Investigation and Modeling of the Preheating Effects on Precipitation and Hot Flow Behavior for Forming High Strength AA7075 at Elevated Temperatures
Department of Mechanical Engineering, Dalian University of Technology, Dalian 116024, China
Department of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China
Department of Material Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK
Authors to whom correspondence should be addressed.
J. Manuf. Mater. Process. 2020, 4(3), 76; https://doi.org/10.3390/jmmp4030076
Received: 17 June 2020 / Revised: 20 July 2020 / Accepted: 21 July 2020 / Published: 23 July 2020
(This article belongs to the Special Issue Hot, Warm and Cold Stamping of High Strength Steel and Aluminium Alloy Parts)
Preheating is the first but critical step for hot stamping high strength precipitate hardened aluminum alloys. To thoroughly understand the effects of preheating conditions—i.e., preheating rate and heating temperature—on the strength and hot deformation of aluminum alloys, a series of thermal–mechanical tests was performed to determine the post-hardness evolution and hot flow behaviors. Typical microstructures with different preheating conditions were also observed through transmission electron microscopy (TEM), with which a unified model of both hot flow and strength based on key microstructural variables was developed, enabling the successful prediction of macroscopic properties using different preheating strategies. The results have shown that for high strength AA7075 at the T6 condition, the dominant mechanism of precipitate evolution with increasing temperature is the coarsening of precipitates first, followed by dissolution when they exceed a critical temperature. A higher heating rate results in a slower coarsening and a relatively higher strength level. In addition, the flow stress of hot deformation is also higher using a quick heating rate, with more significant softening and reduced ductility.