Effects of Welding Speed and Post-weld Hot Rolling on Microstructure and Mechanical Properties of Friction Stir-Welded AZ31 Magnesium Alloy

doi:10.1007/s40195-018-0725-5

Abstract

Abstract:

The as-rolled AZ31 Mg alloy sheets are welded by friction stir welding (FSW) in this study, followed by annealing and hot rolling with different reductions. The effects of welding speed and the rolling reduction on the microstructure, tensile properties, microhardness and fracture surfaces of the specimens are investigated. The results show that the microstructures of the FSW joint in different regions become more and more consistent as the hot rolling reduction increases. The mechanical properties of the FSW joints and the joint coefficient are improved with increasing rolling reduction; however, the hardness value in the stirred zone decreases firstly and then increases. When the rolling reduction is 50%, the tensile strength of FSW joint is close to that of base metal. The sample L₃ achieves the best comprehensive properties, with the yield strength, ultimate tensile strength and elongation of 162, 287.9 MPa, and 17.9%, respectively.

Key words: Magnesium alloy, FSW, Post-weld hot rolling, Microstructure, Mechanical properties

Gang Liu, Li-Na Ma, Zhen-Duo Ma, Xue-Song Fu, Guo-Bing Wei, Yan Yang, Tian-Cai Xu, Wei-Dong Xie, Xiao-Dong Peng. Effects of Welding Speed and Post-weld Hot Rolling on Microstructure and Mechanical Properties of Friction Stir-Welded AZ31 Magnesium Alloy[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(8): 853-864.

Figures/Tables 16

Fig. 1 Schematic illustration of the subsequent rolling process

Table 1 Processing parameters of samples (PWHR parameters at 350 °C)

Sample L	Sample H	Rolling pass	Final reduction in thickness (%)	Final thickness (mm)
L₀	H₀	-	-	7.0
L₁	H₁	1	14	6.0
L₂	H₂	2	32	4.8
L₃	H₃	3	50	3.5

Fig. 2 Schematic illustrations of the tensile specimen

Fig. 3 Cross-sectional macrograph images of a FSW-L0, b FSW-H0

Fig. 4 Optical micrographs of different regions in sample L0: a BM, b TMAZ, c HAZ, d top layer of the SZ, e middle layer of the SZ, f bottom layer of the SZ

Fig. 5 Optical micrographs of different regions in sample H0: a BM, b advancing side of the HAZ, c retreating side of the HAZ, d advancing side of the TMAZ, e retreating side of the TMAZ, f SZ

Table 2 Grain sizes of the different stir zones

Sample	Grain size of top layer (μm)	Grain size of middle layer (μm)	Grain size of bottom layer (μm)	Average grain size (μm)
L₀ (W = 880 rpm, V = 60 mm/min)	6.73	6.08	5.72	6.18
H₀ (W = 880 rpm, V = 80 mm/min)	5.99	5.4	5.2	5.53

Fig. 6 Optical micrographs of the PWHRed sample L0 with different rolling reductions: a 14%, b32%, c 50%

Fig. 7 Optical micrographs of the PWHRed sample H0 with different rolling reductions: a 14%, b 32%, c 50%

Fig. 8 Relationship between the average grain size and rolling reduction: a sample L, b sample H

Fig. 9 SEM images at SZ of a 14%, b 32%, c 50% PWHRed AZ31 sheets

Fig. 10 Stress-strain curves of the tensile test: a sample L, b sample H

Table 3 Tensile properties of the FSW joints and corresponding base metals (φ = UTS of FSW joints/UTS of BM)

Weld joint	YS (MPa)	UTS (MPa)	EL (%)	BM	YS (MPa)	UTS (MPa)	EL (%)	φ(%)
L₀	87	190.8	7.4	L₀-BM	129	217.5	15.3	87.7
L₁	122	265.4	16.1	L₁-BM	156	288.8	31.8	91.9
L₂	153	278.3	17.2	L₂-BM	162	300.1	34.5	92.8
L₃	162	287.9	17.9	L₃-BM	177	305.1	32.8	94.4
H₀	91	179.0	7.3	H₀-BM	129	217.5	15.3	82.3
H₁	109	246.5	13.8	H₁-BM	159	292.1	30.9	84.4
H₂	152	276.2	17.1	H₂-BM	165	305.0	33.6	90.5
H₃	158	284.7	18.7	H₃-BM	180	308.1	32.5	92.4

Fig. 11 Pole figures near SZ-side region of the sample L1

Fig. 12 Hardness profiles along the mid-thickness of joints welded in different conditions: asample H, b sample L

Fig. 13 SEM images of fracture surfaces of sample H: a H0, b H1, c H2, d H3

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