F O R G I N G & E X T R U S I O NPresentation Transcript
FORGING & EXTRUSION BULK DEFORMATION PROCESSES
FORGING Simple forgings can be made with a heavy hammer & an anvil by blacksmiths for centuries. Nowadays a set of dies & a press are used. We have open-die forging & closed-die forging. Drawing out or reducing the cross-section of an ingot or billet to lengthen it. Upsetting or reducing the length of an ingot or billet to a larger diameter. Upsetting, drawing out, and piercing--processes sometimes combined with forging over a mandrel for forging rough-contoured rings.
OPEN-DIE FORGING This form of forging generally involves placing a solid cylindrical work piece between two flat dies (platens) and reducing its height by compressing it. This is known as upsetting. The specimen will develop a barrel shape. Barreling is caused by frictional forces at die-workpiece interfaces that oppose the outward flow of material at these interfaces. Barreling also occurs in upsetting hot work pieces between cold dies.
UPSETTING A RECTANGULAR WORK-PIECE The increase in the length of the specimen is 40% the increase in the width is 230%. The reason for this is that the material flows in the direction of least resistance. a/h ratio is important & effects value of friction.
IMPRESSION-DIE FORGING The sample adopts the shape of the die cavities (impressions) while it is being upset between the closed dies. Some of the material flows out radially & forms a flash. The flash has a high (length/thickness ratio) a/h ratio, because of which it is subjected to high pressure. This means there is high frictional resistance to material flow in the radial direction in the flash gap. The high friction plays an important role in the filling of the die cavities. Hence flash is an important thing. The flash also cools down more quickly. Hence it resists deformation & encourages to fill cavity.
IMPRESSION-DIE FORGING Complex shapes can be made. The forging force increases gradually until flash starts to form. Then the forging load increases rapidly as the dies close gradually. The flash has a finite contact length with the die called land. The land ensures that the flash generates enough resistance to the outward flow of material which aids in die filling.
CLOSED-DIE FORGING Similar to impression die but no flash forms. Work piece is completely surrounded by dies & no excess metal is present for flash. Since no flash can form, proper control of material volume is essential. Under-sized blanks prevent complete filling of die. Over-sized blanks may cause premature die failure or jamming. Precision/flashless forging=near-net-shape production Aluminum & Magnesium alloys are particularly useful for precision forging because of the low forging loads, & temperatures used.
OPEN-DIE FORGING IMPRESSION-DIE FORGING
CLOSED-DIE FORGING Steels & other alloys are more difficult to precision forge. The choice between conventional & precision forging requires an economical analysis. Precision forging requires special dies. Conventional forging requires machining steps to get final product of desired shape.
MISCELLANEOUS FORGING COINING: Minting of coins. High pressures of up to 5-6x material yield point. Produces fine detail & surface finish. HEADING: forming the heads of bolts, screws & nails. Buckling may occur if the l:thick ratio is too high. COGGING: Drawing out. The thickness of the bar is reduced in successive steps, without using large forces. ROLL FORGING: cross-sectional area of bar is reduced & altered in shape by passing it through rolls with grooves of various shapes. Final products like tapered shafts, table knives produced. Also used as a preliminary process followed by other forging processes.
MISCELLANEOUS FORGING SKEW ROLLING: similar to roll forging. Used for making ball bearings. Round wire or rod is used. You can also “upset” a cylindrical piece cut form a round bar. FULLERING & EDGING:
ROLL FORGING FORGING AN INGOT COINING
DEFECTS Surface cracking Excessive material in the web can buckle during forging & form a lap. If the web is thick, the excess material can flow past the forged parts & form internal cracks. Small fillets & large filets. The material can fill large radius better. With small radii, the material folds over itself & produces a lap called cold shut which can later on lead to failure of part.
A FORGING DIE. RIB, WEB, LAND, FILLET, FLASH, GUTTER, PARTING LINE, EXTERNAL (30 -50) & INTERNAL DRAFT ANGLES (70 - 100) Common parts made by forging: crankshafts, connecting rods for engines, turbine disks, gears, bolt heads, hand tools etc.
CRANKSHAFT CONNECTING ROD
EXTRUSION The basic extrusion process, a round billet is placed in a chamber & forced through a die opening by a ram. The die may be of various shapes. Four basic types of extrusion: Direct Indirect Hydrostatic Impact
Hydrostatic Extrusion Indirect Extrusion Direct Extrusion
IMPORTANT FACTORS The extrusion ratio: R = AO /Af AO =billet cross-sectional area Af =area of extruded product The pressure/force acting on material is dependent on R & material yield point. Any friction present will increase the amount of extrusion force. Friction increases if die angle increases. At a certain angle, friction is minimum.
IMPORTANT FACTORS As extrusion speed & temperature increase, pressure increases. Rate of work done. Sample gets heated up very quickly. Incipient melting causes defects. Circumferential cracks due to hot shortness may also develop. The value of R can range from 10 to 100. Ram speeds can go up to 100ft/min (0.5m/s) Lower speeds are maintained for Al, Mg & Cu alloys. Steels, Ti & other refractory metals can use higher speeds.
HOT EXTRUSION Cooling of billet from high temperature needs to be uniform to avoid inhomogeneous deformation. Oxide layers very easily form which will affect the friction coefficient between metal & container. Also, final product may have oxide layer on it. Hence dummy block is made small.