Multiscale Analysis of the Strength and Ductility of AA 6056 Aluminum Friction Stir Welds

Abstract

The number of parameters affecting the friction stir welding process, the subsequent forming operations, and the structural integrity is very large: chemical composition of the two welded materials, welding parameters and thermal history, initial microstructure, flow properties of each alloy, etc. A multiscale analysis based on macro-and micromechanical tests has been conducted in order to determine and quantify the phenomena controlling the mechanical properties of joints made by welding AA 6056 Al alloys in a T4 or T78 state and to construct a predictive model for plasticity and fracture. Small tensile test samples were machined inside the various zones of the welds and parallel to the welding direction to identify the local plastic and fracture properties. Macrotensile tests using samples machined transverse to the welding direction and strain maps obtained by digital image correlation (DIC) provided information about the overall strength, plastic strain localization, and fracture of the joint. Three-dimensional (3-D) finite element (FE) analysis of the deformation of the welded samples loaded transverse to the welding line based on J 2 flow plasticity theory and on the parameters identified on the small test samples was used to quantify the effects of the geometrical, microstructural, and mechanical factors affecting the plastic flow localization process and the evolution of the constraint in the weak zone, which controls the damage rate. Uniform plastic flow is controlled not only by the yield strength mismatch between the weak zone and its surrounding but also by the strain hardening mismatch, both related to the precipitation of the Q phase. The ductility was addressed using a micromechanics-based damage model. A key ingredient of the model was to account for both large primary voids nucleated early on intermetallic particles and small secondary voids nucleated on dispersoı¨ds, which have a first-order effect on the fracture of the AA 6056 Al alloy. The model is shown to capture very well the drop of the overall ductility in the welded joints.

Mechanical properties of Al–Mg–Si alloy sheets produced using asymmetric cryorolling and ageing treatment

“…Maisonnette et al [40] also found a similar size of intermetallics in Al-Mg-Si alloys. Although these intermetallics do not contribute to the hardening of the alloy during the elongation of the material, the particles can be broken into smaller parts during the plastic deformation process [41]. Either because of an intrinsic brittleness or because of a lack of coherence with the matrix, these particles will result in a damage initiation owing to the nucleation of small internal voids, which develop into cracks when exposed to stresses [38,39].…”

Influence of Post Weld Heat Treatment on Strength of Three Aluminum Alloys Used in Light Poles

“…These intermetallic particles result from the presence and availability of residual elements, such as iron and silicon [18,19]. As documented elsewhere, these particles were identified to be Al 12 Fe 3 Si, Al 15 (FeMn) 3 Si and Al 5 FeSi [18][19][20]. The iron-rich intermetallic particles range in size from 1 to 10 microns and are potential sites for the early initiation of microscopic damage during plastic deformation.…”

“…The as-received alloy revealed a random distribution of both coarse and intermediate size intermetallic particles (Figure 3a,b). These intermetallic particles result from the presence and availability of residual elements, such as iron and silicon [18,19]. As documented elsewhere, these particles were identified to be Al 12 Fe 3 Si, Al 15 (FeMn) 3 Si and Al 5 FeSi [18][19][20].…”

Structure‐property investigations on a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications Part I: Local gradients in microstructure, hardness and strength

“…From the literature [31,32,35,[38][39][40] and assuming that below the liquidus the overall precipitation process is similar in friction stir welding and laser beam welding, a schematics of the correlation between hardness variations and precipitates ("Sato et al…”

“…All these factors make the precipitation sequence complex. In AA6056 the major hardening constituents [31,32,35,38] are GP zones in T4 and " ( '/Q') in T6.From the literature [31,32,35,[38][39][40] and assuming that below the liquidus the overall precipitation process is similar in friction stir welding and laser beam welding, a schematics of the correlation between hardness variations and precipitates ("Sato et alDiagram" [41]) is attempted in Fig. 9 for the Al-side.…”