Warm Spray Forming of Ti-6Al-4V

Abstract

Warm spray (WS) is a modification of high-velocity oxy-fuel spraying, in which the temperature of the supersonic gas flow generated by the combustion of kerosene and oxygen is controlled by diluting the combustion flame with an inert gas such as nitrogen. The inert gas is injected into the mixing chamber placed between the combustion chamber and the powder feed ports, thus the temperature of the propellant gas can be controlled from~700 to 2,000 K. Since WS allows for higher particle temperatures in comparison to cold spray, warm sprayed particles are more softened upon impact, thus resulting in greater deformation facilitating the formation of shear instability for bonding. Recently, the combustion pressure of WS has been increased from 1 (low-pressure warm spray) to 4 MPa (high-pressure warm spray) in order to increase the velocity of sprayed particles. Effects of spray parameters on microstructure, mechanical properties, and splats formation of Ti-6Al-4V were systematically studied. Obtained coatings were examined by analyzing the coating cross-section images, microhardness as well as oxygen content. In addition, flattening ratio of splats was calculated as a function of nitrogen flow rate. It was found that the increased particle velocity caused by the increased combustion pressure had significant beneficial effects in terms of improving density and controlling the oxygen level in the sprayed Ti-6Al-4V coatings.

Effects of Spray Parameters and Post-spray Heat Treatment on Microstructure and Mechanical Properties of Warm-Sprayed Ti-6Al-4V Coatings

“…Because of several values for particle velocity and temperature, noticeable difference in the microstructure of assprayed Ti-6Al-4V coatings was found and described in detail in the previous part of Ref 11. The main conclusion was that there is a trade-off relationship between porosity and oxygen content as a function of the nitrogen flow rate and, consequently, particle temperature.…”

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

“…Due to this system the temperature of the propellant gas can be effectively controlled in the range of 800-2300 K so that particle temperature can be kept under its melting point but moderately heated up to cause lager particle deformation upon impact on a substrate in solid state. When it is applied to titanium and its alloys, it can take advantage of thermal softening of the materials to lower the velocity needed to form bonding while avoiding excessive oxidation by keeping the particle temperature below the melting point (Ref [8][9][10].…”

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

“…Thanks to this system, the temperature of the propellant gas can be effectively controlled so that the temperature of the sprayed particle can be kept below its melting point. It can take advantage of thermal softening of the materials to lower the velocity needed to form bonding while avoiding excessive oxidation by keeping the particle temperature under the melting point, especially when it is applied to Ti and its alloys (Ref [15][16][17].…”