Premier aperçu de la fabrication additive de nouveaux alliages de titane beta métastables

Details

Supervisors

Jacques, Pascal

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux

Abstract

enThe SLM is an additive manufacturing method, allowing to build parts in 3D by the addition of material. The limitation of this technique is that only thirty different materials are commercially available. Therefore, new alloys should be developed for SLM preferably using a combination of available powders, as this would have commercial and flexibility benefits. Titanium is a good candidate for this purpose. It combines very interesting properties like a high specific resistance and an excellent corrosion resistance. Its main drawback is the lack of strain hardening. However, a new family of beta metastable Titanium alloys has arisen which presents a higher strain hardening thanks to the combination of TRIP/TWIP effects. This thesis aims at developing a new beta metastable titanium alloy with a higher strain hardening than Ti-6Al-4V based on a powder mixture of cp Ti, Ti-6Al-4V and steel 316L. This work is divided in two parts. First, different compositions of cp Ti, Ti-6Al-4V and steel 316L were casted thanks to the Bo-Md map which allows the prediction of the plastic deformation modes and the stabilised phases of a composition. They were characterized in order to determine the presence of TRIP/TWIP effects. Second, the powders were mechanically mixed to obtain the same composition as the alloy found in the first part of this work. Then, its printing was optimized and characterized. For the first part, it was observed that only one of the casted samples could be entirely beta but no TRIP/TWIP effect was noticed despite the map predictions. Different assumptions can explain these discrepancies : the map does not work with multicomponent alloys, the map does not take into account the influence of the interstitial elements or the Aluminium acts as a beta stabilizing element which favours slips against TWIP. For the second part, the printing of Ti-6Al-4V was mastered. Therefore, it could be used as a comparative basis for the powder mixture. The powder mixture was globally homogeneous. It was printed with success. However, a residual porosity of 2.5 % was obtained with the best parameters. The microstructure was composed of melting pools containing in the middle : elongated beta grains in the radial direction and at the boundary : small equiaxed beta grains with alpha phase at the grain boundary. With regard to the mechanical properties, the stress - strain curve of the printed powder mixture sample did not enter into plasticity due to defects from the 3D printing. The curves of the Ti-5.3Fe-1.3Cr-1Al-0.7Ni-0.6V-0.2Mo-0.1Mn and the Ti-6Al-4V were quite similar and no increase of strain hardening was observed because there was no TRIP/TWIP effect. To conclude, no alloy activating TRIP/TWIP effect was found. However, a functional process to realize the castings was established and the printing of homogeneous powder mixture parts was performed. Some research work is still required to optimize the printing parameters decreasing the porosity of the printed samples.