表面致密化齿轮的滚动接触疲劳

表面致密化给粉末冶金技术提供了制造高强度制品(如汽车变速器齿轮等)的机遇.对这样的应用来说,寿命设计的一个主要技术要求是,确定齿轮齿的滚动接触疲劳的允许工作应力.本文指出,滚动接触疲劳性能是由材料剪切疲劳强度与施加的表面下剪切应力分布的相互作用决定的.适当选取表面致密化层深与热处理渗碳层深度,可使粉末冶金齿轮的滚动接触疲劳耐久性至少达到相当于锻钢的水平.如所指出的那样,一些滚动接触疲劳试验方法,可能低估了表面致密化粉末冶金材料的表面耐久性.本文就表面致密化粉末冶金材料的失效方式、显微结构以及汽车变速器齿轮形状等问题,综述了滚动接触疲劳试验方法.推荐了开发的滚动接触疲劳试验方法,使之产生能代表实际齿轮应用中存在的试验应力分布.     

高负载粉末冶金齿轮选择性表面致密化

本文评述了当前高负载粉末冶金齿轮的发展状况.表明,经选择性表面致密化加工的粉末冶金钢齿轮,其齿的弯曲疲劳强度与接触疲劳强度都和常规钢的相应齿轮相同,完全可替代钢齿轮,如汽车变速器中的钢齿轮.特别是,描述了预成形件的设计与辗压后齿轮中的相应密度分布.最后,表明用选择性表面致密化制造的粉末冶金钢齿轮,既具有相应钢齿轮的使用性能,又具有巨大的节约生产成本的潜力.     

高使用性能粉末冶金零件的表面致密化

为保证机构的可靠性,历史上一直在用锻钢合金批量生产高转矩变速器零件,诸如变速器齿轮、高性能链轮、单向离合器座圈及滚珠轴承座圈。虽然用粉末冶金工艺生产的零件成本低廉,但常规粉末冶金钢的静态、疲劳及耐磨性能的局限性,使粉末冶金零件不能应用于上述领域。为解决这些缺欠,近年来,一直在研发用选择性表面致密化(SSD)改进粉末冶金钢性能的各种生产工艺。特别是,PMG集团开发出了一种拥有专利权的表面致密化工艺——DensiForm。利用这种工艺可使零件表面关键部分形成深度达1mm的完全致密化表面层,而零件心部仍然是多孔性的。这种结合与粉末冶金材料合金化的灵活性,使生产的烧结钢零件的使用性能可与在高应力下使用的锻钢性能相比拟。本文将介绍2个例子,即变速器中应用的表面致密化螺旋齿轮和直齿链条的驱动链轮。本文说明了螺旋齿轮和驱动链轮的生产工艺与显微组织,介绍了轿车变速器用螺旋齿轮的性能(其中包括在三轴总成的成对试验台架上得到的滚动接触疲劳的数据)和表面致密化链轮的材料性能与使用性能。     

螺旋齿轮与正齿轮的表面致密化

过去几十年,用于汽车的粉末冶金零件不断增长,对零件性能的要求也越来越高.汽车用的变速器齿轮的形状复杂,按照齿轮质量要求(DIN,ISO,AGMA等)几何精度很高,按齿面与齿根的耐久性要求力学性能也很高.对于制造复杂形状零件,粉末冶金法对控制成本很有效.通过增加对齿的选择性致密化,在增加很少费用下就能满足精度与力学性能要求.由压制、烧结、碾压致密化及最后热处理组成的工艺路线,可评估用粉末冶金制造的变速器齿轮的可行性.在研究中所用的螺旋齿轮与正齿轮,对致密化及所得到的齿轮的质量与齿根疲劳都进行了试验.     

轿车变速器的粉末冶金齿轮

粉末冶金齿轮由于材料中残留有孔隙,和用切削加工铸锻材料制造的零件相比,强度与耐磨性较差,不适用于重负载应用.通过横向辗压,利用选择性表面致密化,可显著改进粉末冶金齿轮的强度与耐磨性.这篇论文阐述了用压制、烧结、选择性表面致密化、表面硬化及根据要求的品质水平可能还有最后珩磨,制造轿车变速器的粉末冶金螺旋齿轮.研究了各种参数,诸如材料组成、横向辗压与热处理的工艺参数对致密化与硬度深度、孔隙形态、显微组织、齿轮齿廓、DIN品质以及表面粗糙度的影响.初步研究是在3轴的总成成对试验台架上进行的,其表明粉末冶金齿轮具有高负载承载能力.     

烧结钼钢的动力学性能

许多粉末冶金应用，要求烧结零件能耐高的静力学和动力学工作应力。提高烧结密度是提高动力学性能，获得适当显微组织的关键参数之一。在市场上的低合金钢粉中，低钼合金钢粉令人感兴趣，因为这种合金元素仅对粉末压制性有轻微影响，却很有效地提高淬透性和力学性能。此外，像Ni和Cu关键性元素混合到基础钢粉中或在退火处理过程中扩散粘结到钢颗粒上，也会通过改善显微组织使力学性能提高。本文研究了含有不同量的预合金的低Mo合金钢粉的抗拉和动力学性能。也讨论了将Ni和Cu预混合或扩散粘结到这些钼钢中的作用，特别是对轴向疲劳性能和强度的影响。还与其它类型疲劳试样和／或试验方法所得结果进行了对比。     

通用汽车公司将增加新型汽车变速器中粉末冶金制品的使用量

据美国金属市场报道 :美国通用汽车公司的新型 6速前后轮自动变速器将使用一种粉末冶金制行星齿轮托架 ,这种新型行星齿轮托架由几磅重的一些粉末冶金零件组成 ,这种新型汽车用变速器将于 2 0 0 5年投产 ,在这种新型汽车变速器中还将采用粉末冶金制油泵、停车齿轮、离合器踏板等更多的粉末冶金制品 ,据估计到 2 0 0 8年通用汽车公司每年将制造超过 10 0万部这种新型汽车用变速器 ,到那时生产这些汽车变速器每年需要的粉末冶金制品将超过 1万短吨 (90 70t)。信息摘译自美国金属粉末联合会网站http ://www mpif.org/industry/jan0 3news html …     

用于汽车自动变速器的温压涡轮毂

温压是一种可将增高密度与选择高使用性能材料相结合的技术。增高密度有助于提高零件的力学性能,以及整体使用性能。将密度增高与高使用性能相结合,可使制造的零件的使用性能超过相应的铸锻材料产品,同时可制成具有最终形的零件,从而显著减低生产成本。因此,自动变速器的涡轮毂就成为了温压的一个理想对象。这篇论文将评述,将用于高扭矩自动变速器的常规锻件切削加工的涡轮毂转换为用一次压制/一次烧结制造的粉末冶金涡轮毂。粉末冶金涡轮毂使用的材料是FD-0405。在试验室对这种扩散合金化材料进行了评价,和报告了几个密度水平的力学性能。温压可使高应力的内花键区全面达到高密度。为了证明粉末冶金零件的适用性,进行了广泛的力学与零件的特定试验。     

粉末冶金齿轮材料进展

对于生产齿轮与类似的最终形零件,粉末冶金是一种有效工艺。鉴于因固有孔隙度产生的限制和受限制于可用的合金系统,传统的粉末冶金齿轮应用皆用于受力较小者。近年来,采用新压制工艺和新合金材料生产的粉末冶金零件,其屈服强度与抗拉强度皆显著增高,已接近锻造齿轮材料的水平。这篇论文将评述新粉末冶金工艺与新材料,和他们对齿轮类应用的适用性。另外,还将和常用的汽车齿轮材料(包括球墨铸铁、可锻铸铁及锻轧低合金钢)的力学性能进行比较。     

粉末冶金零件在汽车上的应用

列出了我国部分引进牌号汽车每辆汽车上应用的粉末冶金零件数量 ;介绍了汽车工业粉末冶金零件的应用情况 ,包括粉末冶金进、排气门座 ,同步器锥环 ,曲轴正时齿轮和凸轮轴正时齿轮 ,机油泵主、从动齿轮 ,减震器零件 ,烧结铜合金 钢背双金属轴承 ,粉末锻造连杆 ,粉末冶金凸轮轴等     

Contacting surfaces: A problem in fatigue and diffusion bonding

Contact between surfaces usually occurs at asperities under compression or at connecting ligaments, depending on how the interface is formed. This paper deals with the nondestructive evaluation of the topology of contact and with the use of this information to predict the effects that loads borne by these contacts have on mechanical properties. Two specific examples are discussed: a fatigue crack and a diffusion bond. Asperity contact along the fracture surface of a fatigue crack partially shields the crack tip from the externally applied driving force. Using information from acoustic experiments, the geometry of the asperities, the contacting stress, and the shielding stress intensity factor have been estimated. Acoustically, a diffusion bonded interface looks very similar to that joining the two sides of a partially closed crack. In this particular case, the acoustically determined geometry of well-bonded ligaments can be verified by fractography of destructively tested samples whose bond strength has also been determined. Models to determine the bond strength from the ligament geometry are being suggested.     

对称与非对称齿轮齿根弯曲应力对比分析

非对称渐开线直齿圆柱齿轮作为一种新型的齿轮.无法通过现有的解析法对其齿根弯曲应力强度进行计算.通过分析其轮齿齿廓结构特点,以平截面法为基础,建立了一种新的求解方法,推导出该类齿轮齿根弯曲应力解析法计算公式.以相同模数及齿数的对称、非对称齿轮为研究对象,在相同工作状况下,分别通过解析法和有限元法,对对称、非对称渐开线直齿圆柱齿轮正向、反向旋转过程中轮齿齿根的弯曲强度进行了研究.取啮合齿轮对一个啮合周期内的五个特殊位置点,对两种齿轮轮齿两侧齿根弯曲应力进行了对比分析.最后通过阶梯增载疲劳试验法,对两类齿轮进行了轮齿齿根的弯曲疲劳强度试验,通过试验数据对理论分析结果进行了验证.      

Oxidation protection of Ti-aluminide orthorhombic alloys: An engineered multilayer approach

The lack of high-temperature environmental resistance is a major issue in the application of orthorhombic-based titanium aluminide alloys (O alloys) and their composites. Improvement in environmental capability can be achieved by applying diffusion barrier coatings to the surface of the orthorhombic matrix alloy. However, since thin coatings are prone to foreign-object damage, an approach based on thicker multilayer materials may be more prudent for fracture-critical applications. In the present study, foils of the orthorhombic alloy were diffusion bonded on either side with an γ alloy, the latter used in an attempt to provide environmental protection. Mechanical tests suggested that the γ alloy was successful in preventing degradation of the O alloy due to oxidation and interstitial embrittlement under thermal cycling conditions. Processing below the β transus of the O alloy provided an improvement in the stress and strain to failure of the joined material compared to materials processed above the transus. However, in either case, the strengths of the joined materials were significantly lower than that of the uncoated O alloy with similar microstructures. Results suggest that the low strength of the joined materials may be due to cracking of the γ alloy, resulting in premature failure of the O alloy. Finite element analysis (FEA) was performed to understand the stress distribution in the joined material and to investigate approaches for reducing the residual stress. Several approaches for improving the stress and strain to failure of the joined material are presented.     

Delayed Fracture Behavior of CrMo Type High Strength Steel Containing Vanadium

The quenchedandtemperedlowalloysteelswithtensilestrengthexceeding 12 0 0MPaaresus ceptibletohydrogen induceddelayedfracture(HIDF )wheninuse[1,2 ] .Despitetheenormousamountofresearchworkperformedondelayedfrac tureofhighstrengthsteelintheseyears ,thesolu tiontothisproblemstillhasnotbeenobtained .TheresistanceofanalloytoHIDFisstronglyaffectedbytheinteractionofhydrogenwithmicrostructuralhet erogeneitiesthatactashydrogentraps ,andthere foreinasearlyas 1980s ,GMPressouyre[3 ] suggest edtheappli…     

Dynamic properties of high-density low-alloy PM steels

PM steels with density higher than 7.5?g?cm^?3 were prepared from binder-treated powder, and the effects of alloying elements and microstructure on dynamic performance were studied. Although the addition of 1.5?wt-% Cu improves the tensile strength, it impairs toughness of the alloy by the formation of low-toughness pearlite. The 0.5?wt-% Cr addition promotes the formation of Ni-rich martensite and bainite at the expense of pearlite, which is beneficial to strengthen and homogenise the microstructure. The 0.5Cr alloy exhibits improved tensile strength and fatigue strength without sacrificing impact toughness, which exhibits high potential to be used for dynamic and cyclic loading applications. The tensile strength and fatigue endurance strength under 10⁷ cycles of 0.5Cr alloy achieve 1350 and 562?MPa respectively, and impact energy is as high as 21.1?J. The enhanced fatigue life is attributed to the high density, smaller pore size, rounder pores and composite-like microstructure.     

Strength distribution of fatigue crack initiation sites in an Al-Li alloy

The stress-number of cycles to failure (S-N) curves were measured along the short-transverse (S) and rolling (L) directions of a hot-cross-rolled AA 8090 Al-Li alloy plate (45-mm thick). The alloy was solution heat treated, quenched in water, strained by 6 pct, and peak aged. Fatigue tests were carried out in four-point bend at room temperature, 20 Hz, R=0.1, in air. It was found that the fatigue limits in the S and L directions were 147 and 197 MPa, respectively. The crack population on the surface of a sample at failure increased with the applied stress level and was found to be a Weibull function of the applied maximum stress in this alloy. The strength distribution of fatigue weakest links, where cracks were initiated, was derived from the Weibull function determined by the experimental data. The fatigue weakest-link density was defined as the crack population per unit area at a stress level close to the ultimate tensile stress and can be regarded as a materials property. The density and strength distribution of fatigue weakest links were found to be markedly different between the L and S directions, accounting for the difference in fatigue limit between the directions in this alloy. They were also found to be different between S-L and S-T samples, and between L-T and L-S samples of this alloy, which could not be revealed by the corresponding S-N curves measured. These differences were due to the anisotropy of the microstructures in different directions in this alloy.     

A through-process model of an A356 brake caliper for fatigue life prediction

The demand for low-cost, high strength-to-weight ratio components continues to drive the development of aluminum alloy castings for new applications. The automotive brake caliper, which has traditionally been made from cast iron, is one such component where aluminum alloys are being considered. The fatigue performance of the brake caliper is an important consideration in evaluating new designs. The use of aluminum alloys in cast components under cyclic loading conditions necessitates a design approach that, in addition to in-service loading, incorporates the impact of microstructural features on fatigue life. In this investigation, a through-process modeling approach has been employed to link a series of mathematical models describing the processing steps of (1) casting, (2) heat treatment, (3) machining, and (4) in-service performance for a preliminary design of a brake caliper made of aluminum alloy A356, Step (1) includes microstructural predictions (secondary dendrite arm spacing and maximum pore size), which are tracked through to the final component. The final lifing of the component combines the effects of these microstructural features with the complex stress state arising from the combined service loading and residual stresses. It was found that all three factors have a strong influence upon the component’s fatigue performance.     

Effect of Cu infiltration on static and dynamic properties of PM steels

Abstract

Performance improvement that can be achieved by Cu infiltration is quantified in this paper. Tensile and fatigue properties of a Fe–2·0Cu–0·7C powder metallurgy steel were compared to the same alloy infiltrated with 8 wt-%Cu. Microstructural characterisation, using optical and electron microscopies, was carried out to understand the effect of Cu infiltration on mechanical properties. Cu infiltration improves the ultimate tensile strength by 40% by increasing the load bearing structure, decreasing the stress concentrations associated to open porosity and increasing the hardness of the steel matrix. Fractographic observations show the evidence of stress transmission from the sinternecks to the steel particles due to infiltrated Cu. The beneficial effect of Cu infiltration is less pronounced for the fatigue properties as the endurance limit is increased by 10%. This lower improvement is explained by crack initiating at the Cu/steel matrix interface.     

激光冲击强化对Ti6Al4V合金表面完整性和四点弯曲疲劳性能的影响

采用扫描电镜、显微硬度仪、X射线应力测量仪及透射电镜等对激光冲击强化Ti6Al4V合金的表面完整性进行了分析,采用MTS疲劳试验机测试了疲劳性能,并采用扫描电镜分析了疲劳断口,探讨了激光冲击强化机制。结果表明:经功率密度为15.9 GW/cm^2的激光处理后,其四点弯曲中值疲劳寿命较未处理试样提高了4.2~23.5倍;激光功率密度越大,试样的中值疲劳寿命越长。激光冲击强化表现出比喷丸强化更优的疲劳寿命增益效果。经激光冲击强化后,Ti6Al4V合金表面形成了深度为600~1400μm的残余压应力场,表面硬度比未强化区域提高了约10%,且亚表层内部的位错密度也有显著提高。