Solid particle erosion behaviour of Ti6Al4V alloy

Abstract

The effects of particle impingement angle, impingement velocity and erodent particle size on the erosion rate and surface morphology of the Ti6Al4V alloy have been investigated comprehensively in order to evaluate solid particle erosion behaviour of Ti6Al4V alloy. Samples were eroded in a specially designed sandblasting system under various parameters using alumina (Al₂O₃) erodent particles. Surface morphology investigations were examined by scanning electron microscope using various analysis and modes (energy dispersive X-ray analysis, elemental mapping and compositional contrast). Ti6Al4V alloy showed ductile behaviour with a maximum erosion rate at 30° impingement angle. Erosion rate of Ti6Al4V alloy increased with increases in velocity and decreased with increases in erodent particle size. Scanning electron microscopy investigations of eroded surfaces of Ti6Al4V alloy samples reveal the dominant erosion mechanism such as microploughing, microcutting and plastic deformation. Embedded erodent particles on the surfaces of Ti6Al4V alloy nearly at all particle impingement angles and velocities were clearly detected.     

Characterization of LENS-fabricated Ti6Al4V and Ti6Al4V/TiC dual-material transition joints

The design of engineering structures with function-specific material members is on the increase. This requires advanced fabrication technologies with capabilities for multi-material processing. A major challenge however is obtaining effective transition from one material to another. Dissimilar material systems made using laser metal deposition processes have been investigated. The fusion of materials having different physical properties and chemical compositions under high laser power often results in defects at the joints due to thermal expansion mismatch, the formation of intermetallics, or other mechanisms. Some solutions have been proffered in previous works based on evaluations using qualitative techniques. However, quantitative experimental studies are imperative to accurately assess the mechanical behavior of dual-material structures for real-life applications as predictive tools have not yet been validated. In this work, different designs of material transitions from Ti6Al4V alloy to Ti6Al4V/TiC composites were established. Experimental evaluations of their strengths at these joints were made using LENS-fabricated tensile and flexural test samples. The mode of transition from one material to another was found to have a significant effect on the tensile strengths of the structures. Also, material transition designs with optimum strengths were applied for the fabrication of simplified dual-material minimum-weight structures and tested. The structures failed at locations away from the material transition regions, thus proving the effectiveness of the joints.     

Effect of laser power on the microstructure and mechanical properties of TiN/Ti3Al composite coatings on Ti6Al4V

Laser nitriding is one of the effective techniques to improve the surface properties of titanium alloys and has potential application in the life extension of last-stage steam turbine blades. However, cracking of surface coating is a common problem due to heat concentration in laser nitriding process. Conventionally, the cracks can be avoided through heat treatment, which may have an important influence on the mechanical properties of coating. Crack-free TiN/Ti3Al IMC coatings on Ti6Al4V are prepared by plasma spraying and laser nitriding. The microstructures, phase constitutes and compositions of the coating are observed and analyzed with scanning electron microscopy(SEM), X-ray diffraction(XRD) and X-ray energy-dispersive spectroscopy(EDS). Microhardness, elastic modulus, fracture toughness of the coating are measured. The results show that the crackand pore-free IMC coatings can be made through the proposed method; with increasing laser power, the amount and density of TiN phase in the coating first increased and then decreased, leading to the similar trend of microhardness and elastic modulus and the reverse trend of fracture toughness of the coating. Both the average microhardness and elastic modulus of the coating increase three times higher than those of the substrate. The volume fraction of the TiN reinforced phase in composite can be controlled by varying the laser power and the cracking problem in laser nitriding process is successfully solved.     

Several plasma diffusion processes for improving wear properties of Ti6Al4V alloy

Different kinds of diffusion processes, plasma nitriding, oxidizing and oxynitriding as of a combination of other two, have been applied to Ti6Al4V alloy to evaluate the effect of treatment times (1 and 4 h) and temperatures (650 and 750 °C) on wear properties of the alloy. It was observed that a hard modified layer was produced on the surface of the alloy after each diffusion process. While TiN and Ti₂N phases form in the modified layer with plasma nitriding, mainly TiO₂ phase forms after plasma oxidizing treatment. The wear tests performed at different normal loads showed that all treated samples, except for nitrided and oxidized at 650 °C for 1 h, exhibited higher wear resistance than untreated Ti6Al4V alloy. The plasma nitrided samples showed adhesive wear. On the other hand, while the plasma oxidizing samples displayed adhesive wear at lower loads, wear mechanism changed to abrasive wear as the load increased because the oxide film which covers the surface was broken during the sliding at higher loads.     

3D FEM simulation of milling process for titanium alloy Ti6Al4V

Milling is used as one of the most important tools with the complex tool geometry in industry. However, the complex milling process cannot be simulated by 2D finite element method. Therefore, a more real 3D finite element model (FEM) for the complex milling process of titanium alloy Ti6Al4V is firstly developed using the finite element software ABAQUS. This model takes into account the dynamic effects, thermomechanical coupling, material damage law, and contact criterion. Firstly, the Johnson–Cook material constitutive equation was proposed, considering the effects of strain, strain rate, and temperature on material properties. Secondly, the damage constitutive law was adopted as the chip separation criterion. Then, the simulation for the milling process of Ti6Al4V was conducted through ABAQUS based on the established 3D FEM. Finally, chip formation, stress distribution, cutting force, and milling temperature were obtained. Further, a series of milling experiments of Ti6Al4V were carried out to validate the simulation results. It confirms the capability and advantage of 3D FEM simulation in the complex milling process of titanium alloy.     

Ti6Al4V表面激光织构化及其干摩擦特性研究

利用激光加工在Ti6Al4V表面分别构筑直线织构、网格织构和凹坑织构,并对试样的表面粗糙度和横截面硬度进行表征和测试,采用多功能微摩擦磨损试验机评价试样表面织构化对其干摩擦性能的影响,使用三维显微镜和扫描电镜对摩擦实验前后试样的形貌进行分析。研究结果表明:经激光织构化处理后,试样表面硬度显著提高,其中凹坑织构试样的表面硬度最高,直线织构试样的表面硬度最低;当选取适当间距时,凹坑织构试样的摩擦因数小于抛光试样的摩擦因数,而网格织构和直线织构试样的摩擦因数大于抛光试样的摩擦因数。SEM照片显示,织构表面的凹坑起到了捕获磨屑的作用。     

Unlubricated gross slip fretting wear of metallic plasma-sprayed coatings for Ti6Al4V surfaces

Plasma-sprayed Al–bronze or CuNiIn coatings are often applied to protect against fretting wear and extend the operational life of Ti-alloy compressor blades in turbine engines. In order to develop a fundamental understanding of how these coating systems perform under gross slip fretting conditions, bench level fretting wear tests were conducted at room temperature to simulate cold engine startup. Alternative coatings such as plasma-sprayed molybdenum and nickel were also evaluated because of their potential for reducing fretting wear under certain simulated engine conditions. The combination of scanning electron microscopy (SEM), surface profilometry, surface chemistry (EDS), and friction analysis were used to study coating performance and evaluate the interfacial wear mechanisms. In this study, it was determined that all coatings caused significant damage to the mating Ti6Al4V surfaces and that the wear mechanisms were all similar to those of the uncoated baseline case.     

The influence of proteins on the fretting-corrosion behaviour of a Ti6Al4V alloy

The effects of collagen and albumin on the fretting-corrosion behaviour of a Ti6Al4V alloy contacting an Al₂O₃ counter-piece was investigated in pH buffered saline solutions at 37 °C using a tribo-electrochemical apparatus. Phosphate ion and hydroxyethyl-piperazinyl-ethanesulfonic acid (HEPES) were used as the pH buffer agents. Tests were conducted under two applied electrochemical potentials and two loads. Potentiodynamic polarisation curves were measured to assess the effect of proteins and pH buffer agents on the corrosion behaviour. Surfaces were characterised by XPS analysis, secondary electron spectroscopy and laser profilometry.Fretting wear of the Ti6Al4V alloy increased with increasing applied potential and load but was not significantly affected by the presence of collagen or albumin. Only a small lubricant effect of collagen could be observed at cathodic potentials. In phosphate buffer saline (PBS) solutions, those proteins were found to act as cathodic inhibitor by shifting the corrosion potential and the cathodic current towards more cathodic values. Phosphate ions were found to be incorporated on the Ti6Al4V alloy and to cause sedimentation of wear particles around the wear trace. In HEPES solutions wear particles were dispersed away from the wear trace.     

Effect of microstructure and cutting speed on machining behavior of Ti6Al4V alloy

Machining of aerospace and biomedical grade titanium alloys has always been a challenge because of their low conductivity and elastic modulus. Different machining methods and parameters have been adopted for high precision machining of titanium alloys. Machining of titanium alloys can be improved by microstructure optimization. The present study focuses on the effect of microstructure on machinability of Ti6Al4V alloys at different cutting speeds. Samples were subjected to different annealing conditions resulting in different grain sizes and local micro-strains (misorientation). Cutting forces were significantly reduced after annealing; consequently, sub-surface residual stresses were reduced. Deformation twinning was also observed on samples annealed at a higher temperature due to larger grain size. Initial strain free grains and deformation twinning during machining reduces the cutting force at higher cutting speed.     

The effects of cooling conditions on surface integrity in machining of Ti6Al4V alloy

This paper presents results from a comparative study of machining of Ti6Al4V alloy under dry, minimal quality lubrication, and cryogenic cooling conditions using coated tools at varying cutting speeds and feed rates. The influence of the cooling conditions on surface integrity and the product performance was studied in terms of surface roughness, metallurgical conditions, including microstructure, hardness, grain refinement, and phase transformation of the machined product. Results show that cooling conditions affect surface integrity of the product signifying the benefits of cryogenic cooling in improving the overall product performance.     

Surface Integrity of Ultrasonically-Assisted Milled Ti6Al4V Alloy Manufactured by Selective Laser Melting

The Ti6Al4V parts produced by the existing selective laser melting (SLM) are mainly confronted with poor surface finish and inevitable interior defects,which substantially deteriorates the mechanical properties and performances of the parts.In this regard,ultrasonically-assisted machining (UAM) technique is commonly introduced to improve the machining quality due to its merits in increasing tool life and reducing cutting force.However,most of the previous studies focus on the performance of UAM with ultrasonic vibrations applied in the tangential and feed directions,whereas few of them on the impact of ultrasonic vibration along the vertical direction.In this study,the effects of feed rate on surface integrity in ultrasonically-assisted vertical milling (UAVM) of the Ti6Al4V alloy manufactured by SLM were systemically investigated compared with the conventional machining (CM) method.The results revealed that the milling forces in UAVM showed a lower amplitude than that in CM due to the intermittent cutting style.The surface roughness values of the parts produced by UAVM were generally greater than that by CM owing to the extra sinusoidal vibration textures induced by the milling cutter.Moreover,the extra vertical ultrasonic vibration in UAVM was beneficial to suppressing machining chatter.As feed rate increased,surface microhardness and thickness of the plastic deformation zone in CM raised due to more intensive plastic deformation,while these two material properties in UAVM were reduced owing to the mitigated impact effect by the high-frequency vibration of the milling cutter.Therefore,the improved surface microhardness and reduced thickness of the subsurface deformation layer in UAVM were ascribed to the vertical high-frequency impact of the milling cutter in UAVM In general,the results of this study provided an in-depth understanding in UAVM of Ti6Al4V parts manufactured by SLM.     

Tool wear prediction of machining hydrogenated titanium alloy Ti6Al4V with uncoated carbide tools

Tool wear is a problem in the turning of titanium alloy, and it is thus of great importance to understand and quantitatively predict tool wear and tool life. In this paper, a combined tool wear model including abrasive, adhesion, and diffusion wear has been implemented in a commercial finite element (FE) code to predict tool wear. Many key problems in tool wear simulation are presented and discussed such as temperature distribution, the updating of tool geometry, and the smoothing of wear boundary. Subsequently, a finite element method wear prediction model is built, and the results are compared with the experimental value; a good agreement was found. Simulated results showed that cutting force will decrease first and then increase with the increase of the concentration of hydrogen, while tool life varies in the opposite way; therefore, the optimum value of hydrogen content is about 0.3 %. The addition of 0.3 % hydrogen could improve tool life greatly, and its tool life is more than three times that of the as-received material. The hydrogenation process's favorable effect is limited by cutting parameters and cooling conditions. According to the numerical results, an appropriate machining speed and higher feed is the selection criterion for high-efficiency machining of hydrogenated titanium alloy. Furthermore, a reasonable range of cutting parameters is found; the cutting speed is in the range of 50–100 m/min, and the feed is in 0.15–0.25 mm/rev.     

Tribocorrosion of Diamond-Like Carbon Deposited on Ti6Al4V

Tribological and corrosion properties of Ti6Al4V alloy both bare and coated by diamond-like carbon (DLC) were investigated in PBS solution. The films obtained by a PACVD technique present high hardness, good corrosion and wear resistance and lower friction coefficient compared to bare alloy. Tribocorrosion tests on bare alloy showed that when wear stops, the alloy rapidly passivates. DLC films present superior wear resistance under dry conditions. However, film life is greatly reduced during tribocorrosion tests. An erratum to this article can be found at     

钛合金Ti6Al4V铣削力试验分析

对钛合金Ti6Al4V进行了铣削试验,测量了铣削过程中的切削力.利用最小二乘法对试验数据进行回归分析,建立了切削力的经验模型,得出了切削参数对切削力的影响规律,为优化切削参数、研究切削机理提供了参考依据.     

Cutting performance of nano-crystalline diamond (NCD) coating in micro-milling of Ti6Al4V alloy

Hard coatings are an important factor affecting the cutting performance of tools. In particular, they directly affect tool life, cutting forces, surface quality and burr formation in the micro-milling process. In this study, the performance of nano-crystalline diamond (NCD) coated tools was evaluated by comparing it with TiN-coated, AlCrN-coated and uncoated carbide tools in micro-milling of Ti₆Al₄V alloy. A series of micro-milling tests was carried out to determine the effects of coating type and machining conditions on tool wear, cutting force, surface roughness and burr size. Flat end-mill tools with two flutes and a diameter of 0.5 mm were used in the micro-milling process. The minimum chip thickness depending on both the cutting force and the surface roughness were determined. The results showed that the minimum chip thickness is about 0.3 times that of the cutter corner radius for Ti₆Al₄V alloy and changes very little with coating type. It was observed from wear tests that the dominant wear mechanism was abrasion. Maximum wear occurred on NCD-coated and uncoated tools. In addition, maximum burr size was obtained in the cutting process with the uncoated tool.     

A method for identification of geometrical tool changes during machining of titanium alloy Ti6Al4V

There exists an increasing demand for cost and time-efficient cutting tests for describing the performance of different combinations of cutting tools and workpiece materials in the cutting process both in industry and academia. Cutting tools are expected to withstand the heat and the pressure developed during the machining of difficult-to-machine materials such as Ti6Al4V. This article introduces a new test method which may be used in order to analyze both the machinability of a workpiece material as well as the cutting tool behavior. The experiments were performed by using a predefined sequence of feeds, a so-called Stepwise Increased Feed Test. A gradually increased load on the cutting edge was thus applied up to the point where plastic deformation of the cutting edge was obtained. The limit for the initial change in tool geometry was identified through analysis of measured cutting forces.     

置氢钛合金切削变形的实验研究

进行了爆炸落刀实验获取不同置氢量钛合金TC4的切屑根部,通过对切屑根部各变形区和切屑形态的分析研究置氢量影响钛合金切屑变形的规律.分析发现,置氢后钛合金的各变形区的变形量减小了,切屑由明显的锯齿形转变为近似带状的切屑.进一步测量了置氢钛合金切削过程中的切削力和切削温度,并根据切削变形量的减小解释了切削力和切削温度降低的机理.