Microstructure and Strain Hardening of a Friction Stir Welded High-Strength Al–Zn–Mg Alloy

doi:10.1007/s40195-014-0109-4

Abstract

Abstract:

Microstructural evolution and strain hardening behavior of a friction stir welded (FSWed) high-strength 7075Al-T651 alloy were evaluated. The nugget zone was observed to consist of fine and equiaxed recrystallized grains with a low dislocation density and free of original precipitates, but containing uniformly distributed dispersoids. The strength, joint efficiency, and ductility of the FSWed joints increased with increasing welding speed. A joint efficiency of ~91% was achieved at a welding speed of 400 mm/min and rotational rate of 800 r/min, while the ductility remained basically the same as that of the base metal. There was no obvious strain rate sensitivity observed in both base metal and welded joints. While both the base metal and FSWed joints exhibited stage III and IV hardening characteristics, the hardening capacity, strain hardening exponent, and strain hardening rate all increased after friction stir welding.

Key words: Aluminum alloy, Friction stir welding, Mechanical properties, Strain hardening

A. H. Feng, D. L. Chen, Z. Y. Ma, W. Y. Ma, R. J. Song. Microstructure and Strain Hardening of a Friction Stir Welded High-Strength Al–Zn–Mg Alloy[J]. Acta Metallurgica Sinica (English Letters), 2014, 27(4): 723-729.

Figures/Tables 5

Fig. 1 TEM images of FSWed 7075Al-T651 joints: a, c 800 r/min and 400 mm/min, b, d 800 r/min and 100 mm/min

Fig. 2 a YS and UTS, b ductility, c strain hardening exponents versus. welding speed, d strain hardening rate (θ) vs. net flow stress (σ - σy) for the BM and FSWed joints tested at a strain rate of 1 × 10-3 s-1

Table 1 Summary of joint efficiency and hardening capacity for the 7075Al-T651 alloy tested at a strain rate of 1 × 10-3 s-1

Material	Rotational rate ω (r/min)	Welding speed V (mm/min)	Joint efficiency (%)	Hardening capacity H_c
7075Al-T651 BM	–	–	–	0.14
FSWed 7075Al-T651 joint	800	100	77.1	0.49
	800	100	77.8	0.49
	800	200	85.6	0.52
	800	200	86.1	0.48
	800	400	90.5	0.50
	800	400	91.2	0.50

Fig. 3 Effect of strain rate on the tensile properties of FSWed 7075Al-T651 joints made with 800 r/min and tested at different strain rates: a YS, b UTS, c ductility

Fig. 4 A typical FSWed 7075Al-T651 joint made with 800 r/min–400 mm/min and tested at a strain rate of 1 × 10-3 s-1: a a macroscopic image showing the failure location (AS: advancing side; RS: retreating side; and Top: welding plane), b SEM micrograph at a lower magnification, c SEM micrograph at a higher magnification

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