Surface characteristics of self-assembled microporous Ti-6Al-4V compacts fabricated by electro-discharge-sintering in air

A single electro-discharge-sintering (EDS) pulse (1.0 kJ/0.7 g), from a 300 (F capacitor, was applied to atomized spherical Ti-6Al-4V powder in air to produce microporous compact. A solid core surrounded by a porous layer was self-assembled by a discharge in the middle of the compact. X-ray photoelectron spectroscopy was used to study the surface characteristics of the compact material. C, N, O and Ti were the main constituents, with smaller amounts of Al and V. The surface was lightly oxidized and was primarily in the form of TiO₂. A lightly etched EDS sample showed the surface form of metallic Ti, indicating that EDS breaks down the oxide film of the as-received Ti-6Al-4V powder during the discharge process. The EDS Ti-6Al-4V compact surface also contained small amounts of TiN in addition to TiO₂, resulting in the reaction between nitrogen in air and the Ti substrate in times as short as 125 μs.     

MicroRaman study of the effect of oxide layer on nitriding of Ti-6Al-4V

Ti-6Al-4V samples were subjected to nitrogen ion implantation and low pressure RF plasma nitriding in a PIII equipment after chemical etching in Kroll's reagent. The samples were characterized by optical microscopy, AFM, microRaman, XRD and micro hardness measurements. From microRaman, oxides of titanium were found in the inter-granular β region whereas the oxide on the α region was extremely thin. The oxide on the β region was found to be amorphous and from intensity dependent Raman spectra, it was found to have a layered structure. The top layer was rutile and the inner layer anastase. Presence of Ti₂O₃ was also found. After PIII treatment at 600 °C, microRaman showed the presence of nitride in both the regions and the oxide was absent in the β regions. Also, from intensity dependent Raman spectra, it was found that in-take of nitrogen by β regions was higher. The oxide layer remained unaffected after plasma nitriding. Nitride presence in the α was established by microRaman. Even though Raman spectra from β regions were nearly the same as that of oxide, presence of nitrogen was indicated by the spectra. XRD studies of implanted and nitrided samples showed the prsence of TiN and Ti₂N in implanted sample and presence of Ti₂N in the nitrided sample. The β regions were found to have higher microhardness values after PIII and nitriding treatments. This is attributed to the deeper diffusion of nitrogen in these regions.     

Investigation of Ti-6Al-4V alloy acoustic softening

High power ultrasonic vibration is widely used for improving manufacturing processes such as machining and metal forming. High frequency mechanical vibration affects material properties and friction forces in contacting surfaces. Flow stress reduction under superimposed ultrasonic vibration is called as acoustic softening. The amount of this parameter should be determined for ultrasonic assisted metal forming processes. For determination of this parameter for workhorse Ti-6Al-4V alloy, experimental setup was designed and fabricated. Then tensile test under longitudinal ultrasonic vibration was performed for different ultrasonic powers. Results show that ultrasonic vibration has considerable effect on plastic behavior of the alloy and decreases flow stress. Also, increasing ultrasonic power leads to higher acoustic softening. Yield stress reduction up to 9.52%, ultimate stress reduction up to 4.55% and elongation up to 13% were obtained at 340 W ultrasonic power. After applying ultrasonic vibrations and its termination, hardness of specimens were measured in which increase up to 9% was observed.     

Grain refining of Ti-6Al-4V alloy fabricated by laser and wire additive manufacturing assisted with ultrasonic vibration

The formation of the coarse columnar crystal structure of Ti-6Al-4V alloy in the process of additive manufacturing greatly reduces the mechanical performance of the additive manufactured parts, which hinders the applications of additive manufacturing techniques in the engineering fields. In order to refine the microstructure of the materials using the high intensity ultrasonic via the acoustic cavitation and acoustic flow effect in the process of metal solidification, an ultrasonic vibration technique was developed to a synchronous couple in the process of Laser and Wire Additive Manufacturing (LWAM) in this work. It is found that the introduction of high-intensity ultrasound effectively interrupts the epitaxial growth tendency of prior-β crystal and weakens the texture strength of prior-β crystal. The microstructure of Ti-6Al-4V alloy converts to fine columnar crystals from typical coarse columnar crystals. The simulation results confirm that the acoustic cavitation effect applied to the molten pool created by the high-intensity ultrasound is the key factor that affects the crystal characteristics.     

Assessment of precipitates of aged Ti-6Al-4V alloy by ultrasonic attenuation

Abstract

Ti-6Al-4V alloy with different microstructures was investigated by means of ultrasonic attenuation measurements. Widmanstätten and equiaxed microstructures were obtaining by heat treating a Ti-6Al-4V alloy. These two microstructures were over-aged at 545 °C at different ageing times. In order to find out the factors affecting the variation in the ultrasonic attenuation, the heat-treated samples were examined by optical microscopy and scanning electron microscopy. Based on the theory of ultrasonic attenuation in a solid media, the mechanisms of ultrasonic attenuation in the Ti-6Al-4V alloy with different microstructures were analysed. It was found that in both cases with Widmanstätten and equiaxed microstructures, the ultrasonic attenuation increased with frequency. After ageing, the ultrasonic attenuation was mainly attributed to the scattering loss which included the stochastic and the Rayleigh scattering due to the precipitation of Ti₃Al particles homogeneously distributed in the α phase. Data analysis presented in the study showed that ultrasonic attenuation yields more accurate area fractions of precipitates predictions when a polynomial fit is performed.     

Ti-6Al-4V合金中片层组织形成的相场模拟

Ti-6Al-4V是典型的α+β钛合金,不同热处理制度和热加工工艺下可得到形貌各异的微观组织,从而表现出不同的力学性能,深刻理解合金中微观组织的形成机制有助于合金的进一步优化和改造.采用相场方法模拟Ti-6Al-4V合金中片层组织的形成及演化,以热力学数据库和动力学数据库为输入,通过计算定量预测β晶界上已存在初生α相时合金组织随时间的演化.结果表明,在一定条件下,随着时间的延长晶界α向β晶内生长形成片层组织,片状α簇的形貌与界面能各向异性密切相关;晶界取向对片层生长有重要作用,垂直于晶界生长时产生最密集的片层,随倾斜角增大片层加厚且生长缓慢;此外,热处理温度显著改变片层组织形貌,温度越高,片层尖端生长速度越慢,片层间距越大. 关键词： Ti-6Al-4V 相场模拟 片层组织     

A nano-scale mirror-like surface of Ti–6Al–4V attained by chemical mechanical polishing

Metal Ti and its alloys have been widely utilized in the fields of aviation, medical science, and micro-electromechanical systems, for its excellent specific strength, resistance to corrosion, and biological compatibility. As the application of Ti moves to the micro or nano scale, however, traditional methods of planarization have shown their short slabs.Thus, we introduce the method of chemical mechanical polishing(CMP) to provide a new way for the nano-scale planarization method of Ti alloys. We obtain a mirror-like surface, whose flatness is of nano-scale, via the CMP method. We test the basic mechanical behavior of Ti–6Al–4V(Ti64) in the CMP process, and optimize the composition of CMP slurry.Furthermore, the possible reactions that may take place in the CMP process have been studied by electrochemical methods combined with x-ray photoelectron spectroscopy(XPS). An equivalent circuit has been built to interpret the dynamic of oxidation. Finally, a model has been established to explain the synergy of chemical and mechanical effects in the CMP of Ti–6Al–4V.     

Electrical discharge machining of titanium alloy (Ti-6Al-4V)

In this study, the electrical discharge machining (EDM) of titanium alloy (Ti-6Al-4V) with different electrode materials namely, graphite, electrolytic copper and aluminium and process parameters such as, pulse current and pulse duration were performed to explore the influence of EDM parameters on various aspects of the surface integrity of Ti6Al4V. Scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrograph (EDS) and hardness analysis were performed. The experimental results reveal that the value of material removal rate, surface roughness, electrode wear and average white layer thickness are tendency of increase with increasing current density and pulse duration. However, extremely long-pulse durations such as 200 μs led to decrease MRR and surface roughness. Furthermore, the surface hardness is increasing due to the Ti₂₄C₁₅ carbides formed on the surface and obvious cracks are always evident in re-solidified layer when machining copper electrode. The surface crack densities and critical crack lines were determined for the tested material. The graphite electrode is beneficial on material removal rate, electrode wear and surface crack density but relatively poorer surface finish.     

Study on the microstructure and wear resistance of the composite coatings fabricated on Ti-6Al-4V under different processing conditions

Composite coatings mainly containing titanium carbides and borides were produced by laser surface alloying of Ti-6Al-4V with graphite and boron mixed powders. The test results show that the coatings have higher hardness (1600-1700 HV_0.1) and are more resistant to wear than the as-received sample. Laser scanning speed and the content of alloying elements (weight ratio of graphite to boron) have an effect on both the microstructure and the wear resistance of the coatings. TEM results show that strip titanium carbides and borides grow alternately and thus restrain the formation of coarse needle-like TiB and dendritic TiC crystals produced by laser alloying of titanium alloys with boron and graphite separately.     

Laser surface micro-texturing of Ti-6Al-4V substrates for improved cell integration

Biological behaviour of an implant, such as osseointegration, depends on both the chemical composition and the morphology of the surface of the implant. This paper reports the surface modification of Ti-6Al-4V - which is widely used in implantation - by Nd:YAG (λ = 1064 nm, τ = 100 ns) laser irradiation in order to enhance biointegration. Surface parameters are evaluated in terms contact angle measurement and surface roughness. Biocompatibility of the samples is investigated in vitro by monitoring 2T3 osteoblast cell growth on the samples through MTT assay.     

Laser diffusion nitriding of Ti-6Al-4V for improving hardness and wear resistance

Owing to poor tribological properties, titanium (Ti) alloys are usually surface-treated to enhance their surface properties. Laser surface nitriding, among others, is a common method employed to increase hardness and wear resistance for Ti alloys. Conventional laser nitriding involves surface melting of Ti alloys in a nitrogen atmosphere. This inevitably results in a roughened surface and post-treatment might be required. The present study aims at laser diffusion nitriding Ti alloys without surface melting via carefully selecting the laser processing parameters. The nitrided surface was characterized by X-ray diffractometry (XRD), optical microscopy (OM), scanning-electron microscopy (SEM), and profilometry. The nitride layer formed was about 1.62 μm upon repeated passes. The change in surface roughness resulting from laser diffusion nitriding was only minimal. Nanoindentation measurements revealed that the hardness of the nitride layer was around 11.3 GPa, being about 2.3 times that of Ti-6Al-4V. Ball-on-slab sliding wear test recorded a reduction in wear volume by about 8 times. The results of the present work thus demonstrate the feasibility of diffusion nitriding of Ti-6Al-4V by laser treatment for enhancing its surface properties and performance.     

Shear-Banding Evolution Dynamics during High Temperature Compression of Martensitic Ti-6Al-4V Alloy

The isothermal compression dynamics of ternary Ti-6 Al-4 V alloy with initial martensitic structures were investigated in the high temperature range 1083-1173 K and moderate strain rate regime 0.01-10 s^-1.Shear banding was found to still dominate the deformation mechanism of this process,despite its nonadiabatic feature.The constitutive equation was derived with the aid of Zener-Hollomon parameter,which predicted the apparent activation energy as 534.39 kJ/mol.A combination of higher ...     

Growth characteristics of plasma electrolytic oxidation ceramic coatings on Ti-6Al-4V alloy

The aim of this work is to discuss the growth characteristics of the ceramic coatings on Ti alloy by plasma electrolytic oxidation (PEO) technique. Ceramic coatings were prepared on Ti alloy by plasma electrolytic oxidation in different electrolyte solutions under different pulse modes. The composition and the structure of the coatings were investigated by X-ray diffraction and scanning electron microscopy (SEM), respectively. The amount of the dissolved titanium into the electrolytes during PEO process was measured by inductively coupled plasma-atomic emission spectrometer (ICP-AES). The structure and the composition of the coatings were related to the mode of the spark discharge during PEO process. (a) Under the pulsed single-polar mode: In Na₃PO₄ solution, the spark discharge was mainly due to the breakdown of the oxide film, and the coatings prepared were porous and mainly structured by the Ti from the substrate. In K₄ZrF₆-H₃PO₄ and NaAlO₂-Na₃PO₄ solutions, the main mode of the spark discharge was the breakdown of the oxide film at the initial stage, and then changed into the breakdown of the vapor envelope, and the coatings were rough and thick, and mainly structured by the elements from the electrolyte. (b) Under the pulsed bi-polar mode in NaAlO₂-Na₃PO₄ solution, the spark discharge may be mainly due to the breakdown of the oxide film, the coatings prepared were dense in inner layer and loose in outer layer, and structured by the elements from both the substrate and the electrolyte. Besides, the ICP-AES analyses showed that the amount of the dissolved titanium in the electrolyte during PEO process was more under the breakdown of the oxide film than under the breakdown of the vapor envelope, which was consistent with the changes of the structure of the coatings. Cathode pulse in the pulsed bi-polar mode increased the amount of the dissolved titanium in the electrolyte, compared with the pulsed single-polar one.     

CO2 laser gas assisted nitriding of Ti-6Al-4V alloy

Laser gas assisted nitriding of Ti-6Al-4V alloy is carried out and nitride compounds formed and their concentration in the surface vicinity are examined. SEM, XRD and XPS are accommodated to examine the nitride layer characteristics. Microhardness across the nitride layer is measured. Temperature field and nitrogen distribution due to laser irradiation pulse is predicted. It is found that the nitride layer appears like golden color; however, it becomes dark gold color once the laser power irradiation is increased. The δ-TiN and ?-TiN are dominant phases in the surface vicinity. The needle like dendrite structure replace with the feathery like structure in the surface region due to high nitrogen concentration. No porous or microcracks are observed in the nitrided layer, except at high power irradiation, in this case, elongated cracks are observed in the surface region where the nitrogen concentration is considerably high.     

Laser gas-assisted processing of carbon coated and TiC embedded Ti-6Al-4V alloy surface

Laser gas-assisted treatment of Ti-6Al-4V alloy surface is carried out. The alloy surface is initially coated by a carbon layer, in which the TiC particles are embedded prior to laser processing of the surface. The carbon coating with the presence of TiC particles on the workpiece surface is expected to result in carbonitride compound in the surface vicinity after the laser treatment process. Optical and scanning electron microscopes are used to examine the morphological and the metallurgical changes in the laser treated layer. The residual stress formed in the surface region after the laser treatment process is critical for the practical applications of the resulting surface. Therefore, the residual stress formed in the laser treated region is predicted from the analytically equation. The X-ray diffraction technique is incorporated to obtain the residual stress formed in the surface region. It is found that the residual stress predicted agrees with the X-ray diffraction data. The dense structures consisting of TiC_xN_1−x, TiN_x, Ti₂N, and TiC compounds are formed in the surface region of the treated layer. This, in turn, significantly increases the microhardness at the surface.     

Mechanical properties of Ti(C0.7N0.3) film produced by plasma electrolytic carbonitriding of Ti6Al4V alloy

Porous nanocrystalline Ti(C_0.7N_0.3) film on Ti6Al4V substrate was prepared by plasma electrolytic carbonitriding (PECN). The film was characterized and analyzed by using a variety of analytical techniques, such as XRD, SEM, EDX, TEM, FESEM, Rockwell C indenter, scratch tester, Vickers microhardness tester and ring-on-block tribometer. The results showed that the film was about 15 μm thick and its hardness was Hv 2369 at a load of 0.2 N. The adhesion of the film was characterized by Lc and Pc value, and was found to be about 42 N and more than 800 N, respectively. The friction coefficients and wear volume loss of the PECN-treated samples sliding against a steel counterpart were much less than those of the untreated Ti6Al4V. The film possessed a good wear-resistance and antifriction under oil-lubricated condition due to its high hardness, adhesion and fracture toughness. Also, the porous surface morphology of the Ti(C_0.7N_0.3) film contributed to the enhanced tribological resistance by promoting the formation of lubricant film and entrapping wear debris.     

TaC微粒对Ti-6Al-4V合金微弧氧化层结构和性能的影响

为了提高钛合金表面微弧氧化层在海洋环境中的抗腐蚀和耐磨损性能,在硅酸盐系电解液中添加不同浓度粒径在1 μm左右的TaC微粒,制备了 TaC掺杂微弧氧化层.通过扫描电子显微镜、能谱仪和X射线光电子能谱仪等对微弧氧化层的形貌、元素组成及其化学状态进行表征与分析,并对比评价了钛合金表面TaC掺杂微弧氧化层的厚度、表面粗糙度、...     

Combination of laser keyhole and conduction welding: Dissimilar laser welding of niobium and Ti-6Al-4V

Pulsed Nd:YAG laser welding of pure niobium plate to titanium alloy Ti-6Al-4V sheet in butt joint is studied regarding the laser/metal interaction modes. To obtain the optimized process parameters in dissimilar welding of Ti-6Al-4V/Nb, the melting ratio of laser beam energy for each weld counterpart is evaluated experimentally. Different laser welding modes of keyhole and conduction are predicted regarding the absorbed energy from the similar laser pulses on each weld counterpart. Laser keyhole and conduction welding were observed simultaneously through direct visualization of laser interaction with dissimilar metals using High Speed Imaging (HSI) system.