Microstructures And Mechanical Properties Of Mg-Zn-(Cu,Y) Alloy Sheets By Heavy Reduction Rolling Processing

Zn and RE can have both the solution and aging strength effect in the Mg alloys, which makes Mg-Zn-RE system the priority in new alloys of magnesium alloy. This system of alloy can achieve the strengthen effect by adding several alloy elements. However, the fact that a large number of compounds phase existed on the grain boundary can do harm to plasticity of the alloy. Y is one of the elements that have the maximum solid solubility in the magnesium alloys (The maximum content is about12.5%). Put differently, it has the outstanding solution and aging strength effect, as well as the good flame-retardant effect when casted. Adding Cu can act as a method to further improve solution temperature, facilitate solution strength effect, enhance heat stability and boost casting property of alloys. By using heavy reduction rolling, This research studies what effect the rolling parameters can have on microstructure and mechanical properties of Mg-2Zn-Y alloy sheet with low Zn content and micro-Y alloyed, with an aim of developing new magnesium alloys with higher formability. The conclusions can be concluded as:(1) Mg-4Zn-3Cu, Mg-2Zn-3Cu-0.25Y and Mg-2Zn-(0.1-0.15)Y all have superior formability. More specifically, the former two can achieve multi-pass continuous heavy reduction rolling processing with the pass reduction rate is up to30～44%in the range of250℃～380℃, while Mg-2Zn-(0.1-0.15)Y can perform heavy reduction rolling mentioned before even at250℃.(2) With partial heavy reduction rolling process, microstructure of initial rolled Mg-2Zn-(0.1～0.15) Y all fully re-crystallized. As to middle rolling, there exist many twins and shear zones in microstructure; the initial temperature dropped (250℃～380℃) recrystallized microstructure after initial rolling is significantly refined while the microstructure after middle rolling become coarse; initial rolling at low temperature and middle rolling at high temperature or initial rolling at high temperature and middle rolling at low temperature can all enhance strengthen of sheet after final rolling. When Mg-4Zn-3Cu and Mg-2Zn-3Cu-0.25Y are rolled at380℃, the microstructure of initial rolled and middle rolled sheet are fully recrystallized; the temperature of middle rolling number of precipitated phase increased, along with the size of recrystallized grain, indicating that initial and middle rolling at high temperature can improve elongation rate of sheet after final rolling substantially.(3) A relative high temperature is necessary when completely heavy reduction rolling is performed. With only completely heavy reduction rolling, Mg-2Zn-(0.1-0.15) Y sheet has low strengthen and plasticity at high temperature (300℃and350℃), which can be offset by adding more Y. Comparative speaking, Mg-4Zn-3Cu with the same process at the same high temperature(300℃and350℃) has low strengthen but high plasticity. Mg-2Zn-3Cu-0.25Y can achieve heavy reduction rolling only at around350℃but with overwhelmingly strengthen.(4) Mg-2Zn-(0.1-0.15)Y and Mg-2Zn-3Cu-0.25Y sheet with partial heavy reduction and completely heavy reduction system, showed a softening trend in the process of200℃aging, the tensile strength decreased, the elongation improved significantly. However, big increases of both strengthen and elongation rate can be detected in the Mg-4Zn-3Cu in process of200℃aging, with slight better as-annealed performance than AZ31B.(5) Compared with sheet processed by partial heavy reduction rolling, the Mg-2Zn-(0.1-0.15)Y and Mg-4Zn-3Cu with completely heavy reduction rolling exhibits relatively low anisotropy. Mg-2Zn-3Cu-0.25Y, processed by partial heavy reduction rolling, shows low anisotropy. The difference disappears in terms of anisotropy after the process of200℃aging.