材料科学与工程学院 College of Materials Science & Engineering Page 1 Page 1 Fundamental of Materials Forming - Extrusion and Drawing of Metals

材料科学与工程学院 College of Materials Science & Engineering Page 2 Page 2 INTRODUCTION Extrusion is the process squeezing metal in a closed cavity through a tool,known as a die using either a mechanical or hydraulic press.

材料科学与工程学院 College of Materials Science & Engineering Page 3 Page 3 In the extrusion process, a billet (generally round) is forced through a die (Fig. 15. l ), in a manner similar to squeezing toothpaste ( 牙膏） from a tube. Almost any solid or hollow cross-section may be produced by extrusion, which can create essentially semifinished parts （半成品） (Fig. 15.2). Because the die geometry remains the same throughout the operation, extruded products have a constant cross-section.

材料科学与工程学院 College of Materials Science & Engineering Page 4 Page 4 Extrusions Figure 15.2 Extrusions, and examples of products made by sectioning off extrusions. Source: Kaiser Aluminum.

材料科学与工程学院 College of Materials Science & Engineering Page 5 Page 5 Depending on the ductility of the material, extrusion may be carried out at room or at an elevated temperature. Extrusion is often combined with forging operations, in which case it is generally known as cold extrusion. It has numerous important applications, including fasteners and components forautomobiles, bicycles, motorcycles, heavy machinery, and transportation equipment.

材料科学与工程学院 College of Materials Science & Engineering Page 6 Page 6 In the basic extrusion process, called direct, or forward, extrusion, a round billet is placed in a chamber (container) and forced through a die opening by a hydraulically-driven ram or pressing stem (Fig. 15. I). The die opening may be round, or it may have various shapes. Other types of extrusion are indirect( 间接的）, hydrostatic （静水压力的）, and impact extrusion （ 冲挤）. In indirect extrusion (reverse, inverted, or backward extrusion), the die moves toward the billet (Fig. 15.3a).

材料科学与工程学院 College of Materials Science & Engineering Page 7 Page 7 Extrusion: Schematic, Dies Exercise: how can we get hollow parts?

材料科学与工程学院 College of Materials Science & Engineering Page 8 Page 8 The extrusion die is a steel disk with an opening, the size and shape of the intended cross-section of the final extruded product as indicated below. Dies

材料科学与工程学院 College of Materials Science & Engineering Page 9 Page 9 Billet The billet is the starting stock for the extrusion operation.Extrusion billets may be a solid or hollow form, commonly cylindrical, and is the length charged into the extrusion press container.

材料科学与工程学院 College of Materials Science & Engineering Page 10 Page 10 Extrusion operations typically take place with billet heated. However,during the extrusion, the billet is still solid.

材料科学与工程学院 College of Materials Science & Engineering Page 11 Page 11 Types of Extrusion Figure 15.3 Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral.

材料科学与工程学院 College of Materials Science & Engineering Page 12 Page 12 Direct Extrusion Figure 15.1 Schematic illustration of the direct extrusion process.

材料科学与工程学院 College of Materials Science & Engineering Page 13 Page 13 HOT EXTRUSION Extrusion is carried out at elevated temperatures--for metals and alloys that do not have sufficient ductility at room temperature, or in order to reduce the forces required (Table 15.1 ). As in all other hot working operations, hot extrusion has special requirements because ofthehigh operating temperatures.

材料科学与工程学院 College of Materials Science & Engineering Page 14 Page 14 Process Variables in Direct Extrusion 正挤的工艺变量 As can be seen in Fig. 15.4, the geometric variables in extrusion are the die angle, a, and the ratio of the cross-sectional area of the billet to that of the extruded product, Ao/Af, called the extrusion ratio, R. Other variables in extrusion are the temperature of the billet, the speed at which the ram travels, and the type of lubricant used.

材料科学与工程学院 College of Materials Science & Engineering Page 15 Page 15 Circumscribing-Circle Diameter Figure 15.5 Method of determining the circumscribing- circle diameter (CCD) of an extruded cross-section. A parameter describing the shape of the extruded product is the circumscribing-circle diameter (CCD)

材料科学与工程学院 College of Materials Science & Engineering Page 16 Page 16 Extrusion Force The force required for extrusion depends on the strength of the billet material, the extrusion ratio, friction between the billet and the chamber and die surfaces, and process variables such as the temperature of the billet and the speed of extrusion. We can estimate the extrusion force, F, from the formula Where k is the extrusion constant, and A0 and Af are the billet and extruded product areas, respectively. The k values for several metals are given in Fig. 15.6, for a range of temperatures.

材料科学与工程学院 College of Materials Science & Engineering Page 17 Page 17 Extrusion Constant k for Various Metals Figure 15.6 Extrusion constant k for various metals at different temperatures. Source: P. Loewenstein.

材料科学与工程学院 College of Materials Science & Engineering Page 18 Page 18 Metal Flow in Extrusion The metal flow pattern in extrusion, as in other forming processes, is important because of its influence on the quality and the mechanical properties of the final product.

材料科学与工程学院 College of Materials Science & Engineering Page 19 Page 19 Types of Metal Flow in Extruding With Square Dies Figure 15.7 Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction, or in indirect extrusion. (b) Pattern obtained with high friction at the billet- chamber interfaces. (c) Pattern obtained at high friction, or with cooling of the outer regions of the billet in the chamber. This type of pattern, observed in metals whose strength increases rapidly with decreasing temperature, leads to a defect known as pipe, or extrusion defect.

材料科学与工程学院 College of Materials Science & Engineering Page 20 Page 20 Die Design and Die Materials (a) (b) (c) Figure 15.8 Typical extrusion-die configurations: (a) die for nonferrous metals; (b) die for ferrous metals; (c) die for T-shaped extrusion, made of hot-work die steel and used with molten glass as a lubricant. Source for (c): Courtesy of LTV Steel Company.

材料科学与工程学院 College of Materials Science & Engineering Page 21 Page 21 Components for Extruding Hollow Shapes Figure 15.9 (a) An extruded 6063-T6 aluminum ladder lock for aluminum extension ladders. This part is 8 mm (5/16 in.) thick and is sawed from the extrusion (see Fig. 15.2). (b)-(d) Components of various dies for extruding intricate hollow shapes. Source: for (b)-(d): K. Laue and H. Stenger, Extrusion-- Processes, Machinery, Tooling. American Society for Metals, Metals Park, Ohio, Used with permission.

材料科学与工程学院 College of Materials Science & Engineering Page 22 Page 22 Cross-Sections to be Extruded Figure Poor and good examples of cross- sections to be extruded. Note the importance of eliminating sharp corners and of keeping section thicknesses uniform. Source: J. G. Bralla (ed.); Handbook of Product Design for Manufacturing. New York: McGraw-Hill Publishing Company, Used with permission.

材料科学与工程学院 College of Materials Science & Engineering Page 23 Page 23 Cold Extrusion Developed in the 1940s, cold extrusion is a general term often denoting a combination of operations, such as direct and indirect extrusion and forging (Fig ). Cold extrusion has gained wide acceptance in industry, particularly for tools and for components in automobiles, motorcycles, bicycles, appliances, and transportation and farm equipment.

材料科学与工程学院 College of Materials Science & Engineering Page 24 Page 24 Examples of Cold Extrusion Figure Two examples of cold extrusion. Thin arrows indicate the direction of metal flow during extrusion.

材料科学与工程学院 College of Materials Science & Engineering Page 25 Page 25 Cold Extruded Spark Plug 火花塞 Figure Production steps for a cold extruded spark plug. Source: National Machinery Company. Figure A cross-section of the metal part in Fig , showing the grain flow pattern. Source: National Machinery Company.

材料科学与工程学院 College of Materials Science & Engineering Page 26 Page 26 Impact extrusion is similar to indirect extrusion; it is often included in the cold-extrusion category. The punch descends rapidly on the blank (slug), which is extruded backward. Impact Extrusion 冲挤

材料科学与工程学院 College of Materials Science & Engineering Page 27 Page 27 Examples of Impact Extrusion Figure (a) Two examples of products made by impact extrusion. These parts may also be made by casting, by forging, or by machining; the choice of process depends on the dimensions and the materials involved and on the properties desired. Economic considerations are also important in final process selection. (b) and (c) Impact extrusion of a collapsible tube by the Hooker process.

材料科学与工程学院 College of Materials Science & Engineering Page 28 Page 28 EXTRUSION DEFECTS 1.Surface cracking If extrusion temperature, friction, or speed is too high, surface temperatures rise significantly, and this condition may cause surface cracking and tearing. These defects occur especially in aluminum, magnesium, and zinc alloys, although they may also occur in high-temperature alloys. This situation can be avoided by lowering the billet temperature and the extrusion speed.

材料科学与工程学院 College of Materials Science & Engineering Page 29 Page 29 Internal cracking The center of the extruded product can develop cracks variously called center cracking, center-burst, arrowhead fracture, or chevron cracking (Fig a). These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die, a situation similar to the necked region in a tensile-test specimen.

材料科学与工程学院 College of Materials Science & Engineering Page 30 Page 30 The tendency for center cracking: a. increases with increasing die angle; b. increases with increasing amount of impurities; c. decreases with increasing extrusion ratio and friction. These cracks have also been observed in tube extrusion and in tube spinning; they appear on the inside surfaces of tubes and for the same reasons.

材料科学与工程学院 College of Materials Science & Engineering Page 31 Page 31 Chevron Cracking 人字形裂纹 (a) (b) Figure (a) Chevron cracking (central burst) in extruded round steel bars. Unless the products are inspected, such internal defects may remain undetected, and later cause failure of the part in service. This defect can also develop in the drawing of rod, of wire, and of tubes. (b) Schematic illustration of rigid and plastic zones in extrusion. The tendency toward chevron cracking increases if the two plastic zones do not meet. Note that hte plastic zone can be made larger either by decreasing the die angel or by increasing the reduction in cross-section (or both). Source: B. Avitzur.

材料科学与工程学院 College of Materials Science & Engineering Page 32 Page 32 Indirect Extrusion Direct Extrusion DIRECT & INDIRECT EXTRUSION METHODS

材料科学与工程学院 College of Materials Science & Engineering Page 33 Page 33 MECHANICS OF EXTRUSION Reduction, R = A o /A f Ideal work (without friction or redundant work) is obtained by considering a simple tension test, for which  eff =  applied, and  eff =  = lnR. For a non work- hardening material, with  = Y (constant), we then have: With work hardening, use an average Y avg, which gives the same ideal work of deformation with  =lnR. AoAo AfAf LoLo LfLf p

材料科学与工程学院 College of Materials Science & Engineering Page 34 Page 34 MECHANICS OF EXTRUSION The specific work can also be calculated from the ram pressure (p), using: u =( p.A o ).L o /(A o L o ) i.e., the work done in deforming the entire billet. Steady state conditions assumed; i.e., p remains constant during ram displacement. Equating this work to the ideal work in the previous page, we obtain the simple result: p ideal = Y lnR With friction, one obtains:  AoAo AfAf LoLo LfLf p

材料科学与工程学院 College of Materials Science & Engineering Page 35 Page 35 REDUNDANT WORK AND FRICTION EFFECTS In a simple tension test, we have the following change in grid spacing: Before After In actuality, we have the following grid shapes: The effect of modified grid shape requires additional work. This additional work was necessary in order to keep the material primarily under compression loading. The additional work is known as Redundant Work, and results in additional pressure requirement. Ideal conditions Indirect Direct Friction effect also increases the pressure required for extrusion.

材料科学与工程学院 College of Materials Science & Engineering Page 36 Page 36 MECHANICS OF EXTRUSION Often, the die pressure is expressed through the empirical relation: Where a and b are estimated as follows:  ab Efficiency of deformation:  p Where L and D are the length and diameter of the billet in the die, respectively. When lubrication is good, the last term is much smaller. Because of dead zones, angles above 60 degrees are typically approximated with  of 45 degrees.

材料科学与工程学院 College of Materials Science & Engineering Page 37 Page 37 Ideal Work Friction Effect Redundant Work Total Pressure  p EFFECTS OF DIE ANGLE ON THE VARIOUS FORCE PARAMETERS

材料科学与工程学院 College of Materials Science & Engineering Page 38 Page 38 STRAIN RATE DURING EXTRUSION At high temperature, the flow stress is largely a function of the strain rate. Recall: Hence, strain rate is important in most extrusion operations. For high extrusion ratios and  = 45 degrees, this reduces to: In actuality, the strain rates are very non- uniform, resulting in large differences in microstructure and residual stress states.

材料科学与工程学院 College of Materials Science & Engineering Page 39 Page 39 EXAMPLE PROBLEM  p Consider extrusion of pure copper in a square die (  =90 degrees) at 1500 F. The initial diameter and length of the billet are 5" and 10" respectively. Assuming poor lubrication, determine the force required to reduce the diameter to 2" at a ram speed of 10"/sec. R=A o /A f =5 2 /2 2 =6.25 Get, p = 170 ksi. Therefore, force, F=p.A o = 3,338 kip

材料科学与工程学院 College of Materials Science & Engineering Page 40 Page 40 DEFECTS IN EXTRUSION 1. Surface Cracking: High friction and stick-slip lead to fir-tree cracking. At high temperature and high strain rates, surface sticking occurs, and intergranular failures are observed. At low temperatures, high friction with stick slip can result in metal coming out in bursts. The appearance is like bamboo tree, and is often known as bamboo defect. 2. Oxide inclusion drawn in from surface, also called fish- tailing

材料科学与工程学院 College of Materials Science & Engineering Page 41 Page 41 DEFECTS IN EXTRUSION 3. Center-Burst or Chevron Cracking: Caused by accumulation of a mean tensile stress in the center region of the billet. L h High h/L may result in plastic zones not meeting. In order to attain the same length everywhere, the central region is put in tension. This results in cracking.

材料科学与工程学院 College of Materials Science & Engineering Page 42 Page 42 LUBRICATION IN EXTRUSION Lubrication is critical during extrusion. Graphite and molybdenum disulfide are often used. At high temperatures, glass is used. A circular glass pad is used as the starting material, and this softens up at the extrusion temperature. For materials with a tendency to stick, softer metal jackets of Al or Cu are used. These jackets also prevent contamination.

材料科学与工程学院 College of Materials Science & Engineering Page 43 Page 43 The actual extrusion process begins whe n the ram starts applying pressure to the billet within the container, as is shown:

材料科学与工程学院 College of Materials Science & Engineering Page 44 Page 44 Hydraulic-Extrusion Press Figure General view of a 9-MN (1000-ton) hydraulic- extrusion press. Source: Courtesy of Jones & Laughlin Steel Corporation.

材料科学与工程学院 College of Materials Science & Engineering Page 45 Page 45 Drawing 拉拔 Drawing is an operation, developed between A.D and 1500, in which the crosssection of solid rod, wire, or tubing is reduced or changed in shape by pulling it through a die. Drawn rods are used for shafts( 轴类）, spindles( 细长类）, and small pistons and as the raw material （原材料） for fasteners such as rivets （铆钉）, bolts, and screws. In addition to round rods, various profiles can also drawn. The term drawing is also used to refer to making cupshaped parts by sheet forming operations 。

材料科学与工程学院 College of Materials Science & Engineering Page 46 Page 46 Drawing Commonly used to make wires from round bars Similar to extrusion, except: pulling force is applied

材料科学与工程学院 College of Materials Science & Engineering Page 47 Page 47 Process Variables in Wire Drawing Figure Process variables in wire drawing. The die angle, the reduction in cross-sectional area per pass, the speed of drawing, the temperature, and the lubrication all affect the drawing force, F. 拉拔

材料科学与工程学院 College of Materials Science & Engineering Page 48 Page 48 Examples of Tube-Drawing Operations Figure Examples of tube- drawing operations, with and without an internal mandrel. Note that a variety of diameters and wall thicknesses can be produced from the same initial tube stock (which has been made by other processes).

材料科学与工程学院 College of Materials Science & Engineering Page 49 Page 49 Die for Round Drawing Figure Terminology of a typical die used for drawing round rod or wire. Figure Tungsten- carbide die insert in a steel casing. Diamond dies, used in drawing thin wire, are encased in a similar manner.

材料科学与工程学院 College of Materials Science & Engineering Page 50 Page 50 Roll Straightening Figure Schematic illustration of roll straightening of a drawn round rod (see also Fig. 13.7).

材料科学与工程学院 College of Materials Science & Engineering Page 51 Page 51 Cold Drawing Figure Cold drawing of an extruded channel on a draw bench, to reduce its cross-section. Individual lengths of straight rod or of cross-sections are drawn by this method. Source: Courtesy of The Babcock and Wilcox Company, Tubular Products Division.

材料科学与工程学院 College of Materials Science & Engineering Page 52 Page 52 Multistage Wire-Drawing Figure Two views of a multistage wire- drawing machine that is typically used in the making of copper wire for electrical wiring. Source: H. Auerswald.