A novel solution treatment and aging for powder bed fusion–laser beam Ti–6Al–2Sn–4Zr–6Mo alloy: Microstructural and mechanical characterization

Abstract

Ti–6Al–4Zr–2Sn–6Mo alloy is one of the most recent titanium alloys processed using powder bed fusion–laser beam (PBF–LB) technology. This alloy has the potential to replace Ti–6Al–4V in automotive and aerospace applications, given its superior mechanical properties, which are approximately 10% higher in terms of ultimate tensile strength (UTS) and yield strength after appropriate heat treatment. In as-built conditions, the alloy is characterized by the presence of soft orthorhombic α″ martensite, necessitating a postprocessing heat treatment to decompose this phase and enhance the mechanical properties of the alloy. Usually, PBFed Ti6246 components undergo an annealing process that transforms the α″ martensite into an α–β lamellar microstructure. The primary objective of this research was to develop a solution treatment and aging (STA) heat treatment tailored to the unique microstructure produced by the additive manufacturing process to achieve an ultrafine bilamellar microstructure reinforced by precipitation hardening. This study investigated the effects of various solution temperatures in the α–β field (ranging from 800 to 875°C), cooling media (air and water), and aging time to determine the optimal heat treatment parameters for achieving the desired bilamellar microstructure. For each heat treatment condition, different α–β microstructures were found, varying in terms of the α/β ratio and the size of the primary α-phase lamellae. Particular attention was given to how these factors were influenced by increases in solution temperature and how microhardness correlated with the percentage of the metastable β phase present after quenching. Tensile tests were performed on samples subjected to the most promising heat treatment parameters. A comparison with literature data revealed that the optimized STA treatment enhanced hardness and UTS by 13% and 23%, respectively, compared with those of the annealed alloy. Fracture surface analyses were conducted to investigate fracture mechanisms.

1. Introduction

Titanium alloys are widely used in the transportation sector, especially in automotive and aerospace applications, because of their remarkable combination of strength and lightweight properties, excellent corrosion resistance, and high mechanical properties both at room and elevated temperatures [1–3].

Titanium is an allotropic element with a hexagonal closed-pack lattice at temperatures up to 883°C (α phase) and a body-centered cubic lattice (β phase) at temperatures higher than 883°C. Alloying elements can stabilize either the α or β phases, leading to the classification of titanium alloys based on phase proportions at room temperature: α alloys (β phase < 5%), α–β alloys (β phase < 20%), and β alloys (β phase > 30%) [2,4–5]. Among various titanium alloys, Ti–6Al–4V (Ti64), belonging to the α–β alloy family, stands out as the most widely used in industrial environments, accounting for over 50% of the total titanium production [1]. Nevertheless, Ti–6Al–2Sn–4Zr–6Mo (Ti6246), the α–β alloy with a higher percentage of β-stabilizers, has replaced the Ti64 alloy in applications demanding high strength, good creep, and oxidation resistance at elevated operating temperatures, such as in compressor discs and brakes for aero engines [6–7].

Traditionally, Ti alloy components are manufactured using plastic deformation, welding, and machining processes. However, the increasing demand for high-performance components with complex geometries, combined with the challenges of machining titanium using traditional methods, has led to the exploration of titanium alloys in additive manufacturing (AM) processes [8–10]. AM enables the production of components with increased flexibility and freedom, mitigating challenges associated with traditional machining. In particular, the powder bed fusion–laser beam (PBF–LB) technology facilitates the production of near-net-shape structures directly from computer-aided design models, significantly decreasing the necessity for rough machining and thereby reducing the “buy-to-fly” ratio [10]. In addition, the optimized design of structural components can contribute to weight reduction that, in the transportation field, can result in improved fuel efficiency, reduced greenhouse gas emissions, and longer battery life [11–12].