Investigation of a surface engineered additively manufactured titanium alloy

Abstract

Ti-6Al-4V is a titanium alloy widely employed for high-performance applications in the chemical, aerospace, automotive industries. This lightweight alloy offers an impressive combination of properties, featuring good fatigue performance accompanied, a high tensile strength, a low elastic modulus and excellent corrosion resistance. In recent years, there has been a significant increase in the use of additively manufactured components, driving by the advantages it offers over conventional processes. Additive manufacturing (AM) stands out for its ability to rapidly produce components, irrespective of their complexity. Additionally, AM parts are ready for use with minimal post-processing requirements. However, the AM technique also tends to promote porosity, surface roughness and tensile stresses. These factors can compromise the mechanical, corrosion, wear and anti-fouling performance of components in practical applications, particularly impeding its widespread adoption in the maritime industry. However, surface engineering may serve as a viable solution to address some of these limitations. This work revolves around the investigation of a duplex treatment, involving shot peened followed by coating with a Ti/TiN/TiAlN/TiAlCuN layer via physical vapour deposition. The selection of shot peening was driven by its ability to improve the fatigue properties but also increases the material’s wear resistance due to the induced hardening. The coating’s significant hardness enhances the wear resistance, but it also enhances the corrosion resistance through barrier protection. Thus, this investigation aims to shed light on the effect the chosen treatments have on the microstructure, surface condition, hardness, and fatigue properties of AM Ti-6Al-4V. Secondly, this work also aims to show how in turn these properties affect the resulting corrosion, cavitation and cavitation-corrosion response, of the untreated and treated conditions, in a simulated marine environment. All surface treatments yielded a significant increase in hardness. The peening treatment generated compressive stresses extending to a depth of 150 μm, with maximum compressive stresses exceeding 700 MPa at around 30 μm below the surface. The coated-only condition showed excellent corrosion and cavitation-corrosion performance, attributed to the coating's high hardness and good adhesion. The roughness induced by shot peening was however detrimental for the corrosion, cavitation and cavitation-corrosion performance with no or minimal improvements observed. However, despite the presence of scatter, both treatments improved the fatigue life of the substrate.