4. Extrusions Brief introduction Compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross‑sectional shape Process is similar to squeezing toothpaste out of a toothpaste tube In general, extrusion is used to produce long parts of uniform cross-sections Can be hot or cold Hot – steels Cold –soft metals

Extrusions, and examples of products made by sectioning off extrusions.

Extrusion Process Direct – Indirect – Hydrostatic – billet is smaller in diameter than chamber and fluid pressure forces billet by a ram Side (lateral) – extrusion forced at right angle to billet

Schematic illustration of the direct extrusion process.

Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral.

Metal Flow in Extrusion Elongated grain structure Can test with grid test

Types of metal flow in extruding with square dies 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.

Extrusion-Die Configurations (b) (c) Typical extrusion-die configurations: (a) die for nonferrous metals; (b) die for ferrous metals; (c) die for T-shaped extrusion

Hot Extrusion Dies can be preheated to reduce wear and to reduce cooling of the billet Oxide film can form so a dummy block smaller in diameter is placed ahead of ram Square dies used for non-ferrous metals Work piece has high plasticity and low deformation resistance Used for parts with big sizes and high strength

Cold Extrusion Improved mechanical properties by work hardening Good dimensionals Improved surface finish Elimination of billet heating Competitive production rates High stresses on tooling

Two examples of cold extrusion Two examples of cold extrusion. Thin arrows indicate the direction of metal flow during extrusion.

Production steps for a cold extruded spark plug. A cross-section of the metal part above, showing the grain flow pattern.

(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. (b)-(d) Components of various dies for extruding intricate hollow shapes.

Poor and good examples of cross-sections to be extruded Poor and good examples of cross-sections to be extruded. Note the importance of eliminating sharp corners and of keeping section thicknesses uniform.

Impact Extrusion Punch descends rapidly on the blank and extruded backward Collapsible tubes Thin walled parts

Impact Extrusion Schematic illustration of the impact-extrusion process. The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch.

(a) Two examples of products made by impact extrusion (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.

Hydrostatic Extrusion No container wall friction High pressure Defects can be reduced by fluid-to-fluid extrusion Vegetable oils used at room temperature Waxes, polymers, glass used for high temp Limited use due to complex tooling, high pressures, special equipment, and long cycle times

Extrusion Defects Surface cracking Pipe Internal cracking Temperature, friction, or speed too high Sticking along die land Pipe Impurities drawn toward center of billet Can be minimized by controlling flow and by eliminating impurities Internal cracking Tensile stresses Increases with increased impurities Decreases with increased extrusion ratio and friction

(a) (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.

Extrusion Equipment Horizontal hydraulic press Can control stroke and speed Vertical can be used for cold extrusion

General view of a 9-MN (1000-ton) hydraulic-extrusion press.

5. Drawing Brief introduction Cross-section of wire or rod reduced by pulling through die Limit is 63% reduction in cross section Shapes such as flat strips can be drawn

Process variables in wire drawing 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.

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).

Drawing Practice Speeds of 200-500 ft/min for heavy to 10,00 ft/min for thin Reduction of 0-45% Bundling – drawing many wires simultaneously Material is usually tool steel or carbide Wet drawing – completely immersed in lube Dry drawing – surface of rod coated

Die for Round Drawing Terminology of a typical die used for drawing round rod or wire.

Schematic illustration of roll straightening of a drawn round rod

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.

Two views of a multistage wire-drawing machine that is typically used in the making of copper wire for electrical wiring

Questions What are some of the attractive features of the cold extrusion process? What is the unique capabilities and special limitations of hydrostatic extrusion?

6. Sheet metalworking Brief introduction Cold-working The starting stock is sheet metal Higher deformation forces Closer dimensional tolerances Increased strength High production rate High die cost for small quantities

Sheet metal process Shearing Bending Drawing Sheet metal forming

Shearing Mechanical cutting of materials without the formation of chips or the use of burning or melting. Shearing process Sheared edges Burr Break Burnished land Roll over

Sheared edge improvement Clamping the starting stock firmly against the die Maintaining proper clearance and alignment between the punch and the die Restraining the movement of the workpiece Simple shearing Sheets of metal are sheared along a straight line Shear angle is often used to reduce shear force on larger shear

Piercing and blanking Die design Shearing operations where the shear blades are closed In piercing, the punch-out piece is the scrap and the remaining strip is the workpiece. In blanking, the piece being punched out becomes the workpiece and the remaining strip is the scrap. Die design The punch should fit within the die with a clearance between 5% to 7% of the stock thickness. Punches and dies should be in proper alignment so that a uniform clearance is maintained.

Bending The plastic deformation of metals about a linear axis with little or no change in the surface area. Bending process The metal on the outside is stretched while that on the inside is compressed. The location that is neither stretched nor compressed is neutral axis. The outside surface undertakes the maximum tensile stress

Bend radius Springback The smaller the bend radius, the larger the deformation Should be as large as possible to permit easier forming and wider variety of engineering materials Springback Elastic recovery occurs when the bending load is removed The yield strength of the metal and the bending radius are involved in the amount of springback The metal must be overbent by an amount equal to the subsequent springback

Design of bending The smallest bend radius The metal is formed without cracking The value is dependent on both the ductility and the thickness of the metal sheet The process must provide springback compensation when the bend radius is greater than four times of the material thickness The bend axis should perpendicular to the direction of grain flow

Drawing (sheet metal drawing) The forming of closed-bottom cylindrical or rectangular containers from metal sheet Drawing process Shallow drawing and deep drawing Shallow drawing The depth of the product is less than its diameter Deep drawing The depth of the product is greater than the diameter

Trimming of drawn parts Drawing defects Wrinkle and tear Can be limited by multiple operations when deep drawing parts are made Trimming of drawn parts Establish both the size and uniformity of the final part

Sheet metal forming Embossing Bulging Flanging A pressworking process in which raised lettering or other designs are impressed in sheet material Very shallow drawing operation where the depth of the draw is limited to one to three times the thickness of the metal Bulging Locally expand a metal blank or tube outward against a split die Flanging Flanges are rolled on sheet metal

Superplastic sheet forming Metals achieve tensile elongations as high as 2000 to 3000% Ultrafine-grain-size, low strain rates and elevated temperatures are required Can be used to form material into large, complex-shaped products with compound curves by single operation Can be extrusion, drawing, forging, etc. The major limitation is the long cycle times

Questions What is springback? What determines the amount of springback in bending? What factors determine the smallest bend radius for materials? What measures can be employed to improve the quality of a sheared edge? What is the purpose of having a shear angle on a punch? What is the major limitation of the superplastic forming of sheet metal? What are some of the attractive features?