Microstructure and mechanical properties of cold sprayed 7075 deposition during non-isothermal annealing

• Non-isothermal annealing of cold sprayed 7075 Al depositions from RT to 450 °C
• Substantial microstructural changes in the pancaked interface regions
• Accelerated precipitation kinetics by CS in the deposition compared to the powder
• Low hardness of the deposition in less than 370 °C due to the presence of porosity
• Substantially higher hardness of deposition above 370 °C due to GB strengthening

This study presents microstructure and mechanical property relationships of cold-sprayed 7075 aluminum during non-isothermal annealing. Microstructure evolution during non-isothermal annealing from room temperature to 450 °C was performed using in-situ heating via a hot-stage transmission electron microscope. Additional characterization was performed using differential scanning calorimetry and X-ray diffraction. Grain size, dislocation density, microstrain, lattice parameter, and precipitation phenomena were evaluated as a function of annealing temperature. The results showed that cold spray processing accelerated the precipitation kinetics of strengthening phases in the microstructure, compared to the as-received cold spray powder, but did not affect the overall precipitation sequence. Also, pancaked grain structures, found at particle–particle interfaces within the deposition, were converted, due to recrystallization, to ultrafine-grained structures during annealing. The ultrafine-grained structures experienced limited grain growth during the annealing process. This was attributed to the nucleation of grain boundary precipitates in the as-sprayed material, primarily originating from grain boundary solute segregation present in the cold spray powder. Mechanical properties were evaluated using microhardness testing, after annealing, and correlated with microstructural analysis. When subjected to low temperature annealing (below 370 °C), the cold spray processed material had a lower microhardness than that found in conventional 7075 aluminum subjected to the same thermal treatment, due to the presence of inter-particle porosity in the cold spray microstructure. Annealing at temperatures above 370 °C, however, resulted in an increase in hardness, likely due to a reduction in inter-particle porosity and grain boundary strengthening associated with the retention of an ultrafine grain structure at high temperatures.