Public Thesis defense - IMMC

Microstructure-property development in Zr-modified 7075 aluminium alloy processed by laser powder bed fusion by Nicolas Nothomb

Laser Powder Bed Fusion (L-PBF) is the most widespread additive manufacturing method for aluminium alloys. There is a high demand for complex L-PBF parts in the aeronautic sector. Such parts could decrease aircraft weight and allow fuel savings and reduction of the environmental impact of air transportation. However, the aeronautic sector has high standards for fatigue performance and it is a recognised weak point of L-PBF parts. Thus, the aim of this thesis is to improve the fatigue performance of a high strength Al7075 processed by L-PBF.

Firstly, Al7075 has to be processed without excessive residual defects and without hot cracking, a major problem for this alloy. Zirconium was added to Al7075 and, by using an unconventional pre-sintering strategy, a 99.9% dense Al7075+1.8%Zr alloy without hot cracking was successfully processed by L-PBF.

Secondly, EBSD and TEM analyses allowed to understand the effect of a tailored heat treatment (HT) on the strength of Al7075+1.8%Zr. A 1h425◦C followed by 24h120◦C heat treatment allowed to reach a yield strength of 515 MPa. This high strength is mainly due to secondary precipitation of nanometric L12 Al3Zr precipitates.

Thirdly, the effect of building direction (BD) and HT on the tensile and fatigue crack growth (FCG) properties was analysed. Residual lack of fusion (LoF) defects decreased ductility for vertical samples. The HT decreased the strain localisation highly present for the as-built state due to an homogenisation of the microstructure and healing of pores lower than 2 µm. HT also improved the FCG resistance. The lowest ductility of vertical samples as well as faster fatigue crack propagation perpendicular to BD were attributed to a preferential damage of the fine grain zone.

Finally, limited total life fatigue performance of L-PBF Al7075+1.8%Zr was attributed to residual LoF defects. Friction stir processing was applied on L-PBF samples to totally suppress the defects larger than 2 µm. As a result, a fatigue strength of 350 MPa was obtained, competing with the highest values for L-PBF Al alloys described in literature.

Jurys members:

Prof. Aude Simar (UCLouvain, Belgium), supervisor

Prof. Renaud Ronsse (UCLouvain, Belgium), chairperson

Prof. Pascal Jacques (UCLouvain, Belgium), secretary

Prof. Moataz Attallah (University of Birmingham, UK)

Prof. Jean-Yves Buffière (INSA Lyon, France)

Prof. María Teresa Pérez-Prado (IMDEA Materials Institute, Spain)

Pay attention:

The public defense of Nicolas Nothomb scheduled for Thursday November 14 at 04:15p.m. will also take place in the form of a video conference.

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