Understanding the fatigue and fracture mechanisms of cold spray additive repairs

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University of Alabama Libraries

The objective of this research is to understand the underlying mechanisms associated with fatigue and fracture in additive cold spray (CS) repairs of structural components. Because of the low temperatures maintained during deposition, CS is ideal for use in additive repair applications, especially on thermally sensitive materials such as aerospace aluminum alloys. Additive repair functions as a direct alternative to scrapping and replacing the damaged component, offering huge cost savings and reduced maintenance down times. In order for structural additive repairs to be successful, the bulk material that is being used to fill the repair, the adhesion of the repair, and the repaired component must all perform well under the expected loading. While the current literature has investigated these topics, characterization of the fatigue of the freestanding material and the adhesion of CS deposits is significantly lacking. This work examines the freestanding fatigue properties of CS AA7075 and CS AA2024, and shows that they have relatively predictable fatigue performance that compares well with the wrought controls when normalized by UTS. The adhesion strength of CS deposits is of particular importance in aerospace applications where repairs that completely separate can cause foreign object damage elsewhere in the aircraft. Therefore this research seeks to understand the adhesion mechanics of CS through the use of fracture mechanics based adhesion test methods. A strong linear relationship between surface roughness and adhesion toughness was found in CS AA7075 deposits, with interfacial fracture toughness’s ranging from 1.2 to 1.9 MPa√m. Finally realistic fastener hole repairs were made in AA2024 and AA7075, which were found to perform as well or in some cases significantly better than the epoxy filled controls.

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Materials science