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Microstructure Models for Open Architecture Additive Manufacturing

Project Code: 32867
Start date and planned duration: February 2019, 36 months


  • Develop and validate manufacturing process simulations for laser metal deposition (LMD) and wire arc additive manufacturing (WAAM).
  • Manufacture a range of test pieces and samples of increasing geometric complexity using LMD and WAAM.
  • Undertake detailed characterisation of the as-built microstructures.
  • Develop, implement and validate microstructure evolution models for Inconel 718 and 316L stainless steel.
  • Demonstrate the scalability of the model implementation for other alloys not considered in the project using a parent-child paradigm for the phase evolutions.

Project Outline

Two important alloys for LMD and WAAM are 316L stainless steel and Inconel 718. For both IN718 and 316L, the main challenge is that existing metallurgical modelling capabilities are not well-proven for the combination of rapid thermal transients and long durations at elevated temperatures that arise during AM processing. For IN718, these processing conditions often result in the formation of deleterious delta and Laves phase either during initial solidification or when thermal exposure occurs above certain temperatures for long durations. For 316L, long duration at elevated temperatures can cause sensitisation and sigma phase formation. This impacts the corrosion resistance of the as-built part and can lead to poor mechanical properties. As a consequence, expensive heat treatments may be required to make as-built LMD and WAAM IN718 and 316L parts fit for purpose.

The formation of these microstructures is challenging to predict without a validated manufacturing process simulation. Such a manufacturing process simulation has been recently developed for selective laser melting (SLM) at TWI through a CRP project and it has been translated to LMD and WAAM with some success; however, it needs refinement to better match the actual deposition characteristics of these two processes, which are significantly different from SLM. Moreover, once a microstructure evolution model has been coupled with the manufacturing process simulation, process conditions can be varied in the model to understand how changes to processing conditions can prevent the formation of these phases. Thus, a validated process simulation and metallurgical evolution modelling workflow can help optimise the microstructure of the as-built condition by understanding how changing heat input, interpass times, deposition rates, or externally applied heating/cooling can influence the evolution of the microstructure. As a consequence, less trial-and-error experimentation is required to subsequently optimise heat treatments, as the microstructural content will be known.

Therefore, the concept underpinning this project is the development and validation of a robust manufacturing process simulation and microstructure evolution model for LMD and WAAM of 316L and IN718. This involves detailed numerical modelling, experimental work on LMD and WAAM, and extensive quantitative metallography to tie all of the modelling and experimental work together. Whilst focusing on only two alloys, a generic implementation that is transferable to other alloy systems will be used and demonstrated at the end of the project.

Industry Sectors

Benefits to Industry

The project will provide industry with access to state-of-the-art modelling and model validation capabilities. This will enable technology transfer, more rapid qualification of parts, and use of advanced expertise during product and process design. The knowledge gained through a fundamental metallurgical investigation of different materials and different processes will help inform material and process selection. It is anticipated that the models developed will have spill-over benefits for other manufacturing process simulations such as linear friction welding, arc welding, laser welding and electron beam welding.


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