Analysis of brittle layer forming mechanism in Ti6Al4V sloping structures by SLM technology

Recently, selective laser melting (SLM) technology has been applied and promoted into manufacturing industry. However, the geometrical accuracy still hardly meets the requirements. In this paper, we focus on the special surface texture of sloping structures produced by SLM with YLR-200 laser using Ti6Al4V. The four inclined angles of the structures are 30°, 45°, 60°, and vertical 90° to the substrate. The forming mechanism of special surface texture of brittle layer is discussed based on of high intensity laser forming fundamentals. The results showed that the brittle layer was produced in the underside surface of sloping structure, due to the rapid change in thermal properties, the small absorptivity, low thermal conductivity, causing more balling and un-melt powder adherent on the downside surface. The brittle layer thickness increased with the angle decreasing, due to Marangoni Effect and different thermal conductions around outside of the melt pool in the SLM forming processing. Ultimately, it is indicated that we need readjustment of the forming parameters according to analytical or numerical calculations and eventually adding post treatment to reduce the brittle layer, improve the mechanical properties and increase the surface as well as geometrical accuracy of complex structures by SLM.