Experimental investigation of Ti-6Al-4V microstructure and mechanical properties after laser powder bed fusion (LPBF) manufacturing and post-processing

Abstract

Metals and alloys that can maintain desirable mechanical properties in a high-temperature environment are essential in a wide variety of applications. Laser powder bed fusion (LPBF) is an additive manufacturing method that uses a laser to melt and consolidate powder materials layer by layer to build up 3D parts, making complex geometrical components possible in a minimally wasteful manner. Numerous industries are interested in utilising this manufacturing technique as a viable option to manufacture some of their components because of its potential to produce highly complex parts with minimal waste and excellent mechanical properties making it an attractive option for a wide range of applications, the aerospace and biomedical industries having a particular interest in using Ti-6Al-4V produced using LPBF. Despite this, Ti-6Al-4V produced using LPBF often shows porosity, large residual stresses, rough surfaces, and martensitic microstructures, leading to poor fatigue properties. This thesis aims to better understand the structure and properties of parts manufactured using the LPBF process so that the results of this study can be useful for various applications. The research aimed to examine fatigue behaviour, surface topography, and the locations of fatigue cracks on parts, as well as to study the microstructure and mechanical properties of those parts, and particularly the effects of heat and cold exposure. Furthermore, the research aimed to investigate the effect of post-processing in achieving different surface and microstructural conditions. To carry out this work, a test plan was designed to use 67 samples for testing to investigate the fatigue behaviour, surface conditions, mechanical properties and microstructure characterisation under different treatment conditions: stress relief, hot isostatic pressing, β+α solution treatment, and β solution treatment, followed by three different cooling rates, as well as tempering. The study found that the stress-relieving process at 704 °C did not significantly affect the microstructure, while HIP processing reduced porosity by 40% and hardness by 14%. Hardness was generally lower for material treated below the β-transus compared to above. The lowest hardness values were observed after applying hot isostatic pressing, followed by heating at 930 °C and furnace cooling. Martensite was partly transformed after tempering at 800 °C for 30 min while wholly transformed to α + β after one hour. Although building orientation affected fatigue behavior due to columnar grains, surface roughness played a more significant role in fatigue life. Finally, changing the crack mode affected the shape and size of the dimples.

Date of Award	Jul 2023
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Wei Sha (Supervisor), Savko Malinov (Supervisor) & Gasser Abdelal (Supervisor)