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.     

Laser controlled melting of pre-prepared inconel 718 alloy surface

Laser treatment of Inconel 718 alloy surface is carried out. The alloy surface is coated with a carbon layer containing 7% TiC particles prior to the laser treatment. The carbon coating provides increased absorption of the incident laser beam and holds TiC particles. The microstrutural and morphological changes in the laser treated region are examined using optical and scanning electron microscopes, energy dispersive spectroscopy, and X-ray diffraction. The microhardness of the surface is measured and the residual stress formed at the surface vicinity is determined from the XRD technique. It is found that partial dissolution of carbide particles takes place at the surface. The composition of fine grains at the surface vicinity, nitride compounds formed, and dissolution of Laves phase at the surface region enhances the hardness at the treated surface. In addition, laser treated surface is free from the micro-crack network and cavities.     

Laser texturing of Hastelloy C276 alloy surface for improved hydrophobicity and friction coefficient

Laser treatment of Hastelloy C276 alloy is carried out under the high pressure nitrogen assisting gas environment. Morphological and metallurgical changes in the laser treated layer are examined using the analytical tools including, scanning electron and atomic force microscopes, X-ray diffraction, energy dispersive spectroscopy, and Fourier transform infrared spectroscopy. Microhardness is measured and the residual stress formed in the laser treated surface is determined from the X-ray data. The hydrophibicity of the laser treated surface is assessed using the sessile drop method. Friction coefficient of the laser treated layer is obtained incorporating the micro-tribometer. It is found that closely spaced laser canning tracks create a self-annealing effect in the laser treated layer and lowers the thermal stress levels through modifying the cooling rates at the surface. A dense structure, consisting of fine size grains, enhances the microhardness of the surface. The residual stress formed at the surface is compressive and it is in the order of −800 MPa. Laser treatment improves the surface hydrophobicity significantly because of the formation of surface texture composing of micro/nano-pillars.     

Laser bending of AISI 304 steel sheets: Thermal stress analysis

Laser induced bending of steel sheet is carried out and thermal stress developed in the heated region is examined. Temperature and stress fields are predicted using the finite element model. The microstructural changes in the melted region are investigated through scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. The residual stress developed at the surface vicinity of the laser treated region is measured using the X-ray diffraction technique, which is then compared with its counterpart predicted from the simulations. It is found that the residual stress at the surface vicinity is compressive and the prediction of the residual stress agrees well with that obtained from the X-ray diffraction technique. In addition, surface temperature predictions are in good agreement with the thermocouple data. The laser treated region is free from major cracks and large cavities.     

Residual stresses and white layer in electric discharge machining (EDM)

The effect of dielectric liquid and electrode type on white layer structure in electric discharge machined surfaces has been studied in terms of retained austenite and residual stresses using X-ray diffraction method. The machining tests were conducted by using two different tool electrodes (copper and graphite) and dielectric liquid (kerosene and de-ionized water) under same operational conditions. The present work suggests that the surface is saturated with carbon irrespective of the tool electrode material when machining with kerosene dielectric liquid. But, retained austenite is formed on the surface due to carbon uptake from graphite tool electrode when machining with de-ionized water dielectric liquid. On the other hand, even though surface residual stresses increase with structural non-homogeneities in the white layer, no clear consequences have been observed in residual stress distribution beneath the white layer.     

激光冲击诱导弹簧钢丝残余应力状态分析

利用高功率Nd:YAG激光对不同工艺处理的SWOSC-V弹簧钢丝进行单点冲击处理,用X射线应力分析仪测量弹簧内外侧、侧表面的残余应力并计算出残余主应力,建立了激光冲击SWOSC-V弹簧钢丝表面残余应力的产生模型,并利用该模型分析了弹簧钢丝表面残余应力产生的原因。结果表明:弹簧钢丝在经激光冲击处理的表面强化区产生残余压应力,钢丝退火后直接激光冲击处理与经喷丸强化的钢丝激光冲击处理的表面残余应力变化不同,喷丸强化所引起的材料硬化是激光冲击处理弹簧钢丝残余应力变化不同的原因。     

Roles of interfaces between carbon fibers and epoxy matrix on interlaminar fracture toughness of composites

The effect of atmospheric-pressure plasma treatment on high strength PAN-based carbon fibers had been studied in terms of fiber surface energetics and mode I and II interlaminar fracture toughness of unidirectional carbon fibers/epoxy matrix composites. The surface characterization of plasma treated carbon fibers was investigated by X-ray photoelectron spectroscopy (XPS) and contact angles. As a result, the plasma treatment changed the surface properties of the carbon fibers, mainly through formation of oxygen functional groups like hydroxyl, carbonyl, and carboxyl groups. According to contact angle measurements, it was observed that plasma treatment led to an increase in surface free energy of the fibers, mainly due to the increase of its specific component. Fracture toughness test results employing double-cantilever beam (DCB) and end notched flexure (ENF) specimens also showed that the increase in specific components or hydrogen bonding between the –OH groups on carbon fibers and the =O ring in epoxy matrix resins played an important role in improving the degree of adhesion at interfaces, resulting in an increase in the interfacial fracture toughness of the composites studied.     

激光喷丸改善TC6钛合金组织和力学性能

为了使激光冲击强化技术能较好地应用于TC6钛合金的发动机叶片,对TC6钛合金进行试验研究。通过X射线衍射仪、透射电子显微镜等测试技术分析了不同参数下TC6钛合金的微观组织变化,用显微硬度计和残余应力测试仪分别表征表层硬度和残余应力变化,并测试材料冲击后的振动高周疲劳性能。试验结果表明:激光冲击材料后表面组织得到明显细化,随着冲击次数的增加,先后出现了高密度位错、位错胞、亚晶和纳米晶。性能方面,表面硬度在冲击一次即可提高19%,硬度影响深度达到700 m;与此同时表面残余应力最高达到-608.5 MPa,在500 m深度上仍具有-100 MPa左右的应力存在。经三次冲击后,标准疲劳试片的疲劳极限提高近20%。     

基于XRD的镀锌钝化膜残余应力试验研究

利用XRD技术测试了镀锌钝化膜结合界面的残余应力，同时通过电解抛光法检测了其厚度方向残余应力的分布规律，分析了残余应力对镀锌钝化膜结合强度的影响. 试验结果表明，镀锌钝化膜的残余应力均表现为压应力，并随着基体表面残余应力的增大而减小；钝化膜在2—10μm厚度方向的残应力为-274.5—-428.3MPa，其应力为梯度分布；镀锌钝化膜与基体的界面结合强度与其残余应力成反比，减小薄膜残余应力，有利于提高镀锌钝化膜与基体的结合强度. 关键词： X射线衍射法(XRD) 镀锌钝化膜 结合强度 残余应力     

Laser control melting of alumina surfaces and thermal stress analysis

Laser gas assisted melting of alumina surface is carried out and temperature as well as stress fields developed in the irradiated region are predicted using the finite element method (FEM). An experiment is conducted resembling the simulation conditions. Optical and scanning electron microscope (SEM) are used to examine the morphological and the metallurgical changes in the laser treated region. The X-ray diffraction (XRD) technique is used to determine the residual stress developed in the irradiated region. It is found that the residual stress predicted agreed with the measurement result. High heating and cooling rates result in high von Mises stress levels in the surface region.     

多晶铁电薄膜的相变——应力和晶粒尺寸效应

用掠入射X射线衍射法观察到钛酸铅多晶铁电薄膜表面层与体内的相变温度不同,表面层的结构参数也有异于体内;唯象地把多晶铁电薄膜抽象为一个表面层为细晶粒、低应变层,体内为粗晶粒、高应变层的两层结构,根据应力和晶粒尺寸效应对铁电相变的影响,解释了钛酸铅多晶铁电薄膜的相变特征 关键词：     

The structural and optical properties of ZnO/Si thin films by RTA treatments

ZnO/Si thin films were prepared by rf magnetron sputtering method and some of the samples were treated by rapid thermal annealing (RTA) process at different temperatures ranging from 400 to 800 °C. The effects of RTA treatment on the structural properties were studied by using X-ray diffraction and atomic force microscopy while optical properties were studied by the photoluminescence measurements. It is observed that the ZnO film annealed at 600 °C reveals the strongest UV emission intensity and narrowest full width at half maximum among the temperature ranges studied. The enhanced UV emission from the film annealed at 600 °C is attributed to the improved crystalline quality of ZnO film due to the effective relaxation of residual compressive stress and achieving maximum grain size.     

In situ atomic force microscopy observation of hydrogen absorption/desorption by Palladium thin film

Grain structure changes in Pd thin film during hydrogen absorption and desorption were observed by in situ atomic force microscopy. The as-sputtered film had a smooth flat surface with 20-30 nm grains. Film that absorbed hydrogen showed buckling, caused by the compressive stress due to lattice expansion as Pd metal reacted with hydrogen to form the hydride. Grains on the buckles were agglomerated and deformed unlike those on flat areas beside the buckles. Film that absorbed and then desorbed hydrogen still showed some buckling; however, many buckles shrank and flattened when the compressive stress of lattice expansion was released during desorption. On both the remaining and the shrunken buckles, grain agglomeration was retained; whereas, the deformed grains reverted back to their original form. X-ray diffraction indicated compressive residual stress in the as-sputtered film and tensile residual stress in the film after hydrogen absorption/desorption. These results indicate that irreversible grain agglomeration is related to residual tensile stress in the film although agglomeration occurs only on the buckled areas.     

激光冲击对E690高强钢激光熔覆修复微观组织的影响

为研究激光冲击对E690高强钢激光熔覆修复层微观组织的影响,选用专用金属粉末对E690高强钢试样预制凹坑进行激光熔覆修复,并使用脉冲激光对激光熔覆层进行冲击强化处理,同时采用扫描电镜、透射电镜和X射线应力分析仪分别对激光冲击前后激光熔覆层的微观组织和表面残余应力进行检测。结果表明:激光熔覆修复后,激光熔覆层组织为等轴晶,熔覆层与E690高强钢基体之间冶金结合良好,其表面残余应力为均匀分布的压应力。经激光冲击后,激光熔覆层截面晶粒得到细化,并观察到大量的形变孪晶,互相平行的孪晶界分割熔覆层粗大晶粒,在激光熔覆层的晶粒细化过程中发挥着重要作用;试样表层位错在{110}滑移面上发生交滑移,在晶界周围形成了位错缠结。经激光冲击后,激光熔覆层冲击区域表面残余压应力数值相较于冲击前提升了1.1倍。     

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.     

Small-scale fracture toughness of ceramic thin films: the effects of specimen geometry,ion beam notching and high temperature on chromium nitride toughness evaluation

For the implementation of thin ceramic hard coatings into intensive application environments, the fracture toughness is a particularly important material design parameter. Characterisation of the fracture toughness of small-scale specimens has been a topic of great debate, due to size effects, plasticity, residual stress effects and the influence of ion penetration from the sample fabrication process. In this work, several different small-scale fracture toughness geometries (single-beam cantilever, double-beam cantilever and micro-pillar splitting) were compared, fabricated from a thin physical vapour-deposited ceramic film using a focused ion beam source, and then the effect of the gallium-milled notch on mode I toughness quantification investigated. It was found that notching using a focused gallium source influences small-scale toughness measurements and can lead to an overestimation of the fracture toughness values for chromium nitride (CrN) thin films. The effects of gallium ion irradiation were further studied by performing the first small-scale high-temperature toughness measurements within the scanning electron microscope, with the consequence that annealing at high temperatures allows for diffusion of the gallium to grain boundaries promoting embrittlement in small-scale CrN samples. This work highlights the sensitivity of some materials to gallium ion penetration effects, and the profound effect that it can have on fracture toughness evaluation.     

Femtosecond-laser-induced-crystallization and simultaneous formation of light trapping microstructures in thin a-Si:H films

We report the observation of crystallization and simultaneous formation of surface microstructures in hydrogenated amorphous silicon (a-Si:H) thin films as one step laser processing. Light trapping microstructures of around 300 nm in height were formed on a-Si:H films of thickness in the range of 1.5 μm to 2 μm deposited on soda lime glass after exposure to femtosecond laser pulses. Scanning electron microscope (SEM) images show the formation of spikes that are around 1 μm part and their heights could be controlled by the laser fluences. Atomic force microscope (AFM) images were taken to study the roughness created on the surface. The mean roughness of the textured surface increases with laser fluence at smaller power densities, and for power densities beyond 0.5 J/cm² the film removal deteriorates the texturing. X-ray diffraction results indicate the formation of a nano-crystalline structure with (111) and (311) crystal orientation after the laser treatment. The observed black color and enhanced optical absorption in the near infrared region in laser treated films may be due to a combined effect of light trapping in the micro-structured silicon surface because of multiple total internal reflections, phase change in the film, possible defect sites induced after laser treatment and formation of SiO_x. Demonstration of light trapping microstructures in thin a-Si:H films and simultaneous crystallization could provide new opportunities for optoelectronic devices. PACS 42.55.Px; 42.62.Cf; 81.05.Ge     

Changes in the structural phase state of a copper substrate’s surface layer bombarded with titanium ions

The structural-phase state of a copper substrate’s surface layer treated with titanium ions is investigated. It is established via X-ray diffraction analysis that intermetalides of the Cu-Ti type formed in the surface layer following treatment. It is found by means of scanning electron microscopy that a netlike porous structure with characteristic crosswise horizontal and vertical element dimensions of ～1–2 μm forms in the surface layer.     

The effect of nanostructured surface layer on the fatigue behaviors of a carbon steel

A nanostructured surface layer was formed on a carbon steel by means of surface mechanical attrition treatment (SMAT). The microstructure of the surface layer of the SMATed sample was characterized by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness and residual stress distribution along the depth from the SMATed surface layer were measured at the same time. Fatigue behaviors of the carbon steel subjected to the SMAT process were investigated. A nanostructured layer with average grains size of ∼12.7 nm was formed, of which microhardness is more than twice as high as that in matrix and residual compressive stress can reach about −400 MPa with maximum depth of ∼600 μm. The fatigue strength of as-received sample is 267 MPa and that of SMATed sample is 302 MPa based on fatigue life 5 × 10⁶ cycles. The SMAT process has improved the fatigue strength by as much as 13.1% for the carbon steel. It is shown that the SMAT is an effective method to render the material with the features, such as a nanostructured and work-hardened surface layer as well as compressive residual stresses, which can pronouncedly improve the fatigue strength of the carbon steel.     

Laser-induced thermal stresses on steel surface

In laser heat treatment of steels, a thin surface layer of austenite forms during heating and subsequent phase change process in the cooling period. However, thermal stress develops due to high-temperature gradient attainment in the surface vicinity which in turn results in microcrack development at the surface. The present study is carried out to compute the temperature profiles due to step input pulse laser radiation and determine the resulting thermal stresses. The study is extended to include three-step input pulses having the same energy content. This provides the comparison for the influence of the pulse length on the resulting thermal stresses. To validate the theoretical predictions, an experiment is conducted to irradiate the AISI 4142 steel surface by an Nd–YAG laser. Microphotography and EDS analysis of the heated regions are carried out. It is found that considerable thermal stress is eveloped at the workpiece surface due to attainment of high-temperature gradient in this region. In addition, microcracks are observed at the surface of the irradiated spot.