Effect of Heat Input on Microstructure and Mechanical Properties of Joints Made by Bypass-Current MIG Welding-Brazing of Magnesium Alloy to Galvanized Steel

doi:10.1007/s40195-014-0118-3

Abstract

Abstract:

Experiments were carried out with bypass-current MIG welding-brazing of magnesium alloy to galvanized steel to investigate the effect of heat input on the microstructure and mechanical properties of lap joints. Experimental results indicated that the joint efficiency tended to increase at first and then to reduce with the increase of heat input. The joint efficiency reached its maximum of about 70% when the heat input was 155J/mm. The metallurgical bonding between magnesium alloy and steel was a thin continuous reaction layer, and the intermetallic compound layer consisted of Mg-Zn and slight Fe-Al phases. It is concluded that bypass-current MIG welding-brazing is a stable welding process, which can be used to achieve defect-free joining of magnesium alloy to steel with good weld appearances.

Key words: Bypass-current MIG welding-brazing, Dissimilar metals, Joint characteristics, Interface analysis

Miao Yugang, Wu Bintao, Xu Xiangfang, Han Duanfeng. Effect of Heat Input on Microstructure and Mechanical Properties of Joints Made by Bypass-Current MIG Welding-Brazing of Magnesium Alloy to Galvanized Steel[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(6): 1038-1045.

Figures/Tables 12

Table 1 Chemical compositions of AZ31B magnesium alloy and Q235 steel (in wt%)

Material	Si	Fe	Mn	Al	Zn	C	Cr	Ni	Mg
Q235	0.22	Bal.	0.48	0.45	-	0.12-0.2	0.18	-	-
AZ31B	0.1	0.005	0.20-0.50	2.50-3.50	0.50-1.50	-	-	0.005	Bal.

Fig. 1 Schematic diagrams of the BC-MIG process: a block diagram of BC-MIG process; b welding schematic

Table 2 Parameters used for different welding experiments

Experiment	Welding voltage (V)	Welding speed (cm/min)	Heat input (J/mm)
A	18	70	117
B	18	53	155
C	19	53	163
D	19	65	133

Fig. 2 Weld appearances and cross sections of lap joints at different welding voltages (U) and welding speeds (v): a, b U = 18 V, v = 70 cm/min; c, d U = 18 V, v = 53 cm/min; e, f U = 19 V, v = 53 cm/min; g, h U = 19 V, v = 65 cm/min

Fig. 3 Droplet transfer in BC-MIG process: a formation of droplet; b repelled transfer; c droplet falling

Fig. 4 Interface macrographs of lap joints at different welding voltages (U) and welding speeds (v): a U = 18 V, v = 70 cm/min; b U = 18 V, v = 53 cm/min; c U = 19 V, v = 53 cm/min; d U = 19 V, v = 65 cm/min

Fig. 5 EDS analysis results of the joint interface: a SEM image of joint interface; b Mg and Fe distributions along the scanning line; c Zn, Al, and Mn distributions along the scanning line; d EDS result of the denoted point

Fig. 6 Shear strength of the joints obtained by different experiments

Table 3 Calculated welding parameters for different welding experiments

Experiment	Welding current (A)	Welding voltage (V)	Welding speed (cm/min)	Heat input (J/mm)	Weld area (mm²)	Maximum tension Fm (N)	Tensile strength (MPa)	Joint efficiency (%)
A	76	18	70	117	24.25	2,120	87.42	45.63
B	76	18	53	155	21.50	2,860	133.02	70.01
C	76	19	53	163	24.00	2,740	114.17	59.59
D	76	19	65	133	24.00	2,440	101.67	53.06

Fig. 7 Effect of heat input on the joint efficiency

Fig. 8 SEM images of the fracture surface a and the magnified image b of the marked region in a

Fig. 9 Micro-hardness profile along the middle section of the lap joint

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