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Acta Metall Sin  2018, Vol. 54 Issue (12): 1735-1744    DOI: 10.11900/0412.1961.2018.00151
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Research on Prediction Method of Liquation Cracking Susceptibility to Magnesium Alloy Welds
Shujun CHEN, Xuan WANG, Tao YUAN(), Xiaoxu LI
Engineering Research Center of Advanced Manufacturing Technology for Automotive Components, Ministry of Education, Beijing University of Technology, Beijing 100124, China
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Abstract  

Magnesium alloys has a wide application prospect due to their good properties, such as high specific strength and specific stiffness, but the susceptibility of liquation cracking is also pretty high. The liquation in partially melted zone of AZ-series magnesium alloys were investigated with circular-patch welding test. The AZ91, AZ31 base alloys were welded with AZ61 and AZ92 filler wires by using the cold metal transter metal inert-gas (CMT-MIG) welding. The results show that, the liquation occurred along the weld edge of AZ91 with the eutectic reaction occurring between γ (Mg17Al12) phase and Mg-rich phase. The liquation susceptibility of AZ31 was pretty low as γ (Mg17Al12) was not present in base metal of AZ31. Meanwhile, a new method for predicting liquation cracking based on binary phase diagram was proposed. When the initial solidification temperature of weld is higher and the solidification temperature range of weld is shorter than those of base metal, the liquation crack susceptibility of weld is mostly higher. When the initial solidification temperature of weld is close to or below that of base metal, and the solidification temperature range of weld is close to or longer than that of base metal, the liquation cracking susceptibility of weld is lower. This method worked well on predicting the effect of composition of base metal and filler wires on liquation cracking, and the predicting results are consistent with the experimental results. That is, the liquation cracking susceptibility is higher with AZ91 base metal used than that with AZ31 base metal. And, the liquation cracking susceptibility is lower with AZ92 filler wire than that with AZ61 filler wire.

Key words:  magnesium alloy      CMT-MIG welding      liquation cracking      susceptibility prediction      chemical composition     
Received:  19 April 2018     
ZTFLH:  TG401  
Fund: Supported by National Natural Science Foundation of China (No.51704013), Beijing Municipal Education Commission General Research Project (No.KM201810005016) and China Postdoctoral Science Foundation (No.2016M600881)

Cite this article: 

Shujun CHEN, Xuan WANG, Tao YUAN, Xiaoxu LI. Research on Prediction Method of Liquation Cracking Susceptibility to Magnesium Alloy Welds. Acta Metall Sin, 2018, 54(12): 1735-1744.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00151     OR     https://www.ams.org.cn/EN/Y2018/V54/I12/1735

Fig.1  Circular-patch welding experimental setup showing dimension of workpiece and patch (a) and vertical cross-section of the apparatus (b)
Material Alloy Al Zn Mn Mg
Workpiece/patch AZ31 3.0 1.0 0.6 Bal.
AZ91 9.0 0.7 0.2 Bal.
Filler wire AZ61 6.5 1.0 0.3 Bal.
AZ92 9.0 2.0 0.3 Bal.
Table 1  Nominal compositions of materials used for welding (mass fraction / %)
Fig.2  Workpiece (outer piece) and patch (inner piece) used for circular-patch welding from top view (a) and bottom view (b)
Weld # Workpiece Patch Filler wire Wire speed / (mmin-1) Current / A Travel speed / (mmin-1)
1 AZ31 AZ31 AZ61 7.0 89 0.36
2 AZ92 7.0 98 0.36
3 AZ91 AZ91 AZ61 7.0 90 0.36
4 AZ92 7.0 89 0.36
5 AZ91 AZ31 AZ61 7.0 87 0.36
6 AZ92 7.0 87 0.36
7 AZ31 AZ91 AZ61 7.0 98 0.36
8 AZ92 7.0 90 0.36
Table 2  Experimental conditions in circular-patch welding
Fig.3  Transverse microstructures and EDS of weld #3 including OM image (a), magnified image of zone I in Fig.3a (b), SEM image of zone II in Fig.3a (c), SEM image of zone III in Fig.3a of partially melted zone (d) and EDS of the points in Figs.3c and d (e)
Fig.4  Transverse microstructures and EDS of weld #1 including OM image (a), SEM image of zone I in Fig.4a of base metal (b), SEM image of zone II in Fig.4a of partially melted zone (c) and EDS of the points in Figs.4b and c (d)
Fig.5  Schematic showing formation mechanism of liquation cracking (S—solid, L—liquation)
Fig.6  A-B binary phase diagram (I—A-3B, II—A-6B, III—A-9B, TE—eutectic temperature)
Fig.7  Macrographs of circular-patch welds showing no cracking (a) and 47.8% cracking along the outer edge of the weld (b)
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