Certain storage environments risk greater degradation of cold spray powders than others
ADELPHI, Md. -- The quality of metallic alloy powders for additive manufacturing depends heavily on the conditions of the surrounding environment. If stored or processed carelessly, the metal powder may lose its desired material properties at a much faster rate.
At the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory, researchers examined how different environmental conditions affect the performance of aluminum powders used in the cold spray process.
“Aluminum is of great importance to the aerospace industry,” said Dr. Victor Champagne, an Army senior materials scientist. “A lot of our efforts in cold spray repairs are geared toward high-end aerospace components that are made out of high-strength aluminum alloys.”
Champagne and his colleagues, who conducted this research over the course of several years, recently published the results of this study in the journal Acta Materialia.
In order to repair corrosion or wear on parts on anything from aircraft, munitions and vehicles to ships and submarines, high-pressure cold spray systems accelerate metal particles at supersonic speeds so that they adhere to the target substrate upon impact. Once they land, the particles also bind to each other as they deform and flatten to create a structurally sound coating.
Oxides and hydroxides may impede this crucial particle-to-particle bonding process when they form on the surface of the powder.
Repeated exposure to air and moisture causes the metal surface to hydrolyze and produce a thin hydroxide layer that gradually thickens over time. If left unchecked, the layer becomes a barrier to metallurgical bonding that significantly impairs the feedstock powder’s performance.
“Hydrolysis can occur at room temperature when the powder is exposed to humidity,” Champagne said. “It’s when that hydroxide becomes too thick that it actually interferes with consolidation during various additive manufacturing processes, including cold spray.”
Since the quality of a repair relies so heavily on the material property of the feedstock powder, Army researchers wanted to know how exposure to different levels of heat and humidity affects the formation of surface films.
The study tested two experimental conditions, temperature and humidity, with eight different sets of aluminum feedstock powders.
Researchers exposed four sets of powder to temperatures of 300 degrees Celsius for 270 minutes, during which the powder experienced 0, 30, 60 and 240 minutes of dry air exposure, respectively. Researchers subjected two other sets of powders to room temperature for four days with one stored at 50 percent relative humidity and the other stored at 95 percent relative humidity. One set of powder underwent both the high temperature and high humidity treatment, and the remaining batch served as the control.
“This design mimicked how many people in the cold spray industry often handled the powder, even in very humid environments where you get close to 100 percent humidity on a regular basis,” he said. “Prior to this research being done, they would leave the container open for one or two hours as they used the powder before closing it back up, leading to hours of exposure to the air.”
Army researchers gauged the performance of the exposed powders by measuring their critical adhesion velocity, or the velocity at which the particle must travel in order to bond with the substrate. If the treatment condition caused more surface hydroxides to form on the feedstock powder, they would observe a noticeable increase in its critical adhesion velocity.
The results ultimately showed that, while exposure to high temperatures had negligible effects on the performance of the powder, even slight exposure to high levels of humidity increased the critical adhesion velocity by more than 125 m/s, or about 14 percent.
Although the scientific journal officially published these findings just recently, both the U.S. government and the commercial industry acted almost immediately once Champagne and his colleagues released their findings as early as 2017.
“This work resulted in the development of a new manufacturing process to produce these powders in a way that prevents further oxidation and hydrolysis,” Champagne said. “In addition, the powders now come in airtight bags with a valve that attaches to the powder feeder so that the powder never sees the light of day.”
Today, manufacturers, especially for cold spray, handle the feedstock powder in an inert environment at all times in accordance with the results of this research.
CCDC Army Research Laboratory is an element of the U.S. Army Combat Capabilities Development Command. As the Army’s corporate research laboratory, ARL discovers, innovates and transitions science and technology to ensure dominant strategic land power. Through collaboration across the command’s core technical competencies, CCDC leads in the discovery, development and delivery of the technology-based capabilities required to make Soldiers more lethal to win the nation’s wars and come home safely. CCDC is a major subordinate command of the U.S. Army Futures Command.