Development and 3D printing of a new high resistance titanium alloy

Details

Supervisors

Jacques, Pascal

Faculty

Ecole polytechnique de Louvain

Degree label

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

Abstract

The rapid development of additive manufacturing over the past decades opened up new horizons in many industries requiring ever higher performance materials to keep up with the new market demands. An important player in this challenge has been titanium and its alloys. They present an impressive set of properties combining outstanding strength-to-weight ratio, great corrosion resistance and biocompatibility which find applications in the aerospace and biomedical sectors. Nevertheless, the remaining drawback of current titanium alloys is their low work hardening rate and their resulting limited ductility. There are thus strong incentives to develop new titanium alloys to make up for this lack of plastic properties. A recent approach concerning beta-metastable alloys involves twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects to accommodate plastic deformation and increase the material work hardening capabilities. Following this design method, the Ti-12Mo and Ti-8.5Cr-1.5Sn systems have been previously investigated and effectively demonstrated superior work hardening rate and ductility. This master thesis concerns the Ti-8.5Cr-1.5Sn alloy which displayed promising mechanical properties when produced by conventional manufacturing methods. The objective of the study is to produce the alloy by selective laser melting (SLM) in order to bring together the alloy great mechanical behavior with the advantages of additive manufacturing, hopefully fulfilling even more satisfactorily the industrial expectations. The Ti-8.5Cr-1.5Sn alloy was successfully printed by SLM. The influence of laser power and scanning speed on the densification, laser track geometry, hardness and microstructure was investigated along a brief mechanical characterization. It was found that the microstructure is evolving during the SLM process. The large hardness drop observed after a quick heat treatment tends to indicate the presence and growth of omega-Ti phase, leading to material embrittlement. Expected TWIP effects were reported in heat treated samples. Their contribution to the improvement of the strain hardening capability of the alloy is promising but still limited when compared to traditional manufacturing methods. In first instance, a proposition of optimal process window, to obtain proper densification while avoiding major process defects, would lay between 60 and 100 W of laser power and range from 400 to 700 mm/s regarding the laser scanning speed. These process parameters should yield a ratio of linear energy density (P/v) comprised between 130 and 160 J/m.