Dissimilar friction stir lap welding of AA7075 to AZ31B in the presence of Sn interlayer

Abstract

The present study aims to investigate the effect of Sn interlayer on the quality, microstructure and mechanical properties of 7075 T6-AZ31H24 dissimilar joints fabricated by friction stir lap welding. In this regard, various joints were fabricated by changing the interlayer thickness and the number of FSW passes. The microstructure of the joints was studied by optical, scanning and transmission electron microscopes linked to EDS and EBSD detectors. Shear-tensile test was utilized for evaluation of the mechanical properties of the joints. The results showed that the use of 40 μm-thick Sn interlayer was very effective on the improvement of joint quality and joint strength. In this case, the tunneling defect was eliminated and the lap shear fracture load improved by 10% with respect to conventional friction stir lap welding. This was attributed to the facilitated displacement and more proper mixing of dissimilar materials in the nugget due to the lubricating effect of molten tin. Although increase of interlayer thickness showed positive effects on dissimilar material mixing, width of nugget and thickness of mixed region, however, joint strength decreased due to the formation of Mg2Sn intermetallic compound at the out part of the mixed region. Increasing the FSW passes from one to three led to spreading of Mg-Al mixed regions throughout the nugget, thereby the joint strength was deteriorated significantly by formation of various brittle intermetallic compounds (Mg17Al12, Mg2Al3 and Mg2Sn) within the nugget.