Gaseous surface hardening of Ti-6Al-4V fabricated by selective laser melting

The present work investigates the response of different gaseous thermochemical treatments on selective laser melted (SLM) Ti-6Al-4V. The resulting microstructures after thermochemical treatment were investigated with X-ray diffraction, light optical microscopy, scanning electron microscopy and Vickers-microhardness indentation. Nitriding, performed at 1000–1050 °C resulted in a diffusion zone of nitrogen in solid solution and surface compound layers consisting of TiN (and Ti₂N at 1000 °C). Below the compound layer Al-enrichment of the α-zone was observed. Carbo-oxidising in a CO atmosphere at 1000–1050 °C resulted in deep diffusion zones and thick compound layers of the ternary compound TiC1-xOx. Both surface hardness and layer depth were found to increase with temperature and treatment time. Chemically controlled carbo-oxidising, applying the gas redox system CO-CO₂, was performed at temperatures in the range 850–1050 °C, resulting in carbo-oxides and formation of oxides with increasing Ti:O ratio with increasing temperatures (rutile and Magnéli phases). Nitriding followed by (carbo-)oxidising treatment resulted in higher surface hardness owing to the formation of mixed interstitial compounds TiC_1-x-yN_xO_y in the compound layer. The compound layer grew into the Al-rich zone as elongated structures.

The improvement of wear by nitriding, carbo-oxidising and duplex nitriding/(carbo-)oxidising on SLM Ti-6Al-4V was evaluated by dry sliding wear testing. Lowering of the wear volume by up to a factor of 450 compared to an annealed reference sample was realised. Carbo-oxidising in CO at 1000 °C offered the best wear resistance and resulted in a lowering of the friction coefficient, averaging μ = 0.22, compared to μ = 0.45 for an annealed reference sample.