Friction welding of AZ31-SS316L for partially-degradable orthopaedic pins

Abstract

Joining dissimilar metals with friction welding is very challenging due to the differences in physical, mechanical and metallurgical properties of the two parent metals. A careful design of welding parameter is necessary especially for joining two metals with a large difference in melting temperature. In this study, friction welding has been viewed as an attractive process to join degradable metal with corrosion resistant alloy. AZ31 type magnesium alloy was friction welded at a low heat input to 316L type stainless steel to form a partially-degradable pin for temporary bone implants. Meanwhile, 6103 type aluminium alloy was also joined with 202 type stainless steel for comparison. A modified lathe machine at a spindle speed of 1600 RPM was used to perform the friction welding. The strength of the weld joints was evaluated by tensile test and their hardness distribution was measured using Vickers micro hardness tester. The results showed that the optimum parameter for the AZ31-316L joint was obtained at the friction time = 78s, forging pressure = 40kPa and the burn-off = 25mm. Meanwhile, for the 6103-202 joint the optimum parameters were: friction time = 50s, forging pressure = 9.3kPa and burn-off = 12mm. Fracture surface analysis showed a spiral defect on the 6103-202 joint, while the fracture surface of AZ31-316L showed an oxide layer formation that prevent a good interfacial bond. In conclusion welding parameters affect the strength of the joint as a result of elemental diffusion between the two metals. The oxide layer disrupts diffusion in the interface of AZ31 and 316L. Vacuum or inert condition during friction is proposed to avoid the oxide layer formation at the interface of AZ31-316L.