Heterogeneity of crystallographic texture in friction stir welds of aluminum

Abstract

Over the past decade, friction stir welding (FSW) has rapidly become an important industrial joining process, particularly in the aluminum industry. Included among the advantages of FSW are such important attributes as improved weld strength and the elimination of cracking and porosity. During the friction stir process, the metal undergoes a tortuous deformation path that is not yet fully understood. The crystallographic texture that evolves during FSW contains sharp spatial gradients that undoubtedly influence the integrity of the weld and surrounding region in subsequent performance. The locally measured textures are discussed in the context of the material flow required to produce such textures, ultimately resulting in an estimate of the flow field present during FSW.

A microtexture investigation of recrystallization during friction stir processing of as-cast NiAl bronze

“…[19,20] At all SZ locations, the discrete pole figure data show only random orientations in the a phase,i nc ontrast to observations of distinct shear textures in the SZ in FSP/W of aluminum alloys. [24][25][26]31] The observation here of random textures within the SZ is indicativeo fr ecrystallization involving the formation of randomly oriented grain nuclei followed by the long-range migration of high-angle boundaries during the thermomechanical cycle of FSP.The distributions of grain-to-grain misorientation angles for all locations showadistinct randomcomponent, which is similar to that predicted by MacKenzie for randomly oriented cubes, [32] but with additional peaks at 0t o5deg and at 60 deg. Examination of these data shows that the population of the low-angle boundaries (0 to 5d eg) is highest at locations nearest the plate surface in contact with the tool shoulder.These low-angle boundaries may reflectdeformation of newly recrystallized grains by the tool shoulder after passage of the tool pin.…”

“…It has been noted that many previous studies of FSW/P of Al alloys haves hown that fine grains form but distinct shear textures persist throughout weld nuggets or SZs. [24][25][26]31] Such microstructuresand textures haveb een ascribedt oc ontinuous dynamic recrystallization involving the formation of newg rains by dislocation reaction in the deformation microstructure in the absence of the long-range migration of high-angle boundaries. [36] Such processes typically involver etained deformation textures; randomS Z a phase textures suggest that alternativer ecrystallizationm echanisms occur during FSP of these NABm aterials.…”

“…In contrast, Al alloys exhibit fine, equiaxed grains accompanied by deformation-induced shear textures that become evident in the TMAZa nd persist to the centers of the weld nuggets following FSW. [24][25][26]31] The presence of shear texture orientations has been cited as evidence that microstructures evolveb yr ecovery-dominated dynamic continuous recrystallization in FSW/P of aluminum alloys in contrastt ot he PSN reaction during FSP of thisN AB material.…”

“…Such inclination of the {111} slip planes in shear textures has been reportedt oa ppearf rom locations under the shoulder to points at the midplane in the weld zone. [25] Fonda et al [26] havea lso correlated observed lattice orientations in FSW of AA2195, an Al-Cu-Li alloy,w ith shear texture components having {111} aligned with the pin surface. They also reported that these shear textures were present throughoutt he weld zone, and theyc oncluded that refined grains had formed by processes of grain subdivision and the recovery-dominated evolution of high-angle boundaries by dislocation accumulation.…”

Recrystallization mechanisms during friction stir welding/processing of aluminum alloys

“…However, most of the in-depth studies have involved, again, approximately uniform strains and isothermal conditions during SPD. Friction stir welding (FSW) and friction stir processing (FSP) technologies involve SPD but under conditions that include rapid transients and steep gradients in strain, strain rate and temperature [8,9]. Microstructure transformations and restoration processes in the presence of such transients and gradients have only recently begun to be considered [10][11][12].…”