Characterization of microstructural and precipitates evolution in cold sprayed AA7050 after in situ laser heat treatment

Abstract

This research systematically studied the processing-microstructure relationship in cold sprayed AA7050 after laser heat treatment with different levels of heat input (HI). The ability to use in situ laser heating to modify the microstructure and properties of cold sprayed AA7050 is an important goal for repair applications. In this research, a diode laser was used to heat treat the cold sprayed AA7050 in situ, without sectioning samples that are required in the conventional furnace heat treatment. Four AA7050 deposits were cold sprayed with identical spraying parameters and geometry. Three of these deposits were then subsequently heat treated by the laser source with the following linear heat input: 20 J/mm (low-HI), 40 J/mm (medium-HI) and 80 J/mm (high-HI). Electron backscattered diffraction showed that laser heat treatment coarsened the ultra-fine grains in the prior particle boundary regions in the deposit, whereas the larger grains in the prior particle center regions generally remained the same. Scanning transmission electron microscopy revealed precipitates nucleation in the deposits after the laser heat treatment, and the high-HI deposit possessed the largest density of precipitates. In the substrates, a reduction in hardness was observed in the sample with high-HI. The decreased hardness is primarily attributed to the decreased precipitate density. This study shows that with a precise control of HI, laser heat treatment could be a practical approach evolving the microstructure and precipitates in cold sprayed AA7050 deposits without over-aging the precipitates in the underlying substrate.