TEMPERATURE DISTRIBUTION DURING ELECTRO-DISCHARGE ABRASIVE GRINDING

The objective of this work is to develop a finite element method (FEM) based mathematical model to simulate the hybrid machining process of grinding and electric discharge machining (EDM), named as Electro-discharge abrasive grinding (EDAG), for temperature distribution in the workpiece. Two different finite element codes have been developed to calculate the temperature distribution due to grinding heat source and EDM heat source separately. The transient temperature field within workpiece due to cut-off grinding is determined due to moving rectangular heat source. Gaussian heat distribution of power within a spark has been considered in the calculation of temperature distribution due to EDM. Temperature distribution in the workpiece due to combined process is obtained by using superposition. The simulation shows a sudden rise in temperature at the spark location. The predicted results can be used for calculation of thermal stresses, which play a major role as far as high-quality product requirements are concerned.     

断续磨削时工件表层温度场解析

建立了周期变化的移动热源模型,并引进卷积概念,推导了计算断续磨削时工件表层非稳态脉动温度场的理论公式。该公式可以包容连续磨削的情况,且可计算任意时刻的瞬态温度分布。利用推得的公式对断续磨削时的温度场及其降温机理作了深入的研究,并对关于理论公式进行了实验校核。     

EXPERIMENTAL ANALYSIS OF TEMPERATURE IN GRINDING AT THE GLOBAL AND LOCAL SCALES

The purpose of the article is the experimental estimation of the global and local heat fluxes and the corresponding energy partition to the workpiece for regular grinding of 100Cr6 steel with aluminium oxide wheel. By using a grindable thermocouple, the temperature and the real contact length allow determination of the global heat flux and the partition ratio at the wheel scale. The high frequency analysis of the signal has shown maximum flash temperatures of about 1000°C corresponding to the local temperature under the chip-grain unit with very high heating speed of about 100°C/µs. The comparison between theoretical temperature decay and experimental cooling has demonstrated that the time response of the sensor is fast enough for the estimation of the local temperature and power due to the sliding of grain and to the plastic strain of ground materials.     

HFCVD衬底三维温度场有限元法模拟

热丝化学气相沉积(Hot filament chemical vapor deposition,HFCVD)方法制备金刚石薄膜设备简单,成本低廉,适合大面积金刚石膜的产业化生产,其中衬底温度是沉积高质量CVD金刚石膜的重要参数之一.基于此,首先分析大面积HFCVD系统的热交换过程,建立大面积HFCVD系统衬底温度场的三维有限元模型.与传统纯热辐射模型相比,本模型更加接近实际系统,并较好符合试验测定的结果.根据三维有限元模型开展对大面积HFCVD系统衬底温度场的有限元仿真研究,得到HFCVD系统衬底温度场的三维分布规律,并讨论热丝直径、热丝温度、热丝根数、热丝-衬底距离和水冷散热系数等对衬底温度大小及均匀性的影响.仿真结果表明,在适宜金刚石膜生长的参数范围内,热丝参数和衬底接触热阻对衬底温度大小有显著影响,由于衬底内部的三维热传导使得衬底温度场更加均匀,各参数对衬底温度场的均匀性影响不大.研究结果为高质量制备金刚石膜提供理论基础.     

MULTI-OBJECTIVE OPTIMIZATION OF ABRASIVE FLOW MACHINING PROCESSES USING POLYNOMIAL NEURAL NETWORKS AND GENETIC ALGORITHMS

Abrasive flow machining (AFM) is an economic and effective non-traditional machining technique, which is capable of providing excellent surface finish on difficult to approach regions on a wide range of components. With this method, it has become possible to substitute various time-consuming deburring and polishing operations that had often lead to non-reproducible results. In this paper, group method of data handling (GMDH)-type neural networks and Genetic algorithms (GAs) are first used for modelling of the effects of number of cycles and abrasive concentration on both material removal and surface finish, using some experimentally obtained training and testing data for brass and aluminum. Using such polynomial neural network models obtained, multi-objective GAs (non-dominated sorting genetic algorithm, NSGA-II) with a new diversity preserving mechanism are then used for Pareto-based optimization of AFM considering two conflicting objectives such as material removal and surface finish. It is shown that some interesting and important relationships as useful optimal design principles involved in the performance of AFM can be discovered by the Pareto-based multi-objective optimization of the obtained polynomial models. Such important optimal principles would not have been obtained without the use of both GMDH-type neural network modelling and multi-objective Pareto optimization approach.     

MULTI-OBJECTIVE OPTIMIZATION OF ABRASIVE FLOW MACHINING PROCESSES USING POLYNOMIAL NEURAL NETWORKS AND GENETIC ALGORITHMS

Abrasive flow machining (AFM) is an economic and effective non-traditional machining technique, which is capable of providing excellent surface finish on difficult to approach regions on a wide range of components. With this method, it has become possible to substitute various time-consuming deburring and polishing operations that had often lead to non-reproducible results. In this paper, group method of data handling (GMDH)-type neural networks and Genetic algorithms (GAs) are first used for modelling of the effects of number of cycles and abrasive concentration on both material removal and surface finish, using some experimentally obtained training and testing data for brass and aluminum. Using such polynomial neural network models obtained, multi-objective GAs (non-dominated sorting genetic algorithm, NSGA-II) with a new diversity preserving mechanism are then used for Pareto-based optimization of AFM considering two conflicting objectives such as material removal and surface finish. It is shown that some interesting and important relationships as useful optimal design principles involved in the performance of AFM can be discovered by the Pareto-based multi-objective optimization of the obtained polynomial models. Such important optimal principles would not have been obtained without the use of both GMDH-type neural network modelling and multi-objective Pareto optimization approach.     

厚板窄间隙多道埋弧焊温度和残余应力分布

通过有限元数值模拟的方法,分析厚板窄间隙埋弧焊的温度分布,同时给出了厚板上、下表面和厚度方向上的残余应力分布曲线.通过热电偶构成的温度测量系统测得试板特定点上的温度分布,且在焊接完成冷却一段时间后利用盲孔法测量了试板上、下表面特定点上残余应力大小.有限元计算结果和试验测量结果吻合较好,证明了该有限元模型的合理性,为评价焊接接头性能提供了参考.特别是,在厚度方向上不便测量的残余应力分布曲线可以通过有限元数值模拟的方法获得,为实际考查厚板内部残余应力分布提供依据.对于两者结果有较大偏差的地方,分析该偏差出现的原因.     

INVESTIGATION ON NEW TYPE OF DOUBLY BLENDING ABRASIVE WATER JET NOZZLE SYSTEM WITH HIGHER PERFORMANCE

Based on the two existing abrasive water-jet(AWJ) systems, the dia-jet (or pre-jet) and the post-jet, a new type of abrasive water-jet system is put forward, which combines the dia-jet's advantage, low operating system pressure, slender stream jet, and more concentrative abrasive in the blended stream, with merits of post-jet, the less sophisticate apparatus, successive supply of abrasives. The theoretic analysis is brought out in detail, and the nozzle system structure is concisely illustrated. Its relevant experiment results are demonstrated, proving that this new system is effective in various aspects, enlarging penetrating capability without raising system pressure, saving machining power supply, lessening energy loss, etc.     

Application of Nanofluids in Minimum Quantity Lubrication Grinding

This research investigated the wheel wear and tribological characteristics in wet, dry, and minimum quantity lubrication (MQL) grinding of cast iron. Water-based Al₂O₃ and diamond nanofluids were applied in the MQL grinding process and the grinding results were compared with those of pure water. During the nanofluid MQL grinding, a dense and hard slurry layer was formed on the wheel surface and could benefit the grinding performance. Experimental results showed that G-ratio, defined as the volume of material removed per unit volume of grinding wheel wear, could be improved with high-concentration nanofluids. Nanofluids showed the benefits of reducing grinding forces, improving surface roughness, and preventing workpiece burning. Compared to dry grinding, MQL grinding could significantly reduce the grinding temperature.     

EXPERIMENTAL INVESTIGATIONS OF THE FORCE DISTRIBUTIONS IN THE GRINDING CONTACT ZONE

This paper is concerned with the experimental investigations for the prediction of the grinding force distribution in the grinding wheel and workpiece contact zone. A new approach was developed to predict the force distribution using the horizontal and vertical forces measured while grinding a workpiece with a shorter length along the grinding pass. Straight surface grinding experiments were conducted using straight oil as grinding fluid on a nickel-based alloy (IN718) with a 60-grit 150 mm diameter electroplated CBN wheel to implement this approach and to investigate the effects of wheel speeds and workspeeds on the distributions. Grinding power and forces were measured. It was found that the distribution along contact arc increases with a decreasing rate from bottom to top. Wheel speeds have a significant effect on the normal force distribution at the higher removal rate. The grinding power was estimated to verify the predicted force distribution and it was found to be in a good agreement with the measured power.     

MDYB-3有机玻璃裂纹扩展温度效应试验研究

随着对飞机性能要求的提高,对机用有机玻璃的要求愈来愈高.目前飞机舱盖玻璃表面温度在-55℃～100℃范围内变化,因此,研究飞机舱盖MDYB-3有机玻璃温度效应具有十分重要的意义.文中采用CT(compact tension)试样对MDYB-3有机玻璃在-50℃～90℃范围内进行裂纹扩展试验,研究温度对MDYB-3有机玻璃裂纹扩展速率的影响.应用线弹性断裂力学理论,分析不同环境温度下裂纹扩展速率与等效应力强度因子ΔK之间的关系.试验结果表明,MDYB-3有机玻璃随着温度升高,裂纹扩展速率减小,断裂韧度增大;低温时MDYB-3有机玻璃表现为脆性,裂纹扩展十分平滑,断面垂直于拉伸方向,温度较高时MDYB-3有机玻璃表现为粘性,裂纹扩展面凹凸不平.     

Theoretical Analysis of Thermal Stresses in Electro-discharge Diamond Grinding

Excessive heat generated at the machining zone, during Electro-discharge diamond grinding (EDDG), is the major cause of thermal stresses, untempered martensite, overtempered martensite, and cracks. Therefore, the key to achieve good surface integrity in a machined part is to prevent excessive temperature and thermal stresses generated during machining process. A finite element model has been developed to estimate thermal stresses during EDDG when the current is switched-off. First, the developed code calculates the temperature in the workpiece and then the thermal stress field is estimated using this temperature field. Computations were carried out in plane strain condition for different down feeds of the grinding wheel. The effects of time of grinding and feed on thermal stress distribution have been reported. The thermal stresses are found to be higher near top surface at initial time of grinding but shifted away towards bottom after some grinding time.     

FE numerical simulation of mould temperature field during the continuous casting of steel

In this paper, the three-dimensional transient model of the mould temperature is established and the temperature variation is analyzed when the thickness of the mould wall and the cooling rates are different. For the numerical computation, a corresponding finite element method (FEM) program is developed with FORTRAN language, and six-face and eight-node isoparameter elements are applied; the Newton method is used to solve the heat equations. The experimental data have been used to validate the model. It is shown that the appropriate mould wall thickness is 10 mm and the different parts of the mould should correspond with cooling water of different rates.     

选择性激光熔化成形瞬态温度场数值模拟

为揭示选择性激光熔化成形的机理,初步建立了其熔化过程的传热理论模型;考虑温度变化对材料热物性参数的影响,利用ANSYS参数化设计语言(APDL)建立了有限元数值模型并进行了温度场求解.计算结果显示:熔化成形过程中温度场等值线呈椭圆状,已凝固部分在光斑再次扫描至邻近部位和上层粉末时由于热传导作用而发生重熔;熔池的尺寸大小随吸收能量的增加而逐渐增大,由初始宽约0.2mm增至0.25mm左右;温度场分布均匀、温度梯度小可减小零件的翘曲变形;基板温度随时间的推移而逐渐升高.     

Modeling and simulation of magnetic abrasive finishing process

Magnetic abrasive finishing (MAF) is one of the advanced finishing processes, which produces a high level of surface quality and is primarily controlled by a magnetic field. In MAF, the workpiece is kept between the two poles of a magnet. The working gap between the workpiece and the magnet is filled with magnetic abrasive particles. A magnetic abrasive flexible brush (MAFB) is formed, acting as a multipoint cutting tool, due to the effect of the magnetic field in the working gap. This paper deals with the theoretical investigations of the MAF process. A finite element model of the process is developed to evaluate the distribution of magnetic forces on the workpiece surface. The MAF process removes a very small amount of material by indentation and rotation of magnetic abrasive particles in the circular tracks. A theoretical model for material removal and surface roughness is also proposed accounting for microcutting by considering a uniform surface profile without statistical distribution. Numerical experiments are carried out by providing different routes of intermittent motion to the tool. The simulation results are verified by comparing them with the experimental results available in the literature.     

柔性薄管板换热器的结构分析与优化

运用ANSYS对采用柔性薄管板的换热器建立了热分析与结构分析的有限元模型,分析了柔性薄管板的温度场及应力场,得出了管板上的应力分布及最大应力发生部位。根据应力分析结果对影响管板应力的主要结构参数如管板厚度、换热管中心距、不布管区宽度、管板与壳体连接处的转角结构和转角半径进行了分析优化,得到了较合理的设计结果。     

多种数据状态下三参数Weibull分布的极大似然估计

提出一种既适合完全数据,也适合各种截尾数据的三参数Weibull分布极大似然估计的计算方法.首先建立完全数据和截尾数据下统一形式的似然方程;利用二阶收敛Newton-Raphson迭代法求解给定位置参数时含尺度参数和形状参数的两参数Weibull分布的极大似然估计;然后采用稳定快速的Brent搜索法,求解出仅含位置参数的单变量似然函数最优解;最后结合实例说明该算法的稳定性和高效性.该方法具有计算精度高、速度快、适用数据范围广的优点.     

30 m望远镜的三镜Rotator组件轴承概念设计

针对美国30 m望远镜(TMT)三镜系统在工作时的特殊要求,对三镜系统的Rotator组件轴承进行了设计,提出了在载荷连续变化条件下轴系的设计方法.通过分析该系统独特的运动和受力方式并对比现有大型望远镜结构形式,确定了三排滚柱支撑的轴系方案及轴承结构参数.计算中将天顶角定义为变量,确定了轴承的最恶劣工况及此时的望远镜指向.采用了数值计算和有限元仿真的方法对这一条件下的轴承变形和应力同时进行校核,两种方法得到的结果符合得很好,证明了模型的正确性.结果表明,天顶角为0°～65°时,轴承在x、y、z方向上的变形不超过0.015 mm,轴承倾角不超过1.7×10-5 rad,满足设计要求,并留有很大裕度.