A novel test method for evaluation of the abrasive wear behaviour of total hip stems at the interface between implant surface and bone cement

After total hip replacement, some cemented titanium stems show above-average early loosening rates. Increased release of wear particles and resulting reaction of the peri-prosthetic tissue were considered responsible. The objective was to develop a test method for analysing the abrasive wear behaviour of cemented stems and for generating wear particles at the interface with the bone cement. By means of the novel test device, cemented hip stems with different designs, surface topographies and material compositions using various bone cements could be investigated. Before testing, the cemented stems were disconnected from the cement mantle to simulate the situation of stem loosening (debonding). Subsequently, constant radial contact pressures were applied on to the stem surface by a force-controlled hydraulic cylinder. Oscillating micromotions of the stem (+/- 250 microm; 3 x 10(6)cycles; 5 Hz) were carried out at the cement interface initiating the wear process. The usability of the method was demonstrated by testing geometrically identical Ti-6A1-7Nb and Co-28Cr-6Mo hip stems (n= 12) with definite rough and smooth surfaces, combined with commercially available bone cement containing zirconium oxide particles. Under identical frictional conditions with the rough shot-blasted stems, clearly more wear particles were generated than with the smooth stems, whereas the material composition of the hip stems had less impact on the wear behaviour.     

Electric hot incremental forming of Ti-6Al-4V titanium sheet

Electric hot incremental forming of metal sheet is a new technique that is feasible and easy to control to form hard-to-form sheet metals. In the present study, Ti-6Al-4V titanium sheet was studied because it was wildly used in the aeronautics and astronautics industries. Although Ti-6Al-4V titanium can be well-formed in high temperature, the surface quality is a problem. In order to enhance the surface quality, it is very important to select the proper lubricant. At the same time, because Ti-6Al-4V titanium has a lively chemical property, it is very important to choose a processing temperature range in order to acquire excellent plastic property and to prevent oxidation. Various lubricants were selected in processing to compare the effect, and some workpieces were formed at different temperatures to find the best forming temperature. The results show that using the lubricant film of nickel matrix with MoS₂ self-lubricating material, Ti-6Al-4V titanium workpiece was formed with high surface quality, and the optimum thickness of composite coating is 20 μm for Ti-6Al-4V titanium sheet of 1.0-mm thickness. In fact, the lubricant film also does help to prevent oxidation of Ti-6Al-4V titanium sheet. The appropriate temperature range of Ti-6Al-4V forming with slightly oxidized is 500–600°C in processing, and the maximum draw angle formed in this range was 72°.     

切削置氢TC4钛合金切削力及切削温度的有限元分析

利用有限元技术,研究了氢含量对TC4钛合金切削力及切削温度的影响规律,并对高速切削时的切削力、切削温度的规律进行了预测.利用电子万能材料试验机及霍普金森压杆装置获取了不同应变速率及温度时置氢钛合金的流变行为,通过数据的拟合得到了Johnson-Cook(J-C)本构方程,据此建立了切削数值模型.切削力及切削温度的模拟结...     

The Tribological Behavior of a Ti-46Al-2Cr-2Nb Alloy Under Liquid Paraffine Lubrication

The tribological behavior of a Ti-46Al-2Cr-2Nb alloy prepared by hot-pressed sintering was investigated under liquid paraffine lubrication against AISI 52100 steel ball in ambient environment and at varying loads and sliding speeds. For comparison, the tribological behavior of a common Ti-6Al-4V alloy was also examined under the same testing conditions. The worn surfaces of the two alloys were analyzed using a scanning electron microscope. The friction coefficient of the Ti-46Al-2Cr-2Nb alloy in the range of 0.13–0.18 was significantly lower than that of the Ti-6Al-4V alloy (0.4–0.5), but comparable to that under dry sliding, which indicated that TiAl intermetallics could be more effectively lubricated by liquid paraffine than titanium alloys. Applied load and sliding speed have little effect on the friction coefficient of the Ti-46Al-2Cr-2Nb alloy. The wear rate of the Ti-46Al-2Cr-2Nb alloy was about 45–120 times lower than that of Ti-6Al-4V alloy owing to Ti-6Al-4V alloy could not be lubricated effectively. The wear rate of the Ti-46Al-2Cr-2Nb alloy increased with increasing applied load, but decreased slightly at first and then increased with increasing sliding speed. The wear mechanism of the Ti-46Al-2Cr-2Nb intermetallics under liquid paraffine lubrication was dominated by main plowing and slight flaking-off, but that of the Ti-6Al-4V alloy was plastic deformation and severe delamination.     

Experimental versus computational analysis of micromotions at the implant-bone interface

In total hip arthroplasty, micromotions at the implant-bone interface influence the long-term survival of the prosthesis. These micromotions are often measured using sensors that are fixed to the implant and bone at points that are remote from the interface. Given that the implant-bone system is not rigid, errors may be introduced. It is not possible to assess the magnitude of these errors with the currently available experimental methods. However, this problem can be investigated using the finite element method (FEM). The hypothesis that the actual interface micromotions differ from those measured in the experimental manner was tested using a case-specific FE model, validated against deflection experiments. The FE model was used to simulate an 'experimental' method to measure micromotions. This 'experimental' method was performed by mimicking the distance between the measurement points; the implant point was selected at the interface while the bony point was at the outer surface of bone. No correlation was found between the micromotions computed at the interface and when using remote reference points. Moreover, the magnitudes of micromotions computed with the latter method were considerably greater. By reducing the distance between the reference points the error decreased, but the correlation stayed unchanged. Care needs to be taken when interpreting the results of micromotion measurement systems that use bony reference points at a distance from the actual interface.     

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.     

Interfacial properties of diffusion bonded Ti-6Al-4V to AISI 304 stainless steel by inserting a Cu interlayer

The joining characteristics of Ti-6Al-4V with AISI 304 stainless steel by inserting a Cu interlayer was investigated in a vacuum-free diffusion bonding process. The diffusion bonds were carried out in the temperature range of 820, 850 and 870°C for 50, 70 and 90 minutes, respectively, under 1 MPa load in argon atmosphere. The joining performances of diffusion bonded Ti-6Al-4V to AISI 304 were studied experimentally. The influence of the insert layer on the microstructure-formed interface region, bonding quality and mechanical properties have also been estimated. The microstructures formed in the diffusion region were observed and determined by scanning electron microscopy (SEM). The microhardness across and perpendicular to the interface were measured and the strength of the joints were also determined with lap-shear test.     

The compatibility of TiB2 protective coatings with SiC fibre and Ti-6Al-4V

TiB₂ coatings have been studied as prospective protective layers to inhibit the interfacial reaction between SiC fibres and Ti-alloy matrices. This protective coating has been deposited onto SiC monofilament fibres using a chemical vapour deposition (CVD) technique. The fibre-matrix compatibility of these TiB₂-coated SiC fibres in Ti-6Al-4V composites was evaluated by incorporating the coated fibres into Ti-6Al-4V using a diffusion bonding technique. The interfaces of this composite were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis, to evaluate the interfacial microstructures, chemical stability and the efficiency of TiB₂ as a protective coating for SiC fibres in Ti-alloy matrices, and to study the effects of deposition temperature on the interface of the coated fibre. Results show that stoichiometric TiB₂ coatings are stable chemically to both SiC fibres and Ti-6Al-4V and hinder the deleterious fibre-matrix reactions effectively. Boron-rich TiB₂ coatings should be avoided, as they lead to the formation of a needle-like TiB phase at the fibre–matrix interface. These findings provide promising evidence for the value of further exploration of the use of stoichiometric TiB₂ as a protective coating for SiC fibre in Ti-based composites.     

基于变弹性模量的Ti-6Al-4V板材五点弯曲回弹预测

Ti-6Al-4V钛合金材料在弯曲成形过程中会产生较大的回弹,其弹性模量对回弹影响较大,但以往研究均未考虑材料塑性应变变化过程中弹性模量的变化。以Ti-6Al-4V钛合金为对象,进行了材料的单轴拉伸实验和循环加载-卸载实验,以揭示材料各向异性参数及材料弹性模量随塑性应变变化的规律,在此基础上建立了Ti-6Al-4V钛合金变弹性模量数学模型。基于YLD2000-2D屈服准则及变弹性模量和Mises各向同性两种不同的本构模型,对常温下Ti-6Al-4V钛合金板材的五点弯曲过程进行了数值模拟,为了验证数值模拟结果,进行了常温下Ti-6Al-4V板材的五点弯曲实验,结果显示,前者显著提高了Ti-6Al-4V钛合金弯曲回弹预测精度,预测精度相比后者提高了31.18%。     

Finite element modeling of machining of hydrogenated Ti-6Al-4V alloy

The present study is undertaken to investigate the effect of hydrogen on the cutting performance of Ti-6Al-4V alloy by FEM. Mechanical behaviors of hydrogenated Ti-6Al-4V alloy are studied at elevated temperatures and high strain rates with split Hopkinson pressure bar. The Johnson–Cook model was developed combined with quasi-static experimental data. A numerical model is developed to simulate the cutting process. The results of the experiments and simulations agreed well. The results demonstrate that the presence of hydrogen has a significant effect on the cutting forces and temperature, and the cutting forces and temperature increase first and then decreased gradually with the increasing of hydrogen contents. The simulation results show that titanium alloys with 0.3% hydrogen have better machinability at high cutting speed.     

不同摩擦副条件下Ti-6Al-4V合金的微磨粒磨损行为

采用TE66微磨粒磨损实验机对医用Ti-6Al-4V钛合金在不同摩擦副条件下的微磨粒磨损行为进行研究,考察滑行距离、载荷对其微磨粒磨损的影响,通过观察磨斑形貌,分析其磨损机制。研究结果表明:Ti-6Al-4V合金的磨损量随滑移距离和载荷增加而增加,磨损率则相反,并且硬度较高的Si3N4陶瓷球对合金造成的磨损量和磨损率均低于ZrO2陶瓷球;在不同摩擦副条件下,随着滑行距离和载荷的增加,Ti-6Al-4V合金的磨损机制均由三体磨损转变为二三体混合磨损,所不同的是与Si3N4陶瓷球对摩时合金的混合磨损区域要少于与ZrO2陶瓷球对摩时。     

Influence of Deposition Positions on Fretting Behaviors of DLC Coating on Ti-6Al-4V

Abstract

The influence of diamond-like carbon (DLC) coating positions—coated flat, coated cylinder, and self-mated coated surface tribopairs—on the fretting behaviors of Ti-6Al-4V were investigated using a fretting wear test rig with a cylinder-on-flat contact. The results indicated that, for tests without coating (Ti-6Al-4V–Ti-6Al-4V contact), the friction (Q_max/P) was high (0.8–1.2), wear volumes were large (0.08–0.1?mm³) under a large displacement amplitude of ±40 µm and small (close to 0) under a small displacement amplitude of ±20 µm, and the wear debris was composed of Ti-6Al-4V flakes and oxidized particles. For tests with the DLC coating, under low load conditions, the DLC coating was not removed or was only partially removed, Q_max/P was low (≤0.2), and the wear volumes were small. Under high load conditions, the coating was entirely removed, Q_max/P was high (0.6–0.8), and the wear volumes were similar to those in tests without coating. The wear debris was composed of DLC particles, Ti-6Al-4V flakes, and oxidized particles. The DLC coating was damaged more severely when deposited on a flat surface than when deposited on a cylindrical surface. The DLC coating was damaged more severely when sliding against a DLC-coated countersurface than when sliding against the Ti-6Al-4V alloy.     

Finite element simulation and process optimization for hot stretch bending of Ti-6Al-4V thin-walled extrusion

Ti-6Al-4V is widely utilized to manufacture airframe component structures with curvature, because of its excellent strength to weight ratio, outstanding resistance to corrosion, and inherent thermal and electrical compatibility with carbon fiber composite. Hot stretch bending (HSB) is an effective technology to manufacture these kinds of structures. When comparing the thin-walled extrusion with the thick-walled one, however, it is more difficult to form. The reason is that the local temperature of extrusion decreases more because of the heat transfer between extrusion and die. In this study, the material properties of Ti-6Al-4V were measured experimentally, such as the tensile property within the temperatures from 873 to 1023 K and the strain rates from 0.0005 to 0.005 s^?1, the stress relaxation behavior in a wide range of temperatures (773–973 K) and prestrains (0.7–10%), as well as the heat transfer rule between Ti-6Al-4V (extrusion material) and asbestos cement (die material) under different pressures (8–25 MPa). The heat transfer coefficients (HTCs) were determined by an inverse analysis procedure, which was based on the comparison between measured and calculated temperature. Then, the coupled thermomechanical finite element (FE) model considering the effect of heat transfer was established. The influence of preheating temperature of die, initial temperature of extrusion, and dwell time on spring-back was researched based on orthogonal array testing strategy (OATS). The optimized parameters were verified by process test. It was showed that the established FE model could be used to predict spring-back within a relative deviation of 8.05%.     

Ti-6Al-4V钛合金板与304L不锈钢网的扩散焊

分别在800℃、825℃、850℃焊接温度、30 m in保温时间,3 MPa焊接压力下,进行Ti-6A l-4V钛合金板与304L不锈钢网的真空扩散焊接。对接头组织结构与化学元素扩散进行了扫描电镜与能谱分析,并测试了接头的剪切强度。结果表明:不添加中间过渡层金属,可以成功地实现钛合金板与不锈钢网的扩散焊接,并使接头的剪切强度达到90 MPa以上。不锈钢网中的Fe、N、iCr扩散并固溶到钛合金中,稳定了β相,使钛合金在一定深度上,其组织由原来的α+β双相结构转变为单相的β相。不锈钢中的Cr,由于钛合金中Ti的扩散进入,而在界面发生了上坡扩散现象。这种Cr在不锈钢一定深度内的富集,形成窄长的富Cr区域,冷却后转变为硬脆的σ相。但在焊接接头中没有发现明显其它的金属间化合物或氧化物相的生成,使得接头的机械性能得到了很好的保证。     

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.     

Material flow stress and failure in multiscale machining titanium alloy Ti-6Al-4V

Titanium Ti-6Al-4V alloy is a typical difficult-to-machine material due to its unique physical and mechanical properties. The material properties of Ti-6Al-4V play an important role in process design and optimization. However, the dynamic mechanical behavior is poorly understood and accurate predictive models have yet to be developed. This work focuses on the dynamic mechanical behavior of machining Ti-6Al-4V beyond the range of strains, strain rates, and temperatures in conventional materials testing. The flow stress characteristics of strain hardening and thermal softening can be predicted by the Johnson–Cook model coupled with the adiabatic condition. The predicted flow stresses at small strains agree very well with those from the split Hopkinson pressure bar (SHPB) tests, while the predicted flow stresses at large strains also agree with the calculated flow stresses based on the cutting tests with a suitable depth of cut. Heat fraction and temperature parameter control the range of thermal softening and the decrease rate of flow stress. The material may exhibit super plasticity at a small depth of cut with a large radius of the cutting edge in micromachining. Strain rate is one important factor for material fracture close to the cutting edge. The failure strain increases linearly with the increase of homologous temperature, while it only increases slightly with the strain rate.     

Fretting corrosion of orthopaedic implant materials by bone cement

Debonding of bone cement from the stem of the femoral component of a hip prosthesis can result in local tangential oscillatory movement, i.e. fretting, between the two contacting materials as the limb is moved. Patches where such rubbing has occurred are frequently seen on removed implants. Fretting fatigue experiments have been carried out in Hanks solution on austenitic stainless steel and Ti-6Al-4V (IMI 318) with bridges of bone cement clamped to the specimens. Fretting appears to have little effect on the fatigue life of either material but the scanning electron microscope reveals the formation of thick oxide layers which subsequently give rise to loose debris particles by a process of delamination. Further experiments carried out in Hanks solution in an electrolytic cell have shown that there are potential changes when a bone cement rider is fretted against a stainless steel or titanium alloy plate although the change in potential is only one tenth that obtained with a metal-on-metal contact. Fretting by bone cement appears to be producing damage to the metal surfaces which manifests itself as mild wear rather than a diminution in fatigue strength.     

Fatigue life prediction model of Ti-6Al-4V alloy forgings based on forging process parameters

Forging processing parameters have an important impact on the fatigue resistance of metal in the process of plastic forming, which directly threatens the safety and reliability of the metal’s service. Taking Ti-6Al-4V alloy as the research object, this study investigates the effect of processing parameters, including forging temperature and deformation degree on the Young’s modulus, ultimate tensile strength, and reduction of fracture area on the basis of the mechanical property testing of Ti-6Al-4V alloy forgings and obtains the regression equations at the same time. Combined with the existing stress fatigue life prediction formula, the fatigue life prediction based on forging process parameters is realized. Results show that forging temperature and deformation degree have a significant effect on fatigue life. The forging processing parameters of Ti-6Al-4V alloy with optimum fatigue life are as follows: 985 °C–990 °C forging temperature and 46%–50% deformation degree.

     

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.     

Influence of Rapidly Solidified Structures on Wear Behavior of Ti-6AI-4V Laser Alloyed with TiC

In this paper the influence of rapidly solidified structures on the wear behavior of Ti-6Al-4V laser alloyed with TiC is studied by using an unlubricated sliding wear test. The results show that laser surface alloying with TiC can significantly increase wear resistance of Ti-6Al-4V. The wear of Ti-6Al-4V is characterized by adhesive de-lamination and melting showing a severe wear mechanism, while after alloying with TiC, it is changed into a mild wear mechanism characterized by a smooth, worn surface. This is attributed to directional fast-growing dendrites capped by a TiN/TiC film with high hardness and framed α marlensites obtained in rapid solidification.