Microstructure and Mechanical Properties of Pulse MIG Welded 6061/A356 Aluminum Alloy Dissimilar Butt Joints

doi:10.1016/j.jmst.2016.11.004

Abstract

Abstract:

The microstructure and mechanical properties of pulse metal inert-gas (MIG) welded dissimilar joints between 4 mm thick wrought 6061-T6 and cast A356-T6 aluminum alloy plates were investigated. The tensile strength of the joints reached 235 MPa, which is 83% of that of 6061 aluminum alloy, and then decreased with the increase of travel speed while keeping other welding parameters constant. The microstructure, composition and fractography of joints were examined by the optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Grain boundary liquation and segregation occurred in the partially melted zone (PMZ) on 6061 aluminum alloy side, and brittle Fe-rich phases were observed in partially melted zone on A356 aluminum alloy side. The minimum microhardness appeared in heat-affected zone (HAZ) near A356 aluminum alloy substrate. The samples during tensile test failed mainly in PMZ and HAZ on A356 aluminum alloy side through mixed fracture mode with quasi cleavage and dimples on fracture surface.

Key words: Dissimilar aluminum alloy, Butt joint, Pulse MIG, Mechanical property, Microstructure

Fuheng Nie, Honggang Dong, Su Chen, Peng Li, Leyou Wang, Zhouxing Zhao, Xintao Li, Hai Zhang. Microstructure and Mechanical Properties of Pulse MIG Welded 6061/A356 Aluminum Alloy Dissimilar Butt Joints[J]. J. Mater. Sci. Technol., 2018, 34(3): 551-560.

Figures/Tables 16

Table 1 Nominal composition of base metals and filler wire (wt%)

Material	Si	Mg	Fe	Mn	Cu	Cr	Zn	Ti	V	Al
6061	0.58	0.94	0.16	0.03	0.23	0.07	0.01	0.02	—	Bal.
A356	7.98	0.62	0.10	0.006	—	0.0006	0.02	0.07	0.008	Bal.
ER4043	5.0	0.05	0.80	0.05	0.30	—	0.10	0.20	—	Bal.

Table 2 Mechanical properties of the base metals

Material	Tensile strength (MPa)	Yield strength (MPa)	Hardness (HV)
6061-T6	283	274	106
A356-T6	323	263	108

Fig. 1. Microstructure of (a) 6061-T6 aluminum alloy and (b) cast A356-T6 aluminum alloy.

Fig. 2. Photos of butt welded joints between wrought 6061 and cast A356 aluminum alloys produced at different travel speeds.

Fig. 3. Microstructures of joint #1: (a) fusion zone; (b) PMZ on cast A356 aluminum alloy side; (c) the magnified boundary section through PMZ to HAZ on cast A356 aluminum alloy side; (d) liquation on 6061 aluminum alloy side.

Fig. 4. GB liquation bands in PMZ on 6061 aluminum alloy side of (a, b) sample #1 and (c) sample #4.

Fig. 5. The half of cross sectional views of the samples #1 to #4 produced at travel speed: (a) 8, (b) 10, (c) 12, and (d) 16 mm/s.

Fig. 6. (a) Backscattered electron and (b-f) secondary electron images of the microstructure of the PMZ on cast A356 aluminum alloy side of (a-d) joint #2 and (e, f) joint #3.

Table 3 The major compositions and possible phases at different positions

Position	Composition (at.%)				Possible phase
Position	Al	Si	Mg	Fe	Possible phase
I	78.29	21.44	0.22	0.05	Al—Si eutectic
II	86.34	9.57	1.79	2.30	β-Al₅FeSi
III	61.68	22.77	12.04	3.51	π-Al₉FeMg₃Si₅

Fig. 7. Backscattered electron images and the distributions of alloying elements around the regions (a-d) between PMZ and FZ and (e-j) between HAZ and PMZ of sample #1 on A356 aluminum alloy side.

Fig. 8. Backscattered electron images and distribution of alloying elements (a-f) of the region between PMZ and FZ and (g-l) the region within PMZ of sample #1 on 6061 aluminum alloy side.

Fig. 9. Fracture positions of joints #1 to #4.

Fig. 10. Average tensile strength of BMs and samples #1 to #4 produced at travel speed of 8, 10, 12 and 16 mm/s, respectively.

Fig. 11. Transverse microhardness profiles of samples #1 to #4 produced at travel speed of 8, 10, 12 and 16 mm/s, respectively.

Fig. 12. SEM images of fracture surfaces on 6061 aluminum side of (a, b) sample #1 and fracture surfaces on A356 aluminum side of (c, d) sample #2, (e, f) sample #3 and (g, h) sample #4.

Fig. 13. Schematic diagrams illustrate the effect of welding heat input on the width of PMZ and height of weld reinforcement of the joint.

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