Strength and microstructure of laser fusion-welded Ti–SS dissimilar material pair

The ability to efficiently create robust and reliable dissimilar metal joints has the potential to enable new functionalities and reduce the manufacturing costs of medical devices. The need for dissimilar material welds in the medical device industry is driven by the unique properties exhibited by biocompatible materials such as stainless steel and titanium, as well as shape memory materials such as NiTi. Many material pairs, however, suffer from significant intermetallic phase formation during welding which greatly reduces their strength. This study investigates the microstructures and strength of the laser fusion-welded titanium–stainless steel dissimilar material pair as a simplified model of the NiTi–stainless steel pair. Compositional and structural analysis of the weld pool is performed and fracture morphologies are analyzed in different regions of the weld joint. The role of weld pool geometry, heat flow, and quench rate on the resultant phases, microstructures, and strength of the welds is discussed.     

Correlation Between Mechanical Properties and Nanofriction of [Ti–Cr/Ti–Cr–N] n and [Ti–Al/Ti–Al–N] n Multilayers

In this work, thin films deposited by pulsed DC magnetron sputtering of [Ti–Al/Ti–Al–N] _n and [Ti–Cr/Ti–Cr–N] _n multilayers of nanometric periods were analyzed by AFM in contact mode to measure values of lateral and normal forces. From these measurements, the coefficient of friction (COF) of these materials in contact with the AFM tip was calculated. Measurements were made with three types of silicon tips, diamond-coated, Pt–Cr-coated, and bare silicon. Significant differences between the tip materials in contact with the samples, which affected the COF, were observed. The effect of the environmental layer of water covering the surface sample and the tip appears as the most important factor affecting the tribology behavior of the tip-sample contact. For diamond-coated and bare silicon tips there is an additional adherence force increasing the normal load. But for tips platinum–chromium-coated there is a repulsive force due to this water layer, which behaves as a lubricant layer before a threshold load.     

Effect of Plasma Nitriding Environment and Time on Plain Fatigue and Fretting Fatigue Behavior of Ti–6Al–4V

Plasma nitriding was performed on Ti–6Al–4V fatigue test samples at 520°C in two environments (nitrogen and nitrogen–hydrogen mixture in a ratio of 3:1) for two time periods (4 and 18 h). Plain fatigue and fretting fatigue tests were conducted on unnitrided and plasma nitrided samples. Plasma nitriding degraded lives under both plain fatigue and fretting fatigue loadings. The samples nitrided in nitrogen exhibited superior lives compared with the samples nitrided in the nitrogen–hydrogen mixture, possibly due to the relatively higher hardness (and presumably lower toughness) of the nitrided layer of the samples nitrided in the nitrogen–hydrogen mixture environment. For those samples nitrided in the nitrogen–hydrogen mixture, those nitrided for 18 h exhibited superior lives compared with those nitrided for 4 h. This trend was observed for samples nitrided in nitrogen gas at lower stress levels only; the converse was true at higher stress levels of 550 MPa and 700 MPa under plain fatigue loading. However, under fretting fatigue loading, the plasma nitriding time did not influence the lives significantly.     

Tribological performances of ZDDP and ashless triphenyl phosphorothionate (TPPT) as lubricant additives on Ti–N and Ti–Al–N coated steel surfaces

Abstract

In recent years, there has been much attention on the effects of lubricant additives on the friction and wear properties of surface coatings. However, little research has been conducted to investigate the influence of antiwear additives on the tribological performances of titanium nitride (Ti–N) and titanium aluminium nitride (Ti–Al–N) coatings. It has been reported that introducing aluminium into Ti–N coatings enhanced their oxidation resistance. In this study utilising a pin on cylinder tribometer, lubricants containing zinc dialkyl dithiophosphate (ZDDP) or a more environmentally friendly alternative, ashless triphenyl phosphorothionate (TPPT), were used. Experimental results revealed that ZDDP and TPPT helped to reduce wear on both coatings through the formation of a tribofilm, although it was also found that both additives increased the friction coefficient on both surfaces. Based on overall findings, this paper suggests the use of TPPT as a suitable ZDDP replacement for providing wear protection on Ti–N and Ti–Al–N coatings.     

Influence of milling on surface integrity of Ti6Al4V—study of the metallurgical characteristics: microstructure and microhardness

The quality of titanium alloy parts in the aeronautical field demands high reliability, which is largely related to surface integrity. Surface integrity is generally defined by three parameters: a geometric parameter, a mechanical parameter and a metallurgical parameter. The present article addresses the influence of milling on the metallurgical parameter for a surface milled in Ti6Al4V material, focusing in particular on the microstructure and microhardness. Observation of the machined surface from a macroscopic perspective highlight an orange peel phenomenon. This effect is the combined result of redeposition and crushing of the milled material. No plastically deformed layer or lengthening of the grains was observed under the milled surface. As far as microhardness is concerned, a slightly softened region was observed under the milled surface. A diffusion of vanadium from the β phase to the α phase was also noted, but with no change in microstructure.     

The Friction and Wear Properties of Ti–Al–Nb Intermetallics by Plasma Surface Alloying

Both plasma chromizing and carburization following plasma chromizing (duplex treatment) for Ti–Al–Nb alloy were performed, respectively, and the microstructure, dynamic ultra-microhardness, and elastic modulus of the alloying layer were determined. Using silicon nitride (Si₃N₄) balls as the counterface materials, dry sliding friction tests on the substrate, the chromized layer, and the duplex-treated layer were completed by ball-on-disk tribometer at room temperature. The results indicated that the duplex-treated layer was mainly composed of Cr₂₃C₆, Cr₂Nb, pure chromium, and carbon phases, while the chromized layer consisted of Al₈Cr₅ and Cr₂Nb phases. The ultra-microhardness of the duplex-treated layer was higher than that of the chromized layer, whereas the elastic modulus of the duplex-treated layer was lower than that of the chromized layer. The friction coefficient of the duplex-treated layer was about three times lower than that of the chromized layer, while the wear rate was one order of magnitude lower than that of the chromized layer.     

Tribo-layer and its role in dry sliding wear of Ti–6Al–4V alloy

Dry sliding wear tests were performed for Ti–6Al–4V alloy under a load of 50–250 N at 25–500 °C on a pin-on-disk elevated temperature tester. Worn surfaces and subsurfaces were thoroughly investigated for the morphology, composition and structure of tribo-layers. Ti–6Al–4V alloy could not be considered to possess poor wear resistance at all times, and presented a substantially higher wear resistance at 400–500 °C than at 25–200 °C. The tribo-layer, a mechanical mixing layer, was noticed to exist on worn surfaces under various conditions. High wear rate at 25–200 °C was ascribed to no protective tribo-layer containing no or trace tribo-oxides. As more oxides appeared in the tribo-layers, they presented an obviously protective role due to their high hardness, thus giving a reasonable explanation for high wear resistance of Ti–6Al–4V alloy at 400–500 °C.     

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.     

Tribological Performance of Ti–Si-Based in Situ Composites

In this study, a series of Ti–Si-based in situ composites was manufactured by means of a common argon arc melting technique and tribologically evaluated using a sliding ball-on-disc tester under simulated body fluid lubrication. The composite microstructure, mechanical properties, and surface roughness were characterized using light and scanning electron microscopy (SEM), vertical scanning interferometry (VSI), X-ray diffraction (XRD) analysis, and hardness measurements. The evolution of coefficients of friction (COFs) and the appearance of contacting surfaces showed that two the principal wear mechanisms were mixed elastohydrodynamic lubrication (EHL), typically followed by abrasive wear. The mixed EHL was due to the combined effect of serum solution lubrication and surface irregularities, which were produced during the routine surface preparation of samples. The mixed EHL provided the absence of wear and low and stable COFs, which did not depend on the phase composition, microstructure, or hardness of Ti–Si-based alloys. However, in most cases, the change in contact geometry led to the transition from mixed EHL to conventional boundary lubrication, accompanied by increased and unstable friction, adhesive material transfer of metal to the ceramic counterbodies, and abrasive wear. In this respect, the low wear resistance and high adhesion affinity of the titanium matrix of Ti–Si-based alloys should be improved.     

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.     

Study of the intrinsic mechanisms of nickel additive for grain refinement and strength enhancement of laser aided additively manufactured Ti–6Al–4V

It is well-known that grain refiners can tailor the microstructure and enhance the mechanical properties of titanium alloys fabricated by additive manufacturing(AM). However, the intrinsic mechanisms of Ni addition on AM-built Ti–6Al–4V alloy is not well established. This limits its industrial applications. This work systematically investigated the influence of Ni additive on Ti–6Al–4V alloy fabricated by laser aided additive manufacturing(LAAM). The results showed that Ni addition yields three ...     

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.     

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.     

Arc Evaporation of Ti–Al–Ta–N Coatings: The Effect of Bias Voltage and Ta on High-temperature Tribological Properties

Recently, titanium aluminium tantalum nitride (Ti–Al–Ta–N) coatings have been shown to exhibit beneficial properties for cutting applications. However, the reason for the improved behaviour of these coatings in comparison to unalloyed Ti–Al–N is not yet clear. Here, we report on the tribological mechanisms present in the temperature range between 25 and 900 °C for this coating system, and in particular on the effect of the bias voltage during deposition on the tribological response. Based on these results, we provide an explanation for the improved performance of Ta-alloyed coatings. An industrial-scale cathodic arc evaporation facility was used to deposit the coatings from powder metallurgically produced Ti₄₀Al₆₀ and Ti₃₈Al₅₇Ta₅ targets at bias voltages ranging from −40 to −160 V. X-ray diffraction experiments displayed a change with increasing bias voltage from a dual-phase structure containing cubic and hexagonal phases to a single-phase cubic structure. Investigations of the wear behaviour at various temperatures showed different controlling effects in the respective temperature ranges. The results of dry sliding tests at room temperature were independent of bias voltage and Ta-alloying, where the atmosphere, i.e. moisture and oxygen, were the most important parameters during the test. At 500 °C, bias and droplet-generated surface roughness were identified to determine the tribological behaviour. At 700 and 900 °C, wear depended on the coating’s resistance to oxidation, which was also influenced by the bias voltage. In conclusion, Ta-alloyed coatings show a significantly higher resistance to oxidation than unalloyed Ti–Al–N which could be an important reason for the improved performance in cutting operations.     

An experimental study of process variables in turning operations of Ti–6Al–4V and Cr–Co spherical prostheses

Ti–6Al–4V and Cr–Co alloys are extensively used in manufacturing prostheses due to their biocompatibility, high strength-to-weight ratio and high resistance to corrosion and wear. However, machining operations involving Ti–6Al–4V and Cr–Co alloys face a series of difficulties related to their low machinability which complicate the process of controlling the quality levels required in these parts. The main objective of this paper is to study the influence of cutting parameters, machine tool control accuracy and metrology procedures on surface roughness parameters and form errors in contouring operations of Ti–6Al–4V and Cr–Co workpieces. The machining performance of the two biocompatible materials is compared, focusing the study on part quality at low feed per revolution and the stochastic nature of plastic deformations at this regime. The results showed a better surface roughness control for Ti–6Al–4V, whereas for Cr–Co alloys, the performance presents high variability. In the case of form errors (sphericity), contouring errors and metrology procedures are important factors to be considered for quality assurance. In addition, the study analyses the correlation of the machining performance with different sensor signals acquired from a low cost non-intrusive multi-sensor, showing a high correlation of signals from acoustic emission sensors and accelerometers in the machining of spherical features on Ti–6Al–4V parts. The findings of this research work can be taken into account when designing prostheses components and planning their manufacturing processes.     

Relations of counterface materials with stability of tribo-oxide layer and wear behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy

Dry sliding wear tests of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy (TC11 alloy) sliding against AISI 52100 and AISI M2 steels were performed under the load of 50–250 N at 25–600 °C. For two kinds of counterface materials, the titanium alloy presented totally different wear behaviours as the function of temperature. The appreciable variations of the titanium alloy sliding against different counterface materials were attributed the fact that a hard counterface caused unstable existence of tribo-layers by its microcutting action, thus resulting in the increase of wear rate. It is suggested that the hard counterface must be avoided as the counterface for the titanium alloy/steel sliding system.     

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.     

Mechanical and tribological properties of Cr–Nb double-glow plasma coatings deposited on Ti–Al alloy

Double-glow plasma (DGP) coatings are recommended for metallic components to mitigate the damage induced by complex working conditions in previous studies. In this paper, Nb-rich (Cr–Nb₄) and Cr-rich (Cr₄–Nb) -alloyed layers were formed onto the Ti–Al substrate via a DGP process to enhance its wear resistance. Scratch and Nano-indentation tests were used to evaluate the mechanical properties of the coatings. The tribological behaviour of the coatings were investigated using a pin-on-disc tribometer by rubbing against the GCr15 ball. Results from surface analysis techniques showed that the coatings mainly comprised Cr, Nb and Cr₂–Nb phases, and were well bonded to the substrate. The hardness of the Cr–Nb₄ coating was 11.61GPa and the Cr₄–Nb coating was 9.66 GPa which all higher than that of the uncoated Ti–Al which was 5.65 GPa. However, the critical load of the Cr₄–Nb coating ~21.64 was higher than that of the Cr–Nb₄ coating ~17.6. And the specific wear rate of Cr–Nb₄ coating, Cr₄–Nb coating and uncoated Ti–Al were 3.54 × 10^?4, 0.01 × 10^?4 and 1.53 × 10^?4mm³ N^?1 m^?1_, respectively. The low-wear mechanism of the coatings is discussed in detail in this paper.     

Wear mechanisms of K2Ti4O9 whiskers reinforced Al20Si aluminum matrix composites with lubrication of water and tetradecane

The tribological properties of K₂Ti₄O₉ whisker reinforced Al20Si aluminum matrix composites were investigated in a mode of low amplitude reciprocal sliding. The ball-on-disk tests were performed at applied loads of 20–100 N and sliding velocity of 0.09 m s⁻¹. The water lubricated composites demonstrated higher wear resistance and friction coefficient than the tetradecane lubricated composites did. The main wear mechanism is microgrooving at low applied loads and tribochemical wear at high applied loads for the pairs lubricated with water, microgrooving at all test loads for the pairs lubricated with tetradecane.     

RETRACTED ARTICLE: Composition design and mechanical properties of BCC Ti solid solution alloys with low Young’s modulus

(Ti-Zr)-Mo-Nb alloys were studied to find some stabilized BCC solid solution with low Young’s Modulus. We propose a cluster-plusglue-atom model to solve the composition design of multi-component complex alloys from the structure viewpoint. The alloy composition is expressed with [cluster](glue atom)x according to the model, x denoting the number of glue atoms matching one cluster. Alloy structures were identified with XRD and optical microscopy (OM), and mechanical tests were finally carried on to the BCC alloys. The experimental results indicated that a series of β-Ti solid solution alloys with low Young’s modulus and good synthesized mechanical properties can be obtained with the cluster model, where the [CN14 cluster](glue)1 alloys given by of 1:1 cluster model have the optimum properties.