超薄切割片的加工变形研究现状

超薄切割片在工作中极易出现径向加工变形。从应力和变形的理论分析、有限元模拟分析和试验研究等方面,对超薄切割片的加工变形研究现状进行总结。此外,分析研究中存在的问题,介绍具有相似结构的砂轮和圆锯片的相关研究成果。结果发现:切割片转速对超薄切割片变形影响的研究比较系统,但磨削深度和进给速度对其影响的相关研究还有一定的差距。同时,切入工件时测量方法的缺失也限制了研究的深入。因此,需要不断完善理论公式并充分应用有限元模拟,持续推进相关研究,优化和补偿超薄切割片的变形,提高工件的加工精度。      

Laser cladding in repair of IN738 turbine blades

Abstract

Laser cladding with additive powder can be used for repairing high cost components such as industrial turbine blades. Repair of damaged blades is important for electrical power companies to reduce operating costs. The repair of IN738 gas turbine blades using laser cladding is reviewed. A laser cladding system for blade repair and the associated heat treatment procedures before and after repair are discussed. In general, laser cladding with IN625 powder does not require preheating and can be used for repairing IN738 blades in low stress areas. For laser cladding with a higher strength material, e.g. IN738 powder, preheating to high temperatures is necessary to avoid cracking. In a process simulation, test coupons were employed to compare the microstructures and mechanical properties of the IN738 clad region and base metal. Blade repairs with IN738 additive powder were found to be stronger but much more difficult to process than those with IN625 powder. Therefore, extreme caution is required when repairing IN738 blades in high stress areas using a matching filler metal.     

杯形砂轮修整碟形金刚石砂轮磨削力研究

本文通过对杯形砂轮修整碟形金刚石砂轮的试验，对以研磨为主要因素的修整磨削力进行了研究，从磨削模型、砂轮参数、工艺参数等方面研究了修整时的磨削力规律，并用角正回归法推导了磨削力试验公式。实验结果表明：砂轮变速磨削，径向磨削力降低，切深量对磨削力的影响最大，而低速磨削时磨削力最大。磨削力信号是一种平稳的周期振动信号。角正回归法是一种高精度的回归法。变速磨削时修整效率最佳。     

Mechanisms in the generation of grinding wheel topography by dressing

For the process of dressing vitrified bonded grinding wheels with diamond tools it has been unknown how the wheel topography is generated. Moreover, the influence of the kinematical dressing parameters on the wheel wear behavior has not been quantified. In the course of this article the grinding wheel was dealt with as a porous ceramic composite. In FEM simulations common dressing forces and usual dressing tool geometries were applied. The results were verified by dressing tests and grinding wheel scratch tests which show the wheel wear mechanisms. The common practice of decreasing the grinding wheel surface roughness by a finishing dressing stroke has to be reconsidered, because previous dressing strokes with higher depths of cut can weaken the grinding wheel structure and lead to an unsteady phase with high grinding wheel wear after dressing.     

Grinding of microstructured functional surfaces: a novel strategy for dressing of microprofiles

For the grinding of microstructured functional surfaces called riblets, a novel dressing strategy using diamond profile rollers was introduced for the generation of microprofiles on vitrified grinding wheels. The purpose of the present study is to grind special designed riblets (microgrooves with a width of 40 μm and depth of 20 μm) on the surfaces of compressor blades with the purpose of drag reduction in turbulent flow. It is currently not possible to achieve the desired microprofiles on grinding wheels by profile dressing directly, due to the limited producible minimal profile tip geometry on the dressing roller. The new profile dressing strategy using a profile shift kinematic allows generating microscopic roof profiles with an ideal sharp profile tip. Compared with dressing using form rollers, the new dressing method using profile rollers offers a much higher dressing efficiency and process stability at dressing microprofiles.     

Application potential of coarse-grained diamond grinding wheels for precision grinding of optical materials

Fine-grained resin bonded diamond tools are often used for ultra-precision machining of brittle materials to achieve optical surfaces. A well-known drawback is the high tool wear. Therefore, grinding processes need to be developed exhibiting less wear and higher profitability. Consequently, the presented work focuses on conditioning a mono-layered, coarse-grained diamond grinding wheel with a spherical profile and an average grain size of 301 µm by combining a thermo-chemical and a mechanical-abrasive dressing technique. This processing leads to a run-out error of the grinding wheel in a low-micrometer range. Additionally, the thermo-chemical dressing leads to flattened grains, which supports the generation of hydrostatic pressure in the cutting zone and enables ductile-mode grinding of hard and brittle materials. After dressing, the application characteristics of coarse-grained diamond grinding wheels were examined by grinding optical glasses, fused silica and glass–ceramics in two different kinematics, plunge-cut surface grinding and cross grinding. For plunge-cut surface grinding, a critical depth of cut and surface roughness were determined and for cross-grinding experiments the subsurface damage was analyzed additionally. Finally, the identified parameters for ductile-machining with coarse-grained diamond grinding wheels were used for grinding a surface of 2000 mm² in glass–ceramics.     

A multi-objective genetic algorithm (GA) approach for optimization of surface grinding operations

A genetic algorithm (GA) based optimization procedure has been developed to optimize grinding conditions, viz. wheel speed, workpiece speed, depth of dressing and lead of dressing, using multi-objective function model with a weighted approach for surface grinding process. The procedure evaluates the production cost and production rate for the optimum grinding condition, subjected to constraints such as thermal damage, wheel wear parameters, machine tool stiffness and surface finish. New GA procedure is illustrated with an example and optimum results such as production cost, surface finish, metal removal rate are compared with quadratic programming techniques.     

Metallographic and OIM study of weld cracking in GTA weld build-up of polycrystalline,directionally solidified and single crystal Ni based superalloys

Abstract

The repair of gas turbine components is of importance both commercially and scientifically to ensure cost effective repair schemes that will extend the lives of hot end components such as blades and stators. The present communication reports the results of a metallographic and orientation imaging microscopy study of weld cracking observed in the gas tungsten arc repair welds of a polycrystalline (IN738LC), a directionally solidified (Rene 80) and a proprietary single crystal (SX) alloy. The three alloys were welded with low, intermediate and high strength weld fillers, using a weld build-up approach rather than a conventional weld repair of a through thickness crack. This procedure would be applicable for example to worn area on the tips of turbine blades. Inhomogeneous initial microstructures and those from solidification processes led to extensive heat affected zone microfissuring in the IN738LC alloy, associated with MC carbide liquation, liquation of gamma prime (γ′), segregation of boron and strain effects from precipitation of γ′ in both single and double pass welds. As observed previously in a V shaped weld preparation, the extent of microfissuring in alloy IN738LC increased substantially from the use of the low and intermediate strength weld fillers, to extensive heat affected zone microfissuring by using the high strength IN738 filler. In the directionally solidified Rene 80 welds, due to the reduction in grain boundary area per unit volume, only minor heat affected zone cracking was observed, while the SX alloy did not crack at all. The absence of any cracks in the SX alloy welds despite the presence of stray grains in the fusion zone appears to be related to reduced stress levels in the welds due to the choice of welding technique and the welding parameters.     

Special wire guide for on-machine wire electrical discharge dressing of metal bonded grinding wheels

To improve accuracy in wire electrical discharge dressing (WEDD), special attention should be given to wire vibration. In this work, the use of a specially designed wire guide is proposed, which is responsible for both ensuring the stability of the wire and improving the efficiency of dielectric delivery to the dressing zone. For carrying out experiments with metal bonded diamond grinding wheels, a WEDD-device was designed, manufactured and integrated into a grinding machine. High erosion material removal rates and dressing accuracy were achieved, thus demonstrating the feasibility and efficient performance of this in situ dressing process.     

Study on axial-feed mirror finish grinding of hard and brittle materials in relation to micron-scale grain protrusion parameters

An axial-feed mirror finish grinding of hard and brittle materials is proposed by controlling grain protrusion parameters. In this grinding, the grinding wheel feed is along the wheel axial direction rather than in the traditional wheel cutting direction. The objective is to understand how micron-scale grain protrusion parameters influence ductile-mode grinding and ultimately to realize efficient mirror finish grinding using a coarse diamond grinding wheel. In this study, the grain tip truncation (GT-truncation) was performed after dressing to improve grain protrusion topography. First, a formation model of axial-feed ground surface was constructed to analyze the effect of grain protrusion parameters and grinding parameters on the critical cutting depth transferred from brittle-mode removal to ductile-mode removal; then GC dressing and GT-truncation of #180 diamond grinding wheel were experimentally performed to investigate surface roughness and ductile-mode grinding behavior with reference to grinding parameters and grain protrusion parameters; finally, a truncated coarser #60 diamond grinding wheel was employed for mirror finish grinding to observe active grain number and grain protrusion angle. Theoretical analysis shows that this ductile-mode grinding is dominated by active grain number, active grain protrusion angle, wheel rotating speed and axial-feed speed, but it does not depend on the depth of cut assumed to be less than the grain protrusion height. Experimental results indicate that the GT-truncation may increase active grain number and grain protrusion angle for ductile-mode grinding when the axial-feed speed decreases to some extent. Moreover, the micro tip radius of diamond grain also influences the ground surface. It is confirmed that by increasing active grain number and grain protrusion angle synchronously, a truncated #60 diamond grinding wheel can be applied for efficient mirror finish grinding of the SiC ceramic plate at the axial-feed speed of 50 mm/min and the tool path interval of 0.1 mm.     

金刚石砂轮精密修整工艺研究

金刚石砂轮机械磨削是砂轮整形的传统方式。砂轮旋转速度以及工具砂轮的进给量是金刚石砂轮机械精密整形的主要工艺参数。通过在超硬材料砂轮整形机床上的大量实验和砂轮磨削力的分析,得到了金属结合剂和树脂结合剂金刚石砂轮精密整形的比较理想的工艺参数;确定了工具砂轮的线速度应在11 m/s左右,工具砂轮轴转速在1 050～1 800 r/min;金刚石砂轮轴转速设定在400～1 000r/min,金刚石砂轮的线速度为2.6～10.5 m/s。同时,分析比较了机械修锐和喷砂修锐的效果。     

基于磨削法的金刚石磨料工具精密修磨技术

对磨削法修磨复杂形面金刚石磨料工具的技术进行研究。使用光学影像磨床,采用形面组合法对典型复杂形面金刚石工具进行修磨应用试验;通过理论分析、试验以及对修磨形貌显微观察等手段分析修磨机理;对修磨基本工艺、修磨效率和修磨精度的影响进行试验研究,并确定其相关规律。研究结果表明:磨削法修磨去除机理是通过磨粒接触面内局部接触点产生的高应力使金刚石表面发生微破碎,因此采用基于磨削法的精密修磨技术具有高效率和高精度的优点;对修整效果的主要影响因素是修磨切深、修磨速度、修磨砂轮等;单一元素形面修磨精度可以达到0.002 5 mm,角度5′以内,整体形面轮廓精度可以达到0.005 mm以内,修整砂轮后采用切入磨削工件加工,其表面粗糙度可以达到0.120 μm以下。      

Hot tearing in polycrystalline Ni-based IN738LC superalloy: Influence of Zr content

Zirconium is always present in Ni base superalloys as it enhances their creep properties. In the present study, the influence of very small Zr additions, 100–400 ppm, i.e. 0.01–0.04 wt.%, on hot tearing of IN738LC superalloy is experimentally investigated using dedicated turbine blade castings. Although the Zr content remains very small, it has a strong effect on hot tearing tendency. Microstructure of hot tear in as-cast samples reveal that grain size and secondary dendrite arm spacing have no significant effect on hot tearing. On the other hand eutectic phase volume fraction and its dispersion or spreading along grain boundaries drastically affect the hot tearing propensity and strongly increase with increasing amounts of Zr. Hence grain coalescence becomes impossible at grain boundaries covered with eutectic phase films. With increasing Zr content, gain coalescence between two distinct grains with no interdendritic phase requires more undercooling. Coalescence is retarded and occurs deeper in the mush zone, i.e. at lower temperatures resulting in a higher sensitivity to hot tearing. Finally, it is shown that a reduction of Zr content to 0.02 wt.% is required to fully suppress hot tearing in polycrystalline IN738LC blades.     

Mechanism of material removal in electrical discharge diamond grinding

The paper presents the mechanism of material removal in electrical discharge diamond grinding, which integrates electrical discharge machining and diamond grinding for machining electrically conducting hard materials. The role of current and wheel speed on the material removal rate, the grinding forces and power is studied to elucidate the issue. The spark discharges thermally soften the work material in the grinding zone, and consequently decrease the normal force and the grinding power. The improved grinding performance is due to continuous, in-process dressing and declogging of the wheel surface.     

TiAl合金低压涡轮叶片榫头磨削温度场研究

TiAl合金低压涡轮叶片榫头磨削过程中温度分布较为复杂，因此采用有限元法模拟温度分布特征。介绍几何模型建立、传入工件的热量比例的确定以及热源的选择和加载等重要细节。最高磨削温度的仿真和试验结果差异约为15%，验证了模型的合理性。仿真结果表明:开始磨削后，工件表面的温度逐渐升高直至稳定。磨削过程的最高温度出现在齿顶圆弧处，比齿根处的最高温度高约30%～40%，原因是型面各处热传导条件存在差异。磨削温度随磨削速度、工件进给速度升高和切深增大而逐渐升高。      

花岗石异型面高效加工磨削温度场实验研究

石材加工中温度对异型面形成机理和成形刀具的磨破损规律有重要的意义,而理论分析过程比较复杂,准确率还有待修正提高。为有效指导实际生产,掌握金刚石刀具磨削石材的规律,摸索适于高效花岗石磨削的加工工艺,以降低温度对生产的不良影响,提出了基于红外成像的测量和分析方法。实验中,分析了两种石材异型面干切削加工中热量的产生及切削温度的变化,发现花岗石异型面磨削弧区的表面最高温度随主轴转速及刀具切削深度的提高而上升;单独提高刀具的进给速度会使磨削区温度先升后降。     

陶瓷CBN砂轮修整参数的量化分析研究

采用正交试验方法量化分析了陶瓷CBN砂轮的修整参数对工件表面质量的影响。研究结果表明:进给速度对工件表面粗糙度和支承长度率均有明显的影响;修整量对表面粗糙度影响较小,但对支承长度率却有较为明显的影响,且修整时往复次数越少,砂轮越锋利;修整速差对表面质量影响较小。因此,可以通过加大进给速度、降低单次深度和增大往复次数的方法,来达到提高首件表面粗糙度、增加修整间隔、提高砂轮使用寿命的目的。      

cBN砂轮超声振动修整表面地貌实验研究

对cBN砂轮进行了超声振动辅助机械修整试验,研究了砂轮修整后表面地貌特征及砂轮表面静态有效磨粒数。实验结果表明,超声振动辅助修整的cBN砂轮表面的磨粒突出高度随振动频率增大而增大;砂轮表面静态有效磨粒数随着修整导程和修整深度的增加而减少。选择合理的修整参数,采用超声振动修整技术能够获得较理想的cBN砂轮表面。     

纳米结构WC/12Co涂层精密磨削的磨削力研究

本文对金刚石砂轮平面磨削纳米结构WC／12Co涂层材料时得到的磨削力进行了试验研究，研究了涂层材料的磨削力与磨削工艺参数以及砂轮特性之间的关系。通过各项试验研究得出，用金刚石砂轮磨削纳米结构WC／12Co涂层时，磨削力与当量磨削厚度基本成一元线性关系，它随磨削深度、工件速度的增加而增加：当磨粒尺寸减小时。总磨削力增加，但单颗磨粒磨削力减小。材料以非弹性变形的材料去除方式为主。通过试验采集的数据，使用当量磨削厚度作为磨削基本参数建立了法向磨削力理论模型。     

Comparison of numerically and analytically predicted contact temperatures in shallow and deep dry grinding with infrared measurements

Contact zone thermal models of the grinding process are an important tool for the proper selection of process parameters to minimize workpiece damage while improving process efficiency. Validating contact zone thermal models with experimental measurements is difficult due to the high-speed and stochastic nature of the grinding process. In this work an infrared imaging system is used to validate two numerical thermal models, which are then compared to an established analytical contact zone thermal model. The two numerical thermal models consist of a shallow grinding model and a deep grinding model, where the deep grinding model takes the contact angle into account while the shallow grinding model does not. The results show that at small depths of cut both the numerical models and the analytical model perform well; however, as the depth of cut is increased the numerical models’ accuracy increases as compared to the analytical model. The increase in accuracy may be a result of the 2D solution of the numerical models as compared to the 1D solution of the analytical model. Additionally, it was found that the contact angle has very little effect on the contact temperatures. This work also reinforces Rowe's analytical work, using experimental and numerical results, which indicated that the workpiece temperatures are reduced by grinding at higher Peclet numbers for a given material removal rate.