Papers by Author: Liang Chi Zhang

Abstract: Material models in the libraries of commercially available finite element codes often cannot satisfy the needs of some special material property descriptions, such as those of materials under ultra-high-speed cutting. To overcome the difficulty, this paper introduces an efficient explicit algorithm to implement a generalized user-defined model of metal plasticity into the Abaqus/Explicit program. This algorithm makes the computation quite efficient by avoiding updating the tensor of elastic-plastic modulus in each increment.

Abstract: A finite element heat transfer model incorporating a moving heat source has been developed to predict the temperature field in traverse cylindrical grinding. The model was then applied to analyse the grinding-hardening of quenchable steel 1045. It was found that in the region where the grinding wheel had an entire contact with the workpiece, material would experience a heating-cooling cycle, enabling the generation of a uniform hardened layer. In the transient regions at the two ends of the workpiece where the wheel-workpiece contacts were partial, the material was not hardened but experienced an annealing process. The results were in good agreement with the experimental observations.

Abstract: This paper reviews two cooling methods for surface grinding. These include i) replacing the toxic coolant by a mixture of cold air and vegetable oil mist, and ii) minimising the use of coolant using a segmented grinding wheel system. The discussion concludes that the cold air-oil mist provides an environmentally conscious mean for surface grinding. However, due to the limit of its cooling capacity, it is more applicable for fine grinding. Using the segmented wheel system can minimise coolant consumption and can improve grindability.

Abstract: Glass transition is the most important factor in the thermo-forming of glass elements of precise geometries such as optical glass lenses. Among many attempts to model the physics of glass transition, the Master equations based on the potential energy landscape (PEL) appear to be apropos. In this study, we used Monte-Carlo approach to approximately solve the master equations and further implement the Monte-Carlo method in the finite element simulation. We used Selenium as an example since its PEL has been quantified. Through the FEM simulations, it is found that the geometrical replication quality is the best when the forming is performed at the viscosity around 10⁵~10⁶ Pa×s, that the residual stress developed in the cooling process can be minimized in the slow cooling process or through post-annealing process after moulding.

Abstract: This paper uses the molecular dynamics simulation to investigate the quality of nano-scale grooving on mono-crystalline copper by high-speed nano-milling and nano-cutting. The results reveal that nano-milling produces a high-quality nano-groove with smooth surfaces in comparison with a nano-cutting. It is also interesting to note that the machined workpiece subsurface can be free from dislocations.

Abstract: This paper develops a statistical model to analyze the chemical effect on the material removal rate (MRR) in chemo-mechanical polishing of material surfaces (MS). It was considered that the chemical effect comes into play through a passivated layer on the MS. It was found that the effect of such layer on MRR depends on the mean particle radius, the elastic modulus of the pad, and the hardnesses of the passivated layer and the workpiece material.

Abstract: A three-dimensional finite element heat transfer model incorporating a moving heat source was developed to investigate the heat transfer mechanism in grinding-hardening of a cylindrical component. The model was applied to analyze the grinding-hardening of quenchable steel 1045 by two grinding methods, traverse and plunge grinding. It was found that the heat generated can promote the martensitic phase transformation in the ground workpiece. As a result, a hardened layer with a uniform thickness can be produced by traverse grinding. However, the layer thickness generated by plunge grinding varies circumferentially. The results are in good agreement with the experimental observations.

Abstract: A deep understanding of adiabatic shear fracture (ASF) during serrated chip formation is essential to explore the material removal mechanism of high speed cutting (HSC). This paper aims to reveal the microscopic details of ASF in serrated chips. The material to investigate was AISI 1045 steel of different hardness grades, and the micro-structural analysis was conducted using optical and scanning electronic microscopes. The investigation showed that at the hardness of HRC50, most fractured surfaces were covered by a large number of dimples elongated along the shear direction, indicating that the fundamental cause of the serrated chip generation is the deformation localization of the adiabatic shear followed by ductile damage fracture in primary shear zones. The higher the material hardness is, the easier the adiabatic shear and ductile fracture take place. A new model was then proposed to interpret the ductile fracture due to adiabatic shear governed by the nucleation, growth and coalescence of micro-voids during serrated chip formation.

Abstract: Adiabatic shear banding during high speed machining is important to understand material removal mechanisms. This paper investigates the microstructure of adiabatic shear bands (ASBs) in the serrated chips produced during the high speed machining of AISI 1045 hardened steel. Optical microscope, scanning electronic microscope(SEM) and transmission electronic microscope(TEM) were used to explore the microstructural characteristics. It was found that there are two types of adiabatic shear bands. One is the deformed adiabatic shear band composed of a significantly deformed structure generated in a range of low cutting speeds, and the other is the transformed adiabatic shear band composed of very small equiaxed grains generated under high cutting speeds. The results indicated that the deformed band has a tempered martensite structure that formed through large plastic deformation and the transformed band has experienced a dynamic recrystallization process.

Abstract: Carbon nanotubes (CNTs) have been used in making composites because CNTs have high strength, large aspect-ratio and excellent thermal and electrical conductivity. However, to realize the wide applications of CNT composites, further R&D must be carried out. This review will discuss some fabrication, characterisation and application issues of CNT polymer composites. Aspects to outline are purification, dispersion, alignment, stress transfer, interface bonding, wear and friction and rheological properties. Some research challenges will be briefly highlighted as well, including the mass production of long and aligned CNTs at low cost, and the optimization of the microstructures, properties and functioning features of CNT composites.