The microstructure of spray-formed Ti-6Al-4V/SiCf metal-matrix composites

Spray-forming is a possible manufacturing route for the fabrication of Ti alloy fibre-reinforced metal-matrix composites (MMCs) because high rates of alloy-droplet cooling on impact with the fibres prevent excessive fibre-matrix reaction. Ti–6Al–4V matrix MMC monotapes containing TiB₂-coated SiC fibres have been manufactured by electric-arc spray-forming, and the key MMC microstructural characteristics in the as-sprayed monotapes have been investigated by optical and scanning electron microscopy. Fibre infiltration increases with decreasing spraying distance, decreasing atomizing gas pressure and increasing arc current, because of higher temperatures in the Ti alloy spray droplets on impact with the fibres. Too much binder in the fibre preform leads to poor fibre–matrix contact, while removing the binder leads to the fibres becoming misaligned during spraying.     

工件预加载温度对Ti-6Al-4 V车削过程的影响

通过对比仿真和试验的切削力、切屑形态和锯齿化程度验证了仿真模型的可行性,然后用仿真分析的方法研究了工件预加载温度对切屑形貌、切削力和切削温度的影响,获得了锯齿形切屑转变为带状切屑的临界工件预加热温度。结果表明:随着预加载温度的增加,切屑变形减少;在v=50m/min时,锯齿形切屑转变为带状切屑的临界预加载温度为150℃～200℃;切削速度v=110～170m/min时,临界预加载温度为250℃左右。     

置氢对Ti-6Al-4V合金组织和性能的影响

为掌握置氢改善钛合金切削性能的机理,利用光学显微镜与X射线衍射仪分析了置氢Ti-6Al-4V合金的显微组织,采用热膨胀仪与热常数测试仪测定了该合金的热膨胀系数和导热系数,通过高温拉伸试验研究了该合金的力学性能。结果表明:置氢后,随着氢含量的增加,合金中的β相比例提高,并且生成了δ氢化物;与未置氢钛合金相比,在900℃时的热膨胀系数最大降幅为6%,而导热系数在500℃时的增幅最大,为40%;置氮后的钛合金,在800℃时抗拉强度与伸长率的降幅分别为19%和74%。     

Superplastic-like forming of Ti-6Al-4V alloy

Superplastic forming of titanium alloys is used for producing structural components, since it is an effective way to manufacture complex-shaped parts in a one-step operation. An optimized sheet-forming process has been designed incorporating a non-isothermal heating system to establish a fast forming process. This work sought to expand the advantages of the technology to the forming of Ti-6Al-4V alloy at 800 °C and shorter cycle time. The minimum thicknesses area was found at the outward corners, showing a maximum percent thinning of 54 %. In addition to stress variations, the cracks resulting from hot drawing and the oxidation on the sheet surface are the other reasons leading to thickness reduction. From the oxidization behavior of Ti-6Al-4V alloy, it was revealed that the decrease in forming temperature from 900 to 800 °C significantly reduced the formation rate of oxide film on the sheet surface. The study also showed that the main microstructure evolution of Ti-6Al-4V alloy under these conditions was recrystallization.     

SiC_f/Ti-6Al-4V复合材料的界面反应及其影响

采用扫描电镜和透射电镜研究了SiCf/Ti-6Al-4V复合材料的界面反应,并借助纤维顶出试验和有限元方法,分析了界面反应层厚度对界面剪切强度和残余热应力的影响.结果表明:SiCf/Ti-6Al-4V复合材料界面反应产物主要为TiC,其又分为二层,晶粒较粗大的TiC层靠近基体钛合金,晶粒细小的TiC层靠近SiC表面的碳涂层;随着反应层厚度的增加,界面抗剪强度提高,纤维和反应层内的径向残余热应力有所增大,反应层内的环向残余热应力分布有所改变.     

氢对热变形TC4钛合金显微组织的影响

研究了渗氢TC4钛合金热变形后及除氢处理后的显微组织.结果表明:渗氢TC4钛合金组织中的氢化物随氢含量增加逐渐长大;应变速率增加时,亚晶变形增加;除氢后由于动态回复亚晶界变得清晰光滑,亚晶内位错密度降低.     

Enhancing tribological performance of Ti-6Al-4V by sliding process

The exceptional combination of mechanical, physical and anti-corrosive properties of Titanium alloy Ti-6Al-4?V (Ti64) makes it idle material for the applications like aerospace, automobile, chemical, medical etc. However, Ti64 exhibits poor tribological (friction and wear) properties, which limits its implementation in the intended applications. The tribological performance of the Ti64 can be enhanced by developing a protective layer or coating on its surface. It has been reported in literatures that through rubbing process the oxide layers can be achieved at much lower temperature compared to external heating process. Therefore, an endeavour is made in the present work to achieve a tribo-oxide protective layer on the surface of Ti64 through rubbing process. For this, at first the tribological behaviour of tribo pair: Ti64 pin-alumina disc is studied under dry ambient condition for diverse loading and sliding speed conditions, using pin on disc experimental set-up. The obtained results are compared with literatures. The tribological performance is quantified in terms of coefficient of friction (COF) and wear rate. To investigate the tribological mechanism and behaviour, in-situ analysis was performed on the pin’s surface using (i) scanning electron microscopy, and (ii) energy dispersive analysis of X-ray. The mechanical properties like nano-hardness and elastic modulus of the pins surface were also determined. It was envisaged that the tribological behaviour were extremely transient and depend greatly on what the surface has precisely experienced Based on the experimental observations, the experimental conditions providing (i) Case1: deprived tribological properties and (ii) Case 2: higher oxide layer is selected. Now, to enhance the tribological behaviour of Case 1, the pin with high oxide layer, i.e. Case 2 is used. For this experiment is performed initially for Case 2 conditions for the sliding distance of 1000 m (for developing oxide layer) and the experiment is continued for next 1000 m for Case 1 condition. The experimental results in terms of COF and wear rate are presented and corresponding enhancement in their values are discussed.     

Investigation on diffusion wear during high-speed machining Ti-6Al-4V alloy with straight tungsten carbide tools

During high-speed machining Ti-6Al-4V alloy, high-temperature at the tool–chip interface and the concentration gradient of chemical species between tool material and workpiece material support the activation of diffusion process, and therefore the crater wear forms on the rake surface of the cutting tool at a short distance from the cutting edge. In this paper, the diffusion analysis was theoretically proposed. The constituent diffusion at the contact interface between tool material and Ti-6Al-4V alloy at high-temperature environment, the crater wear on the rake surface of the tool, and the chips collected from high-speed milling Ti-6Al-4V alloy with straight tungsten carbide tools were analyzed by the scanning electron microscope with energy dispersive X-ray spectroscopy. The constituents inside the tool could diffuse into the workpiece and the diffusion layer was very thin and close to the interface. Compared with the diffusion of tungsten and carbon atoms, the pulling out and removing of the tungsten carbide (WC) particles due to cobalt diffusion dominated the crater wear mechanism on the rake surface of the cutting tool.     

Fibre laser welding of dissimilar alloys of Ti-6Al-4V and Inconel 718 for aerospace applications

Challenges in dissimilar materials welding are the differences of physical and chemical properties between welding materials and the formation of intermetallic brittle phases resulting in the degradation of mechanical properties of welds. However, dissimilar materials welding is increasingly demanded from the industry as it can effectively reduce material costs and improve the design. In aerospace applications, Ti-6Al-4V titanium alloy and Inconel 718 nickel alloy have been widely used because of their superior corrosion resistance and mechanical properties. In this study, a single-mode continuous-wave fibre laser was used in butt welding of Ti-6Al-4V to Inconel 718. Investigations including metallurgical and mechanical examinations were carried out by means of varying processing parameters, such as laser power, welding speed and the laser beam offset position from the interface of the metals. Simple analytical modelling analysis was undertaken to explain the phenomena that occurred in this process. Results showed that the formation of intermetallic brittle phases and welding defects could be effectively restricted at welding conditions produced by the combination of higher laser power, higher welding speed and shifting the laser beam from the interface to the Inconel 718 alloy side. The amount of heat input and position of laser beam to improve the Ti-6Al-4V/Inconel 718 weld quality are suggested.     

The effect of microstructure on the erosion of Ti-6Al-4V by spherical particles at 90° impact angles

Ti-6A1-4V alloy in two microstructural forms, equiaxed α + β and a basketweave α + β structure, was eroded. The erosion conditions utilized were an impact angle of 90°, spherical silica particles 210 μm in diameter and a velocity of 61 m s^?1. Examination of the eroded surfaces indicated that both microstructures were extensively covered by a layer of glass approximately 2 μm thick prior to the onset of steady state erosion. Thermal softening and plastic flow of erodent on impact formed the glass layer. In this way a large area of the target was protected from direct impact. Consequently little evidence of subsurface damage during erosion was found for either structure. Material loss during steady state erosion was possible only at discontinuities in the glass layer or when it was removed to reveal a fresh metallic surface. The primary mechanism of metal removal was found to be the same in incubation and steady state irrespective of the microstructure. Small platelets were formed from overlapping strain fields by several individual impacts. Other operating mechanisms were cutting due to fractured or preexisting irregular erodent and removal of lips from oblique impacts.     

Mechanical properties of the alloy Ti-6Al-4V irradiated with swift Kr ion

The radiation damage effect on the friction coefficient, wear, and microhardness of the alloy Ti-6Al-4V after 250 MeV krypton ion irradiation was studied. Tribological measurements were made in air, oxygen, and argon atmospheres and in a vacuum. The smallest friction coefficient for the irradiated samples occurred in argon and the vacuum. The wear of the unirradiated samples and those irradiated with low fluences (<10¹³ cm⁻²) increased in the vacuum and argon atmosphere. Wear was significantly reduced after irradiation with the fluence of 10¹⁴ cm⁻². The microhardness of the alloy Ti-6Al-4V increased by over 25% after irradiation with a large fluence.     

Fretting Wear Behavior of Laser Peened Ti-6Al-4V

This work deals with the influence of laser peening on the fretting wear behavior of Ti-6Al-4V. Laser peening was carried out on Ti-6Al-4V. The laser-peened surface was characterized by transmission electron microscopy. Surface roughness, nanoindentation hardness, residual stress, and tensile properties of the material in both laser-peened and unpeened conditions were determined. Fretting wear tests were conducted at different normal loads using a ball-on-flat contact geometry. Laser peening resulted in the formation of nanocrystallites on the surface and near-surface regions, increased hardness, and compressive residual stress. Laser peening did not affect the tensile properties and surface roughness significantly. There was no considerable difference between the values of the tangential force coefficient of laser-peened and unpeened samples. The fretting scar size, wear volume, and wear rate of laser-peened specimens were lower than those of unpeened samples. This may be attributed to an increase in surface hardness due to strain hardening and grain refinement at the surface and near-surface regions, higher compressive residual stress, and higher resistance to plastic deformation of laser-peened samples.     

Fretting fatigue of laser shock peened Ti-6Al-4V

The objective of this paper is to examine fretting fatigue of laser shock peened (LSP) titanium to quantify the influence of LSP on fretting fatigue life. Contact conditions such as loads and pad geometry are chosen to generate fretting fatigue stresses similar to those occurring in blade/disk contacts in gas turbine engines. LSP treated specimens attained 5-, 10- and 25-fold increase in lives compared to untreated specimens. Metallography of the contact area and fractographic analysis of worn pads detail the fretting behavior of LSP treated specimens.     

Effect of electropulsing on springback during V-bending of Ti-6Al-4V titanium alloy sheet

Because the intermediate-stage forming surface and the intermediate-stage forming tool path of the complex shape model are difficult to generate, the existing multi-stage CNC incremental forming takes the regular rotary body model or the square model as the research objects, and all adopt the three-axis CNC incremental forming mode. In this paper, a method for generating the intermediate-stage surface by using a longitude line that can reflect the personality of the surface and the five-axis CNC multi-stage incremental forming tool path was proposed. Firstly, the vertexes of the triangular facets of the STL model are used to generate the longitude lines which can reflect the characteristic of the surface, then the longitude lines are offset according to the multi-stage forming strategy and the characteristics of each surface associated with the longitude lines so that the intermediate-stage longitude lines could be generated, and then the intermediate-stage surfaces are built using the intermediate-stage longitude lines. Finally, the cutter location points of each intermediate stage are obtained by cutting the intermediate-stage surfaces, and the postures of the five-axis CNC pressing tool are determined according to the normal vector of the cutter location points. The case studies show that the proposed method can well generate the five-axis CNC multi-stage incremental forming path for the complex shape sheet metal part. The results of the numerical simulation analysis and forming experiments show that the proposed method is applicable.     

Comparative study of the microstructure of Ti-6Al-4V titanium alloy sheets under quasi-static and high-velocity bulging

Journal of Mechanical Science and Technology - In order to reveal the high-velocity deformation mechanisms of Ti-6Al-4V titanium alloy sheets, the dynamic deformation behavior and the...