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Hot Extrusion of Thin-Wall Multichannel Copper Profiles Frank F. Kraft1 Associate Professor Mem. ASME e-mail: [email protected] Jonathan Kochis e-mail:...

Analyze the five stages of copper Extrusion process. 

Frank F. Kraft 1 Associate Professor Mem. ASME e-mail: [email protected] Jonathan Kochis e-mail: [email protected] Mechanical Engineering Department, Ohio University, Athens, OH 45701 Hot Extrusion of Thin-Wall Multichannel Copper Profiles This paper presents the development of a unique, net shape, hot-extrusion process to pro- duce precision, thin-wall, multichannel copper proFles for high efFciency heat- exchangers. This process is a departure from conventional copper extrusion, which is a nonisothermal process used primarily to produce simple semiFnished products and hol- low proFles requiring cold drawing after hot extrusion. A lab-scale apparatus was devel- oped to simultaneously extrude multiple heated billets through a porthole type hollow die to form the multi-channel proFles. The process is performed at 700–750 ± C, essentially at isothermal extrusion conditions. Temperature and tooling strength considerations neces- sitated the use of superalloys for the apparatus (which included dies, container, ram stems, and support tooling). A 250 kN computer controlled servo-hydraulic MTS V R machine was used to provide the extrusion ram force. Two part designs were extruded to demonstrate process feasibility and versatility. A two-channel design with 0.2 mm wall thicknesses and an 11-channel design with wall-thicknesses of 0.3 mm were extruded. The extrusion ratios for these proFles are 67 and 25, respectively. Experimental data and an approach to analytically model the process are presented. Because solid-state welds in the tube walls are necessitated by the use of hollow extrusion dies, the microstructure in these regions is also presented. [DOI: 10.1115/1.4025496] 1 Introduction and Background Tubular copper profles are generally hot extruded frst into semifnished parts that are subsequently cold drawn to fnal dimensions. Simple seamless round tube For plumbing and reFrig- eration applications accounts For the vast amount produced in this manner, and processing has essentially been optimized For eFf- ciency. The semifnished tube sections are hot extruded at billet temperatures up to 1000 ± C with special presses that pierce a solid billet to Form the single internal passage [ 1 , 2 ]. To prevent over- heating and soFtening oF the tool steel, extrusion must take place at high speeds to minimize the contact time oF the hot billet with the tooling. This also serves to minimize billet cooling which would increase ±ow stress and the ram pressure required. ²rom a practical sense, this limits such extrusion processes to simple hol- low profles with low extrusion ratios and thick walls, hence, the need For fnishing via cold drawing. Extrusion oF thin walled hol- low profles was thought to be prohibitive because oF the inher- ently higher extrusion ratios and slower extrusion speeds needed. Also, extrusion oF copper at these temperatures precludes the typi- cal use oF porthole dies, such as that used to extrude aluminum tubes and hollow profles. The high process temperatures and high stresses on the bridge oF a hollow die present the main challenges that need to be overcome in extruding copper tubes to fnished dimensions. The development oF high-eFfciency heat exchangers For auto- motive climate control systems and other cooling applications has seen the emergence and wide use oF small, thin-walled, and multi- channel aluminum tubes. These tubes are commercially produced, essentially net-shape, via direct hot extrusion From Aluminum Association (AA) 1000 and 3000 series alloys and with porthole type hollow dies. Only post extrusion straightening, sizing, and cutting oF the tube are required. The development oF a process to produce similar tube From copper is the subject oF this paper. Early in the development oF this process, a simple assessment oF crucial parameters For aluminum and copper extrusion was per- Formed. A comparison oF these parameters For aluminum (AA3003 alloy assumed) and copper extrusion is presented in Table 1 . Several conclusions were drawn From this assessment. To achieve a ±ow stress in copper somewhat similar to that For alumi- num extrusion, the temperature should be over 700 ± C. Since thin- walled hollow sections require slower (ram) extrusion speeds and thus longer contact time between the work piece and tooling, the use oF steel tooling (such as AISI H13) is precluded. This also means that the tooling temperature must be maintained at or near the extrusion temperature (as is done with aluminum extrusion), to prevent excessive work-piece cooling. Near-isothermal extru- sion is thus an important goal. To achieve this, superalloys were used For the tooling. These alloys provide strengths at copper extrusion temperatures that are comparable to those oF tool steels at aluminum extrusion temperatures. The basic approach and some oF the initial successes (and chal- lenges) in this endeavor have been previously reported in the liter- ature [ 3 , 4 ]. The work presented herein specifcally Focuses on recent developments in the process, lab apparatus, and modeling. 2 Process and Experimental Apparatus A lab scale apparatus was developed with a 250 kN (56,000 lb), computer-controlled, servo-hydraulic MTS V R machine, which pro- vides extrusion Force and velocity control. It was designed and Fabricated to extrude two billets, simultaneously, to Form multi- channel, thin-wall hollow profles that are typical oF designs From aluminum alloys. The purpose oF the dual billet confguration was to eliminate adverse (axial) stresses on the die bridge during high temperature extrusion oF alloys with such high ±ow stresses. Nickel-based superalloys were used such that a requisite amount oF strength in the tooling at 700–750 ± C was achieved. This eFFec- tively allowed the tooling to be held in this temperature range For near-isothermal extrusion. The relatively slow extrusion speeds and thin tube walls necessitated heating to maintain the extrusion temperature. Thus, this approach overcomes the challenges oF extruding high-ratio hollow copper profles at the high tempera- tures required. A sketch and photo oF the extrusion apparatus are presented in ²ig. 1 ; portions have been previously described in the literature [ 4 , 5 ]. The apparatus uses a 250 kN (28 US ton) servo-hydraulic, PC controlled MTS V R machine to provide the ram pressure. The maximum (specifc) ram pressure available For this confguration 1 Corresponding author. Manuscript received May 1, 2013; fnal manuscript received September 18, 2013; published online November 5, 2013. Assoc. Editor: Yung Shin. Journal of Manufacturing Science and Engineering DECEMBER 2013, Vol. 135 / 061008-1 Copyright V C 2013 by ASME Downloaded From: http://manufacturingscience.asmedigitalcollection.asme.org/ on 02/18/2016 Terms of Use: http://www.asme.org/about-asme/terms-of-use
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is 722 MPa (104.7 kpsi). Ram control is PC-based using MTS’s object-oriented programming. Control can be via either force or ram speed, and both control schemes are used in this effort. The temperatures of the container and die holder are independently controlled via separate controllers. Eight cartridge heaters are used in the container and two are used in the die holder. K-type thermocouples provide the feedback to the temperature control- lers, which provide the control signals to solid state relays. The thermocouples that provide feedback to the respective controllers are located directly adjacent to heaters in the container and in the die holder. The heaters have special grooves in their sheaths to accommodate the thermocouples. Temperature control in this manner ensures that no portion of the tooling will be exposed to excessively high or otherwise unknown temperatures. Figure 2 shows a front view of the apparatus just after an extru- sion trial. The extruded copper tube is protruding from the nitro- gen gas tube in the bottom center of the photo. The separate temperature controllers are shown at the bottom right in Fig. 2 . The process is indeed direct extrusion, even though the stems are stationary (on the top crosshead) and the container is mounted to the bottom hydraulic ram of the machine. The billets are effec- tively pushed “down”, through the die and into the nitrogen-gas tube where cooling takes place while preventing oxidation of the hot tubing. A graphite block directs the tube from the die backer exit to the nitrogen tube. For experimental trials, the extrusion speeds were low enough for suf±cient cooling to take place prior to the tube exiting the nitrogen atmosphere. Also, to isolate the temperature of the apparatus from the MTS machine, water- cooled aluminum heat-exchangers were placed between the loca- tions where the container and stems mount to the machine grips. During the process, the simultaneously extruded billets are deformed in the die to form the internal walls of the tube. Solid state joining of the two halves takes place in the die’s weld cham- bers. This takes place just prior to formation of the tube walls’ ±nal dimensions. Figure 3 illustrates the progression from the con- tainer to the ±nal pro±le, for the 11-channel design with an extru- sion ratio of 25. Similar to typical hollow-die extrusion, the two metal ²ow streams must converge in the weld chambers of the die. In this instance, the surface area of the billet increases by almost 2000% as it is deformed in the die, and this is deemed im- portant to produce good “clean” solid-state welds in the internal walls of the tube. Figure 4 shows photos of a tube section with two billets that were similar to those used to produce it, and the extrusion die. Table 1 Temperature and fow stress For copper and aluminum extrusion, and tooling strengths at these temperatures. Aluminum extrusion (AA3003) Copper extrusion Billet temperature 450—500 ± C 600–800 ± C Billet ²ow stress @ d e /dt ¼ 0.5 s ÿ 1 22–30 MPa 30–70 MPa Tooling material Tool steel (H13) Superalloys Tooling strength at forming temp. ² 850–930MPa ² 900–1070MPa ±ig. 1 Experimental extrusion apparatus. ±igure ( a ) shows an annotated sketch oF the compo- nents For the extrusion tooling, From the US patent [ 4 ]. ±igure ( b ) shows the apparatus aFter an extrusion trial (the copper tube exits the nitrogen-cooling tube, at the bottom right). Detached dummy blocks are used during extrusion. ±ig. 2 Photo oF the extrusion apparatus just aFter an extrusion trial oF copper tube. The insulation that surrounded the heated container has just been removed. 061008-2 / Vol. 135, DECEMBER 2013 Transactions oF the ASME Downloaded From: http://manufacturingscience.asmedigitalcollection.asme.org/ on 02/18/2016 Terms of Use: http://www.asme.org/about-asme/terms-of-use
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First Defined the entire problems? First stage is heating up the material. We will have cylindrical billet that is setting in a die and we heat it up. Heat coming in from anywhere else. How is the temperature changing? What are the energy going in and out and how they influence the temperature change? We have to define the heat transfer problem. What are the boundary condition that apply here and how we can solve this problem. What is the assumption that we can make to reduce the problem to smaller scale. The second one. We have a billet. The billet is transfer into a convicting environment. The billet is at higher temperature and the environment at low temperature. Heat lost. So, we will have different heat profiles or temperature profiles in the materials as the convection coefficient changes and as the time you leave it out changes. So, what again the boundary conditions and how we can solve this problem. The third one: when we have again the billet setting in the die, and heat it again to bring its temperature from T2 to T3. This situation is very close to the first one. How much heat is coming in to take the temperature from T2 to T3. The fourth stage: We have the billet and applying pressure and we have a die. The flow stress will push the copper down and it is going to deform. So, whatever deformation energy and heat that you put in will be dissipated see the picture. We have to consider the Temperature deformation zoon in extrusion. The final stage: sort of wire and the temperature of the wire changes. How much heat does it lose.
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