Low Hardness Zone Research Articles

Change of microstructure and cyclic deformation behavior along the thickness in a friction-stir-welded aluminum alloy

The microstructure and cyclic deformation behavior of three slices of a thick plate of friction-stir-welded Al–Cu alloy were investigated. Friction stir welding resulted in a composite-like nugget zone consisting of fine particles uniformly dispersed in the recrystallized fine-grained matrix. The top slice had larger grains, smaller particles, higher hardness, stronger cyclic hardening, higher hysteresis energy and lower fatigue life than the bottom slice. Fatigue cracks initiated from near-surface particles in the low-hardness zone, and propagated via striation formation mechanisms along with particle cracking.

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

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 β″ 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 friction-stir-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-stir-welded 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.

Microstructure and Cyclic Deformation Behavior of a Friction-Stir-Welded 7075 Al Alloy

Microstructural changes and cyclic deformation characteristics of friction-stir-welded 7075 Al alloy were evaluated. Friction stir welding (FSW) resulted in significant grain refinement and dissolution of η′ (Mg(Zn,Al,Cu)2) precipitates in the nugget zone (NZ), but Mg3Cr2Al18 dispersoids remained nearly unchanged. In the thermomechanically affected zone (TMAZ), a high density of dislocations was observed and some dislocations were pinned, exhibiting a characteristic Orowan mechanism of dislocation bowing. Two low-hardness zones (LHZs) between the TMAZ and the heat-affected zone (HAZ) were observed, with the width decreasing with increasing welding speed. Cyclic hardening and fatigue life increased with increasing welding speed from 100 to 400 mm/min, but were only weakly dependent on the rotational rate between 800 and 1200 rpm. The cyclic hardening of the friction-stir-welded joints exhibiting a two-stage character was significantly stronger than that of the base metal (BM) and the energy dissipated per cycle decreased with decreasing strain amplitude and increasing number of cycles. Fatigue failure occurred in the LHZs at a lower welding speed and in the NZ at a higher welding speed. Fatigue cracks initiated from the specimen surface or near-surface defects in the friction-stir-welded joints, and the initiation site exhibited characteristic intergranular cracking. Crack propagation was characterized by typical fatigue striations along with secondary cracks.

Influence of Tool Dimension and Welding Parameters on Microstructure and Mechanical Properties of Friction-Stir-Welded 6061-T651 Aluminum Alloy

Six-millimeter-thick 6061Al-T651 plates were friction stir welded with varied welding parameters and tool dimensions. The low hardness zones (LHZs), determined by constructing the hardness distribution maps around the heat-affected zones, changed the location and inclination with the welding conditions, but the hardness values of the LHZs were mainly dependent on the welding speed. The thermal cycles experienced by the LHZs had approximately the same peak temperature of 360 °C to 370 °C with varied durations that were governed by the welding speed. The microstructure of the LHZs was characterized by a low density of β′ precipitates, which tended to reduce with increasing the welding speed. The fracture of the welds occurred along the LHZs, and the tensile strength of the welds increased with increasing the welding speed and was independent of the tool dimension and the rotation rate. A heat source zone-isothermal dissolution layer model was proposed to explain the observed effects.