Enhanced strength and sliding wear properties of gas nitrided Ti-6Al-4V alloy by ultrasonic shot peening pretreatment

A novel ultrasonic shot peening (USP) was performed on Ti-6Al-4V alloy as a pretreatment prior to gas nitriding (GN). The samples treated by USP/GN composite processes were systematically investigated in terms of microstructure, strength and sliding wear properties. The results indicated that a surface nitriding layer composed of TiN, Ti2N and alpha-Ti(N) phases with a thickness of 20-30 mu m was formed on the samples after USP/ GN composite process treatment. The USP pretreatment could significantly improve the microhardness and thickness of nitriding layer, and the enhancement is positively correlated with the intensity of USP Almen. Compared with conventional GN without pretreatment, the maximum microhardness (similar to 810HV(0.025)) and thickness of nitriding layer were increased by 30.43 % and 26.92 % by USP/GN composite treatment with Almen intensity of 0.356 mmA, respectively. Meanwhile, the wear resistance was significantly enhanced, and the width of abrasive mark was reduced by 51 %, compared with that without pretreatment. This work offers a potential approach to improve the nitriding process of Titanium alloys.