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Direct -Straightforward steady forward force by hydraulic ram Indirect -Has the advantage that there is no friction between billet and chamber (no movement) -Note dummy block at face of ram to protect tip of pressing stem, particularly in hot extrusion -Used for materials with very high friction, such as high-strength steels Hydrostatic -Fluid pressure results in triaxial compressive stresses acting on workpiece; improving formability -Much less workpiece-container container friction than for direct Lateral -Less common; requires more force Impact -To help overcome initial friction 8
Geometric Variables -Die angle ά -Extrusion Ratio Ratio of x-sect area of billet to that of extrusion A o /A f Other variables: -Temperature of the billet -Speed of ram travel -Type of lubricant 9
The k value is a measure of the strength of the material being extruded and the frictional conditions See example calculation Example 15.1, p.363, text 6/e Extrusion Ratio, R can range from 4 to 400 but usual range is 10 to 100. Softer, non ferrous, metals may reach 400 Ductile metals are easier to extrude 10
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Investigative technique to determine flow pattern: Cut round billet in half lengthwise and then mark one face with a square grid pattern. Braze the two halves together and extrude the billet Melt the braze in a furnace to reveal the extruded grid face Fig. 15.1 show result for direct extrusion with square dies Dead metal zones are areas of no metal flow 13
Can heat in inert atmosphere to prevent formation of surface oxide file which is abrasive 14
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Hollow cross sections Use welding chamber methods and various dies (examples in figure) During extrusion, the metal divides and flows around the supports for the mandrel into strands (separate, profiled threads) As they exit the die, the strands become rewelded under high pressure in the welding chamber The rewelded surfaces have good strength because they have not been exposed to the environment (no surface oxides) Welding chamber process is suitable only for aluminum & some alloys because of their capacity to develop strong fusion welds under pressure Use no lubricants because they prevent rewelding 20
Note: -Importance of symmetry of cross section -Avoidance of sharp corners -Avoidance of extreme changes in die dimensions within the cross section 21
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Heat sinks use many fins to increase surface area and facilitate heat transfer Aluminum is preferred because of high thermal conductivity and superior weightcost ratio Extruded heat sink is preferable to cast item because lower porosity and better thermal conductivity 24
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Note There is a sample calculation on p.369 of text 6/e. This example appears to use an incorrect value for the radius of original slug. The example uses (10 2 /4) as the square of the radius (in 2 ). The correct value for radius is 1/5. Therefore, r 2 = 1/25 or 4/100. 27
Improved mechanical properties provided that the heat generated by plastic deformation and friction does not recrystallize the extruded metal Accurate tolerances reduce the need for subsequent machining or finishing operations Improved surface finish due partly to the absence of an oxide film and provided that lubrication is effective Other methods such as machining; Some machines produce up to 2000 parts/hour 28
Steps -Shear a slug from end of a round rod -Cold extrusion to shape; has a blind hole -Punch out the material at bottom of blind hole Note the respective diameters of the slug and the hole at the bottom of the sectioned part 29
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Note that the blank (slug) is extruded backwards 31
Other typical parts made by impact extrusion: -Collapsible tubes (toothpaste) -Light fixtures -Automotive parts -Small pressure vessels Most nonferrous metals can be impact extruded in vertical presses and at production rates as high as two parts per second Maximum diameter of parts made is about 150 mm (6 ) Process can produce thin-walled tubular sections having thickness-to-diameter ratios as small as 0.005. Therefore, the symmetry of the part and the concentricity of the punch and the blank are important. 32
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Redundant Work of Deformation -Additional energy required because of the non-uniform deformation that occurs within the die zone Eq. 15.4 is a useful equation for this calculation. There are other equations and guidelines in the research literature 42
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Reductions higher than 45% may result in lubricant breakdown May draw hollow or larger sections at higher temperatures to reduce forces Sizing Pass: -Deformation usually occurs at surface only; -This results in non-uniform deformation across the section. -This creates variation in properties of the material depending on location within the cross section 45
Drawing speeds: -Slower for large sizes: 1 2.5 m/s (200 500 ft/min) -Faster for fine wire (for electromagnets): 50 m/s (165 ft/sec) -At higher speeds, may accumulate heat buildup (lack of time to dissipate), possibly leading to detrimental affects on product quality - 46
Bundle -Keep wires separated by suitable metallic material with similar properties, but lower chemical resistance -This enables it to be subsequently leached out from the drawn-wire surfaces 47
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Die materials -Typically tool steels & carbides (Table 5.8) -Use cast-steel dies for hot drawing because of their high resistance to wear at elevated temperatures -Diamond dies used for drawing fine wire -Diamond dies may be made from single crystal diamond or in polycrystalline form with diamond particles in metal matrix (compacts) -Carbide & diamond dies have low tensile strength & toughness -Therefore they are typically used as inserts or nibs within a steel casing 50
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Draw bench -Contains a single die - Pulling force by chain drive or hydraulics -Used for single-length drawing of straight rods and tubes (diameters > 0.75 and lengths, 100 ft. Use rotating drum (bull block or capstan) for very long rods & wire of smaller cross sections 55
Use rotating drum (bull block or capstan) for very long rods & wire of smaller cross sections 56
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