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5441 |
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Post spray heat treatment of cold sprayed C355 deposits for repair: microstructure and mechanical properties |
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Tanvir Hussain* / University of Nottingham, Great Britain M Zuccoli / University of Nottingham , UK A Sabard/ University of Nottingham , UK M Guagliano / Politecnico Milano, Italy H.L. de Villers Lovelock / TWI Ltd, UK |
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Cold gas dynamic spray is increasingly used for dimensional repair in the aerospace sector as it is capable of producing dense, oxide-free deposits of significant thickness and with good levels of adhesion and inherent mechanical strength. There is significant interest in extending the application of cold spray deposits to include structural, load-bearing repairs. However, particularly for high strength aluminium alloys, cold spray deposits can exhibit high levels of porosity and micro-cracks, leading to mechanical properties that are inadequate for most load bearing applications. In this work, heat treatment was investigated as a potential means of improving the properties of a cold sprayed Al alloy C355 deposit. C355 alloy deposits were produced using two process gas temperatures (350°C and 500°C) and three gas pressures (40, 50 and 60 bar) using a commercially available high pressure cold spray system. Microstructural analysis of the coatings revealed that the optimal microstructure (ca. 1% porosity) was obtained at 500°C and 60 bar. Therefore, coatings produced with process conditions of 500°C and 60 bar were heat treated at 175, 200, 225, 250°C for 4h in air and the evolution of the microstructure was analysed using optical microscopy, SEM/ EDX and XRD. The results show that heat treatment at 225°C can decrease porosity (<0.2%) and improve interparticle cohesion. Microhardness as a function of heat treatment temperature was also investigated. The 225°C heat treatment resulted in a microhardness of 105 HV0.5 (as-sprayed hardness 129 HV0.5) and the residual stresses were almost halved from -100 MPa in the as-sprayed deposit to -50 MPa. Therefore, this work has demonstrated that the heat treatment of C355 cold sprayed deposits at 225°C can significantly improve their properties. The paper discusses the fundamental mechanisms in the evolution of the microstructure at various heat treatment temperatures. |