Chapter 16 Bulk Forming Processes (Part 2: Extrusion & Drawing) (Review) EIN 3390 Manufacturing Processes Summer A, 2011 1

16.6 Extrusion Metal is compressed and forced to flow through a shaped die to form a product with a constant cross section A ram advances from one end of the die and causes the metal to flow plastically through the die Pressing ram Figure 16-25 Direct extrusion schematic showing the various equipment components. (Courtesy of Danieli Wean United, Cranberry Township, PA.)

16.6 Extrusion Metal is compressed and forced to flow through a shaped die to form a product with a constant cross section A ram advances from one end of the die and causes the metal to flow plastically through the die Pressing ram Figure 16-25 Direct extrusion schematic showing the various equipment components. (Courtesy of Danieli Wean United, Cranberry Township, PA.)

Extrusion Definition: Purpose: Process of forcing a billet through a die above its elastic limit, taking shape of the opening. Purpose: To reduce its cross-section or to produce a solid or hollow cross section. Analogy: “Like squeezing toothpaste out of a tube”.

Extrusion Extruded products always have a constant cross- section. It can be a semi-continuous or a batch process. Extrusions can be cut into lengths to become discrete parts like gears, brackets, etc. A billet can also extruded individually in a chamber, and produces discrete parts. Typical products: railings, tubing, structural shapes, etc.

Extrusion Can be performed at elevated temperatures or room temperatures, depending on material ductility. Commonly extruded materials include aluminum, magnesium (low yield strength materials), copper, and lead. Steels and nickel based alloys are far more difficult to extrude (high yield strength materials). Lubricants are essential to extrude high strength alloys to avoid tendency of material to weld to die walls.

Advantages of Extrusion Many shapes can be produced that are not possible with rolling No draft is required Amount of reduction in a single step is only limited by the equipment, not the material or the design Dies are relatively inexpensive Small quantities of a desired shape can be produced economically

Extrusion Methods Methods of extrusion: Hot extrusion is usually done by either the direct or indirect methods. Direct extrusion Solid ram drives the entire billet to and through a stationary die Must provide additional power to overcome friction between billet surface and die walls

Extrusion Methods Indirect extrusion A hollow ram pushes the die back through a stationary, confined billet No relative motion and no friction between billet and die walls. Lower forces required, can extrude longer billets. More complex process, more expensive equipment required.

Extrusion Methods Figure 16-27 Direct and indirect extrusion. In direct extrusion, the ram and billet both move and friction between the billet and the chamber opposes forward motion. For indirect extrusion, the billet is stationary. There is no billet-chamber friction, since there is no relative motion.

Variables in Extrusion Die Angle Extrusion Ratio R (A0 / Af): where A0 and Af are billet and extruded product areas. Billet Temperature Ram Velocity Type of Lubricant used.

Variables in Extrusion

Extrusion Parameters defining the extruded shape: CCD (Circumscribing Diameter): Diameter of the smallest circle into which the extruded cross section can fit. Shape Factor = Perimeter / Cross-Area: the larger the shape factor, the more complex the part.

Variables in Direct Extrusion Fig : Process variables in direct extrusion. The die angle, reduction in cross-section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure. Fig : Method of determining the circumscribing-circle diameter (CCD) of an extruded cross-section.

Extrusion Parameters defining the extruded shape Example of Shape Factors between circle and square shapes: Shape factor of a circle = (p .D)/ (0.25p .D2) = 4/D, and shape factor of square = 4 a/a2 =4/a . If areas of the circle and the square are the same: Ac = As , then a2 = (p .D2)/4 , so a = 0.8862D, or D = 1.1284a shape factor of square = 4/a = 4/(0.8862D) = = 1.1884 of shape factor of circle

Extrusion Parameters defining the extruded shape: Reduction Ratio R = Ab/Ap Where, Ab – cross section area of starting stock Ap – across section area of extruded product For a cylinder-to-cylinder extrusion, the area of starting cylinder: Ab = (p .Db2)/4 , and the area of extruded cylinder: Ap = (p .Dp2)/4 . R = Ab/Ap = (0.25p .Db2)/ (0.25p .Dp2) = (Db/Dp)2 if (Db/Dp) = 4, then R = 16

Extrusion Practices Usually billets less than 25’ in length. CCD ranges from ¼” to 40”. Typical values for R range between 10 and 100. Ram speeds up to 100 ft/min, with lower speeds for the most common extruded alloys. Dimensional tolerances (+/- 0.01” to +/- 0.1”) increase with cross section.

Extrusion Force Factors for determining extrusion force: billet strength, extrusion ratio, friction between billet and die surfaces, temperature, extrusion speed. Estimation of Force required: F = A0 k ln (A0/Af) k = extrusion constant Ao, Af billet and extruded product cross section areas

Extrusion Constant K: Fig : Extrusion constant k for various metals at different temperatures

Example for calculation Extrusion Force Given: a 70-30 brass round billet is extruded at 1250 deg. F. Billet diam. = 5”. Extrusion Diam. = 2”. Find: Required force. Assumptions: friction is negligible. Solution: Find k from Fig. 15.6 for 70-30 brass : 30,000 psi at 1250 deg. F. F = p (2.5) 2 (30,000) ln [(p (2.5) 2) / (p (1.0) 2)] = 1.08 x 106 lb = 490 tons.

Forces in Extrusion Lubrication is important to reduce friction and act as a heat barrier Metal flow in extrusion Flow can be complex Surface cracks, interior cracks and flow-related cracks need to be monitored Process control is important Figure 16-28 Diagram of the ram force versus ram position for both direct and indirect extrusion of the same product. The area under the curve corresponds to the amount of work (force x distance) performed. The difference between the two curves is attributed to billet-chamber friction.

Lubrication Essential in drawing to improve die life, reduce drawing forces/temperature, improve surface finish, particularly in hot extrusion. Difficult to maintain a constant lubricant film constant between the die and the workpiece. Can coat with zirconia to add life to die.

Metal Flow Quite complex. Impact quality and mechanical properties of product: must not overlook to prevent defects. Extruded products have elongated grain structure. Metal at center passes through die w/little distortion Metal near surface undergoes considerable shearing. Friction between moving billet and stationary chamber walls impedes surface flow. Result is deformation pattern

Extrusion Methods In Hydrostatic Extrusion: The chamber, which is larger than the billet, is filled with a fluid. The fluid is compressed with the ram and pushes the billet forward. Benefit: no friction to overcome along sides of chamber.

Hydrostatic Extrusion High-pressure fluid surrounds the workpiece and applies the force to execute extrusion Billet-chamber friction is eliminated High efficiency process Temperatures are limited because the fluid acts as a heat sink Seals must be designed to keep the fluid from leaking Figure 16-32 Comparison of conventional (left) and hydrostatic (right) extrusion. Note the addition of the pressurizing fluid and the O-ring and miter-ring seals on both the die and ram.

Extrusion can be Hot or Cold Hot Extrusion Takes place at elevated temperatures. Used in metals that have low ductility at room temperature. Need to pre-heat dies to prolong die life and reduce billet cooling. Hot working tends to develop an oxide film on the outside of the work unless done in an inert environment. Solution: place smaller-diameter dummy block ahead of ram before the billet. A layer of oxidized material is then left in the chamber, and is later removed and final part is free of oxides.

Extrusion can be Hot or Cold Cold Extrusion (also know as Impact Extrusion) Designated as cold when combined with other forging operations. Key variables: Slug dimensions Material property, and lubrication Diameters up to 6” and thin walls can be made. Collapsible tubes can be made this way (toothpaste tubes).

Advantages Cold vs. Hot Extrusion Better mechanical properties due to work- hardening. Good dimensional tolerances & surface finish. No need to heat billet. Competitive production rates & costs. Hot: Larger variety of materials. Less forces required. Better material flow.

Guidelines for Die Design Avoid sharp corners Have similarly sized voids if possible. Have even thickness in walls if possible. General idea is to favor even flow.

Defects in Extrusions Surface Cracking / Tearing Occurs with high friction or speed. Can also occur with sticking of billet material on die land. Material sticks, pressure increases, product stops and starts to move again. This produces circumferential cracks on surface, similar to a bamboo stem. (referred to as bambooing).

16.7 Wire, Rod, and Tube Drawing Reduce the cross section of a material by pulling it through a die Similar to extrusion, but the force is tensile Figure 16-36 Cold-drawing smaller tubing from larger tubing. The die sets the outer dimension while the stationary mandrel sizes the inner diameter. Figure 16-34 Schematic drawing of the rod-or bar-drawing process.

Drawing Definition Variables: Cross section of a round rod / wire is reduced by pulling it through a die. Variables: Die Angle, Extrusion Ratio R (A0 / Af) , Friction between die and workpiece, drawing speed. “There is an optimum angle at which the drawing force is minimum” for a given diameter reduction and friction parameter.

Drawing Estimation of Drawing Force required: F = Yavg Af ln (A0/Af) Yavg = average true stress of material in the die gap. Assumptions: no friction.

Drawing Work has to be done to overcome friction. Force increases with increasing friction. Cannot increase force too much, or material will reach yield stress. Maximum reduction in cross-sectional area per pass = 63%.

Drawing Die Design Die angles range from 6 to 15 degrees. Two angles are typically present in a die: Entering angle Approach angle Bearing Surface (land): sets final diameter of the drawn stock. Back relief angle: exit zone

Figure 16-39 Cross section through a typical carbide wire-drawing die showing the characteristic regions of the contour. Figure 16-40 Schematic of a multistation synchronized wire-drawing machine. To prevent accumulation or breakage, it is necessary to ensure that the same volume of material passes through each station in a given time. The loops around the sheaves between the stations use wire tensions and feedback electronics to provide the necessary speed control.

Figure 16-39 Cross section through a typical carbide wire-drawing die showing the characteristic regions of the contour. Figure 16-40 Schematic of a multistation synchronized wire-drawing machine. To prevent accumulation or breakage, it is necessary to ensure that the same volume of material passes through each station in a given time. The loops around the sheaves between the stations use wire tensions and feedback electronics to provide the necessary speed control.

Defects in Drawing Center cracking. Seams (folds in the material) Residual stresses in cold-drawn products. If % reduction is small: (Compressive at surface / Tensile at Center) If % reduction is larger, opposite occurs: (not desirable- can cause stress corrosion cracking.)

Tube and Wire Drawing Tube sinking does not use a mandrel Internal diameter precision is sacrificed for cost and a floating plug is used Figure 16-37 (Above) Tube drawing with a floating plug. Figure 16-38 Schematic of wire drawing with a rotating draw block. The rotating motor on the draw block provides a continuous pull on the incoming wire.

16.8 Cold Forming, Cold Forging, and Impact Extrusion Slugs of material are squeezed into or extruded from shaped die cavities to produce finished parts of precise shape and size Cold heading is a form of upset forging Used to make the enlarged sections on the ends of rod or wire (i.e. heads of nails, bolts, etc.) Figure 16-41 Typical steps in a shearing and cold-heading operation.

Impact Extrusion A metal slug is positioned in a die cavity where it is struck by a single blow Metal may flow forward, backward or some combination The punch controls the inside shape while the die controls the exterior shape Figure 16-43 Backward and forward extrusion with open and closed dies.

Cold Extrusion Figure 16-44 (a) Reverse (b) forward (c) combined forms of cold extrusion. (Courtesy the Aluminum Association, Arlington, VA.) Figure 16-45 (Right) Steps in the forming of a bolt by cold extrusion, cold heading, and thread rolling. (Courtesy of National Machinery Co. Tiffin, OH.)

Figure 16-46 Cold-forming sequence involving cutoff, squaring, two extrusions, an upset, and a trimming operation. Also shown are the finished part and the trimmed scrap. (Courtesy of National Machinery Co., Tiffin, OH.)

Figure 16-46 Cold-forming sequence involving cutoff, squaring, two extrusions, an upset, and a trimming operation. Also shown are the finished part and the trimmed scrap. (Courtesy of National Machinery Co., Tiffin, OH.)

Summary There are a variety of bulk deformation processes The main processes are rolling, forging, extrusion, and drawing Each has limits and advantages as to its capabilities The correct process depends on the desired shape, surface finish, quantity, etc.