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Keywords
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aluminum alloy, hot stamping, thermo-mechanical processing, graded property,
microstructure.
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Abstract
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In the present study, functionally graded AA7075 components manufactured via hot stamping were investigated by focusing on the effect of different process variables on localized microstructure evolution. To realize gradation through stamping, an active tool was designed and applied. The design of experiments allowed to assess the impact of transfer time from the furnace to the tool, quenching time in the tool, and final quenching media. Related characteristics of mechanical properties throughout the hat-shaped profile were assessed via hardness and tensile tests. As expected, the sections of the samples formed in the cooled part of the tool are characterized by higher mechanical strength following subsequent aging, while sections formed in the heated part exhibited higher ductility. Moreover, the microstructural analysis revealed that fine precipitates with minimum interparticle distances only formed in the cooled section of the samples. Increasing the tool temperature at the heated side to 350 °C resulted in the formation of coarse precipitates in the grain interior and along the grain boundaries. A sharp gradient in terms of microstructural and mechanical properties is found between these conditions. After reducing the transfer time, an increased volume fraction of fine precipitates led to further improvements in hardness and mechanical strengths.
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