AZ31B镁合金板料成形性能研究
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摘要
发展新型材料,降低能耗,保护环境,实现人类的可持续发展,是解决地球资源日趋贫化和环境日益恶化的主要渠道之一,已成为人们关注的主要问题。镁合金作为目前世界上可工程化应用最轻的金属结构材料,具有比强度、比刚度高,导热导电和电磁屏蔽性能优越,与环境相容性良好等优点,在汽车、通讯电子等领域有着广阔的应用前景。
镁合金具有密排六方晶体结构,常温下塑性变形能力差,但随着成形温度的升高,其塑性成形性能将发生根本性改变。利用镁合金板料温热冲压成形工艺不仅可以充分利用镁合金材料优异的性能、环保性以及满足产品薄壁化、轻量化的趋势,而且能够大幅度提高生产效率和产品合格率,因此研究开发镁合金板料冲压成形技术,已经逐渐成为近几年金属塑性加工领域研究的热点。
镁合金在中高温下的流变应力及其数学模型是热冲压数值模拟中不可缺少的信息,它综合反映了流变应力与应变、应变速率、温度以及其它因素之间的关系。针对目前可用的镁合金基本性能数据仍很缺乏的问题,本文开展了AZ31B镁合金板料在室温和中高温下的拉伸试验,获得了AZ31B镁合金板料在不同温度和应变速率下的流变规律。研究表明,AZ31B镁合金板料在室温下拉伸时,其流变曲线对变形速度不敏感,低温变形主要受热效应控制；在中高温拉伸变形中发生明显的软化现象,并且温度越高,应变速率越低,这种软化效应越明显,表现出明显的动态再结晶特征。本文在对高温流变应力变化规律分析的基础上,构建了两类流变应力数学模型,即修正的Fields-Backofen模型和幂为二次函数的指数模型。研究表明,在变形的前半段,两种模型的预测值与试验值均能很好地吻合,但Fields-Backofen修正模型不能反映峰值应力之后的软化效应。而指数形式模型可以很好地反映AZ31B镁合金材料在中高温变形过程中的应变硬化效应和应变软化效应,因而本文所提出的新模型—指数形式模型对于描述镁合金高温下的流变应力软化特征更为适合。
镁合金板料成形的主要失效控制因素不是应力而是极限应变。而极限应变一般通过胀形试验确定。本文进行了AZ31B镁合金板料在三个成形温度150℃、200℃、300℃下的半球形刚性凸模胀形试验,建立了对应温度下的成形极限图,分析获得了成形温度对成形极限图的影响规律。研究表明,AZ31B镁合金的成形极限图受温度影响比较显著,低于100℃时成形极限曲线基本汇集于一点,在150℃时可得到完整的成形极限图,但位置偏低；进一温度升高温度直到300℃,成形极限曲线稳定上升,胀形性能得到显著改善。针对试验制作成形极限图费时费力,且仅能确定几个有限温度点的成形极限图的问题,本文在对实测成形极限曲线变化规律分析的基础上,建立了AZ31B镁合金在不同温度下的成形极限数学模型,为AZ31B镁合金板料冲压成形破裂位置的预测及有限元模拟提供了依据。
极限拱顶高度是反映板料成形性能的一个重要参数,本文主要采用试验方法研究并分析了板料厚度、成形温度、成形速度、润滑条件及预成形等对AZ31B镁合金板料成形性能的影响规律,获得了镁合金板料胀形成形的最优条件,厚度为0.6mm的板料在温度为250℃,采用二硫化钼润滑,且先进行拱顶高度为10mm的预成形,保温时间1.0h时板料的成形性能最好,其极限拱顶高度达到42mm,板料发生完全再结晶。
在板料成形中摩擦是不可避免的,摩擦系数的确定是控制成形工艺及数值模拟计算的必要信息。本文将有限元模拟和试验研究相结合,提出了一种确定摩擦系数的新方法—有限元逆向确定法,分析获得了不同温度下半球形刚性凸模胀形试验过程中镁合金板料与冲头之间的摩擦系数,并结合成形极限图对模拟结果的合理性进行了验证。研究表明,有限元模拟结果与试验结果有着很好的一致性,从而为工程上确定材料接触面间的摩擦系数提供了一种简单、可行的新思路。
拉深成形是一种常见的板料冲压工艺,本文利用Swift拉深试验装置对镁合金板料进行了室温及加热状态下的拉深试验,获得了成形温度、压边力、模具间隙等工艺参数对AZ31B镁合金板料成形性能的影响规律,研究表明,AZ31B镁合金板料室温成形性能较差,在升高的温度下,其成形性能显著改善。AZ31B镁合金板料最佳成形温度范围在210℃-240℃之间。在较低温度下,模具间隙可取材料厚度的1.0～1.05倍,但在较高的温度下,取板料厚度的1.1倍的模具间隙可以得到性能更好的拉深件。利用经验公式计算的压边力并不适用AZ31B镁合金板料,研究表明,作用于试件上的实际压边力应控制在1kN以内。
板料成形过程中的回弹现象是影响成形件精度的主要因素。本文通过90°V形校正弯曲试验,探讨了成形温度、润滑条件、保压时间、最小弯曲半径等对AZ31B镁合金板料弯曲变形及其回弹的影响规律。研究表明,成形温度越高,镁合金的弯曲成形性能越好,其回弹量和最小弯曲半径越小。回弹角随保压时间的变化规律是前期迅速减少,当增加到2分钟左右时基本不再变化。在加热状态下,润滑剂的涂覆部位和有无润滑剂对镁合金的弯曲回弹影响很小。在常温下和加热条件下镁合金板料最小弯曲半径应大于2.0mm而小于8mm,,在较低的温度下,凸模圆角半径宜取的大一些,但温度较高时,可以取较小的临界值。
有限元数值模拟已经成为现代工业设计中主要的辅助分析手段。本文采用有限元法,研究了AZ31B镁合金板料的热成形规律,主要研究了虚拟冲压速度对计算精度和压边力、原始毛坯尺寸、凸模圆角半径对AZ31B镁合金板料拉深性能的影响规律。研究表明,在5000mm/s的虚拟冲压速度下得到系统的动能和沙漏能均远远低于系统内能,能够保证计算精度。但考虑到镁合金板料在实际成形时变形速度比较低,虚拟冲压速度取值不宜太高,建议虚拟冲压速度不要超过2000mm/s。压边方式和压边力大小的合理选取对板料成形起着关键性的作用,并且板料尺寸不同对压边力的敏感程度不同。对于小尺寸试件,由于板料过早地脱离了压边圈的控制,变形后期在径向压力作用下失稳起皱,并且初始压边力越小,起皱越厉害。对于大尺寸试件,压边力低于500N时,试件由于压边力不足而在凸缘区严重起皱,最后在筒形件直臂靠近凹模入口处破裂,压边力超过1000N时,试件在圆筒直臂区被拉裂,并且凸缘区也形成了均匀的折皱。对于本文研究的拉深条件,在良好压边情浣下,200℃时镁合金可拉深成形的最大毛坯尺寸为70mm。随着凸模圆角半径的增大,冲头行程提高,拉深性能得到显著改善。在凸模圆角半径从2mm到4mm变化时,冲头行程迅速提高,当凸模圆角半径超过4mm后冲头行程的增大幅度变缓,这说明AZ31B镁合金板料在200℃拉深时凸模圆角半径取值应在4mm-10mm之间。有限元数值模拟结果与热拉深试验结果基本吻合,表明了本文中测定的材料流变曲线、成形极限曲线及运用有限元逆向法所估算的摩擦系数等数据是合理的,可以较好地保证了有限元数值模拟结果的准确性和可靠性。
The development of new materials with the aim of reducing energy consumption, protecting environment and achieving sustainable human development is one of main ways to solve the depletion of natural resources and deterioration of environment, and has become a major concern of mankind. Magnesium alloys, as the lightest engineering metal material, have many advantages such as high specific strength and specific stiffness, excellent thermal conductivity, strong electromagnetic shielding and good environmental compatibility, and have broad application prospects in automotive, communications, electronics and other fields
Magnesium alloys show low ductility at room temperature because of their typical hexagonal close-packed crystal structure. However, the plastic formability of magnesium alloy would be significantly improved with the increase of temperature. The application of warm and hot stamping technology for magnesium alloy sheet can not only make full use of material's excellent performance, environmental performance and meet the need for thin and lightweight product, but also greatly improve production efficiency and product qualified rate. So the research and development of forming technology for magnesium alloy sheet has been a hotspot in recent years.
Flow stress and constitutive equation of magnesium alloy at high temperatures are indispensable conditions for numerical simulation of hot stamping, which reflect the relationship between flow stress and strain, strain rate, temperature and other factors. Since available basic performance data of magnesium alloy in the present is still very lack, in this paper, uniaxial tensile tests of AZ31B magnesium alloy sheet were conducted at room and high temperatures, true stress-true strain curves at different temperatures and strain rates were obtained. Studies show that the tensile flow curves of AZ31B magnesium alloy sheet at room temperature are not sensitive to deformation velocity and controlled by heating effect. An obvious softening phenomenon was observed during hot tensile deformation of AZ31B magnesium alloy sheet. And as the temperature increases and/or strain rate decreases, the softening effect is more notable, which shows that dynamic recrystallization occurs. Through the analysis of the true stress-strain curves, two mathematical models of flow stress were developed, namely the model based on the Fields-Backofen equation, and the model based on a natural exponential function whose exponent is a quadratic function. The research indicates that, at the first stage of the deformation, two kinds of model are in good agreement with the experimental value. But the model built based on Fields-Backofen equation is inaccurate to describe the softening behavior after the peak stress. While the exponential model can accurately reflect the work hardening and strain softening effect of AZ31B magnesium alloy sheet at high temperatures. Therefore, the new model built in this paper is more appropriate for the prediction of flow stress of AZ31B magnesium alloys at high temperatures.
Major causes of failure in the forming process of Magnesium alloy sheet is not stress but ultimate strain. The ultimate strain can be determined through bulge test. In this paper, the forming limit diagrams of AZ31B magnesium alloy sheet at the temperatures of150℃,200℃and300℃were determined experimentally by conducting hemispherical punch stretching tests. The effects of temperature on forming properties of AZ31B magnesium alloy sheets were discussed. The research indicates that, The forming limit diagrams of AZ31B magnesium alloy sheet are affected by temperatures. When temperature is below100℃, the FLC is almost concentrated to a point. A complete forming limit diagram can be obtained at150℃, but the location is low. When the temperature increases to300℃, the location of forming limit curve rise and the bulging performance is improved significantly. As we all know, experimental determination of forming limit diagram is a time-consuming and laborious work and only can obtain results at given temperature, the mathematical model of the forming limit diagrams for AZ31B magnesium alloy sheet at different temperatures was established on the basis of analysis of the experimental data. It can be used for predicting the locus of failure in stamping for AZ31B magnesium alloy sheet at high temperatures. It also provides an important safety judgment criterion in simulation of AZ31B magnesium alloy sheet metal forming processes.
Limit dome height is one of the important parameters reflecting the forming property of sheet metal. In this paper, the effect of blank thickness, forming temperature, forming speed, lubrication conditions on the forming properties of AZ31B magnesium alloy sheet was analyzed by experimental method and forming optimum conditions was obtained. That is to say, sheet metal with a thickness of0.6mm shows the best forming performance under the conditions of the temperature of250℃, with MoS2lubricating, a pre-forming of10mm dome height and heat holding time of1.0h. The limit dome height reaches42mm with complete recrystallization.
It is well known that friction is inevitable during the forming process of sheet metal and friction coefficient is necessary for controlling sheet metal forming process and numerical simulation. In this paper, a kind of new method for determining the friction coefficient was put forward by the combination of the finite element simulation and experiment. Friction coefficient between magnesium alloy sheet and the punch at different temperatures during hemispherical rigid punch bulging test process is estimated. The rationality of the simulation results are verified by forming limit diagram obtained by experiment. The research indicates that, simulation results are in good agreement with the experimental ones. and so it provides a simple, feasible new idea for the determination of the friction coefficient of the contact position between two surfaces in engineering.
Deep drawing is a common sheet metal stamping process. In this paper, drawing tests of AZ31B magnesium alloy sheet were conducted using Swift drawing experiment device at room and high temperature, the effect law of the process parameters such as forming temperature, blank-holder force, die clearance on the forming performance of AZ31B magnesium alloy sheet are obtained. The research indicates that, the forming property of AZ31B magnesium alloy sheet was poor at room temperature, but it is significantly improved at elevated temperatures. The preferred forming temperature was in the range of210℃-240℃. At lower temperature, die clearance desirable is1.0to1.05times thickness of the material. And good drawing parts can be obtained with gap1.1times thickness of the material at high temperature. The blank-holder force calculated using the empirical formula is not suitable for AZ31B magnesium alloy sheet. BHF acting on the specimen should be controlled less than1kN.
Springback is the main factor influencing the precision of parts during the forming process of sheet metal. The effect of forming temperature, lubrication condition, pressure holding time, the minimum bend radius on the deformation and springback of AZ31B magnesium alloy sheet was explored by V shape bending correction experiment with90°. The results indicate that the higher the temperature is, the better bending performance of magnesium alloy sheet is and the smaller springback and bending radius are. The springback angle decreases rapidly with the holding time in the initial stage, no longer changes when increased to2minutes. The effect of the lubricant on bending springback is less at heating condition. The minimum bending radius should be greater than2.0mm and less than8mm at room and high temperatures for AZ31B magnesium alloy sheet. At lower temperature, a large value is selected, but when the temperature is higher, a smaller critical value can be taken.
Finite element numerical simulation has become main aided analysis method in modern industrial design. Hot forming bahavior of AZ31B magnesium alloy sheet is investigated by finite element method. The effect of virtual stamping velocity on calculation precision and the effect of the blank-holder force, original blank size and punch fillet radius on the deep draw ing performance of AZ31B magnesium alloy sheet are discussed. The results indicate that the kinetic energy of the system and the hourglass energy are far lower than the internal energy of the system at a virtual stamping velocity of5000mm/s, which can ensure the precision of calculation. But considering the low actual deformation rate of magnesium alloy sheet, virtual stamping velocity values should not be too high and it is suggested to be less than2000mm/s. The reasonable selection of blank-holder device and blank-holder force plays a critical role in sheet metal forming. Different sizes of sheet metal have different reflect on the blank-holder force. For small size specimen, because of sheet metal prematurely from the blank holder, instability wrinkling phenomenon appears in the last stage of deformation, and the smaller initial blank-holder force is, the more powerful wrinkling is. For large size specimen, when the blank-holder force is less than500N, severe wrinkling in flange zone of the specimen happens due to lack of blank-holder force, finally results in fracture in the straight wall zone of cylindrical cup close to the entrance of the die. When the blank-holder force is more than1000N, the specimen is pulled crack in the cylinder straight wall zone, and a uniform wrinkle in flange zone is also formed. For deep drawing conditions in this paper under a good blank-holder status, the maximum blank size is70mm at200℃. With the increase of the punch fillet radius, the punch stroke increases, and deep drawability is significantly improved.When punch fillet radius changes from2mm to4mm, punch stroke increases rapidly. When punch fillet radius is larger than4mm, the punch stroke increases at decreased rate. This slows that punch fillet radius should be between4mm and10mm for AZ31B magnesium alloy sheet at200℃. The results of numerical simulation are basically consistent with that of thermal drawing experiment. It indicated that flow curves, forming limit curves and friction coefficient obtained by inverse finite element method were reasonable, which ensures the accuracy and reliability of numerical simulation results.
镁合金具有密排六方晶体结构,常温下塑性变形能力差,但随着成形温度的升高,其塑性成形性能将发生根本性改变。利用镁合金板料温热冲压成形工艺不仅可以充分利用镁合金材料优异的性能、环保性以及满足产品薄壁化、轻量化的趋势,而且能够大幅度提高生产效率和产品合格率,因此研究开发镁合金板料冲压成形技术,已经逐渐成为近几年金属塑性加工领域研究的热点。
镁合金在中高温下的流变应力及其数学模型是热冲压数值模拟中不可缺少的信息,它综合反映了流变应力与应变、应变速率、温度以及其它因素之间的关系。针对目前可用的镁合金基本性能数据仍很缺乏的问题,本文开展了AZ31B镁合金板料在室温和中高温下的拉伸试验,获得了AZ31B镁合金板料在不同温度和应变速率下的流变规律。研究表明,AZ31B镁合金板料在室温下拉伸时,其流变曲线对变形速度不敏感,低温变形主要受热效应控制；在中高温拉伸变形中发生明显的软化现象,并且温度越高,应变速率越低,这种软化效应越明显,表现出明显的动态再结晶特征。本文在对高温流变应力变化规律分析的基础上,构建了两类流变应力数学模型,即修正的Fields-Backofen模型和幂为二次函数的指数模型。研究表明,在变形的前半段,两种模型的预测值与试验值均能很好地吻合,但Fields-Backofen修正模型不能反映峰值应力之后的软化效应。而指数形式模型可以很好地反映AZ31B镁合金材料在中高温变形过程中的应变硬化效应和应变软化效应,因而本文所提出的新模型—指数形式模型对于描述镁合金高温下的流变应力软化特征更为适合。
镁合金板料成形的主要失效控制因素不是应力而是极限应变。而极限应变一般通过胀形试验确定。本文进行了AZ31B镁合金板料在三个成形温度150℃、200℃、300℃下的半球形刚性凸模胀形试验,建立了对应温度下的成形极限图,分析获得了成形温度对成形极限图的影响规律。研究表明,AZ31B镁合金的成形极限图受温度影响比较显著,低于100℃时成形极限曲线基本汇集于一点,在150℃时可得到完整的成形极限图,但位置偏低；进一温度升高温度直到300℃,成形极限曲线稳定上升,胀形性能得到显著改善。针对试验制作成形极限图费时费力,且仅能确定几个有限温度点的成形极限图的问题,本文在对实测成形极限曲线变化规律分析的基础上,建立了AZ31B镁合金在不同温度下的成形极限数学模型,为AZ31B镁合金板料冲压成形破裂位置的预测及有限元模拟提供了依据。
极限拱顶高度是反映板料成形性能的一个重要参数,本文主要采用试验方法研究并分析了板料厚度、成形温度、成形速度、润滑条件及预成形等对AZ31B镁合金板料成形性能的影响规律,获得了镁合金板料胀形成形的最优条件,厚度为0.6mm的板料在温度为250℃,采用二硫化钼润滑,且先进行拱顶高度为10mm的预成形,保温时间1.0h时板料的成形性能最好,其极限拱顶高度达到42mm,板料发生完全再结晶。
在板料成形中摩擦是不可避免的,摩擦系数的确定是控制成形工艺及数值模拟计算的必要信息。本文将有限元模拟和试验研究相结合,提出了一种确定摩擦系数的新方法—有限元逆向确定法,分析获得了不同温度下半球形刚性凸模胀形试验过程中镁合金板料与冲头之间的摩擦系数,并结合成形极限图对模拟结果的合理性进行了验证。研究表明,有限元模拟结果与试验结果有着很好的一致性,从而为工程上确定材料接触面间的摩擦系数提供了一种简单、可行的新思路。
拉深成形是一种常见的板料冲压工艺,本文利用Swift拉深试验装置对镁合金板料进行了室温及加热状态下的拉深试验,获得了成形温度、压边力、模具间隙等工艺参数对AZ31B镁合金板料成形性能的影响规律,研究表明,AZ31B镁合金板料室温成形性能较差,在升高的温度下,其成形性能显著改善。AZ31B镁合金板料最佳成形温度范围在210℃-240℃之间。在较低温度下,模具间隙可取材料厚度的1.0～1.05倍,但在较高的温度下,取板料厚度的1.1倍的模具间隙可以得到性能更好的拉深件。利用经验公式计算的压边力并不适用AZ31B镁合金板料,研究表明,作用于试件上的实际压边力应控制在1kN以内。
板料成形过程中的回弹现象是影响成形件精度的主要因素。本文通过90°V形校正弯曲试验,探讨了成形温度、润滑条件、保压时间、最小弯曲半径等对AZ31B镁合金板料弯曲变形及其回弹的影响规律。研究表明,成形温度越高,镁合金的弯曲成形性能越好,其回弹量和最小弯曲半径越小。回弹角随保压时间的变化规律是前期迅速减少,当增加到2分钟左右时基本不再变化。在加热状态下,润滑剂的涂覆部位和有无润滑剂对镁合金的弯曲回弹影响很小。在常温下和加热条件下镁合金板料最小弯曲半径应大于2.0mm而小于8mm,,在较低的温度下,凸模圆角半径宜取的大一些,但温度较高时,可以取较小的临界值。
有限元数值模拟已经成为现代工业设计中主要的辅助分析手段。本文采用有限元法,研究了AZ31B镁合金板料的热成形规律,主要研究了虚拟冲压速度对计算精度和压边力、原始毛坯尺寸、凸模圆角半径对AZ31B镁合金板料拉深性能的影响规律。研究表明,在5000mm/s的虚拟冲压速度下得到系统的动能和沙漏能均远远低于系统内能,能够保证计算精度。但考虑到镁合金板料在实际成形时变形速度比较低,虚拟冲压速度取值不宜太高,建议虚拟冲压速度不要超过2000mm/s。压边方式和压边力大小的合理选取对板料成形起着关键性的作用,并且板料尺寸不同对压边力的敏感程度不同。对于小尺寸试件,由于板料过早地脱离了压边圈的控制,变形后期在径向压力作用下失稳起皱,并且初始压边力越小,起皱越厉害。对于大尺寸试件,压边力低于500N时,试件由于压边力不足而在凸缘区严重起皱,最后在筒形件直臂靠近凹模入口处破裂,压边力超过1000N时,试件在圆筒直臂区被拉裂,并且凸缘区也形成了均匀的折皱。对于本文研究的拉深条件,在良好压边情浣下,200℃时镁合金可拉深成形的最大毛坯尺寸为70mm。随着凸模圆角半径的增大,冲头行程提高,拉深性能得到显著改善。在凸模圆角半径从2mm到4mm变化时,冲头行程迅速提高,当凸模圆角半径超过4mm后冲头行程的增大幅度变缓,这说明AZ31B镁合金板料在200℃拉深时凸模圆角半径取值应在4mm-10mm之间。有限元数值模拟结果与热拉深试验结果基本吻合,表明了本文中测定的材料流变曲线、成形极限曲线及运用有限元逆向法所估算的摩擦系数等数据是合理的,可以较好地保证了有限元数值模拟结果的准确性和可靠性。
The development of new materials with the aim of reducing energy consumption, protecting environment and achieving sustainable human development is one of main ways to solve the depletion of natural resources and deterioration of environment, and has become a major concern of mankind. Magnesium alloys, as the lightest engineering metal material, have many advantages such as high specific strength and specific stiffness, excellent thermal conductivity, strong electromagnetic shielding and good environmental compatibility, and have broad application prospects in automotive, communications, electronics and other fields
Magnesium alloys show low ductility at room temperature because of their typical hexagonal close-packed crystal structure. However, the plastic formability of magnesium alloy would be significantly improved with the increase of temperature. The application of warm and hot stamping technology for magnesium alloy sheet can not only make full use of material's excellent performance, environmental performance and meet the need for thin and lightweight product, but also greatly improve production efficiency and product qualified rate. So the research and development of forming technology for magnesium alloy sheet has been a hotspot in recent years.
Flow stress and constitutive equation of magnesium alloy at high temperatures are indispensable conditions for numerical simulation of hot stamping, which reflect the relationship between flow stress and strain, strain rate, temperature and other factors. Since available basic performance data of magnesium alloy in the present is still very lack, in this paper, uniaxial tensile tests of AZ31B magnesium alloy sheet were conducted at room and high temperatures, true stress-true strain curves at different temperatures and strain rates were obtained. Studies show that the tensile flow curves of AZ31B magnesium alloy sheet at room temperature are not sensitive to deformation velocity and controlled by heating effect. An obvious softening phenomenon was observed during hot tensile deformation of AZ31B magnesium alloy sheet. And as the temperature increases and/or strain rate decreases, the softening effect is more notable, which shows that dynamic recrystallization occurs. Through the analysis of the true stress-strain curves, two mathematical models of flow stress were developed, namely the model based on the Fields-Backofen equation, and the model based on a natural exponential function whose exponent is a quadratic function. The research indicates that, at the first stage of the deformation, two kinds of model are in good agreement with the experimental value. But the model built based on Fields-Backofen equation is inaccurate to describe the softening behavior after the peak stress. While the exponential model can accurately reflect the work hardening and strain softening effect of AZ31B magnesium alloy sheet at high temperatures. Therefore, the new model built in this paper is more appropriate for the prediction of flow stress of AZ31B magnesium alloys at high temperatures.
Major causes of failure in the forming process of Magnesium alloy sheet is not stress but ultimate strain. The ultimate strain can be determined through bulge test. In this paper, the forming limit diagrams of AZ31B magnesium alloy sheet at the temperatures of150℃,200℃and300℃were determined experimentally by conducting hemispherical punch stretching tests. The effects of temperature on forming properties of AZ31B magnesium alloy sheets were discussed. The research indicates that, The forming limit diagrams of AZ31B magnesium alloy sheet are affected by temperatures. When temperature is below100℃, the FLC is almost concentrated to a point. A complete forming limit diagram can be obtained at150℃, but the location is low. When the temperature increases to300℃, the location of forming limit curve rise and the bulging performance is improved significantly. As we all know, experimental determination of forming limit diagram is a time-consuming and laborious work and only can obtain results at given temperature, the mathematical model of the forming limit diagrams for AZ31B magnesium alloy sheet at different temperatures was established on the basis of analysis of the experimental data. It can be used for predicting the locus of failure in stamping for AZ31B magnesium alloy sheet at high temperatures. It also provides an important safety judgment criterion in simulation of AZ31B magnesium alloy sheet metal forming processes.
Limit dome height is one of the important parameters reflecting the forming property of sheet metal. In this paper, the effect of blank thickness, forming temperature, forming speed, lubrication conditions on the forming properties of AZ31B magnesium alloy sheet was analyzed by experimental method and forming optimum conditions was obtained. That is to say, sheet metal with a thickness of0.6mm shows the best forming performance under the conditions of the temperature of250℃, with MoS2lubricating, a pre-forming of10mm dome height and heat holding time of1.0h. The limit dome height reaches42mm with complete recrystallization.
It is well known that friction is inevitable during the forming process of sheet metal and friction coefficient is necessary for controlling sheet metal forming process and numerical simulation. In this paper, a kind of new method for determining the friction coefficient was put forward by the combination of the finite element simulation and experiment. Friction coefficient between magnesium alloy sheet and the punch at different temperatures during hemispherical rigid punch bulging test process is estimated. The rationality of the simulation results are verified by forming limit diagram obtained by experiment. The research indicates that, simulation results are in good agreement with the experimental ones. and so it provides a simple, feasible new idea for the determination of the friction coefficient of the contact position between two surfaces in engineering.
Deep drawing is a common sheet metal stamping process. In this paper, drawing tests of AZ31B magnesium alloy sheet were conducted using Swift drawing experiment device at room and high temperature, the effect law of the process parameters such as forming temperature, blank-holder force, die clearance on the forming performance of AZ31B magnesium alloy sheet are obtained. The research indicates that, the forming property of AZ31B magnesium alloy sheet was poor at room temperature, but it is significantly improved at elevated temperatures. The preferred forming temperature was in the range of210℃-240℃. At lower temperature, die clearance desirable is1.0to1.05times thickness of the material. And good drawing parts can be obtained with gap1.1times thickness of the material at high temperature. The blank-holder force calculated using the empirical formula is not suitable for AZ31B magnesium alloy sheet. BHF acting on the specimen should be controlled less than1kN.
Springback is the main factor influencing the precision of parts during the forming process of sheet metal. The effect of forming temperature, lubrication condition, pressure holding time, the minimum bend radius on the deformation and springback of AZ31B magnesium alloy sheet was explored by V shape bending correction experiment with90°. The results indicate that the higher the temperature is, the better bending performance of magnesium alloy sheet is and the smaller springback and bending radius are. The springback angle decreases rapidly with the holding time in the initial stage, no longer changes when increased to2minutes. The effect of the lubricant on bending springback is less at heating condition. The minimum bending radius should be greater than2.0mm and less than8mm at room and high temperatures for AZ31B magnesium alloy sheet. At lower temperature, a large value is selected, but when the temperature is higher, a smaller critical value can be taken.
Finite element numerical simulation has become main aided analysis method in modern industrial design. Hot forming bahavior of AZ31B magnesium alloy sheet is investigated by finite element method. The effect of virtual stamping velocity on calculation precision and the effect of the blank-holder force, original blank size and punch fillet radius on the deep draw ing performance of AZ31B magnesium alloy sheet are discussed. The results indicate that the kinetic energy of the system and the hourglass energy are far lower than the internal energy of the system at a virtual stamping velocity of5000mm/s, which can ensure the precision of calculation. But considering the low actual deformation rate of magnesium alloy sheet, virtual stamping velocity values should not be too high and it is suggested to be less than2000mm/s. The reasonable selection of blank-holder device and blank-holder force plays a critical role in sheet metal forming. Different sizes of sheet metal have different reflect on the blank-holder force. For small size specimen, because of sheet metal prematurely from the blank holder, instability wrinkling phenomenon appears in the last stage of deformation, and the smaller initial blank-holder force is, the more powerful wrinkling is. For large size specimen, when the blank-holder force is less than500N, severe wrinkling in flange zone of the specimen happens due to lack of blank-holder force, finally results in fracture in the straight wall zone of cylindrical cup close to the entrance of the die. When the blank-holder force is more than1000N, the specimen is pulled crack in the cylinder straight wall zone, and a uniform wrinkle in flange zone is also formed. For deep drawing conditions in this paper under a good blank-holder status, the maximum blank size is70mm at200℃. With the increase of the punch fillet radius, the punch stroke increases, and deep drawability is significantly improved.When punch fillet radius changes from2mm to4mm, punch stroke increases rapidly. When punch fillet radius is larger than4mm, the punch stroke increases at decreased rate. This slows that punch fillet radius should be between4mm and10mm for AZ31B magnesium alloy sheet at200℃. The results of numerical simulation are basically consistent with that of thermal drawing experiment. It indicated that flow curves, forming limit curves and friction coefficient obtained by inverse finite element method were reasonable, which ensures the accuracy and reliability of numerical simulation results.
引文
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