Artificial aging heat treatment post-effect on profile and mechanical properties of the AA 6061 friction welding joint

Abstract

Friction welding is a solution for metal joint problems that are difficult to joint with fusion welding. The quality results of friction welding are highly dependent on process parameters such as friction pressure, friction time, forging pressure, forging time, and rotating speed. There is often excessive softening, hardening of the metal grains, and formation of an intermetallic phase at the weld interface due to too high friction temperatures and too high forging pressures on the alloy. Several studies on AA 6061 concludes artificial aging heat treatment can provide positive changes on the tensile strength and microstructure of AA 6061. The purpose of this study is to find out the effect of post artificial aging treatment toward profile and mechanical properties of AA 6061 friction welding joint. The varied parameters are forging pressure in friction welding of 325 bar, 350 bar, 375 bar and artificial aging heating temperature post-welding of 100 °C, 150 °C and 200 °C. The fixed parameters observed are the connection profile, microstructure, hardness distribution and tensile strength of the joint. Controlled variables include: 1600 rpm spindle rotation, 65 bar or 65 kg cm−2 friction pressure, friction time tf = 6 seconds, time forging pressure tu = 60 seconds, workpiece contact diameter 15 mm and 15 degrees workpiece chamfer angle. The results are specimens with forging pressures of 325 bar, 350 bar and 375 bar without artificial aging treatment still show clearly interface of Zpl and Zpd zone; whereas in specimens treated with artificial aging, the Zpd and Zpl interface are getting faded which indicating that there is structures homogenization in the Zpd and Zpl zone. The hardness of the joint area increased significantly after artificial aging heat treatment because formed Mg2Si precipitates were dispersed in the Zpl, Zpd and Zud zones. The post-weld artificial aging heat treatment also resulted in a more even distribution of hardness in the joint area. The highest distribution of joint area hardness occurred in specimens with a forging pressure of 350 bar and artificial aging treatment at 200 °C. Artificial aging treatment with a temperature of 200 °C will form Mg2Si (black) precipitation and Fe3SiAl12 (gray) particles which are evenly dispersed in the Al matrix in the Zpd and Zpl zones so that their hardness is high. By applying artificial aging heat treatment, the tensile strength of the specimens increases until the artificial aging temperature is 150 °C; when the artificial aging temperature is raised to 200 °C, the tensile strength tends to drop back.