【作者】 郭彪；

【导师】 葛昌纯；

【作者基本信息】 西南交通大学 ， 材料学， 2012， 博士

【摘要】 粉末冶金技术具有低成本、近净成形等显著优点,已广泛用于生产发动机传动件、通用机械零件和工具等。粉末冶金材料的密度及其分布是影响粉末冶金制品力学性能和尺寸精度的关键因素,开发高密度高性能的粉末冶金制品已成为粉末冶金技术的发展方向和研究重点。粉末锻造技术结合了粉末冶金和精密锻造的优势,提高了传统粉末冶金多孔零件的力学性能,简化了精密锻造的制坯工艺,可实现高性能复杂形状零件的低成本、高效率生产。本文开展了粉末锻造用Fe-0.5C-2Cu烧结合金的制备及粉末锻造和烧结Fe-0.5C-2Cu合金的高温流变和致密化行为研究,建立了粉末锻造和烧结合金的高温流变本构模型,并运用有限元数值模拟技术研究了粉末烧结多孔材料的热变形致密化过程,优化了粉末锻造工艺。同时对材料力学性能与密度的关系,热处理工艺对粉末锻造材料力学性能的影响进行了研究：(1)采用有限元数值模拟技术研究了金属粉末的模压致密化成形机理,在此基础上通过机械混合,双向压制和保护气氛烧结工艺制备了密度为6.43g/cm3的低成本粉末锻造用Fe-0.5C-2Cu合金,评价了粉末烧结合金的力学性能,分析了粉末烧结合金的断裂行为。(2)粉末锻造和烧结Fe-0.5C-2Cu合金的高温热压缩实验表明,高致密的粉末锻造合金由于存在动态回复和动态再结晶软化,流变应力曲线出现明显的峰值应力。而粉末烧结合金在热变形过程中伴随着材料硬化和几何硬化两种硬化方式,随着变形量的增大,材料致密度提高,使得流变应力持续增加。在这一过程中粉末烧结合金的致密化效果同时受到变形速率、变形温度、变形量、初始密度和摩擦的影响,其中变形速率的影响尤为突出。(3)采用应变量耦合的Arrhenius双曲正弦模型和逐步回归模型分别建立了粉末锻造和烧结Fe-0.5C-2Cu合金的本构方程,两个模型对合金高温流变应力的预测平均绝对相对误差分别为3.07%和3.88%,均具有良好的预测能力。(4)分别将粉末锻造和烧结Fe-0.5C-2Cu合金的高温流变本构模型嵌入有限元分析软件模拟合金的恒温恒应变速率压缩过程,模拟结果与热压缩实测载荷—位移曲线吻合较好。粉末烧结合金热压缩致密化过程和密度分布的数值模拟结果与实测值基本一致,平均密度最大绝对相对误差为3.97%,且高应变速率下的预测精度(误差不大于0.53%)比低应变速率下更高。(5)通过粉末锻造过程有限元模拟优化粉末锻造工艺,成功锻造出密度为7.85g/cm3的粉末锻造试样,材料极限拉伸强度和屈服强度分别为932MPa和667MPa,硬度为HB294.4,延伸率为9%,断面收缩率为20%,冲击韧性为22.7J/cm2。对比密度对材料力学性能的影响表明,随着密度增加,材料极限拉伸强度、屈服强度和硬度近似线性增大,延伸率、断面收缩率成幂函数增大,而冲击韧性则随密度增加成指数增大。试样断裂表面随着材料密度提高,撕裂区增大,并在高致密区向穿晶断裂转变,形成包含韧性和脆性的混合断裂。当材料密度接近全致密时,断裂面全部转变为穿晶脆性断裂。(6)粉末锻造Fe-0.5C-2Cu合金不同热处理工艺下的力学性能测试表明,油冷+回火试样的冲击韧性、拉伸强度和硬度均高于空冷+回火试样,但塑性比后者低。试样经油冷和空冷后均在450℃回火时表现出较好的力学性能。更多还原

【Abstract】 Powder metallurgy technology with low-cost, near net shape and other significant advantages has been widely used for production of engine transmission parts, general machinery parts and tools, etc. Density and distribution of powder metallurgy materials is a key factor to affect the mechanical properties and dimensional accuracy of powder metallurgy products. The development of high density and high performance powder metallurgy products has become the powder metallurgy technology development direction and research priorities. Powder forging technology combining the advantages of powder metallurgy and precision forging, improving the mechanical properties of porous powder metallurgy parts, simplifying the process of precision forging billet system, can produce the high performance complex shape parts with high efficiency and low cost. The research contents are as follows:the preparation of Fe-0.5C-2Cu alloy for powder forging; the hot deformation and densification behavior of Fe-0.5C-2Cu alloy. The constitutive equations of flow stress of the powder forged and sintered alloy at elevated temperature were established. The hot deformation and densification process of sintered alloy with pores was investigated, and the process parameters of powder forging were optimized by the means of FENS (finite element numerical simulation). Finally, the relationship between mechanical properties and density of porous materials and the effects of heat treatment process on mechanical properties of powder forged steel were studied. The following main conclusions are obtained:(1) The densification mechanism of metal powder compaction process was studied by the means of FENS. The low cost Fe-0.5C-2Cu alloy samples with density of6.43g/cm3were prepared through mechanical mixing, double-action pressing and sinter in the protection atmosphere. The mechanical properties and fracture behavior of sintered alloy were evaluated and analysed.(2) The hot compressive tests of powder forged and sintered Fe-0.5C-2Cu alloy at elevated temperature were carried out. The results indicated that the peak stresses on flow stress curves of high-density powder forged steel are found due to the effects of dynamic recovery and dynamic recrystallization. While the flow stress of sintered alloy continue to increase in the hot deformation process with the increase of deformation extent and density because of the effects of both material hardening and geometry hardening.(3) Utilizing hyperbolic sine function and introducing the strain with nonlinear fitting, the modified Arrhenius equation of flow stress of powder forged Fe-0.5C-2Cu alloy at high temperature was established. Considering the impact of deformation on the flow behavior, the constitutive equations of flow stress of sintered Fe-0.5C-2Cu alloy were also established using stepwise regression method. The average absolute relative error between predicted results and experimental data of powder forged and sintered alloy are3.07%and3.88%respectively, which indicates that the two constitutive equations have good predication ability.(4) The constitutive equations of flow stress of powder forged and sintered Fe-0.5C-2Cu alloy were implanted into finite elements analysis software to simulate the constant temperature and strain rate compression process. The simulative load-stroke curves agree well with the experimental results. The simulative densification and density distribution of sintered alloy during the hot compression process is basically consistent with the testing results. The max relative error of average density is3.97%, while the accuracy of prediction under the high strain rate condition (no more than0.53%) is higher than that under the low strain rate condition.(5) The powder forged alloy samples with density of7.85g/cm3were prepared under the optimum process conditions by the means of FENS. The values of ultimate tensile strength (UTS), yield strength (YS), elongation (EL), Brinell hardness (HB), reduction in area (RA) and impact toughness (IT) are932MPa,667MPa,9%, HB294.4,20%and22.7J/cm2respectively. The relationships between mechanical properties and density indicated that the UTS, YS and HB increases linearly, and the EL and AR increases in power function, as well as the IT increases exponentially with increasing density respectively. Fracture took place in sintered necks of the material at the low density and the mode of fracture is pure ductile. With the increase of density, the fracture morphology is a combination of ductile rupture as well as brittle fracture from fully dense pearlitic grains. When the density close to full dense, the fracture surface is composed of transgranular cleavage and the mode of fracture is complete brittle.(6) The mechanical properties of powder forged Fe-0.5C-2Cu alloy after different heat treatment were tested. The test results indicate that the IT, UTS, YS and HB of the specimens after oil cooling and tempering is higher than that after air cooling and tempering, while the plasticity is lower than the latter. The specimens cooled in oil and air after tempering at450℃have good mechanical properties.更多还原