HOLKER_2015_conformal_cooling_hot_extrusion_aluminum

Materials Today: Proceedings 2 ( 2015 ) 4838 – 4846

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2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee of Aluminium Two Thousand World Congress and International Conference on Extrusion and Benchmark ICEB 2015doi: 10.1016/j.matpr.2015.10.028

Aluminium Two Thousand World Congress and International Conference on Extrusion and Benchmark ICEB 2015

Hot extrusion dies with conformal cooling channels produced by additive manufacturing

Ramona Hölkera,*, Matthias Haasea, Nooman Ben Khalifaa, A. Erman Tekkayaa, aInstitute of Forming Technology and Lightweight Construction, TU Dortmund University,

Baroper Straße 303, D-44227 Dortmund, Germany

Abstract

The influence of local inner cooling in hot aluminum extrusion dies was investigated. For the manufacturing of the dies with conformal cooling channels, a layer-laminated manufacturing method and a laser melting process were applied. Extrusion trials with and without applying die cooling were performed. Numerical and experimental investigations revealed that, while maintaining the exit temperature of the extrudate, a distinct increase of the production speed up to 300 % can be realized, while the extrusion force increases only slightly. Visioplastic analyses revealed that the rough surfaces, originating from the laser melting, do not disturb the material flow in the welding chamber.

© 2014 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee of Aluminium Two Thousand World Congress and International Conference on Extrusion and Benchmark ICEB 2015.

Keywords: Hot aluminum extrusion; Die cooling; Rapid Tooling; Additive Manufacturing; Productivity

1. Introduction

To improve the productivity in hot metal extrusion, an increase of the extrusion speed is desired. However, the exit temperature of the extrudate increases due to the forming heat and the heat generated by friction and shear [1]. This may result in surface defects, like hot cracks and grain coarsening after extrusion [2]. To compensate the rising material temperature, a precise adjustment of the heat balance is needed.

* Corresponding author. Tel.: +49-231-755-6915; fax: +49-231-755-2489. E-mail address: [email protected],de

© 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee of Aluminium Two Thousand World Congress and International Conference on Extrusion and Benchmark ICEB 2015

4839 Ramona Hölker et al. / Materials Today: Proceedings 2 ( 2015 ) 4838 – 4846

The heat balance in hot extrusion can be controlled by the initial billet temperature. Here, the billet preheating temperature can be reduced globally, or graded by unequal axial heating of the billet in different zones up to different temperatures [3]. In the so called tapered heating, the material extruded firstly is heated up to a higher temperature than the rear end of the billet (at the punch), to compensate the temperature rise during extrusion. However, the disadvantage of lowering the billet temperature is the increase of the flow stress of the material, which requires a higher press load and results in higher tool stresses. Especially in extrusion of high strength alloys, lowering the billet temperature is not possible due to a limited capacity. Other options for controlling the heat balance are lowering the ram speed (isothermal extrusion [4]), which counteracts a raise in productivity, or increasing the heat dissipation, for example by cooling of the container [5], which reacts slowly due to its high mass [6]. Cooling the die is very promising in regard to dissipate the heat close to the forming zone, where the heat is mainly generated. By concentrating the heat dissipation at the end of the forming zone, the extrusion force increases only slightly [7]. In addition, it is possible to cool critical die areas directly by sizing and positioning cooling channels or cooling nozzles adequately to prevent hot cracking (Fig. 1).

Fig. 1. Production related differences in the design of (cooling) channels: a) conventional subtractive manufacturing, b) additive manufacturing.

However, inserting conformal cooling channels close to the main forming zone, which provides the shortest heat conduction path from the forming zone to the cooling zone, is difficult or even impossible for profiles and dies with highly complex geometries. The use of additive manufacturing technologies is a promising approach to allow a conformal cooling, while the structure of the die is not weakened by excessive drilling. This work aims at the integration of conformal cooling channels in extrusion dies to decrease the profile’s exit temperature and to avoid thermally induced surface defects, like hot cracks, in order to raise the productivity in hot aluminum extrusion.

2. Numerical investigations

The effect of the surface-near die cooling close to the forming zone on the resulting profile’s exit temperature and the extrusion force was investigated by finite element simulation using the software SFTC DEFORM 2D. The hot extrusion process of a cylindrical tube profile was simulated by using a simplified axisymmetric model of a bridge die (Fig. 2 a)). The extrusion ratio is R=15:1 and the diameter of the container 66 mm. The cooling channel in the die cap has a cross-section of 2x1 mm. The surface of the cooling channel was modeled with a constant temperature of 60 °C as a boundary condition, which corresponds to the coolant exit temperature (water) measured in experiments. The initial temperatures for container and die were set to 450 °C and for the billet to 500 °C. The cooling was started after the prechamber had been filled with aluminum. EN AW-6060 and H13 were selected as billet and die material, respectively. The flow behavior of the aluminum alloy was modeled by the sinus-hyperbolic flow stress description as a function of temperature and strain rate by Zener-Hollomon and Sellars-Tegart [8]. Between container and billet, as well as between die and billet, sticking friction condition was set except from the area of the die bearings, where a shear friction factor of m=1 was used [9]. During extrusion without cooling at a ram speed of 4 mm/s, the maximum profile’s exit temperature of 530 °C is reached at the end of the process (Fig. 2 b)). By cooling the die cap via cooling channels, the profile’s exit temperature can be reduced by 35 °C to an approximately stationary level of around 495 °C. The extrusion force increases up to 11 % until the end of the process, but remains below the peak value from the beginning (Fig. 2 c)). As mentioned in the state of the art, the heat balance in hot extrusion can be controlled alternatively by using lower preheated billets. In order to reach an approx. comparable profile’s exit temperature as in case of the die cooling (~495 °C), the billet had to be extruded with an initial preheating temperature of only 350 °C (Fig. 2 b)). Because of the temperature-affected increase of the workpiece flow-stress, an increase of the extrusion force of more than 50 % was observed (Fig. 2 c)). By this, the mechanical loads of the dies and the machine are significantly higher.


Fig. 2. a) FE-model, b) comparison of the profile’s exit temperature (b) and the extrusion force (c) using an inner die cooling versus a lower preheating temperature of the billet [8].

The exposure time, determined by the extrusion speed, is one of the key parameters influencing the heat balance in hot extrusion. Next, the results of five different investigated extrusion speeds (1 mm/s, 2 mm/s, 4 mm/s, 8 mm/s, 16 mm/s) without and with die cooling are presented. In case without cooling, each doubling of the extrusion speed leads to an increase of the profile’s exit temperature by the same amount (approx. 18 °C, Fig. 3 a)). As the flow stress of the used aluminum alloy increases by the same amount with each doubling of the strain rate (Fig. 3 c)), the generated deformation energy and therefore the forming heat is proportional to the flow stress, leading to the previously mentioned results.

Fig. 3. a) Profile’s exit temperature at different extrusion speeds for a) without cooling and b) with cooling, c) flow stress in dependence from the strain rate by using the sinus-hyperbolic description of the flow stress (according to [8])


The use of the die cooling (Fig. 3 b)) has the most impact at the lowest investigated extrusion speed of 1 mm/s (highest exposure time). By cooling, the profile’s exit temperature is reduced by around 85 °C at the end of the extrusion process. At the highest applied extrusion speed of 16 mm/s, the die cooling reaches its limitation, because, in spite of cooling, the profile’s exit temperature increases slightly. More heat is generated than the cooling is able to dissipate. The time that is needed for heat conduction is inversely proportional to the extrusion speed. That means, that with increasing extrusion speed, less time for the dissipation of the generated heat is available [10]. At the highest extrusion speed of 16 mm/s the exit temperature of the extrudate can be reduced by 12 °C only in comparison to the case without applying cooling. A comparison of the profile’s exit temperatures at the end of the processes shows, that during extrusion without applying cooling at a ram speed of 1 mm/s and during extrusion with cooling at a ram speed of 4 mm/s, approximately the same profile’s exit temperature is reached (Fig. 3 c)). That means that a four times higher productivity is possible using the inner die cooling. At higher extrusion speeds still a doubling of the productivity is possible (4 mm/s without cooling 8 mm/s with cooling, Fig. 3 c)), reaching the same profile’s exit temperature [8].

3. Manufacturing of extrusion dies by rapid tooling technologies

Two different additive manufacturing processes are used for the manufacturing of extrusion dies with cooling channels. Due to differences in the processes, the layer-laminated manufacturing method is used for the manufacturing of plain and geometrical simple parts, like the die cap of an extrusion die, and the powder metallurgy based method (laser melting) is used to build geometries with very high complexity, like the mandrel of an extrusion die. Experiments were carried out using the near-surface inner cooling in the die cap as well as in the mandrel for the manufacturing of a squared hollow aluminum profile (22 x 22 x 3 mm). These die parts additively manufactured were used in combination with conventionally manufactured die elements.

3.1. Powder metallurgy based methods.

The major advantage of additive manufacturing in comparison to conventional subtractive methods is the geometrical freedom of the components and dies produced as e.g. the manufacturing of shapes with undercut and the manufacturing of conformal cooling channels. The manufacturing process starts with a volume model (3D CAD model) of the geometry of the component. The CAD file is the input information for the slice process, in which a special software slices the component into several layers with a defined thickness (in general between 20 μm and 100 μm). The thickness of the layers determines the surface accuracy of the part as well as the manufacturing time. High thicknesses lead to rough surfaces but a quick manufacturing time while thin layer thicknesses cause the contrary. After transferring the data to the machine, the process starts by deposition a thin layer of the metal powder. The powder is locally melted and solidified layer by layer by using an energy source. After generating the geometry in one layer the building platform is lowered by the thickness of one layer and then a new layer of powder can be deposited. These processes repeat as often as the whole geometry is built up [11].

A mandrel of an extrusion die (Fig. 4 a)) was manufactured in a powder bed of CL50WS (similar to the hot working steel 1.2709) in a nitrogen atmosphere by using a laser cusing machine m3 linear (by Conept Laser GmbH). The fabrication of the die, with a volume of 116 cm³ and a deposition rate of 3.8 cm3/h, took around 30 h (1580 layers à 30 μm). The fitting surfaces and die bearings had an allowance of 0.3 mm and were mechanically refinished. The mandrel was heat treated (ageing 500 °C, 8h) to a final hardness of 55 HRC [11]. Due to the design of the extrusion press, a supply of the cooling medium and the measuring setup is only possible from the exit end face of the die. For feeding the coolant into the mandrel, a multidirectional course of the cooling channel through the mandrel was designed. Two opposite bridges of the mandrel include the cooling channel while in the other two, a channel for thermocouples is positioned (Fig. 4 b)). On top of the mandrel small deflectors were added which prevent a pull out of the thermocouples due to a possible contact between the extrudate and the thermocouples especially during the beginning of the extrusion process (Fig. 4 c)).


Fig. 4. a) Mandrel of an extrusion die manufactured by laser melting, b) geometry of the channels (negative), c) detail top of the mandrel with deflectors and die cap (according to [8])

3.2. Influence of the rough tool surface, resulting from the additive manufacturing process, on the material flow

Due to the step effect and the used powder, parts manufactured by additive manufacturing processes have a rough surface which has to be mechanically refinished in areas which are in contact to other areas, sealing surfaces and the die bearings. The question is, whether the rough surfaces in the die inlet influence the material flow and if these areas can stay as manufactured or if they have to be mechanically refinished. In order to prove this, a visioplastic analysis was carried out. The idea was, to compare the material flow along two different die surfaces which are conventionally and additively manufactured. In order to have equal process boundary conditions, both trials were carried out in one and the same die. For this, a die with different surfaces was developed (Fig. 5 a)).

Fig. 5. Geometry of the die for visioplastic analysis of the material flow, a) 3D-model, b) drawing, longitudinal cut, c) real manufactured die

(according to [8])

To enable the ejecting of the discard out of the die, a die for the manufacturing of a full section profile with a

diameter of Ø 20 mm (instead of a bridge die) was manufactured with an enlarged prechamber (Fig. 5 b)). The die was manufactured with an allowance of 0.3 mm of the hot working steel 1.2709, except for one half of the prechamber. Before ageing, all fitting surfaces and the die bearings were refinished by machining. Finally, the prechamber consists one half each of a machined and of a rough (as laser melted) surface. The macroscopic geometry is fully symmetric. Hence, possible local changes in the material flow can be attributed only to local differences in the surface roughness [8].

In order to facilitate the ejection of the aluminum, the prechamber in the die is tapered by 3° in extrusion direction. The infeed to the die bearings has additionally a die opening angle of 45°. By this, the dead zone shall be minimized and the contact surfaces with relative movement between workpiece and the die shall be enlarged additionally. For visualizing the material flow, the billets were prepared by the indicator pin method (Fig. 6 a)).


Fig. 6. a) Billet for visioplastic analysis (drawing), b) real billet with pins

A finite element simulation (DEFORM 2D) was used to determine a suitable position of the indicator pins as well

as the length to be extruded. Billets of 175 mm in length and 62.5 mm in diameter were prepared, fitting to the prechamber of the die. Indicator pins of EN AW-4043A (AlSI5) with a diameter of 3.1 mm were placed axially and radially in the billet symmetry plane at the tip of the billet. The billet geometry is adapted to the geometry of the die’s prechamber (Fig. 6 b)). By this, directly from starting the process, the indicator pins are in contact with the die surfaces to be investigated. The prepared billets were preheated to 540 °C and put into the container of the extrusion press in this kind, that the indicator pins are placed perpendicular to the transition plane between the machined and the rough surface in the prechamber. The billets were extruded at a ram speed of 0.5 mm/s. After extrusion the billets were ejected and cut in the plane containing the indicator pins. The surface was polished and etched in caustic soda. Fig. 7 shows the prepared billet. The indicator pins are deformed similar in both halves. The rough surface in the prechamber generated by the additive manufacturing process has no influence on the flow behavior. Hence, in regard to the material flow, a machining process is not required.

3.3. Layer Laminated Manufacturing Methods.

For the manufacturing of an extrusion die with integrated cooling channels placed close to the forming zone the Layer Laminated Manufacturing Method was chosen. Single steel sheet layers were cut in a CNC laser cutting center (Trumpf TL 1005) according to the die cap geometry and assembled. The lamellas of the die were cut out sheets of the hot working steel 1.2343/H11 (heat treated to 54 HRC) with a thickness of 1 mm (welding chamber, cooling channel lamellas) and 2 mm (die bearing lamella). The stacked lamellas are supported by a 19 mm thick backer platen made also of the hot working steel 1.2343/H11 (Fig. 8 a)). The cooling channels were aligned around the circumference of the square die opening directly behind the die bearing lamella. To keep each lamella as one single unit, the cooling channels are splitted into two lamellas (Fig. 8 b)).

Fig. 7. Half of the billet with machined pins [8]


Fig. 8. a) Layered die cap, b) geometry of the cooling channel (negative) [12], c) plastic deformation of the die in extrusion direction [8] (optical geometry measurement, GOM-Atos)

Layered dies have a lower stiffness than solid dies. For this reason the right material and layer thickness have to

be chosen. With the former described set-up hollow profiles could be manufactured successfully. After etching the die in caustic soda, by optical measurement (GOM-Atos) of the die bearing lamella it could be seen, that no plastic deformation of the die occurred (Fig. 8 c)). Also the cooling channels were covered sufficiently by the bearing lamella (Fig. 8 c)). This set-up was used for the following experiments carried out.

4. Comparison mandrel cooling vs. die cap cooling

The new extrusion dies were tested on a 2.5 MN direct extrusion press (Collin PLA 250). Fig. 9 shows the comparison of the results of cooling the mandrel or the die cap. The average profile’s exit temperatures (average value in a quasi-steady state after starting the cooling since 1/3 billet length was extruded) for different profile’s exit speeds without and with applying cooling with compressed air as coolant are shown (Fig. 9 a)).

Fig. 9. a) Comparison of profile’s exit temperatures at different exit speeds, with and without mandrel and die cap cooling, b) extrusion forces at vex=0.45 m/min (according to [8])

The reference curves of the profile’s exit temperature without cooling are nearly identically which confirms that the boundary conditions adjusted are comparable and reproducible. In both cases, cooling the mandrel or the die cap, with rising extrusion speed a decreasing amount of heat can be dissipated. However, regarding all exit speeds


investigated, the die cap- and mandrel cooling is able to reduce the profile’s exit temperature at each speed for the same amount. That means, that cooling the mandrel is as effective as cooling the die cap.

Regarding the extrusion forces (Fig. 9 b), exit speed of vex=0.45 m/min) only during starting the process an influence of the rough die surfaces of the additive manufacturing process can be detected. In comparison to the trial using the mandrel conventionally manufactured with finely machined surfaces the extrusion force is only slightly higher. After the beginning, the extrusion forces recorded using the mandrel additively manufactured, are even slightly below the comparative tests. This can be explained by the slightly higher temperature level. By applying the cooling, the extrusion forces increase in both die concepts for the same amount (approximately 0.1 MN), but they are still very close to each other [8].

Since the temperature reduction of cooling the mandrel and cooling the die cap is in both cases the same effective and also the process of extrusion force as well as the increase of the force during applying cooling is similar, next the results from cooling the die cap are presented only.

5. Extension of the process limits

For the experimental investigation of the potential of die cooling in hot extrusion, extrusion trials were carried out at different extrusion speeds. Starting at a low profile’s exit speed, the extrusion speed was increased successively from trial to trial and the surfaces of the profiles were analyzed regarding the formation of hot cracks. As billet material, the difficult-to-extrude aluminum alloy EN AW-7075 was chosen which was preheated to 440 °C. Die and container temperature were set to 450 °C. A square hollow profile (22x22x3 mm) with an extrusion ratio of R=38:1 was extruded on a 10 MN direct extrusion press (SMS) with a container diameter of 105 mm. Water was chosen as cooling medium.

Without cooling, hot cracks occurred at profile exit speeds between 1 m/min and 1.25 m/min (Fig. 10). With rising extrusion speed the size and the dimension of the hot cracks increased. Applying die cooling, profiles can be manufactured at an exit speed of around 3.1 m/min without surface defects. That means that a three times higher productivity can be realized without any thermally induced defects on the extrudate’s surface. The extrusion forces at an extrusion speed of 3 m/min, at the end of extrusion of a billet, were 4.95 MN without cooling and increase by 17 % up to 5.8 MN in case of die cooling.

Fig. 10. Extension of the process limits in hot aluminum extrusion of EN AW-7075 by using an inner die water cooling [13]


6. Conclusions

The manufacturing of components of an extrusion die by selective laser melting and layer laminated manufacturing provides the possibility to integrate multidirectional cooling channels and to integrate thermocouples for temperature measurement into hot extrusion dies close to the forming zone. The temperature reduction by cooling the mandrel or by cooling the die cap is in both cases the same efficient and influences the extrusion force in the same way.

Visioplastic analyses revealed that the material flow in the die, additively manufactured by laser melting, is not influenced by the rough surfaces in the welding chamber. In regard to the material flow, a machining process is not required here.

In numerical and experimental investigations it was proven that the profile’s exit temperature can be reduced significantly by using the inner die cooling. By positioning the cooling channel close to the forming zone, the heat can be dissipated locally and the extrusion force increases only disproportionately in comparison to reference trials without die cooling as well as in comparison to extrusion of lower preheated billets.

The developed tooling technology offers a big potential for increasing the production speed in hot aluminum extrusion.

Acknowledgements

This work was carried out within the projects Te508/27-1 and Te508/27-2, funded by the German Research Foundation (DFG). The financial support is greatly acknowledged.

References

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