导　　师： 周宏

学科专业： 080503

授予学位： 博士

作　　者： ;

机构地区： 吉林大学

摘　　要： 一些重要零件在使用中的服役条件恶劣,其寿命受多种失效形式共同制约。J如制动鼓和热轧辊,主要失效形式为磨损和热疲劳两种共同作用。在一场影响广泛的因灰铁制动鼓破裂而导致的碎片飞出伤人事故中,碎片脱离零件表面出就是热疲劳产生的裂纹和磨损带来的摩擦力共同作用的结果。因此,提高传统灰铁材料热疲劳磨损交互作用下的寿命十分必要。传统研究多为从单一因素出发,解决某一个问题。这使实验室的测试结果与实际工况的差距较大。由于目前实验条件所限,通过研究零件在热疲劳之后的磨损性能能够最大程度贴近实际情况。根据之前的研究,仿生耦合处理能够分别改善试样的抗热疲劳和耐磨损性能,所以仿生耦合处理在强化多因素失效条件下的试样寿命方面,依然有潜力进一步发挥作用。 本文根据耦合仿生学原理,通过模仿在相似环境下生存的贝壳表面耦合特征,使用激光熔凝和熔覆强化方式加工出多种耦合结构,观察了相应特征区域在热疲劳前后的组织变化特点,分析了各试样在疲劳前后磨损性能的变化和相应规律,并探讨了未强化试样和各仿生试样的磨损机理,总结了适宜强化热疲劳和磨损复合条件下零件的强化方法和单元体排列方式。 取得的主要结论如下： 1.通过对试样热疲劳前后磨损性能测试,发现热疲劳降低了灰铁的耐磨性,各试样磨损失重随着热疲劳循环次数逐渐增加。未处理的灰铁试样耐磨性随热疲劳循环次数呈指数形式迅速降低。仿生耦合强化能显著提升灰铁试样疲劳后的耐磨性。仿生熔凝强化试样在热疲劳前后的耐磨性均有显著提高并且其磨损失重随热疲劳循环次数呈线性增加,增幅比未处理灰铁试样明显减小 2.研究同时发现较高的热循环上限温度加速了试样磨损。未处理灰铁试样在上限为700℃热循环后磨损失重是上限500℃热循环后磨损失重的1.8倍。仿生熔凝强化试样在各上限温度热循环后耐磨性强于未处理的灰铁试样。并且其耐磨性随上限温度升高下降幅度较低,700℃热循环后磨损失重是上限500℃热循环后磨损失重的1.3倍。 3.通过熔覆不同粉末可以制得不同组织、不同性能的单元体。单元体性能提升能进一步提高试样疲劳后耐磨性。激光熔覆WC或Cr混合粉末可以获得高硬碳化物或无碳化物的固溶Cr单一基体。其中碳化物型单元体有包裹碳化物颗粒型及析出碳化物型。包裹碳化物颗粒的单元体随热疲劳磨损性能迅速降低,疲劳后试样的耐磨性不如仿生熔凝试样。析出碳化物形单元体试样疲劳前强化效果最高,耐磨性随热疲劳下降幅度有限,疲劳后耐磨性优于仿生熔凝试样。固溶Cr的单元体的疲劳前强化程度不如析出碳化物形单元体。但其随热疲劳循环耐磨性下降幅度最低,这种单元体的试样在多次热疲劳后依然能保持较高耐磨性。 4.单元体形状、排列角度及间隔能够影响试样上的裂纹扩展,进而对疲劳后的磨损产生影响。研究表明网格单元体的试样抗热疲劳性能最好,单元体排列紧密使试样裂化区域减少,当单元体与裂纹扩展方向垂直时,抑制裂纹扩展效果最明显。表面碎裂最轻微的仿生试样疲劳后磨损性能最好。 5.灰铁热疲劳后出现三种变化：表面开裂、硬度降低及发生氧化。这三种因素共同降低了未处理灰铁试样疲劳后的耐磨性。仿生熔凝单元体能够减少试样表面开裂区域、提高疲劳后硬度和减少表面氧化从而提高疲劳后耐磨性。通过调整材料耦元能进一步提高单元体疲劳前后的硬度,调整形状耦元及其特征量能进一步限制试样疲劳后裂纹扩展。两种耦元可以分别针对不同的方面使仿生试样热疲劳后耐磨性进一步提高。 Some important parts work in harsh conditions. Their life is limited by multiple factors. For example, the main failure forms of brake drums and hot work rolls are wear and thermal fatigue. In a fatal accident which caused by a fragment of broken gray iron brake drum, the detach of fragment part from drum is the result of friction form wear and crack from thermal fatigue. Therefore, it's very urgent to make a improvement on the service life of parts, which work under the multiple conditions of wear and fatigue. Most traditional research focus on a single performance by improve from one single factor. It can not solve the problem caused by multiple factors. And this also makes the gap between laboratory test results and the actual conditions larger. Due to the current experimental conditions, it is impossible to apply both wear and thermal fatigue tests simultaneously on a same sample at the same time. In the real working conditions of brake drum and rolls, the working process is the cycling of thermal fatigue→wear→thermal fatigue. So the research can get close to real conditions by study the wear performance after thermal fatigue. According to previous studies, bionic coupling processing can improve the thermal fatigue and wear resistance respectively. And this performance improvement is significantly. According to bionics coupling theory, bio-surface evolved through thousands of years to maximize their ability of adaption to a variety of harsh environments. So bionic coupling process is potential in the research of multi-factor performance enhancement. Based on the bionic coupling theory, this paper researched a bionic laser processing by imitating the shell which survive in similar environments. The shell surface is a construction of multiple coupling characteristics. A variety of coupling structure were processed by laser melting and cladding. The characteristics changes after thermal fatigue were observed. The wear properties and the mechanism before and after fatigue were analyzed. The suitable strengthen method and related parameters were presented. The main conclusions can be concluded as follows: 1. This study found that thermal fatigue increased wear loss of specimens. The wear weight loss of each specimen grow with increasing thermal fatigue cycles. The mass loss of untreated gray iron specimen increased a lot with thermal fatigue cycles. It raises in a form of exponential. Bionic coupling processing can significantly improve the wear resistance after thermal fatigue. Bionic laser remelting processing enhanced the wear resistance before and after thermal fatigue. And mass loss were increased in a form of linear. The wear resistance of bionic specimen drops slower than that of untreated one. 2. This study also found that higher temperatures accelerated the wear resistance drop after thermal fatigue. The mass loss of untreated gray iron specimen after700℃thermal fatigue is1.8times higher than that after500℃thermal fatigue. Bionic laser remelting process enhanced specimen wear resistance at every thermal fatigue on different temperature. And the wear resistance dropped slower with temperature. The mass loss after700℃thermal fatigue is1.3times of that after500℃thermal fatigue. 3. Different bionic units on performance can be processed by laser cladding different powder. By laser cladding mixed WC or Cr powder, hard phases of carbide or solid solute with Cr can be processed. The units with enhanced form of carbide includes carbide particles harden or carbide precipitation harden. The performance of the units with the harden form of carbide particles decreases rapidly with thermal fatigue cycles. Its wear resistance after fatigue is even lower than that of laser remelting unit. The wear resistance of carbide precipitation unit is the best of all others before thermal fatigue. It dropped moderate with thermal fatigue cycles. So it's better than laser remelting unit after thermal fatigue. The enhance amplitude of Cr solid solution unit is not obvious than that of carbide precipitation unit before thermal fatigue. But its wear resistance dropped slower with thermal fatigue. 4. The thermal cracks can be suppressed by adjusting the shape, alignment angle and arrangement interval of bionic units. It reduced the cracked area and the mass loss of bionic processed specimen after thermal fatigue. The studies have proved that the wear resistance can be enhanced by the unit arrangement of closely spaced and perpendicular to the direction of crack on2-dimensional plane. 5. There are three phenomenon found on the gray cast iron after thermal fatigue: surface cracking, hardness reduction and oxidation. These combined three factors reduce the wear resistance of untreated gray iron specimen after fatigue. Bionic laser remelting unit can reduced cracking area on specimen, improve the hardness after thermal fatigue and restrict surface oxidation. By adjusting coupling factor of material can improve the microhardness before and after thermal fatigue further. By adjust the bionic factors of shape and characteristics can limit the cracks on untreated areas in a further degree. The adjustments on two coupling factors above will improve the wear properties of the bionic specimen after thermal fatigue further.

关 键 词： 激光 仿生 灰铁 热疲劳后磨损

分 类 号： [TG143.2]

领　　域： [金属学及工艺] [一般工业技术] [金属学及工艺]