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Journal of Materials Processing Technology 194 (2007) 38–45

Extrusion of sections with varying thickness through pocket dies D. Lesniak ∗ , W. Libura AGH University of Mining and Metallurgy, Krakow, Poland Received 31 August 2006; received in revised form 13 March 2007; accepted 28 March 2007

Abstract The quality of extruded product depends on many technological factors and the proper die design. This study investigates the influence of pocket die geometry on the flow behaviour during extrusion of profiles with varying thickness and, in consequence on its geometrical stability and surface quality. Material microstructure and the mechanical properties on the extrudates cross-section were also explored. The extrusion force was measured for all the dies tested. Based on physical modelling and extrusion trials on AlMgSi (6060) alloy, it was found that pocket dies considerably change the way of the metal flow in comparison to conventional flat dies. Properly designed pocket die allowed obtaining more uniform metal flow through the die, resulting in improved geometrical stability of the profile extruded. The use of pocket dies is also associated with more advantageous state of stresses in the die orifice, which in consequence enhances both material deformability and surface quality of extrudates. The higher values of micro-hardness were obtained while using the pocket dies. Moreover, the pocket dies contribute to the small increase in extrusion force, compared to the flat dies, whereas a considerable decrease in the extrusion force can be expected while extruding hard deformable materials. The criterion for pocket design was proposed and evaluated. © 2007 Elsevier B.V. All rights reserved. Keywords: Extrusion; Pocket dies; Material flow; Product quality; Extrusion force

1. Introduction To-date requirements of the aluminium market are oriented to profiles of complicated shapes demonstrating the highest possible quality. The quality of extrudates depends on many technological factors and the proper die design. In order to obtain an appropriate shape, dimensional tolerances, improved surface quality, the fine homogenous structure and uniform mechanical properties on extrudates cross-section, the metal flow through the die opening must be as uniform as possible. This is especially important when extruding profiles with varying thickness, where a non-uniform metal flow and a high velocity gradient in the die opening occur. The extrusion practice shows that the correctly designed pocket die provides very successful control of metal flow which, in consequence, enhances the product quality. Basically, the pocket die design has been evaluated empirically. Therefore, a great challenge for today is to establish the design rules for the pocket dies with the aim of reducing the costly die trials.

Corresponding author. Tel.: +48 126 173 196; fax: +48 126 172 632. E-mail address: dlesniak@vp.pl (D. Lesniak).

0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.03.123

The available research [1–11] confirms the positive changes in the mode of the metal flow when using the pocket dies of proper geometry. Zasadzinski et al. [1] examined extrusion of section having varying wall thickness and proposed the use of pocket at the thicker part of the section only, guarantying its dimensional stability. Misiolek and Prats [2] determined the pocket height to diameter ratio H/D, that gives the most uniform metal flow and the minimum of extrusion force. Nakanishi et al. [3,5] revealed that the pocket dies affect the geometry of the deformation zone and lead to a more uniform both velocity distribution and strain field. Duplancic et al. [4] have found the beneficial influence of the dies equipped with multi-step pockets on dimensional tolerances of a semi-closed profile. Interesting results of FEM modelling regarding the influence of pocket die configurations on metal flow have been presented by Li et al. [6,7]. In their study, they have shown that pocket angle (angle between square pocket die face and its wall) plays an important role, influencing metal flow velocity, whereas pocket volume has less significant effect on metal velocity. They have stated that conical pocket can more effectively increase metal flow velocity, compared to conventional square and multi-stepped pockets. The latest research regarding the effect of pocket dies on multi-hole die extrusion was performed by Peng and Shepard [8]. Based on the three-dimensional FEM calculations, they


D. Lesniak, W. Libura / Journal of Materials Processing Technology 194 (2007) 38–45

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Table 1 Geometry of pocket dies Profile 1 Sign of die Pocket width w2 , (mm)

A1 1

Profile 2 B1 3

C1 7

A2 2.5

B2 6

C2 13

Table 2 Alloy composition in wt.% Mg

Si

Fe

Zn

Cu

Mn

Al

0.35–0.45

0.33–0.45

≤0.04

0.15

0.1

0.1

Rest

Table 3 Conditions of the extrusion trials Billet material Container diameter Extrusion ratio Ram speed Billet temperature Container and die temperature

6060 110 mm 59.4 (profile 1), 33.9 (profile 2) 2.8 mm/s (profile 1), 4.9 mm/s (profile 2) 470 ◦ C 420 ◦ C

the pocket. The criterion was built on the concept of local volume rates of metal flow. The volume rate of metal flow is defined as: W =S×V where S is cross-section of part of the section, V is velocity of the metal flow.In particular case, when extruding sections with different wall thickness, we have (Fig. 1): Fig. 1. Scheme of the pocket die applied to extrusion of section of varied thickness on its cross-section.

have concluded that balanced both material flow and temperature distribution as well as structural homogeneity of extrudates can be achieved by a suitable location of the pockets. Even a small offset of the pocket can cause a significant change in the metal flow during extrusion and worsen the extrudates quality. The authors of this work carried out the FEM calculations, physical modelling and industrial extrusion of sections with different wall thickness, aimed at evaluating method for optimal pocket geometry [9–11]. Results of these investigations confirmed that metal flow, geometrical stability of extrudates and extrusion force parameters are strongly influenced by geometry of the pocket dies. The criterion for pocket design was built on the concept of local volume rates of the metal flow. The aim of the investigations was to determine the influence of the pocket die geometry on the metal flow during extrusion of sections with varying thickness and consequently on its geometrical stability and surface quality. Mechanical properties on the extrudates cross-section were also explored. The extrusion force was also measured for all the dies used in the experiments.

S1 × V1 = S1 × V2

 SV  1 1 S2 × V2

V1 =V2

and V1 S1 = V2 S2 where S1 and S2 are cross-sections of the thick and thin-walled parts of the profile, S1 and S2 are cross-sections of the pocket parts concerning the thick and thin-walled parts of the profile respectively, V1 and V2 are the velocities of metal leaving the thick and thin-walled parts of the die orifice, V 1 and V/2 are the velocities of metal flowing into different pocket parts. The distance between wall of the pocket and the die oriffice at the thin profile part w2 was varied, while it remained constant at the thick profile part w1 (Fig. 1). Geometry of applied constructional variants of pocket dies is presented in Table 1. Dimensions of the pockets of type B, B1 and B2 were chosen by using the proposed criterion for pocket design. The pocket height H was established to be equal to the thicker profile part. The bearing length of 2 mm was assumed for all the dies. The dies tested are shown in Fig. 2. Extrusion trials were carried out on a 8 MN hydraulic horizontal extrusion press (Fig. 3) having container of 110 mm in diameter. The press was equipped with a modern measuring system, enabling to record the total extrusion force FT . Billets of 107 mm in diameter and 135 mm long were made of 6060 aluminium alloy. The chemical composition of the alloy is presented in Table 2. The billets were extruded to a half of its length to perform further investigations of metal flow. All the process parameters are shown in Table 3.

2. Experimental work

3. Physical modelling Experiments consisted in extrusion of sections, shown in Fig. 1, and having varying thickness—profile 1 (wall thickness ratio g1 /g2 = 6:2) and profile 2 (wall thickness ratio g1 /g2 = 12:2), with the use of flat die and different pocket dies. It was assumed that the distance between the wall of pocket and the die oriffice should base on proper selection of metal expense in different parts of

A mixture of plasticine with chalk [12] was used to model the behaviour of a hard deformable material during extrusion. Such materials are extruded with relatively low speed to prevent


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D. Lesniak, W. Libura / Journal of Materials Processing Technology 194 (2007) 38–45

Fig. 2. Example dies applied to extrusion of profile 1 (on the top) and profile 2 (on the bottom).

(pocket C), while it remained constant at the thick profile part w1 (1 mm). 4. Results and discussion 4.1. Metal flow

Fig. 3. Scheme of the extrusion horizontal press used in experiments.

surface cracking. Usually, the pocket dies are seldom used for extruding hard deformable materials. Physical modelling trials were performed on the vertical hydraulic press, equipped with a measuring system, able to record the total extrusion load. The main goal of these experiments was to check the pocket dies geometry on the formation of cracks and geometrical stability of extrudate. The profile shape was similar to that reported above (wall thickness ratio g1 /g2 = 6:2). Samples of 80 mm in diameter and 120 mm in length were made of black and white layers of chalk-hardened plasticine. All the process parameters are shown in Table 4. A flat die and different pocket dies were used. The distance between wall of the pocket and the die oriffice at the thin profile part w2 was varied; 1 mm (pocket A), 3 mm (pocket B) and 7 mm Table 4 Conditions of the physical modelling of extrusion process Billet material Container diameter Extrusion ratio Ram speed Billet temperature

70% plasticine + 30% chalk 80 mm 50.2 0.5 mm/s 20 ◦ C

Figs. 4 and 5 present the macrostructures of material in the billet rest in case of extruding through flat die (Figs. 4a and 5a) and through different pocket dies (Figs. 4b–d and 5b–d). The material from the left side of the billet feeds the thicker part of the profile. One can notice that there is an insignificant difference in the mode of the material flow, depending on the size of a profile extruded. The flow pattern in Figs. 4a and 5a, obtained for the flat die shows an asymmetrical deformation zone (area bounded with broken lines). This means that a greater volume of the billet material supplies thicker part of the profile. A bigger dead metal zone appears within the billet at the thin profile part (right side). It can be seen in these figures that a large-grained zone inside the deformation zone is observed (bounded with solid lines). This unexpected phenomenon results from the non-uniform metal deformation. The mode of the metal flow changes when the pocket dies are applied. Patterns in Figs. 4b–d and 5b–d indicate that geometry of both deformation zone and dead zone depends on the type of the pocket used. In case of the largest pocket C, there is an almost ideal symmetry of the shape of deformation zone resulting from the uniform material flow. In addition, these dies produce almost symmetrical thick-grained zones inside the billet. Fig. 6a and b present the pictures of material flow in the billet rest in case of extruding through flat die (Fig. 6a) and through the pocket die B (Fig. 6b). The influence of pocket die width w2 on the range of dead zones in the container was shown in Fig. 7.


D. Lesniak, W. Libura / Journal of Materials Processing Technology 194 (2007) 38–45

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Fig. 4. Macrostructures of material in the billet rest while extruding profile 1 by using different die configurations: (a) flat die; (b) pocket die A1 ; (c) pocket die B1 ; (d) pocket die C1 .

Fig. 5. Macrostructures of material in the billet rest while extruding profile 2 by using different die configurations: (a) flat die; (b) pocket die A2 ; (c) pocket die B2 ; (d) pocket die C2 .


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D. Lesniak, W. Libura / Journal of Materials Processing Technology 194 (2007) 38–45

Fig. 6. Pictures of material flow in the billet rest for different die configurations: (a) flat die; (b) pocket die B.

As it can be seen, the use of pocket dies leads to the decrease of dead zones range (increasing the dead zones angle), compared to flat die. Moreover, there is a tendency to even the range of dead zones in the container (metal flow) with increasing the pocket die width.

Fig. 8. Comparison of the extrudates curvature (profile 1), when applying a flat die (on the left), pocket die B1 (in the middle) and pocket die C1 (on the right).

4.2. Shape stability of extrudates Figs. 8 and 9 show the comparison of extrudates curvature (profile 1 and 2)—obtained for different die geometry. These observations indicate that in case of flat die the material flows faster at the thick part of the profile than at the thin one, leading to formation of profile curvature. The significant improvement of geometrical stability of extrudates can be observed while using the optimised pocket die (extrudates in the middle) for both profiles tested. The application of too large pockets at the thin profile part leads to the

Fig. 7. The influence of pocket die width w2 on the range of dead zones in the container.

Fig. 9. Comparison of the extrudates curvature (profile 2), when applying a flat die (on the left), pocket die B2 (in the middle) and pocket die C2 (on the right).


D. Lesniak, W. Libura / Journal of Materials Processing Technology 194 (2007) 38–45

Fig. 10. Influence of the pocket width on extrudates bending.

faster metal exit speed from this very part of the die. In consequence, profile starts to bend again, but towards the opposite side (towards thicker part of its cross-section). The distortion of each extrudates was measured on 1 m length from the die face. The results are presented in Fig. 10. A positive sign means that the profile was bending towards thinner part of its cross-section. Optimal pocket width, to achieve the straight extrudate can be found for both profiles from the pre-

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sented graph. The optimal value of pocket width is equal to 3.5 mm for profile 1 (for g1 :g2 = 6:2) and about 5 mm for profile 2 (for g1 :g2 = 12:2), respectively. Fig. 11a–d show the profiles extruded through a flat die (Fig. 11a) and pocket dies of different geometry (Fig. 11b–d). As it can be seen, a strong distortion of the profile and cyclic large cracks occur on its surface when extruding through a flat die. This is a result of inhomogeneous material flow, generating a distinct velocity gradient and unbeneficial state of stresses in the die orifice. The tensile stresses in the outer layer of the extrudate are responsible for the material cracking. The die with a small pocket (type A) has a small effect on a change in the material flow and on geometrical stability of extrudate. Moreover, many cracks still appear on its surface, especially in the thin-walled part of extrudates. Optimal pocket geometry was found in case of the pocket type B (with a medium width), which ensures good quality of extrudate. This suggests that the most beneficial state of stresses and the homogeneous material flow can be expected for this geometry. As it results from Fig. 11d, too wide pocket worsens the material flow and the surface cracks starts to appear again. Besides, the profile bends again but in the opposite side. 4.3. Extrusion force Figs. 12 and 13 show the results of measured extrusion force as a function of stem stroke for extrusion of profile 1 (Fig. 12)

Fig. 11. Material cracking on extrudates surface when applying dies of different geometry: (a) flat die; (b) pocket die A; (c) pocket die B; (d) pocket die C.


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D. Lesniak, W. Libura / Journal of Materials Processing Technology 194 (2007) 38–45

Fig. 14. Extrusion force as a function of the pocket width. Fig. 12. Extrusion force as a function of stem stroke while extruding profile 1 from 6060 alloy for different die geometry.

and profile 2 (Fig. 13), respectively. The use of all the pocket dies is associated with a little increase in extrusion force compared with traditional flat-faced dies. This is due to additional friction on dead metal zone surfaces in the pocket. As it is seen from Fig. 2 the considerable changes in the geometry of the dead zone within the billet occur, influencing in this way extrusion load. Moreover, the larger changes in extrusion force have been observed in case of the extruding profile 1 (with smaller wall thickness’ ratio) because of the greater value of the extrusion ratio. Influence of the pocket width on the extrusion force is shown in Fig. 14. The highest values of the extrusion force were found for the smallest pocket widths (type A), probably as a result of high friction forces in the narrow pockets. In this case the pocket acts as an additional bearing land and worsens the metal flow to the die orifice. A minimum of the extrusion force was found for pockets of type B, with the medium widths—(3 mm

in case of pocket B1 and 6 mm in case of pocket B2 , respectively). These observations indicate that there is a narrow range of pocket dimensions, which provide optimal force parameters of the process. Extrusion force was measured in the physical modelling for all the dies tested. The results shown in Fig. 15 indicate that the use of pocket dies in extrusion of hard deformable materials contributes also to decreasing the maximal force in the process. This is associated with changes in the material flow in the container. As it was found in the model experiments (Figs. 6 and 7) the range and, at the same time, the volume of the deformation zone decreases when using pocket dies, compared to the flat die. As a result, the lower extrusion force component connected with overcoming the material resistance to flow in the deformation zone can be expected. Moreover, the surface of the dead zones in the container decreases, resulting in lowering the frictional forces. This depicts the opposite tendency to that observed in easily deformable materials. This observation should be verified in industrial test performed on the hard deformable alloys. However, one specific feature is confirmed in both experiments

Fig. 13. Extrusion force as a function of stem stroke while extruding profile 2 from 6060 alloy for different die geometry.

Fig. 15. Maximal extrusion force as a function of the pocket width for plasticine based hard deformable material.


D. Lesniak, W. Libura / Journal of Materials Processing Technology 194 (2007) 38–45

Fig. 16. Material micro-hardness across an extrudate (profile 2) as a function of the pocket width (near thin profile part).

reported in this paper—regardless of the material used, the smallest dimensions of the pocket result in an increase of the extrusion force, as obtained for the pocket of type A. 4.4. Mechanical properties The results of micro-hardness across an extrudate for different dies are presented in Fig. 16. In general, material extruded through the pocket dies reveals a little higher micro-hardness values in comparison with flat die. Thick part of the profile demonstrates higher micro-hardness than the thin one. This tendency tends to increase as the pocket width increases. 5. Conclusion Based on physical modelling and experimental trials performed on the 6060 sections having diversified wall thickness the following conclusions can be drawn: (1) The pocket dies are very useful in control of the metal flow, especially while extruding sections of large differences in wall thickness, they are very efficient in balancing the material exit speed, compared to the traditional flat dies. (2) Pockets affect the range of dead metal zones in the billet, and thereby, enable to even the metal velocity distribution within the deformation zone, resulting in the improved geometrical stability of the profile extruded. (3) There is a narrow range of the pocket width which causes considerable changes in the metal flow. Increasing the pocket width just over the mentioned range slightly influences the metal flow, which was confirmed by the measurements of extrudates bending. (4) The use of pocket dies is also associated with more beneficial state of stresses in the die orifice, leading to higher

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material deformability and better surface quality of extrudates. An optimal pocket die geometry leading to the best surface quality of the product can be found from the criterion of the volume rate control. In addition, applying either too narrow or too wide pockets can worsen the extrudates surface quality. (5) Higher values of micro-hardness were obtained while using the pocket dies with the medium and large widths. (6) The pocket dies contribute to a small increase in the extrusion force, while extruding materials of high extrudability. It was found that the increase in extrusion force is of about 15–20% compared to flat dies. This is due to additional friction in the pocket and changes in dead zone geometry in the billet. (7) A decrease in the maximal extrusion force can be expected during extrusion of hard deformable materials, probably as a result of the changes in material flow in the container and dividing the deformation process into two stages. References [1] J. Zasadzinski, J. Richert, W.Z. Misiolek, Physical modelling pertaining to extrusion of asymmetric shapes, J. Mater. Shaping Technol. 7 (2) (1989) 113–116. [2] A.E. Prats, W.Z. Misiolek, Analysis of metal flow in weld pocket dies, in: Proceedings of 6th International Aluminum Extrusion Technology Seminar, t. I, Chicago, 1996, pp. 75–78. [3] K. Nakanishi, H. Koba, S. Kamitani, Metal flow control in hot extrusion of aluminum alloy using the pocket hole dies, in: Advanced Technology of Plasticity, Proceedings of 6th ICTP, t. III, N¨urnberg, 1999, pp. 1833–1838. [4] I. Duplancic, M. Mioc, Z. Bracic, Case studies on control of metal flow in pre-chamber dies, in: Proceedings of 7th International Aluminium Extrusion Technology Seminar, t. II, Chicago, 2000, pp. 177–186. [5] K. Nakanishi, S. Kamitani, T. Yang, H. Takio, M. Nagayoshi, Material flow characteristics in hot extrusion of aluminium alloy controlled by the flow guide and die bearing, in: Advanced Technology of Plasticity, Proceedings 7th ICTP, t. I, Yokohama, 2002. [6] Q. Li, C.J. Smith, C. Harris, M.R. Jolly, Finite element modelling investigations upon the influence of pocket die designs on metal flow in aluminium extrusion. Part I. Effect of pocket angle and volume on metal flow, J. Mater. Process. Technol. 135 (2–3) (2003) 189–196. [7] Q. Li, C.J. Smith, C. Harris, M.R. Jolly, Finite element modelling investigations upon the influence of pocket die designs on metal flow in aluminium extrusion. Part II. Effect of pocket geometry configurations on metal flow, J. Mater. Process. Technol. 135 (2–3) (2003) 197–203. [8] Z. Peng, T. Sheppard, Effect of die pockets on multi-hole die extrusion, Mater. Sci. Eng. A 407 (1–2) (2005) 89–97. [9] D. Lesniak, W. Libura, V. Pidvysotskyy, A. Milenin, Influence of prechamber die geometry on extrusion of solid sections with different wall thickness, in: Proceedings of 5th International Esaform Conference, Krakow, 2002, pp. 459–462. [10] Lesniak D., Analysis of extrusion trough pocket dies, Ph.D. thesis, AGH University of Science and Technology, Krakow, 2003. [11] D. Lesniak, W. Libura, J. Zasadzinski, Buntoro, K. Muller, J. Fluhrer, Experimental and numerical investigations of aluminium extrusion through pocket dies, in: Proceedings of 8th International Aluminium Extrusion Technology Seminar ET, Orlando, USA, 2004. [12] K. Swiatkowski, Application of model materials to simulation of metal working processes, in: Metallurgy and Foundry Engineering MaFE, 25, no. 1, AGH University of Science and Technology, 1999, pp. 23–29.

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AGHUniversityofMiningandMetallurgy,Krakow,Poland Received31August2006;receivedinrevisedform13March2007;accepted28March2007 ∗ Correspondingau...

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