【作者】 丛大龙；

【导师】 任振安； 周宏；

【作者基本信息】 吉林大学 ， 材料加工工程， 2014， 博士

【摘要】 模具是现代工业中一种广泛应用于塑料产品和金属产品加工的重要工艺装备，由于生产效率高、产品质量优异和生产成本低廉，在现代经济发展中占有非常重要的地位。模具工业的发展水平已经成为衡量一个国家工业水平和产品制造能力的重要标志之一。我国模具生产的整体水平明显低于发达国家,国内模具精度低、复杂程度低、使用寿命低、生产周期长等不利因素是制约其发展的主要障碍。热作模具服役环境恶劣，在热应力和热应变作用下，极易发生热疲劳失效以及疲劳开裂。热疲劳是热作模具的主要失效形式之一，每年由此造成的损失达数亿至数十亿元，浪费严重。因此，修复失效模具，延长模具使用寿命，降低生产成本，节约资源一直是困扰生产者和研究者的技术难题。针对失效热作模具修复的研究一直在广泛开展，但是，目前关于热疲劳裂纹及开裂修复方法及相关设备开发的系统研究还不多见。因此，采用一种行之有效的方法，对细小热疲劳裂纹及时修复，对开裂等大尺寸裂纹修复后进行强化，使模具寿命提高，具有重要研究价值和经济效益。贝壳、植物叶片和蜻蜓翅膀等生物体具有优异的止裂和抗疲劳功能。研究发现他们的结构具有相似特点，即在软质母体上分布有不同形状的硬质单元，两者彼此交替形成软硬相间的耦合结构。通过对这些生物结构的分析、简化和模仿，设计出了适合热作模具裂纹修复及强化的仿生耦合模型。本文的研究目的是将激光修复技术与仿生耦合理论相结合，用于热作模具疲劳裂纹及开裂的修复及强化，改善其抗热疲劳性能，提高模具使用寿命。以疲劳态H13钢为研究对象，利用激光仿生耦合熔凝技术制备了修复止裂单元体，分析了他们的个体特征、耦合形状、分布密度和大小对修复后试样热疲劳性能的影响；在此基础上，采用激光合金化技术强化单元体，进一步提高其止裂和抗疲劳性能，并且探讨了相关机理；研究了激光堆焊修复模具开裂后的仿生耦合强化问题，考察了强化后试样的抗拉强度和热疲劳性能；此外，本研究还制造了一套激光仿生耦合修复系统，用于模具表面热疲劳裂纹的修复，并应用于实际模具的现场试验。结果表明：1.依据贝壳、植物叶片和蜻蜓翅膀的耦合结构，提出的修复止裂仿生模型适合热作模具的裂纹修复和强化。在产生微小热疲劳裂纹的H13钢基体上，采用激光熔凝技术制备呈一定形状分布的修复止裂仿生单元体。单元体组织高度细化，主要由马氏体和残余奥氏体组成，平均显微硬度为510～682HV，具有良好强韧性。单元体成形过程中部分裂纹得到愈合，实现修复的目的，成形后的单元体能够阻碍裂纹扩展，裂纹扩展至单元体受阻后，往往会发生偏折甚至停滞，避免了裂纹对基体的进一步破坏，实现止裂的目的。在一定范围内，扩展受限的残余裂纹有利于裂纹扩展驱动力的释放，有利于抗热疲劳性能的提高。2.单元体形状、分布密度和大小对修复后试样的热疲劳性能有重要影响。通过对比点状、条纹状和网格状单元体修复试样的抗热疲劳性能发现，网格状单元体能够对裂纹形成“封闭效应”，因而具有最佳的止裂抗疲劳能力。单元体高密度分布有利于裂纹修复比例的提高以及减小残余裂纹的扩展空间。体积较大、强度较高的单元体对裂纹阻滞作用更为显著。热疲劳对单元体组织和力学性能产生不利影响，主要表现为组织粗化，碳化物颗粒逐渐析出，显微硬度下降。当组织变化趋于稳定后，单元体硬度仍明显高于基体，可以作为强化结构继续存在，贡献于试样的抗热疲劳性能。3.通过激光合金化Co50和Fe30A自熔性合金粉末改善单元体的化学成分和组织结构，单元体性能进一步提高。其中，Co50合金强化单元体具有最佳的抗氧化性和抗热疲劳性能。4.激光堆焊修复大尺寸裂纹后焊缝热影响区是热疲劳裂纹萌生和扩展的敏感区域。分布于焊缝表面的强化单元体能够抵抗远场拉应力作用延缓焊缝开裂，同时阻碍裂纹扩展。激光仿生耦合强化处理能够显著提高焊缝的抗拉强度和抗热疲劳性能。5.开发了基于6自由度工业机器人和300W光纤传输激光器的激光仿生耦合柔性加工系统，具备了三维空间加工、参数切换和加工工艺程序化等功能，能够满足实际生产中模具结构多样的修复需求。6.对激光熔凝修复参数进行优化，考察了不同参数对单元体尺寸、硬度和表面粗糙度的影响。在本文所选参数范围内，随着平均峰值功率密度的增加，单元体深度增加，硬度下降，表面粗糙度增大；频率的增大或速度的减小造成有效峰值功率密度的增加，单元体深度略有增加，硬度下降，表面粗糙度下降。结合单元体深度、平均硬度和表面粗糙度规划出了适合制备性能优异单元体的参数选择图。7.修复了H13钢铝压铸热作模具表面的热疲劳裂纹。实际生产验证表明，修复止裂单元体在修复裂纹的同时有效阻碍了裂纹的扩展，模具寿命得到提高。更多还原

【Abstract】 Moulds are important equipments and have been widely applied in plastic products and metalproducts in modern industry. Owing to the high efficiency, superior product quality and lower costof production, moulds have occupied an extremely important position in the development ofmodern economic. The level of molding tool industry has now already become one of the importantsigns of national industrial level and product manufacturing capability. The overall level ofmanufacturing moulds in our country falls far behind the advanced countries due to someunfavorable factors such as low accuracy, low complexity, low service life and long productioncycle, which limit its development. Hot working dies works in very bad conditions such as thermalstress and thermal strain which usually induces thermal fatigue failure and cracking. Because ofthermal fatigue failure, which is one of most important failure modes, hot working die steels inChina waste RMB of about several billion yuan and produce a great loss. Therefore, repairingfailure dies, prolonging their service lives, reducing costs and saving resources have beenlongstanding technical problems to producers and researchers. Researchers had been dedicating tothe studies of repairing thermal fatigue failure dies.Therefore,taking an efficient method to repairtiny thermal fatigue cracks in time and strengthen the repaired cracking have important academicvalues and good economic benefits.Some organisms such as shell, plant leaf and dragonfly wing have superior function ofresisting fatigue and cracking. It was found that they have similar structures, that is, there are hardunits with different shapes distributing in the soft base materials. The hard units and soft basematerials form an alternate soft and hard coupling structure. Biomimetic coupling modes weredesigned for the repairing and strengthening of thermal fatigue cracks by analyzing, simplifying andimitating these structures. The paper is aiming to repair failure dies for improving the thermalfatigue resistance and service life by combining laser repair technology and biomimetic couplingtheory. In this paper, the crack repairing and arresting units were fabricated by biomimetic laserremelting, and then the influences of their individual characteristics, distribution shape, size anddistribution density on the thermal fatigue resistance of repaired samples were analyzed on fatigueH13die steel. On that basis, the study of strengthening units fabricated by laser alloying wasconducted to improving the ability of resisting thermal and cracking, moreover the relativemechanism of action was discussed. The cracking of dies was repaired by laser repair with wire andthen the weld was strengthened by laser biomimetic coupling treatment. The tensile strength and thermal fatigue resistance of samples after repair and strengthening were investigated. In addition, alaser biomimetic coupling repair system was manufactured for the repair of thermal fatigue crackson the actual die and the production testing was carried out. The results indicated that:1. The biomimetic models of crack repairing and arresting constructed from couplingstructure of shell, plant leaf and dragonfly wing are accepted for the repair and strengthening of hotworking die. Biomimetic crack repairing and arresting units were fabricated on the fatigue H13diesteel with tiny thermal fatigue cracks. The units possess high strength and toughness because ofmicrostructure transformation of martensite and retained austenite as well as grain refinement. Theaverage microhardness of units is510～682HV. In the process of units formation, partial crackswere repaired. The formed units are able to resist the propagation of cracks and protect the basematerials from further destroy. When cracks propagate and encounter biomimetic units, they dipdeflection and even stop. The residual cracks on the substrate have a beneficial effect on releasingthe driving force of crack propagation in a specific range which improves the thermal fatigueresistance of samples.2. The distribution shape, distribution density and size of units have important effects on thethermal fatigue resistance of repaired samples. In this paper, it was found that, by contrasting thethermal fatigue resistance of repaired samples with spot unit, stration unit and lattice unit, the unitsdistributing in the substrate with lattice shape could generate “closed effect” and thus possess thebest ability of resisting thermal and cracking. The distribution of units with high density means thatmore cracks are repaired and the propagation space of residual cracks are restrained; units withlarge size and high strength have better blocking ability. Thermal fatigue process causes the declineof microstructure and mechanical property of units which manifests as grain coursing, carbideparticles precipitation and hardness decrease. When the microstructure became stable, the hardnessof units was still higher than that of the base materials and the units can still act as strengtheningstructure to contribute to the improvement of thermal fatigue resistance.3. The microstructure and chemical components of units strengthened by laser alloying withself-fluxing alloy powders (Co50and Fe30A) are further enhanced. And of all alloying samples, theones treated by Co50has the best oxidation resistance and thermal fatigue resistance.4. When the cracking of a die was repaired by laser welding with wire, the weld heat affectedzone is always a risk area for thermal fatigue cracks initiation and propagation. The tensile strengthand thermal fatigue resistance of the weld strengthened by laser biomimetic coupling treatment canbe significantly improved. The strengthening units distributing on the weld surface are able to resisttensile stress in the far field and delay the cracking of weld, in the meantime, the cracks startingfrom heat affected zone can be hindered by them.5. On the basis of industrial robots with6degrees of freedom and the300W Nd：YAG laser with optical fiber transmission, a set of laser biomimetic coupling flexible manufacture system wasdeveloped which can be used for the repair of a complex die. The system has functions ofprocessing in three-dimensional space, switching parameters and programmed technology.6. The optimization of laser remelting parameters was conducted. Effects of laser processingparameters on the size, hardness and surface roughness of units have been investigated. In the rangeof this paper, there is a steep increase in both the depth and surface roughness of units and adecrease in microhardness when the average peak power density uplifts; The increased effectivepeak power density caused by raising pulse frequency or decreasing scanning speed leads to a slightincrease in unit depth and a gradual drop in microhardness and surface roughness. Based on areasonable range of unit depth, microhardness and surface roughness, a parameter selection map forlaser biomimetic coupling repair was established.7. Actual failure dies for aluminum casting production were repaired and the manufacturingverification test demonstrated the effectiveness of the laser biomimetic coupling repair units inrepairing thermal fatigue cracks of a casting die and resisting their propagation as well as improvingthe die service life.更多还原