Two-dimensional Mapping of Residual Strains in 6061-t6 Aluminum Alloy Friction Stir Welds

The residual strain profiles were measured through the thickness of friction-stir welded (FSW) plates using neutron diffraction to study the relationship between the angular distortion and the residual strain distribution. Three different weld specimens were prepared from a 6061-T6 aluminum alloy with the purpose of separating the effects of the frictional heat and plastic deformation on the residual strain distribution and the angular distortion in the weld plate: (Case 1) a plate processed with both stirring pin and tool shoulder, i.e., a regular FSW subjected to both plastic deformation and frictional heat, (Case 2) a plate processed only with the tool shoulder, i.e., subjected mainly to the frictional heating, and (Case 3) a plate processed only with the pin, i.e., subjected mainly to the plastic deformation. Case 1 showed little bending of the weld plate about longitudinal (welding) direction, Case 2 exhibited a concave bending, and the Case 3 exhibited a convex bending, suggesting that different residual strain profiles exist through the thickness of the plates. Three principal strain components were measured across the weld line at the face, center, and root of the cross section of the welds. Case 1 showed little variations in the residual strain profiles though the thickness while Case 2 showed significant variations. Unfortunately, results from Case 3 were questionable due to the presence of a groove on the surface of the plate and, hence, it may not truly represent the “pin-only” case. The comparison between Case 1 and Case 2 suggests that an optimal combination of the pin action (plastic deformation and heat transfer through the thickness of the plate) and the shoulder action (heating) could minimize (or provide intentional manipulation of) the through-thickness variation of residual strains and angular distortion in the FSW plates. INTRODUCTION Friction-stir welding (FSW) is a solid-state joining process, developed by The Welding Institute (TWI), UK, which can produce enhanced mechanical properties in welded products compared to the traditional welding techniques [1]. The FSW uses a threaded pin and pressing shoulder to apply severe plastic deformation and frictional heating, respectively, to the base metal to produce a strong metallurgical bond [2]. FSW has many advantages over the traditional fusion welding processes, which include nearly defect-free welds with minimized cracking, fine grain structures, Copyright ©JCPDS International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48. 104