Microstructure and mechanical properties of friction stir lap welds between FeCoCrNiMn high entropy alloy and 6061 Al alloy

Abstract

To investigate the microstructure and mechanical properties of the dissimilar joint between high entropy alloys (HEAs) and Al alloy, FeCoCrNiMn was joined to 6061 Al alloy by friction stir lap welding (FSLW) at tool rotation speeds from 900 rpm to 1800 rpm. Systematic tilting high-resolution transmission electron microscopy patterns were applied to study the interfacial microstructure. A nanoscale (< 250 nm) interfacial layer was found at the 6061/FeCoCrNiMn interface and the phase formed in this layer was identified as Al13Fe4-type intermetallic compound (IMC) with the Fe-sites occupied by HEA elements. Microsegregation of HEA elements was observed in this Al13Fe4-type IMC in two different modes: 1) co-segregation of Cr and Mn and co-depletion of Co and Ni, for the majority of the interfacial layer, and 2) co-segregation of Co and Ni and co-depletion of Cr and Mn, mainly in regions adjacent to the Al alloy side and sporadic in the middle. The maximum tensile lap shear strength reached 151 MPa, which was approximately 73 % of the shear strength of 6061 base material. The joint fracture occurred within Al alloy nearby the interface rather than right along the interfacial layer, suggesting high interfacial metallurgical bonding achieved in this study.