Nondestructive Characterization of Multiscale Defects in an Aluminum Alloy after Cold Spray Repair

Cold spray additive manufacturing presents a rapid solution for repairing metallic systems affected by in-service damage. However, the inherent characteristics of the manufacturing process can result in the development of defects such as pores or inclusions, as well as anisotropy within the repaired volume. These factors can adversely affect the mechanical properties of the repaired components. In this study, aluminum 7050 substrates with representative corrosion damage were repaired by cold spray using feedstock powders of similar composition under varied tool pathing strategies. The cold spray repairs were inspected using immersion ultrasound, microhardness testing, and high-resolution electron microscopy. The objective was to assess and characterize defects in the repair and evaluate their impact on local mechanical properties. Analysis of acoustic impedance differences between the repaired volume and substrate served as an indicator of microstructure dissimilarity. Ultrasonic images were processed to evaluate the size and relative fraction of defects in the repair and microhardness testing was used to correlate ultrasonic results with mechanical properties. Certain tool pathing strategies were found to produce defect-free repairs based on ultrasonic examination. Subsequently, these defect-free samples were further analyzed using high-resolution electron microscopy, which revealed the presence of microporosity at length scales below the ultrasonic detection thresholds. While ultrasonic examination proved effective in detecting the size, shape, and depth of defects with millimeter-scale features, traditional destructive techniques remained necessary to characterize defects at the micron scale.