The Nature of Tensile Ductility as Controlled by Extreme-Sized Pores in Powder Metallurgy Ti-6Al-4V Alloy

Abstract

Tensile properties of Ti-6Al-4V titanium alloy, sintered by a new process (sintering, phase transformation, and dehydrogenation of titanium hydride compacts, termed HSPT process), were investigated to determine how the sintering pores influence the tensile strength and ductility. It was found that the ductility in the sintered alloy is severely affected by the size of the largest pore, referred here as extreme-sized pore, even when the average volume fraction of porosity is nearly constant between a large number of samples. It is shown that the rapid decrease in ductility, with an increase in the extreme pore size, is caused by strain localization around the extreme-sized pore and early crack initiation. This crack initiation leads to fracture of the plane containing the pore thereby limiting the extent of uniform plastic strain that can be attained before fracture. Interestingly, the strength properties are, however, found to be independent of the size of the extreme-sized pore. The results are explained on the basis of stress concentration and strain localization around the extreme-sized pores. The work also reveals that if the extreme-sized pores are eliminated, PM Ti-6Al-4V alloy with high strength (~1100 MPa) and good ductility (~12 pct), which is easily comparable to a wrought Ti-6Al-4V alloy, can be achieved even at oxygen levels up to 0.4 wt pct.

A Review of the As-Built SLM Ti-6Al-4V Mechanical Properties towards Achieving Fatigue Resistant Designs

“…No failure occurred from cracks initiating at lack-of-fusion defects in horizontal coupons, which is a consequence of the defect being located on a plane parallel to the loading direction. This results in the aspect ratio being lower than that of a round pore, causing a lower stress concentration in the vicinity of the defect [82]. However, lack-of-fusion defects are considerably more dangerous in vertically manufactured coupons, due to the aspect ratio being greater than a round pore, which leads to higher stress concentrations in the vicinity of the lack-of-fusion defect.…”

Powder Casting: Producing Bulk Metal Components from Powder Without Compaction

“…56 Large ''rogue'' pores have been found in press-and-sinter HSPT Ti-6Al-4V, which obviously affect the mechanical performance. 54,57 Such pores are also an important concern for powder-cast Ti-6Al-4V. However, if rogue pores can be prevented through careful processing, the densification that results from hydrogen sintering is sufficient to achieve excellent mechanical performance.…”

Identifying Challenges to the Commercial Viability of Direct Powder Rolled Titanium: A Systematic Review and Market Analysis

“…Given the high specific surface area of powdered material, the formation of oxide layers on individual particles can lead to unacceptable levels of contamination. This significantly affects the mechanical properties of the final product, particularly tensile ductility [39,40]. Oxygen contents for PM (powder metallurgy) product made via DPR, CIP (cold isostatic pressing), and die pressing were consolidated from 25 sources of literature.…”