Influence of cavity geometry on impact velocity in cold spraying and qualities in part repair

Structural damage and needed part replacement is a serious source for high maintainment costs and need of resources particularly in aerospace applications. Potentially, cold spraying offers possibilities to restore such damaged parts. However, the needed qualities can only be assured under rather optimum spray and thus powder particle impact conditions. In reality meeting individual damage types and shapes, the particle impact conditions could be inferred by local surface orientation and even the restricted freedom in gas flow over the surface. With respect to the design of repair geometry, this study addresses questions concerning gas flow and possible boundary layers that decelerate power particles by employing 3D computational fluid dynamics and experimental verification. Starting with simple angular, trapezoidal and curved cavity shapes as initial model systems, influences on final particle velocity are investigated for cold spray deposition of Al6061, using nitrogen as process gas at a pressure of 30 bar and a temperature of 500°C. For these examples, bow shock effects in cavities reduce particle impact velocities by up to 85 m/s, being maximum for the angular shape, thus resulting in lower deposit qualities. Further analyses aim on influences by process conditions and optimized cavity shapes for ensuring better repair conditions.