Key Engineering Materials Vols. 589-590

Abstract: Theoretical, experimental and simulation researches are investigated to study the phenomenon of edge chipping in laser assisted milling (LAML) of silicon nitride ceramics. To consider the crack evolution and influencing factors of edge chipping, the Extended Finite Element Method (XFEM) is used to model the process of chipping crack with the advantage of independent of the mesh and needless to remesh with the crack growth. The chipping width measured from experiments are loaded as initial boundary conditions to build the XFEM model, and the results show the edge chipping reduces with cutting forces decreasing and edge toughness increasing. The model reveals the edge chipping of workpiece reduces at elevated temperature through softening and toughening mechanisms.

Abstract: Internal high pressure forming is a kind of the modern plastic processing technology. Using liquid as the pressure transmitting medium, explore the effect of internal high pressure parameter on the three passing pipe technique, internal high pressure forming technique is applied on the three-dimensional shape parts deformation. In this study, we simulate the three-way pipe process by finite element software, analyze the forming force influence to the tube forming quality and get the changeable regulation of the pipe wall thickness with conclusion of the wall thickness variation, which provides the reference data and guidance for the actual production of the three-way pipe.

Abstract: Sapphire, widely used in high-speed integrated-circuit chips, thin-film substrates, and various electronic components, is regarded as one of the most difficult to cut materials owing to its great hardness and low fracture toughness. Rotary ultrasonic machining (RUM) has been regarded as an effective processing method for hard and brittle materials. In this paper, RUM process is introduced into the drilling of sapphire for the first time. The feasibility to machine sapphire using RUM is studied. Moreover, results of a designed experimental study into RUM of sapphire are presented to discuss the main effects as well as interaction effects of process parameters (ultrasonic power, spindle speed, and feedrate) on outputs. The process outputs investigated include cutting force and edge chipping size.

Abstract: The performances of particle reinforced aluminum matrix composite is excellent, but it is hard to be manufactured, which resist its wide spread engineering application. Rotary ultrasonic machining (RUM) is very suit for machining of particle reinforced aluminum matrix composite with moderate or high volume fraction. An aluminum matrix composite reinforced with 45% volume fraction of 3μm silicon carbide particles in a A12 aluminium matrix is experimental studied by RUM, and surface topography, surface roughness and cutting force are analyzed. The experimental results show that machining surface quality is high, surface roughness Ra ranges from 0.131~0.340μm, the cutting process is very smooth and cutting force Fz ranges from 23.33~51.31N, grinding wheel jams and grinding burn do not appear on rotary ultrasonic cutting tool after machining, and tool wear is extremely little.

Abstract: The external efficiency of LED is limited by total internal reflection on the interfacial surface. Surface structure modification is an effective way to solve this problem. In this work, micromachining method was present to manufacture surface microstructures for the GaN based LED light extraction efficiency enhancement. The feasibility of micromachining was discussed theoretically and proved by experimental study in this work. Micro-cutting and micro-forming approach was found be suitable for GaN surface microstructure machining. An experimental study of micro-cutting was carried out. The result shows that no crack and failure occurred during micro-cutting with 1μm cutting depth. This result demonstrated that microstructure can be machined on GaN based LED surface to enhance the LED light extraction efficiency.

Abstract: Two-dimensional finite element model is established by ABAQUS Explicit finite element analysis in this paper. It includes abrasive wear micro cutting process between pair of guide. Johnson-cook constitutive model is used to simulate the guide material deformation of wear process. Based on different sizes and different speed of grain, loading, cutting and unloading process are simulated by finite element, guide surface stress distribution and deformation condition are studied in the guide rail pair wear process. The simulation data is obtained by central composite experiments and regression model is gotten by MATLAB. The influence law of the particle radius and speed to guide residual compressive stress and deformation are gotten by the prediction model.

Abstract: Metallic bipolar plates with unique wave-like microstructure on the bottom of microflow channel have shown promising prospects for the application in proton exchange membrane microfuel cell. A novel method—milling with special thin slotting cutters is developed for fabrication of wave-like microstructure on the bottom of microflow channels. The special thin slotting cutter is manufactured by removing one or several teeth every one tooth of the traditional slotting cutter regularly. Forming mechanism of wave-like microstructure is presented and experimental tests have been conducted for validation of the feasibility of the novel method. Results show that the wave-like microstructure can be successfully fabricated on the bottom of microflow channel. The bottom of bipolar plates with wave-like microstructure is not a flat plane, but a wavy groove. The wavelength and amplitude of wave-like microstructure increase with the increase of feed speed and the number of removed teeth.

Abstract: As a highly efficient heat dissipation unit, a micro heat pipe is widely used in high heat flux microelectronic chips, and its thermal resistance is crucial to heat transfer capacity. Through analyses of the structure and heat transfer performance of a circular heat pipe with sintered wick, the theoretical model of total thermal resistance was established on heat transfer theory, and then simplified, finally a testing platform was set up to test for total thermal resistance performance. The testing results show that when the micro heat pipe is in optimal heat transfer state, its total thermal resistance conform well with that from the theoretical model, and its actual thermal resistance is much lower than that of the rod made of the material with perfect thermal conductivity and of the same geometric size. With the increment of heat transfer capability, the total thermal resistance of a micro heat pipe with sintered wick decreases first, then increases and reaches the minimum when it is in the optimal heat transfer state. The greater total thermal resistance in low heat transfer performance is mainly caused by too much working fluid accumulating in evaporator and the lower velocity in vapor cavity, and the greater total thermal resistance in high heat transfer performance is mainly due to the working fluid drying up in condenser. Total thermal resistance is related to many factors, such as thermal conductivity of tube-shell material, wall thickness, wick thickness, copper powders grain size and porosity, the lengths of condenser and evaporator, and the diameter of vapor cavity etc.. Therefore, the structure parameters of a micro heat pipe with sintered wick should be reasonably designed according to the specific conditions to ensure its heat transfer capacity and total thermal resistance to meet the requirements.

Abstract: As a highly efficient heat transfer component, a micro heat pipe (MHP) has been widely applied to the situations with high heat flux concentration. However, a MHPs heat transfer performance is affected by many factors, among which, working fluid inventory has great influence on the security, reliability and frost resistance of its heat transfer performance. In order to determine the appropriate working fluid inventory for grooved MHPs, this paper first analyzed the working principle, major heat transfer limits and heat flux distribution law of grooved MHPs in electronic chips with high heat flux concentration, then established a mathematic model for the working fluid inventory in grooved MHPs. Finally, with distilled water being the working fluid, a series of experimental investigations were conducted at different temperatures to test the heat transfer performances of grooved MHPs, which were perfused with different inventories and with different adiabatic section lengths. The experimental results show that when the value of α is roughly within 0.40±0.05, a grooved MHP can acquire its best heat transfer performance, and the working fluid inventory can be determined by the proposed mathematic model. Therefore this study solves the complicated problem of determining appropriate working fluid inventory for grooved MHPs.

Abstract: Titanium Carbonitride (TiCN) based cermets are important cutting tools materials. Fabrication of the material is time and cost consuming process for the traditional sintering. In this paper, TiCN based cermets were prepared by using microwave sintering and spark plasma sintering compare to traditional sintering, and the microstructures and properties were investigated. The results show that microwave sintering and the spark plasma sintering can obtain similar properties with traditional sintering with shorter sintering time, this will greatly save energy. The strength and hardness of TiCN based cermets sintered by microwave sintering is 1136Mpa and HRA87, respectively. Microwave wave sintering can obtain finer grain than traditional sintering, and spark plasma sintering have the finest grain, which is in the range of 0.3μm~0.5μm. Due to the present of big pores, the bulk of spark plasma sintering has the lowest bending strength.