Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Science Town, Daejeon 305-701, South Korea
Abstract
In this work, a modified porthole die for tube extrusion has been developed in order to obtain larger welding pressure than that of conventional porthole dies, and the effect of the improved porthole die on welding pressure has been investigated by performing the finite element analysis on aluminum tube extrusion. The comparison of welding strength of tubes extruded by the modified porthole die with that of tubes made by a conventional porthole die on an expanding test has shown that the tubes from the modified die have been improved in welding strength.
#2002 Elsevier Science B.V. All rights reserved.
Keywords:Hot tube extrusion; Porthole die; Solid state bonding; Welding pressure; Expanding test
1. Introduction
For extrusion of hollow shapes, as in tube or pipe extru- sion, dies with complex structure are employed, such as porthole dies, bridge dies, or spider dies. Parts with hollow cross-sections made by any of the above dies have inevitably one or more weld lines in longitudinal directions because the billet is divided into several sections to flow around the core supports and subsequently welds in the welding chamber of the die. Thus, the failure of hollow extruded products mostly occurs along one of the weld lines when the products are subject to severe internal pressure or expansion in the practical use. Therefore, it is important to increase the welding strength. In general, the welding strength is affected considerably by the following factors: billet temperature, extrusion speed, dies shape, etc. Among these factors, the most important factor is welding pressure. Thus, the port- hole die is designed so as to increase the welding pressure in the welding chamber. The flow through the porthole die is so complicated that it is required to manufacture the porthole die in a systematic way.
Mehta et al.[1]obtained velocity, strain distribution and stress distribution based on the theoretical velocity field and on the flow field observed experimentally. Both the theore- tical and experimental results are compared with each other.
Su-Hai and Chao-Shun[2]predicted the deformation beha- viors and forming load of the hot extrusion process for a circular tube by combining the upper bound method with the finite difference method. Moreover, the welding strength of the extruded tube was evaluated by the expanding and flattening tests. Akeret [3] studied the change of micro- structure and mechanical properties of the welding portion of products which are extruded through a porthole die. Based on the steady-state isothermal assumption, Park [4] pro- posed the domain-decomposition method and simulated the extrusion process of an underframe part of a railroad vehicle with multiply-connected profile. Xie et al. [5] divided the extrusion process through the porthole die into a dividing flow stage and a welding stage and could visualize the material flow experimentally at each stage. Mooi et al.
[6] analyzed the circular tube extrusion process by using a thermal FE code based on an ALE algorithm and calcu- lated the deformation of the die.
In this study, a modified porthole die for tube extrusion has been developed in order to obtain larger welding pres- sure than that of conventional porthole dies, and the effect of the improved porthole die on welding pressure has been investigated by performing the finite element analysis on aluminum tube extrusion. In order to reduce time and cost required for die modification, the rigid–plastic finite element method has been adopted to calculate the pressure in the welding chamber. The results of two-dimensional and three- dimensional analyses have shown that the pressure of the
*Corresponding author. Tel.:þ82-42-869-3214; fax:þ82-42-869-3210.
E-mail address:[email protected] (D.Y. Yang).
0924-0136/02/$ – see front matter#2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 7 1 7 - 3
chamber of conventional porthole dies is insufficient to ensure high bonding strength of the welded interface. The comparison of welding strength of tubes extruded by the modified porthole die with that of tubes made by a conven- tional porthole die on an expanding test has shown that the tubes from the modified die exhibit improved welding strength.
2. FE analysis of tube extrusion
In the present study, the conventional porthole die is modified so as to increase the welding pressure of the welding chamber through two-dimensional and three- dimensional FE analyses by using a steady-state extrusion analysis FE code based on the rigid–plastic FE method proposed by Park[4].
2.1. Two-dimensional approximate analysis of tube extrusion using porthole dies
Fig. 1shows a picture of the porthole die that is used in practical application. The portion with an elliptic mark indicates the welding interfaces, at which divide material is bonded together.
The welding chamber of the porthole die is approximated in two-dimensional shape as a longitudinal cross-section containing the leg described inFig. 2.
The welding pressure is investigated with respect to the change of length of the welding chamber and the shape of the leg. These results are reflected on the modified porthole die.
H1,H2,H3,H4,H5,HPare selected as design variables indicated inFig. 2. According as these variables are changed as follows, the welding pressure is investigated.
(1) Change of the width of the leg (change of H1,H3).
(2) Change of the angle of the leg (change of HP).
(3) Increase of the length of the welding chamber (H4þH5 fixed,H4is increased).
(4) Increase of the length of the welding chamber (onlyH4
is increased).
Figs. 3–6 show the pressure distributions for each of above four cases.Z-axis represents a location at the welding interface from the die exit.
From Figs. 3–6, following results are obtained:
(1) H1 does not influence the welding pressure, and the shorterH3is, the higher is the welding pressure.
(2) As the Hpincreases, the angley is also increases and the welding pressure near the tip increases.
Fig. 2. Two-dimensional model of the welding chamber.
Fig. 3. Variation of pressure in welding chamber as change ofH1,H3.
Fig. 4. Variation of pressure in welding chamber as change ofHP.
Fig. 5. Variation of pressure in welding chamber as change ofH4,H5(constantH4þH5).
Fig. 6. Variation of pressure in welding chamber as change ofH4,H5(constantH5).
(3) As the height of the chamber is bigger, the welding pressure increases. In addition, the area on which the divided material is pressurized is increased.
Thus, the above results should be reflected on modifica- tion of the porthole die. However, two-dimensional calcula- tion is performed on a longitudinal cross-section of the welding chamber. In the three-dimensional shape, the real die exit is not on this plane, and material flow moves in the direction normal to this plane. The verification of the two- dimensional approximation must be performed through the three-dimensional analysis.
2.2. Three-dimensional analysis of tube extrusion using porthole dies
A modified porthole die is developed based on the two- dimensional calculation. That is, the height of the welding chamber is added, the angle of the leg,yis increased, and the
tip of the leg is made as sharp as possible. The welding pressure is increased by narrowing the width of area on which material contacts.
Fig. 7 shows the three-dimensional modeling of the design and design variables which should be modified.
The three-dimensional shape is modeled by I-DEAS Master Series.
The amount of the change in the design variables is as follows:
1. The height of the welding chamber,H, is increased from 15 to 20 mm.
2. The width of area on which material contacts, D, is reduced from 18 to 14 mm.
3. The angle of the legy is increased from 148to 208.
4. The tip of the leg,H3, is made as sharp as possible.
Extrusion of a round tube with outer diameter of 15.88 mm and a wall thickness of 1.24 mm is considered.
The analysis is carried out with 1/6 section of the workpiece because of the symmetry. The ram speed is 3 mm/s, the workpiece material used in simulation is Al3003, and the friction factor is taken to be 0.3. The numerical analysis is performed with a steady-state isothermal assumption at 5008C. The constitutive relation is given as follows at 5008C
s¼24:71_e0:1619MPa
Fig. 7. Proposed design variables.
Fig. 8. Pressure distribution in the welding chamber of the both dies.
Fig. 9. Photos of the modified porthole dies.
Computation is performed on Cray 90 computer. It takes 10,000 CPU seconds for the case of the conventional port- hole die and 122,100 CPU seconds for the case of the modified porthole die. Fig. 8shows the pressure distribu- tions in the welding chamber of both dies resulting from three-dimensional calculations. The welding pressure in the chamber of the modified porthole die is higher than that of the conventional porthole die. The higher pressure of the modified die appears on the broader area than that of the conventional porthole die.
3. Strength of extruded tubes
3.1. Extrusion of tubes using the conventional die and the modified die
Based on the two-dimensional and three-dimensional calculations, a modified porthole die is manufactured.
Fig. 9shows the photos of the modified porthole dies.
In order to avoid fracture of tip of the leg, a porthole die is manufactured so that the thickness of the tip is about 2.2 mm. A billet of Al3003 is extruded at 7–8 in./min (2.96–3.39 mm/s) of ram speed at 4808C. After one billet is extruded, a tube of about 120 m in length is obtained. The 9 m of the front part of this long tube is abandoned because of the poor quality by non-uniform flow before arriving at the steady state.
3.2. Expanding test of tubes
The strength of the tube from the modified porthole die is compared with that of the tube from the conventional port- hole die by an expanding test. Test specimens are obtained from the middle and rear parts of the initial long tube.
Specimens of the middle portion and the rear portion are taken from the locations between 30 and 40 m, and between 80 and 90 m, respectively.
The strength of the specimen is examined by the expand- ing ratio. The expanding ratio is defined as a¼dm=d0, wheredmis the maximum diameter when fracture occurs after expanding,d0is the initial outer diameter of tubes.d0of the tube is 15.88 mm.Fig. 10shows a schematic description
of the device for examining the expanding ratio and the expanding process. To avoid the instability of buckling during the expanding process, the device is designed to support the wall of tubes inside and outside. The conical punch is moving upward and tubes are expanded.
Table 1 shows the expanding ratios obtained by the expanding test. Expanding ratio, a, of the tubes for the modified porthole die shows an increase of 0.08 over the tubes from the conventional porthole die for the mid- portion, and 0.1 for the rear-portion, respectively.
4. Conclusions
The improvement of the welding strength of tubes that are extruded using the porthole die has been investigated.
Two-dimensional and three-dimensional analyses of the welding chamber have shown that the welding pressure of the chamber is increased as the welding chamber becomes longer and narrower, and the web sharper.
Thus, the design variables of the porthole die have been changed as follows:
1. The height of the welding chamber,H, is increased from 15 to 20 mm.
2. The width of area on which material contacts, D, is reduced from 18 to 14 mm.
3. The angle of the legy is increased from 148to 208.
4. The tip of the leg,H3, is made as sharp as possible.
The modified porthole die has been developed according to the design variables proposed above. When the welding strength of tubes extruded by the modified porthole die is compared with that of tubes made by a conventional porthole die by the expanding test, the expanding ratio of the tubes from the modified die has the value of 2.03
Fig. 10. Schematic description the expanding device and expanding processes.
Table 1 Expanding ratios
Tubes from the conventional porthole die Mid-portion 1.95 Rear-portion 1.88 Tubes from the modified porthole die Mid-portion 2.03 Rear-portion 1.98
[1] H.S. Mehta, A.H. Shabaik, S. Kobayashi, Analysis of tube extrusion, J. Eng. Ind. Trans. ASME 92 (1970) 403–411.
die deformation calculations of aluminium extrusion, J. Mater.
Process. Technol. 88 (1999) 67–76.
