0.5 3.1 34 92.2 1.8 6.0 51 96.5 0.4 3.1 35 52.4 3.0 44.6 52 55.4 1.7 42.9 36 51.0 3.2 45.8 Fig. 3—Microstructure of eutectic Zn-Al with Mg joints after soldering for 3 min at temperatures of (a) 450, (b) 475, and (c) 500 C. The numbers in the figure denote the points of EDSs presented in Table III. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2687 Fig. 4—Thickness of the interlayer at the interface after soldering process for (a) Zn-Al, (b) Zn-Al0.05Li, (c) Zn-Al0.2Na, respectively, as a function of time for different temperatures. Table IV shows the microhardness of eutectic Zn-Al and Zn-Al alloys with Li and Na on Mg substrate. The two regions investigated were the a-Mg+MgZn eutec- toid structure and the a-Mg solid solution. The average microhardnesses obtained for the a-Mg region were 103, 104, and 101 (HV ) for eutectic Zn-Al, and Zn-Al 0.05 with Li and Na, respectively. The microhardness of the a-Mg region was related to the amount of doped Zn and Al, which was shown in the EDS analysis collected in Tables I through III. The average microhardnesses obtained for the a  Mg + MgZn region were 243, 233, and 237 (HV ) for eutectic Zn-Al, and Zn-Al 0.05 with Li and Na, respectively. The same situation as for the a-Mg region occurs, so the addition of Li and Na [13] [14] increase microhardness to 67.2 and 56.6 (HV ), 0.05 respectively, compared to eutectic Zn-Al (55.0 [13] (HV ) ). However, the greater addition of Na to 0.05 eutectic Zn-Al caused the creation of an area of doped NaZn , where the microhardness is much higher (for [13] Na (3 wt pct) it is 339.0 (HV ) ). In the case of 0.05 [19] soldering AZ31 using the Zn-Al filler metal, the average microhardness values obtained at the interface of the a-Mg solid solution and a  Mg + MgZn eutectoid structure are 93 and 134 (HV ), respectively, 0.05 compared to 81 (HV ) and 130 (HV ), respectively, 0.05 0.05 for Zn-Mg-Al solder, which is much lower than for the a  Mg + MgZn region result obtained by this study. IV. DISCUSSION During the soldering process, Zn-Al solders and the Mg substrate formed an interfacial layer at the interface. This is a diffusion layer, where the a-Mg solid solution is contained by Zn and Al. Such behavior was observed Fig. 5—Device and samples for shear strength measurements. [18] for solder materials of the Zn-Al system, [23] [9] [10] Zn-Mg-Al, Zn-Mg, and Sn-Zn. The Mg sub- strate dissolves more easily compared to a Cu sub- [15–18] interface reaction area, which results in the Mg substrate strate, and the kinetics of creation of the IMC layer being dissolved by solders at the high temperature of are different. However, the interlayer after soldering for 500 C. 8 minutes at a temperature of 500 C is not as thick as 2688—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 6—Shear strength of joints for (a) eutectic Zn-Al, (b) Zn-Al0.05Li, and (c) Zn-Al0.2Na, respectively. b—CuZn are formed at the interface during soldering, Table IV. Microhardness Value Obtained at the Interface of causing the pure Cu to dissolve in the solder but only as Mg Joints for Eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na particles of e, which could with time transform to the c Alloys [16,25,26] phase as the phase with lower Gibbs free energy. Microhardness (HV) The observed microstructure of Mg joints (Figures 1, 2 and 3) shows that the Mg from the substrate dissolved in Alloys a-Mg Region a  Mg + MgZn Region the solder as particles of Mg (EDS points: 6, 9, 10, 17, 19, and so on), and as the matrix of solder a  Mg + Zn-Al. 103 ± 4 243 ± 7 MgZn (area EDS: 4, 5, 11, 12, 16, 18, and so on). The Zn-Al0.05Li 104 ± 4 233 ± 2 Zn-Al0.2Na 101 ± 3 237 ± 11 solder’s chemical composition transforms from the Zn-Al system to the Mg-Zn system in the event of diffusion of Mg from the substrate. A different character [18] of changes is observed for Cu joints, where the Cu dissolves in the solder but the matrix of the solder is still the Zn-Al system. For such changes, temperature controls the reaction when soldering on Mg substrate. [19] For the Mg-Zn system, the temperature of the soldering process is above that of eutectic reaction, so the Mg will dissolve faster in the solder. As observed in References 2, 23, and 24 and in our study, the Mg substrate is dissolved in the solder in its entirety, and the amount of Mg increased with increasing soldering time. The results show that the relative content of Al and Zn elements in the solder decreases. The Mg substrate dissolves in a similar manner as in the dissolving e phase in the solder, with ‘‘scallops’’ detaching from the interlayer and diffusing deeper in the solder. The amount of Mg in the solder matrix close to the interlayer was similar to the amount in Reference 10 in which the composition of the solder was 47.8 of Mg, 48.9 of Zn and 3.3 of Al (wt pct), as it was in this study (EDS analysis, Tables I through III). However, as presented in Reference 10, for soldering at 390 C for 30 seconds, with solder composed of 97.7 Zn, 0.8 Mg, 1.1 Al and 0.4 Fig. 7—Microstructures of eutectic Zn-Al with Mg substrates joints Mn (wt pct), the Zn reach region is observed within the after soldering at a temperature of 475 C for 8 min. [10] solder. This changes with a higher amount of Mg in the solder (83.0 of Zn, 15.2 of Mg, 1.3 of Al, and 0.5 Mn that of the same solder used for soldering Cu (87.1 lm (wt pct)), and the Zn reach region almost disappears at a [24] [15] for Zn-Al, 85.5 lm for Zn-Al0.05Li, and 79.3 lm slightly higher soldering temperature (420 C). In this [16] for Zn-Al0.2Na, compared to this study: 19.4, 22.5, study, after 3 minutes and at a soldering temperature of and 24.8 lm, respectively). In the case of the Cu 475 C, the chemical composition of solder was substrate, three IMCs e—CuZn , c—Cu Zn , and 4 5 8 Mg+MgZn, as shown in Figure 7. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2689 [19] As observed in Reference 10, the weld width (joint area) compared to 93 (HV ) for the a-Mg solid solution. 0.05 is increased from 378 to 722 lm, indicating that the As presented in Reference 23 for Mg-based solder, the diffusion of Mg is higher than that of Zn and Al. However, fracture of the joint exhibits intergranular features, and the the process of dissolution of the Mg substrate is very crack originates from the a  Mg + MgZn eutectoid dynamic, as it happens spontaneously rather than uni- structure. The microhardness of the a-Mg solid solution is formly, creating a place where there is a fast path of Mg 81 (HV ), and that of the a  Mg + MgZn eutectoid 0.05 [23] diffusion to the solder (as shown in Figure 7). The model structure is 130 (HV ). Where an external force acts on 0.05 of microstructure evaluation of Mg/Zn/Mg joints was the solder joint, the stress concentration is generated easily [23] presented in Reference 27. At first, in the case of dissolving from the hard a  Mg + MgZn eutectoid structure. [19] [23] Mg substrate by Zn, an interlayer starts to be created at the The same effect as in the case of Zn-Al and Mg-based [24] interface. The ‘‘scalloped’’ structure of the interlayer is solders was observed for Al-based solder, where the hard correlated with the grain boundary penetration phe- phase of b-Mg Al (which is much harder than a-Mg, at 17 12 nomenon: liquid would penetrate the grain boundary 120 (HV ) and 91(HV ), respectively)is responsible for 0.05 0.05 along the depth direction of the Mg-base metal, since the the fracture. The same observation is made in Reference Mg atoms at the grain boundary had higher chemical 8—that, with increasing Al in Mg-Sn-In-Al, the hardness [28] potential than those in the grains. At higher tempera- also increases, from 96 (HV )to123 (HV ) for 0 and 6 0.05 0.05 tures, the diffusion coefficient of Mg increased exponen- (wt pct) of Al, respectively. For the higher microhardness tially, causing an increase in the Mg contained in the liquid values for a-Mg (~ 100 (HV )) and a-Mg+MgZn (~ 230 0.05 [27] and increasing the width of the Mg(Zn) diffusion layer. (HV )) obtained in this study, the fact that the soldering 0.05 The same character of dissolving substrate and formation process was performed without flux had an impact. This of ‘‘scallops’’ at the interface as that presented in the could be compared with References 19 and 23,where literature for Cu substrate with Zn-Al alloys was soldering was carried out in the presence of QJ201 [15–18] observed. However, in the case of Cu, three IMCs flux—which in our observations and measurements were formed at the interface and the process was controlled reduced the mechanical properties of the obtained joints. by the diffusion of Cu to the solder (Table IV). Taking into account that the thickness of the IMC V. CONCLUSIONS layer is correlated with diffusion and growth rate in the Eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na alloys Mg joint, the character of growth rate changes from were designed to join pure magnesium substrate. The volume diffusion to grain boundary. However, a lot of experiment shows the mechanism and morphology Mg particles are observed in the solder. Similar changes for Mg, forming the basis of and explain the microstructure and properties for Mg joints soldered [9,19,23] behavior of Mg when soldering Mg alloys. From this with Zn-based alloys were observed in the literature [24] study, the following important conclusions were derived: for Al-based filler metal. The diffusion layer at the interface is observed, and a huge amount of Mg dissolves (1) Magnesium substrate can be successfully joined in solder and changes its chemical composition to by eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na a-Mg + b-Mg Al . The results of the performed 17 12 alloys in an argon gas shield. mechanical tests indicate that the average shear strength (2) The original solder, eutectic Zn-Al, Zn-Al0.05Li, [24] is 45 MPa. Furthermore, the fractographic analysis of and Zn-Al0.2Na alloys were consumed after the the soldered Mg joint for both Zn and Al-based solder soldering process. The cross section microstruc- [9,18,23,24] shows a brittle fracture pattern. ture showed that a  Mg + MgZn eutectoid The results of mechanical property testing showed that structures were formed in the soldering region. the addition of Li and Na increases the shear strength of Mg (3) The average shear strengths are 28, 32, and 50 MPa joints to ~ 32 and ~ 50 MPa, respectively, compared to for eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na 28 MPa for eutectic Zn-Al. Furthermore, the obtained alloys, respectively, for soldered Mg joints. The [19] shear strength is slightly higher compared to 19 MPa for microhardness for the a-Mg region is 103, 104, and a Zn-Al alloy filler with 19.2 (wt pct) Al. However, this is 101 (HV ), and for a  Mg + MgZn is 243, 233, [10] 0.05 much lower compared to 70 MPa for Sn40Zn and and 237 (HV ) for eutectic Zn-Al, Zn-Al0.05Li, [23] 0.05 56 MPa for Mg3.0Al1.0Zn0.4Mn0.1Si. Moreover, as and Zn-Al0.2Na, respectively. The fracture is was observed in all cases, the fracture proceeds from the intergranular, and the crack originates from the eutectic a  Mg + MgZn region close to the interfacial hard a  Mg + MgZn eutectoid structure. layer. The observation in Reference 19 shows that the fracture of the soldered joint has a brittle pattern. The dissolution of Al andZninthe a-Mg has a solid solution OPEN ACCESS strengthening effect, which improved the mechanical prop- erties. The stress concentration effect in solder joints on Mg This article is distributed under the terms of the substrate accumulates in the a  Mg + MgZn eutectoid Creative Commons Attribution 4.0 International [19] structure, where the fracture will start. The authors show License (http://creativecommons.org/licenses/by/4.0/), that the high microhardness and brittleness of the a  which permits unrestricted use, distribution, and Mg + MgZn eutectoid structure in the case of Zn-Al reproduction in any medium, provided you give solder are the main reasons for the fracture of the soldered appropriate credit to the original author(s) and the Mg joints. The fracture-causing high microhardness of the source, provide a link to the Creative Commons a  Mg + MgZn eutectoid structure is 134 (HV ), license, and indicate if changes were made. 0.05 2690—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A 15. T. Gancarz, J. Pstrus, G. Cempura, and K. Berent: J. Electron. REFERENCES Mater., 2016, vol. 45, pp. 6067–78, https://doi.org/10.1007/s11664- 1. K.U. Kainer, ed., Magnesium Alloys and their Applications, ed., 016-4815-8. Wiley-Vch, New York, 2006. 16. T. Gancarz and J. Pstrus´ : K. BerentJ. Mater. Eng. Perform., 2016, 2. A.A. Luo: Inter. Mater. Rev., 2004, vol. 49, pp. 13–30. vol. 25, pp. 3366–74, https://doi.org/10.1007/s11665-016-2075-7. 3. Y. Zhang, Z. Luo, Y. Li, Z.-M. Liu, and Z.-Y. Huang: Mat. Des., 17. J. Pstrus´ , P. Fima, and T. Gancarz: J. Mater. Eng. Perform., 2012, 2015, vol. 75, pp. 166–73. vol. 21, pp. 606–13. 4. J. Hirsch and T. Al-Samman: Acta Mater., 2013, vol. 61, 18. J. Pstrus and T. Gancarz: J. Mater. Eng. Perform., 2014, vol. 23, pp. 818–43. pp. 1614–24. 5. M. Mezbahul-Islam, A.O. Mostafa, and M. Medraj: J. Mater., 19. L. Ma, D.Y. He, X.Y. Li, and J.M. Jiang: Mater. Letter., 2010, 2014, vol. 2014, pp. 1–33. vol. 64, pp. 596–98. 6. F. Hayat: Mater. Des., 2011, vol. 32, pp. 2476–84. 20. T.Massalski,H.Okamoto,P.Subramanian, and L. Kacprzak: Binary 7. L.M. Zhao and Z.D. Zhang: Scr. Mater., 2008, vol. 58, alloy phase diagrams, ASM International, Ohio, 2001, p. 2445. pp. 283–86. 21. J.R. Terbush, N. Stanford, J.-F. Nie, and M.R. Barnett: Metall. 8. L.M. Liu and H.Y. Wang: Mater. Sci. Eng. A, 2009, vol. 507, Mater. Trans. A, 2013, vol. 44A, pp. 5216–24. pp. 22–28. 22. M. Prazmowski: _ Arch. Metall. Mater., 2014, vol. 59, pp. 1137–42. 9. L. Liu and Z. Wu: Mater. Character., 2010, vol. 61, pp. 13– 23. L. Ma, D. He, X. Li, and J. Jiang: J. Mater. Sci. Technol., 2010, vol. 26, pp. 743–46. 10. Z. Wang, H. Wang, and L. Liu: Mat. Des., 2012, vol. 39, 24. L. Ma, P. Qiaoa, W. Longa, D. Heb, and X. Lib: Mater. Des., pp. 14–19. 2012, vol. 37, pp. 465–69. 11. T. Watanabe, 8 – Brazing and soldering of magnesium alloys, 25. T. Gancarz, J. Pstrus, P. Fima, and S. Mosinska: J. Alloy Compd., Welding and joining of magnesium alloys, (2010) 97-121. 2014, vol. 582, pp. 313–22. 12. M.K. Kulekci: Int. J. Adv. Manuf. Technol., 2008, vol. 39, 26. Y. Takaku, L. Felicia, I. Ohnuma, R. Kainuma, and K. Ishida: J. pp. 851–65. Electron. Mater., 2008, vol. 37, pp. 314–23. 13. T. Gancarz, G. Cempura, and W. Skuza: Mater. Charact., 2016, 27. R. Xie, X. Chen, Z. Lai, L. Liu, G. Zou, J. Yan, and W. Wang: vol. 111, pp. 147–53. Mater. Des., 2016, vol. 91, pp. 19–27. 14. T. Gancarz and G. Cempura: Mater. Des., 2016, vol. 104, 28. M.F. Wu, C. Yu, and J. Pu: Mater. Sci. Technol., 2008, vol. 24, pp. 51–59. pp. 1422–26. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2691 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Metallurgical and Materials Transactions A Springer Journals
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Abstract

TOMASZ GANCARZ, KATARZYNA BERENT, WOJCIECH SKUZA, and KATARZYNA JANIK Magnesium has applications in the automotive and aerospace industries that can significantly contribute to greater fuel economy and environmental conservation. The Mg alloys used in the automotive industry could reduce mass by up to 70 pct, providing energy savings. However, alongside the advantages there are limitations and technological barriers to use Mg alloys. One of the advantages concerns phenomena occurring at the interface when joining materials investigated in this study, in regard to the effect of temperature and soldering time for pure Mg joints. Eutectic Zn-Al and Zn-Al alloys with 0.05 (wt pct) Li and 0.2 (wt pct) Na were used in the soldering process. The process was performed for 3, 5, and 8 minutes of contact, at temperatures of 425 C, 450 C, 475 C, and 500 C. Selected, solidified solder-substrate couples were cross-sectioned, and their interfacial microstructures were investigated by scanning electron microscopy. The experiment was designed to demonstrate the effect of time, temperature, and the addition of Li and Na on the kinetics of the dissolving Mg substrate. The addition of Li and Na to eutectic Zn-Al caused to improve mechanical properties. Higher temperatures led to reduced joint strength, which is caused by increased interfacial reaction. https://doi.org/10.1007/s11661-018-4617-0 The Author(s) 2018 [3,6] I. INTRODUCTION techniques such as resistant spot welding, reactive [7] [8] [9] brazing, laser welding, ultrasonic-assisted soldering, CHANGES to the electrical engine, which can bring and so on. In order to eliminate the negative effect of about a reduction in its mass, are of significant interest to Mg–Al IMCs on the strength of the joints, an interlayer [1,2] the automotive industry. Al and Mg alloys are increas- [3] intended to block the formation of brittle IMC was used. [3] ingly used to reduce structural mass. Taking into [3] [10] [11] Zn, Sn-Zn alloys, Mg-Zn-Al, and Mg-In-Zn were account the lightest applications, the density of structural used to join Al with Mg alloys. 3 3 [4] materials (q =2.7 gcm , q =1.7 gcm ) is Al Mg In view of the high oxidation and corrosion of Mg very important. The aerospace industry’s requirements alloys, an addition characterized by high electrode [9] for lightweight materials to operate under increasingly potential compared to Mg ( 2.37 V) is made to the demanding conditions calls for reduced mass and solder. In this case, the layers of chromium and Teflon [12] improved mechanical properties. Compared to the Al protected the Mg against corrosion. Mg alloys sol- alloys, the Mg alloys display better physical and mechan- dered at high temperature displayed worse mechanical ical properties, such as high strength-to-weight ratio, high properties, so the joining process temperature should be damping capacity, and a high recycling potential. These lower than 450 C. For lower temperature soldering, flux facts could have a decisive effect on the application of Mg was used to increase wettability, which also protected the alloys in the automotive, electronics, and aerospace alloy surface from the formation the stable magnesium [11] industries. However, similar atomic mass and melting oxide. The application of an Zn interlayer when temperature in the Al-Mg system formed brittle inter- joining Al/Mg alloys using resistant spot welding caused metallic compounds (IMCs) such as Al Mg and 3 2 a reduction in welding current compared with traditional [5] Al Mg in the solidified metal. Furthermore, joining [3] 12 17 welding. Al dissolving into the fusion zone caused the the Al with Mg alloys is difficult, but possible using several increased formation of solid particles, which improves [3] the quality of the joints. Sn-Zn used at low tempera- tures for soldering Al/Mg joints caused the amount of Zn to rise by up to 30 pct, increasing shear strength. In TOMASZ GANCARZ, WOJCIECH SKUZA, and KATARZYNA addition, the dispersive distribution of the Al-Sn-Zn JANIK are with the Institute of Metallurgy and Materials Science, Polish solid solution within the solder reduced the brittleness of Academy of Sciences, 30-059 Krakow, Poland. Contact e-mail: the joint, thus greatly improving the mechanical prop- t.gancarz@imim.pl KATARZYNA BERENT is with the AGH erties. The conducted study, joining AZ31B alloy using University of Science and Technology, Academic Centre for Materials and Nanotechnology, 30-059 Krakow, Poland. Zn, caused the formation of an IMC from the Zn-Mg Manuscript submitted May 24, 2017. system at the interface. However, a higher cooling rate Article published online April 18, 2018 2684—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A was beneficial for increasing the tensile shear strength of microstructure and IMCs occurring at the interface. [9] joints, as it caused the formation of equiaxed dendrites For all samples, three measurements were made at and refined eutectic structure. different areas to improve statistics and to check the To show the influence at the interface during solder- homogeneity of the joints. Mechanical tests were con- ing, research should be carried out for pure Mg. This ducted with an INSTRON 6025 testing machine mod- study demonstrates the effect on the microstructure of ernized by Zwick/Roell. Testing conditions were in joints and mechanical properties of joining pure Mg accordance with ASTM A 264-03, with a strain rate of using eutectic Zn-Al and Zn-Al alloys with additions of 0.00025 (1/s) at room temperature. The mechanical tests Li and Na. were performed for each of the joints with the eutectic Zn-Al alloys, and with Na and Li additions, and three successful measurements were taken. After mechanical testing, the real area of the joints (the area of soldering II. EXPERIMENTAL between Mg substrates) was measured using CorelDraw [13] [13] Cast alloys of eutectic Zn-Al, Zn-Al0.05Li, and with the GetArea module. The shear strength is calcu- [14] Zn-Al0.2Na were used in this study, and the solder lated using the obtained force from mechanical tests [15,16] was tested while soldering Cu substrate. The cast divided by real area of the joints. The samples for the alloys were rolled to a thickness of 1 mm and cut into tests were taken after soldering, where the flow solder 8 9 10 mm pieces. The base material used for soldering from the substrate was removed. The microhardness of was pure Mg (99.9 pct) with dimensions of the formed phases in the soldering region was measured 8 9 4 9 25 mm. The Mg substrate and solder pieces by means of microhardness tests carried out at a load of were cleaned using emery paper and acetone before the 0.5 (N) and over a time of 15 (seconds). soldering process, to remove the oxide from the surface. Soldering the Mg joints was carried out using the sessile [17] drop method, with protective gas Ar (5N), for times III. RESULTS of 3, 5, and 8 minutes of contact and at temperatures of 425 C, 450 C, 475 C, and 500 C. The samples were A. Microstructure Observation moved from the cold zone to the hot zone of the furnace, where a type K thermocouple was touching the bottom All the Mg joints were obtained under a suitable pro- tective gas without flux. However, an experiment using of the holder and the melting process was observed by flux and without protective gas was also performed. CCD camera. After a predetermined time, the sample Fluxes such as ALU12, QJ201, F380Mg, and F390Mg was moved to the cold zone, then removed and placed caused the joints’ mechanical properties to worsen, and on a stone table. A special holder (the same as was used even led to the joints themselves breaking. Taking this in Reference 18) was employed during the process, to into account, a protective atmosphere was used instead keep together the Mg substrate. After the soldering of flux, and this resulted in stable joints. The microstruc- process, the overflow of solder was gently removed in ture is presented in Figure 1, for eutectic Zn-Al and Mg order to preserve the dimensions of the samples. Four joints after soldering for 3, 5, and 8 minutes of contact, samples were prepared for each temperature and time, at a temperature of 425 C. The solder dissolved Mg three for testing of mechanical properties and one for substrates during the soldering process, and formed at microstructure observation. The specimens for cross section were mounted in resin, than grand and polished. the interface an interfacial Mg-Zn layer, which was Microstructural and elemental analyses were performed confirmed by the EDS analysis presented in Table I. using scanning electron microscopy (FEI Quanta 3D With increasing soldering time, the Mg-Zn layer grew. FEG-SEM) coupled with energy dispersive X-ray spec- Furthermore, particles of Mg with a small amount of Zn trometry (EDS), in order to study the interfacial dissolved inside the filler solder. Fig. 1—Microstructure after soldering process for eutectic Zn-Al and Mg joints for (a)3,(b) 5, and (c) 8 min of contact, at a temperature of 425 C. The numbers in the figure denote the points of EDSs presented in Table I. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2685 The amount of Al is on the same level for all of an MgZn phase by a eutectoid reaction at 325 C: [19] measured points, at around 2 to 3 (wt pct). However, Mg Zn fi a  Mg + MgZn. However, in our case, 7 3 the Zn in the Mg-Zn particles increased slightly from the Mg was dissolving and supplied to the melt solder, Mg substrate (~ 3 wt pct) to inside the solder which is observed in the microstructure (Figure 1). With (~ 8 wt pct), which correlates with Mg dissolving in increasing time, a rising number of Mg particles is the solder. The Mg particles in all soldered joints were observed, with the highest being for 8 min. A similar observed. Most importantly, the matrix of the solder microstructure for all eutectic Zn-Al and Zn-Al with Li changed from eutectic Zn-Al to an Mg + MgZn and Na alloys was observed. However, under the same eutectoid structure. This reaction is correlated with the conditions, the highest amount of dissolved Mg in the high amount of dissolved Mg, which, according to the interface, and the greatest number of Mg particles for [5] Mg-Zn phase diagram, formed the lamellar a  Na content, was observed for Zn-Al0.05Li compared to Mg + MgZn eutectoid structure distributed on the eutectic Zn-Al (see Figure 2). In Figure 2, (a) Zn-Al, (b) boundary of the black a-Mg solid solution in the Zn-Al0.05Li, and (c) Zn-Al0.2Na joints after soldering soldering region. EDS analysis (Table I—gray area) at 450 C and 8 minutes are presented. shows that, with increasing time, the amount of Mg in Figure 3 shows the Mg joints soldered by eutectic these eutectoid areas also increases. According to the Zn-Al after 3 minutes and at temperatures of 450 C, [5] Mg-Zn phase diagram, the Mg Zn phase is formed by 475 C, and 500 C. With increasing temperature, the 7 3 a eutectic reaction at 340 C: L fi a  Mg + Mg Zn . Al-Mg interlayers obtained greater thickness and a 7 3 Further reduction of temperature causes the formation higher number of Mg inclusions are observed. [20] According to the Mg-Li phase diagrams, Li con- tent up to 5 (wt pct) dissolved in the solid solution. For [21] Na content (in the whole range) as shown in Mg-Na, Table I. EDS Analysis of Marked Points in Fig. 1 of the liquid occurring across almost the entire Mg range Soldering Joints with Increasing Time of 3, 5, and 8 Min at a Temperature of 425 C of Mg-Na alloys after 97.8 C probably accelerates the dissolving process of Mg substrates. The greatest Weight Percent dissolution with soldering time for Li and Na-containing alloys compared to eutectic Zn-Al is presented in MgK AlK ZnK Figure 4. 1 91.1 2.1 6.8 The measured thickness of the ‘‘interlayer’’ is depen- 2 90.5 2.1 7.4 dent on the time. With increasing temperature and time, 3 90.8 2.1 7.1 the thickness of the interlayer also increased. This effect 4 49.4 3.2 47.4 caused the Mg substrate to dissolve by the grain 5 49.5 3.0 47.5 boundary, and all grains to move to the solder, which 6 90.0 2.0 8.0 is shown for Na content in eutectic Zn-Al in Figure 2(c). 7 91.3 2.1 6.6 The data presented in Figure 4 confirm that, with time 8 90.9 2.4 6.7 and temperature of soldering, the effect of dissolving Mg 9 90.9 2.2 6.9 substrates is highest for Na, then for Li additions, and 10 90.5 2.2 7.3 lowest for eutectic Zn-Al. 11 50.2 3.4 46.4 12 50.1 3.6 46.3 The microstructure for 425 C is presented in 13 91.3 2.3 6.4 Figure 1(a). With increasing temperature, the amount 14 92.0 2.3 5.7 of dissolved Mg in the solder also increases. At the 15 90.4 2.4 7.2 highest temperature (500 C), dissolution of the Mg 16 54.0 3.6 42.4 substrate by the grain boundary is observed after 17 91.2 2.4 6.4 3 minutes, to the same degree as after 8 minutes at a 18 51.6 3.8 44.6 lower temperature, of 450 C. For all alloys, eutectic 19 90.5 2.5 7.0 Zn-Al, and Zn-Al with Li and Na additions, the Fig. 2—Microstructure after soldering process for (a) Zn-Al, (b) Zn-Al0.05Li, (c) Zn-Al0.2Na, respectively, at 450 C for 8 min. The numbers in the figure denote the points of EDSs presented in Table II. 2686—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A observed microstructures are similar. However, the during applied force. Such a solution ensuring the thickness of the interlayer at the interface increases with correct shear strength of joints was applied. additions of Li and Na compared to eutectic Zn-Al, as To determine the shear strength of joints, the device shown in Figure 4. For the highest soldering tempera- presented in Reference 22 and Figure 5 was used, along ture, of 500 C, the dissolution of all Mg grains is with samples of different diameters. However, especially observed for all alloys, which caused increasing con- for the high temperature of 500 C, the Mg pads are sumption of the Mg substrate, an increase in the dissolved by the solder during the soldering process, thickness of the interlayer, and a resulting increase in causing the area of interface reaction to move and the thickness of the area of the joints. expand. The obtained results for the shear strengths of joints with eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na, after soldering times of 3, 5, and 8 minutes B. Mechanical Properties and for temperatures of 425 C, 450 C, 475 C, and At first, tensile tests were used to determine the shear 500 C are presented in Figure 6. [9,10,19] strength of joints. However, during our tensile Zn-Al0.2Na showed the highest shear strength value testing the sample was twisted, causing the Mg substrate (50.7 MPa), followed by Zn-Al0.05Li (32.2 MPa) and to break close to the soldering zone, which increased the eutectic Zn-Al (28.2 MPa). For all solders, the highest errors in calculating the shear strength of the joints. As a values of shear strength of joints for temperatures of result, a different method of tensile testing was proposed 425 C, 450 C, and 475 C were obtained for all times. in order to obtain the correct shear strength of joints. As These values are all similar and fall within error limits. described in Reference 22, the soldered pad is sheared For a soldering temperature of 500 C, shear strength reduces significantly, by as much as 50 pct, in all cases. Such behavior could be caused by the increasing Table II. EDS Analysis of Marked Points in Fig. 2 of Table III. EDS Analysis of Marked Points in Fig. 4 of Soldering Mg Joints at a Temperature of 450 C and a Time Soldering Joints with Increasing Temperatures of 450 C, of 8 Min, by Eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na 475 C, and 500 C and a Time of 3 Min Weight Percent Weight Percent MgK AlK ZnK MgK AlK ZnK 20 91.5 1.9 6.6 37 96.0 0.2 3.8 21 90.9 1.9 7.2 38 95.2 0.2 4.6 22 90.4 1.9 7.7 39 49.3 0.9 49.8 23 49.7 3.0 47.3 40 49.9 0.9 49.2 24 49.4 3.0 47.6 41 94.4 0.3 5.3 25 92.7 1.8 5.5 42 95.5 0.6 3.9 26 92.8 1.7 5.5 43 95.6 0.6 3.8 27 92.3 1.7 6.0 44 95.7 0.5 3.8 28 92.7 1.7 5.6 45 57.2 1.6 41.2 29 49.1 3.1 47.8 46 56.0 1.6 42.4 30 48.7 3.0 48.3 47 96.5 0.5 3.0 31 92.9 1.8 5.3 48 95.5 0.6 3.9 32 92.6 1.9 5.5 49 96.5 0.5 3.0 33 92.2 1.8 6.0 50 96.4 0.5 3.1 34 92.2 1.8 6.0 51 96.5 0.4 3.1 35 52.4 3.0 44.6 52 55.4 1.7 42.9 36 51.0 3.2 45.8 Fig. 3—Microstructure of eutectic Zn-Al with Mg joints after soldering for 3 min at temperatures of (a) 450, (b) 475, and (c) 500 C. The numbers in the figure denote the points of EDSs presented in Table III. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2687 Fig. 4—Thickness of the interlayer at the interface after soldering process for (a) Zn-Al, (b) Zn-Al0.05Li, (c) Zn-Al0.2Na, respectively, as a function of time for different temperatures. Table IV shows the microhardness of eutectic Zn-Al and Zn-Al alloys with Li and Na on Mg substrate. The two regions investigated were the a-Mg+MgZn eutec- toid structure and the a-Mg solid solution. The average microhardnesses obtained for the a-Mg region were 103, 104, and 101 (HV ) for eutectic Zn-Al, and Zn-Al 0.05 with Li and Na, respectively. The microhardness of the a-Mg region was related to the amount of doped Zn and Al, which was shown in the EDS analysis collected in Tables I through III. The average microhardnesses obtained for the a  Mg + MgZn region were 243, 233, and 237 (HV ) for eutectic Zn-Al, and Zn-Al 0.05 with Li and Na, respectively. The same situation as for the a-Mg region occurs, so the addition of Li and Na [13] [14] increase microhardness to 67.2 and 56.6 (HV ), 0.05 respectively, compared to eutectic Zn-Al (55.0 [13] (HV ) ). However, the greater addition of Na to 0.05 eutectic Zn-Al caused the creation of an area of doped NaZn , where the microhardness is much higher (for [13] Na (3 wt pct) it is 339.0 (HV ) ). In the case of 0.05 [19] soldering AZ31 using the Zn-Al filler metal, the average microhardness values obtained at the interface of the a-Mg solid solution and a  Mg + MgZn eutectoid structure are 93 and 134 (HV ), respectively, 0.05 compared to 81 (HV ) and 130 (HV ), respectively, 0.05 0.05 for Zn-Mg-Al solder, which is much lower than for the a  Mg + MgZn region result obtained by this study. IV. DISCUSSION During the soldering process, Zn-Al solders and the Mg substrate formed an interfacial layer at the interface. This is a diffusion layer, where the a-Mg solid solution is contained by Zn and Al. Such behavior was observed Fig. 5—Device and samples for shear strength measurements. [18] for solder materials of the Zn-Al system, [23] [9] [10] Zn-Mg-Al, Zn-Mg, and Sn-Zn. The Mg sub- strate dissolves more easily compared to a Cu sub- [15–18] interface reaction area, which results in the Mg substrate strate, and the kinetics of creation of the IMC layer being dissolved by solders at the high temperature of are different. However, the interlayer after soldering for 500 C. 8 minutes at a temperature of 500 C is not as thick as 2688—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 6—Shear strength of joints for (a) eutectic Zn-Al, (b) Zn-Al0.05Li, and (c) Zn-Al0.2Na, respectively. b—CuZn are formed at the interface during soldering, Table IV. Microhardness Value Obtained at the Interface of causing the pure Cu to dissolve in the solder but only as Mg Joints for Eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na particles of e, which could with time transform to the c Alloys [16,25,26] phase as the phase with lower Gibbs free energy. Microhardness (HV) The observed microstructure of Mg joints (Figures 1, 2 and 3) shows that the Mg from the substrate dissolved in Alloys a-Mg Region a  Mg + MgZn Region the solder as particles of Mg (EDS points: 6, 9, 10, 17, 19, and so on), and as the matrix of solder a  Mg + Zn-Al. 103 ± 4 243 ± 7 MgZn (area EDS: 4, 5, 11, 12, 16, 18, and so on). The Zn-Al0.05Li 104 ± 4 233 ± 2 Zn-Al0.2Na 101 ± 3 237 ± 11 solder’s chemical composition transforms from the Zn-Al system to the Mg-Zn system in the event of diffusion of Mg from the substrate. A different character [18] of changes is observed for Cu joints, where the Cu dissolves in the solder but the matrix of the solder is still the Zn-Al system. For such changes, temperature controls the reaction when soldering on Mg substrate. [19] For the Mg-Zn system, the temperature of the soldering process is above that of eutectic reaction, so the Mg will dissolve faster in the solder. As observed in References 2, 23, and 24 and in our study, the Mg substrate is dissolved in the solder in its entirety, and the amount of Mg increased with increasing soldering time. The results show that the relative content of Al and Zn elements in the solder decreases. The Mg substrate dissolves in a similar manner as in the dissolving e phase in the solder, with ‘‘scallops’’ detaching from the interlayer and diffusing deeper in the solder. The amount of Mg in the solder matrix close to the interlayer was similar to the amount in Reference 10 in which the composition of the solder was 47.8 of Mg, 48.9 of Zn and 3.3 of Al (wt pct), as it was in this study (EDS analysis, Tables I through III). However, as presented in Reference 10, for soldering at 390 C for 30 seconds, with solder composed of 97.7 Zn, 0.8 Mg, 1.1 Al and 0.4 Fig. 7—Microstructures of eutectic Zn-Al with Mg substrates joints Mn (wt pct), the Zn reach region is observed within the after soldering at a temperature of 475 C for 8 min. [10] solder. This changes with a higher amount of Mg in the solder (83.0 of Zn, 15.2 of Mg, 1.3 of Al, and 0.5 Mn that of the same solder used for soldering Cu (87.1 lm (wt pct)), and the Zn reach region almost disappears at a [24] [15] for Zn-Al, 85.5 lm for Zn-Al0.05Li, and 79.3 lm slightly higher soldering temperature (420 C). In this [16] for Zn-Al0.2Na, compared to this study: 19.4, 22.5, study, after 3 minutes and at a soldering temperature of and 24.8 lm, respectively). In the case of the Cu 475 C, the chemical composition of solder was substrate, three IMCs e—CuZn , c—Cu Zn , and 4 5 8 Mg+MgZn, as shown in Figure 7. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2689 [19] As observed in Reference 10, the weld width (joint area) compared to 93 (HV ) for the a-Mg solid solution. 0.05 is increased from 378 to 722 lm, indicating that the As presented in Reference 23 for Mg-based solder, the diffusion of Mg is higher than that of Zn and Al. However, fracture of the joint exhibits intergranular features, and the the process of dissolution of the Mg substrate is very crack originates from the a  Mg + MgZn eutectoid dynamic, as it happens spontaneously rather than uni- structure. The microhardness of the a-Mg solid solution is formly, creating a place where there is a fast path of Mg 81 (HV ), and that of the a  Mg + MgZn eutectoid 0.05 [23] diffusion to the solder (as shown in Figure 7). The model structure is 130 (HV ). Where an external force acts on 0.05 of microstructure evaluation of Mg/Zn/Mg joints was the solder joint, the stress concentration is generated easily [23] presented in Reference 27. At first, in the case of dissolving from the hard a  Mg + MgZn eutectoid structure. [19] [23] Mg substrate by Zn, an interlayer starts to be created at the The same effect as in the case of Zn-Al and Mg-based [24] interface. The ‘‘scalloped’’ structure of the interlayer is solders was observed for Al-based solder, where the hard correlated with the grain boundary penetration phe- phase of b-Mg Al (which is much harder than a-Mg, at 17 12 nomenon: liquid would penetrate the grain boundary 120 (HV ) and 91(HV ), respectively)is responsible for 0.05 0.05 along the depth direction of the Mg-base metal, since the the fracture. The same observation is made in Reference Mg atoms at the grain boundary had higher chemical 8—that, with increasing Al in Mg-Sn-In-Al, the hardness [28] potential than those in the grains. At higher tempera- also increases, from 96 (HV )to123 (HV ) for 0 and 6 0.05 0.05 tures, the diffusion coefficient of Mg increased exponen- (wt pct) of Al, respectively. For the higher microhardness tially, causing an increase in the Mg contained in the liquid values for a-Mg (~ 100 (HV )) and a-Mg+MgZn (~ 230 0.05 [27] and increasing the width of the Mg(Zn) diffusion layer. (HV )) obtained in this study, the fact that the soldering 0.05 The same character of dissolving substrate and formation process was performed without flux had an impact. This of ‘‘scallops’’ at the interface as that presented in the could be compared with References 19 and 23,where literature for Cu substrate with Zn-Al alloys was soldering was carried out in the presence of QJ201 [15–18] observed. However, in the case of Cu, three IMCs flux—which in our observations and measurements were formed at the interface and the process was controlled reduced the mechanical properties of the obtained joints. by the diffusion of Cu to the solder (Table IV). Taking into account that the thickness of the IMC V. CONCLUSIONS layer is correlated with diffusion and growth rate in the Eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na alloys Mg joint, the character of growth rate changes from were designed to join pure magnesium substrate. The volume diffusion to grain boundary. However, a lot of experiment shows the mechanism and morphology Mg particles are observed in the solder. Similar changes for Mg, forming the basis of and explain the microstructure and properties for Mg joints soldered [9,19,23] behavior of Mg when soldering Mg alloys. From this with Zn-based alloys were observed in the literature [24] study, the following important conclusions were derived: for Al-based filler metal. The diffusion layer at the interface is observed, and a huge amount of Mg dissolves (1) Magnesium substrate can be successfully joined in solder and changes its chemical composition to by eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na a-Mg + b-Mg Al . The results of the performed 17 12 alloys in an argon gas shield. mechanical tests indicate that the average shear strength (2) The original solder, eutectic Zn-Al, Zn-Al0.05Li, [24] is 45 MPa. Furthermore, the fractographic analysis of and Zn-Al0.2Na alloys were consumed after the the soldered Mg joint for both Zn and Al-based solder soldering process. The cross section microstruc- [9,18,23,24] shows a brittle fracture pattern. ture showed that a  Mg + MgZn eutectoid The results of mechanical property testing showed that structures were formed in the soldering region. the addition of Li and Na increases the shear strength of Mg (3) The average shear strengths are 28, 32, and 50 MPa joints to ~ 32 and ~ 50 MPa, respectively, compared to for eutectic Zn-Al, Zn-Al0.05Li, and Zn-Al0.2Na 28 MPa for eutectic Zn-Al. Furthermore, the obtained alloys, respectively, for soldered Mg joints. The [19] shear strength is slightly higher compared to 19 MPa for microhardness for the a-Mg region is 103, 104, and a Zn-Al alloy filler with 19.2 (wt pct) Al. However, this is 101 (HV ), and for a  Mg + MgZn is 243, 233, [10] 0.05 much lower compared to 70 MPa for Sn40Zn and and 237 (HV ) for eutectic Zn-Al, Zn-Al0.05Li, [23] 0.05 56 MPa for Mg3.0Al1.0Zn0.4Mn0.1Si. Moreover, as and Zn-Al0.2Na, respectively. The fracture is was observed in all cases, the fracture proceeds from the intergranular, and the crack originates from the eutectic a  Mg + MgZn region close to the interfacial hard a  Mg + MgZn eutectoid structure. layer. The observation in Reference 19 shows that the fracture of the soldered joint has a brittle pattern. The dissolution of Al andZninthe a-Mg has a solid solution OPEN ACCESS strengthening effect, which improved the mechanical prop- erties. The stress concentration effect in solder joints on Mg This article is distributed under the terms of the substrate accumulates in the a  Mg + MgZn eutectoid Creative Commons Attribution 4.0 International [19] structure, where the fracture will start. The authors show License (http://creativecommons.org/licenses/by/4.0/), that the high microhardness and brittleness of the a  which permits unrestricted use, distribution, and Mg + MgZn eutectoid structure in the case of Zn-Al reproduction in any medium, provided you give solder are the main reasons for the fracture of the soldered appropriate credit to the original author(s) and the Mg joints. The fracture-causing high microhardness of the source, provide a link to the Creative Commons a  Mg + MgZn eutectoid structure is 134 (HV ), license, and indicate if changes were made. 0.05 2690—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A 15. T. Gancarz, J. Pstrus, G. Cempura, and K. Berent: J. Electron. REFERENCES Mater., 2016, vol. 45, pp. 6067–78, https://doi.org/10.1007/s11664- 1. K.U. Kainer, ed., Magnesium Alloys and their Applications, ed., 016-4815-8. Wiley-Vch, New York, 2006. 16. T. Gancarz and J. Pstrus´ : K. BerentJ. Mater. Eng. Perform., 2016, 2. A.A. Luo: Inter. Mater. Rev., 2004, vol. 49, pp. 13–30. vol. 25, pp. 3366–74, https://doi.org/10.1007/s11665-016-2075-7. 3. Y. Zhang, Z. Luo, Y. Li, Z.-M. Liu, and Z.-Y. Huang: Mat. Des., 17. J. Pstrus´ , P. Fima, and T. Gancarz: J. Mater. Eng. Perform., 2012, 2015, vol. 75, pp. 166–73. vol. 21, pp. 606–13. 4. J. Hirsch and T. Al-Samman: Acta Mater., 2013, vol. 61, 18. J. Pstrus and T. Gancarz: J. Mater. Eng. Perform., 2014, vol. 23, pp. 818–43. pp. 1614–24. 5. M. Mezbahul-Islam, A.O. Mostafa, and M. Medraj: J. Mater., 19. L. Ma, D.Y. He, X.Y. Li, and J.M. Jiang: Mater. Letter., 2010, 2014, vol. 2014, pp. 1–33. vol. 64, pp. 596–98. 6. F. Hayat: Mater. Des., 2011, vol. 32, pp. 2476–84. 20. T.Massalski,H.Okamoto,P.Subramanian, and L. Kacprzak: Binary 7. L.M. Zhao and Z.D. Zhang: Scr. Mater., 2008, vol. 58, alloy phase diagrams, ASM International, Ohio, 2001, p. 2445. pp. 283–86. 21. J.R. Terbush, N. Stanford, J.-F. Nie, and M.R. Barnett: Metall. 8. L.M. Liu and H.Y. Wang: Mater. Sci. Eng. A, 2009, vol. 507, Mater. Trans. A, 2013, vol. 44A, pp. 5216–24. pp. 22–28. 22. M. Prazmowski: _ Arch. Metall. Mater., 2014, vol. 59, pp. 1137–42. 9. L. Liu and Z. Wu: Mater. Character., 2010, vol. 61, pp. 13– 23. L. Ma, D. He, X. Li, and J. Jiang: J. Mater. Sci. Technol., 2010, vol. 26, pp. 743–46. 10. Z. Wang, H. Wang, and L. Liu: Mat. Des., 2012, vol. 39, 24. L. Ma, P. Qiaoa, W. Longa, D. Heb, and X. Lib: Mater. Des., pp. 14–19. 2012, vol. 37, pp. 465–69. 11. T. Watanabe, 8 – Brazing and soldering of magnesium alloys, 25. T. Gancarz, J. Pstrus, P. Fima, and S. Mosinska: J. Alloy Compd., Welding and joining of magnesium alloys, (2010) 97-121. 2014, vol. 582, pp. 313–22. 12. M.K. Kulekci: Int. J. Adv. Manuf. Technol., 2008, vol. 39, 26. Y. Takaku, L. Felicia, I. Ohnuma, R. Kainuma, and K. Ishida: J. pp. 851–65. Electron. Mater., 2008, vol. 37, pp. 314–23. 13. T. Gancarz, G. Cempura, and W. Skuza: Mater. Charact., 2016, 27. R. Xie, X. Chen, Z. Lai, L. Liu, G. Zou, J. Yan, and W. Wang: vol. 111, pp. 147–53. Mater. Des., 2016, vol. 91, pp. 19–27. 14. T. Gancarz and G. Cempura: Mater. Des., 2016, vol. 104, 28. M.F. Wu, C. Yu, and J. Pu: Mater. Sci. Technol., 2008, vol. 24, pp. 51–59. pp. 1422–26. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2691

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Metallurgical and Materials Transactions ASpringer Journals

Published: Apr 18, 2018

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