激光熔覆成形马氏体不锈钢应力演化及调控机制

发布时间：2019-05-29 19:25

【摘要】：残余应力问题是激光熔覆增材制造及再制造大规模商业化应用最亟待解决的问题之一。激光熔覆过程剧烈的温度场演化必然伴随应力、应变演化,导致零件存在高水平的残余应力,残余应力将影响零件的服役性能和安全,甚至使零件在制造过程中由于开裂和变形而报废。因此,研究激光熔覆过程中应力演化规律及机理,更好地实现残余应力水平及分布的调控,具有非常重要的科学意义和现实意义。本文系统研究了马氏体不锈钢激光熔覆过程中的结构、物性、应力演化,阐述了马氏体相变对应力场演变的影响规律及机理,探讨了通过调控相变来调整残余应力水平及分布的方法,研制了激光熔覆用可免热处理高强高韧铁基合金粉末,并将该粉末用于大型压缩机叶轮的激光熔覆再制造。研究了马氏体不锈钢激光熔覆成形过程中的物性演化,分析了固态相变对物性的影响。采用MTS810万能材料实验机、DIL801热膨胀仪、Setaram Setsys Evo同步热分析仪及耐驰LFA427激光闪射法热导仪,确定了马氏体不锈钢不同温度和物相下的物性参数,分析了固态相变对各参数的影响,具体参数包括屈服强度、弹性模量、塑性模量、膨胀系数、比热容、导热系数等。指出了相变温度和高温相化学元素含量是影响相变体积效应大小的关键因素。研究了奥氏体化条件、降温条件、载荷对马氏体不锈钢激光熔覆成形过程中固态相变的影响。采用L78 RITA淬火相变仪研究不同温度循环对相变动力学系数、相变点、相变后组织特点的影响,结果表明,在实验条件下,室温下材料均为马氏体组织,但不同温度历程会导致材料中有效元素含量不同、奥氏体相稳定性不同,进而影响固态相变点及相变动力学系数。采用Gleeble 3500研究不同外加载荷对马氏体相变点、相变动力学系数及相变塑性的影响,当外载小于屈服强度的时候,相变塑性可用Greenwood-Johoson机制解释。讨论了相变塑性及其对应力演化的影响。较大值的恒定外载条件下,相变塑性应变可远远超过体积效应应变,激光熔覆Fe-Cr-Ni-Mo-B-Si钢室温下为脆性材料,其拉伸实验表明,相变塑性可以使其获得高达30%的延伸率。激光熔覆过程中,刚开始发生固态相变的时候,相变塑性对应力演化影响显著,但随着相变过程的进行,应力水平降低,相变塑性的影响逐渐削弱。给出了马氏体不锈钢激光熔覆成形宏观模拟的温度场、应力场、固态相变处理方法,并给出了边界、初始条件及模型参数。考虑了应力对相变动力学系数及相变温度的影响,给出了相变体积效应及相变塑性的处理方法,依据Greenwood-Johoson相变塑性机制,给出了相变塑性参数。给出了混合相物性的处理方法,分别给出了高低温相的弹性模量、屈服强度、塑性模量、膨胀系数、比热容、热导率、密度等参数同温度及物相的关系。采用宏观模拟和实验结合的方法,研究了单道、多层多道激光熔覆过程中应力的演化规律。分别考虑了无固态相变、发生固态相变、不同温度固态相变、低温预热等多种情况下应力场的演化,结论认为:固态相变点较低的材料,固态相变对最终残余应力分布具有决定性的影响,固态相变将显著降低纵向残余拉应力,甚至导致残余压应力出现;相变完全的情况下,固态相变点越低,则残余拉应力越小,若出现压应力,则残余压应力越大;若材料在熔覆结束后整体发生固态相变,则熔覆部分及附近区域应力场分布均匀性较好,残余拉应力水平较低,此情况下,即便是固态相变点较高,固态相变对残余应力的主导作用也很显著。提出了一种表征激光熔覆成形材料残余拉应力积累水平的方法,拉应力积累系数R L0.2???-?。针对大型压缩机叶轮激光熔覆再制造的残余应力问题,讨论了影响马氏体相变点的因素,研制了激光熔覆用可免热处理高强高韧Fe-Cr-Ni-Mo-Mn-Nb合金粉末,在适当的工艺条件下,熔覆层内部残余应力水平较低,沉积态材料机械性能同FV520B锻件相当,该材料被用于大型压缩机叶轮的激光熔覆再制造。
[Abstract]:The problem of residual stress is one of the most urgent problems to be solved in the manufacture and re-manufacture of large-scale commercial application of laser cladding. The temperature field evolution of the laser cladding process is inevitably accompanied by stress and strain evolution, resulting in a high level of residual stress in the part, the residual stress will affect the service performance and safety of the part, and even the part is scrapped due to cracking and deformation during the manufacturing process. Therefore, it is of very important scientific and practical significance to study the law and mechanism of stress evolution during laser cladding, and to realize the control of residual stress level and distribution. The structure, physical property and stress evolution of the martensitic stainless steel laser cladding process are studied in this paper. The influence rule and mechanism of the martensitic transformation on the evolution of the stress field are described. The method of adjusting the residual stress level and distribution by controlling the phase change is discussed. The high-strength and high-toughness Fe-based alloy powder for laser cladding can be heat-treated, and the powder is used for laser cladding and remanufacturing of large-scale compressor impeller. The physical property evolution of the martensitic stainless steel laser cladding process is studied, and the effect of the solid phase transition on the physical properties is analyzed. By using the MTS810 universal material experimental machine, the DIL801 thermal expansion instrument, the Setaram Setsys Evo synchronous thermal analyzer and the NETZSCH LFA427 laser flash-shooting method, the physical parameters of the martensitic stainless steel at different temperatures and phases are determined, the influence of the solid-state phase change on the parameters is analyzed, the specific parameters include the yield strength, Elastic modulus, plastic modulus, expansion coefficient, specific heat capacity, thermal conductivity, and the like. It is pointed out that the content of the chemical elements in the phase-change temperature and the high-temperature phase is a key factor which influences the size of the phase-change volume. The effect of the austenitizing condition, the cooling condition and the load on the solid phase transition in the laser cladding of the martensitic stainless steel was studied. The effect of different temperature cycle on the phase change dynamics coefficient, phase change point and the microstructure of the phase change is studied by the L78 RITA quenching phase change instrument. The results show that under the experimental conditions, the material at room temperature is the martensite structure, but the different temperature course can lead to the difference of the effective element content in the material. The stability of the austenite phase is different, which further affects the solid-state phase-change point and the phase-change kinetic coefficient. The effect of different applied loads on the martensitic transformation point, phase change dynamics coefficient and phase change plasticity is studied by Gleeble 3500, and when the external load is less than the yield strength, the phase change plasticity can be explained by the Greenwood-Joson mechanism. The effect of phase change plasticity and its effect on the stress evolution is discussed. Under the condition of constant external load of the larger value, the plastic strain of the phase change can far exceed the volume effect strain, and the laser cladding Fe-Cr-Ni-Mo-B-Si steel is a brittle material at room temperature, and the tensile test shows that the phase-change plasticity can make it obtain the elongation of up to 30%. In the process of laser cladding, the effect of phase change plasticity on the stress evolution is significant when the solid-state phase change occurs, but with the process of phase change, the stress level is reduced, and the effect of the phase change plasticity is gradually weakened. In this paper, the temperature field, the stress field and the solid-state phase change processing method for the laser cladding of the martensitic stainless steel are given, and the boundary, the initial conditions and the model parameters are also given. In this paper, the effect of stress on the phase-change dynamics coefficient and the phase-change temperature is considered, the phase-change volume effect and the treatment method of the phase-change plasticity are given, and the phase-change plastic parameters are given according to the Greenwood-Joson phase-change plastic mechanism. The relationship between the elastic modulus, yield strength, plastic modulus, expansion coefficient, specific heat capacity, thermal conductivity and density of the high and low temperature phase is given. The evolution of stress in single-channel and multi-layer multi-channel laser cladding is studied by means of macro-simulation and experimental combination. The evolution of the stress field in many cases, such as the solid phase transition, the solid phase transition, the solid phase transition at different temperature and the low-temperature pre-heating, is considered. The results show that the solid phase transition has a decisive influence on the final residual stress distribution. The solid-state phase change significantly reduces the longitudinal residual tensile stress, and even leads to the occurrence of residual compressive stress; in the case of the complete phase change, the lower the solid phase transition point, the smaller the residual tensile stress, and if the compressive stress is present, the greater the residual compressive stress; and if the material undergoes a solid-state phase change as a whole after the end of the cladding, Then the distribution of the stress field in the cladding and the surrounding area is good, the residual tensile stress is low, in which case, even if the solid-state phase change point is high, the leading role of the solid-state phase change on the residual stress is also significant. A method for characterising the residual tensile stress accumulation level of a laser cladding forming material is presented. -?. In the light of the residual stress problem of the laser cladding and remanufacturing of the large-scale compressor, the factors influencing the martensitic transformation point are discussed, and the heat-treated high-toughness Fe-Cr-Ni-Mo-Mn-Nb alloy powder for laser cladding is developed. Under the appropriate process conditions, the residual stress level in the cladding layer is low. The mechanical property of the deposited material is equivalent to that of the FV520B forging, and the material is used for laser cladding and remanufacturing of the large-scale compressor impeller.
【学位授予单位】：哈尔滨工业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.71;TG665

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