Effects of rotation tool-induced heat and material flow behaviour on friction stir lapped Al/steel joint formation and resultant microstructure

Abstract

Reliable joining of Al/steel via friction stir lap welding (FSLW) is strongly dependent on interfacial macro-/micro-structures. However, how the underlying thermo-mechanical process affects them and how they can be controlled remain unclear. In this study, a 3D coupled Eulerian–Lagrangian finite element model was integrated with a tracer particle technique to simulate the FSLW of Al alloy 5052 and high-strength DP590 steel, to elucidate the role of tool-induced heat and material flow. The results showed that the material flow recirculated near the shoulder and pin, which mainly originated from the advancing side of the Al and the retreating side of the steel. Increasing the rotational velocity intensified the overall material flow, resulting in increased migration on the steel side. Insufficient steel migration around the pin at lower rotational velocities was mainly responsible for the micro-voids and non-bonding defects at the lap interface. The asynchronous convergence of Al and steel flowing from the advancing and retreating sides, respectively, refilled the instantaneous gap, thereby normally bonding the lapped interface behind the pin. The rotational pin affected the hook structure and steel fragment formation through the shearing and squeezing of the steel migration flow. Material inter-migration produced an intercalated structure at the lapped Al/steel interface. The extremely high temperature, approximately above Al 5052 solidus temperature, under the pin bottom caused the formation of a thicker intermetallic compound (IMC), thereby considerably reducing the interfacial strength. Multi-scale mechanical strength assessments indicated that an intercalated interfacial structure with a thickness of Al-rich IMC layer less than ∼1.0 μm was desirable because it exhibited a higher local interfacial strength than the Al matrix.