Studies of friction stir welds

Friction stir welding (FSW) is a solid state welding process in which a spinning tool is forced along the joint line, heating the abutting components by friction, and producing a weld joint by strong plastic mixing (stirring) of material from the two components. Compared to classical fusion welding techniques, FSW can overcome problems such as weld porosity and cracking in the heat affected zones. As opposed to dissimilar material combinations, the joining of similar materials (mainly Al alloys) by FSW is becoming widely implemented in industrial production. In the framework of the EU funded JOIN-DMC project (JOINing Dissimilar Materials and Composites), FSW of dissimilar materials were characterised, applying for the first time a novel non-destructive strain and phase mapping technique, developed at ID15B. This technique provides depth-resolved information about intensity and position of all the Bragg reflections emanating from the sample within the angular acceptance given by the experimental setup. Bulk investigations of several mm thick Al plates are realised by using high energy x-rays. A sketch of the experimental setup was shown in Figure 14. This is first technique allowing the mapping of a weld zone in a non-destructive manner with depth resolution, material composition and strain distribution. The results presented relate to a FSW between AA2024-T3 and AA6082-T6 (single phase Al alloys). The strain tensor was derived from the diffraction-cone distortions, recorded at two different angular sample positions. The differences in the diffraction spectra of both materials were used as an indicator for the material mixing in the stirring zone. Part of the results is shown in Figure 38. For a better comprehension the welding tool is shown schematically (tool pin marked by thick dashed line). A good mixing of the two materials is observed, resulting in a smoother residual stress profile in the stirring zone as compared to results (not shown here) from a weld with reversed material combination.

R. Martins and V. Honkimäki - Depth resolved strain and phase mapping of dissimilar friction stirwelds using high energy synchrotron radiation - Textures and Microstructures 35, no 3/4, 145 –152 (2003).


 

a) Depth resolved residual macro-strains in longitudinal direction (parallel to weld); b) through thickness longitudinal residual stress component, calculated from the strain measurements in three dimensions; c) enlarged view of the distribution of the plate material (AA6082 and AA2024) in the stirring zone.