alloy extrusion process to determine the state of stress, strain and the temperature through square and round dies. Li et al. [2004] studied the temperature evolution during the extrusion of 7075 aluminum alloy by means of 3D FEM simulations.
To simulate the bending of the extruded products in extrusion process, Muller [2006] used an Arbitrary Lagrangian Euler (ALE) finite element code, Press Form ver.1.4. Author observed that the friction force inside the bending device do not influence the outflow velocity difference and thus does not influence the desired radius, but it increases the strain within the bending fixture and the required press capacity. Lepadatu et al. [2006] used finite element model to predict wear of the die.
They presented the investigation on the statistical process variation of the tool wear progression during metal extrusion process. Chen et al. [2007] used finite volume method to simulate irregular Al alloy profile extrusion process. As the extrusion ratio is too large (more than 25), the plastic deformation is quite severe. Hence, it is very difficult to get satisfactory results using traditional Lagrangian finite element method to simulate that kind of deformation. Authors built a computer aided optimization (CAO) model based on orthogonal experiments, artificial neural network (ANN) and genetic algorithm (GA) and utilized them to obtain optimal parameters of Al alloy profile extruding process.
The finite element method is widely used for analyzing the extrusion
methods used for improving the performances of the extrusion process are discussed here.
2.6.1 Optimization of Die Profile
An extensive literature exists on optimal die profile design based on the power minimization by using slip-line field method, upper bound method and more recently by finite element method.
2.6.1.1 Slip-line field method
Sotrais and Kobayashi [1968] obtained an optimal streamlined die profile for frictionless axisymmetric extrusion by using slip-line field method. They conducted experiments on pure lead. The power required by the designed streamlined die was found to be less compared to the conical die of same axial length.
2.6.1.2 Upper bound method
Chen and Ling [1968] obtained the optimal die length for axisymmetric extrusion of rods through cosine, elliptic and hyperbolic dies, whilst Chen [1970] carried out same study for the plane strain extrusion. Zimerman and Avitzur [1970] obtained the optimal die angles for axisymmetric extrusion of rods through the conical dies by assuming generalized plastic boundaries. Yang et al. [1987] and Yang and Han [1987] obtained the optimal die length for the fourth order polynomial dies. Reddy et al. [1996] combined the upper bound and finite element method and obtained the optimal die profiles for axisymmetric extrusion process.
Juneja and Prakash [1975] obtained the optimal die angle using spherical velocity field for extrusion of polygonal sections from similar shaped billets through conical dies. Yang and Lange [1984] obtained the optimal length of the streamlined die with upper bound method at various process conditions. Gunasekera and Hoshino [1982, 1985] proposed an upper bound method and obtained the optimal condition to extrude the polygonal sections through conical and streamlined die. The streamlined die was found better.
2.6.1.3 Finite element method
Balaji et al. [1991] proposed a model which predicts the deformation field, optimal die geometry and plastic boundary using finite element method. They obtained the optimal die profile which minimizes the redundant work during extrusion. Joun and Hwang [1993] developed an optimization scheme for obtaining optimal die profile which consumes the least power in axisymmetric extrusion process.
In most of the research works, the optimal die design is obtained by minimizing the extrusion power. The reduction of extrusion power is definitely advantageous for extrusion industry but some amount of redundant work during the process is also necessary to improve the strength and quality of the extruded products. Less number of research works has been carried out on the optimal die design based upon the improvement of strength and quality of the products. Jo et al.
[2001] designed the optimal tool shape in metal forming for the improvement of the mechanical properties of the products by providing uniform microstructure distribution. The proposed method was validated with hot extrusion experiment and finite element analysis. The control of exit velocity of the extruded product is necessary to minimize the distortion of profile. Lin and Ransing [2009] proposed a layout design approach using geometry based die and length design methodology to minimize the variation in exit velocity of the extruded product. This proposed methodology is for single-hole extrusion of any die profile.
2.6.2 Reduction of Friction
The friction encountered at the die-billet interface and billet-container interface effect significantly the extrusion load and quality of the extruded product. The optimized die profile can solve the problem up to some extent but researchers also focused to reduce the friction at the die length and die land region. Bjork et al.
[1999] coated the extrusion dies by TiC+TiN by chemical vapour deposition technique for hot extrusion. The extrusion load was found to be less in extrusion with coated dies as compared to the extrusion load obtained with uncoated dies. The coating also enhances the die life.
2.6.3 Use of Vibration in Extrusion Process
It has also been observed that vibrating the die can reduce the friction encountered during metal forming. The external and internal friction occurs in the plastic forming of metallic materials. These frictions can be reduced by ultrasound oscillation under suitable conditions. Few studies have been published relating to the behavior of metallic materials during plastic forming with ultrasound. Pohlman and Lehfeldt [1966] studied experimentally the influence of ultrasonic vibration in wire drawing process in reducing the internal friction of the metal and the external friction between tool and work-piece. It was observed that the reduction in drawing force is proportional to the amplitude with which the drawing die oscillates.
Eaves et al. [1975] reviewed the application of ultrasonic vibration to deforming metals. It was observed that the change in coefficient of friction often occurs as a result of (i) pumping of lubricant, (ii) increase in chemical reactivity of the surface or lubricant, (iii) softening or melting of asperities and (iv) separation on surface allowing redistribution of lubricant. Work can be done against friction by reversing the friction vector or by reducing the component of friction vector along the direction of deformation. The authors also concluded that the rise in temperature due to dissipation of vibration energy can be efficiently used to change the metallurgical properties of the deforming metal.
Siegert and Ulmer [2001] studied the influence of the ultrasonic vibration on the reduction of friction during wire and tube drawing process. The wire drawing die was oscillated parallel to the drawing direction at ultrasonic frequencies in range of 20-22 kHz. The reduction in drawing force was due to the ultrasonic amplitude. In case of tube drawing, the mandrel was vibrated and results obtained were similar to that of wire drawing. Authors observed that with certain drawing velocity range, smoother wire surface was obtained with higher vibration amplitude. Murakawa and Jin [2001] investigated the effect of radial and axial ultrasonic vibration on the wire drawing die and compared the results with conventional drawing process. Radial vibration was found to be an effective measure against the use of lubricant and also for increasing the critical drawing speed in ultrasonic drawing process. Hayashi et al. [2003] carried out finite element analysis of conventional, axial and radial vibration in wire drawing process. The drawing force and stress-strain distributions
in the drawn wire were studied. No significant difference in equivalent plastic strain for conventional, axial and radial vibration drawing process was observed. The reduction of drawing speed depends on the amplitude of vibration.
The above discussed research works on wire drawing process and upsetting process shows the effectiveness of ultrasonic vibration in reduction of forming load and better product quality. The effect of ultrasonic vibration on extrusion process was studied by Akbari Mousavi et al. [2007]. They carried out the finite element analysis using ABAQUS to find out the effect of extrusion speed, vibration amplitude, vibration frequency and frictional condition on the extrusion force. It was observed that when the extrusion speed is below the critical speed, the extrusion force and the material flow stress were reduced by using the ultrasonic vibrations.
The average extrusion force decreases by reducing the extrusion speed or increasing the amplitude of the vibration. The vibration frequency was found to be less effective than the vibration amplitude in reducing the extrusion force. The axial ultrasonic vibration influences the friction force between the die shoulder and the flow material.
Huang et al. [2002] studied the influence of ultrasonic vibration on the interfacial boundary condition and thermal effects in soft solid forming operation.
Plasticine was used in the upsetting experiments. The upsetting process was also simulated with finite element package, ABAQUS. The authors observed that by applying short longitudinal ultrasonic pulse to the die, reduction in the mean forming force takes place.
The effective use of ultrasonic vibration can be a better alternative in eliminating the lubrication in forming process. However, the experimental research on the effective use of vibration in extrusion process is very limited and needs to be explored.