A Review of Post-Build Heat Treatments of Ti-6Al-4V PBF Material
By K Georgilas
Ti-6Al-4V (Ti64) is an alpha-beta alloy possessing a high strength-to-weight ratio and excellent corrosion resistance. Ti64 is both solution strengthened as well as precipitation strengthened. The mechanical properties largely depend on the crystal microstructure which can be manipulated through heat treatment. Due to the extensive use of Ti64 in industry, several ‘traditional’ heat treatments exist. The way L-PBF manufacturing process takes place, microstructure, and thus material properties, differs from ‘traditionally’ manufactured components. During the PBF process several defects, including porosity and lack of fusion, can arise. Similarly, microstructure in PBF material is anisotropic, differing between the vertical and horizontal directions, leading to varying mechanical properties in different loading directions. However, heat treatments currently in use by industry have originally been developed with a completely different starting point in terms of material microstructure e.g. cast material. Hence, their applicability and repeatability when performed on AM material is uncertain.
The objectives of this report are to describe the metallurgy of Ti64 and the necessity of post thermal treatment to achieve final properties. Review the current state-of-the-art in terms of microstructure and mechanical properties after treatment of the L-PBF Ti64 material. Identify gaps and opportunities in the post thermal treatment of L-PBF Ti64 material.
- · Heat treating L-PBF Ti-6Al-4V material is necessary to improve the mechanical properties of the material due to the extensive presence of martensite in the as-built condition.
- · Low temperature heat treatments (<900°C) indicate that martensite is decreased, but not completely dissolved.
- · Higher temperature treatment in the sub transus region (900-1000°C) shows that the presence of martensite in the microstructure is possible.
- Martensite has been shown to completely dissolve when processed above the β transus (>1000°C) with clear evidence of grain growth.