【作者】 刘毅；

【导师】 曹晓卿；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2012， 硕士

【摘要】 镁合金作为一种新型的轻质金属结构材料,具有很高的比强度、比刚度、导热性以及电磁屏蔽性等优良特性,其薄板构件在航空航天、交通运输、电子通讯等领域具有很大的应用潜力。然而,由于镁合金是密排六方结构(HCP)金属,镁合金板材在室温下的冲压性能差、成品率低,极大地限制了镁合金的应用。成形极限图(Forming Limit Diagram,简称FLD)是判断和评定板材成形性最为简便和直观的方法,且可用于预测和判断冲压工艺的成败,协助确定冲压工艺参数,是一种解决板材冲压问题的有效工具。近年来,成形极限图也被国内外学者广泛地用来评价镁合金板材的综合成形性能。但目前对镁合金成形极限图的研究主要通过试验进行,且对镁合金成形极限图的应用研究很少。因此本文通过试验结合有限元数值模拟的方法研究镁合金板材在热态下的成形极限图及其应用,对促进镁合金的进一步发展具有重要意义。本文采用北航研制的板材热成形通用试验机BCS50-AR以及网格应变自动测量系统GMASystem,在150～250℃温度范围内进行了AZ31镁合金的成形极限图试验,研究了成形温度、应力状态及凸模速度对镁合金成形性的影响；基于有限元软件Dynaform,模拟了AZ31镁合金板材在150～250℃温度范围内的成形极限图试验,研究了不同判断准则对确定仿真试验过程中极限应变的影响,并讨论了摩擦条件及应变速率等因素对成形极限图的影响；将试验确定的成形极限图导入Dynaform软件中,作为AZ31镁合金成形过程中的破裂判据,模拟了AZ31镁合金在相应温度下的筒形件拉深过程,并与直接用Dynaform软件中默认的Keeler’s公式作为判据时的模拟结果进行了比较,研究了成形极限图在板材成形数值模拟中的适用性。通过研究得到如下结论：(1)成形温度对AZ31镁合金成形极限图的影响很大,成形极限图中成形极限曲线的高度随成形温度的升高而提高。150℃时,凸模速度对AZ31镁合金成形性的影响较小,但受应力状态的影响很大；200～250℃时,凸模速度对AZ31镁合金成形性的影响增大,而应力状态对成形性的影响减小。(2)基于有限元软件Dynaform能较好地模拟AZ31镁合金在不同温度下的成形过程,通过采用综合判断方法,即根据两条判断准则：1、拉压区,试样缩颈时试样所受的载荷出现峰值；2、双拉区,试样缩颈时应力状态向平面应变路径突变,能较准确地确定AZ31镁合金仿真试验过程中的极限应变。摩擦条件对AZ31镁合金成形极限图的影响很大,成形极限图中成形极限曲线的高度随摩擦系数的升高而显著降低,且摩擦系数对成形极限图右半部分的影响更显著。应变速率对AZ31镁合金成形性的影响与成形温度有关,150℃下应变速率的影响要明显小于250℃,与不同温度下凸模速度对镁合金成形性影响的试验结果规律相同。(3)将试验获得的成形极限图作为Dynaform软件模拟时的破裂判据,比直接用Keeler’s公式作为判据能更准确地预测AZ31镁合金薄板在成形过程中的破裂问题。更多还原

【Abstract】 Magnesium alloys, light weight metallic structural materials with advantages of low density, high specific strength and specific stiffness, high thermal conductivity, excellent electromagnetic shielding and so on, their sheets will be comprehensively used in aviation and spaceflight, transportation vehicles, Electronic communications fields and so on. Due to the close-packed hexagonal structure of magnesium, the magnesium alloy sheets have poor formability under room temperature and low rate of finished products, which has restrict its application.Forming limit diagram (FLD) is the most simple and intuitive method to estimate and evaluate the formability of sheet metal, and also can be used to predict and judge the success or failure of stamping process, to assist for the determination of stamping process parameters. So it is a very effective tool to solve the problem of sheet metal stamping. In recent years, FLDs were used widely to evaluate the comprehensive properties of magnesium alloy sheets by domestic and foreign scholars, but most focus on experimental study and little investigation on their application. So the study of FLD of magnesium alloy sheet at elevated temperatures has important significance to promote its application.The hot sheet formability test machine and the grid measurement&analysis system which developed by BUAA were used to determine the FLD of AZ31magnesium alloy sheet at the forming temperature range of150～250℃ and the influence of forming temperature, stress state and strain rate on the formability of AZ31magnesium alloy were analyzed. The FLD experiments of AZ31magnesium alloy at the temperature range of150～250℃were simulated using DYNAFORM, and the influence of judging criterion on the acquisition of limit strain data as discussed. In order to research into the application of FLD in finite element simulation, the deep drawing processes were simulated using DYNAFORM with the FLDs and Keeler’s function as fracture criterion respectively in the forming temperature range of150～250℃.Following conclusions were obtained from the research:(1) The forming temperature has a great influence on the FLD of AZ31magnesium alloy, the height of FLD curve increases with the increase of forming temperature. The punch speed has little influence on the formability of AZ31magnesium alloy at150℃, yet the influence at the temperature between200℃to250℃is rather huge. The stress state has great influence on the formability of AZ31magnesium alloy at150℃, while it has less influence at the temperature range of200～250℃(2) The forming process of AZ31magnesium alloy can be predicted accurately using DYNAFORM at different temperatures, and the limit strain date can be determined more precisely by using the maximum load judging method and the strain path judging method in the process of simulation test. The effect of friction condition on the FLD of AZ31magnesium alloy is remarkable, the height of FLD curve decreases with the increase of friction coefficient. The influence of strain rate on the formability of AZ31magnesium alloy is related to forming temperature, it increases with the forming temperature increasing from150℃to250℃, which agrees well with the experimental results.(3) It can be predicted more precisely that whether the fracture will happen or not in the forming process by using the FLD as the fracture criterion than by auto Keeler’s fracture criterion when simulating with DYNAFORM.更多还原