Department of Mechanical and Aerospace EngineeringNEWS & EVENTS

A Mechanistic Study on the Effects of Ultrasonic Vibrations to the Microstructure and Mechanical Properties of the Selective Laser Melting Process
Mr George Alexandru TILITA
Department of Mechanical and Aerospace Engineering, HKUST
Date  :  22 Jan 2018 (Mon)
Time  :  3:00 p.m.
Venue  :  Room 2571B, HKUST (2/F., Lift #27/28)


Selective laser melting (SLM) is a layer by layer powder bed additive manufacturing process that can be used to create high complexity parts. Current state of technology limits accessibility as parts printed are subject to anisotropy, high residual stresses, low ductility, and low fatigue limits; limitations which introduce the need for thermal post-processing such as High Isostatic Pressing (HIP) which increase the costs and adoption of SLM. Stainless steel 304L is a material which can be used in a wide range of applications including medical, aerospace, and automotive industries due to its high corrosion resistance. These same industries are also the first users of SLM and research into the combined use of the two can provide benefits for both. Ultrasonic excitation is a procedure which generates vibrations at frequencies of 20 kHz and above and are used in multiple manufacturing techniques to increase productivity or surface finishing. Ultrasonic excitation in casting has shown to decrease grain size due to the effect of the oscillating pressure waves on the nucleation conditions. While ultrasonic excitation has been used the solidifying of metals made through casting, it has not been used before in the SLM process. 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 defects present in Selective laser melting.

This work begins by designing and constructing a selective laser melting device which uses a custom Laser Cutting Machine for the laser system and optics which is then used to 3D print samples that would allow characterization of mechanical properties and microstructure of samples with and without ultrasonic excitation. Single layers were printed on a substrate of the same material to observe the effect on microstructure. Using SEM microscopy it was observed that microstructure follows a more uniform distribution of grain shapes, reducing the presence of elongated grains by up to 54% which can lead to reduction in anisotropy 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 nucleation part of the model is detailed in the current thesis while the grain growth part is detailed in Wenhao Chen’s MPhil Thesis. The combined model shows very good agreement with experimental samples and thus simulation Robust Design of Experiments was conducted to determine the effects that vibrational amplitude, frequency, thermal gradient, and cooling rate 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.

To further determine the effects introduced by ultrasonic excitation, and also to determine the effects that laser parameters have on mechanical properties, multiple Robust Design of Experiments were conducted analyzing hatch spacing, laser power, scanning speed, substrate temperature, and ultrasonic vibration amplitude. The multilayer samples were separated by direction of printing and then subjected to nano-indentation in order to extract the hardness values, Young’s modulus, and creep behavior. 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. In some directions, Hardness was also increased by about 5%.

The overall results suggest that ultrasonic excitation could be a solution for SLM in the context of reducing anisotropy, improving mechanical properties, and potentially reducing or even eliminating the need for thermal post-processing. Even so, further research is needed in order to assess the extent to which these improvements can be taken. For that a new SLM device will need to be constructed to include more control over the thermal conditions.

Keywords: Selective Laser Melting; Ultrasonic Excitation; Cavitational bubbles; Anisotropy; Hardness; Young’s Modulus; Volumetric Energy Density.

(Supervisors: Prof. Matthew Yuen, Prof. Yongsheng Gao and Prof. Robin Ma)