Effect of mean stress on fretting fatigue of Ti-6Al-4V on Ti-6Al-4V

Fretting fatigue tests of Ti‐6Al‐4V on Ti‐6Al‐4V have been conducted to determine the influence of stress amplitude and mean stress on life. The stress ratio was varied from R=−1 to 0.8. Both flat and cylindrical contacts were studied using a bridge‐type fretting fatigue test apparatus operating either in the partial slip or mixed fretting regimes. The fretting fatigue lives were correlated to a Walker equivalent stress relation. The influence of mean stress on fretting fatigue crack initiation, characterized by the value of the Walker exponent, is smaller compared with plain fatigue. The fretting fatigue knockdown factor based on the Walker equivalent stress is 4. Formation of fretting cracks is primarily associated with the tangential force amplitude at the contact interface. A simple fretting fatigue crack initiation metric that is based on the strength of the singular stress field at the edge of contact is evaluated. The metric has the advantage in that it is neither dependent on the coefficient of friction nor the location of the stick/slip boundary, both of which are often difficult to define with certainty a priori.     

Biaxial deformation of Ti-6Al-4V and Ti-6Al-4V/TiC composites by transformation-mismatch superplasticity

Gas-pressure bulge forming of unreinforced Ti-6Al-4V and TiC-reinforced Ti-6Al-4V was performed while cycling the temperature around the allotropic transformation range of the alloy (880–1020 °C). The resulting domes exhibited very large strains to fracture without cavitation, demonstrating for the first time the use of transformation-mismatch superplasticity under a biaxial state of stress for both an alloy and a composite. Furthermore, much faster deformation rates were observed upon thermal cycling than for control experiments performed under the same gas pressure at a constant temperature of 1000°C, indicating that efficient superplastic forming of complex shapes can be achieved by transformation-mismatch superplasticity, especially for composites which are difficult to shape with other techniques. However, the deformation rate of the cycled composite was lower than for the alloy, most probably because the composite exhibits lower primary and secondary isothermal creep rates. For both cycled materials, the spatial distribution of principal strains is similar to that observed in domes deformed by isothermal microstructural superplasticity and the forming times can be predicted with existing models for materials with uniaxial strain rate sensitivity of unity. Thus, biaxial transformation-mismatch superplasticity can be modeled within the well-known frame of biaxial microstructural superplasticity, which allows accurate predictions of forming time and strain spatial distribution once the uniaxial constitutive equation of the material is known.     

Fatigue Behavior of Ti-6Al-4V

Low-cycle-fatigue texts in vacuum and air were performed. Under cyclic loading the Ti-6Al-4V showed both cyclic hardening and cyclic softening depending on heat treatment, stress amplitude, and microstructure. Plastic deformation of the β-phase in the unaged condition due to stress induced martensitic transformation caused cyclic hardening. Cyclic softening was observed if the α-phase hardened by coherent Ti₃Al particles was plastically deformed. Equiaxed microstructures exhibited a stronger cyclic softening than lamellar structures. This behavior could be explained by the pronounced texture of the equiaxed microstructures, whereas the lamellar structures were texture-free. The fatigue life was influenced by the cyclic softening process mainly in the low-cycle-fatigue regime. The fatigue life at normalized stress amplitude (σ_a/σ_y) was shorter for microstructures with strong cyclic softening as compared to microstructures with lower cyclic softening.     

Thermomechanical Behavior of Ti-6Al-4V Alloy

Ti-6Al-4V, among the Ti alloys, is the most widely used. In the present work, the behavior of Ti-6Ak-4V alloy has beeninvestigated by the uniaxial hot isothermal compression tests and a series of dilatometric experiments were also carried out todetermine the transformation temperatures at different cooling rates. Specimens for hot compression tests were homogenizedat 1050℃ for 10 min and then quickly cooled to different straining temperatures from 1050 to 850℃. Cooling rates were chosenfast enough to prevent high temperature transformation during cooling. Compression tests were conducted at temperaturesfrom 1050 to 850℃ in steps of 50℃ at constant true strain rates of 10~(-3) or 10~(-2) s~(-1). The apparent activation energy forcompression in two-phase region was calculated 420 kJ·mol~(-1). Partial globularization of cr phase was observed in the specimendeformed at low strain rates and at temperatures near the transformation zone and annealed after deformation.     

Superplastic deformation of strongly textured Ti-6 Al-4 V

Stress and strain anisotropy of a strongly textured Ti-6 Al-4 V alloy bar during superplastic deformation at 880 and 928° C has been investigated. After 0.9 superplastic strain at 928° C the deformation tended to become isotropic. The anisotropic superplastic deformation was found to be dependent upon the aligned microstructure and not influenced by the original -phase crystallographic texture. The room-temperature anisotropy before and after superplastic plastic deformation was controlled by the original -phase texture, which was still present even after 1.48 strain (344% elongation) at 928° C.     

Laser surface melting of Ti-6Al-4V alloy

Surface hardening of Ti-6Al-4V alloy with laser surface melting (LSM) in a nitrogen atmosphere has been studied. In LSM, hardness increased almost three-fold in comparison to that of the substrate, the latter having a Vickers hardness of 350, by the formation of TiN in the range of 100m of melt depth. Hardness, then, decreased slowly and reached a minimum of 580 VHN at a maximum melt depth of 750m. -Ti was formed in the heat-affected zone (HAZ) with a VHN of 450. Ageing treatments were performed for all specimens at 450 °C and different ageing times (1–20h). Short ageing treatments increased the hardness in the melted zone as well as in the HAZ (1–3h). Long ageing treatments (7–20h) resulted in uniform hardness distribution in the melted zone.     

Superplastic deformation of strongly textured Ti-6Al-4V

Changes in microstructure and texture during superplastic deformation of strongly textured Ti-6Al-4V bar have been determined in order to establish the cause of stress and strain anisotropy. The effect of strain on the microstructure of the alloy was to cause a progressive break-up, due to grain-boundary sliding, of an initially directional microstructure containing contiguous-phase. The texture of the-phase, however, changed very little with superplastic strain but that of the-phase was randomized. Shape changes predicted by permitted deformation modes in the-phase did not correlate with the observed shape changes, whereas the observed anisotropy could be explained by the break-up of the contiguous-phase. A model to account for this anisotropy is described briefly, together with a typical microstructure which should exhibit isotropic superplastic deformation.     

Laser surface modification of Ti-6Al-4V alloy

Hardening of Ti-6Al-4V alloy with laser surface melting (LSM) and laser surface alloying (LSA) techniques was attempted. Both LSM and LSA were carried out in a nitrogeneous atmosphere. Niobium, molybdenum and zirconium were used as alloying elements in the LSA. A hardness increase was observed for both LSM and LSA. Maximum hardness was obtained for LSM and zirconium alloy addition. In LSM, hardness increased almost three-fold in comparison to the substrate, which has a Vickers hardness of 350, by the formation of TiN in the region of 100 m melt depth. Hardness then decreased slowly and reached a minimum of 580 VHN at the maximum melt depth of 750 m. However, hardness for the zirconium alloy addition was uniform throughout the melted zone. Ageing treatments were performed for all specimens at 450C and different ageing times. Hardness measurements and X-ray diffraction were utilized to delineate the features associated with the hardening of the melted zone.     

Ti-6Al-4V组织相变研究进展

钛合金因具有密度小、延展性好、疲劳性优异和断裂性能以及比强度高等优点而被广泛应用于航空航天等领域。综述了近年来国内外学者在Ti-6Al-4V固态相变理论和实验研究方面的进展情况,总结了其由β相区在不同冷却速率下所得到的相变组织,得到了hcp-α向bcc-β转变时具有伯格斯取向关系,介绍了包括马氏体相变、块型转变以及扩散相变的转变温度,当冷却速率达到525℃/s时发生马氏体相变,β相将转变为α￠相,当冷却速率为20~410℃/s时发生竞争扩散型相变β→α_m,低于20℃/s发生扩散控制相变,原始β晶粒中出现不规则的αGB、初生α集束和魏氏α组织。最后探究了新技术在Ti-6Al-4V微观相变组织中的应用。     

Microstructural Features of Friction-Welded Ti-6Al-4V Joint

1. IntroductionTi-6Al--4V is presently the most widely used Tialloyt especially in aerospace and marine industries.The fabrication of many structural components madeof Ti--6Al-4V alloy often requires welding. The highheat input and fast cooling rate during welding frequently lead to local microstructure changes which,in turn, change the local mechanical behavior in thewelded joint and, thereby, the overall performanceof the l'velded component[1'2]. In Ti-6Al-4V jointswelded with the laser …     

Hydrogen Treatment of Cast Ti-6Al-4V Alloy

This paper presents the results of an investiga-tion of the effect of hydrogen treatment onmicrostructures and tensile and low cycle fatigueproperties of a Ti-6Al-4V cast alloy.The phasetransformation and the refining mechanism of thecast microstructure during the process of hydrogentreatment were studied.It was found that afterhydrogen treatment,the coarse Widmanstttenstructure of the as-cast Ti alloy was transformedinto a very fine and equiaxed α+β microstructurewithout any GBα phase.The tensile strength andductility and the low cycle fatigue life of thehydrogen treated specimens were significantly im-proved.     

Superplastic deformation of Ti-6Al-4V extruded tube

Superplastic forming using conventional alloys and product forms may be a cost effective route for component manufacture. In this paper superplastic deformation of Ti-3Al-4V extruded tube is examined in terms of the strain anisotropy, microstructural changes, the texture and the post-formed tensile properties. Superplastic deformation with low flow stresses over a wide range of strain rates was observed. Strain anisotropy and surface roughening occurred during deformation and was associated with the two-phase, aligned and banded microstructure. A small reduction in the tensile properties was found and changes in the texture were noted after superplastic deformation.     

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54ψ%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.     

Microstructural assessment of laser nitrided Ti-6Al-4V alloy

A microstructural study of the phases developed during the laser nitriding of a Ti-6Al-4V alloy by, using a CL5 continuous CO₂ laser with a spinning beam and concentration of 80% nitrogen, was undertaken. The vertical sections, perpendicular to the melt track were examined by optical microscopy and scanning electron microscopy (SEM), while specimens for X-ray diffractometry (XRD), X-ray photospectroscopy (XPS) and transmission electron microscopy/selected area electron diffraction (TEM/SAED), were taken parallel to the melt track. In this way the variation in microstructure as a function of depth from the laser treated surface, was studied. This supplemented XRD and XPS investigations undertaken previously. Two zones were identified. Zone 1, within 50 m of the surface, contained well defined dendrites of fcc TiN_0.8, plus hcp TiN_0.3 and hcp Ti. Zone 2, below 50 m, consisted of needles of hcp Ti. From a consideration of the hardness profiles in Zone 2, it is suggested that at the top of the zone, the phase is, in fact, a solid solution containing 3–4% N, which decreased to <1% N at the bottom of the zone. The TEM/SAED study permitted the three phases fcc TiN_0.8, hcp TiN_0.3 and hcp Ti to be identified through a combination of morphology and SAED patterns. This also showed that the fccTiN_0.8 contained fringes, which were considered to be stacking fault fringes and allowed this phase to be readily recognized in the TEM. The presence of stacking faults may be associated with the high nitrogen concentration of 80% used for the laser nitriding in this work.     

Ti-6Al-4V合金的室温蠕变行为

研究了Ti-6Al-4V钛合金板材的室温蠕变行为及其对合金后续使用性能的影响.结果 表明:合金的宏观织构、应力水平以及预塑性应变都显著影响其室温蠕变行为.在加载方向上合金的<0001>峰值极密度越高,则其加工硬化指数越大、蠕变指数越小、室温蠕变性能越好.足够大的应力,是合金发生室温蠕变的必要条件.只有在蠕变应力不小于0...     

Laser-beam welding of SiC fibre-reinforced Ti-6Al-4V composite

Three- and ten-ply SiC fibre-reinforced Ti-6Al-4V composites were joined using a laser beam. With a 300 m thick Ti-6Al-4V filler metal, fully penetrated welds without apparent fibre damage, could be obtained in welding directions both parallel and transverse to the fibre direction by controlling the welding heat input. Excess heat input resulted in the decomposition of SiC and subsequent TiC formation, and also caused degradation of joint strength. The welding of the three-ply composite in which full penetration was achieved at lower laser power, exhibited higher flexibility in heat input than that of the ten-ply composite. Heat treatment at 1173 K after welding improved the joint strength because of the homogenization of the weld metal and decomposition of TiC. The strengths of the transverse weld joints after the heat treatment were approximately 650 and 550 MPa for the three- and ten-ply composites, respectively. With the welding direction parallel to the fibre direction, the strengths both parallel and transverse to the weld joint were equivalent to those of the base plate.     

In-vivo degradation mechanism of Ti-6Al-4V hip joints

In-vivo exposed Ti-6Al-4V implants were investigated to determine the degradation mechanism occurring during the articulating movements of the hip joint in the human body. Failed implants were compared to Ti-6Al-4V samples, which were tested in the laboratory for their tribocorrosion performance. The results strongly indicate that degradation of Ti-6Al-4V has occurred with the same mechanism for both the implants and the laboratory tested samples and, hence, block-on-ring tribocorrosion testing was found to be a useful tool for mimicking the degradation occurring in the body.The degradation mechanism was concluded to be of combined mechanical and chemical nature. Wear debris is formed and accumulated in large flakes (> 100 μm). Upon further sliding the flakes are pressed into the surface and ultimately crushed into small, brittle particulate debris (< 5 μm), which is released from the surface. It is suggested, that the small brittle particulate debris has a more detrimental effect in the hip joint than the large flakes due to a stronger reaction of the peri-prosthetic tissue. Also, Al-oxide particles are formed as part of the degradation mechanism.     

The high-cycle fatigue behavior of Ti-6Al-4V alloy

The cyclic stress-strain behavior and the S/N behavior of a “pseudo-elastic” alloy, Ti-6Al-4V of coarsened microstructure and of a “plastic” material, copper, are compared. In accord with views recently put forth by Freudenthal, important differences are noted between these two types of materials. These differences include the mode of crack initiation, the intensity of the Bauschinger effect, and the level below macroscopic yield at which long-life fatigue data lie.     

Ti-6Al-4V合金铸件拉伸性能差异原因分析

目的 研究Ti-6Al-4V合金两种不同结构铸件产品之间,拉伸性能存在着较大差异的原因。方法分别对同批次母合金浇注的两种熔模精密铸件进行化学成分分析、金相组织检查和拉伸试棒宏微观断口形貌观察。方法 分析发现,肋板和支承座铸件两者化学成分没有明显差异,断口表面未见明显冶金缺陷。金相组织检查表明：肋板的原始β晶粒尺寸和晶粒内部的α片层间距均,小于支承座的原始β晶粒尺寸和晶粒内部α片层间距。结论 分析结果表明,拉伸性能数据存在差异主要与其晶粒度和晶内片层间距差异有关,该组织差异主要由铸造工艺中不同的冷却速度引起的。通过改变取样位置,试样的拉伸强度可获得明显提高。