Research On The Technics And Mechanisms Of ZK Series Magnesium Alloys During High Strain Rate Forging

Magnesium alloys are the lightest metallic structure materials with high specific strength and stiffiness, superior properties for absorbing vibration or shock energy, insulating electromagnetic interference, and excellent machining property, etc, which have been considered as the environment-friendly engineering nviterial with the greatest prospects for development in the21st century. Especially, forged magnesim alloy have broad application prospects in aircraft and automobile industries where energy conservation and emissions reduction need to be achieved ascribe to their excellent combination properties. However, the undesirable forgeability because of the narrow range of forging temperature and high sensitivity of forging strain rate in magnesium has limited their widespread application. Therefore, it is of important values both in theory and engineering to develop a more reliable forging process for magnesium alloys with high excellent combination properties. High strain rate forging was carried out on ZK (Mg-Zn-Zr) series alloys in the present study, and the forgeability should be significantly improved by twin induced dynamic recrystallization (TDRX) during high strain rate forging. A forging technique of the high strain rate multiple forging (HSRMF) was developed on the basis of grain refinement and texture weaken during multiple forging (MF), and the influence of forging technics and alloy element on HSRMF were also investigated in this paper.Firstly, hot compression was carried out on ZK series alloy in the consideration that temperature and strain rate play the most important role during forging process. The influence of temperature and strain rate on microstructure and flow behavior was analysed, and the processing parameters were optimized. Moreover, the dynamic recrystallization (DRX) mechanism during high strain rate compression was also investigated, the result showed that:(1) During low strain rate (ε≤1s-1) compression, DRX initiated mainly at original grain boundaries and resulted in grain refinement. During high strain rate(ε≥10s-1) compression, DRX initiated maily at the extensively developed twins in original grain cores, and the TDRX was the main grain refinement mechanism.(2) The hot workability of ZK series alloys increased with the strain rate increasing, the optimum processing parameters for the three alloys were temperature range of250℃～350℃and strain rate range of ε≥10s-1. (3) During High strain rate compression of ZK series alloy, the DRX at intial grain boundaries was characteristic of dis-continuous DRX mechanism which including grain boundary bulging, the formation of suh-grain at the place bulging out and the development of high-angle grain boundaries, while the DRX at intial grain cores was characteristic of continuous DRX mechanism which including the formation of dislocation cells due to dislocation pile-ups, the development of sub-grain caused by dislocation rearrangement and the formation of high-angle grain boundaries.Secondly, high strain rate forging at different conditions were studied to improve the forgeability and mechanical properties of ZK21alloy. The forging technics were further optimized by the investigation of deformation characteristics and regularities of different forging routes (uniaxial, biaxial and triaxial). Moreover, the influence forging parameters (including strain rates, pass strain and colling rates) during multiple forging were also investigated, the results showed that:(1) DRX at initial grain boundaries and twin induced DRX (TDRX) were the main grain refinement mechanism during uniaxial forging, which resulted in an incomplete DRX structure and small improvement of mechanical properties due to the limited deformation limit under uniaxial load. In biaxial forging samples, TDRX was the main DRX mechanism. A homogeneous ultrafine grained structure formed after biaxial forging wich show high strength but low ductility. In triaxial forging samples, TDRX and rotation DRX were the main grain mechanisms. A novel mixed structure of honeycomb-like coarse DRX grains and island-like ultrafine grains formed after triaxial forging. This novel mixed structure show substantial improvements in mechanical properties, and tenslie testing gave an ultimate tensile strength (UTS) of341MPa and an elongation of25%.(2) During high strain strain rate multiple forging, the decrease of strain rate and increase of pass strain could decrease the strength owing to the reduction of grain refinement, small pass strain HSRMF has therefore identified as an effective technique for producing high strength ZK21alloy. On the other hand, the increase of colling rate after HSRMF could decrease the ductility while simultaneous preserving even high strength, therefore air colling was more feasible for producing stronger and more ductile ZK21alloy.Thirdly, microstructure and mechanical properties of the HSRMFed ZK series alloy were studied based on the optimized forging parameters, then the influence of Zn content on twinning and DRX was analysed, moreover, the strengthen mechanism was also investigated. The results showed that: (1) The initial grains were refined significantly ater HSRMF. A novel mixed structure of honeycomb-like coarse DRX grains and island-like ultrafine grains formed, and the grain refinement was inflenced by alloy element Zn. The grain size of the coarse grains and ultrafine grains were determined by Zn content, while the fraction of the coarse grains and ultrafine grains were determined by the distribution of Zn element.(2) The mechanical properties improved substantially after HSRMF, and grain refinement, twins and second phase were the main strengthen mechanism. Increasing of Zn content in ZK series alloy coule reduce the contribution of grain refinement and twins decrease on strengthen materials, but increase the contribution of second phase.Fourthly, HSRMF was carried out based on the optimizd forging paramenter and alloy content, and the microstructure homogeneity and mechanical anisotropy was studied. The results showed that:(1) Micro structural imhomogenity was detected in the HSRMF samples owing to the friction. However, The UTS and ductility of the HSRMFed ZK21and ZK60alloys in different positions at the accumulated strain of3.3were315.2-325.6MPa,19.4～24.3%and313.1～326.5MPa,20.7-27.6%, respectively.(2) The UTS and ductility of the HSRMFed ZK21and ZK60alloys in different directions at the accumulated strain of3.3were299.3-325.3MPa,20.5～36.6%and310.6～323.9MPa,21.9-29.7%, respectively. HSRMF was therefore identified as a potential technique for producing magnesium alloy with low anisotropy.(3) HSRMF was a potential technique for producing stronger and more ductile magnesium alloy.