Microstructure and Low-Cycle Fatigue of a Friction-Stir-Welded 6061 Aluminum Alloy

Abstract

Strain-controlled low-cycle fatigue (LCF) tests and microstructural evaluation were performed on a friction-stir-welded 6061Al-T651 alloy with varying welding parameters. Friction stir welding (FSW) resulted in fine recrystallized grains with uniformly distributed dispersoids and dissolution of primary strengthening precipitates b¢¢ in the nugget zone (NZ). Two low-hardness zones (LHZs) appeared in the heat-affected zone (HAZ) adjacent to the border between the thermomechanically-affected zone (TMAZ) and HAZ, with the width decreasing with increasing welding speed. No obvious effect of the rotational rate on the LHZs was observed. Cyclic hardening of the friction-stir-welded joints was appreciably stronger than that of base metal (BM), and it also exhibited a two-stage character where cyclic hardening of the frictionstir-welded 6061Al-T651 alloy at higher strain amplitudes was initially stronger followed by an almost linear increase of cyclic stress amplitudes on the semilog scale. Fatigue life, cyclic yield strength, cyclic strain hardening exponent, and cyclic strength coefficient all increased with increasing welding speed, but were nearly independent of the rotational rate. Most friction-stirwelded joints failed along the LHZs and exhibited a shear fracture mode. Fatigue crack initiation was observed to occur from the specimen surface, and crack propagation was mainly characterized by the characteristic fatigue striations. Some distinctive tiremark patterns arising from the interaction between the hard dispersoids/inclusions and the relatively soft matrix in the LHZ under cyclic loading were observed to be present in-between the fatigue striations.

Microstructural evolution of cast Mg–Al–Zn during friction stir processing and subsequent aging

“…The grain sizes were obviously smaller than those estimated from optical micrographs [26]. This may be because the LAGBs could not be completely etched and therefore were not visible under optical microscopy.…”

“…More recently, a detailed comparative study indicated that single-pass FSP on AZ80 casting produced a heterogeneous microstructure with low strength and ductility; however, both two-pass FSP and single-pass FSP with a pre-solution treatment (referred to hereafter as Pre-ST FSP) resulted in significantly improved microstructural uniformity and mechanical properties [26]. These studies demonstrate that two-pass FSP is a highly effective approach to enhancing the mechanical properties of AZ magnesium alloy castings.…”

“…These studies demonstrate that two-pass FSP is a highly effective approach to enhancing the mechanical properties of AZ magnesium alloy castings. Furthermore, it was revealed that the two-pass FSP AZ80 exhibited more uniform microstructure and better mechanical properties than the Pre-ST FSP AZ80 [26]. Considering the fact that the two-pass FSP AZ80 sample had quite different microstructures prior to the final FSP pass, compared to the Pre-ST FSP sample [26], it is very likely that two FSP AZ80 samples would exhibit different deformation and recrystallization mechanisms.…”

“…Furthermore, it was revealed that the two-pass FSP AZ80 exhibited more uniform microstructure and better mechanical properties than the Pre-ST FSP AZ80 [26]. Considering the fact that the two-pass FSP AZ80 sample had quite different microstructures prior to the final FSP pass, compared to the Pre-ST FSP sample [26], it is very likely that two FSP AZ80 samples would exhibit different deformation and recrystallization mechanisms. Furthermore, only limited investigation of the precipitation behavior of FSP AZ magnesium alloys has been undertaken [26].…”

“…Considering the fact that the two-pass FSP AZ80 sample had quite different microstructures prior to the final FSP pass, compared to the Pre-ST FSP sample [26], it is very likely that two FSP AZ80 samples would exhibit different deformation and recrystallization mechanisms. Furthermore, only limited investigation of the precipitation behavior of FSP AZ magnesium alloys has been undertaken [26]. Therefore, it is worthwhile to conduct in-depth microstructural examinations on the AZ magnesium alloys after FSP and subsequent aging.…”