Effect of sintering atmosphere and carbon content on the densification and microstructure of laser-sintered M2 high-speed steel powder

In the present work, the role of graphite addition and sintering atmosphere on the laser sintering of M2 high-speed steel (HSS) powder was studied. The test specimens were produced using a continuous wave CO2 laser beam at power of 200 W and at varying scan rate ranges from 50 to 175 mm s−1 under nitrogen and argon atmospheres. It was found that laser scan rate and sintering atmosphere strongly influence the densification of M2 HSS powder. Opposite to the conventional sintering that the sinterability of HSS is improved by graphite addition, the result of this work demonstrated that graphite addition decreases the sintered density of M2 HSS at relatively low laser scan rates, i.e. <100 mm s−1, whilst at higher scan rates the effect is not very conspicuous. Furthermore, sintering under argon atmosphere yielded better densification compared to nitrogen atmosphere, particularly at higher scan rates. The microstructure of laser-sintered parts consisted of large and elongated pores and a heterogeneous metal matrix. The matrix structure includes martensite, high carbon austenite and inter-cellular or inter-dendrite carbide network. Addition of graphite to the M2 HSS powder increases the heterogeneity of the microstructure, as it was noticed from the variation in the micro-hardness throughout the sintered specimens.