Hong Hocheng

Comprehensive analysis of delamination in drilling of composite materials with various drill bits

Abstract Beside the twist drill, the effects of various drill geometries were rarely discussed in analytical fashion. This study presents a comprehensive analysis of delamination in use of various drill types, such as saw drill, candle stick drill, core drill and step drill. In this analysis, the critical thrust force at the onset of delamination is predicted and compared with the twist drill.

Optimization of electrochemical polishing of stainless steel by grey relational analysis

Abstract Grey relational analysis is useful for the multi-input, discrete data and uncertain experimental study. Developed in this paper is the application of the grey relational analysis for optimizing the electropolishing of 316L stainless steel with multiple performance characteristics. The processing parameters (temperature, current density, and electrolyte composition) are optimized with considerations of the multiple performance characteristics (surface roughness and passivation strength). The conducted experiments approve the effectiveness of the grey relational analysis.

Preliminary study of material removal in electrical-discharge machining of SiC/Al

Abstract Metal matrix composite (MMC) materials have increasingly widened their use due to the merits of possessing high specific strength and modulus of elasticity while carrying good deformability and conductivity comparable to metals in addition to its use structural and functional components for high-performance applications, such as aerospace vehicles and racing automobiles, MMC also has potential for molds. Especially for the large and mid-size molds in precision manufacturing, where handling is difficult due to the heavy weight, MMC can improve the productivity by saving the cost of loading, positioning and stocking. MMC is difficult to machine due to serious tool wear caused by the hard reinforcement. To exploit the potential industrial applications and investigate proper manufacturing processes, the machinability of electrical discharge machining (EDM) of MMC needs to be studied for reliable and economical production. The fundamental analysis starts from the material removal of MMC by a single spark. This paper presents the correlation between the major machining parameters, electrical current and on-time, and the crater size produced by a single spark for the representative material SiC/Al. The experimental results not only show the predicted proportionality based on heat conduction model, but are also compared with common steels regarding the material removal rate. Though the crater size of SiC/Al is larger than steel, the SiC particles can interfere the discharges. For effective EDM, large electrical current and short on-time are recommended. Based on the obtained knowledge, one can proceed to the study of machinability of MMC by EDM for optimal production cycle.

A material removal analysis of electrochemical machining using flat-end cathode

Abstract Electrochemical machining (ECM) has been increasingly recognized for the potential for machining, while the precision of the machined profile is a concern of its application. A process to erode a hole of hundreds of micrometers on the metal surface is analyzed in the current paper. A theoretical and computational model is presented to illustrate how the machined profile evolves as the time elapses. The analysis is based on the fundamental law of electrolysis and the integral of a finite-width tool. The paper also discusses the influence of experimental variables including time of electrolysis, voltage, molar concentration of electrolyte and electrode gap upon the amount of material removal and diameter of machined hole. The results of experiment show the material removal increases with increasing electrical voltage, molar concentration of electrolyte, time of electrolysis and reduced initial gap. The time of electrolysis is the most influential factor on the produced diameter of hole.

The anisotropic heat-affected zone in the laser grooving of fiber-reinforced composite material

Abstract A heat-affected zone is frequently associated with the laser processing of materials. In the shaping of composite material after curing, thermal damage is found, such as matrix recession, matrix decomposition, and delamination, which lead to poor assembly tolerance and long-term performance deterioration. The current study analyzes the peculiar growth of the heat-affected zone due to laser energy and the anisotropy of heat conductivity from the anisotropic arrangement of the fiber reinforcement in the composite material. It further applies the concept that the cut quality can be improved by reducing the resulting temperature of the workpiece, which leads to laser machining at low temperature. The paper investigates the cutting of unidirectional laminated carbon/epoxy composite perpendicular to and parallel to the fiber axis, using a cool nitrogen jet to reduce thermal damage. A theoretical analysis based on moving-point heat-source theory is adopted to determine the extent of thermal damage in correlation with process parameters and material properties. The analytical solution is modified by the ‘mirror image method’ for a specimen of finite thickness. The range of the heat-affected zone is estimated by the isotherm of the matrix char temperature. Heat conduction is maximum along the carbon fibers, the heat-affected zone shape thus being affected by the beam scanning direction relative to the fiber orientation.

Electropolishing of cylindrical workpiece of tool materials using disc-form electrodes

Abstract This research presents a new application of electropolishing using a low-cost disc-form electrode offering fast improvement of the surface roughness of SKD61. It requires no expensive special-purpose equipment or heavy material removal as conventional electrochemical machining does, and it also avoids the complex pre-polishing of the workpiece before the electropolishing. Round bars or round tubes produced by traditional turning, drawing, form rolling, or extrusion, can be successively electropolished using the designed disc-form electrodes. Five electrode designs are discussed. The experimental parameters include rotational speed of workpiece, electrical current rating and pulse period, electrode geometry, and electrode feed rate. Thinner disc and larger disc taper angle are associated with larger discharge space for the electrode, thus the polishing is more effective. A smaller end radius of the disc electrode produces higher current density and provides faster feed rate and a better polishing effect. A disc with discharge flute performs better, and larger flute back rake angle, side rake angle, wider flute, and deeper flute depth are also advantageous. Although the use of pulsed current slightly outperforms the fluted electrode using continuous current, it sacrifices both machining time and cost. The best electrode design is identified. A guideline for the design of electrodes is provided based on the experimental results.

Material removal analysis in abrasive waterjet cutting of ceramic plates

Abstract Fine ceramics have been recognized increasingly in structural and functional applications on account of their merits of hardness, corrosion resistance, electromagnetic response and bio-compatibility. Due to the need for dimensional control or production optimization, post-sintering machining can be required. Cutting by conventional means is most often practiced, but the associated heavy tool wear is hard to overcome. A waterjet at transonic speed carrying abrasive particles provides an effective means for hard-material removal. Undesired material fracture at entrance and exit can be reduced significantly by sequential abrasive micro-machining. The present paper discusses the kerf formation of a ceramic plate cut by an abrasive waterjet. The mechanism and the effectiveness of material removal are studied first. Different materials are found to possess different removal rates in machining and there also exists a critical combination of hydraulic pressure, abrasive flow rate and traverse speed, below which through-cut for a certain thickness cannot be achieved. The wall finish achieved is determined by the mesh size of the abrasives: sufficient hydraulic pressure with fine abrasives will produce a smooth surface comparable to that from grinding. The kerf is slightly tapered with wider entry due to decreased cutting energy with kerf depth. A high-power input per unit length produces a small taper but a wide slot.

Modeling and experimental analysis of the material removal rate in the chemical mechanical planarization of dielectric films and bare silicon wafers

Historically chemical mechanical planarization ~CMP! has been used to polish optically flat surfaces. As the wiring density in highperformance chips increases and the device size scales down to submicrometers ~as shown in Table I!, planarization technology becomes indispensable during both device fabrication and formation of multilevel interconnects. CMP has emerged recently as a processing technique for a higher degree of planarization in submicrometer very large scale integration ~VLSI!, and is widely accepted for planarizing interlevel dielectrics and selective removal of aluminum, tungsten, copper, and titanium overburden following metal filling of studs and interconnects. A schematic diagram of a CMP is shown in Fig. 1 ~view from top and side!. The wafer is held face down by a rotating carrier pressed against the polishing pad, which is rotating as well. A down force is applied onto the wafer. The polishing colloidal slurry, which consists of chemical reagents, is dispensed onto the pad surface. The slurry is distributed across the pad by the centrifugal force of the rotating pad to wet the polishing pad and the wafer. The chemical reaction in CMP softens the dielectric or hardens the metal surface material of the wafer. The treated material is then removed mechanically by fine abrasives. In balance between the chemical and mechanical actions, the process removes material continuously. Although quite a few papers discuss the experimental results and the effects of the process parameters, the material removal mechanism is addressed less. Some authors consider the material removal mechanism at the direct contact between the wafer and the pad, while others consider that there is a thin fluid film between the wafer and the pad. Warnock presented the dependence of the polish rate on the wafer shape, though it is completely a phenomenological model. 1 Yu et al. presented a physical CMP model that includes the effects of polishing pad roughness and dynamic interaction between pad and wafer. Two new feature-scale-polishing mechanisms based on asperity theory are proposed and investigated experimentally. However, it is not clear whether or how the asperity affects the global quality of planarization. 2 Warnock and Yu et al. both derive the reduction rate of the step height assuming direct contact between pad and wafer without considering the role of abrasive flow in between. Liu assumes that the abrasive cuts the wafer under the loaded pressure and the relative velocity between the wafer and the pad. The model is capable of delineating the role of the mechanical properties of the slurry particles and the surface film to be polished. Liu assumes that the deformed volume to be composed of three regions, namely, the regions of elastic deformation, plastic deformation, and microcutting. He also assumes the deformation volume during contact to be approximately equal to the volume of microcutting and calculates the removal rate by the Hertzian elastic equation. The results were also compared with the wear model of Cook, 3 which is derived from the glass polishing process. 4 Cook and Liu both believe that the removal of wafer is primarily due to the direct contact between abrasives and wafer, and the Hertzian contact is adopted. Cook’s results did not reveal the detail of material removal rate, and Liu did not characterize the role of flow. Runnels presented two CMP models in 1994. One is the tribology analysis of CMP, and the other is the feature-scale fluid-based erosion model for CMP. Runnels considers that the knowledge of the stress distribution on the wafer surface is required in order to explain the variation of material removal rate on a wafer during CMP. This study analyzes the fluid film between the wafer and pad, and demonstrates that hydroplaning is possible for standard CMP processes. The importance of wafer curvature, slurry viscosity, and rotation speed on the thickness of the fluid film is also demonstrated. Although the analysis is novel, the model can only estimate the thickness of the film between pad and wafer, and Runnels admitted that the assumption of the spherical wafer needs modification. 5 The

Preliminary study of ultrasonic drilling of fiber-reinforced plastics

Abstract Composite materials posses advantages in structural application thanks to their high specific strength and directional properties. Although in many applications, composites are cured to final shape, machining can be required at both the prepreg and product stages. In traditional drilling, due to the anisotropy and the lamination of composite materials, delamination and splintering at hole edge often occur. Ultrasonic machining is suitable for these materials for its nature of material removal by small individual abrasives. Experiments were conducted on the ultrasonic drilling of Carbon/Epoxy and Carbon/PEEK. The examination of surfaces and abrasives after machining illustrates the hammering and impact of the abrasive particles on the workpiece. Brittle fracture of fibers and plastic deformation of matrix are seen. The research highlighted the influence of the concentration of abrasives, the size of abrasive grains, the energy of ultrasonic oscillation and the feed rate of tool on the machinability in respect of material removal rate, surface roughness and hole clearence. Dimensional analysis synthesizes the significant parameters in machining. Ultrasonic machining produced better surface finish and hole quality than conventional drilling. The cost factors of ultrasonic machining, investment, annual revenue, annual expenditures, economic life and salvage value are analyzed with annual and present worth methods. It shows that ultrasonic machining can earn higher profit than other non-conventional machining processes.

Kinematic Analysis and Measurement of Temperature Rise on a Pad in Chemical Mechanical Planarization

Pad temperature measurement using a thermal camera during chemical mechanical planarization (CMP) process is an effective means for monitoring the pad life and removal uniformity. In this study, we present a novel kinematic analysis method based on the principle of energy transformation and conservation to account for the pad temperature rise. The relative speed between the wafer and the pad determines the energy content causing the temperature rise. The pad temperature rise is measured by the thermal image method, with those results agreeing with the prediction of kinematic calculations. The pad temperature is closely related to the outcome of CMP, such as removal rate, uniformity, and pad life. Results in this study provide further insight into the relation between the pad temperature distribution, wafer uniformity, and pad life, thereby contributing to a more effective process control.