Erosion resistance of laser clad Ti-6Al-4V/WC composite for waterjet tooling

In waterjet operations, milled surfaces are left with some undesirable dimensional artefacts, thus the use of abrasion resistant mask has been proposed to improve the surface quality of machined components. In this study, the erosion performance of laser clad Ti-6Al-4V/WC composite coating subjected to plain water jet (PWJ) and abrasive water jet (AWJ) impacts to evaluate its potentials for use as waterjet impact resistant mask material and coating on components was investigated. Results showed that composite with 76 wt.% WC composition subjected to PWJ and AWJ impacts offered resistance to erosion up to 13 and 8 times that of wrought Ti-6Al-4V respectively. Scanning electron microscopy (SEM) examination of the eroded composite surfaces showed that the erosion mechanism under PWJ impacts is based on the formation of erosion pits, tunnels and deep cavities especially in the interface between the WC particles and the composite matrix owing to lateral outflow jetting and hydraulic penetration. Composite suffered ploughing of the composite matrix, lateral cracking and chipping of embedded WC particles and WC pull-out under AWJ impacts. The composite performance is attributed to the embedded WC particles and the uniformly distributed nano-sized reaction products (TiC and W) reinforcing the ductile β-Ti composite matrix, with its mean hardness enhanced to 6.1 GPa. The capability of the Ti-6Al-4V/WC composite coating was demonstrated by effective replication of a pattern on a composite mask to an aluminium plate subjected to selective milling by PWJ with an overall depth of 344 μm. Thus, composite cladding for tooling purpose would make it possible to enhance the lifetime of jigs and fixtures and promote rapid machining using the water jet technique.     

Flow field and its effect on microstructure in cold crucible directional solidification of Nb containing TiAl alloy

A 3D finite element model was established to investigate the flow field in TiAl melt under different process parameters that include the position of the solidification interface, the meniscus height, the heating power and the current frequency. A square cold crucible with an inner cross section of 36 mm × 36 mm was employed to directionally solidify Ti–46Al–6Nb–B (at.%) ingots. The flow pattern in the melt is highly parameter-related. Generally, there are two flow swirls with opposite directions in the melt. The flow near the solidification interface can be minimized with an optimal meniscus height. The flow velocity is decreased with a decrease of heating power and an increase of current frequency. A weaker lateral flow near the solidification interface is beneficial for the continuous growth of columnar grains. The microsegregation of a directionally-solidified ingot can be reduced by controlling the flow in the melt, which is one of the advantages of cold crucible directional solidification.     

Experimental study of heat transfer coefficient on hot steel plate during water jet impingement cooling

Experiments were performed, under transient conditions, to investigate the heat transfer phenomena of stationary hot steel plate under multiple top circular jets on run-out table. Based on inverse heat conduction model, a two-dimensional finite difference program was developed to calculate the local surface convective heat transfer coefficients and corresponding temperatures. The cooling water jet flow rate was varied from 15 L/min to 35 L/min and its effect on the convective heat transfer coefficient and surface temperature was analyzed. The results show that heat transfer coefficients are nonlinear functions of surface temperature. The cooling flow rate has no effect on heat transfer coefficient and surface temperature at stagnation point. Within 70 mm distance from stagnation line, heat transfer coefficient ratio changes slightly from 0.87 to 0.97. Beyond surface temperature of 350 °C, heat transfer coefficient ratio decreases with increasing distance from stagnation line.     

铝板在双面喷流加热和辊底摆动过程中的温度均匀性

基于辊底式铝板淬火炉加热工艺,针对板材双面多喷口气体喷流换热与内部导热的耦合问题,建立了板材喷流加热过程数理模型,研究了喷口排布、热风流量、铝板厚度和辊底摆动周期对板材表面换热及板温均匀性的影响。结果表明,喷口垂直喷流的换热效果优于倾斜喷流;垂直喷流时喷口以1/4间距错排布置时,铝板表面温度均匀性最好;喷流造成的铝板表面温度不均匀主要集中在加热前期,易造成板材的变形,且对于薄板更加显著;可采用较小的热风流量及较短的辊底摆动周期以改善铝板表面温度均匀性,减小材料的应力和应变。     

Effects of squeeze casting parameters on the microstructure of LM13 alloy

Effects of applied pressure and melt and die temperatures on the microstructure of squeeze cast LM13 alloy were examined. The results showed that application of pressure during solidification decreased the grain size and SDAS of the primary α phase and modified the eutectic silicon particles. With application of an external pressure of about 100 MPa, the average SDAS and the average aspect ratio of eutectic silicon particles were reduced from 47 μm and 5 to about 34 μm and 1.5, respectively. SDAS of the primary α phase and the average aspect ratio of eutectic silicon particles decreased slightly with a drop in the melt or die temperatures, reaching to 32 μm and 1.25, respectively, for the best conditions.     

Abrasive slurry jet micro-machining of holes in brittle and ductile materials

This paper investigated the effects of elasticity and viscosity, induced by a dilute high-molecular-weight polymer solution, on the shape, depth, and diameter of micro-holes drilled in borosilicate glass and in plates of 6061-T6 aluminum alloy, 110 copper, and 316 stainless steel using low-pressure abrasive slurry jet micro-machining (ASJM). Holes were machined using aqueous jets with 1 wt% 10 μm Al₂O₃ particles. The 180 μm sapphire orifice produced a 140 μm diameter jet at pressures of 4 and 7 MPa. When the jet contained 50 wppm of dissolved 8 million molecular weight polyethylene oxide (PEO), the blind holes in glass were approximately 20% narrower and 30% shallower than holes drilled without the polymer, using the same abrasive concentration and pressure. The addition of PEO led to hole cross-sectional profiles that had a sharper edge at the glass surface and were more V-shaped compared with the U-shape of the holes produced without PEO. Hole symmetry in glass was maintained over depths ranging from about 80–900 μm by ensuring that the jets were aligned perpendicularly to within 0.2°. The changes in shape and size were brought about by normal stresses generated by the polymer. Jets containing this dissolved polymer were observed to oscillate laterally and non-periodically, with an amplitude reaching a value of 20 μm. For the first time, symmetric ASJM through-holes were drilled in a 3-mm-thick borosilicate glass plate without chipping around the exit edge.The depth of symmetric blind holes in metals was restricted to approximately 150 μm for jets with and without PEO. At greater depths, the holes became highly asymmetric, eroding in a specific direction to create a sub-surface slot. The asymmetry appeared to be caused by the extreme sensitivity of ductile materials to jet alignment. This sensitivity also caused the holes in metals to be less circular when PEO was included, apparently caused by the random jet oscillations induced by the polymer. Under identical conditions, hole depths increased in the order: borosilicate glass > 6061-T6 aluminum > 110 copper > 316 stainless steel. The edges of the holes in glass could be made sharper by machining through a sacrificial layer of glass or epoxy.     

Separation of PRMMC into matrix alloy and reinforcements by nozzle filtering method

The SiC_P/Al–4 mass%Cu alloy composites fabricated by a low-pressure infiltration process (LPI process) were remelted and separated by nozzle filtering method. In the separation process, the PRMMC specimen was placed in a container with a small nozzle at the bottom. The molten PRMMC was forced to flow out through the nozzle by applying a certain pressure of Ar gas. Most of the molten matrix alloy flowed out through the nozzle and the remainder in the container consisted of SiC particles and a part of the matrix alloy. The particle volume fraction of the remainder was higher than that of the original PRMMC and the remainder would work as a filter to separate SiC particles from the matrix alloy melt. When nozzle tip angle was ranged from 60° to 120°, about 80% of matrix alloy in the PRMMC was separated and few SiC particles were observed in the separated matrix alloy. The surface of recovered SiC particles became slightly roughened due to the reaction with the molten matrix during the separation process. However, this is not expected to affect their reuse.     

Detection and 3-D positioning of small defects using 3-D point reconstruction,tracking, and the radiographic magnification technique

In this paper the capability of a 3-D point reconstruction algorithm based on multiple hypothesis tracking is experimentally explored on a setup consisting of a microfocus X-ray source and a digital detector array. The algorithm is verified to detect and 3-D position steel particles with diameters 0.16–0.26 mm radiographed behind 4.7 mm Inconel 718 at two to ten times magnification. At ten times magnification the algorithm is verified to detect and 3-D position with an average error of 0.1 mm pore defects with diameters 0.05–0.25 mm in 4.7 mm thick titanium alloy laser welds.     

Effects of ultrasonic treatment on microstructure and tensile strength of AZ91 magnesium alloy

In this study, the effects of ultrasonic treatment on microstructural features and tensile strength of AZ91 magnesium alloy were investigated. AZ91 melts were subjected to ultrasonic waves of different power levels for 5 min using an ultrasonic device with frequency of about 20 kHz and maximum power of 600 W and cast in sand moulds. The results showed that ultrasonic treatment of the melt prior to casting had a significant effect on the size and sphericity of α-Mg dendrites as well as on the size, continuity, sphericity and distribution of intermetallic particles formed during cooling and solidification of the alloy. Increasing the applied ultrasonic power resulted in smaller, more rounded and better distributed grains and particles. The microstructural effects were mainly attributed to the cavitation and streaming phenomena which took place during ultrasonic treatment in the melt. Tensile strength of the alloy was significantly improved by ultrasonic treatment of the melt. Discontinuity and refinement of Mg₁₇Al₁₂ particles in the ultrasonically treated samples is thought to be the main reason for this improvement. The paper also examines different possible mechanisms responsible for microstructural modification of different phases under ultrasonic treatment conditions.     

Effect of Y2O3 crucible on contamination of directionally solidified intermetallic Ti–46Al–8Nb alloy

The effect of Y₂O₃ crucible on contamination of Ti–46Al–8Nb (at.%) alloy directionally solidified (DS) in a Bridgman-type apparatus was studied. Directional solidification experiments were performed in dense Y₂O₃ crucibles using different growth rates, melt temperatures and various reaction time between the melt and the crucible. The main mechanism responsible for the contamination of the DS samples is diffusion controlled dissolution of the Y₂O₃ crucible in the melt which leads to an increase of oxygen and yttrium content in γ(TiAl) + α₂(Ti₃Al) matrix and precipitation of non-metallic particles in interdendritic region. Transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD) showed that these particles are Y₂O₃ phase. The oxygen content and volume fraction of Y₂O₃ particles increase with increasing melt temperature and reaction time. The activation energy for increase of oxygen content is calculated to be Q_O = 412.1 kJ/mol and the kinetics of this process is suggested to be controlled by long-range diffusion with the oxygen content exponent of 3. The activation energy for Y₂O₃ particle formation is calculated to be Q_Y = 421.8 kJ/mol and the time exponent is determined to be m = 0.55. Vickers microhardness measurements in lamellar γ + α₂ matrix without Y₂O₃ particles can be used as an indirect evidence of the level of contamination of DS samples with statistically identical α₂–α₂ interlamellar spacing.     

Facile one-step route to polyaniline–silver nanocomposite particles and their application as a colored particulate emulsifier

Polyaniline–silver nanocomposites were synthesized in the form of colloidal particles by the facile one-step aqueous chemical oxidative dispersion polymerization of aniline using silver nitrate as an oxidant and poly(vinyl alcohol) as a colloidal stabilizer. Aniline monomer was oxidized by silver ions, yielding polyaniline and elemental Ag simultaneously. The synthesized nanocomposite particles were colloidally stable over 2 years and transmission electron microscopy studies indicated the production of spherical, plate and rod-shaped polyaniline–silver nanocomposite particles with a silver core–polyaniline shell morphology. The conductivity of a pressed pellet of the nanocomposite particles using the conventional four-point probe technique was 1.4 × 10^?2 S/cm at 25 °C. The nanocomposite particles behaved as a ‘colored’ particulate emulsifier for the stabilization of transparent oil-in-water emulsions.     

Abrasive enhanced electrochemical slurry jet micro-machining: Comparative experiments and synergistic effects

Abrasive enhanced electrochemical slurry-jet machining (ESJM) is presented as a new approach to the micro-machining of metals using a combination of abrasive slurry-jet machining (ASJM) and electrochemical jet machining (ECJM). A novel ESJM prototype was developed to generate a charged slurry jet consisting of a mixture of Al₂O₃ abrasive particles and an electrolytic solution of NaCl and NaNO₃. A DC potential of 30 V was applied between the nozzle and specimen. A series of micro-channels were machined in Stellite 12 using ASJM, ECJM and ESJM processes to investigate the relative effects of erosion and anodic dissolution on the material removal rate and surface finish in the combined process of ESJM. The results illustrated that the ESJM process results in significantly greater target mass loss rate than the separate erosion and corrosion processes. The magnitude of the synergistic effect on the rate of mass loss was found to vary from positive to negative as the erosion component increased with increasing particle kinetic energy (jet pressure) and particle concentration. The roughness of the channels machined using ESJM was between that obtained with ASJM and ECJM. The roughness decreased as the erosion component of the total mass loss increased.     

针型喷嘴射流特性研究

运用FLUENT流体仿真软件的VOF两相流模型对针型喷嘴在不同入口压力、不同打击距离的喷嘴射流流场进行数值分析,得到针型喷嘴射流流场的速度和被清洗件承受的压力分布云图和压力分布曲线,研究针型喷嘴射流参数的变化对射流流场速度及压力的影响规律。结果表明:随着入口水压的增加,射流最大速度和被清洗件上承受的压力值增加,但增加趋势逐渐减小;随着打击距离的增加,清洗范围略微增大,被清洗件上承受的压力值减小,但减小程度逐渐下降;进而得出当入口压力为30 MPa,打击距离在90～100 mm范围内的针型喷嘴具有最佳清洗效果。     

水射流喷嘴内表面形状对切割性能的影响研究

磨料水射流在喷嘴中运动并得到加速,喷嘴内表面形状对射流影响显著。通过仿真与试验比较圆柱形、圆锥形、锥直形和流线形4种喷嘴的切割性能,结果表明:圆柱形喷嘴射流加速过程短,衰减快,切割效果差;圆锥形喷嘴射流加速过程长,衰减过程缓慢,有较长的射流核心段,具有一定的应用价值;锥直形、流线形两种喷嘴的加速过程与衰减过程无显著差异,射流核心段较长,实际切割能力并没有显著差别;在锥直形喷嘴中,出口圆柱段喷嘴长径比为3～4较合适。     

Effects of isothermal process parameters on the microstructure of semisolid AZ91D alloy produced by SIMA

The effects of isothermal process parameters on the microstructure evolution of semisolid AZ91D alloy produced by strain-induced melt activation (SIMA) were investigated using the self-programmed analysis software based on quantitative metallography. The results showed that long isothermal time could make the semisolid particles more globular, but the size of the particles would grow larger; high semisolid isothermal temperature would reduce the solid volume fraction and accelerate the spherical evolution of the solid particles. It was found that the optimal process parameters should be 570 °C and 10–20 min of isothermal temperature and time respectively based on the conditions of this paper. The mechanism of the particles’ formation was also discussed during the isothermal treatment.     

高压二氧化碳载送干冰喷射去污流场分析及关键参数优化

为提升核设施退役去污能力,提出高压二氧化碳载送干冰喷射去污方法。基于Fluent软件建立了喷射去污系统中关于喷嘴与去污箱体内部流场的分析模型,通过流场分析获得了干冰颗粒速度分布。以靶距、喉半径、喷嘴长度、干冰粒径、压力为关键参数,以干冰颗粒对污染表面的冲击应力与分布直径的乘积为去污评价指标,分别开展了1.5、3 mm口径喷嘴流场分析的正交试验。通过均值响应分析和主效应分析掌握了各关键参数对去污性能的影响规律,获得最优参数组合。分析结果表明：关键参数优化后,1.5 mm口径喷嘴去污性能提升了5.64%,3 mm口径喷嘴去污性能提升了15.8%,后者去污性能是前者的1.3倍,但前者的二氧化碳利用率是后者的3倍,具有更好的经济性。该分析结果可为喷嘴选型和喷射去污系统参数计算提供支撑。     

Microstructure and wear resistance of Stellite-6/WC MMC coatings produced by laser cladding using Yb:YAG disk laser

The paper focuses on the study results of Stellite-6/WC metal matrix composite coatings (MMC coatings) produced by laser cladding technology using a 1 kW continuous wave Yb: YAG disk laser with powder feeding system. Specimens were preparation using CNC machining center equipped with a laser nozzle. Powder mixtures containing 60% tungsten carbides particles and 40% commercial Stellite-6 powder were used. In this study, three different values of laser beam power (400 W, 550 W and 700 W) and three different powder feed rate (5.12 g/min, 10.24 g/min and 15.36 g/min) were used. For all specimens, the same scanning speed of laser beam were applied. Changes in roughness, microstructure as well as wear resistance were investigated. It was found that increasing laser beam power caused a decrease in wear resistance of coating. Furthermore in described process appeared the best value of the powder feed rate which potentially resulting in better wear resistance. Exceeding this value influence on more intensive wear of coating. Special attention was given to the wear mechanism of MMC coatings.     

An innovative method to perform maskless plain waterjet milling for pocket generation: a case study in Ti-based superalloys

This paper presents an innovative methodology/jet path on which plain waterjet (PWJ) can generate pockets of good dimensional/geometrical definition (minimised under/over-erosion) while the proposed method leads to the avoidance of grit embedment on the target workpiece and the elimination of extra cost and time related to the use of mask.The novelty of the paper relies on the proposal of jet-path strategy that minimises the variations in jet dwell time by providing “continuous” relative movement during the jet-part interaction (through minimisation of accelerations/decelerations of the machine head) and by removing a controlled amount of material in a series of layers using special techniques. The proposed method is powerful in its approach from which it ensures (quasi)equal exposure time for each zone of material over which the jet passes, so that the jet path is “totally contained” within the form to be generated; hence, no masking is necessary to define the contour/shape.This approach has been employed for generating pockets on two Ti-based superalloys commonly used in aerospace industries, followed by dimensional, geometrical and surface quality analysis. The results proved that this approach can produce milled surfaces of straightness of the pocket bottom (<200 μm), tolerance on depth of cut per layer (<20 μm), tolerance on the radii at the bottom of the pockets (<100 μm), surface roughness (Ra=4–14 μm) and waviness (Wa=10–13 μm) characteristics in conditions of high surface integrity (no cracks, contaminations, etc.).     

Synthesis and microwave absorbing properties of FeCo alloy particles/graphite nanoflake composites

Exfoliated graphite was processed into nano-sized flakes by jet milling, ultrasonication and acid treatments, and then FeCo alloy particles were uniformly deposited onto the surface of the prepared graphite nanoflakes (GNFs) by a co-deposition and subsequent annealing process in reducing atmosphere. The FeCo-GNFs composites demonstrate good soft magnetic performance and effective microwave absorption, especially in the lower frequency range. The strongest absorption reaches ?30.6 dB at 7.4 GHz with a coating thickness of 2 mm. Furthermore, effective microwave absorption at different frequency bands can be acquired by adjusting the coating thicknesses and filling ratios of FeCo-GNFs absorber. Considering the low cost and high efficiency, the FeCo-GNFs composites have great potential as microwave absorber for practical applications.