In order to obtain acceptable quality and a high production rate in the hot extrusion process, the deformation behavior related to strength, forming load, temperature, and wear and fatigue properties of the die must be properly considered. Also die designs must properly consider bearing lengths to maintain uniform workpiece velocity at the die exit such that straight extruded parts can be produced. However, the process design of hot extrusion is extremely difficult since these numerous process parameters must be considered simultaneously. The present study for die design of hot extrusion can be divided into three major parts. First, design rules were developed based on experimental and finite element simulation results. Second, in order to reduce the time and cost of initial design, a design system based on the above design rules was developed for the flat die hot extrusion process. Finally, the wear distribution and elastic deformation of the designed dies were evaluated through finite element analysis.
The material properties of aluminum alloys were obtained from hot compression experiments in this study. Using the measured material property, the temperature of workpiece and forming load were estimated from the energy equilibrium equation and upper bound method, respectively. Also, design rules to determine the appropriate number of extrusion exits, the location of exits, and the thickness and diameter of dies were established in consideration of the above calculations of temperature and forming load along with the input data of extrusion geometry and press capacity.
Another design consideration that is required is the possibility of surface cracks. Thus, a limit diagram as a function of temperature and damage factor was established based on hot upsetting experimental results. And finally an investigation to establish a design rule for assignment of bearing lengths in flat die hot extrusion was made in the current study. This process made use of finite element analysis along with an automatic remeshing module. The proposed design rule for bearing lengths of die exit was formed as a function of the cross-sectional thickness and distance from the die center. Also, consideration of the high friction level in end regions of die exits was also included.
Based on these design rules, a design system for the flat die hot extrusion process was developed. This design system is composed of input module for specifying extrusion shape and selecting press capacity, material type, process parameters, and extrusion conditions and design module for determining the appropriate number of extrusion exits, the location of exits, and the thickness and diameter of dies in consideration of the above input data. For verification of the developed design system, the forming load between the design and finite element analysis results were compared. The design system was found to over-predict the forming load by 13 % error. Also, the limit diagram for predicting the possibility of surface cracks was found to be useful for actual extrusion processes and the designed bearing lengths were found to lead to fairly uniform workpiece velocity at the die exit. The current die design system was applied to single/twin channel-section and H-section dies from industry and it was found that the design results were consistent with industrial findings.
Finally, the designed dies were evaluated in terms of wear distribution and amount of elastic deformation. Reliable prediction of wear is very important since it directly affects die life. Archard`s wear model was implemented into the current finite element analysis program for metal forming. The simulation results were found to be consistent with the trend of experimental results available in the literature. Also, with the pressure distribution at the die/workpiece interface obtained from metal forming finite element analysis, further elastic analyses were carried out to investigate the stress distribution and elastic deformations of extrusion dies. From the current study, it was found that the main source of die failure can be either wear or fatigue depending on the die geometry.
