Manufacturing Processes Chap. 15 - Extrusion & Drawing

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. Extruded materials include lead, copper, aluminum, magnesium (low yield strength materials). 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.

Direct, Indirect, Hydrostatic, Lateral. Extrusion Types Types of extrusion: (Fig. 15.3) Direct, Indirect, Hydrostatic, Lateral. In Direct Extrusion: Solid ram drives the entire billet to and through a stationary die. Must provide additional power to overcome frictional resistance between billet surface and die walls. In Indirect Extrusion: A hollow ram drives the die back though a stationary confined billet. No relative motion: >> no friction between billet and die walls. Lower forces required, can extrude longer billets. More complex process, more expensive equipment required.

Extrusion Types In Hydrostatic Extrusion: (Fig. 15.3) 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. In Lateral Extrusion: Force is applied in one direction and extruded product leaves die in normal direction.

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

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

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 f (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 (psi)

F = P (2.5) 2 (35,000) ln [(P (2.5) 2) / (P (1.0) 2)] Example 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 figure for 70-30 brass :: 35,000 psi at 1250 deg. F. F = P (2.5) 2 (35,000) ln [(P (2.5) 2) / (P (1.0) 2)] = 1.26 x 106 lb = 630 tons.

Metal Flow Is quite complex. Will 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 seen (fig. 15.7).

Extrusion can be Hot or Cold Hot Extrusion (Fig. 15.3a) 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. (Fig. 15.11) Slugs having less than 1.5” diam. are sheared / ends ground; larger slugs are machined. Punch descends on a blank, which is extruded backward. Slug dimensions and material, as well as lubrication are key variables. 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.

Hollow Shape Designs Can make extrusion designs having multiple longitudinal cavities. (Fig. 15.2) Spider, porthole or bridge dies are used. Hot metal divides and flows around the internal spider shaped mandrel into strands. Further reduction forces seams to close and re-weld given the high pressure and temperature in the chamber. Since no metal has been exposed to contamination, perfect weld results. Process is only good for aluminum / its alloys.

Die Designs Non-ferrous metals use ‘square’ dies (α = 90 deg) Non-ferrous metals are more softer/ductile than ferrous metals. A dead metal zone will not be as catastrophic as for ferrous metals. Using small die angles with ferrous metals causes increased pressure on the tooling. Tubing can be made by having solid or hollow billets with or without the use of a mandrel. (fig. 15.19)

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. See fig. 15.7

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.

Benefits & Limitations Benefits include: Ability to extrude brittle materials. Low friction. Can use small die angles / high R values. Can extrude metals / polymers. Limitations Limited industrial applications. Complex tooling required. Need specialized equipment.

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).

Defects in Extrusions Pipe When dead zones are produced, oxides /impurities are drawn to the center of the billet, like a funnel. Requires scrapping the product. Can minimize by modifying flow pattern to make it more uniform, modifying friction and temperature gradients. Can also machine billet surface or chemical etch prior to extrusion.

Defects in Extrusions Internal Cracking Center of extrusion tends to develop cracks of various shapes. Center, center-burst, arrowhead, chevron cracking. Due to hydrostatic stress at CL in the deformation zone in the die. Center cracking: Increases with increasing die angle. Increases with impurities. Decreases with increasing R and friction.

Drawing Definition (See Fig. 15.18) Cross section of a round rod / wire is reduced by pulling it through a die. Variables: Die Angle a 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. Friction increases, drawing force increases. 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. Back relief angle

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.)