Compréhension et optimisation des post-traitements de pièces en Scalmalloy® produites par fabrication additive

Details

Supervisors

Simar, Aude

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux, à finalité spécialisée

Abstract

enAdditive manufacturing is a technology with many advantages such as the complexity and diversity of the geometries that can be obtained, a density close to 100 % or the reduction of the weight of the structures produced compared to more traditional processes such as casting. Aluminum alloys are good candidates for additive manufacturing because they provide good static mechanical properties, corrosion resistance and good fatigue behavior. Among them, Scalmalloy®, an Al-Mg-Sc-Zr alloy patented by APWorks, is being studied more and more. Indeed, this alloy offers very interesting properties, especially after a heat treatment, called HT0, at 320°C for 4 hours allowing to create Al3(Sc,Zr) nanoprecipitates, responsible for the important strength of this alloy (σ_y = 480 MPa). However, like many alloys produced by additive manufacturing, the presence of porosities results in lower fatigue strength. To compensate for this effect, an FSP treatment can be applied, unfortunately leading to a decrease in static mechanical properties. Through SEM and TEM analysis, it was possible to observe the presence of Al3(Sc,Zr) nanoprecipitates in samples treated with HT0, FSP and FSP+HT0. The size of the precipitates is about 28, 35 and 27 nm respectively. Therefore, it cannot be assumed that an increase in their size is responsible for the decrease in hardness when FSP treatment is applied. This phenomenon is due to a heterogeneous distribution of precipitates along the grain boundaries and dislocations after FSP treatment. Two post-treatments were applied to improve the hardness of the FSP treated samples. The first one consists of solution heat treatment at 585°C for 15 or 30 hours, followed by artificial aging at temperatures ranging from 250 to 320°C and different times. This post-treatment did not increase the hardness because the temperature of 585°C is not sufficient to dissolve the Al3(Sc,Zr) precipitates. The second type of post-processing is the optimization of the FSP parameters using a lower rotational speed and/or a higher advancing speed. It did not improve the hardness because the precipitates are still heterogeneously distributed even when cooler parameters are used. In addition, tunnel-like defects appeared after the application of these new FSP treatments. The FSP treatment allows to improve the fatigue properties of Scalmalloy® but it seems difficult to recover static mechanical properties close to those after HT0. It will be necessary to know which criterion is the most important in the application in order to choose the appropriate post-treatment.