Ting Fai Kong

Prediction of a Billet Shape for Axisymmetric Warm Forming Using Variational Analysis

When designing the warm-forming process, it is the usual practice for the preliminary shapes of billets, blanks, or preforms to be predicted by work experience, the geometry of the finished part, and the law of volume constancy. There was no guarantee that the process would be successful. Therefore, in order to verify the reliability of these traditional estimation methods in preparing the billet shapes, it was necessary to conduct a more intensive study. A set of tools in various combinations of punch and die were used both for numerical simulation and for physical modeling. This produced a number of variations in the die cavity. These variations were analyzed to evaluate the feasibility of axisymmetric warm forming with a predicted billet, made of stainless steel AISI 316L. A comparison of the results of the die fillings and load-stroke curves by simulations with experiments showed that the variational analysis is a promising method to use for predicting the critical shapes of billets in axisymmetric warm forming.

Qualitative study of bimetallic joints produced by solid state welding process

Abstract Forge welding (FOW) and hot pressure welding (HPW) are solid state welding processes. They are capable to be used for joining dissimilar metals. This paper presents a qualitative study of bimetallic joints produced by these two processes under various welding temperatures T and depths of deformation d. The test materials are stainless steel 316L and 6063 aluminium alloy. This combination can take advantage of both excellent wear resistance and light weight. The joint quality was examined by tensile tests and metallographic observation. The quality characteristics such as tensile strength σ and the continuity of diffusion zone are discussed. The results show that the bimetallic joints produced by the FOW are superior in quality to those produced by the HPW.

Numerical Simulation and Experimental Study on Profile-Rolling Process for Micro-Teeth Components

This paper presents a numerical simulation and experimental study on profile-rolling process for micro-teeth components. The target component was made of AISI 304 stainless steel with 6.0 mm outer diameter, and its number of teeth was 25. Three raw material rods with different diameters 5.6 mm, 6.0 mm, and 6.4 mm were investigated in the finite-element simulation. The spindle speed of the rolls was 60 rpm, and the material forward speed was 1.8 mm/s, which were reasonably compatible with the practical profile-rolling conditions. The simulation results showed that the different numbers of teeth were produced by employing different diameters of raw materials. Only the rod with the 6.0 mm diameter could be rolled into the near desired shape. Experimental verification was also undertaken. The numerical simulation was found to be in good agreement with the measured profile of the actual rolled component. The microstructure of the material after rolling was improved also as the hardness of the teeth was 40% higher than that of the core.

Microstructural Effect and Material Homogenization of Thermal-Hydroforming Magnesium Alloy Tubes

The microstrctural evolution pre and post heat treatment is critical to achieve a successful product for metal forming process. This paper aims to investigate the microstructual effect of the magnesium alloy tubes undergone various heat treatment conditions to achieve material homogenization. The heat treatment conditions under various periods of time (1, 2, 6, 12 and 30 hours) at 400 °C were employed to investigate the microstructural effect on hydroforming magnesium tubes. The greatly reduced impurity embedded in grain boundaries and more uniform grain sizes do indicate the improvement of material strength and ductility. To validate the conclusion, corresponding tensile tests at the different temperatures (20 °C and 200 °C) were carried out. The increased engineering strain in two directions (hoop and longitudinal) implies that the microstructural evolution is unquestionably useful to enhance the ductility of the magnesium tubes. Subsequently, the tubes after optimal heat treatment condition at 400 °C for 6 hours were used to further carry out the thermal hydroforming process for validation. The defect-free hydroformed tubes were produced under the same working condition, which is unable to be achieved for tubes without the heat-treatment process.

Flow Stress Experimental Determination for Warm-Forming Process

Warm forming is a manufacturing process in which a workpiece is formed into a desired shape at a temperature range between room temperature and material recrystallization temperature. Flow stress is expressed as a function of the strain, strain rate, and temperature. Based on such information, engineers can predict deformation behavior of material in the process. The majority of existing studies on flow stress mainly focus on the deformation and microstructure of alloys at temperature higher than their recrystallization temperatures or at room temperature. Not much works have been presented on flow stress at warm-forming temperatures. This study aimed to determine the flow stress of stainless steel AISI 316L and titanium TA2 using specially modified equipment. Comparing with the conventional method, the equipment developed for uniaxial compression tests has be verified to be an economical and feasible solution to accurately obtain flow stress data at warm-forming temperatures. With average strain rates of 0.01, 0.1, and 1 /s, the stainless steel was tested at degree 600, 650, 700, 750, and 800 °C and the titanium was tested at 500, 550, 600, 650, and 700 °C. Both materials softened at increasing temperatures. The overall flow stress of stainless steel was approximately 40 % more sensitive to the temperature compared to that of titanium. In order to increase the efficiency of forming process, it was suggested that the stainless steel should be formed at a higher warm-forming temperature, i.e. 800 °C. These findings are a practical reference that enables the industry to evaluate various process conditions in warm-forming without going through expensive and time consuming tests.Copyright

Experimental Study of Optimal Process Parameters for Deformation Welding of Dissimilar Metals

The market trends of automobile, aerospace and marine are towards greater demand on light, strong, economical, and corrosion-resistant parts and components. However, no single metal or alloy can perfectly satisfy all these requirements. This has led to the rapid growth in interest of joining dissimilar metals. For example, inexpensive materials can be conserved to combine with high-strength, high-toughness, light-weight, or excellent corrosion-resistant materials. Solid-state welding is particularly suitable for producing such dissimilar-metal joints because the melting of base metals is nearly avoided, the metals being joined can retain their original properties with less effect on heat-affected zone, and the metallurgical damage of the joints can be minimized. Diffusion bonding has been developed for many years among the solid-state welding processes. Since its processing time is very long (from minutes to hours) and the specific work environment such as a vacuum, inert gas, or reducing atmosphere is required, this process has not been widely accepted and implemented in the industry. Deformation welding is another solid-state welding process. It is similar to the diffusion bonding in which both processes require the application of heat and pressure. Comparing to the diffusion bonding, the processing time of deformation welding is relatively short (within seconds) and the requirements of working conditions are less stringent. However, most literatures on joining dissimilar metals tend to use of diffusion bonding, the previous publications of deformation welding of dissimilar metals are very rare as most studies are focusing on joining similar metals. Hence, deformation welding of dissimilar metals is immature for the industrial applications and the optimal process parameters such as welding temperature, amount of deformation, and contact time are difficult to be determined precisely.

Computer simulation of material flow in warm-forming bimetallic components

Bimetallic components take advantage of two different metals or alloys so that their applicable performance, weight and cost can be optimized. However, since each material has its own flow properties and mechanical behaviour, heterogeneous material flows will occur during the bimetal forming process. Those controls of process parameters are relatively more complicated than forming single metals. Most previous studies in bimetal forming have focused mainly on cold forming, and less relevant information about the warm forming has been provided. Indeed, changes of temperature and heat transfer between two materials are the significant factors which can highly influence the success of the process. Therefore, this paper presents a study of the material flow in warm‐forming bimetallic components using finite‐element (FE) simulation in order to determine the suitable process parameters for attaining the complete die filling. A watch‐case‐like component made of stainless steel (AISI‐316L) and aluminium alloy (AL‐...