Research On Deformation Behavior And Forming Performance Of Fine-grained ZK60 Alloy Sheet At Medium And Lower Temperature

Magnesium and its alloys have been widely used in transportation,aerospace,3C electronic products and other fields,because of its low density,high specific strength,high specific stiffness and good damping properties.However,the grain structure of conventional rolled magnesium alloy sheet is generally coarse?>10?m?and its forming properties are not ideal.Fine-grained Magnesium alloy sheet has higher strength and plasticity,and can be formed at lower temperature,it has better stretch formability at high temperature,and even exhibits superplasticity.At present,there are few reports on the forming properties of fine-grained magnesium alloy sheets.Therefore,it is of great significance to study the deformation behavior and formability of fine-grained Magnesium alloy sheets at different temperatures for its application.In this paper,ZK60 magnesium alloy was studied.Fine-grained sheets with average grain size of 0.9²?m were obtained by high strain rate rolling?HSRR?and pre-rolling and high strain rate rolling?PR-HSRR?.Then the rheological behavior,work hardening and softening behaviors of HSRR fine-grained sheets under strain rate of 1?10^-3s^-11?10^-1s^-1 at 298⁵73K were studied.The plastic deformation mechanisms of the fine-grained sheet at elevated temperatures were analyzed.The critical conditions of dynamic recovery?DRV?and dynamic recrystallization?DRX?are obtained by calculating the work hardening rate.The softening mechanism of sheet metal at different temperatures was analyzed in order to improve the stretch formability of sheet metal by controlling the deformation conditions.Then the stamping properties of sheet metal at elevated temperatures were studied by Erichsen cupping test and hot drawing test.The effects of temperature on the forming properties of the fine-grained sheets are studied.The main conclusions are listed as follows:?1?Fine-grained sheets with grain size less than 2?m can be prepared by HSRR and PR-HSRR methods.Both sheets exhibit excellent mechanical properties at room temperature.The tensile strength,yield strength and elongation of HSRR sheets along the rolling direction were 353MPa,285MPa and 26%,respectively.The tensile strength,yield strength and elongation of PR-HSRR sheets were 394MPa,337MPa and 24%,respectively.The room temperature anisotropy of HSRR sheet was lower;?2?The influence of temperature and strain rate on the shape characteristics of the rheological stress curve of ZK60 fine-grained sheet was expounded.At RT³73K,the rheological stress reached its maximum value and breaks immediately with almost no softening behavior.At 423⁵73K,the rheological curves showed typical characteristics of DRX.?3?The strain rate sensitivity index?m?increased with the increasing deformation temperature,from 0.0115 at 298K to 0.387 at 573K.By calculating the apparent activation energy?Q?at different temperatures,it can be seen that,at 423K and below,dislocation slip and other intracrystalline deformation are the main deformation mechanisms.At 473K,the deformation mechanism began to change into a diffusion control process,and the dislocation creep controlled by climbing dominated the deformation process.The temperature was further increased to523⁵73K,and the grain boundary sliding?GBS?controlled by grain boundary diffusion was the main deformation mechanism.Under the action of GBS,the plate obtained a relatively large elongation.At 573K and with strain rate of 1?10^-3s^-1,the elongation is the largest,reaching 344%.?4?The plastic strain ratio?r?gradually decreases with increasing temperature,and the anisotropy of ZK60 fine-grained sheet also gradually decreases,the tensile formability is improved.At 573K,the elongation of the sheet in three directions is more than 341%.At 423K and above,the cup protrusion forming performance of fine-grained ZK60 magnesium alloy sheet is greatly improved.At 523K,the IE value of cupping test is the largest,which is 18.7mm.The better IE value forming temperature is 473⁵23K.The drawing ability of sheet metal also increases with the increase of drawing temperature,and its LDR value starts to decrease at 573K,and its better hot drawing temperature range is 523⁵73K.