Hot deformation behaviors of Fe-microalloyed Ti-6Al-4V based on experiments and calculations

Our previous results have shown that comprehensive mechanical properties of titanium alloys can be effectively improved by addition of Fe[1]. We systematically investigate hot deformation behaviors of Ti-6Al-4V-0.35Fe in this study, which is significant to improve plastic deformation ability of titanium alloys. In experiment, we use a Gleeble 3800 thermo-mechanical simulator to obtain the relationship between thermomechanical parameters and flow stress in a range of temperatures (800-950 °C) and strain rates (0.001-10 s-1). The single-peak profiles of the flow curves indicate that dynamic recrystallization (DRX) mechanism dominates the deformation. TEM analysis indicate that the grain size in DRX changes under different deformation temperatures, and finer grains are formed at relatively lower temperature due to the dynamic globularization. The dislocation walls are formed in subgrain boundaries due to dislocation slipping-climbing. The Avrami-type DRX model and the strain compensated multivariable regression model have been applied to fit the experimental stress-strain data during hot deformation. A comparative study between these two types of constitutive models is conducted to represent the flow behavior. It is found that both models have good accuracy in predicting the flow stress of Ti-6Al-4V-0.35Fe alloy. A processing map based on dynamic material model (DMM) at the strain of 0.8 (steady-state flow stage) has been established to identify the flow instability regions and stability regions. The strain rate range of stability region is 0.001-0.6s-1 which has been expanded compared to the range of 0.0003-0.1s-1 of Ti-6Al-4V. Optimal hot working parameters are confirmed to be 920-950 °C and 0.001-0.005 s-1, and nearly complete DRX has taken place. Our results indicate that hot working property of Fe-microalloyed Ti-6Al-4V is better than that of Ti-6Al-4V alloy in 800-950 °C temperature scale, and processing cost has been decreased.     

Corrosion resistance properties of plasma nitrided Ti-6Al-4V alloy in hydrochloric acid solutions

The corrosion properties of TiN layers obtained by glow discharge on Ti-6Al-4V samples were investigated by polarisation curves, cyclic voltammetry and loss weight tests, in different HCl solutions. The results were compared with those obtained on the untreated alloy tested in the same conditions. The corrosion resistance of nitrided samples depends on the quality of the film and increases as the thickness increases. At potential lower than 1000 mV (SSE) nitrided samples show passive behaviour even in concentrated HCl solution (4M). If the potential is higher than a threshold, which depends on the acid concentration, it is observed a high anodic current related to the TiN oxidation. This enhanced oxide growth promotes cracking of the oxide layer and the exposure of fresh TiN surface.     

钛合金热等静压模拟本构关键参数确定及工艺优化

对钛合金粉末热等静压进行数值模拟,以预测材料的成形性能;采用本构为MSC.Marc软件中嵌入的Shima模型,通过对该数值模型的分析;制定了本构模型中TC4材料模拟所需材料参数的测定方法。针对航空类典型环形零件的成形,应用数值模型,通过对多种不同工艺制度参数的对比,得到了热等静压过程中的关键参数压力、温度曲线的优化制度。利用热等静压制备了实际的环形样件,数值模拟与试验结果吻合较好,说明数值模拟可对零件的热等静压工艺过程进行预测,同时也为复杂零件的成形预测奠定了基础。     

TC4钛合金高速铣削力研究

钛合金高速铣削因具有高效率、高质量的优点,被广泛应用于航空航天制造业。为了研究高速铣削参数对钛合金高速铣削力的影响,利用专业金属切削加工有限元软件AdvantEdge,对TC4钛合金高速铣削过程进行二维模拟仿真,建立了高速铣削TC4钛合金时铣削力的预测模型,获得了不同铣削参数对铣削力的影响规律,并对仿真结果进行了试验验证。结果表明:高速铣削TC4钛合金的铣削力比较小,基本不超过100 N,铣削力最大值达到140 N;铣削合力对铣削宽度的变化最为敏感,对铣削速度和铣削深度变化的敏感次之,对每齿进给量最不敏感。研究结果为优化高速铣削工艺提供理论分析和试验依据。     

Corrosion resistance properties of plasma nitrided Ti-6Al-4V alloy in nitric acid solutions

The corrosion resistance of plasma nitrided Ti-6Al-4V titanium alloy in nitric acid solutions was investigated using polarisation curves, cyclic voltammetry, XRD analysis and surface microhardness. For comparison, untreated alloy samples were tested under the same conditions. While the untreated alloy shows a passive behaviour, the compound layer obtained by glow-discharge nitriding treatment, which is composed of an outer TiN and an inner Ti₂N layer, shows a high electrochemical inertia; however, if the polarisation potential is higher than a threshold or the period of immersion in high oxidising acid solution is sufficiently long, then the compound surface layer will be attacked and removed with corrosion rate higher than in the case of the untreated Ti-6Al-4V alloy. The corrosion resistance of Ti₂N layer results higher in comparison with that of TiN layer.     

Dynamics of chip formation during orthogonal cutting of titanium alloy Ti-6Al-4V

Polished and etched disks of titanium alloy Ti-6Al-4V were machined in a series of continuous and interrupted orthogonal cutting tests on a specially adapted lathe. A high-speed imaging system with a microscope lens and strobed copper-vapour laser illumination system enabled direct observation of the chip formation zone at a recording rate of 24,000 frames/s. The chip formation cycle was recorded for cutting speeds from 4 to 140 m/min. Segmented chips were observed throughout. Image analysis of the recorded video sequences examined the resulting chip segment geometry, the segmentation frequency and the critical strain required to initiate shear band formation.     

Influence of spark plasma sintering on microstructure and wear behaviour of Ti-6Al-4V reinforced with nanosized TiN

Ti-6Al-4V/TiN composites were successfully consolidated by spark plasma sintering (SPS). TiN addition to Ti-6Al-4V was varied from 1% to 5% (volume fraction). The effect of TiN addition on the densification, microstructure, microhardness and wear behaviour of Ti-6Al-4V was studied. Experimental results showed reduction in sintered density of the compacts from 99% to 97% with increase in TiN content. However, an increase in microhardness value was recorded from HV_0.1 389 to HV_0.1 488. X-ray diffraction (XRD) analysis showed that the intensity of diffraction peaks of TiN phase in the composites increased also with formation of small amount of secondary Ti₂N phase. SEM analysis of SPS sintered nanocomposites possessed a refinement of α/β phase microstructure in Ti-6Al-4V with the presence of uniformly dispersed TiN particles. The worn surface of the composite showed improved abrasive wear resistance with non-continuous grooves as compared to the sintered Ti-6Al-4V without TiN addition.     

Bulk titanium nitride material obtained from SHS starting powder: Densification, mechanical characterization and tribological approach

Powders used for this study were synthesized by the self-propagating high temperature synthesis (SHS) process. This method allows producing titanium nitride powders in an economic and easy way. Densification of these original powders is carried out by the hot pressing process. Sintering is achieved, under a 50 MPa pressure, at different temperatures (1400 °C, 1600 °C and 1800 °C). At 1600 °C properties are optimised and bulk specimen density is superior to 98%. In addition, microhardness reaches the average value of 1790 ± 225 H_V and Young modulus measured is 430 ± 10 GPa. A tribological characterization is then carried out with an adapted four balls machine tribometer. Titanium nitride tribological behavior is compared with the one of other hard materials such as 100C6 steel, titanium carbide and two cemented carbides of different compositions. Parameters such as toughness, hardness and wear volume are correlated in order to understand wear mechanisms origins.     

Thermal expansion and lattice parameters of shaped metal deposited Ti-6Al-4V

Thermal expansion and lattice parameters are investigated up to 1100 °C for Ti-6Al-4V components, fabricated by shaped metal deposition. This is a novel additive layer manufacturing technique where near net-shape components are built by tungsten inert gas welding.The as-fabricated SMD Ti-6Al-4V components exhibit a constant coefficient of thermal expansion of 1.17 × 10⁻⁵ K⁻¹ during heating up to 1100 °C, not reflecting the α to β phase transformation. During cooling a stalling of the contraction is observed starting at the β transus temperature. These high temperature experiments denude the α phase of V and enrich the β phase.The development of the lattice parameters in dependence on temperature are observed with high temperature X-ray diffraction. The unit cell volumes derived from these parameters are at room temperature larger for the α than for the β phase. With increasing temperature the unit cell volume of the β phase increases stronger than the one of the α phase resulting in a similar unit cell volume at the β transus temperature.These observations are interpreted as an indication for as-fabricated the SMD components being in a non-equilibrium state and reaching equilibrium during the slow heating and cooling during of the two different high temperature experiments.     

基于Ti-6Al-4V的热等静压与温压成形数值模拟对比研究

为对比研究钛合金的热等静压和温压成形工艺特点,以典型圆柱件成形为例,采用MSC.MARC软件,选取了Shima模型嵌入Ti-6Al-4V材料模型进行数值模拟.通过分析两种工艺过程中粉末体的流动情况和致密度分布,得出粉末体的热等静压是一个先向外膨胀再向中心收缩,最终达到全致密的过程,致密度分布均匀;而温压成形是沿着冲头方向直接进行收缩,致密度不高并存在明显的梯度现象.通过两种成形特点的对比,能分别预测成形方法的变形以及指导两种成形方法的选取.     

Dehydrogenation of Hydrogenated Ti-6Al-4V Alloy

对未置氢及置氢Ti-6Al-4V合金进行了TG/DSC试验,研究了置氢钛合金的除氢行为。结果表明,当温度超过600℃时,置氢钛合金的失重规律与未置氢钛合金具有较大的差别。当加热温度在600~900℃之间时,置氢钛合金的失重随着氢含量的增加而增加。这是由于置氢合金中的亚稳相发生了分解。不考虑合金氧化的影响,置氢钛合金的最大失重与合金中的氢含量一致。置氢钛合金的最佳除氢温度为750℃。对于不同氢含量的置氢钛合金,其除氢工艺是相同的。     

Ti及Ti-6Al-4V的水基凝胶注模成形研究

将凝胶注模工艺应用于金属Ti及Ti-6Al-4v合金粉末的坯体成形，研究了高固相含量的Ti粉和Ti-6Al-4V合金粉末的料浆的制备。结果表明，用凝胶注模工艺制备出了固相含量为54φ%的钛合金粉末料浆和形状复杂的坯体；粉末的颗粒形状是影响料浆固相含量的重要因素，球形粉末配制成的浆料固相含量最高，近球形次之，片状最低；分散剂柠檬酸铵也可以显著提高浆料的固相含量。     

Composition and Structure of Ti-6Al-4V Alloy Plasma-based Ion Implanted with Nitrogen

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Ti-6Al-4V钛合金等通道转角挤压有限元模拟

对Ti-6Al-4V钛合金的等通道转角挤压过程进行三维有限元模拟,分析不同的凸模下压速度对等通道转角挤压过程的影响,并结合实际的挤压过程考虑挤压后残留在模具出口通道内的残余试样对挤压下一根试样的影响.结果表明:挤压速度的提高对应力、载荷和温升的影响很大,对应变速率很敏感的钛合金应在有效细化晶粒的前提下降低挤压速度,试验中挤压速度取0.3mm·s-1;残余试样的存在使变形更均匀,但增加了挤压下一根试样时的初始阶段的载荷.     

Ti-6Al-4V基体上等离子喷涂羟基磷灰石涂层的热-力耦合和残余应力

基于热弹塑性理论建立了等离子喷涂羟基磷灰石涂层的热-力耦合模型。一方面,通过引入变形对温度的影响项对经典的Fourier瞬态热传导方程进行了修正,另一方面,在考虑了温度软化效应、应变硬化效应、和应变率强化效应后,采用适用于高温、高应变率条件下的Johnson-Cook模型作为本构方程。基于上述耦合模型,采用有限元方法对涂层内的残余应力场进行了模拟,同时还模拟了基体预热温度以及涂层厚度对残余应力的影响。为了验证数值模拟的可靠性,还采用“材料去除”的实验技术测试了涂层在界面处的残余应力。结果表明：在界面边缘有明显的应力集中;适当提高基体的预热温度有利于减小残余应力;涂层的残余应力随厚度的增加而增大。     

Ti- 6Al-4V/hydroxyapatite composite coatings prepared by thermal spray techniques

The poor mechanical properties of hydroxyapatite (HA) can be enhanced by forming a composite with a bioinert and mechanically strong metal alloy such as Ti-6A1-4V. Biomedical composites composed of titanium alloys and HA can offer concomitant bioactive properties as well as good mechanical strength and toughness. This paper describes an attempt to improve coating mechanical properties by forming a composite composed of HA and Ti-6A1-4V. Several compositions (20, 33, and 80 wt % HA) were prepared. Subsequent examination of the plasma-sprayed coatings revealed alternating HA-rich and titanium-rich lamella microstructures. The HA-rich regions appeared porous as a result of poor interparticle adhesion, with the 80 wt% HA coatings having the highest porosity. Mechanical property analysis showed the 20 wt% HA coating to have the highest storage modulus (∼60 GPa). This coating also had the highest bond strength (≥20 MPa max). The coatings tended to exhibit increased bond strength at thicknesses less than or equal to 60 μm. The excellent bond strength of the Ti-6A1-4V/HA composite is caused by the superior interfacial bond between the Ti-6Al-4V-rich splats and the substrate. The encouraging development of this composite raises the possibility of its use as a bond coat for plasma-sprayed HA on titanium-alloy implants.     

Laser treatment and PVD TiN coating of Ti-6Al-4V alloy

Improved adhesion between a coating and the base substrate is necessary for successful engineering applications. This may be achieved through generating intermediate zone between the coating and the substrate. In the present study, laser gas alloying of Ti-6Al-4V alloy was carried out. Nitrogen was introduced co-axially with the laser beam while the workpiece is scanned at a constant speed in the x-direction during the laser heating process. The laser-treated workpieces were then slightly ground and TiN PVD coated. Thickness of the coating layer was limited to 2 μm. The wear tests were conducted on the laser-treated TiN-coated workpieces. To examine the metallurgical changes, microphotographs of workpiece cross-sections were taken. To monitor the nitride compounds in the surface region, XRD was carried out. It was found that three wear stages occur. The steady wear stage is relatively longer for laser-treated TiN-coated workpiece compared to the TiN-coated workpiece.     

Tool development based on modelling and simulation of hot sheet metal forming of Ti-6Al-4V titanium alloy

In the aero engine industry alternative manufacturing processes for load carrying aero engine structures imply fabrication. The concept of fabrication involves simple forgings, sheet metals and small ingots of e.g. titanium alloys which are welded together and heat treated. In the concept phase of the product development process, accurate evaluations of candidate manufacturing processes with short lead times are crucial. In the design of sheet metal forming processes, the manual die try out of deep drawing tools is traditionally a time consuming, expensive and inexact process. The present work investigates the possibility to design hot forming tools, with acceptable accuracy at short lead times and with minimal need for the costly die try out, using finite element (FE) analyses of hot sheet metal forming in the titanium alloy Ti-6Al-4V. A rather straightforward and inexpensive approach of material modelling and methods for material characterisation are chosen, suitable for early evaluations in the concept phase. Numerical predictions of punch force, draw-in and shape deviation are compared with data from separate forming experiments performed at moderately elevated temperatures. The computed responses show promising agreement with experimental measurements and the predicted shape deviation is within the sheet thickness when applying an anisotropic yield criterion. Solutions for the hot forming tool concept regarding heating and regulation, insulation, blank holding and tool material selection are evaluated within the present work.     

Electrochemical behavior of the Ti6Al4V alloy implanted by nitrogen PIII

Plasma surface treatments have been used very often to enhance the surface properties of metallic materials. In this work, Ti6Al4V titanium alloy was treated by nitrogen plasma immersion ion implantation (NPIII) in order to obtain improvements in its surface properties, such as corrosion resistance evaluated here. The microstructure and corrosion behavior of the implanted and unimplanted samples were evaluated, using, XRD, GDOES and potentiodynamic polarization and impedance electrochemical spectroscopy tests in 0.6 M NaCl solution. It was verified that the NPIII created resistant layers to corrosive attacks. In corrosion tests by polarization, the implanted samples showed corrosion current density reduction of about 10 times compared to the Ti6Al4V alloy without treatment. Besides that, it was also observed a reduction of the passive current density of one order of the magnitude. In all the studied cases, the polarization curves were shifted to more positive values of potentials, indicating a lower tendency of these PIII treated surfaces to corrosion. The implantation process produced a thin TiN surface layer followed by Ti₂N and then a layer with nitrogen in solid solution, all detected by GDOES combined with X-ray diffraction. These layers promoted an excellent polarization resistance of the Ti6Al4V surfaces on impedance spectroscopy tests also. This better performance in these tests can be correlated with the formation of continuous nitride layer, which could retard chloride ions ingress into the substrate.