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.

Keywords

powder bed fusion-laser beam / titanium alloys / heat treatments / mechanical properties / fractographic analysis

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