Part Temperature Effects in Powder Bed Fusion Additive Manufacturing of Ti-6Al-4V
To ensure the widespread adoption of metal Additive Manufacturing (AM) processes, a complete understanding of the interactions between process variables is necessary. The process variables of beam power, beam velocity, deposition geometry, and beam diameter have been shown in prior works to have major effects on resultant melt pool and solidification characteristics, but this list is incomplete. Without accounting for part temperatures prior to deposition, unintended outcomes may result. In the current work, Ti-6Al-4V is studied in the Powder Bed Fusion (PBF) processes to gain an in-depth understanding of how part temperature interactions with other process variables affect physical properties of the process such as melt pool size and variability, part distortion, porosity, and microstructural characteristics. This research is performed through a combination of finite element modelling, single melt track experiments, full part production, and in-situ monitoring in order to gain a full understanding of the underlying relationships between part temperature and part outcomes. In the Arcam Electron Beam Melting (EBM®) process, this knowledge is used to generate a feedback control strategy to constrain prior beta grain width to remain constant while part surface temperatures are allowed to vary. In the Laser Powder Bed Fusion (LPBF) process, deposition is investigated at elevated substrate temperatures and several findings show that unintended part temperature increases can lead to undesirable consequences while prescribed part temperature changes can increase the available processing window and allow for more uniform deposition. This work also shows that both global temperature changes due to substrate heating and local temperature changes due to the choice of scan strategy can be combined into one metric: the temperature encountered by the melt pool during deposition. A combination of destructive and non-destructive characterization methods are utilized to understand and measure the changes to the melt pool and microstructural development that are seen during deposition. The feasibility of using a commercial high speed camera as a tool for thermography is characterized and the ability to discern cooling rates and thermal gradients within and surrounding the melt pool provide validation for trends in melt pool properties generated from simulations. This work provides a greater understanding of the role of part temperature during deposition and presents methodologies to account for the changes to the melt pool and resultant part due to both prescribed and unintended temperature changes during deposition.