汽车覆盖件用6451铝合金板材热变形行为研究

Hot Deformation Behavior of 6451 Alloy Sheet for Automotive Cover

通过Gleeble-3500热压缩模拟试验机对6451铝合金进行热压缩试验,分析了合金在不同温度(350~560℃)、不同应变速率(0.01~20 s^(−1))下平面热压缩流变应力与应变关系,建立了热加工图,确定了最佳热加工工艺窗口,并借助扫描电镜(SEM)及电子背散射衍射(EBSD)研究了其热变形机制。结果表明,6451铝合金流变应力呈正的应变速率敏感性和负的温度敏感性。在较低温度和低应变速率下,应变主要集中于心部,表面与心部变形差距大,易造成失稳。在350~480℃、1~20 s^(−1)及480~560℃、0.1~10 s^(−1)条件下,6451合金具有较好的热加工性能。当变形温度较低、应变速率较高时,6451合金的软化机制为动态回复;随温度升高、应变速率降低,小角度晶界比例减少,大角度晶界比例增大,亚晶尺寸增大,出现再结晶晶粒,合金的软化机制逐渐由动态回复转变为动态回复与动态再结晶共存。

The hot compression test was conducted on 6451 aluminum alloy by Gleeble-3500 thermal simulation test machine to investigate relationship between hot compression flow stress and strain of alloy under the deformation tem⁃peratures of 350~560℃and strain rate of 0.01~20 s^(−1).The processing map was established and optimal hot process window was determined.Furthermore,hot deformation mechanism was explored by scanning electron microscopy(SEM)equipped with electron backscatter diffraction(EBSD).The results indicate that the flow stress of 6451 alloy exhibits positive strain rate sensitivity and negative temperature sensitivity.Under lower temperature and strain rate,strain is concentrated at the center,and great difference between the center and surface leads to the flow instability.Under the condition of 1~20 s^(−1) and 350~480℃or 0.1~10 s^(−1) and 480~560℃,6451 alloy exhibits excellent hot pro⁃cessing performance.With lower deformation temperature and higher strain rate,the soft mechanism of 6451 alloy is characterized by dynamic recovery.With the increase of temperature and decrease of strain rate,the fraction of small angle grain boundary is decreased and that of large angle grain boundary is increased.Meanwhile,subcrystalline size is increased,and recrystallized grains can be observed,where the soft mechanism is converted from dynamic recovery to the co-exist of dynamic recovery and recrystallization.