Effect of heat input on microstructure and properties of CP-Ti/mild steel lap joints produced by friction-stir welding

Friction stir lap welding was employed for joining a ۲mm-thick commercially pure titanium sheet on mild steel plate. The effect of interface temperature on microstructure and mechanical properties was investigated. The measured peak temperature of the joint interface during thermal cycle was found to be in the range of ۶۴۱-۱۱۱۰°C as a function of tool rotation speed (۲۸۰-۹۰۰rpm). Sound joints were achieved using tool rotation speed of ۵۰۰rpm (interface temperature of ۷۷۵°C). A maximum shear strength value of ~۲۷۴ MPa was resulted for the joint with an interface peak temperature of ~۷۷۵°C (i.e., ۹۵% of the strength of the weaker base material). The joint interface was reinforced by a composite-type microstructure of mild steel particles in titanium matrix. Further increase in tool rotation speed resulted in a decrease of shear strength value; for joints with interface peak temperatures of ۹۷۰ °C and more, the interface microstructure contained various reaction products (FeTi, Fe۲Ti, β-Ti and eutectic phases) due to remarkable diffusion of Ti and Fe across the interface. The decrease in shear strength was attributed to the formation of these brittle phases.

کلیدواژه ها:

Interfaces ، Friction stir welding ، Titanium ، Steel ، Lap joints