Study of the intrinsic mechanisms of nickel additive for grain refinement and strength enhancement of laser aided additively manufactured Ti-6Al-4V

It is well-known that grain refiners can tailor the microstructure and enhance the mechanical properties of titanium alloys fabricated by additive manufacturing (AM). However, the intrinsic mechanisms of Ni addition on AM-built Ti-6Al-4V alloy is not well established. This limits its industrial applications. This work systematically investigated the influence of Ni additive on Ti-6Al-4V alloy fabricated by laser aided additive manufacturing (LAAM). The results showed that Ni addition yields three key effects on the microstructural evolution of LAAM-built Ti-6Al-4V alloy. (a) Ni additive remarkably refines the prior-beta grains, which is due to the widened solidification range. As the Ni addition increased from 0 to 2.5 wt. %, the major-axis length and aspect ratio of the prior-beta grains reduced from over 1500 mu m and 7 to 97.7 mu m and 1.46, respectively. (b) Ni additive can discernibly induce the formation of globular alpha phase, which is attributed to the enhanced concentration gradient between the beta and alpha phases. This is the driving force of globularization according to the termination mass transfer theory. The aspect ratio of the alpha laths decreased from 4.14 to 2.79 as the Ni addition increased from 0 to 2.5 wt. %. (c) Ni as a well-known beta-stabilizer and it can remarkably increase the volume fraction of beta phase. Room-temperature tensile results demonstrated an increase in mechanical strength and an almost linearly decreasing elongation with increasing Ni addition. A modified mathematical model was used to quantitatively analyze the strengthening mechanism. It was evident from the results that the alpha lath phase and the solid solutes contribute the most to the overall yield strength of the LAAM-built Ti-6Al-4V-xNi alloys in this work. Furthermore, the decrease in elongation with increasing Ni addition is due to the deterioration in deformability of the beta phase caused by a large amount of solid-solution Ni atoms. These findings can accelerate the development of additively manufactured titanium alloys.

Automated summary

Ni addition in LAAM-built Ti-6Al-4V alloy can refine prior-beta grains, induce the formation of globular alpha phase, and increase the volume fraction of beta phase, leading to improved mechanical strength and decreased elongation. These findings contribute to the development of additively manufactured titanium alloys.