Effects of Al Content and Addition of Third Element on Fabrication of Ti-Al Intermetallic Coatings by Heat Treatment of Warm-Sprayed Precursors

Abstract

Four powder mixtures of titanium and aluminum with 50:50, 40:60, 30:70, and 20:80 atomic ratios were used as feedstock for Warm Spray process to produce composite coatings. A two-stage heat treatment at 600 and 1000°C was applied to the deposits in order to obtain titanium aluminide intermetallic phases. The microstructure, chemical, and phase composition of the as-deposited and heat-treated coatings were investigated using SEM, EDS, and XRD. It was found that the Al content affects on the thickness expansion of the heat-treated Ti-Al coatings significantly and also has a major influence on the porosity development, which is caused by the Kirkendall effect. The effects of adding a third element Si and heat treatment with pressure to produce denser Ti-Al intermetallic coating were also examined. The investigated hot-pressed coatings with addition of Si exhibited much denser microstructure and contained TiAl intermetallic phases with titanium silicide precipitates.

Fabrication and Oxidation Resistance of TiAl Matrix Coatings Reinforced with Silicide Precipitates Produced by Heat Treatment of Warm Sprayed Coatings

“…Due to the relatively low temperature during WS, no aluminum or titanium oxides were detected by XRD in the as-sprayed deposits, which was confirmed in our previous studies (Ref 10,11).…”

“…2. These coatings consist of highly deformed aluminum-silicon ''matrix'' with hardly distinguishable particle boundaries, into which relatively undeformed Ti particles were embedded (Ref 10,11). Al underwent much more deformation than Ti because of its lower yield strength and much lower melting point.…”

“…However, there are some problems with production of coatings using Ti and Al powder mixture as a feedstock. In our previous study, pure Ti and Al powders were deposited by warm spray, which is a modification of high-velocity oxy-fuel (HVOF) spray process, also called as a two-stage HVOF, and Ti-Al phases were synthesized by two-stage heat treatment ( Ref 10,11). Accordingly, Ti-Al intermetallic phases were formed with a porosity up to 30% mostly due to the Kirkendall effect.…”