Microstructure, hardness and cavitation erosion behaviour of Ti–6Al–4V laser nitrided under different gas atmospheres

Abstract

The present paper reports on a comparative study of the microstructure, hardness and cavitation erosion behaviour of Ti–6Al–4V laser nitrided under different gas atmospheres with nitrogen concentrations of 0–25%. By varying the nitrogen content of the gas atmosphere, the structure and hence the hardness and cavitation erosion behaviour of the laser nitrided layers can be controlled in a wide range. With increasing nitrogen content of the gas atmosphere, the structure within the laser nitrided layers characteristically changed from martensitic α′-Ti to a fine mixture of α- and β-Ti grains, finally leading to the formation of different titanium nitrides. The study revealed that the increase of hardness and cavitation erosion resistance can be mainly attributed to solid solution hardening of the nitrogen. It is shown that optimum cavitation erosion resistance is only achieved if crack free layers containing no brittle titanium nitrides are produced.