Thermo-mechanical Finite Element Method Research Articles

Modeling high-temperature glass molding process by coupling heat transfer and viscous deformation analysis

Glass molding is as an effective approach to produce precision micro optical elements with complex shapes at high production efficiency. Since glass is deformed at a high temperature where the mechanical and optical properties depend strongly on temperature, modeling the heat transfer and high-temperature deformation behavior of glass is an important issue. In this paper, a two-step pressing process is proposed according to the non-linear thermal expansion characteristics of glass. Heat transfer phenomenon was modeled by considering the temperature dependence of specific heat and thermal conductivity of glass. Viscosity of glass near the softening point was measured by uniaxially pressing cylindrical glass preforms between a pair of flat molds using an ultraprecision glass molding machine. Based on the numerical models and experimentally measured glass property, thermo-mechanical finite element method simulation of temperature rise during heating and material flow during pressing was carried out. The minimum heating time and pressing load changes were successfully predicted.

3D FE analysis on the product defects of composites formed by liquid–solid extrusion process

The liquid–solid extrusion process for forming composite products was simulated by the three-dimensional thermo-mechanical finite element method. The mechanism for forming defects was analyzed based on the distribution of temperature and flowage of the work piece. The surface annular cracks occurring at the initial stage was caused by the high deformation temperature together with the axial tensile stress which was caused by uneven axial flow rate. However, the low deformation temperature in the terminal stage led to high resistance of deformation and interfacial strength, resulting in the fracture of fibres during severe deformation. The deformation condition in the middle stage was feasible to obtain composite products that were defect free. The simulation was verified through the comparison of the deforming force between the calculated and the measured one in laboratory conditions. The results indicate that the forming quality depends on the thermal and deformation state of the work piece, which can be controlled by the selection and adjustment of the process parameters.

Viscous pressure bulging of aluminium alloy sheet at warm temperatures

Improving the formability of aluminium alloy sheet metal by using warm or elevated temperature has become a valid approach. In this paper, viscous pressure bulging (VPB) at warm temperature is proposed. The coupled thermomechanical finite element method and experimental method were used to investigate the VPB of aluminium alloy AA3003 at warm temperature. The temperature distributions of sheet metal and viscous medium were analyzed for non-isothermal VPB. The influence of forming temperature on thickness distribution, forming load and failure location of sheet metal were investigated. Research results show the temperature gradient field in sheet metal forms when the initial temperature of viscous medium is lower than that of sheet metal. The formability and failure location of sheet metal changes with initial temperature of viscous medium.

An investigation of tearing failure in fine-blanking process using coupled thermo-mechanical method

In fine-blanking processes, localized deformation is prone to cause tearing failure at the surface of shearing edges, which can influence the quality and functionality of the blanked part. In this paper, a numerical simulation of a fine-blanking process has been conducted by using a coupled thermo-mechanical finite element method. The thermal effect on the material properties has been taken into account in the simulation. On the basis of the results of the simulation, the distributions of equivalent plastic strain and stresses in a fine-blanking process have been analyzed. Moreover, examinations of metallurgical microstructure by means of optical and scanning electron microscopy have been performed. It has been observed that the grains near the shearing edge were highly distorted within a narrow shearing zone. The formation of microvoids under large shear deformation supports the argument that material damage initiates and develops around this local area during the fine-blanking process. It is suggested that tearing failure could initiate from the damaged area of the workpiece adjacent to the tool tips where cracking occurs due to excessive local tensile stress produced by fine blanking attributes and interfacial friction.

Die design for axisymmetric hot extrusion

A comprehensive finite element model is presented to obtain the temperature distribution in the workpiece as well as in the tooling in hot and warm extrusion processes. Thermo-mechanical analysis of hot extrusion process is carried out by combining the comprehensive finite element thermal model with the deformation models [upper bound and rigid-plastic finite element models presented earlier (N. Venkata Reddy, P. M. Dixit and G. K. Lal, J. Mater. Process. Technol. 55, 331 (1995); N. Venkata Reddy, P. M. Dixit and G. K. Lal, ASME J. Engng Ind. 118 (1996)) [1, 2]. The predictions of the combined thermo-mechanical finite element method (TMFEM) are first compared with experimental results to validate the method. Then it is shown that the temperature distribution and the extrusion power obtained by the combined upper bound/finite element method (UBFEM) are in good agreement with those of TMFEM. Since UBFEM takes significantly less computational time than TMFEM, it is used to obtain the optimal die profile at various process conditions by minimizing the extrusion power. A simple fracture criterion proposed by Venkata Reddy et al. [ ASME J. Engng Ind. 118 (1996)] [2] based on the concept of the hydrostatic stress component in the deformation zone falling to zero is used along with TMFEM to predict the die lengths at which the initiation of internal defects takes place. Finally, it is shown that the optimal die profiles satisfy the conditions for prevention of internal defects.