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5446 |
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Friday, June 09, 2017, Hall 28 2:00 PM Cold Gas Spraying IV |
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Solution heat treatment of gas atomized aluminium alloy powders: Microstructural changes and resultant mechanical properties |
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Alexandre Sabard* / University of Nottingham, Great Britain Heidi Lovelock / TWI Ltd, United Kingdom Phil McNutt/ TWI Ltd, United Kingdom Dave Harvey/ TWI Ltd, United Kingdom Tanvir Hussain/ University of Nottingham / Faculty of Engineering, United Kindgom... |
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Cold gas dynamic spray has significant potential for load-bearing repairs in the aerospace sector, as it is capable of producing pore and oxide-free deposits of significant thickness and with good levels of adhesion and mechanical strength. However, recently published research has shown that the rapid solidification experienced by gas atomised powders during manufacturing leads to a non-equilibrium powder microstructure, including clusters of dislocations as well as significant localised segregation of important alloying elements within each particle. This effect is particularly pronounced in precipitation hardenable aluminium alloys. Therefore, we have recently started investigating the solution heat treatment of aluminium alloy powders in order to adjust the powder phase composition and microstructure for cold spraying. This is also expected to lead to a more predictable heat treatment response of the cold spray deposits. In this work, aluminium alloy 7075 gas atomized powders were solution heat treated at 450 °C for 5 hours in a sealed glass vial under vacuum and quenched in water. The powder particle microstructures were investigated using scanning electron microscopy with electron back scatter diffraction (SEM/ EBSD) and optical microscopy. These investigations revealed that the dendritic microstructure and solute segregation in the gas atomized powders was altered by the heat treatment, with the particles exhibiting a homogeneous distribution of solute atoms. To gain a thorough understanding of the precipitation states of the powders, samples were studied using transmission electron microscopy (TEM) in the as-quenched state and after 30 and 60 days respectively of room temperature ageing. The influence on the mechanical properties of the powder particles was studied using nano-indentation. The treated powders exhibited a hardness decrease of nearly 23% compared to the as-received powders. This paper further discusses the evolution of the microstructure during the conventional ageing cycles. Keywords: Aluminium alloy, Al 7075, solution heat treatment, precipitation hardening, cold spray, powder |