Wear behaviors of an (TiB + TiC)/Ti composite coating fabricated on Ti6Al4V by laser cladding

A titanium-based composite coating reinforced by in situ synthesized TiB whiskers and TiC particles was successfully fabricated on Ti6Al4V by laser cladding. The coating is mainly composed of α-Ti cellular dendrites and a eutectic in which a large number of needle-shaped TiB whiskers and a few equiaxial TiC particles are uniformly embedded. The wear resistance of the coating is significantly superior to that of Ti6Al4V under the dry sliding wear condition at room temperature.     

Laser boronising of Ti–6Al–4V as a result of laser alloying with pre-placed BN

This paper discusses the effect of CO₂ laser alloying of pre-placed BN coating with Ti–6Al–4V alloy. The formation of titanium boride and titanium nitride investigated using energy dispersive X-ray diffraction (EDXRD) result were related to the microhardness and microstructure. The nitrogen and boron diffusion during the laser boronising process identified using secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectrometry (XPS) analysis was compared with the EDXRD results. The surface hardness HV1500–1700 observed at the boronised layer was five to six times higher than that of untreated Ti–6Al–4V alloy. This was compared with needle platelet and dendrite type microstructures. Theoretically estimated surface temperature values were used to interpret the compound formation in the laser alloyed layer.     

Ti6Al4V合金激光原位制备TiN枝晶增强梯度复合表面层

利用连续波2kW Nd-YAG激光在Ti6Al4V合金表面原位制备TiN枝晶增强梯度金属基复合材料表面层，并研究了该表面层的显微组织和磨损性能。结果表明：该表面层沿激光熔化深度具有明显的梯度结构，表面层与Ti6Al4V基体之间呈现良好的冶金结合，Ti6Al4V的表面硬度及耐磨性得到了显著增强．     

Changes in surface morphology of Ti6Al4V alloy and ZrO2-Y2O3 ceramic coating modified by high-intensity pulsed ion beams

The high-intensity pulsed ion beam (HIPIB) technique is developed to treat metallic and ceramic surfaces to improve materials performance. The processing is based on the beam-material interactions: remelting and/or ablation of a top layer on the irradiated surfaces (extreme surface heating effect); subsequently, the molten states may be frozen at an ultra-fast re-solidification rate after termination of the ion beam pulse. Surface smoothing and reconstruction of titanium alloys and ZrO₂-Y₂O₃ coatings have been observed as one of the typical outcome under high-intensity pulsed ion beam irradiation. It is demonstrated that the changes in surface morphology may significantly contribute to the improvements of overall performance of the materials.     

Microstructures and wear properties of in situ formed composite coatings produced by laser alloying technique

Laser-surface alloying of titanium alloy Ti-6Al-4V with C and Si mixed powders has been carried out. The composite coatings, thickness of about 0.7 mm, mainly consisting of titanium carbides and silicides, have a hardness of about 1500 HV_0.1, and the wear resistance is 4 times more than that of the as-received.     

In situ synthesis of TiN/Ti3Al intermetallic matrix composite coatings on Ti6Al4V alloy

The aim of this paper was to develop an in situ method to synthesize the TiN reinforced Ti₃Al intermetallic matrix composite (IMC) coatings on Ti6Al4V alloy. The method was divided into two steps, namely depositing pure Al coating on Ti6Al4V substrate by using plasma spraying, and laser nitriding of Al coating in nitrogen atmosphere. The microstructure and mechanical properties of TiN/Ti₃Al IMC coatings synthesized at different laser scanning speeds (LSSs) in laser nitriding were investigated. Results showed that the crack- and pore-free IMC coatings can be made through the proposed method. However, the morphologies of TiN dendrites and mechanical properties of coatings were strongly dependent on LSS used in nitriding. With decreasing the LSS, the amount and density of TiN phase in the coating increased, leading to the increment of microhardness and elastic modulus and the decrement of fracture toughness of coating. When the LSS was extremely high (i.e., 600 mm/min), only a thin TiN/Ti₃Al layer with thickness around of 100 μm was formed near the coating surface.     

Effect of ultrasonic vibration-assisted laser surface melting and texturing of Ti-6Al-4V ELI alloy on surface properties

Ultrasonic vibration-assisted laser surface processing that involves application of vertical ultrasonic vibrations to the Ti-6 Al-4 V alloy substrates while being irradiated with a CO_2 laser was performed for the development of laser melted and textured surfaces with potential applications in biomedical implants.The laser processing resulted in very consistent repeating undulating grooved surfaces, and the undulations were significantly more pronounced in the samples processed with higher ultrasonic power outputs.The phase evolution, studied by x-ray diffraction, confirmed that the laser processing triggered transformation of globular α→ acicular α and martensitic α' as well as increased amounts of retained α phases,which were also reflected in the microscopic analysis. The surface texture developed by laser processing resulted in increased surface wettability with increasing ultrasonic power output. The textured surfaces exhibited marked decrease in coefficients of friction during sliding wear testing performed under simulated body fluid due to lubricant entrainment within the textured grooves. The texturing also resulted in significant reduction in surface contact area during the wear process, which considerably reduced the overall wear rates due to abrasive wear.     

Ti6Al4V合金激光原位合成自润滑复合涂层高温摩擦学性能

为提高Ti6Al4V合金的高温摩擦学性能,采用激光熔覆技术在其表面原位合成多相混杂金属基高温自润滑耐磨复合涂层,熔覆粉末的成分为Ni60-16.8%TiC-23.2%WS_2(质量分数,下同),系统地研究复合涂层的显微组织、物相结构及其在20,300,600,800℃下的摩擦学性能和相关磨损机理。结果表明:复合涂层的显微硬度(701.88HV_0.5)约为基体(350 HV_0.5)的2倍;由于原位合成固体润滑相(Ti_2SC/TiS/NiS/TiO/TiO_2/NiCr_2O_4/Cr_2O_3)和硬质相(W,Ti)C_1-x/TiC/Cr_7C_3的协同作用,复合涂层的耐磨减摩性能明显优于基体。随着温度升高,涂层和基体的摩擦因数和磨损率均呈下降趋势,在800℃时复合涂层和基体的摩擦因数分别为0.32和0.43,磨损率分别为1.80×10^-4,2.92×10^-5mm/Nm。在800℃下塑性变形、分层和氧化磨损为基体主要磨损机理,复合涂层以氧化磨损和轻微的黏着磨损为主。     

Formation of nanocrystalline structure of Ti–6Al–4V alloy by cyclic hydrogenation–dehydrogenation treatment

Ti–6Al–4V (Ti64) sheet specimens were cathodically hydrogenated in sulfuric acid solution at ambient conditions. The hydrogenated specimens were then sent to go through the designed thermohydrogen processing (THP) twice to obtain a nano-sized grain structure. The average grain size of resulted microstructure was found to be 10–20 nm obtained by TEM. Qualitative and quantitative analyses performed by employing X-ray diffractometry (XRD) and elemental analysis (EA) showed that the addition of As₂O₃ as hydrogenation promoter in electrolyte significantly increased the hydrogen uptake. The high concentration of hydrogen arising from promoter action is the key factor in grain refinement. The optimal processing parameter found for grain-refining Ti64 was: (1) electrolytic hydrogenation at 100 mA cm⁻² for 3 h in 1 N H₂SO_4(aq) by adding 0.1 g L⁻¹ As₂O₃; (2) β transformation carried out at 850 °C for 1 h in air furnace, followed by a furnace cooling to 590 °C and held for 6 h; (3) oxide film removed and then dehydrogenated at 650 °C and 1.0 × 10⁻⁶ Torr for 10 h; (4) repeated the same processes once more.     

Effect of various surface conditions on fretting fatigue behavior of Ti–6Al–4V

An experimental investigation was conducted to explore the fretting fatigue behavior of Ti–6Al–4V specimens in contact with varying pad surface conditions. Four conditions were selected: bare Ti–6Al–4V with a highly polished finish, bare Ti–6Al–4V that was low-stress ground and polished to RMS #8 (designated as ‘as-received’), bare Ti–6Al–4V that was grit blasted to RMS #64 (designated as ‘roughened’) and stress relieved, and Cu–Ni plasma spray coated Ti–6Al–4V. Behavior against the Cu–Ni coated and as-received pads were characterized through determination of a fretting fatigue limit stress for a 10⁷ cycle fatigue life. In addition, the behavior against all four-pad conditions was evaluated with S-N fatigue testing, and the integrity of the Cu–Ni coating over repeated testing was assessed and compared with behavior of specimens tested against the as-received and roughened pads. The coefficient of friction, μ, was evaluated to help identify possible crack nucleation mechanisms and the contact pad surfaces were characterized through hardness and surface profile measurements.

An increase in fretting fatigue strength of 20–25% was observed for specimens tested against Cu–Ni coated pads as compared to those tested against as-received pads. The experimental results from the S-N tests indicate that surface roughness of the coated pad was primarily responsible for the increased fretting fatigue capability. Another factor was determined to be the coefficient of friction, μ, which was identified as ˜0.3 for the Cu–Ni coated pad against an as-received specimen and ˜0.7 for the bare as-received Ti–6Al–4V. Specimens tested against the polished Ti–6Al–4V pads also performed better than the specimens tested against as-received pads. Fretting wear was minimal for all cases, and the Cu–Ni coating remained intact throughout repeated tests. The rougher surfaces got smoother during cycling, while the smoother surfaces got rougher.     

Effect of 0.5 wt.% hydrogen addition on microstructural evolution of Ti–6Al–4V alloy in the friction stir welding and post-weld dehydrogenation process

The α + β titanium alloy, Ti–6Al–4V, was thermohydrogen processed with 0.5 wt.% hydrogen and friction stir welded using a W–Re pin tool. Defect-free joints were obtained with proper parameters. Hydrogen was removed from the joint through a post-weld dehydrogenation process. The microstructures of the as-welded and dehydrogenated joints were examined. The effect of 0.5 wt.% hydrogen addition on microstructural evolution of Ti–6Al–4V alloy in the friction stir welding and post-weld dehydrogenation process was revealed.     

Research on in situ synthesised (Ti,V)C/Fe composite coating by laser cladding

Laser cladding is an effective way to improve the wear resistance of mechanical components. In this study, the composite carbide (Ti,V)C reinforced Fe based coating was successfully synthesised by laser cladding the powder mixtures of ferrotitanium, ferrovanadium and graphite. The samples were analysed to assess the microstructure, microhardness and wear properties. Results indicate that high quality composite coating with metallurgical joint to the steel substrate was obtained. During laser cladding processes, it is found that the (Ti,V)C composite particles were in situ synthesised and distributed evenly in the coating. The microhardness and wear properties of the clad coating were improved significantly in comparison to the steel substrate due to the presence of the hard reinforcement (Ti,V)C.     

Effects of hydrogenation temperature on room-temperature compressive properties of CMHT-treated Ti6Al4V alloy

Compression tests were performed at room temperature to investigate the effects of hydrogenation temperature on compressive properties of Ti6Al4V alloy treated by continuous multistep hydrogenation treatment (CMHT).Pressure-composition isotherms and microstructures were also studied.Results showed that the equilibrium hydrogen pressure increased,and the hydrogen absorption rate decreased with the increase of hydrogenation temperature.The amounts of β phase and α'martensite increased first and then decreased when Ti6Al4V alloy was treated by four times CMHT with the increase of hydrogenation temperature.Hydrogenation temperature played a different role on the compressive properties of CMHT-treated Ti6Al4V alloy.The ultimate compression of Ti6Al4V alloy treated by 11 times CMHT at 850 ℃ increased by 83.3 ％ as compared to the as-received Ti6Al4V alloy.The compressive properties of Ti6Al4V alloy were dependent on the amounts of different phases and microstructures when Ti6Al4V alloy was treated by CMHT at different temperatures.     

Laser metal deposition of functionally graded Ti6Al4V/TiC

Functionally graded materials (FGMs) are advanced materials with improved properties that enable them to withstand severe working environment which the traditional composite materials cannot withstand. FGM found their applications in several areas which include: military, medicine and aerospace. Various manufacturing processes are used to produce functionally graded materials that include: powder metallurgy, physical vapour deposition, chemical vapour deposition process and laser metal deposition process. Laser metal deposition (LMD) process is an additive manufacturing process that can be used to produce functionally graded material directly from the three dimensional (3D) computer aided design (CAD) model of the part in one single process. LMD process is a fairly new manufacturing process and a highly non-linear process. The process parameters are of great importance in LMD process and they need to be optimized for the required application. In this study, functionally graded titanium alloy composite was produced using optimized process parameters for each material combination as obtained through a model that was developed in an initial study and the FGM was characterized through metallurgical, mechanical and tribological studies. The results show that the produced FGM has improved properties when compared to those produced at constant processing parameters for all material combinations.     

The optimization of friction spot welding process parameters in AA6181-T4 and Ti6Al4V dissimilar joints

Friction spot welding is a relatively new solid-state joining process able to produce overlap joints between similar and dissimilar materials. In this study, the effect of the process parameters on the lap shear strength of AA6181-T4/Ti6Al4V single joints was investigated using full-factorial design of experiment and analyses of variance. Sound joints with lap shear strength from 4769 N to 6449 N were achieved and the influence of the main process parameters on joint performance was evaluated. Tool rotational speed was the parameter with the largest influence on the joint shear resistance, followed by its interaction with dwell time. Based on the experimental results following response surface methodology, a mathematical model to predict lap shear strength was developed using a second order polynomial function. The initial prediction results indicated that the established model could adequately estimate joint strength within the range of welding parameters being used. The model was then used to optimize welding parameters in order satisfy engineering demands.