Additive manufacturing through the selective laser melting of metal powders allows for the printing of large aircraft parts, with complex geometries that cannot be constructed using conventional processes. Due to the violent nature of the laser melting process, porosity and stress-distortion can occur in a printed part, rendering it unsuitable for use in aircraft manufacture. Numerical simulations can allow us to understand the properties of the finished part, and hence can be used to develop better techniques.
Sponsor
This project is funded by Boeing Research & Technology. 
The challenge
Selective laser melt of titanium powders is a complex process which is well-suited for multi-physics modelling
Additive manufacturing is a complex multiphysics problem, involving a phase change of a powder (and some of the substrate beneath the powder) to liquid directly beneath the laser. This then subsequently cools, forming a new `layer’ of manufactured material. The controlled conditions under which this process must occur makes dynamic observation difficult, thus numerical simulations can allow greater understanding of the melting/solidification process.
The research
Understanding the processes behind selective laser melting of a powder requires the resolution of the formation of an individual layer of material. The thermal effects (propagation of the melt pool) are dominant in determining the material properties of the final part, hence we have initially focussed on these. However, fluid flow and surface tension can have affect the final distribution of the melted region, thus we will also consider these effects in the future.
Due to the geometry of the printed parts, additive manufacturing requires three-dimensional simulations, therefore for efficiency, we have developed a thermal model within a commercial multiphysics package. This allows us to track the dynamic evolution of the thermal profile within a material substrate (with temperature-dependent properties) as a laser source is passed over the top of the domain.
Results
The material properties of a metal (and powder) have a significant effect on the heat distribution from a laser heat source. By including the fully thermally dependent properties (including latent heat contributions), we are able to correctly capture the behaviour of a laser melting a titanium alloy substrate, validated using experimental results from the Institute for Manufacturing at the University of Cambridge. This enables us to investigate the effects of different experimental parameters on the melt behaviour of the substrate.
Publications
- Multiphysics simulation of selective laser melt of titanium powders., Millmore S. and Nikiforakis N. 2019 (in preparation)