Design of titanium alloy for efficient sintering to low porosity

Abstract

Few Ti alloys have been designed for ease of sintering. This paper considers the design of alloys for processing using the mixed elemental technique, in which powders are mixed, cold pressed in a die to near net shape and sintered under vacuum at high temperature. The authors describe steps in the process of developing a Ti-Ni-Sn alloy, able to be sintered to near full density at a sintering temperature as low as 1100uC without requiring unusually fine powder or high compaction pressure. Higher sintering temperature allows the Ni content of the alloy to be reduced, but swelling of the alloy probably imposes an upper limit on practical sintering temperature. The increase in green density, and hence sintered density, conferred by Sn in Ti-Sn alloys, and the increase in sintered density due to the high diffusivity of Ni in Ti-Ni alloys, are combined in Ti-Ni-Sn alloys.

A comparison of Ti–Ni and Ti-Sn binary alloys processed using powder metallurgy

“…The density improvement when sintering in the lower temperature range (i.e. below 1250 °C) is more likely due to the higher green densities of the compacts [12]. In this case the density gain surpasses the loss from the skeleton expansion.…”

“…Ti-Ni-Sn PM systems [12,[22][23][24][25], although a slightly higher compaction pressure of 400 MPa was used in those studies, along with more coarse initial elemental powder sizes. The addition of Sn improves the green density when higher contents are used (i.e.…”

“…Ultimately it would be of interest to examine a combination of the two systems studied here, for example an initial ternary composition with 2.5 wt.% Sn and 5 wt.% Ni, with the aim of gaining the best characteristics of each additive. With that in mind, preliminary studies by Schaffer and colleagues have demonstrated some promise for such an approach[12].Finally, it should be noted that more analyses are needed to fully understand the detailed sequence of liquid phase sintering reactions occurring for the case of a persistent liquid phase (and to a lesser extent the transient liquid). Consequently, future DSC investigations would be helpful in determining the liquid formation in-situ, while the use of dilatometry could identify the swelling/shrinkage phenomena arising during the sintering process.…”

“…As can be anticipated, the liquid phase volume will therefore vary with both the Ni content and sintering temperature. In addition to introducing a liquid phase at elevated temperature, Ni has been found to reduce the diffusion activation energy [14], and therefore has a high diffusivity in Ti-Ni alloys [12], which promotes greater inter-diffusion. Some densification improvements have been confirmed in PM processed…”

“…To boost the sintering densification, alloy additions are sometimes introduced to generate a secondary liquid phase; for example adding Cu or Fe to Ti [9][10][11][12]. The presence of the liquid phase during sintering provides an effective diffusion route for mass transport.…”