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.     

Laser surface coating of Mo–WC metal matrix composite on Ti6Al4V alloy

Laser surface alloying of Mo, WC and Mo–WC powders on the surface of Ti6Al4V alloys using a 2 kW Nd-YAG laser was performed. The dilution effect upon the microstructure, microhardness and wear resistance of the surface metal matrix composite (MMC) coating was investigated. With a constant thickness of pre-placed powder, the dilution levels of the alloyed layers were found to increase with the incident laser power. The fabricated surface MMC layer was metallurgically bonded to the Ti6Al4V substrate. The microhardness of the fabricated surface layer was found to be inversely proportional to the dilution level. The EDAX and XRD spectra results show that new intermetallic compounds and alloy phases were formed in the MMC layer. With the existence of Mo content in the pre-placed powder, the β-phase of Ti in the MMC coating can be retained at the quenching process. With increasing weight percentage content of WC particles in the Mo–WC pre-pasted powder, the microhardness and sliding wear resistance of the laser surface coating were increased by 87% and 150 times, respectively, as compared with the Ti6Al4V alloy. The surface friction of the laser-fabricated MMC coatings was also decreased as compared with the worn Ti6Al4V substrate.     

基于TiC_(0.5)N_(0.5)/Si_3N_4复相陶瓷基体的PVD及CVD涂层刀具性能

首先,以15vol%或25vol%的TiC0.5N0.5粉体为导电第二相,利用热压烧结法制备了TiC0.5N0.5/Si3N4复相陶瓷;然后,分别通过物理气相沉积(PVD)和化学气相沉积(CVD)技术在TiC0.5N0.5/Si3N4陶瓷刀具表面沉积了CrAlN和TiN/Al2O3/TiN涂层;最后,通过对TiC0.5N0.5/Si3N4刀具进行连续切削灰铸铁实验,研究了TiC0.5N0.5含量和涂层类型对刀具磨损特征的影响,并探讨了刀具的磨损机制。结果表明:TiC0.5N0.5含量的增加有利于提高TiC0.5N0.5/Si3N4复相陶瓷刀具基体的硬度和电导率,但对耐磨性和切削寿命的影响较小;采用PVD技术沉积CrAlN涂层时,随着TiC0.5N0.5含量的增加,涂层的厚度、结合强度和硬度都得到提高,涂层刀具的磨损性能显著提高,切削寿命也明显延长;而采用CVD技术沉积TiN/Al2O3/TiN涂层时,TiC0.5N0.5含量的变化对涂层的厚度、结合强度和硬度基本没有影响,TiN/Al2O3/TiN涂层刀具整体切削性能变化不大。CrAlN涂层和TiN/Al2O3/TiN涂层都可明显改善TiC0.5N0.5/Si3N4复相陶瓷刀具的耐磨性和切削寿命;相对于TiN/Al2O3/TiN涂层,CrAlN涂层具有更高的涂层硬度和粘着强度,但TiN/Al2O3/TiN涂层具有较大的涂层厚度,TiN/Al2O3/TiN涂层刀具表现出更加优异的耐磨性和切削寿命。TiC0.5N0.5/Si3N4复相陶瓷刀具的磨损机制以机械摩擦导致的磨粒磨损为主,伴随有少量的粘结磨损。     

Structure and hardness of surface of Al18B4O33w/Al composite by laser surface melting

As a new material, aluminum borate whisker reinforced aluminum composites have attracted interest and have been considered for a wide range application because of their high specific strength, high modulus and low cost. The study included a detailed characterization of the laser melting surface in terms of microstructures, phase analysis and a ratio of Al₂O₃ (the decomposition products of whiskers during laser process) and Al in intensity with tests parameters as an indication of Al₂O₃ distribution. Microhardness of the laser layer was also studied in detail. The results indicated that the most of γ-Al₂O₃ exists at the bottom of the laser pool, which led to a maximum value in the hardness be obtained. Microhardness of the laser layer was improved to 294 Hv as compared to 178 Hv of the as-cast composite, because of the existence of Al₂O₃ particles, solid solution hardness and the grain refinement of the laser layer following rapid quenching associated with the process.     

Development of wear resistant composite surface on mild steel by laser surface alloying with silicon and reactive melting

The present study concerns laser surface alloying with silicon of mild steel substrate using a high-power continuous wave CO₂ laser with an objective to improve wear resistance. The effect of surface remelting using nitrogen as shrouding environment (with and without graphite coating) on microhardness and wear resistance has also been evaluated. Laser surface alloying leads to formation of a defect free microstructure consisting of iron silicides in laser surface alloyed mild steel with silicon and a combination of silicides and nitrides when remelted in nitrogen. Carbon deposition prior to remelting leads to presence of a few martensite in the microstructure. A significant improvement in microhardness is achieved by laser surface alloying and remelting to a maximum of 800 VHN when silicon alloyed surface is melted using nitrogen shroud with carbon coating. A detailed wear study (against diamond) showed that a significant improvement in wear resistance is obtained with a maximum improvement when remelted in nitrogen atmosphere followed by carbon coating.     

Si3N4/TiC纳米复合陶瓷材料R曲线行为

在微米Si3N4基体中加入亚微米Si3N4及纳米TiC颗粒,热压烧结制备出力学性能良好的Si3N4/TiC纳米复合陶瓷材料。采用压痕-弯曲强度法测定了复合材料的裂纹扩展阻力曲线(R曲线)。结果表明:材料呈现出上升的阻力曲线特性,显示出增强的抗裂纹扩展能力。其中,加入质量分数为10%亚微米Si3N4颗粒和15%纳米TiC颗粒的复合材料显示出较为优越的抗裂纹扩展能力,其阻力曲线上升最陡,上升幅度最大。分析表明:弥散的TiC粒子同基体之间弹性模量和热膨胀失配以及Si3N4类晶须拔出与桥联补强协同增韧,有助于纳米复合材料抑制主裂纹失稳扩展,导致复合材料的阻力曲线行为。     

Effect of laser surface hardening on the hydrogen embrittlement of AISI 420: Martensitic stainless steel

The susceptibility of stress corrosion cracking (SCC) of AISI 420 which was surface transformed hardened by a pulsed Nd:YAG laser, was investigated in 5% sodium chloride + 0.5% acetic acid solution by the U-Bend method, in the range of pH value from 3.5 to 6, in the absence and presence of 1 ppm thiosulphate ion, at 25 and 60 °C. The results showed that the laser-treated areas are more susceptible to SCC than the base metal. Hydrogen embrittlement (HE) is the main cause of crack propagating, mostly effective on the grain boundaries and the interface between carbide particles and second phases; tempered martensite or ferrite.     

Prediction of mechanical properties and modeling on sliding wear behavior of LM25/TiC composite using response surface methodology

Aluminum LM25/TiC (10?wt%) metal matrix composite was developed using the liquid metallurgy route. The microstructural examination and the mechanical properties such as hardness and tensile strength were investigated on the composite specimens. The tribological behavior of the composite was studied using central composite design (CCD) based on response surface methodology (RSM) under the influence of wear process parameters such as applied load, sliding velocity and sliding distance. Pin-on-disc tribometer was utilized for conducting the experimental runs and the model was constructed based on the obtained wear rates. Confirmation experiments and analysis of variance were performed to ensure the adequacy of the constructed model. Microstructural examination reveals that uniform dispersion was attained in the composite, which enhances the hardness and the tensile strength. The wear results showed that the wear rate increased with increase in load, decreases with increase in velocity and varies nonlinearly with sliding distance. Scanning electron microscopic (SEM) analysis was performed to examine the worn surface morphologies and the worn surfaces revealed that TiC reinforcement protects the matrix from more material removal at all conditions. The developed composite can be utilized for the tribological applications like engine block, cylinder liners and pistons.     

Investigation of C/Si3N4 Nanocomposites

Nanocrystalline carbon black and graphite micrograins have been added to silicon nitride starting powder and sintering additives (Al₂O₃ and Y₂O₃). These mixtures were mechanochemically activated several hours in a planetary type alumina ball‐mill in order to achieve a homogenous mass. As an alternative to nano‐ and micrograins, carbon fibres have been added to carbon free silicon nitride batches. Sinter‐HIP has been applied to rectangular bars which were earlier dry pressed. Structural and morphological analysis were performed on as‐prepared samples. Bending strength and elastic modulus have been found to be influenced by amount of carbon black and graphite introduced in silicon nitride matrix.     

Optimization of cold-sprayed AA2024/Al2O3 metal matrix composites via friction stir processing: Effect of rotation speeds

In this study, friction stir processing (FSP) was employed to modify cold-sprayed (CSed) AA2024/Al₂O₃ metal matrix composites (MMCs). Three different rotation speeds with a constant traverse speed were used for FSP. Microstructural analysis of the FSPed specimens reveals significant Al₂O₃ particle refinement and improved particle distribution over the as-sprayed deposits. After FSP, a microstructural and mechanical gradient MMC through the thickness direction was obtained. Therefore, a hybrid technique combining these two solid-state processes, i.e. CS and FSP, was proposed to produce functionally gradient deposits. The Guinier-Preston-Bagaryatskii zone was dissolved during FSP, while the amounts at different rotation speeds were approximately the same, which is possibly due to the excellent thermal conductivity of the used Cu substrate. Mechanical property tests confirm that FSP can effectively improve the tensile performance and Vickers hardness of CSed AA2024/Al₂O₃ MMCs. The properties can be further enhanced with a larger rotation speed with a maximum increase of 25.9% in ultimate tensile strength and 27.4% in elongation at 1500 rpm. Friction tests show that FSP decreases the wear resistance of CSed MMCs deposits due to the breakup of Al₂O₃ particles. The average values and fluctuations of friction coefficients at different rotation speeds vary significantly.     

Characterizations of 21-4N to 4Cr9Si2 stainless steel dissimilar joint bonded by electric-resistance-heat-aided friction welding

A new welding process, electric-resistance-heat-aided friction welding (ERHAFW), was introduced in this study. To further improve the joint quality and energy-saving, electric resistance welding was combined with the conventional continuous-drive friction welding. 21-4N (austenitic stainless steel) and 4Cr9Si2 (martensitic stainless steel) valve steel rods of 4 mm diameter were used as base metals. The results show that electric-resistance-heat-aided friction welding can be applied to join thin rods within a relatively short time, which is very difficult for conventional friction welding (FW). The microstructure of ERHAFW bonded 21-4N to 4Cr9Si2 presents non-uniform across the joint. Different structure zones are observed from the weld line to both sides, which are the weld center, thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). These regions exhibit different structures owing to the difference in the thermophysical and mechanical properties of these two steels under the fast heating and cooling during welding. The variation of microhardness in the joint is attributed to the microstructure change. The higher microhardness is obtained in the weld center and TMAZ of 4Cr9Si2 corresponding to the presence of fine tempered martensite and carbides.     

The influence of lubrication on tribological properties of Si3iNi4/1045 steel sliding pairs

Compared with cermet cutting tools, ceramic cutting tools have much better thermal stability and higher hardness. Si₃N4-based composite ceramics have been used progressively more for machining cast iron at very high speed. However, they have been found to be unsuitable for machining steel. One of the factors influencing this difference in behaviour is thought to be serious chemical wear of the ceramic at the high temperatures reached at the cutting tools. Lubrication could reduce the friction and the high temperature of the cutting tool/workpiece contact zone. A simulation test was carried out on a pin-on-disc tribometer in order to investigate the effect of lubrication on the friction and wear of Si₃N₄/1045 steel sliding pairs; the effects of load and speed on friction and wear were also investigated. The results show that suitable lubrication could greatly reduce the friction coefficient and wear rate of Si₃N₄ ceramic. Scanning electron microscopy, Auger electron spectroscopy and X-ray photoelectron spectroscopy were used for examinations of the worn surfaces. A wear mechanism of the Si₃N₄ ceramic in sliding contact with 1045 steel is proposed. This revised version was published online in November 2006 with corrections to the Cover Date.     

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.     

反应烧结制备Si3N4/SiC复相陶瓷及其力学性能研究

以两种不同配比Y₂O₃/Al₂O₃ (A, 2:3; B, 3:1, 总量15 wt%)为烧结助剂, 通过添加不同质量分数的SiC粉体,反应烧结制备了高强度的氮化硅/碳化硅复相陶瓷。并对材料的相组成、相对密度、显微结构和力学性能进行了分析。结果表明: 在1700℃保温2 h情况下, 烧结助剂A 与B对应的样品中α-Si₃N₄相全部转化为β-Si₃N₄; 添加5wt% SiC, 烧结助剂A对应样品的相对密度达到最大值94.8%, 且抗弯强度为521.8 MPa, 相对于不添加SiC样品的抗弯强度(338.7 MPa)提高了约54.1%。SiC能有效改善氮化硅基陶瓷力学性能, 且Si₃N₄/SiC复相陶瓷断裂以沿晶断裂方式为主。     

化学表面处理对微型化的氮氧化硅驻极体层电荷储存性能的影响

研究了化学表面处理对微型化的氮氧化硅膜电荷稳定性的影响,考察了表面处理对脱阱面电荷横向扩散的抑止作用。借助对微型化前后的样品在高温和高湿环境中的表面电位衰减测量的对比,研究了不同试剂的表面处理对微型化样品的电荷储存及其动态特性的影响。     

Effect of ceramic conversion surface treatment on fatigue properties of Ti6Al4V alloy

Samples of Ti6Al4V alloy were ceramic conversion (CC) treated. The fatigue properties of untreated and CC treated samples were evaluated with a conventional Wöhler cantilever rotating-bending fatigue machine under both plain fatigue and fretting fatigue conditions. Experiment results showed that CC treatment had a detrimental effect on plain fatigue properties and reduced the plain fatigue limit of the investigated Ti6Al4V alloy by about 24%. However, the fatigue property loss could be fully recovered by an additional shot peening after CC treatment. More importantly, shot peening after CC treatment resulted in considerably longer fatigue life over untreated and CC treated materials in the finite-life region. In contrast to its detrimental effect on plain fatigue, CC treatment appeared to be beneficial to fretting fatigue property. It increased fretting fatigue strength (FFS at 10⁷ cycles) of the Ti6Al4V alloy by approximately 10%. Shot peening after CC treatment further enhanced the fretting fatigue properties, with an improvement in FFS (10⁷ cycles) of about 20% over the untreated material under the present treatment and testing conditions.     

不同裂解升温速率制备的Cf/Si3N4复合材料性能研究

以聚硅氮烷为先驱体，研究先驱体转化过程中不同裂解升温速率对制备3D-BCf/Si3N4复合材料性能的影响。结果表明：随着裂解升温速率的提高，陶瓷基复合材料的力学性能明显提高，以10℃/min裂解升温速率制得的陶瓷基复合材料的弯曲强度达604MPa。     

Microstructure and wear properties of laser melted γ-Ni/Mo2Ni3Si metal silicide “in situ” composite

Wear resistant γ/Mo₂Ni₃Si metal silicide “in situ” composite consisting of Mo₂Ni₃Si primary phase and lamellar γ/Mo₂Ni₃Si eutectics was fabricated by the laser melting process. The γ/Mo₂Ni₃Si composite has an excellent wear resistance under metallic dry sliding wear test conditions mating with hardened 1.0% C-1.5% Cr bearing steel. The excellent tribological properties are attributed to the high hardness of Mo₂Ni₃Si and the toughening effect of the ductile γ solid solution. Wear of the γ/Mo₂Ni₃Si composite is governed by the micro-cracking and spalling of the dendritic Mo₂Ni₃Si primary phase and the preferential wear of the interdendritic γ/Mo₂Ni₃Si eutectics.