Friction Stir Lap Welding: material flow, joint structure and strength

Friction stir welding has been studied intensively in recent years due to its importance in industrial applications. The majority of these studies have been based on butt joint configuration and friction stir lap welding (FSLW) has received considerably less attention. Joining with lap joint configuration is also widely used in automotive and aerospace industries and thus FSLW has increasingly been the focus of FS research effort recently. number of thermomechancal and metallurgical aspects of FSLW have been studied in our laboratory. In this paper, features of hooking formed during FSLW of Al-to-Al and Mg-to-Mg will first be quantified. Not only the size measured in the vertical direction but hook continuity and hooking direction have been found highly FS condition dependent. These features will be explained taking into account the effects of the two material flows which are speed dependent and alloy deformation behaviour dependent. Strength values of the welds will be presented and how strength is affected by hook features and by alloy dependent local deformation behaviours will be explained. In the last part of the paper, experimental results of FSLW of Al-to-steel will be presented to briefly explain how joint interface microstructures affect the fracturing process during mechanical testing and thus the strength. From the results, tool positioning as a mean for achieving maximum weld strength can be suggested.