Bonding interfaces in friction-and explosive-welded aluminum and steel joints

Abstract

Interfaces of friction-and explosive-welded aluminium-mild steel joints, before and after annealed for 1h at temperatures between 350° and 500°C, were examined by tensile test for bonding strength, and by optical and electron micrographies, and EPMA for phases in intermediate reaction-layers. Results obtained are as follows:
(1) Bonding strength of friction-welded joint in the as-welded state is lower but that of explosive-welded joint is higher than the tensile strength of wrought aluminium, say 86 MPa.
(2) The bonding strength of friction-welded joint decreases to almost the same level, say 56 MPa, by annealing at temperatures between 350° and 500°C, while that of explosive-welded joint decreases with increase in annealing temperature and becomes lower than the fracture strength of annealed aluminum matrix after annealing at and above 450°C, which is also lower than that of friction-welded joint annealed at the same temperatures.
(3) The intermediate reaction-layer of friction-welded joint consists mainly of fine aluminum and Fe₄Al₁₃ grains at the aluminum side, and ion and columlar Fe₂Al₅ grains at the steel side in the layer. Meanwhile, the interface of explosive-welded joint consists of three kinds of interfacial zones; those are massive fused zone, thin fused zone, and non-layer zone.
(4) The fused zone consists of two parts; one, named as matrix, is composed with Al phase and network of fine Fe₂Al₅ particles, and the other is such as it looks as if it were a one-phase particle with lower Fe concentration than that of Fe₄Al₁₃ but is a mixture of the matrix and compounds, mainly Fe₄Al₁₃ and seldom Fe₂Al₅ in large particles.
(5) By annealing, the intermediate layer of friction-welded joint does not grow largely. In explosive-welded joint, however, the layer of Fe₂Al₅ grows by annealing at 450°C in the non-layer interface, and this lowers the bonding strength.