Composition control for laser solid forming from blended elemental powders

Laser solid forming (LSF) from blended elemental powders is a powerful tool for the synthesis of novel materials. Accurate composition control is critical for the application of this technique. It is found that the chemical composition of as-deposited sample can be controlled the same as the premixed elemental powders by keeping the identity of the divergence angles of the elemental powder streams. A mathematical model was established to describe the powder delivery process during LSF from blended elemental powders. Based on the consistency condition for divergence angles of different elemental powder streams, the match condition among the elemental powder characteristics (including particle size and density) can be obtained, which ensures the consistency in composition between the laser deposits and the premixed elemental powders. LSF experiments were carried out using a blend of Ti, Al, and V powders and the composition analysis was performed on as-deposited samples. The experimental results demonstrate the validity of the developed mathematical model.     

Chemical composition analysis for laser solid forming of titanium alloys from blended elemental powders

Laser solid forming （LSF） from blended elemental powders is an advanced technique to investigate new alloy systems and to create innovative materials. Accurate composition control is critical for the applications of this technique. In this letter, the composition analysis is performed on LSF titanium alloys from blended Ti, Al, and V powders. It is found that the composition of as-deposited sample can be controlled by keeping the identity of the divergence angle of each elemental powder stream. Based on the consistency condition for divergence angles of different elemental powder streams, the matching relation among the Ti, Al, and V powder characteristics （particle size and density） can be obtained, which ensures the consistency in composition between the laser deposits and the blended elemental powders under different laser processing parameters.     

The effect of thermal aging on the strength and the thermoelectric power of the Ti-6Al-4V alloy

When the Ti-6Al-4V alloy is overaged at 500-600°C, nanometer-sized α₂ (Ti₃Al) particles can be homogeneously precipitated inside a phases, thereby leading to strength improvement. Widmanstätten and equiaxed microstructures containing fine α₂ (Ti₃Al) particles were obtained by overaging the Ti-6Al-4V alloy. Precipitation of α₂ (Ti₃Al) particles was monitored using thermoelectric power measurements for different aging conditions in the Ti-6Al-4V alloy. Overaging heat treatments were conducted at 515, 545 and 575°C for different aging times. In addition, overaging samples were examined by optical microscopy, scanning electron microscopy and hardness measurements. It was found that the thermoelectric power is very sensitive to the aging process in the two studied Ti-6Al-4V structures.     

Microstructure characterization of Al matrix composite reinforced with Ti-6Al-4V meshes after compression by scanning electron microscope and transmission electron microscope

Compressive properties of Al matrix composite reinforced with Ti-6Al-4V meshes (TC4(m)/5A06 Al composite) under the strain rates of 10(-3)S(-1) and 1S(-1) at different temperature were measured and microstructure of composites after compression was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). Compressive strength decreased with the test temperature increased and the strain-rate sensitivity (R) of composite increased with the increasing temperature. SEM observations showed that grains of Al matrix were elongated severely along 45° direction (angle between axis direction and fracture surface) and TC4 fibres were sheared into several parts in composite compressed under the strain rate of 10(-3)S(-1) at 25°C and 250°C. Besides, amounts of cracks were produced at the interfacial layer between TC4 fibre and Al matrix and in (Fe, Mn)Al(6) phases. With the compressive temperature increasing to 400°C, there was no damage at the interfacial layer between TC4 fibre and Al matrix and in (Fe, Mn)Al(6) phases, while equiaxed recrystal grains with sizes about 10 μm at the original grain boundaries of Al matrix were observed. However, interface separation of TC4 fibres and Al matrix occurred in composite compressed under the strain rate of 1S(-1) at 250°C and 400°C. With the compressive temperature increasing from 25°C to 100°C under the strain rate of 10(-3) S(-1), TEM microstructure in Al matrix exhibited high density dislocations and slipping bands (25°C), polygonized dislocations and dynamic recovery (100°C), equiaxed recrystals with sizes below 500 μm (250°C) and growth of equiaxed recrystals (400°C), respectively.     

Al-Ti-B细化工业纯铝凝固组织演变过程数值模拟

采用耦合群体动力学方法与元胞自动机方法建立了细化处理条件下铝合金凝固微观组织演变的数值模型.该模型考虑了a-Al的非均匀形核过程、晶粒的初始球形长大以及之后的枝晶生长过程.利用建立的模型模拟了Al-5Ti-1B中间合金细化工业纯铝凝固组织演变过程.结果表明:形核初始阶段,熔体中存在充足数量的有效形核粒子, a-Al形核率随着熔体过冷度的增大逐渐增高;形核开始不久后, a-Al的异质形核过程由熔体中有效形核粒子数量控制,直到再辉发生,形核停止.模拟分析了中间合金添加量以及熔体冷却速度对工业纯铝凝固组织演变过程的影响,模拟结果与实验结果相符,验证了模型的准确性.     

Significant influences of metal reactivity and oxide films at particle surfaces on coating microstructure in cold spraying

Based on large amount of experimental observations, the effects of metal reactivity and oxide films at particle surfaces on coating deposition behavior in cold spraying were presented and discussed. The oxygen contents in as-sprayed Ti, Ti-6Al-4V and Al coatings were higher than those in the corresponding starting powders. The obvious flashing jets outside nozzle exit during deposition of Ti and Ti-6Al-4V were caused by the reaction of the particles with oxygen in the entrained or the adopted air. For Ti and Ti-6Al-4V coatings, their porous structures are predominantly attributed to the surface reactivity (defined as reactivity with oxygen). This surface reaction could be helpful for formation of a metallurgical bonding between the deposited particles. For Al, even though it is more reactive than Ti, the oxide films at Al particle surfaces suppress the surface activity.     

XPS study of porous dental implants fabricated by electro-discharge-sintering of spherical Ti-6Al-4V powders in a vacuum atmosphere

A single electro-discharge-sintering (EDS) pulse (0.7-2.0 kJ/0.7 g), from a 300 μF capacitor, was applied to atomized spherical Ti-6Al-4V powder in a vacuum to produce a porous-surfaced implant compact. A solid core surrounded by a porous layer was formed by a discharge in the middle of the compact. X-ray photoelectron spectroscopy was used to study the surface characteristics of the implant material. C, O, and Ti were the main constituents, with smaller amounts of Al, V, and N. The implant surface was lightly oxidized and was primarily in the form of TiO₂ with a small amount of metallic Ti. A lightly etched EDS implant 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 implant surface also contained small amounts of aluminum oxide in addition to TiO₂. However, V detected in the EDS Ti-6Al-4V implant surface did not contribute to the formation of the oxide film. The small amount of N in the implant surface resulted from nitride material that was also found in the as-received Ti-6Al-4V powders.     

A study on wear resistance and microcrack of the Ti3Al/TiAl + TiC ceramic layer deposited by laser cladding on Ti-6Al-4V alloy

Laser cladding of the Al + TiC alloy powder on Ti-6Al-4V alloy can form the Ti₃Al/TiAl + TiC ceramic layer. In this study, TiC particle-dispersed Ti₃Al/TiAl matrix ceramic layer on the Ti-6Al-4V alloy by laser cladding has been researched by means of X-ray diffraction, scanning electron microscope, electron probe micro-analyzer, energy dispersive spectrometer. The main difference from the earlier reports is that Ti₃Al/TiAl has been chosen as the matrix of the composite coating. The wear resistance of the Al + 30 wt.% TiC and the Al + 40 wt.% TiC cladding layer was approximately 2 times greater than that of the Ti-6Al-4V substrate due to the reinforcement of the Ti₃Al/TiAl + TiC hard phases. However, when the TiC mass percent was above 40 wt.%, the thermal stress value was greater than the materials yield strength limit in the ceramic layer, the microcrack was present and its wear resistance decreased.     

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     

Microstructure characteristics of laser forming repaired Ti60 alloy

The microstructure characteristics of laser forming repaired (LFR) Ti60 (Ti-5.6Al-4.8Sn-2Zr-1Mo-0.35Si-0.3Nb) as-deposited and annealed samples are analyzed. The microstructure of as-deposited repaired zone (RZ) consists of epitaxial columnar prior β grains, in which fine woven α laths and β-phase between α laths exist. The heat-affected zone (HAZ) experiences a continuous microstructural transition from duplex microstructure of the base metal zone (BMZ) to the microstructure of RZ. The presence of silicide precipitates is observed in both RZ and BMZ in an annealed sample by transmission electron microscopy. They are identified as (Ti, Zr) 6 Si 3 distributed mainly at the α/β interface with the size of 100 300 nm. The fine αprecipitates are detected in BMZ by electron diffraction; there was no α detected in RZ.     

Hydrogen peroxide treatment on Ti-6Al-4V alloy: A promising surface modification technique for orthopaedic application

Ti-6Al-4V alloy was treated with various concentrations (5 wt.%, 15 wt.% and 25 wt.%) of hydrogen peroxide (H₂O₂) and then heat treated to produce an anatase titania layer. The surface modified substrates were immersed in simulated body fluid (SBF) solution for the growth of an apatite layer on the surface and the formed apatite layer was characterized using various surface characterization techniques. The results revealed that titania layer with anatase nature was observed for all H₂O₂ treated Ti-6Al-4V alloy, irrespective of the H₂O₂ concentrations. Ti-6Al-4V alloy treated with 15 wt.% and 25 wt.% of H₂O₂ induced apatite formation, however 5 wt.% of H₂O₂ treated Ti-6Al-4V failed to form apatite layer on the surface. The electrochemical behaviour of H₂O₂ treated specimens in SBF solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy. Ti-6Al-4V alloy treated with 25 wt.% of H₂O₂ solution exhibited low current density and high charge transfer resistance values compared to specimens treated with other concentrations of H₂O₂ and untreated Ti-6Al-4V alloy.     

Surface characteristics of TiN/ZrN coated nanotubular structure on the Ti-35Ta-xHf alloy for bio-implant applications

In this study, we investigated the surface characteristics of the TiN/ZrN-coated nanotubular structure on Ti-35Ta-xHf ternary alloys for bio-implant applications. These ternary alloys contained from 3 wt.% to 15 wt.% Hf contents and were manufactured in an arc-melting furnace. The Ti-35Ta-xHf alloys were heat treated in Ar atmosphere at 1000 °C for 24 h, followed by water quenching. Formation of the nanotubular structure was achieved by an electrochemical method in 1 M H₃PO₄ electrolytes containing 0.8 wt.% NaF. The TiN coating and ZrN coating were subsequently prepared by DC-sputtering on the nanotubular surface. Microstructures and nanotubular morphology of the alloys were examined by FE-SEM, EDX and XRD. The microstructure showed a duplex (α′′ + β) phase structure. Traces of martensite disappeared with increasing Hf content, and the Ti-35Nb-15Hf alloy had an entirely equiaxed structure of β phase. This research has shown that highly ordered, high aspect ratio, and nanotubular morphology surface oxide layers can be formed on the ternary titanium alloys by anodization. The TiN and ZrN coatings formed on the nanotubular surfaces were uniform and stable. The top of the nanotube layers was uniformly covered with the ZrN film compared to the TiN film when the Ti-35Ta-xHf alloys had high Hf content.     

Effect of microstructure on ~3He migration in TiT_(1.9) films

Two kinds of films were prepared to study the effect of microstructure on helium migration in Ti tritides. Both films showed different release behaviors and helium bubble distributions. In the film consisting of columnar grains, a twolayered structure was observed. Inclusions with a strip feature were found at the grain boundary, and no helium bubbles were distributed in these inclusions. However, helium preferred to migrate to the boundaries of these inclusions. Bubble linkage as a ribbon-like feature developed parallel to the film surface in the film consisting of columnar grains. More cracks were developed at the grain boundaries of the film consisting of columnar grains, although the helium content in the film consisting of columnar grains was less than that in the film consisting of equiaxed grains. A surface region with a small number of bubbles, or "depleted zone", was observed near the surface. The cracks extending to the film surface were the pathways of the critical helium released from the film. The helium migration was strongly influenced by the grain microstructure.     

Growth characteristics of micro-plasma oxidation ceramic coatings on Ti alloy by inductively coupled plasma-atomic emission spectrometer technique

The aim of this work was to study the growth characteristics of micro-plasma oxidation ceramic coatings on Ti-6Al-4V alloy. Compound ceramic coatings were prepared on Ti-6Al-4V alloy by pulsed micro-plasma oxidation (MPO) in NaAlO₂ solution. The phase composition and surface morphology of the coating were investigated by X-ray diffractometry and scanning electron microscopy. The solution of Ti from the substrate and the content of Al in the electrolyte were studied by inductively coupled plasma-atomic emission spectrometer (ICP-AES) technique. Ti from the substrate dissolved and came into the coating and the electrolyte during MPO process. The content of Ti in the electrolyte under the pulsed bi-polar mode was more than that of the pulsed single-polar mode. The phase composition and structure of the coating was attributable to the space steric hindrance of Al congregated on the electrode surface due to the effect of the electric field and the electrolyte characters. For the pulsed single-polar mode, the coating was mainly composed of a large amount of α-Al₂O₃ and a small amount of γ-Al₂O₃. And the coating was mainly structured by Al from the electrolyte. However, the coating was composed of a large amount of Al₂TiO₅ and a little α-Al₂O₃ and rutile TiO₂ for the pulsed bi-polar mode. And the coating was structured both by Ti from the substrate and Al from the electrolyte.     

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.     

激光熔覆TiC陶瓷涂层的组织和摩擦磨损性能研究

采用激光熔覆技术在TC4合金表面上制备了TiC陶瓷涂层,分析了熔覆层的微观组织,测试了熔覆层的硬度和摩擦磨损性能。结果表明:TiC激光熔覆层分为熔覆区和稀释区两个区域,熔覆区未受到基底的稀释,由TiC颗粒和TiC树枝晶组成;稀释区受到了基底的稀释,由TiC树枝晶和钛合金组成;TiC激光熔覆层的显微硬度在HV700~1500之间,明显地改善了TC4合金表面的摩擦和磨损性能。     

Effects of heat treatment on evolution of microstructure of boron free and boron containing biomedical Ti-13Zr-13Nb alloys

In the present study, the effects of heat treatment on the microstructure of Ti-13Zr-13Nb (TZN) and Ti-13Zr-13Nb-0.5B (TZNB) alloys have been investigated. Depending on the heat treatment conditions, the microstructure of the heat treated TZN alloy consisted mainly of elongated and/or equiaxed α, β or martensite. Slow cooling (furnace or air cooling) from the solution treatment temperature produced α and β phases in the microstructure. Rapid cooling (water quenching) resulted in martensite and retained β when the solution treatment temperature was above or close to β transus. However, martensite was not formed after water quenching from a solution treatment temperature which was below β transus due to partitioning effect of the alloying elements. Increasing the cooling rate from the furnace cooling to the air cooling led to finer microstructure. Aging of water quenched samples transformed the martensite, if present, into α and β, and the morphology of α phase changed from elongated to equiaxed and enhanced the growth of α. The microstructure of all the TZNB samples consisted of dispersed precipitated particles of TiB in the matrix. The majority of the boride particles showed an acicular (needle like) morphology. The other phases present in the TZNB alloy were similar to those in the similarly heat treated TZN alloy. Moreover, a growth of α phase was observed in the microstructure of TZNB alloy when compared with that of TZN alloy.     

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.     

Study on impact fusion at particle interfaces and its effect on coating microstructure in cold spraying

This paper deals with the impact melting phenomenon at the interfaces between the deposited particles in cold-sprayed coatings and its effect on coating microstructure and particle bonding mechanism. Al-12Si, Al2319, Ti, Ti-6Al-4V, Ni and NiCoCrAlTaY powders were selected as feedstocks, which have various thermal and mechanical properties. The analytical results showed that most of the used materials possibly experienced the local melting at the contact interfaces of particles under certain impact conditions. Low melting point, relatively high gas temperature and chemical reaction with the atmosphere are the main factors contributing to the impact fusion during cold spraying. The results also indicated that the local melting would benefit the formation of a metallurgical bonding between the deposited particles and enhance the coating cohesion.