EXPERIENCE WITH THERMOMECHANICAL FATIGUE UNDER SERVICE-TYPE LOADING

The thermomechanical fatigue behaviour of different high temperature alloys has been investigated and is under investigation respectively. The creep-fatigue behaviour of heat resistant steels was investigated by long-term service-type strain cycling tests simulating thermomechanical fatigue (TMF-) loading conditions at the heated surface of e.g. turbine rotors. Single-stage as well as three-stage cycles leads to similar results at the application of the damage accumulation rule. Life prediction which simulates typical combinations of cold starts, warm starts and hot starts has been established successfully for isothermal service-type loading and will be exceeded for thermomechanical loading. Long-term thermomechanical fatigue testing of Thermal Barrier Coating systems show typical delamination damage. An advanced TMF cruciform testing system enables complex multiaxial loading.     

Effect of the hot-rolling microstructure on texture and surface roughening of Al-Mg-Si series aluminum alloy sheets

This study investigates the effect of microstructural changes induced by hot rolling on the formation of the texture in high-strength aluminum alloy sheets used in automotive applications. Fully (S2) and partially (S1) re-crystallized samples were fabricated by controlling the final hot-rolling temperature. Optical microscopy (OM) was used to observe the microstructure of the transverse direction (TD)-plane of the hot-rolled strips, and the electron back-scattered diffraction (EBSD) technique was used to evaluate the texture of the normal direction (ND)-plane of the finished sheets that were subjected to hot rolling, followed by cold rolling and a solid-solution heat treatment (SSHT). The grains in S1 showed a low-angle grain boundary, and the concentrations of Cube {001}〈100〉 and Goss {011}〈100〉 orientations were detected at different sheet thicknesses. In contrast, a randomized texture was observed in S2. On the basis of the above results, this paper discusses how the microstructure and texture achieved after hot rolling can influence the final microstructure, texture and surface roughening behavior.     

Static and Cyclic Load-Deflection Characteristics of NiTi Orthodontic Archwires Using Modified Bending Tests

Near-equiatomic nickel-titanium (nitinol) has the ability to return to a former shape when subjected to an appropriate thermomechanical procedure. One of the most successful applications of nitinol is orthodontic archwire. One of the suitable characteristics of these wires is superelasticity, a phenomenon that allows better-tolerated loading conditions during clinical therapy. Superelastic nitinol wires deliver clinically desired light continuous force enabling effective tooth movement with minimal damage for periodontal tissues. In this research, a special three-point bending fixture was invented and designed to determine the superelastic property in simulated clinical conditions, where the wire samples were held in the fixture similar to an oral cavity. In this experimental study, the load-deflection characteristics of superelastic NiTi commercial wires were studied through three-point bending test. The superelastic behavior was investigated by focusing on bending time, temperature, and number of cycles which affects the energy dissipating capacity. Experimental results show that the NiTi archwires are well suited for cyclic load-unload dental applications. Results show reduction in superelastic property for used archwires after long-time static bending.     

Effect of thermo-mechanical processing on filiform corrosion of aluminium alloy AA3005

The effect of laboratory scale thermo-mechanical processing of aluminium alloy AA3005, supplied as 2.9 mm sheet at the hot roll transfer gauge, on filiform corrosion (FFC) susceptibility, electrochemical properties and near surface microstructure was investigated. Corrosion testing showed that the as hot rolled material exhibited poor FFC resistance and that heat treatment of both the as hot rolled and subsequently cold rolled samples resulted in a drastic loss of corrosion resistance. Microstructural characterisation showed that both hot and cold rolling resulted in enhanced surface shear deformation and the formation of a thin layer of metal in the surface with different properties and microstructural characteristics than the underlying bulk material. Enhanced surface deformation resulted in a redistribution of intermetallic particles in this region. Poor FFC properties were correlated with the presence of a higher density of fine intermetallic precipitates in the surface layers than in the underlying bulk. It is suggested that enhanced secondary precipitation of intermetallic particles in the deformed surface layers during high temperature exposure, e.g., during hot rolling or annealing steps, is the main contributing factor to the observed poor FFC resistance.     

基体表面异质材料滚动接触疲劳性能与失效机理的研究进展

滚动接触疲劳性能是评价膜层性能的重要指标之一。影响零件滚动接触疲劳性能的因素主要分为膜层自身结构完整性和服役条件两大类。膜层自身结构完整性又受制于涂覆工艺、材料体系、后处理方式等因素。在服役工况确定的情况下,膜层自身结构完整性对零件的接触疲劳性能起决定性的作用。不同的涂覆工艺、材料体系、后处理方式对零件的滚动接触疲劳性能及失效机理的影响不尽相同。本文综述了涂覆工艺、材料体系、后处理方式对基体表面异质材料滚动接触疲劳性能与失效机理的影响,发现对滚动疲劳失效机理也存在作用。最后,总结了目前关于膜层滚动接触疲劳研究中存在的问题,探讨了解决问题的方法,以期为基体表面膜层的接触疲劳寿命预测奠定良好的基础。     

Enhanced laser weldability of an aerospace superalloy by thermal treatment

ABSTRACT

Weldability improvement of the Inconel 738 alloy, subjected to different pre-weld heat treatments and, thermomechanical fatigue of the unwelded and welded alloy is investigated. A new pre-weld heat treatment is found to significantly reduce the susceptibility of the extremely difficult-to-weld alloy to weld cracking. The heat treatment also produces welded material with enhanced resistance to thermomechanical fatigue failure at high mechanical strain ranges, in comparison to the unwelded alloy. Furthermore, by comparing polycrystalline and single crystal materials, without altering the chemistry/microstructure of the superalloy, the presence of grain boundaries in the alloy is found to aid damage mechanisms during in-phase thermomechanical fatigue.     

重型汽车发动机曲轴断裂分析

某重型汽车在正常行驶过程中,发动机曲轴突然发生断裂。对失效曲轴进行硬度测试、金相组织检查及断口宏微观观察等综合分析,结果表明：该曲轴断裂性质为弯曲-扭转疲劳断裂,其断口明显分为3个区域,即疲劳源区、扩展区和瞬断区;曲轴表面硬度比规定硬度值低,问时,材料表层和内部存在较多弥散分布的气孔及Al2O3、MnS等氧化物和硫化物夹杂,在弯矩和扭矩的共同作用下,疲劳裂纹从曲轴轴径油孔下方过渡圆角处等应力集中区域开始萌生,并沿与轴径约呈45°的方向扩展,最终导致曲轴断裂失效。     

钛合金热轧板高应变率塑性行为的研究

以10mm厚度的热轧钛合金板TC4为原料,通过Hopkinson压杆实验和强迫剪切实验,对高应变率下的厚板钛合金的塑性行为进行了研究和分析,得到了不同热处理制度下材料的真实应力-应变曲线,并观察了不同热处理制度下试验后材料的微观组织。     

Cr5֧�й�ʧЧ����

 采用光学金相显微镜、超声波探伤和力学性能测试等方法对Cr5支承辊大面积剥落的原因进行了分析。宏观断口和探伤结果表明，该轧辊的剥落面经过了一段疲劳延伸过程，裂纹由硬化层底部起源，沿轧辊圆周，逆轧制方向发展。剥离块为辊身的硬化层，硬化层外表为马氏体，中部为索氏体，硬化层的底部组织为屈氏体。轧辊辊身传动侧存在的轧辊制造问题和热处理工艺不当是该次剥落的原因。     

Influence of the post-deformation annealing heat treatment on the low-cycle fatigue of NiTi shape memory alloys

Shape memory alloys (SMA) suffer from the same impairing mechanisms experienced during cycling loading by classic alloys. Moreover, SMA fatigue behavior is greatly influenced by thermomechanical cycling through the zone of thermoelastic phase transformation, which is the basis of shape memory and superelasticity effects. Since the fatigue resistance of any material can be improved by an appropriate thermomechanical treatment, in the present work combined differential scanning calorimetry and microhardness testing were used to determine an optimum annealing temperature for the cold-worked Ni-50.1%Ti alloy. The optimization is based on the assumption that latent heat of transformation is proportional to the mechanical work generated by SMA upon heating, while material hardness is related to the yield stress of the material. It is supposed that an optimum trade-off in these two properties guarantees the best dimensional and functional stability of SMA devices. The level and stability of the mechanical work generated by the material during low-cycle fatigue testing are considered criteria for the material performance and thus of the validity of the proposed optimization procedure.     

Effect of processing parameters on the earing and mechanical properties of strip cast type 3004 Al alloy

Electrical resistivity, superficial hardness, tensile testing, and quantitative metallography techniques were used in this study. The strip cast type 3004 aluminum alloy received sixteen different thermomechanical treatments before cups were drawn. The top edges of the drawn cups were not flat. Rather, there were high points or ears with valleys between them. The homogenization temperature varied from 510 to 621 °C at 24 h. Some samples received an additional 426 °C/24 h homogenization anneal. Most specimens were rolled along the longitudinal direction of the as-cast material, and some were rolled in the transverse direction. Most samples were recrystallized at 454 °C for 24 h in addition to the homogenization treatment. Some were recrystallized for 168 h. All samples were subsequently rolled to 0.33 mm for cup drawing and percent earing determination. The percent earing results of some samples were less than 1.5%, but the mechanical strength was also lowered. The high-temperature recrystallization anneal of 454 °C was the controlling factor in determining the earing and mechanical strength of the final rolled sheet.     

Effect of processing parameters on the earing and mechanical properties of strip cast type 3004 Al alloy

Electrical resistivity, superficial hardness, tensile testing, and quantitative metallography techniques were used in this study. The strip cast type 3004 aluminum alloy received sixteen different thermomechanical treatments before cups were drawn. The top edges of the drawn cups were not flat. Rather there were high points or ears with valleys between them. The homogenization temperature varied form 510 to 621 °C at 24 h. Some samples received an additional 426 °C/24 h homogenization anneal. Most specimens were rolled along the longitudinal direction of the as-cast material, and some were rolled in the transverse direction. Most samples were recrystallized at 454 °C for 24 h in addition to the homogenization treatment. Some were recrystallized for 168 h. All samples were subsequently rolled to 0.33 mm for cup drawing and percent earing determination. The percent earing results of some samples were less than 1.5%, but the mechanical strength was also lowered. The high-temperature recrystallization anneal of 454 °C was the controlling factor in determining the earing and mechanical strength of the final rolled sheet.     

Texture development and hardening characteristics of steel sheets under plane-strain compression

Crystallographic texture development and hardening characteristics of a hot-rolled, low-carbon steel sheet due to cold rolling were investigated by idealizing the cold rolling process as plane-strain compression. The starting anisotropy of the test material was characterized by examination of the grain structure by optical microscopy and the preferred crystal orientation distribution by x-ray diffraction. Various heat treatments were used in an effort to remove the initial deformation texture resulting from hot rolling. The plastic anisotropy of the starting material was investigated with tensile tests on samples with the tensile axis parallel, 45°, and perpendicular to the rolling direction. The grain structure after plane-strain compression was studied by optical microscopy, and the new deformation texture was characterized by x-ray diffraction pole figures. These figures are compared with the theoretical pole figures produced from a Taylor-like polycrystal model based on a pencil-glide slip system. The uniaxial tensile stress-strain curve and the plane-strain, compressive stress-strain curve of the sheet were used to calibrate the material parameters in the model. The experimental pole figures were consistent with the findings in the theoretical study. The experimental and theoretical results suggest that the initial texture due to hot rolling was insignificant as compared with the texture induced by large strains under plane-strain compression.     

喷丸对25CrNi2MoV钢滚动接触疲劳性能的影响

目的提高25CrNi2MoV钢的滚动接触疲劳性能。方法对25CrNi2MoV钢进行表面喷丸处理,并采用3D形貌仪、光学显微镜、显微硬度仪、X射线应力分析仪与滚动接触疲劳试验机等仪器,对试样表面形貌、表面显微组织、显微硬度、表面残余压应力与滚动接触疲劳性能等进行测试分析。结果与未处理试样相比,经喷丸处理后,试样表面形貌由磨削加工槽型向酒窝状的弹坑转变,表面粗糙度增大,表面显微硬度由503HV0.2增大到577HV0.2,增加了14.7%,表面残余压应力由-90.0 MPa增大到-758.0 MPa。当喷丸强度为0.445 mmA时,试样具有最好的滚动接触疲劳寿命,其额定寿命(L10)、中值寿命(L50)、特征寿命(L63.2)分别为4.973×10^6次、6.578×10^6次和6.945×10^6次,分别是未处理试样对应寿命的11.1倍、7.3倍和7.0倍,试样滚动接触疲劳失效形式主要为疲劳剥落。当喷丸强度为0.596 mmA时,试样表面出现微裂纹,导致滚动接触疲劳寿命降低,此时试样疲劳失效形式主要为点蚀与疲劳剥落。未处理试样疲劳失效形式主要为分层。结论喷丸处理能细化试样表层晶粒组织,增大试样表面粗糙度、表面硬度与表面残余压应力。合适强度的喷丸处理可以抑制试样表面与次表面裂纹的萌生与扩展,显著提高滚动接触疲劳性能。     

An experimental and numerical study of prestrained AA5754 sheet in bending

This study focuses on the effects of prestrain magnitude on 3 mm AA5754 sheet in bending at nominal applied strain rates of 0.001/s and 0.1/s. The necessity of incorporating prestrain and strain rate effects into numerical simulations of bending is also evaluated. A series of experimental bend tests using axial compression in the longitudinal material direction were performed following plane strain prestrain in the transverse material direction. An inelastic buckling mode of deformation was produced with the peak buckling load increasing and the minimum load decreasing with larger magnitudes of prestrain. A semi-empirical material model, referred to as the Voce-MA model, was developed which incorporates strain rate-sensitivity of the flow stress, the prestrain magnitude and their interaction. Simulations of the bend tests using this material model were then performed in LS-DYNA at nominal applied strain rates of 0.001/s and 0.1/s for samples with 0, 3, 6 and 12% plane strain prestrain. It was shown that for AA5754 sheet in bending, prestrain effects must be considered in terms of current sheet thickness and material hardening. While the peak and minimum loads are not strain rate sensitive at the low rates used in this study, a rate-dependant material model is still necessary in order to account for the deviations in local strain rate from the applied strain rate. The Voce-MA material model was capable of representing prestrain and strain rate effects for all cases of AA5754 sheet in bending considered in this study.     

退火温度对新型锆合金薄板带材析出相和织构的影响

采用扫描电镜和超高分辨透射电镜,对具有良好冲制性能的新型锆合金薄板成品带材进行含晶粒、第二相粒子等在内的显微组织研究,并探索真空退火处理条件下温度对带材显微组织的影响。结果显示:新型锆合金薄板成品带材晶粒平均尺寸2.17 μm,存在{0001}<1010>和{0001}<1120>两种织构,大部分晶粒<1120>平行带材RD方向,较少晶粒<1010>平行带材RD方向;第二相粒子分布在晶粒内部及晶界,平均尺寸114 nm,尺寸较大的为不规则椭圆形的Zr-Nb-Fe相,尺寸较小的为圆形的β-Nb相;热处理退火温度降低,带材晶粒尺寸减小,第二相粒子细小弥散分布;新型锆合金薄板成品带材良好冲制性能主要源于轧制积累应变诱发再结晶过程进行充分,导致晶粒细小及孪晶发生破碎;相对轧制变形,退火对带材冲制性能影响不显著。     

Regulation Mechanism of Novel Thermomechanical Treatment on Microstructure and Properties in Al-Zn-Mg-Cu Alloy

Scanning electron microscopy, transmission electron microscopy, tensile test, exfoliation corrosion test, and slow strain rate tensile test were applied to investigate the properties and microstructure of Al-Zn-Mg-Cu alloy processed by final thermomechanical treatment, retrogression reaging, and novel thermomechanical treatment (a combination of retrogression reaging with cold or warm rolling). The results indicate that in comparison with conventional heat treatment, the novel thermomechanical treatment reduces the stress corrosion susceptibility. A good combination of mechanical properties, stress corrosion resistance, and exfoliation corrosion resistance can be obtained by combining retrogression reaging with warm rolling. The mechanism of the novel thermomechanical treatment is the synergistic effect of composite microstructure such as grain morphology, dislocation substructures, as well as the morphology and distribution of primary phases and precipitations.     

Effect of surface rolling on fatigue behavior of a pearlitic ductile cast iron

Surface rolling is a mechanical treatment usually used in parts fabricated with steel and ductile cast iron, specifically in stress concentration regions, to improve fatigue properties. This process hardens and introduces compressive residual stresses to the surface of the material through the application of controlled strains, thus provoking a reduction of resulting tensile stress at its surface under cyclic loading. This work deals with the effect of surface rolling on high cycle fatigue behavior of a pearlitic ductile cast iron used in crankshafts by the automotive industry. Rotating bending fatigue tests were performed in both smooth and notched specimens, the latter either with or without a surface rolling treatment. Compressive residual stresses and heavy plastic deformation imposed on the surface grains due to cold work made difficult the nucleation and propagation of the crack at the rolled surface of the notch. As a consequence, surface-rolled notch testpieces presented a higher endurance limit (478 MPa) than both smooth (299 MPa) and notched (166 MPa) testpieces did. The surface rolling apparatus developed for this work proved to be very efficient and simple, providing good control of parameters involved in the process (i.e., rolling load, frequency, and number of revolutions).     

小型堆直流蒸汽发生器传热管工艺研究及质量控制

针对小型堆直流蒸汽发生器,分析了其传热管的工艺过程.冷轧时,优化了冷轧工模具孔型的开口设计、孔型曲线设计及与之匹配的顶头变形段及定径段设计;TT处理时,控制温度在715 ℃、保温时间为5h以上,以提高抗晶间腐蚀的能力;弯制时,采用反变形法和特殊的模具设计,确定模具的开槽高度为9.91 mm、反变形量为0.09 mm,以...     

18CrNiMo7-6钢旋转弯曲疲劳试样磨削工艺优化

渗碳淬火可以提高材料的疲劳性能,但在对热处理后旋转弯曲疲劳试样的磨削加工中存在废品率高、效率低等问题,为了高效高质量磨削出标准疲劳试样,提出一种径向圆跳动变形量检测方法以衡量热处理后变形程度,基于国标GB/T4337-2015中尺寸、形位公差以及表面质量要求,提出两种磨削加工方案,并进行同轴度误差检测与比较,对渗碳淬火后18CrNiMo7-6钢旋转弯曲疲劳试样毛坯在外圆磨床上的磨削加工工艺进行优化,确定最终磨削方案。磨削后试样同轴度误差为9.3μm,表面残余应力为-233.84 MPa,表面算术平均偏差Sa为0.208μm,符合国标要求,为疲劳试验中标准旋转弯曲疲劳试样磨削加工提供依据。