AZ31B镁合金板材电致塑性渐进成形研究

发布时间：2018-06-16 19:57

本文选题：镁合金板材 + 电致塑性　；参考：《山东大学》2016年博士论文

【摘要】：作为21世纪的“朝阳金属”,镁合金具有低密度、高比强度／刚度、强阻尼、易回收等诸多优点,有着非常广阔的发展前景。近年来,结构轻量化和环境保护需求的加剧,更加促进了镁合金在航空航天、交通运输、电子通讯等制造业领域的推广应用。然而,镁合金晶格的密排六方结构导致其室温延展性较差,严重制约了镁合金的产业化发展。板材渐进成形作为一种柔性近净成形加工工艺,通过对板材逐次局部成形代替整体成形以提高板材成形性能,是当今先进制造技术的重要发展方向之一。作为高能量场在塑性成形中的应用,电致塑性效应能够显著提高难成形材料的成形性能,被广泛关注,但其电流密度阈值要求高,限制了其应用。单点渐进成形工艺中,工具头与板材接触面积小,能有效满足电致塑性高电流密度阈值的要求。电致塑性效应与渐进成形技术的结合,能够显著提高镁合金板材的成形性能和制件成形质量,推动镁合金成形工艺的发展,具有重要的理论意义与工程价值。本文以提高镁合金板材成形性能为目的,将电致塑性效应引入板材渐进成形工艺,提出镁合金板材电致塑性渐进成形技术,并对其成形工艺、成形机理以及电致塑性效应作用机制进行了实验研究与机理分析,主要工作和研究成果如下：(1)提出了镁合金板材电致塑性渐进成形方法,论证了其可行性与技术优势。基于理论分析,对成形系统进行自主设计研究,搭建了电致塑性渐进成形实验平台。针对具体工况,对成形过程摩擦机理进行实验研究与理论分析,研究结果表明,电致塑性渐进成形摩擦方式为载流混合摩擦,二硫化钼涂层是其较理想润滑剂,在实现稳定导电的前提下能起到良好的润滑作用。结合具体实验平台,对成形实施过程和成形质量控制进行了理论分析,为后续试验研究提供了理论指导。(2)基于响应曲面分析方法,研究了电致塑性渐进成形各关键工艺参数对板材成形性能的影响规律及显著性。研究结果表明,电脉冲参数对板材成形极限有着最显著的影响,随着电脉冲参数的增加,板材成形极限明显增大；进给速率和层间步进量均与成形极限响应值呈负相关关系,且显著性水平相近,工具头直径对板材成形极限无显著影响；电脉冲参数与进给速率／工具头直径、进给速率和层间步进量存在较显著的交互作用。基于正交试验,研究了电致塑性渐进成形工艺参数对制件成形几何精度和表面硬度的影响规律及显著性。研究结果表明,电脉冲参数是制件成形精度的最显著影响因素,随着电脉冲参数增加,制件几何精度明显提高；较低的进给速率、合适尺寸的中等层间步进量与工具头直径可明显减小制件成形高度误差,而较小的工具头直径可导致斜壁过多弹性变形积累,明显增大成形回弹角度；对于制件表面硬度,只有选择合适尺寸的中等工具头直径时可见其较明显提高,其他因素对其无显著影响。经综合分析,选取电脉冲参数、进给速率、层间步进量和工具头直径分别为80V/400Hz、800mm/min、0.2mm和8mm的参数组合时,板材成形性能和制件成形质量较好。相对室温不加电渐进成形,此工艺参数下的电致塑性渐进成形可大幅提高镁合金板材成形能力。(3)基于成形工艺研究结果,选择合理成形参数,开展了电致塑性渐进成形实验,结合理论分析,从成形过程温度、成形力、应变状态三方面对电致塑性渐进成形机理进行了研究。成形温度研究结果表明,成形板材定点温度由工具头实时位置与工具头实时温度决定,仅在该点处于当前成形区附近时温度较高,且高温阶段持续时间很短；成形温升速率受电脉冲参数、工具头直径、制件成形角度影响,与进给速率和层间步进量无关,与电流值呈正相关,与工具头尺寸和制件成形角度呈负相关。成形力研究表明,板材轴向成形力随加工深度增加,先逐渐增大,后逐渐趋于稳定,最后在电脉冲持续作用下呈现出降低趋势；横向成形力只在工具头运动轨迹的切向产生,无径向力存在,因此,横向力在X,Y方向的分力随工具头圆周运动过程呈正弦规律交替变化且总相差π/2相位；电脉冲参数、层间步进量以及成形角度是影响成形力的较显著因素,其次是进给速率,工具头直径对成形力影响不显著；最大轴向力与电脉冲参数变化呈负相关,与进给速率、层间步进量、工具头直径以及成形角度成正相关。通过试验手段获取了成形板材不同加载路径下的应变场分布以及极限应变点的应变历史,研究结果表明,渐进成形过程板材成形区应变历史为快速急剧变形过程,对于方锥台形件模型直线加载区域,其应变状态基本为平面应变,而圆锥台形件模型为圆周加载,其应变状态向双向拉伸形式发展；较优参数下的电致塑性渐进成形可使镁合金板材达到很高的成形极限。(4)基于控制电脉冲参数的渐进成形试验和单向拉伸试验,研究了不同电脉冲参数对镁合金电致塑性效应的影响规律,并结合理论模型,对电致塑性效应模式进行了分析。研究结果表明,镁合金板材的电致塑性效应主要取决于脉冲有效值电流密度,随着脉冲有效值电流密度的增加,板材渐进成形性能明显提高。对于电致塑性单拉变形,板材流动应力随脉冲有效值电流密度的增加逐渐降低,而板材的断裂延伸率在达到一定值后不再增加,甚至呈现出降低趋势,表明镁合金板材的电致塑性拉伸变形存在一个最优电脉冲参数范围。另外,脉冲有效值电流密度对电致塑性效应的决定作用,以及各工艺参数与电流密度的关系,是引发各工艺参数对电致塑性效应存在交互影响作用的原因。电致塑性效应存在非热效应,且其大小由脉冲峰值电流密度决定。在脉冲有效值电流密度相同或相近时,具有高峰值电流密度的电脉冲能产生更加显著的电致塑性效应。电致塑性存在阈值效应,当脉冲有效值电流密度低于阈值时,板材成形性能随脉冲有效值电流密度变化较小,电致塑性效应不明显,而当其高过阈值后,电致塑性效应会显著增加。(5)针对电致塑性渐进成形制件与电致塑性单拉试样,开展了微观组织研究,探讨了电脉冲提高镁合金板材成形性能的微观机理。研究结果表明,电脉冲可降低镁合金板材动态再结晶温度,加速动态再结晶过程,同时,可抑制断裂孔洞的产生,从而提高镁合金板材成形性能。电致塑性渐进成形过程快速加热、高温持续时间短的特点,抑制了晶粒长大,促进镁合金超细等轴晶粒的产生,为晶界滑移创造了先决条件,促使晶界滑移在较低温度下发生。电致塑性拉伸变形过程中,大参数电脉冲可引发缩颈变形区瞬间高温,使材料发生逆共晶反应(α+β=L),从而在材料内部变形区引发一定量液相的产生。合适数量的液相可优化材料变形机制,使材料延展性提高,但当液相数量过多时,可使晶界弱化,从而导致材料整体延展性的降低。
[Abstract]:As the "sunrise metal" in twenty-first Century, magnesium alloy has many advantages, such as low density, high specific strength / stiffness, strong damping, easy recovery and so on. It has a very broad prospects for development. In recent years, the demand for structure light weight and environmental protection is aggravated, and it has promoted magnesium alloy in aerospace, transportation, electronic communication and other manufacturing fields. However, the six square structure of the lattice of magnesium alloy leads to the poor ductility of the magnesium alloy, which seriously restricts the development of the magnesium alloy. As a flexible and near net forming process, the plate progressive forming is an advanced manufacturing technology by replacing the whole forming of sheet metal to improve the forming performance of the sheet. As one of the important development directions, as the application of high energy field in plastic forming, the electroplastic effect can significantly improve the formability of the refractory material, and it is widely concerned, but its current density threshold is high and its application is limited. In the single point progressive forming process, the contact area of the tool head and the plate is small, and the high electroplasticity can be effectively met. The requirement of the current density threshold. The combination of the electro plastic effect and the progressive forming technology can significantly improve the forming properties of the magnesium alloy sheet and the forming quality of the parts, and promote the development of the magnesium alloy forming process. It has important theoretical significance and engineering value. In this paper, the purpose of improving the forming performance of the magnesium alloy sheet is to make the electroplastic effect. The progressive forming process of sheet metal should be introduced, and the electroplastic progressive forming technology of magnesium alloy sheet is put forward, and its forming process, forming mechanism and the mechanism of electroplastic effect have been studied and analyzed. The main work and research results are as follows: (1) the method of progressive forming of magnesium alloy sheet is put forward. Feasibility and technical advantages. Based on theoretical analysis, the independent design of the forming system is carried out and an experimental platform for the progressive forming of electric plastic is set up. The experimental research and theoretical analysis of the friction mechanism of the forming process are carried out according to the specific working conditions. The results show that the friction mode of the progressive forming process is the mixed friction of the carrier flow and the molybdenum disulfide. The coating is an ideal lubricant, and it can play a good lubrication effect on the premise of stable conduction. Combining the concrete experimental platform, the forming implementation process and the forming quality control are analyzed theoretically. (2) based on the response curved surface analysis method, the electroplastic progressive forming is studied. The influence of the key process parameters on the sheet forming performance is found. The results show that the electric pulse parameters have the most significant influence on the sheet forming limit. With the increase of the electric pulse parameters, the forming limit of the sheet is obviously increased, and the feed rate and the interlayer step are all negatively related to the forming limit response value, and the significance is significant. The diameter of the tool head has no significant influence on the sheet forming limit, and there is a significant interaction between the electrical pulse parameters and the feed rate / tool head diameter, the feed rate and the interlayer step. Based on the orthogonal test, the influence of the technological parameters of the electroplastic progressive forming process on the geometric precision and the surface hardness of the parts is studied. The results show that the parameters of electric pulse are the most significant factors affecting the forming precision of parts. With the increase of the parameters of the electric pulse, the geometric precision of the workpiece is obviously improved. The lower feed rate, the suitable size of the middle step and the diameter of the tool head can obviously reduce the height error of the workpiece, and the smaller diameter of the tool head. It can lead to the accumulation of over elastic deformation of the inclined wall and obviously increase the springback angle of the forming. For the surface hardness of the workpiece, it is obvious that it is obviously improved when the diameter of the medium tool head is suitable for the selection of the suitable size, and the other factors have no significant influence on it. When the parameters of 80V/400Hz, 800mm/min, 0.2mm and 8mm are combined, the sheet forming properties and the forming quality of the parts are better. The forming ability of the magnesium alloy sheet can be greatly improved by the incremental forming of the electroplastic forming under this process. (3) based on the results of the forming process, the reasonable forming parameters are selected and the electroplasticity is carried out. The progressive forming mechanism of the forming process is studied from three aspects: the forming process temperature, the forming force and the strain state. The results of the forming temperature study show that the fixed point temperature of the formed sheet is determined by the real time position of the tool head and the real time temperature of the tool head, and the temperature is only at the time temperature near the current forming zone. The temperature rise rate is very short. The forming temperature rise rate is influenced by the parameters of electric pulse, the diameter of the tool head and the forming angle of the workpiece. It is not related to the feed rate and the interlayer step. It has a positive correlation with the current value. It has a negative correlation with the size of the tool head and the forming angle of the workpiece. In addition, it gradually increases, then gradually tends to stabilize, and finally shows a decreasing trend under the continuous action of electric pulse. The lateral forming force is only produced by the tangential direction of the tool head movement path, and there is no radial force. Therefore, the force of lateral force in the direction of X and Y is alternately changed with the sine law of the tool head circle movement and the difference of the phase of PI /2 is always different. The parameters of electric pulse, interlayer step and forming angle are the most significant factors affecting the forming force, followed by the feed rate, the diameter of the tool head has no significant influence on the forming force; the maximum axial force is negatively correlated with the change of the electrical pulse parameters, and is positively related to the feed rate, the interlayer step, the diameter of the tool head and the forming angle. The strain field distribution under different loading paths and the strain history of the limit strain point are obtained. The results show that the strain history of the sheet forming zone in the progressive forming process is a rapid and rapid deformation process. For the linear loading area of the square cone platform model, the strain state is basically plane strain, while the cone model is the model. For circumferential loading, the strain state of the alloy develops to the form of biaxial tension, and the electroplastic progressive forming under the superior parameters can make the magnesium alloy sheet reach a very high forming limit. (4) based on the progressive forming test and uniaxial tensile test, the influence of different pulse parameters on the electroplastic effect of magnesium alloys is studied. According to the theoretical model, the model of the electroplastic effect is analyzed. The results show that the electroplastic effect of the magnesium alloy sheet mainly depends on the current density of the pulse effective value. With the increase of the current density of the pulse effective value, the progressive forming performance of the plate is obviously higher. The increase of the current density of the pulse effective value gradually decreases, and the elongation at break of the plate no longer increases, or even shows a decreasing trend. It shows that there is an optimal electric pulse parameter range for the electroplastic tensile deformation of the magnesium alloy sheet. In addition, the effect of the current density of the pulse effective value on the electroplastic effect is determined. The relationship between the process parameters and the current density is the cause of the interaction of the process parameters on the electroplastic effect. The electroplastic effect has a non thermal effect and its size depends on the pulse peak current density. The electric pulse energy with the peak current density is the same or the phase of the pulse current density. There is a more significant electroplastic effect. There is a threshold effect on the electroplasticity. When the current density of the pulse effective value is lower than the threshold, the forming performance of the plate varies with the current density of the pulse effective value, and the electroplasticity effect is not obvious, and the electroplastic effect will increase significantly after the high threshold. (5) the progressive forming of the electroplastic forming process is made. The microstructural study was carried out and the microstructure of the magnesium alloy sheet was studied. The results showed that the electric pulse could reduce the dynamic recrystallization temperature of the magnesium alloy sheet and accelerate the dynamic recrystallization process. At the same time, it could inhibit the formation of the fracture cavity and thus improve the formation of magnesium alloy sheet. With the characteristics of rapid heating and short duration of high temperature, the characteristics of rapid heating and short duration of high temperature can inhibit grain growth and promote the production of superfine equiaxed grains in magnesium alloys. It creates a prerequisite for grain boundary slip and leads to the occurrence of grain boundary slip at a lower temperature. The high temperature in the shape area makes the material reverse the eutectic reaction (alpha + beta =L), which leads to the production of a certain amount of liquid phase in the internal deformation area of the material. The appropriate amount of liquid phase can optimize the deformation mechanism of the material and improve the ductility of the material, but when the amount of liquid phase is too large, the grain boundary can be weakened and the overall ductility of the material is reduced.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG306

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