Laser Powder Bed Fusion (L-PBF) 3D printing of Titanium-based alloys: Effects of process parameters and post-fabrication treatments

Abstract

Laser Powder Bed Fusion (LPBF), also known as Selective Laser Melting (SLM), as one of additive manufacturing techniques for fabrication of metallic parts, is being actively developed to manufacture intricate metallic parts for a range of applications. This has resulted in generating great research interests in understanding the principles of the LPBF fabrication and the effects of the process parameters on the as-printed surface topography and microstructure. This is an important step in characterising the capabilities of the manufacturing route due to the correlation between the surface topography-microstructure and mechanical properties of printed metallic parts. Moreover, study of some post treatments and their effects on the microstructure and mechanical properties are as important. Therefore, the main aim of this research project is to study the characteristics of LPBF fabricated titanium based parts and explore the effects of manufacturing parameters and post-process treatments on the microstructure and associated mechanical properties. Amongst the metallic alloy powders used as the starting material during LPBF process, the fabrication of Ti-6Al-4V (also known as Ti64) components has drawn considerable attention in diverse industrial fields such as aerospace and biomedical. Therefore, the above-mentioned alloy, as one of the most widely researched titanium based alloy, was selected as the material of interest in this project. In the initial stage of this research, an attempt was made to establish an empirical method for optimisation of process parameters of LPBF by exploring a correlation between the morphology of top surface and the residual porosity of the parts. By categorising the surface morphology into two groups of surfaces with meso roughness and micro roughness, it is proved that top surface morphology is a most reliable measure for prediction of internal porosity of the Ti64 parts. Moreover, in the optimisation process, a range for the process parameter with their threshold is provided and discussed which can be found in Chapter 3. Then a fundamental study was conducted to highlight the differences between the powder metallurgy as a traditional near net shape manufacturing route with the layer-wise LPBF technique. It is found that sintered Ti64 parts, have lamellar microstructure of α and β while the microstructure of LPBF fabricated parts has single phase of α′ as a result of diffusionless transformation from parent β which had an epitaxial columnar growth. Moreover, it is discovered that the higher strength of LPBF fabricated parts compared to sintered parts is owing to only morphology and refinement of α′ because the nano-hardness of bulk α′ compared to α is nearly the same. The discussions about these findings are presented in Chapter 4. For better understanding of the effects of build orientations and surface conditions on mechanical properties of LPBF fabricated parts, a novel design overcame the challenge of fabrication of straight horizontal samples without any post treatment. It is revealed that the vertically deposited samples suffered from premature failure in their truly as-built conditions. The reason behind this problem and the most viable post treatment method is discussed in two journal papers which are represented in Chapters 5 and 6. The outcome of this research study is five peer-reviewed journal articles and one international conference paper. The thesis is based on four main journal papers presented as four chapters (Chapter 3 to 6) to highlight the main findings and contributions of this study to the field. While all original published papers (as parts of Chapter 4, 5 and 6) are presented in the Appendix A, the other article which is as a part of this study but not presented in the chapters, along with conference paper are in the Appendix B.