Abstract
Extrusion is a compression process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. The open-die extrusion is performed in state the billet is not entirely guided in the die. There are several advantages of the cold op...
Abstract
Extrusion is a compression process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. The open-die extrusion is performed in state the billet is not entirely guided in the die. There are several advantages of the cold open-die extrusion. Grain structure and mechanical properties are enhanced in cold extrusion. And a fairly close tolerances are possible, especially in cold open-die extrusion. Comparing with hot working, little or no wasted material is created.
On the contrary, owing to the considerable deformation associated with extrusion operations, a number of defects can occur in extruded products.
Among the various defects, the barreling defect caused by excessive reduction of area is the most common. The maximum reduction of area at which no barreling defect occur is called as the forming limit. Forming limit can be reduced to trial-error and time of development for cold open-die extrusion process design. Therefore, forming limits is required in the process design for cold open die extrusion.
In the past, this aim has been depend on only the skill and the experience deriving from the shop practise, but the necessity to produce a wide range of complex items requires the use of the finite element method.
The key parameter, strength of material, ratio of length to diameter, die half-angle, reduction rate of extrusion section, etc., which largely influence forming process and the limits range are analyzed by numerical simulation , and experiment has been done to verify the results.
In addition to the study of the forming limits, it should be attention on the method of dividing the cold extrusion dies. Especially in cold extrusion of a non-axially symmetrical shape such as a regular hexagonal section, it is necessary to relieve the concentration of die stress. A split type of dies is required to mitigate the buildup of stress concentrations located at the corners of the hexagonal hole. Generally, the insert of a hexagonal die is made by cutting each corner of a cylinder using a hexagonal hole and then put together into a die and shrink-fitted.
However, split dies face problem when extruding material at the corners of the hexagonal split dies. To address this problem, two types of split dies of a circular-hexagonal die and an double-hexagonal die were evaluated. The effects of the pre-stress ring on the die was compared and analyzed for the two types of die.
In this thesis, predictions of forming limits and die stress in cold open-die extrusion process has been simulated by using commercial FEM code DEFORM-3D. The simulated results were compared with experimental ones and conclude as follows;
1. As the extrusion stress reach the yield strength of the material, barreling defect occur. Therefore, it will be possible to predict forming limits of cold open-die extrusion process by using the yield strength of the materials. These forming limits also apply to non-axisymmetric cold open-die extrusion.
2. Improving the strength of the material, reducing the length-to-diameter ratio, and reducing the half-angle of extrusion die can be increasing the forging limit included. Therefore, forming limits predicted by FEM will be useful for reducing the trial error and optimizing the process design of the open-die extrusion process.
3. In the effects of circle-hexagonal dies and double-hexagonal dies, analyzed results show that using the double-hexagonal dies can extend the lifetime of forging dies. Therefore, it is proposed to apply a new die splitting method of the die design for cold open-die extrusion.
Abstract
Extrusion is a compression process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. The open-die extrusion is performed in state the billet is not entirely guided in the die. There are several advantages of the cold open-die extrusion. Grain structure and mechanical properties are enhanced in cold extrusion. And a fairly close tolerances are possible, especially in cold open-die extrusion. Comparing with hot working, little or no wasted material is created.
On the contrary, owing to the considerable deformation associated with extrusion operations, a number of defects can occur in extruded products.
Among the various defects, the barreling defect caused by excessive reduction of area is the most common. The maximum reduction of area at which no barreling defect occur is called as the forming limit. Forming limit can be reduced to trial-error and time of development for cold open-die extrusion process design. Therefore, forming limits is required in the process design for cold open die extrusion.
In the past, this aim has been depend on only the skill and the experience deriving from the shop practise, but the necessity to produce a wide range of complex items requires the use of the finite element method.
The key parameter, strength of material, ratio of length to diameter, die half-angle, reduction rate of extrusion section, etc., which largely influence forming process and the limits range are analyzed by numerical simulation , and experiment has been done to verify the results.
In addition to the study of the forming limits, it should be attention on the method of dividing the cold extrusion dies. Especially in cold extrusion of a non-axially symmetrical shape such as a regular hexagonal section, it is necessary to relieve the concentration of die stress. A split type of dies is required to mitigate the buildup of stress concentrations located at the corners of the hexagonal hole. Generally, the insert of a hexagonal die is made by cutting each corner of a cylinder using a hexagonal hole and then put together into a die and shrink-fitted.
However, split dies face problem when extruding material at the corners of the hexagonal split dies. To address this problem, two types of split dies of a circular-hexagonal die and an double-hexagonal die were evaluated. The effects of the pre-stress ring on the die was compared and analyzed for the two types of die.
In this thesis, predictions of forming limits and die stress in cold open-die extrusion process has been simulated by using commercial FEM code DEFORM-3D. The simulated results were compared with experimental ones and conclude as follows;
1. As the extrusion stress reach the yield strength of the material, barreling defect occur. Therefore, it will be possible to predict forming limits of cold open-die extrusion process by using the yield strength of the materials. These forming limits also apply to non-axisymmetric cold open-die extrusion.
2. Improving the strength of the material, reducing the length-to-diameter ratio, and reducing the half-angle of extrusion die can be increasing the forging limit included. Therefore, forming limits predicted by FEM will be useful for reducing the trial error and optimizing the process design of the open-die extrusion process.
3. In the effects of circle-hexagonal dies and double-hexagonal dies, analyzed results show that using the double-hexagonal dies can extend the lifetime of forging dies. Therefore, it is proposed to apply a new die splitting method of the die design for cold open-die extrusion.
주제어
#Extrusion, Forming Limits, Open-Die
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