Mechanism of enhanced ductility of Ti–6Al–4V alloy components deposited by pulsed plasma arc additive manufacturing with gradient-changed heat inputs

Abstract

Titanium alloys deposited by high-energy-density beam additive manufacturing (AM) technologies possess higher strength and lower ductility in the as-built condition. To improve the ductility of the as-built deposited components, this study employed the pulsed plasma arc AM（PPAM） technology with a gradient-changed heat input strategy to study the effect of microstructure evolution on the mechanical properties. The results indicate that the grain evolution in the height direction of the thin-walled Ti–6Al–4V alloy components is affected by the pulse stirring and cooling gradient. These two factors lead to three modes of nucleation growth at the solid–liquid interface of the solidification front and prevents coarsening microstructure growth along a single direction. After being deposited by multiple thermal cycles under the high pulse frequency process, the deposited microstructure decomposed to form a dispersed nano secondary α phase, which benefits the growth along the close-packed plane {10 1‾ 1}at the platelet colony α or the grain boundary direction. Consequently, the average elongation rate (A) of 12.7 ± 1% and area reduction rate (Z) of 56.5 ± 2% were higher than the forging standard, and the average tensile properties were also commensurate with the forging strength. This result is because the secondary α phase precipitates and grows along the same orientation to form a fine platelet α phase, which is advantageous for being slipped during the stretching process and enhances the plasticity. Thus, the thin-walled components of the as-built specimens fabricated via PPAM exhibit superior comprehensive mechanical properties.