A mechanistic study on the effects of ultrasonic vibrations to the microstructure and mechanical properties of the selective laser melting process

Abstract

Selective laser melting (SLM) is a layer by layer powder bed additive manufacturing process that is used to create high complexity parts. The process suffers from anisotropy, high residual stresses, and low ductility which creates the need for thermal post-processing that increases the costs and adoption of SLM. Ultrasonic excitation in casting has shown to decrease grain size due to the effect of the oscillating pressure waves on the nucleation conditions. The primary objective of this thesis is to build an SLM device which applies ultrasonic excitation to the workspace, and characterize the effects of the vibrations on the limitations present in Selective laser melting. Single layers of 100 μm were printed on a substrate of the same material to observe the effect of ultrasonic excitation on microstructure. Using SEM, it was observed that microstructure follows a more uniform distribution of grain shapes, reducing the presence of elongated grains by up to 54% and approximately 20% in overall grain size. A nucleation and grain growth model was also created to predict the resulting microstructure of the SLM process which also takes into account the changes in chemical potential introduced by ultrasonic excitation. The theoretical model shows very good agreement to experimental samples and a DOE was conducted to determine the effects that input parameters have on the shape, size, and type of grain formed in different conditions. These simulations also allow for the prediction of the optimum parameters which lead to the best mechanical properties. Sample DOEs were conducted to identify the optimum input parameters. The samples showed a significant decrease in the anisotropy of both hardness and young’s modulus, as well as an overall increase in the young’s modulus. Overall, ultrasonic samples showed a 5% increase in both hardness and young’s modulus.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology