High-temperature compressive deformation of additive manufactured Ti6Al4V

Abstract

Machining titanium alloys such as Ti6Al4V to near-net-shapes is difficult and costly because of their low thermal conductivity, high strength and hardness at elevated temperatures, high chemical reactivity, and relatively low elastic modulus that hinder their usage. Additive manufacturing of Ti6Al4V provides the opportunity to produce complex geometries with minimum machining and short production times, thereby reducing manufacturing costs. However, there is limited research available on the high-temperature compressive deformation of additive manufacturing Ti6Al4V to fully replace conventionally manufactured Ti6Al4V. In this study, as-built Ti6Al4V additive manufactured (AM) using selective laser melting (SLM) and laser engineered net shaping (LENS) techniques were subjected to hot compression tests at temperatures ranging from 750 to 900˚C and strain rates of 0.1 - 10 s⁻¹ using the Gleeble 3500® thermo-mechanical simulator and power dissipation maps to characterise deformation behaviour. Microstructural analyses on the additively manufactured Ti6Al4V before and after hot compression tests were performed using optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). An Arrhenius-type equation with a hyperbolic sine function was utilised to compare the experimental flow stress obtained by the Gleeble 3500® thermo-mechanical simulator to the calculated flow stress, with good correlation. In all cases a wrought Ti6Al4V was used as a control. The results obtained by SEM and XRD showed that additive manufactured Ti6Al4V exhibited larger β-grain boundaries with finer α-phase in between compared to the wrought Ti6Al4V displaying a network of β-phase with equiaxed and elongated α-phase. The α- and β-phase composition showed a notable dependence on manufacturing techniques, with no significant changes in the chemical composition after deformation, but only in the percentages of phases present. Flow stress curves obtained at 10 s⁻¹ showed that the peak strength of wrought Ti6Al4V decreased by 54.7% when the deformation temperature was increased from 750 to 900˚C but decreased by a maximum of 70.6% for the AM Ti6Al4V, with the most significant decrease observed on the samples produced by LENS. At higher strain rates, the flow stress of all the samples, regardless of manufacturing technique, exhibited flow softening after achieving the peak stress value, with stress decreasing by at least 298 MPa at 0.1 s⁻¹, compared to a maximum decrease of 400 MPa at 10 s⁻¹. Decreasing the strain rate from 10 to 0.1 s⁻¹ resulted in serrated flow stress curves, consistent with the Portevin-Le Chatelier effect, which became more pronounced with an increase in temperature. In addition, the Portevin-Le Chatelier effect was observed most frequently on the additive manufactured Ti6Al4V, suggesting that the wrought microstructure with coarser elongated and equiaxed α-grains hinders dislocation movement and results in a lower flow stress. Power dissipation maps showed that the predominant mechanisms in the high-power efficiency percentages of deformation were dynamic recrystallisation (DRX) and globularisation for wrought Ti6Al4V; dynamic globularisation, recovery (DRV), and DRX for the samples prepared by SLM; and dynamic globularisation and DRX for the samples produced using LENS. These results imply that additive manufactured Ti6Al4V would be more prone to failure by creep or stress rupture compared to wrought Ti6Al4V.