In-situ nitrogen strengthening of selective laser melted Ti6Al4V with superior mechanical performance

Abstract

The in-situ strengthening approach has attracted increasing attention in the selective laser melting (SLM) of high-performance alloy. In the present work, the argon-nitrogen (Ar-N2) reactive atmosphere was introduced in the SLM of Ti6Al4V alloy to achieve in-situ strengthening. The microstructure evolution and mechanical performance were studied under various atmosphere conditions and laser processing parameters, respectively. The results show that the solid-solution of 871 ppm ~ 1460 ppm nitrogen was obtained in the titanium matrix through the SLM in the Ar-N2 atmosphere, whereas no trace of nitride was found. Compared with the pure Ar atmosphere condition, an increasing ratio of nano-scale β phase and coarser martensites could be found after the addition of 5 vol% N2 in the Ar atmosphere, and the twinning phenomenon becomes more conspicuous as well. Under the Ar-N2 reactive atmosphere, the as-built Ti6Al4V alloy exhibits a significantly high yield strength of 1336 MPa and ultimate tensile strength of 1418 MPa, which are notably higher than previously reported SLM and conventionally manufactured Ti6Al4V alloys. Meanwhile, the fracture elongation of ~6% was obtained while attaining the high tensile strength. Analysis exhibits that the solid solution of the nitrogen atom is the main reason for improving strength in the nitrogen-strengthened Ti6Al4V samples. The strength of Ti6Al4V alloy was improved without excessive sacrifice of ductility, which was attributed to the multiple effects of the nitrogen solid-solution and the intrinsic heat treatment. This work adopts a practical methodology with designed low nitrogen content in the SLM process to achieve an in-situ strengthening of Ti6Al4V alloy without the formation of brittle nitrides.