Alexandre MARGUERET – Strategies for the development of locally controlled metallic materials through additive manufacturing

Jury

Dr. Marc THOMAS, ONERA DMAS - Rapporteur
Prof. Eric HUG, Université de Caen Normandie - Rapporteur
Prof. Muriel VERON, Université Grenoble Alpes - Examiner
Dr. Partice PEYRE, ENSAM Paris - Examiner
Dr. Pierre LHUISSIER, Université Grenoble Alpes - Superviser
Dr. Guihlem MARTIN, Université Grenoble Alpes - Co-superviser

Abstract

Powder bed fusion processes (additive manufacturing) are known for their ability to produce parts with complex geometries and have reached a certain level of maturity. Significant efforts have been made to control manufacturing defects and their impact on mechanical properties, optimize fabrication strategies, and select appropriate geometries. Recently, open powder bed fusion additive manufacturing machines have been developed, providing the opportunity to control all beam parameters during fabrication. This freedom to locally control parameters can be exploited to control porosity and/or microstructure within parts produced by these processes.

The objective of this thesis is to study strategies for locally controlling porosity and microstructure during manufacturing. To achieve this, an "open-source" electron beam powder bed fusion machine (allowing full control over all electron beam parameters) was used (this is the only machine of its kind in France). The selected materials for this study are model materials: Ti-6Al-4V alloy (one of the most emblematic materials in additive manufacturing) and pure copper (a pure metal that presents unique challenges when processed through additive manufacturing). Local control of porosity in Ti-6Al-4V samples is achieved by varying the scanning speed to locally reduce energy input. Meanwhile, local control of pure copper's microstructure relies on fabrication strategies that take advantage of in-situ  recrystallization.
For both porosity control in Ti-6Al-4V and microstructure control in pure copper, a similar approach was adopted. First, the impact of modifying processing parameters on porosity or equiaxed grain fraction is analyzed. Then, the ability to generate uniform zones (steady-state regimes), transition zones (transient regimes), and gradual variations in porosity or equiaxed grain fraction is demonstrated. Porosity gradients or variations in the proportion of recrystallized grains were achieved along the build direction as well as within the construction plane.