Microstructural Evolution in Solution Heat Treatment of Gas-Atomized Al Alloy (7075) Powder for Cold Spray

Abstract

Cold gas dynamic spray is being explored as a repair technique for high-value metallic components, given its potential to produce pore and oxide-free deposits of between several micrometers and several millimeters thick with good levels of adhesion and mechanical strength. However, feedstock powders for cold spray experience rapid solidification if manufactured by gas atomization and hence can exhibit non-equilibrium microstructures and localized segregation of alloying elements. Here, we used sealed quartz tube solution heat treatment of a precipitation hardenable 7075 aluminum alloy feedstock to yield a consistent and homogeneous powder phase composition and microstructure prior to cold spraying, aiming for a more controllable heat treatment response of the cold spray deposits. It was shown that the dendritic microstructure and solute segregation in the gas-atomized powders were altered, such that the heat-treated powder exhibits a homogeneous distribution of solute atoms. Micro-indentation testing revealed that the heat-treated powder exhibited a mean hardness decrease of nearly 25% compared to the as-received powder. Deformation of the powder particles was enhanced by heat treatment, resulting in an improved coating with higher thickness (* 300 lm compared to * 40 lm for untreated feedstock). Improved particlesubstrate bonding was evidenced by formation of jets at the particle boundaries.

On Coating Techniques for Surface Protection: A Review

“…As a result, this method is categorized Figure 11. Top: SEM micrographs of cold spray-coated AA6061 substrate by (a) as-received and (b) heat-treated AA7075 particles with a mechanical interlocking between the coating and the substrate [105]. Bottom figure: a schematic view of cold spray coating process [106].…”

Phase Transformations in Thermally Treated Gas-Atomized Al 7075 Powder

“…Because features in the powder are retained during consolidation, the as-atomized microstructure can be controlled using thermal treatments. Recent research has shown the retention of feedstock powder microstructure during consolidation for other alloys, though that research included a limited analysis of the internal microstructure of the powders, not fully investigating the effect of the thermal treatments on the powders [10][11][12][13][14]. These researchers assumed the network of solute elements at the boundaries in the as-atomized structure, as seen in scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), was segregation.…”

“…Solutionization times for Al 7075 are typically on the order of 4-16 h for wrought components, depending on part size [9]. Given this timescale, other researchers chose to use long heat treatment times for their powders; however, other researches performed on Al 6061 indicated faster diffusion times in powders, resulting in the need for shorter thermal treatments [5,10].…”

Gas-Atomized Al 6061 Powder: Phase Identification and Evolution During Thermal Treatment

“…With this in mind, Sabard et al applied a heat treatment to the as-atomized powders prior to use in cold spray consolidation. 7 They found that, with a heat treatment of 4 h at 450°C, particle deformation was enhanced, leading to improved particle-substrate bonding and thicker coatings. Additionally, they found that the thermally-treated powders had a reduction in solute segregation and a higher porosity than the as-atomized condition.…”

“…2 However, with the advent of AM, the focus was first on creating the process and then on optimizing the processing parameters for the consolidated part, and it is only in recent years that research has been conducted analyzing the effects of the feedstock on the consolidated parts. [3][4][5][6][7][8][9][10] Many metal AM techniques that utilize powder as feedstock employ melting as a means of consolidating the feedstock into a solid material. These processes use different methods to melt the feedstock, for example, electron beams or lasers, which affect the melt pool temperature, 11,12 and differences in melt pool temperature can influence the microstructure.…”