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Hot Cracking in AZ31 and AZ61 Magnesium Alloy
J. Mater. Sci. Technol., 2011, 27(7), 633-640.
Hot Cracking in AZ31 and AZ61 Magnesium Alloy C.J. Huang1)† , C.M. Cheng2) , C.P. Chou1) and F.H. Chen2) 1) Department of Mechanical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, China 2) Department of Industrial Education, National Taiwan Normal University, Taipei 106, Taiwan, China [Manuscript received October 26, 2010, in revised form January 5, 2011]
This paper examined the impact of the number of thermal cycles and augmented strain on hot cracking in AZ31 and AZ61 magnesium alloy. Statistical analyses were performed. Following observation using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) was used for component analysis. Results showed that Al content in magnesium alloy has an effect on hot cracking susceptibility. In addition, the nonequilibrium solidification process produced segregation in Al content, causing higher liquid Mg-alloy rich Al content at grain boundaries, and resulting into liquefied grain boundaries of partially melted zone (PMZ). In summary, under multiple thermal cycles AZ61 produced serious liquation cracking. AZ61 has higher (6 wt%) Al content and produced much liquefied Mg17 Al12 at grain boundaries under multiple thermal cycles. The liquefied Mg17 Al12 were pulled apart and hot cracks formed at weld metal HAZ due to the augmented strain. Since AZ31 had half the Al content of AZ61, its hot-cracking susceptibility was lower than AZ61. In addition, AZ61 showed longer total crack length (TCL) in one thermal cycle compared to that in three thermal cycles. This phenomenon was possibly due to high-temperature gasification of Al during the welding process, which resulted in lower overall Al content. Consequently, shorter hot cracks exhibited in three thermal cycles. It was found the Al content of AZ31 and AZ61 can be used to assess the hot-cracking susceptibility. KEY WORDS: Varestraint test; Thermal cycles; Hot-cracking susceptibility; Gas tungsten arc welding (GTAW); AZ31; AZ61
1. Introduction Magnesium has high damping capability, excellent electromagnetic interference shielding properties, and superior machinability. It is widely used in many industries, such as automobiles, aircraft, and 3C electronics products[1–3] due to its excellent material characteristics, including low density, good thermal conductivity, strong shock absorption, and recyclable properties. Welding technologies play an important role in joining applications. Laser beam welding (LBW), gas tungsten arc welding (GTAW), electron beam welding (EBW), and friction stir welding (FSW)[4–7] have been used in welding magnesium alloys. The most common defects of magnesium alloys during welding are: weakening at heat-affected zone (HAZ)[8] , hot cracking, distortion, and porosity[9] . † Corresponding author. Ph.D.; E-mail address: [email protected] (C.J. Huang).
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Welding defects could weaken the joint after welding. Hot cracking is related to the composition of alloy itself, and is closely associated with the contractions caused by regional heating and cooling during welding[10,11] . Previous studies did not focus on magnesium alloy s hot cracking susceptibility; most of them only discussed the mechanical properties and microstructure of fusion zone and HAZ after welding. Researches of magnesium alloy s hot cracking susceptibility have not been published in present literature. The current project examines hot cracking produced in AZ31 and AZ61 with thermal cycles and augmented strain, analyzing the reasons for their occurrence. 2. Experimental 2.1 Experiment material The materials were hot rolled plate of AZ31 and AZ61 magnesium alloys. Specimen dimensions were
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Table 1 Chemical composition of AZ31 and AZ61 magnesium alloys, wt% Al 3.15 6.13
Zn 0.99 1.01
Mn 0.43 0.24
Si 0.044 0.027
Cu 0.0023 0.0026
Fe 0.0044 0.0039
Mg Bal. Bal.
Table 2 Parameters for GTAW Current/A 100
Voltage/V 10
Travel speed/(mm/min) 60
Argon flow rate/(L/min) 10
Table 3 Parameters for Spot Varestraint test Current/A 100
Welding time/s 3
Argon flow rate/(L/min) 10
Augment strain/% 1, 3, 5
200 mm × 35 mm × 3 mm; the tensile strength of the base metal was 210 and 300 MPa. Table 1 shows the base metal compositions. This experiment used semi-automatic gas tungsten arc welding (GTAW) as a welding method to perform the single pre-welding (one thermal cycle) and triple pre-welding (triple thermal cycles) of specimens without filler materials using a W-ThO2 electrode of 2.4 mm in diameter. Following experimental welding testing and correction, the optimal welding parameters were uncovered. Table 2 shows the relevant welding parameters. 2.2 Spot varestraint test The hot cracking test involved a spot Varestraint test, which was carried out using the GTAW method. The instrument used in this test was a multiple Varestraint test instrument developed by the authors. The gun of the GTAW was controlled using a computer program to move along the x and y axes; die-blocks with various radii were set on the x or y axis. The stroke was adjustable to bend the materials to obtain the radius of the blocks. The block was changeable and the longitudinal Varestraint test was performed by setting the stroke using a single mechanism. Therefore, this instrument used for the spot Varestraint test, the longitudinal Varestraint test and the Varestraint test of x and y axis welding was a multiple hot cracking test instrument. 2.3 Experiment procedure Performing spot-welding on pre-welded specimens enabled spot varestraint tests, which involved spot welding of non-pass (non-thermal cycles), single pass (one thermal cycle), and triple pass (three thermal cycles) specimens (welding diagrams are shown in Fig. 1); 1%, 3%, and 5% augmented strains were applied. Multiple thermal cycles could simulate thick plate or repair welding which needed many times welding, and augmented strains could imitate residual stress of fusion zone after welding. Table 3 shows the spot Varestraint parameters. Following the tests, SEM was used to examine hot cracking in the fusion
Fig. 1 Welding diagram
zone and HAZ of specimens. The lengths of hot cracking specimens under different augmented strains and thermal cycles were analyzed to examine hot cracking susceptibility. The total crack length (TCL)[12] for each specimen represented an indicator for hot cracking susceptibility. Etchant was produced by mixing 3.5 g of picric acid+6.5 ml acetic acid+20 ml water and 100 ml ethyl alcohol for AZ31[13] , and 4 vol% HNO3 +96 vol% ethyl alcohol for AZ61[14] , and was used to etch cross-sections of the specimen. 3. Results and Discussion 3.1 SEM observation Figure 2 shows the SEM results of AZ31 and AZ61 under the varestraint test with no thermal cycle. As seen in the figure, while AZ31 and AZ61 did not undergo thermal cycle, hot cracking concentrated in the fusion zone (FZ). Figure 3 shows the SEM picture of AZ31 and AZ61 during one thermal cycle. It can be seen in Fig. 3(a) for AZ31 that hot cracking
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Fig. 4 Schematic drawing location of weld metal HAZ and base metal HAZ
Fig. 2 Hot cracking in fusion zone of AZ31 (a) and AZ61 (b) (no thermal cycle)
Fig. 3 Hot cracking in fusion zone and HAZ of AZ31 (a) and AZ61 (b) (one thermal cycle)
mostly exists in fusion zone, with only small parts extending to HAZ. In Fig. 3(b) hot cracks appeared in HAZ after thermal cycle, a portion of cracks appeared in FZ, while most hot cracks were concentrated in the HAZ. The hot cracking in weld metal HAZ (W.M. HAZ) is visibly longer than the hot cracking in base metal HAZ (B.M. HAZ). In this experimental design,
HAZ were divided into weld metal HAZ and base metal HAZ based on HAZ positions (as in Fig. 4). Figure 5 shows the total crack lengths in the HAZ of AZ31 and AZ61 under different augmented strain and multiple thermal cycles. Figure 5(a) shows AZ31 s HAZ total cracking length was both short at three thermal cycles and one thermal cycle, while the total crack length invariable as thermal cycles accumulated. Figure 5(b) shows that AZ61 s total cracking length at one thermal cycle was longer than that at three thermal cycles. This result contradicts the hot cracking theories which posit under normal circumstances. TCL in HAZ of AZ61 increased with the increase in augmented strain and the number of thermal cycles. Our experimental results showed the hot cracking of AZ61 s HAZ were mainly located at W.M. HAZ, while the length of hot cracking at B.M. HAZ were significantly shorter, as shown in Fig. 3(b). To examine this phenomenon, hot crack lengths were collected from W.M. HAZ and B.M. HAZ of AZ61, respectively, as shown in Fig. 6. Figure 6 shows that the AZ61 s TCL of W.M. HAZ after three thermal cycles was shorter than one thermal cycle, which was not the case for B.M. HAZ. It was likely caused by a lack of thermal refining after pre-welding before undergoing varestraint tests. As a result, weld metal after heat was applied again became W.M. HAZ. Grain coarsening and precipitation segregated in grain boundaries of W.M. HAZ both appeared in this region. Mg-Al binary phase diagram explains these differences between hot crack length in the HAZ of AZ31 and AZ61[15] . Figure 7 shows that AZ31 s and AZ61 s solidus and liquidus temperature range depends on Al content. This temperature range is called the partially melted zone (PMZ), which is wider for AZ61 that has higher Al content than AZ31. In PMZ the precipitation at grain boundaries could be liquefied under welding. Therefore, during varestraint test, the HAZ becomes a solid-liquid coexistence region with α Mg and liquid Mg-alloy. Being pulled by the augmented strain, the unsolidified liquid Mg-alloy at grain boundaries were pulled away forming hot cracks in W.M. HAZ. Due to higher Al content in AZ61, liquid Mg-alloy with low melting point was easily formed to yield wider grain boundaries liquefied region in PMZ. It stays within grain boundaries and creates wider liquefied regions, resulting in longer hot cracks.
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Fig. 5 Effect of number of thermal cycles on hot cracking in HAZ: (a) AZ31, (b) AZ61
Fig. 6 Effect of number of thermal cycles on HAZ hot cracking under 1% (a), 3% (b) and 5% (c) augmented strain
3.2 Hot cracking observation Specimens of AZ31 and AZ61 were taken from the hot cracking cross sections of B.M. HAZ and W.M. HAZ. The specimens were polished and etched before being placed under SEM. Figure 8 shows the AZ61 hot cracking cross section after three thermal cycles. Precipitation was clearly visible between hot cracks and grains. Figure 8(a) shows a crack and a line of white precipitation. The precipitation around the edges of the cracks indicated the liquid Mg-alloy had not solidified. This liquid Mg-Alloy was under augmented strain and later formed hot cracks during solidification. This corresponds to the mechanism of hot crack formation asserted by Borland[16] . The white precipitation around the grain boundaries should be
Mg17 Al12 [17] , which showed no clear cracks at low magnification. The possible cause was that the major part of the tension was located to the left of the crack, so the rest of the tension was unable to pull the liquid Mg-alloy on the right; hence, the liquid Mg-alloy became precipitation (Mg17 Al12 ) during the cooling process. The precipitation (Mg17 Al12 ) was magnified to 5,000 times in Fig. 8(b) and small cracks between precipitation and grains were visible, which indicates clear signs of tension that was not strong enough to separate the grains. There were signs of Mg17 Al12 around the grain boundaries alongside AZ31 s W.M. HAZ cracks (Fig. 9(a)). Smaller cracks only appeared between Mg17 Al12 and α-Mg (Fig. 9(b)). In the Mg-Al binary phase diagram (Fig. 7), there was no Al element precipitating in the solidification
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Fig. 7 Mg-Al binary phase diagram and schematic drawing of PMZ of arc weld: (a) AZ31, (b) AZ61 Fig. 9 (a) AZ31 hot cracks and precipitation, (b) small hot cracks around precipitation
Fig. 10 Relationship between critical solidification range and hot cracking susceptibility
AZ31 and AZ61, whereas pure Mg metal was not suitable. In other words, if liquefied Mg17 Al12 was not present within the grain boundaries during the final solidification process, hot cracks would not be formed on grain boundaries of pure Mg metal or Mg alloy. Fig. 8 (a) AZ61 hot cracks and precipitation, (b) small hot cracks around precipitation
process of pure magnesium (without solid-liquid coexistence region), as solid state α-Mg existed below melting point. During the solidification process, AZ31 and AZ61 increasingly segregated Al elements and formed the liquefied Mg17 Al12 , so there were liquid Mg alloy and solid state α-MgCo existing above eutectic temperature[18,19] . According to Borland s hot cracking theory[16] , the critical solidification range (CSR) of solid-liquid coexistence region is broader and the hot cracking forming is easier (Fig. 10). Thus, wider CSR would form liquation hot cracking of
This proves that low melting point phase of Mg17 Al12 around the grain boundaries was one of the causes for hot cracking in magnesium alloys. AZ61 have much Al and much Mg17 Al12 could precipitate, so AZ61 has higher hot-cracking susceptibility in W.M. HAZ. 3.3 Grain boundary analysis Hot cracking of HAZ may be liquation hot cracking or ductility-dip hot cracking. In this study, AZ61 produced liquation hot cracking. Two types of HAZ liquation hot cracking existed: the first was just HAZ s grain boundaries liquefaction during welding.
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to find out Cu precipitation at the grain boundaries in PMZ, with respect to AZ61. The nonequilibrium solidification process of liquid Mg-alloy segregated increasingly large amounts of Al element, lowering the melting points of liquid Mg-alloy rich in Al. Liquid Mg-alloy, which had not solidified, was then torn by augmented strain, forming cracks[21] . These results suggest that Mg17 Al12 at grain boundaries liquefaction f produces hot cracks. When 1%, 3% and 5% augmented strain was applied to AZ61 magnesium alloy, Al (7.7 wt%) and Zn (0.3 wt%) precipitation at grain boundaries was far lower than the content produced by one thermal cycle (Al 13.3 wt% Zn 0.82 wt%). Figure 13 shows W.M. HAZ in one and three thermal cycle specimens analyzed by EDS. The initial results (shown in Table 4) show that for Table 4 Chemical composition for EDS analyses of Fig. 12 (wt%)
Fig. 11 EDS analysis of spot of path AB across grain boundary: (a) one thermal cycle; (b) three thermal cycles
The melting point of grain boundaries was the lowest, so grain boundaries were the last part to solidify during the solidification process; the second type included precipitation or low-melting point impure components at HAZ grain boundaries, which lowered melting points at grain boundaries and thereby produced liquefied metal membranes[20] . This experiment involved AZ61 specimens that had been processed in one and three thermal cycles. EDS analysis analyzed grain boundaries of PMZ approaching the hot cracking. Figure 11 shows the results of this analysis. Line AB is the scanning path crossing over grain boundaries; the line AB could be divided into six equal parts at the length of 3 μm each. Figure 12(a) depicts the component analysis of Al element along line AB; it shows that Al precipitation occurred at the grain boundary. With one thermal cycle, the Al content was 13.3 wt% at the grain boundary, significantly higher than the 6.0 wt% of the original base metal; with three thermal cycles, Al content was reduced to 7.7 wt%, only 1.7 wt% higher than the base metal. Figure 12(b) depicts component analysis for Zn along line AB. The results clearly show that precipitation of Zn occurred at the grain boundary. In one thermal cycle, the Zn content was 0.82 wt%, close to the 1 wt% of the base metal. These results indicate that AZ61 magnesium alloy experienced Al precipitation at the grain boundaries in W.M. HAZ. Furthermore, the results of this study corresponds to the findings in Kou and Cheng s research where they[10,11] investigated aluminum alloy
Element (a) (b) (c) (d)
Al 6.92 6.94 11.06 8.7
Zn 0.71 0.64 1.49 0.71
Mg Bal. Bal. Bal. Bal.
both specimens Al content was 6.9 wt% and Zn content was approximately 0.7 wt%. In other words, no differences were found. Because there were grains and grain boundaries on the surface analyzed, grains occupied most area of the analyzing surface. This caused that Al content for Table 4(a) and (b) was almost the same and was similar to base metal. However, for a component analysis of torn hot cracking surfaces, Al content in crack surfaces with one thermal cycle was 11 wt%, while Al content in cracking surfaces with three thermal cycles was 8 wt%. These results indicate a clear reduction in Al content. Cracking surface formed near the grain boundaries which had high Al content liquid alloy. During the solidification process grains solidified first, and fusion zone s grains growth would be identical to base metal which has the same component and size from epitaxy growth mechanism, which prevented the A1 content of grains from being affected by gasification of Al and Zn. Hightemperature gasification reduced Al content of liquid Mg-alloy at grain boundaries which solidified slowest, so gasification of Al and Zn occurred on the cracking surface of W.M. HAZ. This study posits that the reduction in Al content was the result of the additional pre-welding from three thermal cycles, which caused high-temperature gasification of some Al and Zn. The melting and evaporation points of Al and Zn were 660◦ C/2060◦ C and 420◦ C/750◦ C, while the temperature of a created electric arc could reach beyond 8000◦ C[22] . This explains the evaporation of Al and Zn after multiple welding processes. After three thermal cycles, gasification of Al and Zn reduced Al and Zn content of liquid Mg-alloy, resulting in higher
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Fig. 12 EDS analysis data of path AB across grain boundary: (a) Al, (b) Zn
Fig. 13 Location of AZ61for EDS analyses: (a) 1C-W.M. HAZ, (b) 3C-W.M. HAZ, (c) 1C-hot cracking surface, (d) 3C-hot cracking surface [1C means one thermal cycle; 3C means three thermal cycles
melting point of liquid Mg-alloy. Hence, AZ61 s TCL (9.23 mm) after three thermal cycles was less than that after one thermal cycle (10.5 mm), as shown in Fig. 6(b). 4. Conclusions (1) The Al content of AZ31 and AZ61 can be used
to assess the hot-cracking susceptibility, because the more Al content, the more precipitation (Mg17 Al12 ) with low melting point was produced at grain boundaries. During the welding process, Mg17 Al12 melted at grain boundaries and caused liquefaction. At the same time, augmented strain pulled away the grain boundaries, and in turn formed the hot cracking.
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AZ61 was more prone to form hot cracking. In three thermal cycles AZ61 s Al content was lower than one thermal cycle according to the EDS results. This is attributable to the evaporation of Al related to the number of heat input, resulting in less Al in the liquid Mg-alloy (having a higher melting point). Thus, the region of grain liquefaction was narrower than in one thermal cycle. (2) Following one and three thermal cycles for AZ61 magnesium alloy, severe liquation hot cracking was produced in W.M. HAZ. EDS analysis revealed precipitation of alloy elements Al and Zn at grain boundaries. After one thermal cycle, precipitation of Al reached 13.3 wt%, representing a doubling of Al components compared to the base metal. This phenomenon might be the result of AZ61 having twice the Al content of AZ31, leading to wider liquefied grain boundaries region in PMZ. So AZ61 produced serious liquation hot cracking in W.M. HAZ following multiple thermal cycles. (3) Spot varestraint tests can be used to find serviceable material for repairing welding or multi welding on thick plates. Our findings suggest that AZ31 can be used for repairing welding (or thick plate welding), whereas AZ61 is not suitable. REFERENCES [1 ] H. Westengen: Light Metal Age, 2000, 58, 44. [2 ] B.L. Mordike and T. Ebert: Mater. Sci. Eng. A, 2000, 302, 37. [3 ] A. Weisheit, R. Galun, and B.L. Mordike: Weld. J., 1998, 77, 149. [4 ] C.C. Chang, C.P. Chou, S.N. Hsu, G.Y. Hsiung and
J.R. Chen: J. Mater. Sci. Technol., 2010, 26, 276. [5 ] S.F. Su, J.C. Huang, H.K. Lin and N.J. Ho: Mater. Sci. Eng. A, 2002, 33, 1461. [6 ] L.M. Liu, Y. Shen and Z.D. Zhang: Sci. Technol. Weld. Joining, 2009, 11, 398. [7 ] K.S. Arora, S. Pandey, M. Schaper and R. Kumar: J. Mater. Sci. Technol., 2010, 26, 747. [8 ] W.A. Baeslack, S.J. Savage and F.H. Froes: Mater. Sci. Lett., 1986, 6, 935. [9 ] J. Shen, G.Q. You, S.Y. Long and F.S. Pan: Mater. Chartac., 2008, 59, 1059. [10] C. Huang and S. Kou: Weld. J., 2001, 1, 9. [11] C.M. Cheng, C.P. Chou, I.K. Lee and H.Y. Lin: Sci. Technol. Weld. Joining, 2005, 10, 344. [12] B. Thomas: Hot Cracking Phenomena in Welds II, EB, Springer-Verlag, Berlin, 2008, 49. [13] M.B. Kannan, W. Dietzel, C. Blawert, S. Riekehr and M. Kocak: Mater. Sci. Eng. A, 2006, 444, 220. [14] D. Min, J. shen, S. Lai and J. Chen: Mater. Charact., 2009, 60, 1583. [15] Y. Quan, Z. Chan, X. Gong and Z. Yu: Mater. Sci. Eng. A, 2008, 496, 45. [16] J.C. Borland: Brit. Weld. J., 1960, 7, 508. [17] I.A. Yakubtsova, B.J. Diaka, C.A. Sagera and B. Bhattacharyab: Mater. Sci. Eng. A, 2008, 496, 247. [18] M.C. Flemings: Solidification Processing, McGraw Hill, New York, 1974. [19] D.A. Porter and K.E. Easterling: Phase Transformations in Metals and Alloys, 2nd edn, Chapman and Hall, New York, 1992. [20] B. Hemsworth, T. Boniszewski and N.F. Eaton: Brit. Weld. J., 1969, 1, 5. [21] S. Kou: Welding Metallurgy, John Wiley and Sons, New York, 2003. [22] B. Ribic, T.A. Palmer and T. DebRoy: Int. Mater. Rev., 2009, 54, 223.
Hot Cracking in AZ31 and AZ61 Magnesium Alloy C.J. Huang1)† , C.M. Cheng2) , C.P. Chou1) and F.H. Chen2) 1) Department of Mechanical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, China 2) Department of Industrial Education, National Taiwan Normal University, Taipei 106, Taiwan, China [Manuscript received October 26, 2010, in revised form January 5, 2011]
This paper examined the impact of the number of thermal cycles and augmented strain on hot cracking in AZ31 and AZ61 magnesium alloy. Statistical analyses were performed. Following observation using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) was used for component analysis. Results showed that Al content in magnesium alloy has an effect on hot cracking susceptibility. In addition, the nonequilibrium solidification process produced segregation in Al content, causing higher liquid Mg-alloy rich Al content at grain boundaries, and resulting into liquefied grain boundaries of partially melted zone (PMZ). In summary, under multiple thermal cycles AZ61 produced serious liquation cracking. AZ61 has higher (6 wt%) Al content and produced much liquefied Mg17 Al12 at grain boundaries under multiple thermal cycles. The liquefied Mg17 Al12 were pulled apart and hot cracks formed at weld metal HAZ due to the augmented strain. Since AZ31 had half the Al content of AZ61, its hot-cracking susceptibility was lower than AZ61. In addition, AZ61 showed longer total crack length (TCL) in one thermal cycle compared to that in three thermal cycles. This phenomenon was possibly due to high-temperature gasification of Al during the welding process, which resulted in lower overall Al content. Consequently, shorter hot cracks exhibited in three thermal cycles. It was found the Al content of AZ31 and AZ61 can be used to assess the hot-cracking susceptibility. KEY WORDS: Varestraint test; Thermal cycles; Hot-cracking susceptibility; Gas tungsten arc welding (GTAW); AZ31; AZ61
1. Introduction Magnesium has high damping capability, excellent electromagnetic interference shielding properties, and superior machinability. It is widely used in many industries, such as automobiles, aircraft, and 3C electronics products[1–3] due to its excellent material characteristics, including low density, good thermal conductivity, strong shock absorption, and recyclable properties. Welding technologies play an important role in joining applications. Laser beam welding (LBW), gas tungsten arc welding (GTAW), electron beam welding (EBW), and friction stir welding (FSW)[4–7] have been used in welding magnesium alloys. The most common defects of magnesium alloys during welding are: weakening at heat-affected zone (HAZ)[8] , hot cracking, distortion, and porosity[9] . † Corresponding author. Ph.D.; E-mail address: [email protected] (C.J. Huang).
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Welding defects could weaken the joint after welding. Hot cracking is related to the composition of alloy itself, and is closely associated with the contractions caused by regional heating and cooling during welding[10,11] . Previous studies did not focus on magnesium alloy s hot cracking susceptibility; most of them only discussed the mechanical properties and microstructure of fusion zone and HAZ after welding. Researches of magnesium alloy s hot cracking susceptibility have not been published in present literature. The current project examines hot cracking produced in AZ31 and AZ61 with thermal cycles and augmented strain, analyzing the reasons for their occurrence. 2. Experimental 2.1 Experiment material The materials were hot rolled plate of AZ31 and AZ61 magnesium alloys. Specimen dimensions were
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Table 1 Chemical composition of AZ31 and AZ61 magnesium alloys, wt% Al 3.15 6.13
Zn 0.99 1.01
Mn 0.43 0.24
Si 0.044 0.027
Cu 0.0023 0.0026
Fe 0.0044 0.0039
Mg Bal. Bal.
Table 2 Parameters for GTAW Current/A 100
Voltage/V 10
Travel speed/(mm/min) 60
Argon flow rate/(L/min) 10
Table 3 Parameters for Spot Varestraint test Current/A 100
Welding time/s 3
Argon flow rate/(L/min) 10
Augment strain/% 1, 3, 5
200 mm × 35 mm × 3 mm; the tensile strength of the base metal was 210 and 300 MPa. Table 1 shows the base metal compositions. This experiment used semi-automatic gas tungsten arc welding (GTAW) as a welding method to perform the single pre-welding (one thermal cycle) and triple pre-welding (triple thermal cycles) of specimens without filler materials using a W-ThO2 electrode of 2.4 mm in diameter. Following experimental welding testing and correction, the optimal welding parameters were uncovered. Table 2 shows the relevant welding parameters. 2.2 Spot varestraint test The hot cracking test involved a spot Varestraint test, which was carried out using the GTAW method. The instrument used in this test was a multiple Varestraint test instrument developed by the authors. The gun of the GTAW was controlled using a computer program to move along the x and y axes; die-blocks with various radii were set on the x or y axis. The stroke was adjustable to bend the materials to obtain the radius of the blocks. The block was changeable and the longitudinal Varestraint test was performed by setting the stroke using a single mechanism. Therefore, this instrument used for the spot Varestraint test, the longitudinal Varestraint test and the Varestraint test of x and y axis welding was a multiple hot cracking test instrument. 2.3 Experiment procedure Performing spot-welding on pre-welded specimens enabled spot varestraint tests, which involved spot welding of non-pass (non-thermal cycles), single pass (one thermal cycle), and triple pass (three thermal cycles) specimens (welding diagrams are shown in Fig. 1); 1%, 3%, and 5% augmented strains were applied. Multiple thermal cycles could simulate thick plate or repair welding which needed many times welding, and augmented strains could imitate residual stress of fusion zone after welding. Table 3 shows the spot Varestraint parameters. Following the tests, SEM was used to examine hot cracking in the fusion
Fig. 1 Welding diagram
zone and HAZ of specimens. The lengths of hot cracking specimens under different augmented strains and thermal cycles were analyzed to examine hot cracking susceptibility. The total crack length (TCL)[12] for each specimen represented an indicator for hot cracking susceptibility. Etchant was produced by mixing 3.5 g of picric acid+6.5 ml acetic acid+20 ml water and 100 ml ethyl alcohol for AZ31[13] , and 4 vol% HNO3 +96 vol% ethyl alcohol for AZ61[14] , and was used to etch cross-sections of the specimen. 3. Results and Discussion 3.1 SEM observation Figure 2 shows the SEM results of AZ31 and AZ61 under the varestraint test with no thermal cycle. As seen in the figure, while AZ31 and AZ61 did not undergo thermal cycle, hot cracking concentrated in the fusion zone (FZ). Figure 3 shows the SEM picture of AZ31 and AZ61 during one thermal cycle. It can be seen in Fig. 3(a) for AZ31 that hot cracking
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Fig. 4 Schematic drawing location of weld metal HAZ and base metal HAZ
Fig. 2 Hot cracking in fusion zone of AZ31 (a) and AZ61 (b) (no thermal cycle)
Fig. 3 Hot cracking in fusion zone and HAZ of AZ31 (a) and AZ61 (b) (one thermal cycle)
mostly exists in fusion zone, with only small parts extending to HAZ. In Fig. 3(b) hot cracks appeared in HAZ after thermal cycle, a portion of cracks appeared in FZ, while most hot cracks were concentrated in the HAZ. The hot cracking in weld metal HAZ (W.M. HAZ) is visibly longer than the hot cracking in base metal HAZ (B.M. HAZ). In this experimental design,
HAZ were divided into weld metal HAZ and base metal HAZ based on HAZ positions (as in Fig. 4). Figure 5 shows the total crack lengths in the HAZ of AZ31 and AZ61 under different augmented strain and multiple thermal cycles. Figure 5(a) shows AZ31 s HAZ total cracking length was both short at three thermal cycles and one thermal cycle, while the total crack length invariable as thermal cycles accumulated. Figure 5(b) shows that AZ61 s total cracking length at one thermal cycle was longer than that at three thermal cycles. This result contradicts the hot cracking theories which posit under normal circumstances. TCL in HAZ of AZ61 increased with the increase in augmented strain and the number of thermal cycles. Our experimental results showed the hot cracking of AZ61 s HAZ were mainly located at W.M. HAZ, while the length of hot cracking at B.M. HAZ were significantly shorter, as shown in Fig. 3(b). To examine this phenomenon, hot crack lengths were collected from W.M. HAZ and B.M. HAZ of AZ61, respectively, as shown in Fig. 6. Figure 6 shows that the AZ61 s TCL of W.M. HAZ after three thermal cycles was shorter than one thermal cycle, which was not the case for B.M. HAZ. It was likely caused by a lack of thermal refining after pre-welding before undergoing varestraint tests. As a result, weld metal after heat was applied again became W.M. HAZ. Grain coarsening and precipitation segregated in grain boundaries of W.M. HAZ both appeared in this region. Mg-Al binary phase diagram explains these differences between hot crack length in the HAZ of AZ31 and AZ61[15] . Figure 7 shows that AZ31 s and AZ61 s solidus and liquidus temperature range depends on Al content. This temperature range is called the partially melted zone (PMZ), which is wider for AZ61 that has higher Al content than AZ31. In PMZ the precipitation at grain boundaries could be liquefied under welding. Therefore, during varestraint test, the HAZ becomes a solid-liquid coexistence region with α Mg and liquid Mg-alloy. Being pulled by the augmented strain, the unsolidified liquid Mg-alloy at grain boundaries were pulled away forming hot cracks in W.M. HAZ. Due to higher Al content in AZ61, liquid Mg-alloy with low melting point was easily formed to yield wider grain boundaries liquefied region in PMZ. It stays within grain boundaries and creates wider liquefied regions, resulting in longer hot cracks.
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Fig. 5 Effect of number of thermal cycles on hot cracking in HAZ: (a) AZ31, (b) AZ61
Fig. 6 Effect of number of thermal cycles on HAZ hot cracking under 1% (a), 3% (b) and 5% (c) augmented strain
3.2 Hot cracking observation Specimens of AZ31 and AZ61 were taken from the hot cracking cross sections of B.M. HAZ and W.M. HAZ. The specimens were polished and etched before being placed under SEM. Figure 8 shows the AZ61 hot cracking cross section after three thermal cycles. Precipitation was clearly visible between hot cracks and grains. Figure 8(a) shows a crack and a line of white precipitation. The precipitation around the edges of the cracks indicated the liquid Mg-alloy had not solidified. This liquid Mg-Alloy was under augmented strain and later formed hot cracks during solidification. This corresponds to the mechanism of hot crack formation asserted by Borland[16] . The white precipitation around the grain boundaries should be
Mg17 Al12 [17] , which showed no clear cracks at low magnification. The possible cause was that the major part of the tension was located to the left of the crack, so the rest of the tension was unable to pull the liquid Mg-alloy on the right; hence, the liquid Mg-alloy became precipitation (Mg17 Al12 ) during the cooling process. The precipitation (Mg17 Al12 ) was magnified to 5,000 times in Fig. 8(b) and small cracks between precipitation and grains were visible, which indicates clear signs of tension that was not strong enough to separate the grains. There were signs of Mg17 Al12 around the grain boundaries alongside AZ31 s W.M. HAZ cracks (Fig. 9(a)). Smaller cracks only appeared between Mg17 Al12 and α-Mg (Fig. 9(b)). In the Mg-Al binary phase diagram (Fig. 7), there was no Al element precipitating in the solidification
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Fig. 7 Mg-Al binary phase diagram and schematic drawing of PMZ of arc weld: (a) AZ31, (b) AZ61 Fig. 9 (a) AZ31 hot cracks and precipitation, (b) small hot cracks around precipitation
Fig. 10 Relationship between critical solidification range and hot cracking susceptibility
AZ31 and AZ61, whereas pure Mg metal was not suitable. In other words, if liquefied Mg17 Al12 was not present within the grain boundaries during the final solidification process, hot cracks would not be formed on grain boundaries of pure Mg metal or Mg alloy. Fig. 8 (a) AZ61 hot cracks and precipitation, (b) small hot cracks around precipitation
process of pure magnesium (without solid-liquid coexistence region), as solid state α-Mg existed below melting point. During the solidification process, AZ31 and AZ61 increasingly segregated Al elements and formed the liquefied Mg17 Al12 , so there were liquid Mg alloy and solid state α-MgCo existing above eutectic temperature[18,19] . According to Borland s hot cracking theory[16] , the critical solidification range (CSR) of solid-liquid coexistence region is broader and the hot cracking forming is easier (Fig. 10). Thus, wider CSR would form liquation hot cracking of
This proves that low melting point phase of Mg17 Al12 around the grain boundaries was one of the causes for hot cracking in magnesium alloys. AZ61 have much Al and much Mg17 Al12 could precipitate, so AZ61 has higher hot-cracking susceptibility in W.M. HAZ. 3.3 Grain boundary analysis Hot cracking of HAZ may be liquation hot cracking or ductility-dip hot cracking. In this study, AZ61 produced liquation hot cracking. Two types of HAZ liquation hot cracking existed: the first was just HAZ s grain boundaries liquefaction during welding.
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to find out Cu precipitation at the grain boundaries in PMZ, with respect to AZ61. The nonequilibrium solidification process of liquid Mg-alloy segregated increasingly large amounts of Al element, lowering the melting points of liquid Mg-alloy rich in Al. Liquid Mg-alloy, which had not solidified, was then torn by augmented strain, forming cracks[21] . These results suggest that Mg17 Al12 at grain boundaries liquefaction f produces hot cracks. When 1%, 3% and 5% augmented strain was applied to AZ61 magnesium alloy, Al (7.7 wt%) and Zn (0.3 wt%) precipitation at grain boundaries was far lower than the content produced by one thermal cycle (Al 13.3 wt% Zn 0.82 wt%). Figure 13 shows W.M. HAZ in one and three thermal cycle specimens analyzed by EDS. The initial results (shown in Table 4) show that for Table 4 Chemical composition for EDS analyses of Fig. 12 (wt%)
Fig. 11 EDS analysis of spot of path AB across grain boundary: (a) one thermal cycle; (b) three thermal cycles
The melting point of grain boundaries was the lowest, so grain boundaries were the last part to solidify during the solidification process; the second type included precipitation or low-melting point impure components at HAZ grain boundaries, which lowered melting points at grain boundaries and thereby produced liquefied metal membranes[20] . This experiment involved AZ61 specimens that had been processed in one and three thermal cycles. EDS analysis analyzed grain boundaries of PMZ approaching the hot cracking. Figure 11 shows the results of this analysis. Line AB is the scanning path crossing over grain boundaries; the line AB could be divided into six equal parts at the length of 3 μm each. Figure 12(a) depicts the component analysis of Al element along line AB; it shows that Al precipitation occurred at the grain boundary. With one thermal cycle, the Al content was 13.3 wt% at the grain boundary, significantly higher than the 6.0 wt% of the original base metal; with three thermal cycles, Al content was reduced to 7.7 wt%, only 1.7 wt% higher than the base metal. Figure 12(b) depicts component analysis for Zn along line AB. The results clearly show that precipitation of Zn occurred at the grain boundary. In one thermal cycle, the Zn content was 0.82 wt%, close to the 1 wt% of the base metal. These results indicate that AZ61 magnesium alloy experienced Al precipitation at the grain boundaries in W.M. HAZ. Furthermore, the results of this study corresponds to the findings in Kou and Cheng s research where they[10,11] investigated aluminum alloy
Element (a) (b) (c) (d)
Al 6.92 6.94 11.06 8.7
Zn 0.71 0.64 1.49 0.71
Mg Bal. Bal. Bal. Bal.
both specimens Al content was 6.9 wt% and Zn content was approximately 0.7 wt%. In other words, no differences were found. Because there were grains and grain boundaries on the surface analyzed, grains occupied most area of the analyzing surface. This caused that Al content for Table 4(a) and (b) was almost the same and was similar to base metal. However, for a component analysis of torn hot cracking surfaces, Al content in crack surfaces with one thermal cycle was 11 wt%, while Al content in cracking surfaces with three thermal cycles was 8 wt%. These results indicate a clear reduction in Al content. Cracking surface formed near the grain boundaries which had high Al content liquid alloy. During the solidification process grains solidified first, and fusion zone s grains growth would be identical to base metal which has the same component and size from epitaxy growth mechanism, which prevented the A1 content of grains from being affected by gasification of Al and Zn. Hightemperature gasification reduced Al content of liquid Mg-alloy at grain boundaries which solidified slowest, so gasification of Al and Zn occurred on the cracking surface of W.M. HAZ. This study posits that the reduction in Al content was the result of the additional pre-welding from three thermal cycles, which caused high-temperature gasification of some Al and Zn. The melting and evaporation points of Al and Zn were 660◦ C/2060◦ C and 420◦ C/750◦ C, while the temperature of a created electric arc could reach beyond 8000◦ C[22] . This explains the evaporation of Al and Zn after multiple welding processes. After three thermal cycles, gasification of Al and Zn reduced Al and Zn content of liquid Mg-alloy, resulting in higher
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Fig. 12 EDS analysis data of path AB across grain boundary: (a) Al, (b) Zn
Fig. 13 Location of AZ61for EDS analyses: (a) 1C-W.M. HAZ, (b) 3C-W.M. HAZ, (c) 1C-hot cracking surface, (d) 3C-hot cracking surface [1C means one thermal cycle; 3C means three thermal cycles
melting point of liquid Mg-alloy. Hence, AZ61 s TCL (9.23 mm) after three thermal cycles was less than that after one thermal cycle (10.5 mm), as shown in Fig. 6(b). 4. Conclusions (1) The Al content of AZ31 and AZ61 can be used
to assess the hot-cracking susceptibility, because the more Al content, the more precipitation (Mg17 Al12 ) with low melting point was produced at grain boundaries. During the welding process, Mg17 Al12 melted at grain boundaries and caused liquefaction. At the same time, augmented strain pulled away the grain boundaries, and in turn formed the hot cracking.
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AZ61 was more prone to form hot cracking. In three thermal cycles AZ61 s Al content was lower than one thermal cycle according to the EDS results. This is attributable to the evaporation of Al related to the number of heat input, resulting in less Al in the liquid Mg-alloy (having a higher melting point). Thus, the region of grain liquefaction was narrower than in one thermal cycle. (2) Following one and three thermal cycles for AZ61 magnesium alloy, severe liquation hot cracking was produced in W.M. HAZ. EDS analysis revealed precipitation of alloy elements Al and Zn at grain boundaries. After one thermal cycle, precipitation of Al reached 13.3 wt%, representing a doubling of Al components compared to the base metal. This phenomenon might be the result of AZ61 having twice the Al content of AZ31, leading to wider liquefied grain boundaries region in PMZ. So AZ61 produced serious liquation hot cracking in W.M. HAZ following multiple thermal cycles. (3) Spot varestraint tests can be used to find serviceable material for repairing welding or multi welding on thick plates. Our findings suggest that AZ31 can be used for repairing welding (or thick plate welding), whereas AZ61 is not suitable. REFERENCES [1 ] H. Westengen: Light Metal Age, 2000, 58, 44. [2 ] B.L. Mordike and T. Ebert: Mater. Sci. Eng. A, 2000, 302, 37. [3 ] A. Weisheit, R. Galun, and B.L. Mordike: Weld. J., 1998, 77, 149. [4 ] C.C. Chang, C.P. Chou, S.N. Hsu, G.Y. Hsiung and
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