【作者】 马炳东；

【导师】 赵宇光；

【作者基本信息】 吉林大学 ， 材料加工工程， 2014， 博士

【摘要】 在脉冲电流的作用下，电能、热能和应变能被瞬时输入到金属材料中。在电-热-力多场耦合作用下，材料中将引发一系列瞬时非平衡动态过程及其伴生现象。深入研究这些非平衡过程、伴生现象、演变规律和作用机理，将进一步揭示金属材料的电致强化本质，探索与发展新的材料强韧化方法，对于大幅度提高金属材料的综合力学性能具有重大的理论意义和实用价值。高强度钢是现代工业中用量大、使用范围广的重要基础材料，深入挖掘其性能潜力是长久以来材料研究热点和永恒主题。本文采用瞬时高能脉冲电流对高强度钢的组织与力学性能进行了研究。同时，利用数值模拟方法模拟了在脉冲电流处理过程中，试样中各个部位的电流密度分布、温度场以及应力场状态，研究脉冲电流与其所形成的物理场对新相在母相中形成时影响。在脉冲电流处理下，硬化后的冷轧硼钢具有优异的综合力学性能。在最优参数处理下，其抗拉强度超过了2GPa，同时具有良好的延展性能。而且，其加工硬化能力也要好于模压硬化处理的硼钢。部件的高强度与韧性，将提高汽车的安全性能。由于脉冲电流处理的热的以及非热的效应，其极大地提高了相变奥氏体的形核率。而且，由于脉冲电流处理的时间非常短，以及通水铜电极的急速冷却作用，所以新形成奥氏体晶粒的长大并不容易。从而，硼钢在随后的淬火冷却过程中，形成了细小的板条马氏体组织。这种组织状态，极大地提高了硬化硼钢的综合力学性能。同时，在本实验条件下，脉冲电流促进冷轧硼钢再结晶而导致的其最终组织细化的效果，要好于脉冲电流循环热处理效应而导致的组织细化效果。数值模拟结果表明，在试样的弧形过渡处，电流密度分布较为集中。试样中部的温度最高，而其两个端部的温度最低，且在试样中部与端部的连接区域存在着梯度温度分布。随着脉冲电流处理时间的延长，这种温度梯度越来越大。试样中的应力场数值模拟表明，弧形过渡区域的应力集中最为明显，而试样中部和其两个端部的应力值较低。随着脉冲电流处理时间的延长，在试样中部与端部的连接区域，其应力梯度也越来越大。高电流密度脉冲电流的作用下，在冷轧硼钢样品端部与中部的连接区域，形成了局域的奥氏体纳米晶结构。这种奥氏体纳米晶结构是单相纯净的，并没有其它相与之共存。从数值模拟的结果看，在局域纳米晶结构被发现的地方，其具有高电流密度、较高温度以及高应力的特点。快速的温升只是提供了相变的温度条件，而其本身并不能引起材料中纳米晶结构的形成。从脉冲电流的特殊效应、奥氏体相变的特点以及实验结果中的较大尺寸纳米晶结构看，奥氏体晶粒发生再结晶过程而纳米化，形成局域奥氏体纳米晶结构的可能性是存在的。合金元素含量较多的区域以及快速冷却的特点，为局域奥氏体纳米晶结构能够保持到室温提供了热力学和动力学条件。同时，在试样连接区的电-热-力三场耦合作用，对其局域奥氏体纳米晶结构形成是有利的。从数值模拟的非均匀物理场分析结果看，当母相中形成一个高电导率的新相晶核时，体系中的电流密度分布、温度场以及应力场，将有利于其长大而不利于其分解。同时，当母相中形成一个低电导率的新相晶核时，体系中的电流密度分布、温度场以及应力场，将有利于其分解而不利于其长大。脉冲电流的回火处理提高了高碳钢锯条的力学性能，其齿部的硬度较大而锯条背部的硬度较低，在齿槽部位的硬度最低。齿部的高硬度，提高了锯条的切割能力，而锯条背部的较低硬度则提高了锯条的韧性。同时，在齿槽部位的塑性区域阻止了锯条的在此裂纹的萌生和扩展。因此，钢锯条的强度和韧性得到明显的改善。锯条的脉冲电流处理数值模拟表明，由于脉冲电流的绕流现象，齿槽部位较大的电流密度造成了较大的温升，以及由此而产生的较大应力。在齿槽部位的高电流密度和温度以及较大应力可引起此部位的电致塑性效应较为明显，裂纹的愈合效应更加有效，从而提高钢锯条的强韧性。调质态12.9级高强度螺栓的低温脉冲电流处理，可以提高其综合的力学性能，即抗拉强度和塑性得到同时改善。利用脉冲电流对螺栓进行回火处理时，螺栓的综合力学性能也得到了较大的提升。螺栓的脉冲电流低温处理数值模拟表明，由于电流的绕流现象，在螺纹根部的电流密度最大，而螺纹部位的电流密度较小，而螺栓中部的电流密度介于其间且分布均匀。在螺纹根部的温度较高，而齿尖位置温度最低，这样的温度分布可在提高螺纹的强度的同时提高螺栓整体的综合力学性能。同时，螺栓的脉冲电流回火处理数值模拟表明，在螺纹部位的应力值较大，而螺栓中部的应力值最低，这样的应力分布可提高螺纹以及整体螺栓抵抗疲劳破坏的能力。更多还原

【Abstract】 Under the treatment of electropulsing, the energy of electricity, heat and stress can beinputted into the metallic materials instantaneously. With the coupling effect of theelectricity-heat-stress multi-field, a series of instantaneous, non-equilibrium and dynamicprocesses and its associated phenomena can be brought about. In-depth study of thesenon-equilibrium processes, associated phenomena, evolution laws and the mechanism couldfurther reveal the essence of electro-strengthening on the metals, exploring and developingthe new material strengthening methods, and being significant theoretical and practical valueof greatly improving the mechanical properties of the metallic materials. The high-strengthsteels are widely used in the modern industrial in a large amount as the important basicmaterials. To dig their performance potential is the hotspot and eternal theme for the materialresearches for a long time.In this paper, the effects of transient high-energy electropulsing onthe microstructure and mechanical properties of high-strength steels were studied.Meanwhile, the numerical simulation was employed to analyse the current densitydistribution, temperature and stress field in the materials during the electropulsing treatment.The effects of electropulsing and the physical fields caused on the crystallographicorientation selection of the new nucleating phase and the decomposition of the differentphase pre-existing in the matrix were investigated as well.After the electropulsing treatment with the finest parameters, the cold-rolled22MnB5boron steel exhibites the excellent mechanical properties, reaching the tensile strength of2GPa with good ductility. Moreover, the hardening capacity of the electropulsing hardenedboron steel is also better than that of the steel hardened by the hot-stamping process. Thehigh strength and toughness of the parts could improve the safety performance of the carwhen suffering a collision accident. Due to the thermal and athermal effects of the pulse current, the austenite nucleation rate was improved greatly. For the extremely shortelectropulsing duration with the rapid cooling effect of the water-cooled copper electrodes,the size of the newly formed austenite grains could not be enlarged easily. Therefore,thesmall lath-martensite microstructure was obtained in the boron steel during the subsequentquenching process, which greatly improves the mechanical properties of the hardened boronsteel. The electropulsing cyclic treatment can cause the refinement effect in the boron steel.However, this refinement trend is limited. That is to say that the electropulsing cyclictreatment could not refine the grains infinitely. Moreover, under the same experimentalconditions, for the effect of the grain refinement, the cyclic electropulsing treatment isinferior to the electropulsing induced recrystallization in the cold-rolled boron steel.Numerical simulation results for the specimen of cyclic electropulsing treatment showed that,during the electropulsing treatment, for the cross-sectional area of the current channel at thearc-transition region decreases abruptly, some current will choose the closest distance to passthrough, resulting in the significantly increased current density at this location. Thetemperature field numerical simulation results indicates that in the middle part of thespecimen the average temperature is the highest, while the average temperature at the ends isthe lowest. At the arc-transition area between the middle and ends of the specimen, thetemperature distribution is gradiental, and with the extension of the electropulsing duration,the slope of this gradient is increasing. The results of the stress field numerical simulationshowed that the stress concentration at the arc-transition region is most obvious, while in themiddle and two ends of the specimen the stress value is small. In the same way, theelectropulsing duration is longer, the the stress gradient slope is bigger, which is resultingfrom the temperature gradient obviously.The local austenite nanocrystalline structure formed at the transition region between themiddle and ends of the specimen from the cold-rolled boron steel when subjected to thetransent electropulsong in the high current density and the conditios of rapid cooling. Thisaustenite nanocrystalline structure is pure in the single phase, and no other phases orimpurities coexisting. Numerical simulation results showed that in the location where thenanocrystalline structure was found, the current density, temperature and stress values are alllarger relatively. On the formation mechanism of the local austenite nanocrystalline structure,we believe that the rapid temperature rising and the lowered thermodynamic barrier are thetwo major reasons. Meanwhile, a high concentration of alloying elements into thenanocrystalline structure and the rapid cooling provide the thermodynamic and kineticconditions for keeping it to the room temperature. Judging from the numerical simulation results, the instant coupling of electricity, hot and stress exsit in this arc-transition region.Therefore, as to the formation of local austenitic nanocrystalline structure, the mechanism ofhigh temperature recrystallization has the certain rationality. After the analysis of thereported orientational nanocrystalline, we believe that the kinetics process is critical to theformation of nanocrystals. Through the numerical simulation of the current densitydistribution, temperature and stress field in the system of the matrix with the different phasecoexisting, it was provided data for studying the impact of kinetics on the process of thenanocrystals formation. From the numerical simulation results of the physical fields, it canbe concluded that, when there is a different phase with the high conductivity in the matrix,the current density distribution, temperature and stress field could be beneficial to itsgrowing up in the shape of columnar along the current direction and detriment of itsdecomposition. However, when there is a different phase with the low conductivity in thematrix, the physical fields could be beneficial to its decomposition instead of growing up.Electropulsing tempering treatment of the quenched high-carbon steel saw bladeresulted in that the mechanical properties of it was increased, the teeth has the highesthardness which is bigger than that at the back saw blade, and the hardness is the lowest at thetooth root. The high hardness in the tooth could improve the cutting performance, while thelow hardness in the back could increase the toughness of the saw blade. Meanwhile, theplastic area at the tooth root could prevent the initiation and propagation of cracks thereeffectively. Thus, the strength and toughness of the saw blade had been significantlyimproved. Simulation results show that due to the detour effect of the pulse current, thevalue of current density at the tooth root is the highest, resulting in the largest temperaturerising and consequently the biggest thermal stress there. The high current density,temperature and the stress at the tooth root could make electroplasticity there carry out moreeffectively, improving the effect of crack healing and thereby increasing the strength andtoughness of the saw blade. When the12.9-grade high-strength bolt in the state of quenchedand tempered subjected to the low-temperature electropulsing, the mechanical propertiescould be improved, having the high tensile strength and ductility simultaneously. Numericalsimulation results of the bolt subjecting to the electropulsing showed that due to the detoureffect of the pulse current, at the thread root the value of the current density is the highest,and that at the thread tip is the lowest, while inside the bolt the current density value issomehow between them uniformly. Thus, the highest temperature occurs at the thread root,and at the thread tip the temperature is the lowest. Such temperature distribution couldimprove the toughness of the bolt as well as the strength at the thread at the same time. Meanwhile, the stress value at the thread tip is larger relatively, and that is lower at thethread root, while inside the bolt the stress value is the lowest, which is in favor of thefatigue damage resistance of the thread and the whole bolt as well.更多还原