Influence of screen height and bias voltage on the active screen plasma nitriding of shot-peened Ti-6Al-4V titanium alloy

The effects of screen height and bias voltage on the active screen plasma nitriding (ASPN) of Ti-6Al-4V titanium alloy were investigated in this paper. Before nitriding, the Ti-6Al-4V alloy was pretreated by shot peening to allow the ASPN performed at a low temperature of 500 degrees C. The ASPN process variables concerned in this paper included screen height (20 and 100 mm) and bias voltage (0, 400 and 1100 V). The results show that the increase in active screen height effectively increased the yield of nitride particles transferred to the specimen surface by expanding the screen sputtering area and promoted the thickening of the nitrided layer. When the bias voltage increased from 0 V to 1100 V, the nitrided layer thickness and surface hardness of the nitrided Ti-6Al-4V showed a tendency of increasing first and then decreasing. Among the three selected bias voltages (0 V, 400 V, 1100 V), the thickest and hardest nitrided layer was obtained at 400 V, which balanced better between the deposition of nitride particles and the sputtering thinning of the nitrided layer. Meanwhile, the thickness and hardness of the nitrided layer on the shot-peened Ti-6Al-4V specimens were greatly improved compared to the original specimen. The thickest (5.30 mu m) and hardest (1672 HV) nitrided layer was obtained on the shot-peened specimen at ASPN 500 degrees C-20 h with a bias voltage of 400 V and a screen height of 100 mm.