【作者】 孟超；

【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】 热疲劳失效是热作模具主要失效形式之一。国产模具使用寿命较低，我国每年消耗热作模具近百亿元人民币。因此，提高热作模具的热疲劳性能，降低模具的制造成本,具有重要的经济意义。以往研究表明，通过激光仿生耦合技术模仿生物体表的耦合作用，已经对处理国内常用的3Cr2W8V热作模具钢的热疲劳性能起到了很好的效果。然而，激光仿生耦合技术中耦元及其特征参数均对处理材料的热疲劳性能产生不同影响，并且不同种类热作模具钢经激光仿生耦合处理后的热疲劳性能响应不同，同时热循环温度也是处理后材料产生不同程度热疲劳破坏的重要因素，因此需要对其影响机制进行更深入地探讨与揭示。因此，考察了形状耦元截面形态对激光仿生耦合试样热疲劳性能的影响；研究了在不同热循环温度下不同种类热作模具钢经激光仿生耦合处理后热疲劳性能的响应及热疲劳循环后母材与单元体的组织和硬度的变化规律；研究了材料耦元强化处理对仿生耦合试样热疲劳性能的影响；模拟了未处理试样热疲劳过程冷却后的应力场并结合热疲劳试验前后激光仿生耦合试样和未处理试样的拉伸性能，探讨了激光仿生耦合处理提高热疲劳性能的机理。主要研究结果如下：1.根据形状耦元截面形态的轮廓及尺寸定义了三种单元体截面形态，即单元体宽度与深度比大于4倍将其命名为―扁长‖形单元体、单元体截面轮廓类似于抛物线形将其命名为―U‖形单元体、单元体截面轮廓类似于三角形将其命名为―V‖形单元体。揭示出形状耦元截面形态对热疲劳性能的影响规律，即―U‖形单元体对热疲劳性能提高效果最佳，之后为―V‖形单元体和―扁长‖形单元体。2.不同热循环温度条件下，不同种类热作模具钢经激光仿生耦合处理后对热疲劳性能响应规律不同，即经激光仿生耦合处理后其热疲劳性能均比未处理试样有不同幅度提高；热循环上限温度为600℃时，激光仿生耦合处理HD钢具有最佳的热疲劳性能，其次为处理HHD钢、H13钢；热循环上限温度为650℃、700℃及750℃时，激光仿生耦合处理HHD钢具有最佳的热疲劳性能，其次为处理H13钢、HD钢。不同热循环温度条件下，形状耦元对提高热疲劳性能的影响为网状＞条状＞点状。3.揭示了不同热循环温度条件下，不同种类热作模具钢经激光仿生耦合处理后热疲劳性能产生不同响应的原因，即在热循环上限温度为600℃时热疲劳循环后HD钢母材及其单元体的硬度仍然最高，并且不同种类热作模具钢母材及单元体的微观组织较热疲劳试验前并无显著变化，因此，在热循环上限温度为600℃时激光仿生耦合处理HD钢具有最佳的热疲劳性能。在热循环上限温度为650℃、700℃、750℃时热疲劳循环后在不同种类热作模具钢中HHD钢母材及其单元体具有最佳的抗热循环软化能力及组织热稳定性，因此，在热循环上限温度为650℃、700℃、750℃时激光仿生耦合处理HHD钢具有最佳的热疲劳性能。4.揭示出材料耦元强化处理对仿生耦合试样热疲劳性能的影响规律a)激光仿生耦合试样再经脉冲电流处理后，单元体组织细化、硬度提高，并且激光未处理区域也发生相变，产生细小的隐晶马氏体组织。脉冲电流处理后提高了仿生耦合试样的整体性能，因此进一步提高了仿生耦合试样的热疲劳性能。b)激光熔覆(LC)处理与激光熔凝(LM)处理相比改变了单元体的组织和化学组成。热疲劳后LC单元体的抗热循环软化能力、组织热稳定性都优于LM单元体，因此，LC处理进一步改善了仿生耦合试样的热疲劳性能。5.揭示出激光仿生耦合处理提高材料热疲劳性能的机理a)材料经激光仿生耦合处理后组织和性能都要优于未处理材料，即通过个体强化机制与双相混合机制使处理后的材料展现出优异的抗热疲劳裂纹萌生能力。另一方面，形状耦元对热疲劳裂纹的扩展表现出不同程度的阻滞行为，降低了热疲劳裂纹的扩展速度，提高了处理表面的抗热疲劳裂纹扩展能力。b)当单元体形状相同时，由于热疲劳裂纹的萌生与扩展是在材料表层的一定深度范围内发生，并且热疲劳裂纹的深度随着热疲劳循环次数的增加而逐渐增加，因此不同单元体截面形态对热疲劳性能的提高幅度不同。―扁长‖形单元体在三种单元体截面形态中面积最小，且深度最浅，因此其对热疲劳性能提高幅度最小。―U‖形单元体截面面积大于―扁长‖形单元体，并且随着深度的增加―U‖形单元体的宽度要大于―V‖形单元体，因此―U‖形单元体具有最佳的热疲劳性能。更多还原

【Abstract】 Thermal fatigue damage is one of the major reasons which lead to the failure ofhot-work dies. There is consumption of hot-work die about10billion RMB in Chinadue to low using life of domestic dies. Therefore, improvement in thermal failureproperty and reduction in fabricating cost of hot-work die possesses economicsignificance. In previous studies, thermal fatigue property of3Cr2W8V steel, which isthe common material for hot-work die in China, has been improved effectively by laserbiomimetic coupling (LBC) technique which imitates the coupling effect of flora andfauna surfaces. However, for the investigation of thermal fatigue properties of hot-workdies treated by LBC technique, there are many coupling elements and characteristicparameters which can influence the thermal fatigue properties. Moreover, the responsesof thermal fatigue properties of different hot-work dies materials treated by LBCtechnique are different. On the other hand, the thermal cycles temperatures are alsoimportant for the thermal fatigue properties of materials treated by LBC technique.Thus, the influence mechanisms of these factors on the thermal fatigue properties needto be further explored and revealed.Therefore, the effects of morphologies of shapes coupling elements on the thermalfatigue properties of LBC samples were studied. During different thermal cyclestemperatures, the responses of thermal fatigue properties of different hot-work diessteels treated by LBC technique were investigated. Moreover, variations ofmicrostructures and microhardnesses for parent metal and units were researched afterthermal fatigue cycles. In addition, the influence of materials coupling elementsstrengthening treatment on the thermal fatigue property of LBC samples was studies.The stress field of untreated sample after cooling during thermal fatigue cycles wassimulated, and combined with the tensile property of LBC samples and untreated samples before and after thermal fatigue cycles to analyze the mechanism forimproving the thermal fatigue property of LBC sample. The main results indicate that:1. According to the contours and dimensions of morphologies of shapes couplingelements, three kinds of units morphologies were defined, i.e.―prolate‖morphology unit which the ratio of width and depth of unit was greater than4times and―U‖morphology unit which unit contour was similar to parabola, aswell as―V‖morphology unit which contour was similar to triangle. Theinfluence of morphologies of shapes coupling elements on the thermal fatigueproperties of LBC samples were revealed, i.e.―U‖morphology unit had the bestthermal fatigue property, followed by―V‖morphology unit and―prolate‖morphology unit.2. During different thermal cycles temperatures, the responses of thermal fatigueproperties of different hot-work dies steels treated by LBC technique weredifferent. Three kinds of hot-work die steels treated by LBC technique hadbetter thermal fatigue property than untreated samples during different thermalfatigue cycles temperatures. During the thermal cycles temperature at600℃, thethermal fatigue properties of LBC samples sorted HD＞HHD＞H13. During thethermal cycles temperatures at650℃,700℃and750℃, the thermal fatigueproperties of LBC samples sorted HHD＞H13＞HD. During different thermalcycles temperatures, the thermal fatigue behavior of LBC sample with griddingshape is the best, followed by striation shape and spot shape among the shapescoupling elements.3. During different thermal cycles temperatures, different responses of thermalfatigue properties of different hot-work dies steels treated by LBC techniquewere revealed. During the thermal cycles temperature at600℃, after thermalfatigue testing, the hardness of HD steel and unit of HD steel were still thehighest, moreover the microstructure of different kinds of hot-work dies steelsand units had no significant change. Therefore, during the thermal cyclestemperature at600℃, HD steel LBC sample had the best thermal fatigue property. During the thermal cycles temperatures at at650℃,700℃and750℃,after thermal fatigue testing, thermal cycling softening resistance andmicrostructure thermal stability of HHD steel and unit of HHD steel were thebest. Thus, during the thermal cycles temperatures at650℃,700℃and750℃,HHD steel LBC sample had the best thermal fatigue property.4. The influences of materials coupling elements strengthening treatment on thethermal fatigue property of biomimetic coupling samples were revealed.a) The microstructures of units of tempered or annealed H13steel were refinedby electropulsing. Cryptocrystalline martensite were obtained when H13steel was treated by electropulsing. The thermal fatigue property of LBCsample was improved further due to better overall performance after treatedby electropulsing.b) Comparing with laser melting (LM), the microstructures and chemicalcompositions of units were changed by laser cladding (LC). Microstructurethermal stability and thermal cycling softening resistance of LC unit werebetter than that of LM unit. LC biomimetic coupling sample was moreeffective to improve thermal fatigue behavior than LM biomimetic couplingsample.5. The influence mechanisms of improved thermal fatigue property of materialstreated by LBC technique were revealed.a) The materials treated by LBC technique had finer microstructures and bettercomprehensive properties than that of untreated materials. On the basis ofindividual strengthening of the units and double-phases compoundstrengthening, the thermal fatigue cracks initiation resistance was improved.On the other hand, shapes coupling elements had different effect on thethermal fatigue cracks propagation resistance which decrease thepropagation speed of thermal fatigue cracks and improve the thermal fatiguecracks propagation resistance.b) Owing to the thermal fatigue cracks initiate and propagate within a certaindepth of materials surface, as well as the depth of thermal fatigue cracks increases with the increasing thermal fatigue cycles, different unitsmorphologies had different degrees of improvement in thermal fatigueproperty.―Prolate‖morphology unit had the smallest area and lowest depthwhich lead to the worse thermal fatigue property among differentmorphologies units. The area of―U‖morphology unit was bigger than thatof―prolate‖morphology unit, and―V‖morphology unit is narrower than the―U‖morphology unit indepthdirection, therefore,―U‖morphology unit hadthe best thermal fatigue property.更多还原