Papers by Author: Liang Chi Zhang

Abstract: Unlike the traditional silicate glasses, borosilicate glasses behave differently because of the addition of boron atoms. Extensive studies have been carried out to understand the abnormal function of boron in glass network. However, it is not clear how the atomic structure of borosilicate glass changes under loading. This paper investigates the behaviour of borosilicate glass under uniaxial compression with the aid of ab initio simulations. Sodium borosilicate glass having 160 atoms and a mass density of 2.51 g/cm³ with composition 3Na₂O-B₂O₃-6SiO₂ were equilibrated first at 3500K, then at 2500K, 1500K, 1200K, 1000K, 825K and 625K. Structural analysis showed that at higher temperatures the sodium borosilicate liquid does not have a specific structure. At around 825 K (i.e. around T_g), boron network and silicon network form and remain stable even at a temperature of 625 K. When the supercooled sample at 825K was subjected to uniaxial compression, the stress along the compression direction first increases and then decreases with a change in boron structure, which could modify the behaviour of the borosilicate glass.

Abstract: This paper presents a transient thermal analysis of cold strip rolling, integrating both the microscale asperity deformation and macroscale strip deformation. A statistical characterisation was carried out to obtain the contact pressure and thermal contact conductance at the roll-strip interface. To address the effects of rolling speed and temperature rise, Johnson-cook constitutive model of yielding strength was employed to incorporate the strain rate and temperature variation in the analysis. It was found that the developed model can successfully predict both the interface contact stress and temperature rise in the rolling bite.

Abstract: Relaxation oscillation is a nonlinear dynamic phenomenon, commonly observed in viscous-plastic deformation of materials. However, it is the first time that we observed this phenomenon in the viscous flow of borosilicate glass in its super-cooled liquid region. Our investigation identified that the oscillation is caused by the particular microstructure of borosilicate glass. Specifically, the structure of borosilicate glass consists of borate-rich and silicate-rich networks. During the viscous flow, the fast deformation in borate network tends to be localized. However, the network mixing reaction between the borate-rich and silicate-rich networks can slowly relax the fast localized deformation. These two processes occur simultaneously and as a result bring about the relaxation oscillation. Based on this mechanism, the study established a physical constitutive model to predict the relaxation oscillation during the compression of borosilicate glass.

Abstract: Thermoplastic forming is a promising method for fabricating metallic glass (MG) products with complex shapes. This method can avoid the difficulties encountered in other manufacturing processes, such as very high cooling rate required by casting and catastrophic cracking in machining. However, during thermoplastic forming the adhesion between dies and MGs restricts the production. It is therefore important to explore the underlying adhesion mechanisms during forming and establish guidelines for selecting proper die materials. In this paper, we comprehensively studied the adhesion between La-based MG and some widely-used die materials (electroless Ni-P, Si, alumina and silicon nitride) in the thermoplastic forming process. It was found that, among these die materials, alumina has the best performance, which is attributed to its strong chemical bonds and low surface energy. The study concludes that the surface energy and the type of chemical bonds can be proper indicators for selecting die materials.

Abstract: This paper investigates the effect of the grinding-induced cyclic heating on the hardened layer properties generated in plunge cylindrical grinding. It was found that by increasing the number of grinding cycles, the hardened layer becomes thicker. The cyclic grinding stressing together with the heating initiates a plastic deformation zone of a highly oriented microstructure. The surface residual stresses of the layer are compressive. Under a lower infeed rate, the tempering of the hardened layer material will occur more severely.

Abstract: To effectively machine fibre-reinforced polymer composites using a simple tool, the authors have developed an elliptic vibration-assisted (EVA) cutting technique by applying micro-scale vibrations to a tool tip. This investigation aims to understand the effect of vibration frequency and amplitude on the EVA cutting performance. With the aid of a microstructure-based 3D finite element analysis, this study found that an increased vibration frequency or amplitude can accelerate the fracture of fibres, and reduce cutting forces in both the cutting and normal directions. The fracture mechanism was found to be dominated by the bending of fibres when the vibration frequency or amplitude in the cutting direction was small. With increasing the frequency or amplitude, impact-induced fracture becomes dominant, which reduces subsurface damage. It was found that to promote the performance of EVA cutting, the vibration frequency and amplitude of the cutting tool should be high. However, a too large frequency can bring about severe subsurface damage and bending fracture of fibres beneath the cutting path. A too large amplitude in the cutting direction, however, can accelerate the tool wear, while that in the vertical direction can worsen the fibre-matrix debonding.

Abstract: Grinding-hardening is a thermo-mechanical process capable of simultaneous material removal and surface enhancement. However, there are difficulties in dealing with cylindrical workpieces, because of the complex 3D-helical movement of the grinding-induced heat source. This paper investigates the effects of some key grinding parameters on the profile and microstructure variations of the hardened layer generated by a traverse cylindrical grinding, both experimentally and numerically. It was found that the ratio of the longitudinal wheel feed to the rotational speed of the workpiece R is most influential on the hardened layer profile. A larger R results in a thicker hardened layer but a more pronounced discontinuity of two adjacent hardening zones. Although a smaller R yields a relatively thinner hardened layer, the non-uniformity could be minimised by the overlapping of the grinding heating. The microstructure of the hardened layer can be altered by the tempering in subsequent grinding heating. A thermal analysis with the aid of the finite element method in conjunction with an experimental microstructural examination showed that the temperature distribution in the tempered region would lead to a variation of the precipitated carbide, and that a higher tempering temperature could result in a higher degree of the carbide precipitation and a more significant reduction of hardness.

Abstract: Precision glass moulding is a technique that enables the production of optical lenses of complex geometries in a single step. However, it has been reported that the product quality highly depends on the properties of a raw material, the design of a die, and the selection of a processing program. This paper aims to reveal the formation mechanism of the residual stresses by optical lens moulding. To this end, a modulus-based constitutive model was developed to integrate with the deformation and stress analyses by the finite element method. The investigation showed that the residual stresses are caused by the variability and heterogeneity of thermal expansion in a lens, but that they can be reduced effectively by decreasing the rate of cooling within the stage from the molding temperature to the glass transition temperature.

Abstract: This paper investigates the effect of the plastic deformation of surface asperities on the interface friction in metal forming involving multi-scale deformation with random surface topography. The equivalent interfacial layer (EIL) introduced by the authors previously was used to integrate the Reynolds equation with the plastic deformation of the randomly distributed surface asperities. The contributions of solid-lubricant interaction, lubricant viscosity and microscopic deformation were therefore included efficiently in a conventional macroscopic finite element analysis. The merit of the method was demonstrated by an investigation into the metal strip rolling, whose friction, lubrication and pressure distribution are otherwise hard to be characterized accurately.

Abstract: High speed cutting (HSC) has been used widely in the metal machining industry. However, applications and investigations have shown that to optimize an HSC process, many issues need to be understood, such as the fluctuations of cutting forces and residual stresses. Extensive studies have found that these fluctuations are originated from the shear banding during chip formation and from the work-material properties influenced by the coupled attack of high strain rate and high temperature rise during cutting. Due to the complexity of the material deformation mechanisms during HSC, both experimental examination and theoretical analysis are essential. This keynote presentation reviews an integral approach of the author’s team to establishing the investigation chain of experimental analysis, constitutive modelling and numerical simulation to tackle the intricacy of HSC-induced deformation. It points out that while an experimental examination can provide insightful understanding of the deformation mechanisms, it is often limited to a narrow range of testing conditions. A numerical simulation can overcome such experimental difficulties through large-scale parametric studies, but can also bring about erroneous results if the constitutive behavior of a workpiece material is improperly described.