Effects of sliding velocity on tribo-oxides and wear behavior of Ti–6Al–4V alloy

Dry sliding wear tests were performed for Ti–6Al–4V alloy on a pin-on-disc wear tester. The wear behavior of Ti–6Al–4V alloy at sliding velocities of 0.5–4 m/s was studied and the tribo-oxides and their function were explored. Ti–6Al–4V alloy presented a marked variation of wear rate as a function of velocity. With the rise and fall of wear rate, Ti–6Al–4V alloy underwent the transitions of wear mechanisms from the combination of delamination wear and oxidative wear at lower speeds to delamination wear at 2.68 m/s, and then to oxidative wear at 4 m/s. These phenomena were attributed to the appearance and disappearance of tribo-oxides. In spite of trace or a small amount, tribo-oxides would change the wear behavior, and even wear mechanism.     

Micro-abrasion wear testing of triode plasma diffusion and duplex treated Ti–6Al–4V alloy

In this paper micro-abrasion wear testing is used to evaluate the wear resistance of triode plasma diffusion-treated, single-layered TiN-, CrAlN-, and WC/C-coated and duplex-diffusion and coated Ti–6Al–4V under uniform three-body rolling abrasion. Nanoindentation, Knoop microhardness, mechanical surface profilometry, optical microscopy, scanning electron microscopy and atomic force microscopy, were used to characterise the surfaces under investigation. Optimum testing conditions for rolling abrasion were established by varying the test parameters and resultant severity of contact. Very low normal loads and high volume fractions of particles in the abrasive slurry are necessary to obtain predictable and reproducible results. Relatively coarse SiC abrasive particles, having a mean diameter of around 3 μm, appear more suitable for micro-abrasion testing of the samples investigated, compared to finer Al₂O₃ particles. Problems associated with the measurement of the scar volume and subsequent calculation of the wear rate for hard coatings deposited on relatively soft metals like titanium are identified, and suitable testing and measurement techniques are suggested. Three-dimensional wear scar maps generated by mechanical stylus profilometry were used to measure the wear volumes. Under the test conditions used, wear coefficients can be determined from perforating and non-perforating tests, although perforating tests provide more consistent results. Triode plasma diffusion treatments, plasma-assisted (PA) PVD TiN and PAPVD CrAlN can reduce the specific wear rate of Ti–6Al–4V, while PACVD-based WC/C coatings do not provide suitable protection against abrasive wear. The combination of triode plasma oxynitriding diffusion treatments and PVD coatings to create duplex treatments can also lead to further reductions in the coating wear coefficient when compared to non-duplex coatings deposited on non-pretreated substrates.     

Minimal quantity lubrication-MQL in grinding of Ti–6Al–4V titanium alloy

Titanium and its alloys are attractive materials due to their unique high strength–weight ratio that is maintained at elevated temperatures and their exceptional corrosion resistance. The major application of titanium has been in the aerospace industry. On the other hand, titanium and its alloys are notorious for their poor thermal properties and are classified as difficult-to-machine materials. The problems that arise during grinding of titanium alloys are attributed to the high specific energy and high grinding zone temperature. Significant progress has been made in dry and semidry machining recently, and minimal quantity lubrication (MQL) machining in particular has been accepted as a successful semidry application because of its environmentally friendly characteristics. A number of studies have shown that MQL machining can show satisfactory performance in practical machining operations. However, there has been few investigation of MQL grinding of special alloys like titanium alloys and the cutting fluids to be used in MQL grinding of these alloys. In this study, vegetable and synthetic esters oil are compared on the basis of the surface quality properties that would be suitable for MQL applications. The cutting performance of fluids is also evaluated using conventional wet (fluid) grinding of Ti–6Al–4V. As a result, synthetic ester oil is found to be optimal cutting fluids for MQL grinding of Ti–6Al–4V.     

Ductile fracture locus of Ti–6Al–4V titanium alloy

The simulation of ductile fracture in real components is becoming a strategic issue in numerical simulations. Numerical simulations of crashes, forming processes, impacts and fractures are reliable only if carried out with an accurate material calibration. The topics involved in this kind of simulation require a complete calibration of both the true stress–strain curve and the failure. The focus of this work is the accurate calibration of the constitutive relations of the titanium alloy Ti–6Al–4V. The approach proposed is based on different experimental tests supported by numerical simulations performed by means of detailed FE models. The Bao–Wierzbicki ductile failure criterion is calibrated using a total of 11 specimens. These specimens are tested on a multiaxial test machine to investigate the failure at different stress triaxialities. Furthermore, the sensitivity to the mesh size and the assessment of the calibration accuracy are analysed in detail on different components in order to verify the geometry transferability.     

Characterization of dry sliding wear of AlSi alloys

Controversy exists in the published literature as to the effect of silicon content and pressure on the dry sliding wear of AlSi alloys. The present paper attempts to clarify the question by reporting a statistical analysis of data obtained from factorially designed experiments conducted on a pinon-disc machine in the pressure range 0.105–1.733 MPa and speed range 0.19–0.94 m s⁻¹. Under these conditions it was found that, in the range 4–24 wt.% Si, wear of binary unmodified alloys does not significantly differ between the alloys. However, it is significantly less than that corresponding to an alloy containing no silicon. The effect of pressure on wear rate was found to be linear and monotonie and, over the narrow range of speeds used, the wear rate was found to be unaffected by speed. The coefficient of friction was found to be insensitive to variations in silicon content, pressure and speed.     

Influence of plasma nitriding on fretting wear behaviour of Ti–6Al–4V

Plasma nitriding was performed on Ti–6Al–4V samples at 520 °C in two environments (pure nitrogen and a mixture of nitrogen and hydrogen in the ratio of 3:1) for two different time periods (4 and 18 h). Fretting wear tests were conducted on unnitrided and nitrided samples for 50,000 cycles using alumina ball counterbody. Plasma nitriding reduced the tangential force coefficient of Ti–6Al–4V. The samples nitrided for 4 h exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided for 18 h. The samples nitrided in nitrogen–hydrogen mixture environment exhibited higher hardness and lower tangential force coefficient compared to the specimens nitrided in pure nitrogen. The samples plasma nitrided in nitrogen–hydrogen mixture for 4 h exhibited the highest hardness and the lowest tangential force coefficient. The wear volume and specific wear rate of the plasma nitrided samples were lower than those of the unnitrided samples. A consistent trend was not observed regarding which nitriding condition would result in lower wear volume and specific wear rate at different loads.     

Dry sliding wear behaviour of Ti–6Al–4V pin against SS316L disc in vacuum condition at high temperature

ABSTRACT

Ti–6Al–4V alloy exhibit a unique combination of mechanical, physical, and chemical properties; that pronounced its desirability for implementation in the fields of aerospace, automobile, and chemical industries. The mechanisms, namely, strain rate response/adiabatic shear band (ASB) – effect of plastic deformation, tribo-chemical reaction and formation of the mechanically mixed layer (MML), can control wear behaviour of the alloy. Hence, the present work investigates the influence of these mechanisms in governing the tribological characteristics of the Ti6Al4V alloy aganist SS316L steel. The experiments were executed on a pin-on-disc tribometer under vacuum (2?×?10^?5?Torr), by varying the temperature (25, 100, 200, 300 and 400°C) at constant sliding speed (0.01?ms^?1) and load (137.3?N) conditions. Compression test was carried out at distinct strain rate (0.001 and 10 s^?1) and temperature (25, 100 and 400°C) values, to investigate the occurrence of ASB. The scanning electron microscopy and energy dispersive X-ray spectroscopy analyses were used to evaluate the formation of appendage layers (oxides and MML) and the composition of wear debris, respectively. The wear rate of the Ti–6Al–4V alloy decreased with increment in temperature (room condition to 400°C) inside vacuum environ, governed by ASB and the presence of oxide layers.     

Combined rough and finish machining of Ti–6Al–4V alloy by electrochemical mill-grinding

Abstract

This study proposes a combined method for the electrochemical mill-grinding of Ti–6Al–4V alloy to achieve a high material removal rate, high machining accuracy and good surface quality based on rough and finish machining. In the rough machining stage, a maximum feed rate of 2.7?mm min^?1 and a material removal rate of 248.3?mm³ min^?1 were achieved experimentally at a 10?mm cut depth using an abrasive tool with five rows of tool-sidewall outlet holes. In the finish machining stage, there were almost no overcuts or stray corrosions produced. The sidewall surface roughness and sidewall flatness were Ra = 1.06 and 76.8?μm after the finishing stage, which represent a 68% and 79.2% improvement compared with the rough machining stage, respectively. Finally, we fabricated a 1-mm-thick thin-walled structure using the combined machining operations, in which approximately 96% of the total material removal volume was performed at the rough machining stage.     

Failure mechanisms of a PCD tool in high-speed face milling of Ti–6Al–4V alloy

High-speed milling tests were carried out on Ti–6Al–4V titanium alloy with a polycrystalline diamond (PCD) tool. Tool wear morphologies were observed and examined with a digital microscope. The main tool failure mechanisms were discussed and analyzed utilizing scanning electron microscope, and the element distribution of the failed tool surface was detected using energy dispersive spectroscopy. Results showed that tool flank wear rate increased with the increase in cutting speed. The PCD tool is suitable for machining of Ti–6Al–4V titanium alloy with a cutting speed around 250 m/min. The PCD tool exhibited relatively serious chipping and spalling at cutting speed higher than 375 m/min, within further increasing of the cutting speed the flank wear and breakage increased greatly as a result of the enhanced thermal–mechanical impacts. In addition, the PCD tool could hardly work at cutting speed of 1,000 m/min due to the catastrophic fracture of the cutting edge and intense flank wear. There was evidence of workpiece material adhesion on the tool rake face and flank face in very close proximity to the cutting edge rather than on the chipped or flaked surface, which thereby leads to the accelerating flank wear. The failure mechanisms of PCD tool in high-speed wet milling of Ti–6Al–4V titanium alloy were mainly premature breakage and synergistic interaction among adhesive wear and abrasive wear.     

In-process prediction of surface roughness in turning of Ti–6Al–4V alloy using cutting parameters and vibration signals

In this work, an attempt has been made to use vibration signals for in-process prediction of surface roughness during turning of Ti–6Al–4V alloy. The investigation was carried out in two stages. In the first stage, only acceleration amplitude of tool vibrations in axial, radial and tangential directions were used to develop multiple regression models for prediction of surface roughness. The first and second order regression models thus developed were not found accurate enough (maximum percentage error close to 24%). In the second stage, initially a correlation analysis was performed to determine the degree of association of cutting speed, feed rate, and depth of cut and the acceleration amplitude of vibrations in axial, radial, and tangential directions with surface roughness. Subsequently, based on this analysis, feed rate and depth of cut were included as input parameters aside from the acceleration amplitude of vibrations in radial and tangential directions to develop a refined first order multiple regression model for surface roughness prediction. This model provided good prediction accuracy (maximum percentage error 7.45%) of surface roughness. Finally, an artificial neural network model was developed as it can be readily integrated into a computer integrated manufacturing environment.     

Micro-drilling of Ti–6Al–4V alloy: The effects of cooling/lubricating

This paper presents a series of experimental investigations of the effects of various machining conditions [dry, flooded, minimum quantity lubrication (MQL), and cryogenic] and cutting parameters (cutting speed and feed rate) on thrust force, torque, tool wear, burr formation, and surface roughness in micro-drilling of Ti–6Al–4V alloy. A set of uncoated carbide twist drills with a diameter of 700 μm were used for making holes in the workpiece material. Both machining conditions and cutting parameters were found to influence the thrust force and torque. The thrust force and torque are higher in cryogenic cooling. It was found that the MQL condition produced the highest engagement torque amplitude in comparison to the other coolant–lubrication conditions. The maximum average torque value was obtained in the dry drilling process. There was no substantial effect of various coolant–lubrication conditions on burr height. However, it was observed that the burr height was at a minimum level in cryogenic drilling. Increasing feed rate and decreasing spindle speed increased the entry and exit burr height. The minimum surface roughness values were obtained in the flood cooling condition. In the dry drilling process, increased cutting speed resulted in reduced hardness on the subsurface of the drilled hole. This indicates that the surface and subsurface of the drilled hole were subject to softening in the dry micro-drilling process. The softening at the subsurface of drilled holes under different cooling and lubrication conditions is much smaller compared to the dry micro-drilling process.     

Surface quality improvement of EDMed Ti–6Al–4V alloy using plasma etching and TiN coating

This paper seeks to improve the surface quality of electrical discharge machining (EDM) Ti–6Al–4V using plasma etching treatment and TiN coating. The EDM parameter setting is optimized firstly based on grey-Taguchi method. Four EDM parameters, including current (A), voltage (V), pulse duration (μs), and duty factor (%), are selected for multiple performance of lower electrode wear rate (EWR), higher material removal rate (MRR), and better surface roughness (SR). An orthogonal array, signal-to-noise (S/N) ratios, and analysis of variance (ANOVA) are used to analyze the effects of these EDM parameters. Normality tests show that all the distributions fit normality assumption with p?=?0.276, 0.688, and 0.663, respectively. The EDM process is stable over time monitored by Shewhart control charts. It is observed that there is an EDM damaged layer on the surface consisting of debris, microcracks, molten drops, and solidified metals by scanning electron microscopy. The plasma etching and TiN coating are employed to improve surface quality of the EDMed Ti–6Al–4V alloys. The results demonstrate that using the oxygen plasma etching treatment, the damaged phenomena are decreased, and the mean SR value is reduced from Ra?=?2.91 to Ra?=?2.50 μm. In addition, when the plasma-treated alloy is coated with Ti buffer/TiN coating by physical vapor deposition, the surface morphology exhibits less defects and a better surface finish. The mean SR values are further reduced from Ra?=?2.50 μm to Ra?=?1.48 μm (for 740 nm TiN film) and Ra?=?0.61 μm (for 1450 nm TiN film), respectively.     

Adhesive wear of stir cast hypereutectic Al–Si–Mg alloy under reciprocating sliding conditions

This paper describes an attempt to enhance the wear properties of hypereutectic cast aluminium–silicon alloys produced by semi-solid metal (SSM) processing technique. The rheological experiments on SSM slurries were performed under continuous cooling condition from liquidus temperature. Wear characteristics of alloy under investigation were studied using pin on flat wear system over a range of normal load (10–40 N) at constant average sliding speed (0.2 m/s) against cast iron and stainless steel counter surface. Stir cast alloy showed lesser weight loss compared to conventional cast alloy. Stir cast and conventional cast alloys showed higher weight loss against the stainless steel as compared to that against cast iron counter surface. Optical microscopy of the conventional cast and stir cast hypereutectic alloy has shown that stir casting causes refinement of primary silicon particles and modification of eutectic silicon compared to conventional cast alloy. The scanning electron microscopy of wear surfaces was carried out to investigate the mode of wear.     

Influence of graphite content on the dry sliding and oil impregnated sliding wear behavior of Al 2024–graphite composites produced by in situ powder metallurgy method

The influence of graphite content on the dry sliding and oil impregnated sliding wear characteristics of sintered aluminum 2024 alloy–graphite (Al/Gr) composite materials has been assessed using a pin-on-disc wear test. The composites with 5–20 wt.% flake graphite particles were processed by in situ powder metallurgy technique. For comparison, compacts of the base alloy were made under the same consolidation processing applied for Al/Gr composites. The hardness of the sintered materials was measured using Brinell hardness tester and their bending strength was measured by three-point bending tests. Scanning electron microscopy (SEM) was used to analyze the debris, wear surfaces and fracture surfaces of samples. It was found that an increase in graphite content reduced the coefficient of friction for both dry and oil impregnated sliding, but this effect was more pronounced in dry sliding. Hardness and fracture toughness of composites decreased with increasing graphite content. In dry sliding, a marked transition from mild to severe wear was identified for the base alloy and composites. The transition load increased with graphite content due to the increased amount of released graphite detected on the wear surfaces. The wear rates for both dry and oil impregnated sliding were dependent upon graphite content in the alloy. In both cases, Al/Gr composites containing 5 wt.% graphite exhibited superior wear properties over the base alloy, whereas at higher graphite addition levels a complete reversal in the wear behavior was observed. The wear rate of the oil impregnated Al/Gr composites containing 10 wt.% or more graphite particles were higher than that of the base alloy. These observations were rationalized in terms of the graphite content in the Al/Gr composites which resulted in the variations of the mechanical properties together with formation and retention of the solid lubricating film on the dry and/or oil impregnated sliding surfaces.     

Wear and corrosion behavior of electrodeposited nickel–carbon nanotube composite coatings on Ti–6Al–4V alloy in Hanks′ solution

Carbon nanotubes (CNT) have received considerable interest in many industries, but composite coatings of CNTs have not yet been sufficiently developed for use in biomedical implants. This investigation elucidates the wear and corrosion behavior of electroplated Ni/CNT composite coatings on Ti–6Al–4V alloy in Hanks’ solution. Experimental results indicate that the CNTs in an electroplated Ni/CNT composite coating increase its hardness to 98.5% higher than that of a pure Ni coating. Additionally, an Ni/CNT composite coating can form stable and dense passive film, which significantly improves wear and corrosion in Hanks′ solution.     

Chemistry enhanced shear thickening polishing of Ti–6Al–4V

To obtain the great surface quality of Ti–6Al–4V and achieve high efficiency in the polishing process, the chemistry enhanced shear thickening polishing (C-STP) was proposed, and the polishing performance of different pH slurry was studied. The results show that the material removal rate gradually increases as the pH value decreases from 10 to 1, and the best surface quality is obtained at pH 2. The corrosion current density and potential were measured by potentiodynamic polarization under three typical pH values. It is confirmed that the most massive corrosion rate presents at pH 2, and the passive film is most susceptible to be produced at pH 10. The reaction resistance was measured by electrochemical impedance spectroscopy to clarify the polishing mechanism. Under acidic conditions, the chemical reaction product on the surface can be quickly removed by mechanical action of the abrasive. On the contrary, the passive film formed on the surface under the alkaline condition is difficult to be removed. The corrosion reaction products were determined by X-ray photoelectron, and the chemical reaction under acid-base environment was derived. MRR reached 107.3 nm/min under the selected process parameters, and the surface roughness (S_a) is reduced from 124 nm to 8.6 nm within 15 min.     

Suppression of forced vibration due to chip segmentation in ultrasonic elliptical vibration cutting of titanium alloy Ti–6Al–4V

Ultrasonic elliptical vibration cutting of titanium alloy Ti–6Al–4V is investigated in this research. Because products made of Ti–6Al–4V alloy are usually designed for possessing low-rigidity structures or good-quality cut surfaces, machining requirements such as low cutting forces and slow rate of tool wear need to be fulfilled for realization of their precision machining. Therefore, the ultrasonic elliptical vibration cutting is applied as a novel machining method for those products. Machinability of Ti–6Al–4V alloy by the ultrasonic elliptical vibration cutting with cemented carbide tools is examined to figure out suitable cutting conditions for precision machining of Ti–6Al–4V alloy. As experimental results, generated chips, cutting forces, and profiles of cut surfaces are indicated. A forced vibration problem occurred due to the segmented chip formation, which is also well-known in the ordinary non-vibration cutting. Therefore, characteristics of the forced vibration due to the chip segmentation are investigated in this research. Through the experiments, it is found that the frequency and magnitude of the forced vibration have relation with the average uncut chip thickness and cutting width. Especially, it is found that the averaging effect can suppress the forced vibration, i.e. the chip segmentation tends to occur randomly over the large cutting width, and hence the force fluctuations with random phases tend to cancel each other as the cutting width increases relatively against the average uncut chip thickness. Based on the investigations, a new practical strategy to suppress the forced vibration due to chip segmentation is proposed and verified. Using the proposed method significantly decreased cutting forces and good quality of surfaces are obtained when the forced vibration is suppressed compared to the ordinary non-vibration cutting results. Therefore, the results suggest that the precision machining can be realized without sacrificing the machining efficiency by increasing the width of cut and decreasing the average uncut chip thickness.     

Grain wear of brazed polycrystalline CBN abrasive tools during constant-force grinding Ti�C6Al�C4V alloy

Constant-force grinding experiments of titanium alloy Ti?C6Al?C4V were carried out with brazed polycrystalline cubic boron nitride (PCBN) abrasive wheel heads. Stock removal and tool wear of PCBN abrasive tools were evaluated by comparison with that of monocrystalline CBN counterparts. Grain wear of the brazed PCBN abrasive tools were examined using optical microscopy and scanning electron microscopy. Fracture mechanism of the PCBN abrasive grains was discussed. The results show that the brazed abrasive tools with PCBN grains give larger stock removal by 25% and the longer service life by 30% than the tools with CBN grains. PCBN abrasive grains have the capability to maintain sharp cutting edges by means of the fracture behavior of microcrystalline CBN particles. In case of strong fixing of the PCBN grains in the Ag?CCu?CTi filler layer, the crack of PCBN abrasive grains are mainly dominated in the intergranular mode dependent upon the joining effects of AlN binder material among the adjacent microcrystalline CBN particles.     

Sliding friction and wear of Cu–graphite against 2024, AZ91D and Ti6Al4V at different speeds

Cu–graphite composite fabricated by powder metallurgy art is no longer novel material. However, it might be a versatile self-lubricating material sliding against different metals and alloys. In this connection, understanding towards its tribological behavior and wear mechanism is very important. Sliding tribological behaviors of Cu–graphite composite against different counterparts, specified as 2024 aluminium alloy, AZ91D magnesium alloy, and Ti6Al4V titanium alloy, were investigated over varied sliding speeds at room temperature in air. The friction and wear tests were conducted on a pin-on-disk tribo-meter. Tribological performance of Cu–graphite composite strongly depended on its counterpart materials. Cu–graphite composite could provide friction reduction in sliding against 2024 and Ti6Al4V. Cu–graphite composite was a good self-lubricating material in sliding against AZ91D at low speeds but not at 0.25 and 0.50 m/s. Wear mechanism of Cu–Gr composite was related to the transfer, counter-transfer, mechanical mixing and tribo-oxidation at tribo-interface. Sliding speed had influences on tribo-interface and thereby wear mechanism. Finally, the effects of naturally occurred oxide film and sliding speed were discussed.