Cryogenic Processing High-Strength 7050 Aluminum Alloy and Controlling of the Microstructures and Mechanical Properties

The high strength or flow stress as well as low plastic deformability of 7000 series Al alloysmakes it difficult to improve their microstructures and mechanical properties by cold processing, and many advanced alloying methods and processing technologies are continually developed for higher mechanical properties and acceptable elongation. In this work, the cryogenic deformation (rolling) was applied to process high-strength 7050 Al alloys, and its effects on the microstructures and mechanical properties were studied. The results showed that after the pre-cooling with liquid nitrogen, the quenched 7050 Al alloy can obtain much higher rolling reduction, similar to that under warm or hot rolling, and a great number of substructures and high-density dislocations were formed which greatly increased the strength. The higher cryogenic deformability would be mainly related with the higher work-hardening ability at low temperature, while the strength enhancement would be largely attributed to the solution strengthening and dislocation strengthening. The cryogenic deformation can obviously stimulate the ageing process of the quenched 7050 Al alloy, but the direct ageing of the cryogenic-rolling 7050 Al alloy can assure higher strength and acceptable elongation, which would be greatly attributed to the precipitation strengthening and dislocation strengthening, while the recovery and ageing-induced precipitates help improving the tensile elongation. During room-temperature rolling, the formation of GP zones and eta' phases caused by the heats transformed from the deformation as well as their interaction with dislocations leads to the appearance of amounts of shear bands (instability areas), which will easily cause the cracking or edge-cracking of the rolling sheets. However, the cryogenic rolling with distinctly impeding the solute diffusion can result in the suppression of precipitation of the strengthening phases so as to decrease the occurrence of the shear instability areas, and uniform and stable plastic deformation or good work-hardening as well as high-quality rolling sheets are obtained. The excellent plastic deformability of high-strength Al alloys at cryogenic temperatures could be suggested as an effective way to improve the processing of high-strength Al alloys.