钛合金焊接过程应力应变特点分析

采用有限元分析和试验测试相结合的方法对钛合金薄板钨极氩弧焊的焊接应力应变进行分析.结果表明,在焊接不均匀加热和冷却的综合过程中,熔池金属是在周边已经产生了压缩塑性应变的框架内熔化,并在已经发生了压缩塑性应变的近缝区框架内冷却.焊缝冷却过程中,在"力学熔化"区间,有拉伸塑性应变的卸载过程,但残余状态仍保留了压缩塑性应变.焊缝在冷却、凝固降温过程中,其纵向拉应力的峰值始终低于材料在相应温度下的屈服强度,直至残余状态.     

组织不均匀性对TA15钛合金电子束焊焊接接头热影响区应变集中的影响

通过对TA15钛合金电子束焊焊接接头热影响区进行纳米压痕试验和小试样拉伸试验,结合基于组织的有限元模拟,研究了热影响区中α相和α'相不均匀性对热影响区应变集中的影响. 结果表明,α'相的屈服强度和加工硬化系数均高于α相,α'相是热影响区中较“硬”的组织,α相是热影响区中较“软”的组织,两种组织具有不同的力学性能,因此热影响区组织具有不均匀性. 组织不均匀性导致其细观应力应变分布不均匀,α'相承受较高的应力而α相有较大的应变,从而引起局部塑性变形的不协调,最终形成沿α相的塑性应变集中带.     

不同应变速率条件下Al67Ti25Mn8金属间化合物的高温拉伸力学行为

研究了Al67Ti25Mn8金属间化合物在高温不同应变速率条件下的拉伸力学行为.结果表明在3．34×10－5-6.68×10－4s－1范围,随着应变速率的降低,材料的屈服强度显著下降,而拉伸塑性明显升高,塑性最高可达到21％;在低应变速率区高拉伸塑性的获得是由于材料在高温变形过程中发生的动态回复和动态再结晶所致;在高温拉伸时Al67Ti25Mn8金属间化合物是以沿晶方式发生断裂在较高应变速率下材料的低塑性沿晶断裂表明Al67Ti25Mn8的晶界结合强度仍较弱     

循环热处理对喷射沉积Al-27%Si合金组织和性能的影响

采用喷射沉积技术制备Al-27%Si(质量分数)合金,研究循环热处理对其组织、残余应力和力学性能的影响。采用Ansys有限元模拟结合X射线检测评估Al-27%Si合金残余热应力的分布和大小。结果表明,喷射沉积Al-27%Si合金由均匀细小的初晶Si和α-Al基体组成,初晶Si的尺寸随热循环温度升高和次数增加而增大。Ansys有限元模拟结果显示喷射沉积Al-27%Si合金增强体Si颗粒受径向拉应力,Al基体受径向压应力,平均残余热应力为121 MPa,小于X射线法测的平均残余应力131.7 MPa。Al-27%Si合金屈服强度与残余热应力、Si相形貌和大小有关。循环热处理后,当Si相形貌和尺寸相当时,材料的屈服强度随残余热应力的降低而升高;当Si相尺寸随热循环增大时,在残余应力相当的情况下材料的屈服强度降低。     

连续SiC纤维增强Ti基复合材料中的残余热应力

提出了用"残余应变不平衡法"来检测SiCf/Ti基复合材料的残余热应力.即在复合材料中单边多加入基体合金而形成层状材料,冷却中因残余热应变不平衡而产生弯曲,测量弯曲变形量,计算复合层间的热应力和应变,得出产生残余热应力的起始温度为704℃.采用Schapery模型计算不同温度下SiCf体积分数为35%的SiCf/Ti6Al4V复合材料热膨胀系数,获得了SiCf/Ti6Al4V复合材料中Ti6Al4V基体和SiC纤维中的平均残余热应力分别为349 MPa和648MPa.     

Ti—6A1—4V厚板的高温塑性各向异性

在25—704℃下,研究了工厂退火的、双重退火的和交叉轧制的Ti—6Al—4V厚板的平面塑性各向异性参数和法向塑性各向异性参数(分别为△R与R)与温度的关系。这两个参数与合金的织构和显微组织有关,并以最大负荷下拉伸试样的塑性应变比(R),即其宽度应变对厚度应变的比值来表征,最大负荷约相当于0.065纵向应变,试样取向与轧向成0°、45°和90°。随着温度的升高,具有α变形型基面织构的厚板的各向异性变化比具有β转变型织构厚板的各向异性变化要大。此种性质与织构的漫散程度有关,也与主要塑性变形方式由孪晶转变成滑移的热诱致转变有关。由于存在织构,上述结果有力表明:控制加工温度可能有利于改变Ti—6Al—4V厚板的塑性各向异性,以满足给定加工工序对其成形性的要求。     

应变速率对Ti1023合金室温拉伸性能的影响

研究了在室温拉伸试验过程中,不同应变速率对Ti1023钛合金力学性能的影响。结果表明,抗拉强度、断后伸长率和断面收缩率受应变速率变化的影响较小。屈服强度对应变速率比较敏感,随着应变速率增加,屈服强度不断提高,本试验条件下的屈服强度变化范围超过15%。并根据不同应变速率下的屈服强度,得到了应变速率和屈服强度的关系。根据应力-应变曲线以及拉伸前后试样的显微组织变化,发现Ti1023合金拉伸过程伴随应力诱发马氏体相变。     

微米尺寸不锈钢的形变与疲劳行为的尺寸效应

采用聚焦离子束溅射蚀刻加工了微米尺寸304不锈钢悬臂梁试样．利用静态及动态弯曲加载研究了微米尺寸材料的形变与疲劳开裂行为．结果表明：随薄膜厚度的减小,材料的屈服强度升高,塑性下降．屈服强度随悬壁梁厚度的变化关系与Hall—Perch晶粒强化关系相似．微小悬壁梁屈服强度的升高来源于小尺度材料在非均匀变形下引起的应变梯度贡献的增加;而塑性下降则归因于较薄薄膜的晶粒内较少的可动位错．疲劳裂纹从尖缺口处萌生的门槛值接近块体材料．     

基于有限元方法的硬质合金/Ti/TiC/Ta-c多梯度复合涂层的残余热应力分析

基于有限元方法,通过Ansysworkbench软件分析了硬质合金梯度涂层的热应力缓和机制。比较了单层Ta-c涂层、Ti/Ta-c复合涂层及Ti/Ti C/Ta-c多梯度复合涂层对热应力场的影响。分析了Ti/Ti C/Ta-c多梯度复合涂层中Ti、Ti C、Ta-C层厚度和涂层沉积温度等因素对热应力的影响。结果表明:3种涂层中,等效应力最大值皆出现在最表层的Ta-c层中,且Ti/Ta-c复合涂层所受等效应力最大,为354.74 MPa;单层的Ta-c涂层等效应力值最小,为132.89MPa;在Ti和Ta-c之间增加Ti C过渡层时,等效应力减小为251.76 MPa。Ti C、Ta-C层厚度对热应力的影响较大;涂层热应力随着涂层沉积温度的升高逐渐呈线性关系递增,且随温度的增加,Ta-c层中的sp~3键会向sp~2键转化,转化过程中碳sp~3键释放应力,会使涂层残余应力有所减小。     

TiC/Ti3AlC/Ti3Al三相自生复合材料热应力分析

根据TiC/Ti3AlC/Ti3Al三相自生复合材料的结构,建立了热应力分析的4层嵌套模型,推导了热应力计算公式.并利用该模型计算了TiC/Ti3AlC/Ti3Al三相复合材料的热应力.计算结果表明各相及对应界面的法向应变和切向应变均较TiC/Ti3Al两相结构材料有所降低,这有利于防止材料发生界面脱粘损坏;在Ti3AlC界相中只产生较小的切向压应力,可大幅度降低Ti3Al基体相中的切向压应力,有利于防止Ti3Al基体发生拉裂破坏.对具有包覆结构的TiC/Ti3AlC/Ti3Al合金的显微力学性能进行了测试,Ti3AlC界相的存在,使增强体中心到基体的显微硬度和弹性模量呈梯度变化,有利于应力传递.     

Effects of Traverse Scanning Speed of Spray Nozzle on the Microstructure and Mechanical Properties of Cold-Sprayed Ti6Al4V Coatings

Cold spray has the potential to restore damaged aerospace components made from titanium alloy, Ti6Al4V at low temperature (200-400 °C). Traverse scanning speed during deposition is one of the key factors that affect the quality of the Ti6Al4V coatings as it influences the thermal build-up and coating thickness per pass. As there are fewer reported studies on this, this work investigated the effects of different traverse scanning speeds (100, 300 and 500 mm/s) of cold spray nozzle on the microstructure and mechanical properties of cold-sprayed Ti6Al4V coatings. The cross-sectional analysis showed coating porosities reduces with slower traverse speed, from 3.2 to 0.5%. In addition, the microhardness of the coatings increased from about 361-385 HV due to strain hardening. However, the adhesion strength of the coatings to the substrates significantly decreased with reduced traverse speed from about 60 MPa (glue failure) at 500 mm/s to 2.5 MPa (interface failure) at 100 mm/s. Therefore, this study revealed that the control of heat build-up and thickness per pass during the cold spray deposition of the Ti6Al4V coatings is crucial to attain the desirable properties of the coatings.     

Finite element analysis of thermal stress in magnetron sputtered Ti coating

The thermal, shear and radial stresses generated in the Ti coating deposited on glass and Si substrates were investigated by finite element analysis (ANSYS). The four-node structural and quadratic element PLANE 42 with axisymmetric option were used to model the Ti coating on glass and Si substrates. The influence of deposition temperature, substrate and coating properties on the generation of thermal stress in Ti is analyzed. It is found that the thermal stress of Ti coating exhibits a linear relationship with deposition temperature and Young's modulus of the coating, but it exhibit an inverse relationship with the coating thickness. The results of simulated thermal stress are in accordance with the analytical method. The radial stress and shear stress distribution of the coating–substrate combination are calculated. It is observed that high tensile shear stress of Ti coating on glass substrate reduces its adhesive strength but high-compressive shear stress of Ti on Si substrate improves its adhesive strength.     

Study on the subsonic speed flame spraying coating of hydroxyapatite

A new method of preparation of biomaterial composite coating by the technique of subsonic thermal spraying was discussed in this paper. Ti6Al4V and pure Ti were chosen as substrate and sublayer material respectively and the working layer was sprayed with biomaterial hydroxyapatite (HAP), forming the composite coating. The experiments of heat shock and tensile strength showed that the bonding strength between coating and substrate is almost as same as that of specimen in which Ni/Al powder was adopted as sublayer. The phases of TiN, TiO2, and Ti2O3 were formed in the sublayer, which are free of toxic and have no side effects. The powder of working layer HAP was decomposed partly during spraying, but it can be solved by later treatment.     

Effects of processing conditions on the mechanical properties and deformation behaviors of plasma-sprayed thermal barrier coatings: Evaluation of residual stresses and mechanical properties of thermal barrier coatings on the basis of in situ curvature measurement under a wide range of spray parameters

A modified analytical model has been developed to describe the nonlinear elastic response and residual stress in plasma sprayed thermal barrier coatings (TBCs) on the basis of the measured curvature–temperature plot during the spraying process. Evolution of residual stresses and the nonlinear stress–strain relation of the coating during the cooling stage after deposition were identified and the strain-dependent coating modulus was obtained. Wide ranges of deposition temperature from 200 to 850 °C as well as that of passage thickness from 6 to 58 μm were explored in the experiments because significant changes in the TBC microstructure could be expected. The analyzed results were related to the processing parameters in spraying, such as the substrate temperature and passage thickness. A complicated interplay among the coating microstructure, residual stresses and mechanical properties was identified. Generally with increasing deposition temperature or passage thickness, denser microstructures were observed with an increase in elastic modulus. The nonlinear strain–stress curves of TBCs indicated that the coating modulus increased with compressive residual stress due to closing of microcracks and inter-splat sliding. Moreover, the coating modulus depended not only on the magnitude of residual stress but also on the coating thickness and it was found that the axial force, which is the product of the residual stress and coating thickness, could be used to express their synergistic effect.     

Effects of coating thickness and residual stresses on the bond strength of ASTM C633-79 thermal spray coating test specimens

Wire-arc-sprayed nickel-aluminum is widely used in the aircraft industry for dimensional restoration of worn parts and as a bond coat for thermal barrier coatings and other top coats. Some repair applications require thick coatings, which often result in lower bond strength. A mechanism being investigated to ex-plain this decrease in bond strength is the free edge effect, which includes both coating residual stresses and coating thickness. The layer-removal method was used to determine experimentally the residual stresses in wire-arc-sprayed nickel-aluminum coatings of different thicknesses. Bond strength evalu-ations were performed using an improved ASTM C 633-79 test specimen. Finite-element analysis and fracture mechanics were used to investigate the effects of coating thickness and residual stress state on coating bond strength.     

Deposition of thick TiAlN coatings on 2024 Al/SiCp substrate by Arc ion plating

Aluminum-matrix composites with particulate SiC ceramic reinforcements (Al/SiC_p) have received much attention for space and aircraft propulsion applications. It is imperative to deposit thick hard coatings on these composites for protection. TiAlN coatings with a Ti interlayer were deposited by arc ion plating (AIP) on 2024 Al/SiC_p substrates at various nitrogen flow rates. It was found that when the nitrogen flow rate is increased from 100 sccm to 250 sccm, the deposition rate decreases, the coating hardness increases and the adhesion strength decreases. Based on the above results and the principle of gradient materials, the thick gradient TiAlN coatings with a Ti interlayer were successfully deposited on a 2024 Al/SiC_p substrate to a thickness of 60 μm by continuously increasing the nitrogen flow rate during deposition. Such an achievement can be attributed to the gradient distribution of elements, hardness, and stresses across the coating thickness.     

GH4169表面电火花沉积NiCrAlY涂层的制备

采用电火花沉积技术在GH4169合金上制备不同铝含量的NiCrAlY涂层，通过真空退火处理高铝的NiCrAlY涂层厚度.测试电火花沉积涂层的增重曲线，用扫描电子显微镜观察涂层的组织，用X射线分析仪测试涂层的物相组成，研究退火以及不同涂层的元素含量对NiCrAlY涂层组织的影响规律.结果表明,高铝含量的NiCrAlY涂层存在涂覆极限厚度；退火处理后再次涂覆可进一步增加涂层厚度.在四次涂覆+退火后涂层达到极限厚度，厚度约为45 μm.低铝含量的NiCrAlY涂层没有涂覆到极限厚度.对高铝含量的NiCrAlY涂层XRD结果表明,退火前主要物相为NiAl，退火后出现Ni3Al，涂层可以继续涂覆.低铝的NiCrAlY涂层物相为Ni3Al，没有涂覆极限.和NiAl相比，Ni3Al和GH4169的线膨胀系数接近，使含有Ni3Al相的涂层能够继续涂覆.     

Residual stress development in cold sprayed Al,Cu and Ti coatings

Residual stresses play an important role in the formation and performance of thermal spray coatings. A curvature-based approach where the substrate–coating system deflection and temperature are monitored throughout the coating deposition process was used to determine residual stress formation during cold spray deposition of Al, Cu and Ti coatings. The effect of substrate material (carbon steel, stainless steel and aluminium) and substrate pre-treatment (normal grit blasting, grit blasting with the cold spray system and grinding for carbon steel substrate) were studied for all coating materials with optimized deposition parameters. Mainly compressive stresses were expected because of the nature of cold spraying, but also neutral as well as tensile stresses were formed for studied coatings. The magnitudes of the residual stresses were mainly dependent on the substrate/coating material combination, but the surface preparation was also found to have an effect on the final stress stage of the coating.     

Influence of pre-plastic deformation on thermal residual stress and yield st rength of SiCw/Al composites

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Microstructure and physical properties of PVD TiN/（Ti, Al）N multilayer coatings

Magnetron sputtered （Ti, Al） N monolayer and TiN/（Ti, Al） N multilayer coatings grown on cemented carbide substrates were studied by using energy dispersive X-ray spectroscopy （EDX）, scanning electron microscopy （SEM）, nanoindentation, Rockwell A indentation test, strength measurements and cutting tests. The results show that the （Ti, Al）N monolayer and TiN/（Ti, Al）N multilayer coatings perform good affinity to substrate, and the TiN/（Ti, Al）N multilayer coating exhibits higher hardness, higher toughness and better cutting performance compared with the （Ti, Al）N monolayer coating. Moreover, the strength measurement indicates that the physical vapour deposition （PVD） coating has no effect on the substrate strength.