Fabrication of TiAl intermetallic phases by heat treatment of warm sprayed metal precursors

• Warm Spray was used to deposit Ti–Al composite coatings.
• In order to obtain Ti–Al intermetallic phases two-stage heat treatment was carried out.
• The heat-treated coating increased porosity up to 25%.
• The relationship between the porosity and the volume expansion was described.

Warm Spraying is an atmospheric coating process based on high-velocity impact bonding of powder particles. By decreasing the temperature of combustion gas via mixing with nitrogen the oxidation of feedstock powder can be effectively controlled. This is particularly important for Ti-based coating materials, which rapidly oxidize at elevated temperatures.In this study, Ti–Al composite coatings were fabricated by the Warm Spray process using a mixture of titanium and aluminum powders as a feedstock and applying a two-stage heat treatment at 600 and 1000 °C to obtain intermetallic phases. The microstructure, chemical and phase composition of the deposited and heat-treated coatings were investigated using SEM, EDS and XRD. The experimental results show that TiAl3 was the first intermetallic phase formed during the first-stage heat treatment. The growth of TiAl3 layer occurred mainly by diffusion of Al into Ti particles. Significant porosity that developed during the heat treatment was caused mainly by Kirkendall effect. After the second-stage heat treatment, a coating layer with TiAl as the dominant phase was obtained with about 20 vol % porosity.