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.     

Enhancement of surface roughness in electrochemical machining of Ti6Al4V by pulsating electrolyte

Electrochemical machining (ECM) is widely used in machining a variety of components used in aerospace, defence, automotive and medical applications. The surface roughness of the ECM process has become important because of increased quality demands. Considerable attention has been paid to achieving low surface roughness in ECM. Surface roughness is closely related to the distribution of gases and Joule heat produced during the ECM process, which affect the electrolyte electric conductivity and directly determine the surface roughness. In this report, a pulsating electrolyte, which is one of the unsteady flows that are characterized by periodic fluctuations of the mass flow rate and pressure, is first introduced to the ECM process. The ECM process is affected by the pulsating electrolyte because it can modify the heat transfer. The goal of this report is to present experimental results of the surface roughness obtained on Ti6Al4V samples using a developed pulsating electrolyte supply system in ECM. It is observed that a lower surface roughness and higher material removal rate could be obtained by using a pulsating electrolyte with proper pulsating frequency and amplitude. In direct current ECM, the surface roughness Ra is 5.7 μm, the material removal rate is 0.85 g/min at a constant electrolyte, the lowest surface roughness is 3.69 μm and the largest material removal rate is 0.92 g/min, which are obtained at a pulsating frequency of 10 Hz and amplitude of 0.2 MPa. In pulsed current ECM, the surface roughness Ra and material removal rate are 0.67 μm and 0.38 g/min at a constant electrolyte, respectively, and both the minimum surface roughness Ra of 0.53 μm and maximum material removal rate of 0.39 g/min are observed when the proper pulsating electrolyte flow frequency and amplitude are used.     

Adaptive fuzzy sliding-mode control for the Ti6Al4V laser alloying process

It is well known that surface alloying quality may vary significantly with respect to process parameter variation. Thus a feedback control system is required to monitor the operating parameters for yielding a good quality control. Since this multi-input and multi-output (MIMO) system has nonlinear coupling and time-varying dynamic characteristics, it is very difficult to establish an accurate process model for designing a model-based controller. Hence an adaptive fuzzy sliding-mode controller (AFSMC) which combines an adaptive rule with fuzzy and sliding-mode control is employed in this study. It has an on-line learning ability for responding to a system’s nonlinear and time-varying behaviours. Two adaptive fuzzy sliding-mode controllers are designed for tuning the laser power and the traverse velocity simultaneously to tackle the absorptivitiy and geometrical variations of the work pieces. The simulation results show that good surface lapping performance is achieved by using this intelligent control strategy.     

超声振动铣削加工Ti6Al4V的表面残余应力研究

建立了包含振动铣削、移除铣刀、载荷释放及降温等多个分析步的超声振动铣削三维有限元模型,对铣削表面的残余应力进行了仿真。通过有限元计算,得到了不同振动参数和铣削用量条件下加工表面的残余应力大小与分布情况,并根据振动切削表面残余应力的影响机制对仿真结果进行了分析。研究结果表明,对工件施加超声振动后,切削表面残余拉应力值明显减小;振动参数和铣削速度对残余应力的影响较大,而进给量对残余应力的影响较小,并通过相关实验验证了有限元仿真结果的正确性。     

选区激光熔化工艺参数对Ti6Al4V粉末成型性的影响

以Ti6Al4V钛合金粉末为研究对象,在单层扫描和单道扫描实验的基础上,研究SLM工艺参数对Ti6Al4V合金材料成型性的影响,并进行了块体成型实验,通过设计正交试验及观察试样的形貌和致密度分析,最终得到Ti6Al4V合金粉末SLM块体成型的最佳工艺参数为:激光功率400W、搭接率1、扫描速度750mm/min,其致密度可以达到96.17％.     

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.     

Experimental study on micro EDM-drilling of Ti6Al4V using helical electrode

There is a growing interest in the machining of micro-holes with high aspect-ratio in difficult-to-machine alloys for the aerospace industry. Processes based on electro discharge machining (EDM) and developed for the manufacture of both micro-electrode and micro-hole are actually used, but most of them involve micro-EDM machines. In this work, the influence of EDM parameters on material removal rate, electrode wear, machining time and micro-hole quality when machining Ti6Al4V is studied. Due to an inefficient removal of debris when increasing hole depth, a new strategy based on the use of helical-shaped electrodes has been proposed. The influence of helix angle and flute depth with respect to process performance has been addressed. Main results include 37% reduction in machining times (hole diameter 800 μm) when using electrode helix angle of 45° and flute-depth of 50 μm, and an additional 19% with flute-depth of 150 μm. Holes of 661 μm diameter and as much as 6.81 mm depth, which yields in aspect ratio of 10:1, have successfully been machined in Ti6Al4V.     

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.     

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     

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.     

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.     

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.     

Slurry abrasion response of surface engineered Ti6Al4VELI

Titanium alloys, by virtue of their outstanding combination of properties, continue to evolve as direct replacements for steels in offshore production systems to meet the challenging offshore application conditions. However, the wider use of titanium and its alloys is frequently retarded by their reputation for poor tribological behaviour. It is because of this simple engineering scenario that comparative tribological behaviour of surface engineered Ti6Al4VELI in abrasive slurry has been evaluated to identify treatments capable of improving its wear behaviour. To best simulate potential application conditions, sliding wear tests were carried out using a block-on-wheel test configuration in abrasive mud slurry. The wear volumes lost from the surface engineered Ti6Al4VELI test blocks ranged from negligibly small for substrates plasma sprayed with either WC–Co, Ni–Cr or Mo, to approximately twice that measured for a steel test block in the cases of untreated, shot peened, electroless Ni plated and anodised Ti6Al4VELI. Hard chrome plated material and thermochemically treated material demonstrated a certain degree of wear, which however, was significantly less than that found for the steel block.     

Ti6Al4V表面生物活性陶瓷涂层的摩擦学性能

人工合成的生物活性陶瓷被广泛应用于金属内植入体表面用以修复或置换缺损的骨组织。使用自制的往复试验机,在6.5 mm/s的滑动速度和不同载荷(1 N和2 N)下对溶胶-凝胶法制备的生物活性陶瓷羟基磷灰石(HAP∶Ca10(PO4)6(OH)2)和二氧化钛涂层,以及羟基磷灰石/二氧化钛复合涂层进行了摩擦学性能的研究。结果显示,由不同工艺路线制备的HAP涂层的抗磨性有一定的差异,其中采用有机磷酯前驱体法制备的HAP涂层的抗磨性较好,但它们的抗磨性均差于二氧化钛涂层;羟基磷灰石/二氧化钛复合涂层的抗磨性与两者的复合方式有关,其中非晶态结构的复合涂层的抗磨性最佳。     

Fretting of CoCrMo and Ti6Al4V alloys in modular prostheses

Implantation of a total hip replacements (THR) is an effective intervention in the management of arthritis. Modularity at the taper junction of THR was introduced in order to improve the ease with which the surgeon could modify the length of the taper section and the overall length of the replacement. Cobalt chromium (Co–28Cr–6Mo) and titanium (Ti–6Al–4V) alloys are the most commonly used materials for the device. This study investigates the fretting behaviour of both CoCr–CoCr and CoCr–Ti couplings and analyses their damage mechanisms. A reciprocating tribometer ball on plate fretting contact was instrumented with in situ electrochemistry to characterise the damage inflicted by tribocorrosion on the two couplings. Fretting displacements amplitudes of 10, 25 and 50?μm at an initial contact pressure of 1?GPa were assessed. The results reveal larger metallic volume loss from the CoCr–CoCr alloy compared to the CoCr–Ti alloy, and the open circuit potential indicates a depassivation of the protective oxide layer at displacement amplitudes >25?μm. In conclusion, the damage mechanisms of CoCr–CoCr and CoCr–Ti fretting contacts were identified to be wear and fatigue dominated mechanisms respectively.