Impact on Cold spray Repairs

Abstract

This report investigates the damage behavior of cold spray repairs subjected to low velocity impact (LVI) loading, with a focus on their potential application for aircraft cowling inlet lip repairs. Cold spray technology is increasingly considered as a promising alternative to traditional thermal spray and Laser- clad technologies due to its ability to significantly reduce repair costs, decrease residual tensile stresses in parts and decrease turn-around time during maintenance and overhaul. Unlike conventional thermal spraying methods, cold spray operates at temperatures below the melting point of the feedstock powder, allowing particles to remain in the solid state throughout the deposition process. As a result, it avoids the formation of heat-affected zones, preserves material properties, and induces favorable compressive residual stresses. These advantages make cold spray particularly suited for both structural and non- structural aerospace repairs. However, insufficient research exists to verify the quality of cold spray repairs on aircraft components to obtain the technology certified.
The primary objective of this research was to identify and characterize the dominant damage mecha- nisms that occur in cold spray coatings and repairs under LVI conditions. Six types of cold sprayed aluminum alloy coupons were evaluated, including both flat coatings and double curved blend-out re- pairs and cold spray depositions of AA1XXX and AA2024. The depositions were applied using either a robotic system or a manual gun. All specimens were subjected to controlled impact loading via drop weight and/or quasi static indentation tests. The results were analyzed using high-speed camera imaging, optical microscopy, and scanning electron microscopy of both surface and cross-sectional features.
Key output parameters such as dent depth, dent width, absorbed energy, and crack formation were analyzed and compared across different coupon types and impact energies. Results show that:
• Deposition geometry plays a critical role in how the material deforms under impact.
• The ductility of the deposition and substrate materials (e.g., AA1XXX vs. AA2024) influences both energy absorption and crack resistance for varying impact location.
• Coarse-gritsurfacepreparationenhancesbondingandincreasesresistancetodelaminationcom- pared to finer surface treatments.
• Barely visible impact damage does not apply to cold spray depositions as a dent is always visible. However, the substrate (non-impact side) showed different crack thresholds and damage mech- anisms compared to the cold spray deposition. The substrate cracked more severely compared to the cold spray deposition.
• Manual gun-sprayed depositions exhibit non-uniform properties, leading to variable crack thresh- olds across the repair area.
To compare performance independently of thickness, the impact data was normalized by the deposi- tion thickness. This revealed that thin coatings (like AA1XXX on top of AA2024) show lower energy absorption per millimeter compared to blend-out repairs.
In conclusion, the study highlights the critical influence of deposition material, geometry, thickness, and surface treatment on the performance of cold spray repairs under impact. It demonstrates key differences between robotic and manual spray systems, particularly in terms of uniformity and damage treshodls. The results show that cold spray depositions are able to withstand low velocity impact and can thus be used to repair dents in cowling inlet lips. The insights contribute to the ongoing research to integrate manual cold spray gun systems into aerospace repairs.