电流对金属合金力学性能的影响及机理研究

发布时间：2018-04-20 05:25

本文选题：金属成形 + 电辅助成形　；参考：《大连理工大学》2016年博士论文

【摘要】：在金属材料的塑性成形过程中通入电流,能够显著地提高金属材料的延伸率,降低流动应力。根据这一特点,开发出的电辅助成形技术以高效节能的特点逐渐受到工业界和学术界的重视,具有广阔的发展前景。电流对金属材料性能的影响主要由两方面要素引起：一方面是电流产生的焦耳热影响材料的热力学性能；另一方面则是电流本身在不考虑温度作用的情况下亦能够引起材料应力下降和伸长率提高。后者被称为纯电致塑性效应,对该方向的研究在相关领域内存在争议,部分学者认为并没有电致塑性效应的存在,仅仅是温度本身的作用引起材料力学性能发生改变。为了更加深入地研究电流对金属材料的影响,本文建立了一套新型电辅助拉伸测试系统,提出了一种新的对比试验方法,研究电流对金属材料的焦耳热效应和纯电致塑性效应的影响。本文完成的主要研究工作包括：(1)设计开展了连续电流作用下的电辅助拉伸试验,研究了电流密度、晶粒尺寸和流动应力之间的影响关系。和相同温度下的热辅助拉伸试验结果相比,电流诱导下的金属材料产生更低的应力,并且流动应力随着电流密度的增加而进一步降低。通过对不同晶粒尺寸的铜合金进行对比试验,发现晶粒尺寸越小,流动应力越高,电流引起的流动应力幅值下降越大；对于铝合金的试验结果表明,随着电流密度的增加,通电引起的应变率敏感指数值由负值变为正值并持续上升,并且电流引起的应变率敏感指数值显著高于温度引起的指数值。同时,将电流对铝合金塑性失稳效应(PLC效应)的影响和相同温度下的应力曲线比较,可以得出电流有助于消除铝合金的锯齿流变的结论。(2)设计开展了脉冲电流作用下的电辅助拉伸试验,研究了相同温度峰值下不同脉冲电流密度和流动应力之间的关系。研究结果表明,脉冲电流产生的峰值温度引起铜合金产生应力回复;当峰值温度高于600℃时,应力回复值与初始屈服应力值接近。铝合金的试验结果揭示,在产生相同温度的条件下,随着脉冲电流密度的增加,虽然最大应力值不会产生较大波动,但是脉冲电流会引起瞬时应力幅值将下降,并且伸长率显著增加。这说明了电流对金属材料有额外的作用。(3)建立了电辅助成形中适用于多种材料的热效应模型。考虑连续电流作用的焦耳热效应,建立与材料熔点相关的热软化参数和归一化电流密度之间的关系。根据适用于多种材料的热软化模型,得到不同材料的电流密度敏感系数,并有效解释了金属材料成形中的电流密度阈值。此外,建立了脉冲电流中金属材料应力幅值瞬时下降的预测模型,综合考虑电流产生的焦耳热效应和材料本身的应变强化作用,模拟整个拉伸区间的应力幅值下降,与试验结果吻合。(4)揭示了金属材料在连续电流和脉冲电流中的微观组织演变规律,包括晶粒尺寸、晶界效应、动态再结晶和空洞长大等。高电流密度下的连续电流拉伸试验结果显示,晶界区域出现空洞和熔融现象,这种现象是由电流产生的不均匀焦耳热和应变诱导的晶界熔化产生。铜合金在拉伸过程中发生动态再结晶,并在断裂区域出现疑似等轴铸态组织和粗大树枝态组织。此外,电流有助于抑制铝合金锯齿流变的产生,是由电流在沉积物附近团聚并促进沉积物的扩散引起的。(5)最后,本文提出了一种基于新型表面微纹理制造技术的电辅助微轧制成形工艺。该工艺将电流和微轧制技术结合起来,快速实现金属表面微结构的制造。通过试验和仿真分析对比研究了通电和未通电工艺下的成形力和轧制形状。结果表明,采用电辅助微轧制成形技术得到的铝合金表面成形深度高。通过单搭接头的剪切强度试验发现,含有微轧制表面的试样粘接性能显著强于未经处理的试样。
[Abstract]:In the plastic forming process of metal materials, the penetration of current can significantly increase the elongation of metal materials and reduce the flow stress. According to this characteristic, the developed electric auxiliary forming technology is gradually paid attention to in the industry and the academia with the characteristics of high efficiency and energy saving. It has a broad development prospect. The response is mainly caused by two factors: one is that the Joule heat generated by the current affects the thermodynamic properties of the material; on the other hand, the current itself can cause the decrease of the material stress and the increase of the elongation at the same time without considering the temperature. The latter is called the pure electric plastic effect, and the research on this direction is in the related field. In dispute, some scholars believe that there is no electroplastic effect, only the effect of temperature itself causes the mechanical properties of materials to change. In order to further study the influence of current on metal materials, a new type of electrical auxiliary tensile test system was established, and a new comparative test method was put forward to study electricity. The influence of the Joule heat effect and the pure electroplastic effect on the metal material is studied. The main research work completed in this paper includes: (1) the electric auxiliary tensile test under the action of continuous current is designed and carried out. The relationship between the current density, the grain size and the flow stress is studied. Compared with the metal material induced by current, the metal material produces lower stress and the flow stress is further reduced with the increase of current density. By comparing the copper alloys with different grain sizes, the smaller the grain size, the higher the flow stress, the greater the flow stress amplitude of the current, and the test knot for the aluminum alloy. The results show that, with the increase of current density, the strain rate sensitivity index value changes from negative to positive and continues to rise with the increase of current density, and the strain rate sensitivity index caused by current is significantly higher than that caused by temperature. At the same time, the effect of current on the plastic instability effect (PLC effect) of aluminum alloy and the stress curve at the same temperature are compared. The conclusion is that the current can help eliminate the sawtooth rheology of the aluminum alloy. (2) the electric auxiliary tensile test under the pulse current is designed and the relationship between the different pulse current density and the flow stress at the same temperature peak is studied. The results show that the peak temperature produced by the pulse current causes the stress of the copper alloy. When the peak temperature is higher than 600 C, the stress recovery value is close to the initial yield stress value. The experimental results of aluminum alloy show that the maximum stress value will not produce large fluctuation with the increase of the pulse current density at the same temperature, but the pulse current will cause the instantaneous stress amplitude to decrease, and the elongation will be extended. This shows a significant increase in the effect of current on metal materials. (3) a thermal effect model suitable for various materials in electric assisted forming is established. Considering the Joule heat effect of continuous current, the relationship between the heat softening parameters related to the melting point of material and the normalized current density is established. The current density sensitivity coefficient of different materials is obtained, and the current density threshold in metal forming is explained effectively. In addition, the prediction model of the instantaneous drop of stress amplitude in metal material in pulse current is established, and the Joule heat effect produced by the current and the strain strengthening effect of the material itself are taken into consideration, and the whole drawing is simulated. The stress amplitude of the interval decreases with the experimental results. (4) the microstructure evolution of the metal material in continuous current and pulse current is revealed, including grain size, grain boundary effect, dynamic recrystallization and cavity growth. The results of continuous current tensile test under high current density show that the grain boundary area appears cavity and melting phenomenon. This phenomenon is produced by the melting of the grain boundary caused by the uneven Joule heat and strain induced by the current. The dynamic recrystallization of the copper alloy during the stretching process and the appearance of the doubtful equal axis cast structure and the coarse branch structure in the fracture region. In addition, the current helps to suppress the formation of the aluminum alloy saw tooth flow, and the current is near the sediments. (5) finally, an electric assisted micro rolling process based on a new surface micro texture manufacturing technology is proposed. This process combines the current and the micro rolling technology to quickly manufacture the micro structure of the metal surface. Through the experiment and simulation analysis, the electricity and the unelectricity are compared and studied. The forming force and rolling shape under the process show that the surface forming depth of the aluminum alloy is high. It is found that the bonding property of the sample containing the micro rolling surface is better than that of the untreated sample.

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG113.25

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