Sintering mechanism and mechanical properties of powder metallurgy Ti-6Al-4V alloy

Abstract

Ti-6Al-4V is crucial to improved performance in aircraft and aerospace systems owing to the excellent combination of specific mechanical properties and outstanding corrosion behavior. Its high production cost, however, is the main challenge against the widespread application in comparison to competing materials. These cost inefficiencies have led to developing near net shape powder metallurgy (P/M) Ti-6Al-4V techniques. Of the processes, the blended elemental (BE) approach, more feasible and cost-effective than other routes, is composed of powder compaction, sintering, and, if necessary, additional full-density processing. Sintering might be largely responsible for densification of the mixed Ti-6Al-4V powders, and the sintering conditions can determine the microstructural properties of as-sintered samples. Furthermore, since the mechanical properties, one of the most important properties for structural applications, should be controlled by these characteristics, the performance of P/M Ti-6Al-4V components might be determined by sintering conditions. The solid-state sintering of mixed Ti-6Al-4V powders includes the densification and the homogenization of Ti and Al/V particles. The role of specific surface energies and chemical concentration gradients during sintering is investigated by comparing the densification behaviors of pure Ti powders and the mixed powders with coarser Ti particles. The addition of Al/V particles into Ti matrix results in the enhanced densification by a synergetic effect of the self-diffusion of Ti and the inter-diffusion of alloying elements, especially the migration of Al into Ti matrix. The application of larger Ti powders in the mixed powders leads to retard the densification of the mixtures. It seemed to be attributed to the reduced surface energies of Ti particles plus the slowdown in the inter-diffusion of alloying particles. These demonstrations are confirmed by conducting a series of dilatometry tests and microstructural analyses of various sintering temperature. Furthermore, it is also consistent with the predicted activation energies for sintering

for example, the addition of alloying particles reduces the energies from 299. 35 to $135.48 kJ \cdot mol^{-1}$ and the use of coarser Ti powders increases them from 135.48 to $181.16 kJ \cdot mol^{-1}$. The hot deformation behavior of Ti-6Al-4V sintered preforms was investigated by hot compression tests over temperatures from $800-1100^\circ C$ and strain rates from $0.001-10.0 s^{-1}$. The flow curves for the $\alpha$+$\beta$ regimes exhibited continuous flow softening and broad oscillation after the peak stress under all of the processing conditions. For the $\beta$ field, steady-state flow stress was obtained or stress oscillation appeared after the peak stress. In the $\alpha$+$\beta$ field, the dynamic globularization and platelet buckling/kinking were observed at lower and higher strain rates, respectively. Unlike in the $\alpha+\beta$ region, dynamic recrystallization was found at higher strain rates in the $\beta$ domain. The apparent activation energies for hot deformation in the alpha+beta and beta regimes from kinetic modeling technique were estimated to 147.35 and $233.95 kJ mol^{-1}$, respectively. The efficiency of power dissipation over all of the deformation conditions was also calculated by using dynamic material models and schematized into a processing map. The highest value was predicted to 46% at a temperature of $945^\circ C$ and at a strain rate of $0.001 s^{-1}$, and the processing condition seemed to be optimal for hot deformation of the Ti-6Al-4V sintered preforms. The mechanical properties of deformed Ti-6Al-4V specimens are affected by the porosity and the microstructures. As the forging temperature increases from 900 to $1100^\circ C$, the properties also increased due to the reduction of porosity. The influence of interstitial elements is found to be negligible since the contents of impurities are unaffected with the forging temperature. In case of $\alpha+\beta$ deformed specimens, the enhanced mechanical properties are attributed to the higher dislocation density determined by KAM values and newly introduced nanosized $\alpha$ grains. For $\beta$ forged samples, the strong texture perpendicular to the basal plane and twin boundaries might lead to the higher strength and moderate ductility.