CHAPTER 15 EXTRUSION 擠壓成形 AND DRAWING OF METALS Reference Book: 1) Kalpakjain, S. and Schmid, S.R., “Manufacturing Engineering and Technology”. 4th ed., Prentice-Hall, New Jersey, 2001, pp 3
69 – 391 List of Contents 15.1 Introduction 15.2 The Extrusion Process 15.3 Extrusion Practice 15.4 Hot Extrusion 15.5 Cold Extrusion 15.6 Impact Extrusion 15.7 Hydrostatic Extrusion 15.8 Extrusion Defects 15.9 Extrusion Equipment 15.10 The Drawing Process 15.11 Drawing Practice 15.12 Defects and Residual Stresses 15.13 Drawing Equipment
15.1 INTRODUCTION In extrusion process, a billet (generally round) is forced through a die (Fig. 15.1), in a manner similar to squeezing toothpaste from a tube. Almost any solid or hollow cross-section may be produced by extrusion, which can create essentially semifinishied parts (Fig. 15.2). Because the die geometry remains the same through the operation, extruded products have a constant cross-section. Extrusion is often combined with forging operations, in which case it is generally known as cold extrusion. Typical products made by extrusion are: railings for sliding doors, tubing having various cross-sections, structural and architectural shapes, and door and window frames. Drawing is an operation in which the cross-section of solid rod, wire, or tube is reduced or changed in shape by pulling it through a die. Drawn rods (being larger diameter of wires that normally has smaller diameter with sizes down to 0.01 mm) are used for shafts, spindles, and small pistons and as the raw material for fasteners such as rivets, bolts, and screws. 15.2 THE EXTRUSION PROCESS Direct (or forward) extrusion
In the basic direct (forward) extrusion process, a round billet is placed in a chamber (container) and forced through a die opening by a hydraulically-driven ram or pressing stem (Fig. 15.1). The die opening may be round, or it may have various shapes. Indirect (reverse, inverted, or backward) extrusion In indirect (reverse, inverted, or backward) extrusion, the die moves toward the billet of which the material immediately around the die is effectively deformed and moves backwardly in relation to the direction of the tool stem (Fig. 15.3a). Hydrostatic extrusion In hydrostatic extrusion (Fig. 15.3b), the billet is smaller in diameter than the chamber, which is filled with a fluid, and the pressure is transmitted to the billet by a ram. Since the billet is surrounded with fluid that isolates it from the contact with container, there is no friction to overcome along the container walls. Lateral extrusion In lateral extrusion (Fig. 15.3c), the billet in container is squeezed to flow out from die opening in die situated at the side wall of the container. Process Variables in Direct Extrusion Referring to Fig. 15.4, the geometric variables in extrusion are: 1) The die angle α 2) The ratio of the cross-sectional area of the billet to that of the extruded product Ao/Af, which is called extrusion ration R Other variables in extrusion are: 1) Temperature of the billet, 2) The speed at which the ram travels, and 3) The type of lubricant used Circumscribing-Circle Diameter (CCD) A parameter describing the shape of the extruded product is the circumscribing-circle diameter (CCD), which the diameter of the smallest circle into which the extruded crosssection will fit (Fig. 15.5). CCD for a square cross-section is its diagonal dimension. Shape Factor The complexity of an extrusion is a function of the ratio of the perimeter of the extruded product to its cross-sectional area, known as the shape factor. Obviously, a solid round extrusion has the smallest shape factor, whereas the parts shown in Fig. 15.2 have high shape factor. Extrusion Force
The force required for extrusion depends on: (i) strength of the billet material, (ii) the extrusion ratio, (iii) friction between the billet and the chamber and die surface, and (iv) process variables such as the temperature of the billet and the speed of extrusion. The extrusion force F can be estimated from: F = Ao k ln(Ao/Af)
where k is the extrusion constant, and Ao 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 temperature. Example: Calculation of Force in Hot Extrusion Around billet made of 70-30 brass is extruded at a temperature of 675oC (1250oF). The billet diameter is 5 in. (125 mm) and the diameter of the extrusion is 2 in. (50 mm). Calculate the extrusion force required. Solution: The extrusion force is calculated using Eq. (15.1), in which the extrusion constant, k, is obtained from Fig. 15.6. For this material, we find that k = 35,000 psi (250 MPa) at the extrusion temperature. Thus, F = Ao k ln(Ao/Af)= π(2.5)2 (35,000) ln[π(2.5)2/ π(1.0)2] =1.26 x 106 lb =630 tons = 5.5 MN
Metal Flow in Extrusion Like incompressible fluid flow in a channel, extruded products have an elongated grain structure (preferred orientation). A common technique for investigating the flow pattern is to section the round billet in half lengthwise and then mark one face with a square grid pattern. The two halves are placed in the chamber together and extruded. The products are then taken apart and studied. Fig. 15.7 shows typical flow patterns obtained by this technique in direct extrusion with square dies (90o die angle). Types of Metal Flow in Extruding With Square Dies Fig. 15.7a shows the flow pattern obtained at low friction, or in indirect extrusion. It is noted that, except those 5 columns of grid-meshes closer to the inlet to the die opening, the other grid-meshes almost remain as what they originally were. This suggests the effective deformation zone is relatively small. It is seen that the level of distortion severity of meshes increases when they are closer to the die. The closer space for the meshes around the die opening means materials to be compressed while the longer lengthwise of meshes along the central line of the r.h.s. indicates the material is under tensile deformation.
Fig. 15.7b shows the distortion of grid-meshes to be more severe and the effective deformation is effectively propagating to the upstream of the chamber. Also there is region of dead zone around the die angle when friction is high. Fig. 15.7c shows the deformation to be very severe when the extrusion is conducted at high friction and/or with cooling of the outer regions of the billet in the chamber. The deformation type shows the flow pattern likely to lead to a defect named pipe. 15.4 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. Page 15-9 tabulates the extrusion temperature ranges for various metals, which can be used as reference purpose. Die Design and Die Materials Die design (Fig. 15.8) requires considerable experience. Square dies (shear dies) are used in extruding nonferrous metals, especially aluminum. Square dies develop dead-metal zones, which in turn form a die angle (see Fig. 15.7b and c) along which the material flows in the deformation zone. The dead-metal zones produce extrusion with bright finishes. Tubing is extruded from a solid or hollow billet to wall thickness as small as 1 mm. For solid billets, the ram is fitted with a mandrel that pierces a hole in the billet. Billets with a previously pierced hole may also be extruded in this way. Because of friction and the severity of deformation, thin-walled extrusions are more difficult to produce than thickwalled extrusions (For Al, thickness limited to 1 mm; for carbon steel to 3 mm; for stainless steel to 5 mm). Hollow cross-sections (Fig. 15.9a) can be extruded by welding-chamber methods and the use of various dies known as spider dies, porthole dies, and bridge dies (Fig. 15.9b). During extrusion, the metal divides and flows around the supports for the internal mandrel into strands; these strands are then re-welded under the high pressures existing in the welding chamber, before they exit through the die. This condition is much like that of air flowing around a moving car and rejoining downstream. Guidelines for proper die design in extrusion are illustrated in Fig. 15.10. Note the importance of symmetry of cross-section; note also the avoidance of sharp corners and of extreme changes in die dimensions within the cross-section. Extrusion-Die Configurations Fig. 15.8a is the configuration of a die configuration for extruding relatively softer nonferrous metals. Fig. 15.8b is the counterpart for extruding ferrous metals. The arrow shows the direction of metal flow. Comparison of Fig. 15.8a with Fig. 15.8b shows that the die for nonferrous metals has angle almost 90o and long land section, whilst for ferrous metals they are 60o and shorter respectively. In extrusion, the die in Fig. 15.8a is likely to generate larger dead-metal zone than in Fig. 15.8b as ferrous metals are harder.
Fig. 15.8c shows a die for T-shaped extrusion, made of hot-work die steel and used with molten glass as a lubricant to reduce friction. Components for Extruding Hollow Shapes Fig. 15.9 shows an extruded 6063-T6 aluminum ladder lock for aluminum extension ladders. The part is 8 mm thick and is sawed from the extrusion. Figs. 15.9b and d shows the components of various dies for extruding intricate hollow shapes. The figures illustrate the configuration of various die-parts for achieving different extrusion geometries. Cross-Sections to Be Extruded Fig. 15.10 gives the examples of poor and good cross-sections to be extruded. The poor one (Fig. 15.10a) has: (i) sharp corners, (ii) knife edge, (iii) unbalanced section wall, (iv) unbalanced voids, (v) inadequate section thickness between the two unbalanced voids, (vi) unbalanced die tongue. All these make the extrusion difficult to be produced. The good one (Fig. 15.10b) rectifies those poor designs in Fig. 15.10a, in which the die tongue becomes more balanced, the sharp corners are properly filled, the wall thickness is balanced and adequately uniform and large, and the voids are also suitably balanced. Cold Extrusion Cold extrusion is a general term often denoting a combination of operations, such as direct and indirect extrusion and forging (Fig. 15.11). The process uses slugs cut from cold-finishes or hot-rolled bar, wire, or plate. Slugs that are less than about 40 mm in diameter are sheared, and their ends are squared by grinding or upsetting. Cold extrusion has the following advantages over hot extrusion: 1) improved mechanical properties resulting from work-hardening, provided that the heat generated by plastic deformation and friction does not recrystallized the extruded metal; 2) good control of dimensional tolerances, reducing the need for subsequent machining or finishing operations; 3) improved surface finish, due partly to lack of an oxide film, provided that lubrication is effective; 4) elimination of the need for billet heating; 5) production rates and costs that are competitive with those of other methods of producing the same part; some machines are capable of producing more than 2000 parts per hour. The magnitude of the stresses on the tooling in cold extrusion, however, is very high, especially with steel workpieces, being on the order of the hardness of the workpiece material. Example of Cold-Extruded Part Fig. 15.12 shows the production steps for a cold extruded spark plug. Firstly, a slug is sheared off the end of a round rod (on the left of Fig. 15.12); it is then cold extruded (Middle in Fig. 15.12), in an operation similar to those shown in Fig. 15.11, but with a
blind hole. The material at the bottom of the blind hole is then punched at the bottom of the sectioned part, respectively. The grain structure for the corresponding bi-sectional cross-section is shown in Fig. 15.13. The grain flow pattern indicates the favorable properties likely to obtain. Generally, the studying of material flow during deformation helps avoid defects and leads to improvements in punch and die design. 15.6 IMPACT EXTRUSION Impact extrusion is similar to indirect extrusion. The punch descends rapidly on the blank (slug), which is extruded backward (Fig. 15.14). Because of volume constancy, the thickness of the tubular extruded section is a function of the clearance between the punch and the die cavity. The extruded parts tend to stick to the punch and are thus stripped by the used of stripper plate (see: left and right diagrams in Fig. 15.14). Typical products made by impact extrusion are shown in Fig. 15.15a. Another example is the production of collapsible tubes, such as toothpaste (Fig. 15.15b and c). Observing that the punch (Fig. 15.15b) has a larger diameter shoulder and gradually increasing size from its tip-section. The former provides clearance for the reduction of contact friction (Fig. 15.15c) and the shallow die cavity facilitating the production of free surface, which subsequently lowers the extrusion force. Most non-ferrous metals can be impact-extruded in vertical presses and at production rates as high as two parts per second. 15.8 EXTRUSION DEFECTS Below are some possible defects likely to occur in extrusion. 1) Surface cracking. If extrusion temperature, friction, or speed is too high, surface temperature rise significantly, and this condition may cause surface cracking and tearing (fir-tree cracking or speed cracking). These cracks are intergranular (along the grain boundaries and are usually caused by hot shortness. 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. - Surface cracking may also occur at lower temperatures, where it has been attributed to periodic sticking of the extruded product along the die land. When the product being extruded sticks to the die land, the extrusion pressure increases rapidly. Shortly thereafter, the product moves forward again and the pressure is released. The cycle is then repeated continually, producing periodic circumferential cracks on the surface. Because of the similarity in appearance o the surface of a bamboo stem which it causes, it is known as bamboo defect. 2) Pipe. The type of metal-flow pattern shown in Fig. 15.7c tends to draw surface oxides and impurities toward the center of the billet, much like a funnel. This defect is known as pipe defect, also tailpipe or fishtailing. As much as one-third of
the length of the extruded product may contain this type of defect and have to be cut off as scrap. - Piping can be minimized by modifying the flow pattern to a more uniform one; for example, by controlling friction and minimizing temperature gradients. Another method is to machine the billet’s surface prior to extrusion, so that scale and surface impurities are removed. These impurities can also be removed by chemical etching of the surface oxides prior to extrusion. 3) Internal cracking. The center of the extruded product can develop cracks variously called center cracking, center-burst, arrowhead fracture, or chevron cracking (Fig. 15.16a). These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die (Fig. 15.16b), a situation similar to the necked region in a tensile-test specimen. - The tendency for center cracking: a. increases with increasing die angles; b. increases with increasing amount of impurities; and c. decreases with increasing extrusion ratio and friction.
15.9 EXTRUSION EQUIPMENT The basic equipment for extrusion is a horizontal hydraulic press (Fig. 15.17). These presses are suitable for extrusion, because the stroke and speed of the operation can be controlled. They are capable of applying a constant force over a long stroke; thus, long billets can be used, and the production rate increased. Fig. 15.17 shows the general view of a 9-MN (1000-ton) hydraulic extrusion press.
15.10 THE DRAWING PROCESS In drawing, the cross-section of a round rod or wire is typically reduced or changed by pulling it through a die (Fig. 15.18). Fig. 15.18 shows the process variables in wire drawing. The major variables are similar to those in extrusion: reduction in crosssectional area, die angle, friction along the die-workpiece interfaces, and drawing speed. Drawing Force F The expression for the drawing force under frictionless conditions is similar to that for extrusion; it is given by the expression F = Yavg Af ln(Ao/Af) (15.2)
where Yavg is the average true stress of the material in the die gap, and Ao and Af are the billet and drawn product areas, respectively. Because more work has to be done to overcome friction, the force increases with increasing friction.
Shapes of Drawing
Various solid cross-sections can be produced by drawing through dies with different profiles. The initial cross-section is usually round or square. Proper die design and the proper selection of reduction sequence per pass require considerable experience to ensure proper material flow in the die, reduce internal or external defects, and improve surface quality. The wall thickness, diameter, or shape of tubes that have been produced by extrusion or by other processes can be further reduced by tube drawing processes (Fig. 15.19). Fig. 15.19 gives examples of tube-drawing operations, with and without an internal mandrel. Note that a variety of diameters and wall thickness can be produced from the same initial tube stock (which has been made by other processes). 15.11 DRAWING PRACTICE Die design The characteristic features of a typical die design for drawing are shown in Fig. 15.20. Die angles usually range from 6o to 15o. Note, however, that there are two angles (entering and approach) in a typical die. The basic design for this type of die was developed through years of trial and error. The purpose of the bearing surface (land) is to set the final diameter of the product (called sizing). Also, when a worn die is reground, the land maintains the exit dimension of the die opening. A set of dies is required for profile drawing for various stages of deformation. Die materials Die materials for drawing are usually tool steels and carbides; diamond dies are used for find wire. For improved wear resistance, steel dies may be chromium plated, and carbide dies may be coated with titanium nitride. Mandrels for tube drawing are usually made of hardened tool steels or of carbides. Diamond dies are used for drawing fine wire with diameters ranging from 2 ?m to 1.5 mm. They may be made from a single-crystal diamond or else in polycrystalline form, with diamond particles in a metal matrix (compacts). Because of their cost and their lack of tensile strength and toughness, carbides and diamond dies are used as inserts or nibs, which are supported in a steel casing (Fig. 15.21). 15.11 DEFECTS AND RESIDUAL STRESSES Defects: Typical defects in drawn rod and wire are similar to those observed in extrusion. They are: 1) Center cracking (Fig. 15.16) 2) Seams (an additional type of defect, which is longitudinal scratches or folds in the material and may open up during subsequently forming operations (like upsetting, heading, bending, etc.) 3) Other surface defects like scratches and die marks due to improper selection of the drawing parameters, poor lubrication, or unsatisfactory die condition Residual stresses: Residual stresses significantly cause stress-corrosion cracking of the part over a period of time.
Straightening: Rods and tubes that are not sufficiently straight (or are supplied as coil) can be straightened by passing them through an arrangement of rolls placed at different axes (Fig. 15.22). The rolls subject the product to a series of bending and unbending operations, a process similar to roller leveling (see Fig. 13.7). 15.12 DEFECTS AND RESIDUAL STRESSES The equipment for drawing is basically of two types: 1) Draw bench. A draw bench contains a single die, and its design is similar to that of a long horizontal tension-testing machine (Fig. 15.23). The pulling force is supplied by a chain drive or is activated hydraulically. Draw benches are used for signle-length drawing of straight rod and tube with diameters larger than 20 mm and length up to 30 m. 2) Bull block. Very long rod and wire (many kilometers) and wire of smaller crosssections, usually less than 13 mm, are drawn by a rotating drum (bull block, capstan, Fig. 15.24). The tension in this setup provides the force required for drawing the wire, usually through multiple dies.