【摘要】：殘余應力問題是激光熔覆增材制造及再制造大規(guī)模商業(yè)化應用最亟待解決的問題之一。激光熔覆過程劇烈的溫度場演化必然伴隨應力、應變演化,導致零件存在高水平的殘余應力,殘余應力將影響零件的服役性能和安全,甚至使零件在制造過程中由于開裂和變形而報廢。因此,研究激光熔覆過程中應力演化規(guī)律及機理,更好地實現(xiàn)殘余應力水平及分布的調(diào)控,具有非常重要的科學意義和現(xiàn)實意義。本文系統(tǒng)研究了馬氏體不銹鋼激光熔覆過程中的結構、物性、應力演化,闡述了馬氏體相變對應力場演變的影響規(guī)律及機理,探討了通過調(diào)控相變來調(diào)整殘余應力水平及分布的方法,研制了激光熔覆用可免熱處理高強高韌鐵基合金粉末,并將該粉末用于大型壓縮機葉輪的激光熔覆再制造。研究了馬氏體不銹鋼激光熔覆成形過程中的物性演化,分析了固態(tài)相變對物性的影響。采用MTS810萬能材料實驗機、DIL801熱膨脹儀、Setaram Setsys Evo同步熱分析儀及耐馳LFA427激光閃射法熱導儀,確定了馬氏體不銹鋼不同溫度和物相下的物性參數(shù),分析了固態(tài)相變對各參數(shù)的影響,具體參數(shù)包括屈服強度、彈性模量、塑性模量、膨脹系數(shù)、比熱容、導熱系數(shù)等。指出了相變溫度和高溫相化學元素含量是影響相變體積效應大小的關鍵因素。研究了奧氏體化條件、降溫條件、載荷對馬氏體不銹鋼激光熔覆成形過程中固態(tài)相變的影響。采用L78 RITA淬火相變儀研究不同溫度循環(huán)對相變動力學系數(shù)、相變點、相變后組織特點的影響,結果表明,在實驗條件下,室溫下材料均為馬氏體組織,但不同溫度歷程會導致材料中有效元素含量不同、奧氏體相穩(wěn)定性不同,進而影響固態(tài)相變點及相變動力學系數(shù)。采用Gleeble 3500研究不同外加載荷對馬氏體相變點、相變動力學系數(shù)及相變塑性的影響,當外載小于屈服強度的時候,相變塑性可用Greenwood-Johoson機制解釋。討論了相變塑性及其對應力演化的影響。較大值的恒定外載條件下,相變塑性應變可遠遠超過體積效應應變,激光熔覆Fe-Cr-Ni-Mo-B-Si鋼室溫下為脆性材料,其拉伸實驗表明,相變塑性可以使其獲得高達30%的延伸率。激光熔覆過程中,剛開始發(fā)生固態(tài)相變的時候,相變塑性對應力演化影響顯著,但隨著相變過程的進行,應力水平降低,相變塑性的影響逐漸削弱。給出了馬氏體不銹鋼激光熔覆成形宏觀模擬的溫度場、應力場、固態(tài)相變處理方法,并給出了邊界、初始條件及模型參數(shù)�？紤]了應力對相變動力學系數(shù)及相變溫度的影響,給出了相變體積效應及相變塑性的處理方法,依據(jù)Greenwood-Johoson相變塑性機制,給出了相變塑性參數(shù)。給出了混合相物性的處理方法,分別給出了高低溫相的彈性模量、屈服強度、塑性模量、膨脹系數(shù)、比熱容、熱導率、密度等參數(shù)同溫度及物相的關系。采用宏觀模擬和實驗結合的方法,研究了單道、多層多道激光熔覆過程中應力的演化規(guī)律。分別考慮了無固態(tài)相變、發(fā)生固態(tài)相變、不同溫度固態(tài)相變、低溫預熱等多種情況下應力場的演化,結論認為:固態(tài)相變點較低的材料,固態(tài)相變對最終殘余應力分布具有決定性的影響,固態(tài)相變將顯著降低縱向殘余拉應力,甚至導致殘余壓應力出現(xiàn);相變完全的情況下,固態(tài)相變點越低,則殘余拉應力越小,若出現(xiàn)壓應力,則殘余壓應力越大;若材料在熔覆結束后整體發(fā)生固態(tài)相變,則熔覆部分及附近區(qū)域應力場分布均勻性較好,殘余拉應力水平較低,此情況下,即便是固態(tài)相變點較高,固態(tài)相變對殘余應力的主導作用也很顯著。提出了一種表征激光熔覆成形材料殘余拉應力積累水平的方法,拉應力積累系數(shù)R L0.2???-?。針對大型壓縮機葉輪激光熔覆再制造的殘余應力問題,討論了影響馬氏體相變點的因素,研制了激光熔覆用可免熱處理高強高韌Fe-Cr-Ni-Mo-Mn-Nb合金粉末,在適當?shù)墓に嚄l件下,熔覆層內(nèi)部殘余應力水平較低,沉積態(tài)材料機械性能同F(xiàn)V520B鍛件相當,該材料被用于大型壓縮機葉輪的激光熔覆再制造。
[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.
【學位授予單位】：哈爾濱工業(yè)大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：TG142.71;TG665

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