WATER DROPLET EROSION AND MICROSTRUCTURE OF LASER-NITRIDED TI-6AL-4V

The ability of laser nitriding to improve water droplet erosion resist ance of Ti-6Al-4V alloy was studied. Using a CO2 continuous laser, a l ayer of about 400 mu m thickness was nitrided and another layer of 400 -500 mu m was only heat affected. Electron microscopy observations sho wed that the microstructure of nitrided lavers consists essentially of TiN compounds which are embedded in Ti(alpha) matrix. Depending on th e nitrogen concentrations within the feeding gas, the titanium nitride s exhibited plate-like shape or dendritic morphologies. Below the nitr ided layer a thickness of 50-100 mu m of samples underwent martensitic structure which in its turn gives rise progressively to bimodal (alph a + beta) base material. Laser nitriding increased microhardness from 370-400 to 650-800 kgf mm(-2), and enhanced erosion resistance signifi cantly compared with untreated Ti-6Al-4V and hardened 12% Cr stainless steel. The mechanism of material removal by erosion was changed from work hardening and platelets detachment in untreated samples to brittl e fracture by formation of large Bakes and spalling in nitrided layers . Advanced stages of erosion are accompanied by the appearance of macr ocracks often in the nitrided zone, but some of them propagate even in to heat-affected area. The annealing at 650 and 700 degrees C of the l aser-nitrided samples results in precipitation of beta phase rich in v anadium.