激光刻蚀法制备GH4169超疏水表面及性能研究

采用光纤激光打标机在GH4169基底上刻蚀出微观结构,并通过在1.0%氟硅烷(FAS)乙醇溶液中改性降低表面能制备GH4169超疏水表面。该超疏水表面能达到接触角大于160°且滚动角小于10°。通过扫描电子显微镜(SEM)、能谱仪(EDS)、接触角测量仪对样品的微观结构、元素组成及润湿性进行表征和分析。采用单因素实验探究光纤激光打标机的扫描频率、功率以及速度对GH4169表面润湿性的影响规律,并对制备出的超疏水表面的粘附性和自清洁性进行了测试。     

Fabrication of concave micro lens array using laser patterning and isotropic etching

Micro lens arrays are widely used in optical communication and laser-fiber coupling applications. In this paper, a technique to fabricate concave micro lens arrays on glass substrate using a third harmonic Nd:YAG laser direct patterning and followed by chemical wet isotropic etching is presented. The patterning process was done on gold film, which was coated on a glass substrate by using a NC controlled laser ablation tool paths. The glass substrate is then etched by using hydrofluoric acid (HF) solutions whereby the exposed area will be dissolved away by chemical reaction with HF. The type of etching process is an isotropic etching which the etching rate is equal at all direction thus produce hemispherical concave profile on glass. The optimum laser patterning parameters is obtained and the effect of different types of HF solutions on etching efficiency is studied. The surface morphology, 2D and 3D profiles are also measured. Various micro lens diameters are fabricated with different values of lens sag.     

Hot cracking in pulsed laser processing of a nickel based superalloy built up by electrospark deposition

A cast precipitation hardened nickel based superalloy was built up by electrospark deposition and then subjected to pulsed laser welding. The resistance of electrospark deposition layer to liquation and solidification cracking in pulsed laser welding was significantly higher than that of the cast alloy. It is shown that the extreme cooling rates involved in the solidification of electrospark deposition droplets result in the prevention of formation of grain boundaries terminal solidification constituents, which are the sources of liquation cracking in the cast alloy.     

Surfactant-controlled damage evolution during chemical mechanical planarization of nanoporous films

The integration of nanoporous organosilicate thin films involving chemical mechanical planarization (CMP) is a significant challenge due the evolution of defects in the films during CMP in the form of cracking and delamination. This study shows that small changes in CMP electrolyte chemistry and surfactant additions can have dramatic effects on crack growth rates in the films. Crack growth rates were sensitive to the type of electrolyte and decreased in the presence of electrolytes that caused crack tip blunting. Growth rates were also sensitive to nonionic surfactant additions where molecular structure and weight were demonstrated to be important variables. An optimized blend of surfactants and electrolytes can significantly retard defect evolution due to molecular bridging. Surfactant self-assembly and resulting molecular bridging were characterized by in situ atomic force microscopy and used to quantify the molecular bridging observed.     

Internal residual stress measurement on inertia friction welding of nickel-based superalloy

The internal residual stress in the narrow inertia friction welding (IFW) welds of FGH96 nickel-based superalloy was measured with the contour method (CM). The as-welded internal hoop and axial residual stresses were obtained after two cuts and the detailed steps of the CM measurement were described. In addition, the hole-drilling method was used to obtain the surface stress. Furthermore, the internal hoop residual stress of a FGH96 superalloy IFW specimen after post-weld heat treatment (PWHT) was also measured with the CM after a single cut. The measured results show that the peak hoop residual stress is not symmetrical about the weld centreline. Axial residual stress on the outer surface at the weld centreline is compressive stress, while tensile stress appears on the inner surface, and it varies linearly along the thickness. The peak values of hoop tensile and compressive stresses decrease dramatically after PWHT.     

Deep UV laser etching of GaN epilayers grown on sapphire substrate

The laser etching of GaN epilayers on sapphire substrate was carried out using a deep ultraviolet pulsed laser (157 nm wavelength, 20 ns pulse width). The quality and morphology of the etched GaN surface were evaluated by scanning electron microscopy, atomic force microscopy and scanning profilometer. Quadrate micro-hole and micro-trenches etched by the 157 nm laser exhibited clean and smooth edges, sharp side walls and very small heat affected zone (HAZ). In order to achieve controllable high-quality etching, the laser and processing parameters, such as laser repetition rate, scan speed, were systematically investigated and optimized. The mechanism analysis shows that, direct photoionization or photo-chemical reaction play predominant role within 157 nm laser etching of GaN epilayers.     

Thermomechanical analysis of multi-bead pulsed laser powder deposition of a nickel-based superalloy

In laser powder deposition (LPD) repair of nickel-based turbine blades, hot cracking is the most common defect. The cracking has been found to be associated with thermal stress concentration and low-melting constituents on the grain boundaries. For directionally solidified and single-crystal blades, a positive correlation is established between cracking and the “stray grain” formation. Control of the deposit molten pool shape has been proposed as an effective method to limit the stray grains. However, in multi-bead, multi-layer LPD with a pulsed laser, due to a much more complex bead geometry, appearance of stray grains seems to be random. To obtain insight into cracking and stray grain formation during multi-bead LPD process on nickel-based superalloys, a 3-D transient finite element (FE) model has been developed and a thermomechanical analysis is performed. Time-dependent temperature and thermal strain fields have been predicted. The developed thermal model has been applied to predict and analyze the distribution of thermal stress concentration and the tendency of stray grain formation. The reliability and accuracy of the model are verified experimentally by the measured temperature field profile and the observation of microstructure.     

Phase-field modelling of as-cast microstructure evolution in nickel-based superalloys

A modelling approach is presented for the prediction of microstructure evolution during directional solidification of nickel-based superalloys. A phase-field model is coupled to CALPHAD thermodynamic and kinetic (diffusion) databases, so that a multicomponent alloy representative of those used in industrial practice can be handled. Dendritic growth and the formation of interdendritic phases in an isothermal (2-D) cross-section are simulated for a range of solidification parameters. The sensitivity of the model to changes in the solidification input parameters is investigated. It is demonstrated that the predicted patterns of microsegregation obtained from the simulations compare well to the experimental ones; moreover, an experimentally observed change in the solidification sequence is correctly predicted. The extension of the model to 3-D simulations is demonstrated. Simulations of the homogenization of the as-cast structure during heat treatment are presented.     

Laser-assisted friction stir processing of IN738LC nickel-based superalloy: stir zone characteristics

Friction stir processing (FSP) of high softening-temperature materials such as nickel-based superalloys is considered to be difficult. Laser heating of a localised area ahead of the FSP tool was used to provide sufficient plasticity during the FSP of IN738LC nickel-based superalloy. The stir zone (SZ) microstructure of the friction stir processed and laser-assisted friction stir processed were characterised. Laser-assisted friction stir processing (LAFSP) produced a defect-free pass, but FSP resulted in generation of a discontinuity in the SZ. Both lower volume fraction of partially dissolved γ′ precipitates and coarser grain structure of SZ in LAFSP led to more ductility of the SZ material and elimination of the defects.     

Lattice misfit during ageing of a polycrystalline nickel-base superalloy

The temporal evolution of the lattice parameters and lattice misfit of an advanced polycrystalline nickel-base superalloy have been studied in situ during an ageing heat treatment using synchrotron X-ray diffraction. During ageing, the γ and γ′ lattice parameters were both observed to decrease, a trend that cannot be attributed to a loss of coherency alone. Phase-extracted γ′ replicated this behaviour. Atom probe tomography was used to measure the compositional changes between the start and end of the ageing heat treatment. Using these data, a thermodynamic assessment was made using the software ThermoCalc of the structural change across the interface between γ and γ′. Subsequently, the unconstrained lattice parameters were estimated and were shown to be in good agreement with the X-ray diffraction measurements. Thus, the observed anomalous lattice misfit behaviour was concluded to be dominated by elemental exchange between the γ and γ′ phases during ageing.     

铝合金表面电解刻蚀图纹化过程与机制

通过电解刻蚀在铝合金表面形成图纹, 然后再阳极氧化、电解着色, 可使铝合金具有多彩的装饰性。电解刻蚀液由形成阻挡层的物质和具有溶解（ 刻蚀） 作用的物质组成。电解刻蚀图纹的形成与电极表面析出的氢气有直接关系, 氢气泡尺寸越大, 在电极表面滞留的时间越长, 则图纹的刻痕越深、越宽。图纹处氧化膜的孔径、孔密度和厚度较大, 电解着色时可沉积较大量的金属, 呈现出较深的颜色, 从而使铝合金表面具有对比鲜明的彩色图案外观。     

Sequential laser and mechanical micro-drilling of Ni superalloy for aerospace application

Laser percussion drilling is inherently associated with poor geometry and thermal defects. While mechanical micro-drilling produces good quality holes, premature drill breakage often occurs and it is difficult to drill holes at acute angles. This paper presents the feasibility and basic characteristics of a new approach for micro-drilling In718 alloy sheets at an acute angle, using sequential laser and mechanical drilling. The results demonstrate that sequential laser-mechanical micro-drilling alleviates the defects associated with laser-drilled holes, reduces burr size and machining time and increases the tool life compared with mechanical drilling.     

Effects of heat treatment on mechanical properties of ODS nickel-based superalloy sheets prepared by EB-PVD

A Y2O3 dispersion strengthened nickel-based superalloy sheet(0.15 mm thick) was prepared by electron beam physical vapor deposition(EB-PVD) technology.Different heat treatments were used to improve the mechanical properties of the alloy sheet.Differential thermal analysis(DTA) was used to examine the thermal stability of the as-deposited sheet.Element contents,phase composition and microstructure investigations on as-deposited and heat treated specimens were performed by X-ray fluorescence spectrometer(XRF),X-ray diffraction(XRD) and scanning electron microscopy(SEM).Tensile tests were conducted at room temperature on specimens as-deposited and heat treated.The results show that the as-deposited sheet is composed of equiaxed grains on the substrate side and columnar grains on the evaporation side.The as-deposited sheet shows poor ductility due to micropores between columnar grains.The strength and ductility can be improved effectively by annealing at 800°C for 3 h.For samples treated at 1100°C,the strength drops down due to the precipitates of Y3Al5O12(YAG).     

超声冲击细化FGH95镍基高温合金激光熔覆层组织

采用超声冲击工艺对FGH95镍基高温合金激光熔覆层进行处理. 利用金相显微镜分析超声冲击前后激光熔覆层微观组织的变化特征,采用图像分析软件Image-Pro Plus测量晶粒尺寸,并对超声冲击处理前后熔覆层截面的显微维氏硬度进行了对比分析. 结果表明, 超声冲击处理后熔覆层发生塑性变形,并沿深度方向晶粒具有不同程度的细化. 强塑性变形区内位错塞积现象显著,熔覆层组织形成细化的小晶块;次塑性变形区深度范围为0.1~0.25 mm,其微观组织中存在大量细化的树状枝晶;微塑性变形区内等轴晶细化至5.4 μm,细化深度达0.7 mm. 超声冲击处理后熔覆层显微维氏硬度显著增大,表面显微维氏硬度值最大为594.25 HV,相比未经冲击熔覆层提高1.3倍.     

激光选区熔化GH3536镍基高温合金的微观组织和晶体取向

采用激光选区熔化方法制备了高致密度GH3536镍基高温合金块状试件,分析了激光选区熔化成形GH3536合金显微组织和晶体取向。结果表明：随着激光能量密度的升高,成形试样的致密度先升高后降低,当激光能量密度为180~230J.m_^-1时,致密度达到99.55%以上;其组织存在着明显的各向异性,垂直于构建方向的微观组织呈“棋盘状”形貌,晶粒大多数为等轴晶(长径比为1.828μm)且得到了细化(d_mean=11.226μm),尤其熔池搭接区域晶粒更加细小(5μm以下),而平行于构建方向为“鱼鳞状”形貌,大多数为柱状晶(长径比为2.831μm),晶粒直径较大(d_mean=25.964μm)。同时SLM成形GH3536镍基高温合金存在明显的择优取向,横截面上晶粒具有较强的取向< 100 >取向,垂直于构建方向和平行于构建方向均为立方织构{100}<001>。此外SLM凝固成形中晶粒生长对晶粒内晶体取向演变有着显著影响,纵截面变形晶粒内的晶体取向变化不明显,纵截面变形晶粒内的晶体取向变化比较明显,这是由于SLM成形具有极高的温度梯度和快速的冷却速率(10_⁵K/s)导致的。     

Micro-machinability of monocrystal silicon by direct etching using excimer laser

The high flexibility of laser direct writing and its potential capability of fabricating micro-mechanical structure are discussed. Aiming at providing knowledge for micro-engineering application, experiments on direct etching of monocrystal silicon with focusing KrF excimer laser beam (λ = 248 nm) were carried out. The validity of non-thermal excimer laser material processing was examined by SEM observation on the “heat-affected zone (HAZ)”. An empirical formula establishing the relationship of etching depth versus laser energy per pulse and the number of laser pulses has been derived from the experiments. Impact damage at the underside of silicon wafer has been observed. Its relevant causes are analyzed.     

Direct laser fabrication of nickel alloy samples

Direct laser fabrication (DLF) is an advanced manufacturing technology which can build full density metal components directly from CAD files without using any modules or tools. An open-loop controlled hardware system and associated control software for the DLF process of nickel alloy samples was constructed in our work. The hardware system is consisted of a CO₂ laser, a 4-axis CNC table, a coaxial powder nozzle and a powder recycler. In order to achieve the maximum flexibility and extensibility for the fabrication of metal parts, path plug-ins was introduced into the control software. The effect of the specific energy on the cross-section shape of nickel alloy cladding was studied by a single-track cladding experiment with different laser processing parameters. The comprehensive effect of the optimized laser processing parameters was also studied by an orthogonal experiment. The experimental results showed that the specific energy for laser processing is the most important factor, which controls the component qualities. There is an appropriate range for the specific energy in which the nickel alloy samples can be fabricated layer by layer with a uniform height. If the specific energy is too low, the inner height of a sample is lower than its contour height. Banding structure can be observed in the microstructure of nickel alloy samples. The gain size in the light zones of the bandings is much smaller than those in the dark zones.     

Approach for bidirectional laser bending of sheet metal with one-sided accessibility

Laser bending by means of the temperature gradient mechanism (TGM) has so far been limited to bending the free end of a sheet metal strip toward the irradiating laser beam. This investigation aims to present a new approach which, despite one-sided accessibility, allows targeted incremental laser bending in both directions by the TGM and the upsetting deformation mechanism (UM). The method is based on blind holes along the bending line, whereby the bend direction can be determined by irradiating either the blind hole bottoms or the bridges between the holes. The possibility of bidirectional and alternating laser bending is demonstrated.