Understanding Grain Growth and Pore Elimination in Vacuum-Plasma-Sprayed Titanium Alloy
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INTRODUCTION
VACUUM plasma spray deposition is a well-established process and has been used to deposit coatings and layers for more than 20 years. To extend the process to deposit free standing materials to use as structural materials instead of coatings, several problems must be solved in both the spraying process and the material properties obtained. In the present study, one of the most important titanium alloys, Ti-6Al-4V, has been deposited using the vacuum plasma spray forming (VPSF) process to form a near-net-shape structure. Previous results have shown that the physical and mechanical properties of the vacuum-plasma-sprayed structures are lower than those of conventionally processed materials due to their internal microstructure.[1,2,3] For instance, the elongation of spray-formed Ti-6Al-4V is less than 1 pct and the modulus of elasticity is about 30 pct lower than for the conventionally processed material due to the existence of about 3 vol pct porosity, interlamellar boundaries, and internal defects. Therefore, to achieve the desired level of mechanical performance from the VPSF Ti-6Al-4V components, the level of porosity and internal defects must be reduced, interlamellar boundaries must be eliminated, and optimum phase compositions must be achieved through postdeposition heat treatments.
The response of VPSF titanium alloy to heat treatments has not been reported previously. There are little data available in the literature on the densification and grain growth of conventionally processed titanium alloys.[4–7] Because the microstructure of VPSF Ti-6Al-4V alloy differs from the types of structures found in conventionally processed structures, such as produced by powder metallurgy, the densification and grain growth behavior in the sprayed structure will be different. The plasma-sprayed deposits typically consist of billions of individual splats that are connected together by mechanical and chemical bounding. The microstructure in VPSF is mainly dependent on solidification of the individual splats and their position in the deposit. The microstructure of the as-sprayed VPSF Ti-6Al-4V alloy consists of fine columnar grains within the splats, oriented in the direction of heat transfer into the prior splats (Figure 1). A significant level of porosity across the interlamellae boundaries is observed, which depends on the thermal spray process parameters. In the following sections, the results of studies on grain growth, densification, and pore elimination during isothermal heat treatments of vacuum plasma-sprayformed Ti-6Al-4V materials are reported.
II. H.R. SALIMI JAZI, Postdoctoral Student, T.W. COYLE, Professor of Materials Science and Engineering, and J. MOSTAGHIMI, Professor of Mechanical, Industrial Engineering, are with the Center for Advanced Coating Technologies, University of Toronto, M5S 3G8 Toronto, Canada. Contact e-mail: [email protected] Manuscript submitted June 8, 2006. Article published online March 27, 2007. 476—VOLUME 38A, MARCH 2007
EXPERIMENTAL METHOD
The sprayed-formed Ti-6Al-4V samples we
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