| dc.description.abstract | The influence of input parameters on the coalescence of bubbles during Directed Energy De- position of metals is studied. Additionally, the in-situ infrared images captured during the process were dynamically calibrated using the surface temperature at the boundary of the melt-pool and a temperature scaling factor.
Porosity is a key obstacle in the qualification and certification of metal additive manufactured parts. Previous researchers have quantified the effect of porosity on part performance for parts cre- ated using directed energy deposition, formation mechanisms of porosity and the effect of process parameters on final part porosity. However, due to the complex system of heat and mass flow in the melt pool, there is limited research that discusses the morphological evolution of the bubbles inside the melt pool. This work uses the in-situ X-ray and IR images taken by Dr. Wolff and her col- leagues at the Argonne National Laboratory and analyses the morphological changes that bubbles undergo before the solidification front makes them into final part pores. Coalescence of bubbles is the most profound change that leads to the formation of large pores. Coalescence occurs both leading the laser keyhole and trailing it. It is found that the final part porosity originated mostly in the wake of the keyhole. Additionally, the partially fused powders from the first track, increase the magnitude of the bubbles formed during the second track, which results in more instances of coa- lescence. Porosity due to instability at tip of the keyhole does not create dense clusters of bubbles required for bubble coalescence.
In-situ thermal imaging of a complex and stochastic process such as DED is indispensable, however, using IR cameras for this purpose can be challenging as the emissivity of the substrate is not constant. One of the observations made in this research was a difference in the magnitude and direction of the change in emissivity between the first track and the second track. The emissivity values were higher for the second track and further investigation into this shall be done in the future. | en |
