MIL-STD-3021 CHG 1

Abstract

<u>Purpose</u>. The procedures covered by this standard are intended to ensure that cold spray coating operations, either manual or automated, meet prescribed requirements. This process can be used to restore dimensionally discrepant parts, or parts requiring protection from corrosion and wear (e.g. abrasion, cavitations, and erosion) but not limited to these applications. <br><u>Process</u>. This standard describes the process requirements for surfacing by means of cold spray deposition. The term "cold spray" has been used to describe this process because both the temperature of the powder-laden gas jet and the temperature of the powder material are low enough to prevent a phase change or stress in the deposit or substrate. Cold spray is a process whereby metal powder particles are utilized to form a deposit by means of ballistic impingement upon a substrate. The metal powders typically range in particle size from 1.0 to 100 micrometers (&#956;m) and are accelerated by injection into a high-velocity stream of gas. The high velocity gas stream is generated through the expansion of a pressurized, preheated, gas through a nozzle. The nozzle may be a contracting-expanding supersonic type or a contracting sonic type. The pressurized gas is expanded in order to achieve high velocity, with an accompanying decrease in pressure and temperature. The powder particles, initially carried by a separate gas stream, are injected into the nozzle either at the nozzle entrance (High Pressure Powder Injection) or at a lower pressure point downstream of the entrance (Low Pressure Powder Injection). The particles are then accelerated by the main nozzle gas flow and are impacted onto a substrate after exiting the nozzle. The solid particles that impact the substrate above a threshold (critical) velocity for the powder and substrate combination will deform and bond in a dense layer. As the process continues, particles continue to impact the substrate and form bonds with the previously deposited material resulting in a uniform deposition with very little porosity and high bond strength. <br><u>Types</u>. The two principal cold spray system configurations are depicted by Figures 1 and 2. The two configurations differ in the carrier gas, gas pressure, and powder injection location. Figure 1 shows a High Pressure Powder Injection System in which the main gas stream and the powder stream are both introduced into the inlet chamber of the nozzle. This configuration requires that the powder feeder be capable of high gas pressure. A low molecular weight gas, such as helium, is sometimes used as the accelerating gas when particles must be brought to very high velocity. Figure 2 shows a Low Pressure Powder Injection System in which the powder stream is injected into the nozzle at a point where the gas has expanded to low pressure. This system generally utilizes readily available compressed air, but can utilize nitrogen and helium, as well. <br><u>Applications</u>. The superior qualities of cold sprayed deposits are often required by the application. For example, the high heat transfer coefficient and electrical conductivity of cold sprayed deposits favor its use in electronic applications. Applications for cold spray technology often occur in situations where conventional thermal metal spray technology cannot be successfully used and where cold spray will result in an improved deposit. These situations often occur when high temperatures cannot be tolerated by the substrate. Good corrosion protection is achieved by dense, impermeable cold sprayed deposits. Wear resistant, hard surfaces, such as MCrAlYs, can be deposited by cold spray when operated at its hightemperature end.<b><u>Intended Use:</u></b> This military standard is intended to ensure cold spray coating operations on parts for military components meet prescribed requirements.<br>