Colloquia

Summary

Aluminium alloys are used extensively in modern industries due to beneficial properties. However, these properties can create challenges for the use with AM. Among others, the thermal conductivity, and reflectivity together with the oxidation of the alloys can lead to defects like cracks, pores, and unmelted zones. Thus, there is a need to develop AM processes and to understand and control the process and material behaviours.

The goal of this research is to develop a new experimentally validated computational platform to realize the performance of a new AM process called Molten Metal Deposition (MMD). MMD is a liquid metal printing method used with aluminium alloys where a wire feedstock is fed in an extruder and deposited on a heated substrate. The controlled and external heat source (up to 900oC) and heated substrate (up to 600oC) allow for the control of the thermal behaviour like peak temperatures, cooling rates, and solidification rates. The controlled heat input avoids common difficulties in Metal AM processes like hot and cold cracking, metal vaporization and heat accumulation. In addition to the goal, a fundamental understanding is necessary to address the following knowledge gap – to what extent do the MMD process parameters (e.g., nozzle temperature, substrate temperature and printing speed) dictate the thermal history and performance of fabricated aluminium parts?  First, a new finite element model is developed based on hot element addition in combination with a volumetric heat source. Second, experimental characterisations are conducted to validate the numerical model and leverage the found guiding principles for the optimisation of the MMD process. The experimental study is carried out based on Design of Experiments to obtain the effect of the input parameters on characteristics like surface waviness, porosity, deposition efficiency, and mechanical properties.

The thermal model is validated with experimentally measured temperatures to observe a root mean square error of 10 – 30.5oC. Based on the results, ensuring a substrate temperature below 500oC with high nozzle temperatures (850-900oC) and printing speeds (160 – 180 mm/min) will lead to the best print quality. With the lower substrate temperature, the guideline favours faster solidification and uses printing speed and nozzle temperature to obtain high temperatures for layer fusion. The result is a fundamental understanding of how the input parameters affect the measured characteristics and thermal response in Molten Metal Deposition.