【作者】 王传伟；

【导师】 周宏；

【作者基本信息】 吉林大学 ， 材料加工工程， 2014， 博士

【摘要】 随着我国铁路系统的不断提速，列车天线支架在剧烈变速时承受的应力应变相应提高，服役环境较提速前更为严苛，这对天线支架的强韧性能提出了更高的要求。目前，天线支架的使用材料和零件结构一般沿用2007年第6次提速前的设计标准。其材料一般为铁素体/珠光体型低碳钢，这类材料因良好的低温塑性、焊接性能等优点而在工程领域广泛使用，但其较低的强韧性能已无法满足列车提速后的运行需求。另一方面，如果对天线支架进行材料更换或重新进行结构设计，不仅涉及到一系列生产工艺、零件尺寸和安装位置的变动，而且对现有天线支架进行报废处理也必然造成巨大的资源浪费和经济损失。因此如何在不替换现用材料、不改变零部件结构的同时，以低能耗、低成本提高低碳钢的强韧性能已成为制约我国轨道交通事业发展的一个难题，也是工业工程领域目前普遍面临的关键性问题。科学发展的历史表明，科学技术的突破性进展往往从天然有机体对自然界的适应性规律中寻求答案。同样，低碳钢强韧化技术的研究在天然生物材料优异的强度、韧性的组合中得到了启示。本文依据仿生耦合理论，从工程仿生学实用角度出发，将树叶、昆虫翅鞘等具有良好强韧性能的生物体作为原型，设计了仿生耦合强韧化模型，并以高速动车天线支架用钢S355钢为试验材料，采用激光处理技术实现了仿生耦合试样的制备，为强韧化技术的研究开拓了新思路，并通过拉伸性能的变化系统研究了形态、材料耦元对仿生耦合强韧化效果的影响规律和作用机理。研究发现，采用激光熔凝技术对S355钢进行仿生耦合处理能够同时提高材料的强度与塑性。仿生耦合处理后，具有一定形态规律的激光处理区形成了仿生单元体，其显微组织较母材得到明显细化、位错密度提高了一个数量级。单元体熔凝区的显微组织为细化的板条马氏体，板条宽度介于300nm～600nm之间，平均显微硬度较母材提高了125%；热影响区由马氏体和未熔铁素体组成，平均硬度提高46%。仿生耦合处理的强化作用主要来源于单元体中的上述显微组织变化和拉伸过程中拉应力由基体向单元体的传递。由于单元体和基体之间的应力传递，在相同载荷下仿生试样基体中承受的拉应力要低于未处理试样，只有进一步提高外加载荷才能使其达到相应的临界应力，因而使仿生试样获得了更高的强度。仿生耦合处理的韧化作用在于单元体对切应力集中的阻碍，在拉伸过程中能够抑制颈缩处的形变收缩，使切应力分布更均匀，延长试样的均匀形变过程，从而提高了仿生试样的塑性形变能力，使其获得了更高的延伸率。形态耦元对仿生试样拉伸性能具有重要影响。同比之下，网格状仿生试样强韧化效果最好，屈服强度和抗拉强度较未处理试样分别提高14.4%和13.1%，且延伸率同时提高11.1%。降低单元体间距能够使网格状仿生试样的强韧化效果进一步提高，在本文考察范围内横向间距3mm、纵向间距5mm时仿生试样拉伸性能最佳：其对应仿生试样屈服强度、抗拉强度、延伸率分别较未处理试样提高了16.3%、15.4%和19.8%。在此基础上进一步降低单元体间距尽管能够提高试样的强度，但会导致仿生试样的塑性急剧下降。此外，条纹状单元体由于具有较强的方向性，当它沿不同方向排列分布时，仿生试样的拉伸性能差异显著：单元体沿拉伸方向排列，试样强化效果最好；随着单元体角度的提高，仿生试样的强度提高幅度降低，但延伸率提高幅度逐渐增加；当单元体完全垂直于拉伸方向时，仿生试样韧化效果最好，获得了最大的延伸率提高百分比。母体材料的马氏体含量、合金元素含量以及试样厚度均严重影响仿生耦合处理的强韧化效果，进而决定仿生试样拉伸性能提高百分比。随着马氏体含量的提高，单元体和基体之间应力传递作用减弱，造成仿生试样强度提高百分比的逐渐降低。然而母材马氏体含量越高，仿生耦合处理后韧化效果越明显，延伸率提高百分比持续增加，直至马氏体含量达到90%时，才略有下降。S355钢、45#钢、H13钢三种母材仿生耦合处理后强度和塑性均同时提高，且母材中碳元素和合金元素含量越高，单元体的硬度和强度也就越高，仿生试样的强化效果越好；然而45#钢和H13钢受热影响区未熔珠光体和粗大碳化物颗粒的影响，韧化效果较S355钢则有所下降，延伸率提高百分比相应降低。而对不同厚度的仿生试样拉伸性能的研究表明，对S355钢而言，母材厚度在3.0mm内时，利用单元体轮廓数学模型和双相结构体积分数法则计算的仿生试样屈服强度与实际测量平均值相差在10MPa以内。当母材厚度改变但其他参数不变时，可近似使用17.6%作为单元体体积百分比的临界值，在此范围内进行仿生耦合处理可获得较好的强韧化效果。通过激光熔丝技术制备组织和材料均不同于低碳钢母材的合金化单元体，有利于突破激光熔凝仿生试样的强度极限，进一步提高仿生耦合处理的强化效果；但激光熔丝仿生试样的延伸率相对于未处理试样均有所降低，这是因为合金化单元体的界面尽管具有较高的硬度和强度，但形变能力却难以满足要求，甚至出现了塑性较差的柱状晶组织，拉伸过程中界面易发生局部脱粘，影响了仿生耦合处理的韧化效果。为提高激光熔丝仿生试样的塑性，尝试了激光熔丝技术结合脉冲电流处理，不仅使试样获得了高于激光熔凝处理的强化效果，而且脉冲电流能够改善合金化单元体的界面组织和硬度分布，具有较好的韧化效果。在脉冲电流作用时间220ms时，激光熔丝仿生试样屈服强度和抗拉强度相对于未处理试样分别提高28.2%和25.9%，且延伸率同时提高6.9%。本文依据仿生耦合原理，利用激光技术制备的仿生耦合强韧化模型能够同时提高低碳钢材料的强度与塑性，为强韧化技术研究开拓了新思路，具有重要的科学意义和实用价值。同时，仿生耦合强韧化技术只是针对零部件表面进行局部处理而保持心部不变，不需改变零部件的使用材料和结构设计，因此具有广阔的应用前景，显著的经济价值和社会效益。更多还原

【Abstract】 With the acceleration of China’s railway system, antenna mounting brackets of thetrains suffer from the increased powerful stress and strain due to the rapid starting orsudden braking, and their service environment tends to be more intricate and harsh. Inthis condition, higher strength and toughness of the mounting bracket is needed to meetthe requirements of the accelerated trains. Currently, materials and component structureused in the mounting brackets follow the design standards in2007before the6thspeedup. Ferrite/pearlite low carbon steel is widely used in mechanical engineering andhas always been employed to produce the mounting brackets for its superior plasticityand good weldability. However, a major shortcoming of low carbon steel is its poorstrength and toughness performance, which places a limit on its service life in theaccelerated trains. On the other hand, replacing the materials or redesigning thecomponent structure not only leads to a series of changes in the production process butalso to the scrap of the present mounting bracket, which would cause huge waste ofresources and economic losses. Therefore, how to improve the strength and toughnessof low carbon steels but without changing the present materials and componentstructure has imposed restrictions on China’s railway transportation and has also been akey issue in the engineering field.The phylogeny of science indicated that many great progresses of science andtechnology came from inspirations of the adaptability of biology to the naturalenvironment. In the same way, strengthening and toughening technique of low carbonsteels has been enlightened by the unsurpassable advanced performance of biologicalmaterials. In this paper, inspired by some tree leaf and insect elytrum with excellentcombination of strength and toughness, biomimetic coupling model for low carbonsteels has been designed to improve the service life of mechanical components. S355steel, which is wildly used in the antenna bracket of High-speed EMU, was chosen asexperimental material and the biomimetic coupling model was applied in it by lasertechnique. Though systematic study on the tensile properties of the laser processed biomimetic samples, we discussed the influences of morphology and material couplingelements on the strengthening and toughening effects as well as the mechanism behindthem. Based on this work, we expected to provide valuable information for thestrengthening and toughening technique of low carbon steels.Results indicate that the laser biomimetic coupling method can improve thestrength and ductility of S355steel simultaneously. After the treatment, laser affectedarea with certain morphology makes up the biomimetic units. Microstructures of theunits are significantly refined than the matrix and the dislocation density increases by anorder of magnitude. Remelted zone is mainly composed of fine martensitic lath with itslath width ranging between300nm and600nm. Compared with that of the matrix, theaverage microhardness of Remelted Zone is increased by125%. Heat affected zoneconsists of martensite and incompletely dissolved ferrite, and its microhardness isincreased by46%in contrast to that of the matrix. Strengthening effect of thebiomimetic coupling treatment is attributed to the microstructure changes in the unitsand the stress transition from the substrate to the units. Because of the stress transfer inbiomimetic specimens, the units would carry higher tensile stress, and thus stressdistributed in the substrate is much lower than that in the untreated specimens with thesame external loading. Therefore, higher external tensile load will be applied when thesubstrate of the biomimetic specimens reaches its yield point and failure point. Thiscontributes significantly to the enhancement of strength, leading to the higher yieldstrength and tensile strength of the biomimetic specimens. Toughening effect of thebiomimetic coupling treatment rests on that the units have a beneficial effect onredistributing the shear stress throughout the specimen and resisting the initiation andgrowth of the necking. Due to this toughening effect, uniform deformation process isprolonged and the regions away from the neck could perform a larger strain, thusenhancing the elongation of biomimetic samples.Morphological coupling element has an important impact on the tensile propertiesof biomimetic samples. Compared comprehensively on the strength and ductility of thesamples, biomimetic specimens with gridding-shaped units exhibit the most desirable strengthening and toughening effects, generating a superior increment relative to theuntreated specimen in yield strength, tensile strength and elongation by14.4%,13.1%and11.1%, respectively. Meanwhile, it is found that the strengthening and tougheningeffects increase with the reduction of unit distance. Among all the samples investigatedin the present study, gridding units with lateral distance of3mm and longitudinaldistance of5mm contribute to the most integrated improvement in the tensile propertiesof biomimetic specimens, producing a development relative to the original material inyield strength, tensile strength and elongation by16.3%,15.4%and19.8%, respectively.And further decreasing the unit distance would result in a sharp deterioration of theductility. The tensile properties of striation biomimetic specimens are dependentcritically on the orientation of striation unit. When the units are aligned along the loaddirection (unit angle=0°), the biomimetic sample exhibits the greatest improvement instrength. As the unit angle rises, the strengthening effect is decreased while thetoughening effect increases. The maximum elongation is achieved in the sample with itsunits aligned at a90°angle to the load.Martensite content, chemical compositions and the thickness of the matrix have agreat influence on the strengthening and toughening effects of biomimetic couplingtreatment and thus determine the tensile property increments of biomimetic samples.With the increase of martensite content, stress transition between the substrate and unitsis diminished, resulting in the decrease of strength increment in biomimetic samples.However, toughening effect increases with the martensite content, and the elongationincrement of biomimetic samples enhances continuously until the martensite contentreaches90%volume fraction. Both the strength and ductility of S355steel,45#steeland H13steel are improved after the biomimetic coupling treatment. The hardness andstrength of the units increase with the carbon and alloy content of the matrix, andtherefore the strengthening effect of biomimetic specimen increases as well. But on theother hand, the strengthening effect depends to a great extent on the properties ofinterfacial bond between the unit and substrate. The incompletely dissolved pearlite(45#steel) and especially the coarse particles (H13steel) in the heat affected zone impair the toughening effect, and the elongation of their corresponding biomimeticspecimens is relatively lower than that of S355steel. Study on the tensile properties ofthe biomimetic specimens with different thickness indicates that the mathematicalmodel and volume fraction of the unit could be applied to calculate the yield strength ofbiomimetic coupling processed S355steel, and the calculation errors are less than10Mpa when the thickness of samples is below3mm. It is also found that17.6%could beapproximately considered as the threshold value of the units’ volume fraction when thethickness of samples varies but the other processing parameters are fixed. Under thisthreshold value, desirable strengthening and toughening effect could be obtained afterthe biomimetic coupling treatment.Laser wire process was applied to fabricate alloying units which possess differentmicrostructures and materials with the matrix of low carbon steels. The alloying unitsbreak the limit of strength increment in laser remelting processed samples and furtherimprove the strengthening effect of biomimetic coupling treatment. However, theelongation of laser wire processed samples is decreased and lower than their untreatedcounterpart. This is because that though the interface between alloying unit andsubstrate has considerable hardness and strength, its deformability is reduced by thecolumnar grains and could not meet the requirement of biomimetic specimens. It is alsonoticed that localized interfacial debonding occurs during the tensile deformationprocess, which impairs the toughening effect of biomimetic coupling treatment. In thiscondition, electropulsing treatment on the laser wire processed samples was attemptedto improve their ductility. After the electropulsing treatment, interfacial microstructuresand hardness distribution of the alloying units could be improved. And consequently thetoughening effect of laser wire processed samples is considerably enhanced after thetreatment. The electropulsing stimulation with220ms discharing duration generates asuperior increment of laser wire processed samples relative to the untreated specimen inyield strength, tensile strength and elongation by28.2%,25.9%and6.9%, respectively.The idea for improving the strength and ductility of low carbon steels bybiomimetic coupling method in this paper has provided an original research thought for solving strengthening and toughening problems of mechanical components which existwidely in the field of mechanical engineering. The studies on the tensile properties oflow carbon steels processed by laser biomimetic coupling treatment have academicsignificance and applicable value in engineering. Meanwhile, the application ofbiomimetic coupling technique can not only improve the strength and ductility of lowcarbon steels simultaneously, but also remain the present materials and originalcomponent structure of mechanical components. And therefore, the development of thebiomimetic coupling technique is of profound societal and remarkable economic value.更多还原