【作者】 陆宇衡；

【导师】 肖志瑜；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2014， 博士

【摘要】 铁基粉末冶金材料和零件是粉末冶金工业的主导产品，发展高性能低成本铁基粉末冶金材料是粉末冶金的研究重点之一。针对铁基烧结材料低合金化和低成本化的发展趋势，本文采用低成本的锰合金元素替代贵重的Ni、Mo等元素，优化制备出高性能Fe-2Cu-0.5Mn-0.9C烧结材料。研究了材料的力学性能、压制和烧结行为，探讨了锰对材料制备、性能及烧结过程的影响，研究了材料的超高周超声疲劳行为，为含锰铁基烧结材料的应用提供了技术指导，具有重要的学术意义和实用价值。本文的主要研究结果如下：锰含量影响Fe-Cu-Mn-C系列材料的性能和烧结行为。采用部分预合金法加入铁锰合金并采用模壁润滑温压，制备得到的Fe-2Cu-0.5Mn-0.9C材料性能较佳。其烧结组织均匀，烧结密度可达到7.3g/cm3以上，抗拉强度达到715MPa，硬度达到97HRB，冲击功达到23J，拉伸断裂类型为韧-脆混合断裂。在冷压和温压压制下，压力增大可提高Fe-2Cu-0.5Mn-0.9C压坯密度。其中，700MPa下进行模壁润滑120℃温压，相对密度达94.9%。高速压制压坯的相对密度达到96.0%。过量的锰在烧结时会造成材料内部氧化，降低材料强度。适量加入锰有良好的合金化效果。烧结中锰在材料中发生转移，主要路径为烧结初期材料中连通的孔隙网络通道。蒸发凝聚是锰的烧结机制，锰蒸汽使材料发生膨胀，造成了烧结密度的轻微下降。烧结时间延长，材料内部的孔隙发生闭合，烧结密度逐渐升高。另外，烧结温度提高时，材料发生轻微的烧结收缩现象，部分抵消由于锰的作用而导致的材料膨胀。喷丸对温压Fe-2Cu-0.5Mn-0.9C材料有良好的强化效果，能够提升的材料表面致密度、显微硬度和残余应力，但对抗拉强度和断裂特征影响不大。材料与GCr15滚珠轴承钢的微动磨损试验表明，在油脂润滑下，摩擦表面主要的磨损机制在低载荷时为疲劳剥落，高载荷时为磨粒磨损。干摩擦下表现出多种磨损机制共存，包括磨粒磨损、粘着磨损和氧化磨损，并伴随塑性流动现象。喷丸强化能够降低表面磨损面积，加载载荷较低时，喷丸能够提升材料的微动磨损性能，加载载荷高时，可提升磨损初期的微动磨损性能。喷丸强化前后材料摩擦表面的磨损机制相似，但低载荷下的喷丸材料磨损表面粘着磨损程度要小于未经喷丸材料。铁基烧结材料中运用超声疲劳测试技术可以大幅提高疲劳测试效率。材料超声疲劳S-N曲线为连续下降，存在条件疲劳极限。在106，107和108周次下，温压Fe-2Cu-0.5Mn-0.9C材料相应的拉压超声疲劳强度为393MPa，289MPa和213MPa。孔隙或夹杂物为超声疲劳裂纹源，瞬断区特征与材料拉伸断口相似，超高周疲劳断口中出现不规则分布的疲劳辉纹。建立的疲劳断裂模型表明，超声疲劳测试中，疲劳裂纹扩展阶段的振动周次与疲劳载荷的平方成反比，裂纹扩展寿命只占整个超声疲劳断裂中很小一段寿命，而疲劳裂纹形成寿命占了大部分的疲劳寿命。Fe-2Cu-0.5Mn-0.9C的超声疲劳性能略高于Fe-2Cu-2Ni-1Mo-1C烧结材料。在106，107和108周次下，Fe-2Cu-2Ni-1Mo-1C相应的拉压疲劳强度为312MPa，249MPa和199MPa。铁基烧结材料的超声疲劳断口特征相似。在高应力低疲劳循环周次时，疲劳裂纹源区位于靠近试样表面的缺陷处，在低应力和大于107周次的超高循环周次下，裂纹源区主要位于材料内部。疲劳断裂周次与裂纹源缺陷的尺寸大小并没有明显关系，缺陷处应力强度因子随着疲劳周次的提高而降低。在106，107和108周次下，Fe-2Cu-0.5Mn-0.9C材料相应的对称弯曲疲劳强度分别为402MPa，331MPa和273MPa。更多还原

【Abstract】 Iron-based powder metallurgy sintered material and parts are widely used in powdermetallurgy industry. One of the research targets for powder metallurgy technique isdeveloping the iron-based sintered material with high performance and low cost. In order tocater the dominant trend of manufacturing low alloy, low cost iron-based sintered materials, inthis paper, iron-based powder metallurgy sintered material with high performance and lowcost was studied. The purpose of this study is to replace Ni, Mo and other expensive elementsby using low cost Mn element and then high density Fe-2Cu-0.5Mn-0.9C sintered materialwas optimized. The mechanical properties, densification mechanism，sintering behaviour andultrasonic fatigue behaviours of high density iron-based powder metallurgy materials werestudied, as well as the influence of Mn addition on the preparation, properties and sinteringprocess. Our research provides technical guidance for the application of Mn-containingiron-based sintered material and it has important academic significance and practical value.The main results show that:The mechanical properties and sintering behaviour of Fe-Cu-Mn-C series materials areaffected by different manganese content. Partially pre-alloyed Fe-2Cu-0.5Mn-0.9C sinteredmaterial was fabricated by die-wall lubricated warm compaction with good mechanicalproperty and uniformly distributed microstructure. The sintered material has a density ofhigher than7.3g/cm3, tensile strength of715MPa, Rockwell hardness of97HRB and impactenergy of23J. The tensile fracture mode is tough-brittle mixed fracture. The green densityof material under cold, warm compaction increases with increasing pressure. The greenrelative density of Fe-2Cu-0.5Mn-0.9C is able to reach at94.9%and96.0%under warmcompaction (700MPa,120℃) and high velocity compaction respectively. The content ofmanganese also influences the sintering process. Excess of manganese within sinteredmaterial will probably lead to internal oxidation and material strength reduction. But it hasgood strengthening effects if adding appropriate amount of manganese. During initialsintering period of Fe-Cu-Mn-C series materials, manganese alloying elements transfers intoiron matrix through the connecting pore-networks. Additionally, evaporation condensation ofMn is the diffusion mechanism during sintering process. The manganese vapor causesmaterial expansion and lowers sintered density slightly during the pre-sintering and initialsintering period, while density increases due to pores closing and porosity networksdisappearing gradually as sintering time extended. What is more, elevated sintering temperature contributes to slight sintering shrinkage of the material, which partially offsetmaterial expansion.Surface properties of warm compacted Fe-2Cu-0.5Mn-0.9C material such as the surfacedensity, micro hardness and residual stress are improved by shot peening. However, thetensile strength and fracture characteristics are not changed after shot peening. Fretting weartests between sintered materials and GCr15steel show that the main wear and frictionmechanism is fatigue spalling (at low load) and abrasive wear (at high load) under oillubrication. However, observation of dry friction surfaces indicates that mixed wearmechanism, including abrasive wear, adhesive wear, oxidation wear and plastic transfer. Thesurface wear area of specimen can be reduced by shot peening. The fretting wear property ofshot peen can be improved under lower loading, while only early stage friction of which canbe elevated under higher loading. The wear mechanism of shot peened material is similar withsintered material, but adhesive wear degree of shot peened material is significantly less thansintered material.Ultrasonic fatigue testing method is a high efficiency technology, which can be successfullyapplied in very high cycles fatigue testing of iron-based powder metallurgy sintered materials.The ultrasonic fatigue S-N curve decreases continuously. The ultrasonic fatigue limit existsand the axial fatigue strength of Fe-2Cu-0.5Mn-0.9C are393,289and213MPa for thecorresponding conditions of106,107and108cycles, respectively. The ultrasonic fatigue cracksources of fracture are located at voids or inclusions. Dimples and cleavage planes areobserved in the fatigue fracture, which is similar with that of tensile fracture. Some ultrasonicfatigue striations are distributed in fracture under ultra-high cycle. Fatigue fracture modelshows that the number of fatigue cycles of crack growth stage is inversely proportional to thesquare of fatigue stress in high-cycle fatigue ultrasonic test. The crack growth stage is only asmall part of the whole ultrasonic fatigue life. Crack initiation stage accounts for the most ofthe fatigue life. The axial ultrasonic fatigue strengths of Fe-2Cu-2Ni-1Mo-1C are312,249and199MPa for the corresponding conditions of106,107and108cycles, respectively. Thefatigue strength is less than that of Fe-2Cu-0.5Mn-0.9C sintered material. There is no obviousrelationship between the defect size and the fatigue cycles. In addition, the stress intensityrange factor of defects reduces with the gradual increase of the number of fatigue cycles. Thefatigue fracture characteristics of iron-based sintered material are similar with each other.Crack sources of fracture are located at voids or inclusions near the surface under high cyclebut it moves to the internal sites under ultra-high cycle. What is more, the symmetricalbending ultrasonic fatigue strengths of Fe-2Cu-0.5Mn-0.9C are402,331and273MPa for the corresponding conditions of106,107and108cycles, respectively.更多还原