Metal Extrusion Flashcards
Direct Extrusion
A billet is placed in a chamber (container) and forced through a stationary die opening by a hydraulically driven ram (pressing stem or punch).
Billets are extruded individually.
Small portion of the billet remains in the die (scrap, butt)
Significant friction between the billet surface and the container walls during the extrusion process require higher ram forces.
Increase of friction if material is extruded hot due to the presence of an oxide layer on the surface of the billet
Indirect Extrusion
A ram pushes the die back through a stationary, confined billet.
Metal is forced to flow through the clearance in a direction opposite to the motion of the ram.
Hollow (tubular) sections can be produced by metal flowing around the ram and the die.
No friction at the container walls as there is no relative movement between billet and container.
Lower rigidity of hollow ram.
Difficulty in supporting extruded product as it exits die.
Hydrostatic Extrusion
Pressure is transmitted by the ram to an incompressible fluid medium.
Billet diameter is smaller than container dimensions and any billet-chamber friction is eliminated.
High pressure fluid surrounds the billet and applies the force to extrude it through the die.
Hot Extrusion
Heating of billet above its recrystallisation temperature.
Reduces strength and increases ductility of the metal, permitting more size reductions and more complex shapes.
Die materials used hot worked tool and die steel for wear resistance, hot hardness and thermal conductivity.
Special lubricants – glass to support material flow during extrusion, surface finish and integrity and reduction of extrusion forces. Jacketing (canning) – enclosure of the billet in a thin-walled container of
softer material.
Cold Extrusion
Cold or warm extrusion generally produce discrete parts made from slugs which are cut from bars, wires or plates.
Increases material strength due to strain hardening, close tolerances,
absence of oxide layer improves surface finish.
High production rates as no heating of billets is required.
Impact extrusion – high speed cold extrusion alternative.
Die materials use tool steel for wear resistance and to retain shape under high stress.
Lubricants use is critical because of seizures. Use phosphate-conversion followed by coating of soap or wax.
Advantages
Versatile shapes can be produced.
Economical for both large and short runs.
Low tooling costs.
Dimensional tolerances ± 0.025 – 2.5mm.
High billet-to-product cross-sectional area ratios (~100:1 for weaker metals).
Limitations
High temperature generated by massive deformation.
Lubrication issues when extruding complex cross-section.
Surface cracking
(firtree or speed cracking)
High workpiece temperatures caused by a too high extrusion speed or surface temperatures.
High friction and surface chilling of high temperature billet.
Pipe
(tailpipe, fishtailing)
Surface oxides and impurities are drawn into the centre of the billet; usually associated with direct extrusion.
Internal cracking
(center burst, arrowhead fracture, chevron cracking)
Tensile stress at the centreline in the deformation zone in the die - large deformation in regions away from the central axis will stretch material along the centre of the work.
Tubing
Extruded from a solid or hollow billet; when extruded from solid the ram is
fitted with a mandrel piercing a hole into the billet. Thin-walled extrusions are more difficult to produce than thick walls.
Hollow Cross-sections
Welding-chamber method allows to extrude hollow cross-sections of various shapes by using dies known as porthole, spider, bridge or torpedo dies.
Metal divides and flows around the mandrel and the strands of metal re-weld under pressure before exiting the die.
Bar and Wire Drawing
Cross-section of a bar, rod, or wire is reduced by pulling it through a die opening. Tensile and compression stresses because of the drawing operation and indirect compression of the material in the die.
Usually performed as a cold-working operation.
Preparation of starting stock includes annealing, cleaning and pointing.
Reduction of cross-sectional area per pass can be up to 50% (bar) and 30% (wire); the smaller the initial cross-section, the smaller the reduction per pass.
Drawing speeds range from 1-3m/s to 50m/s for fine wire.
Limited time to dissipate the heat generated at high drawing speeds can affect the product quality.
Die material is typically made from tool steel or single crystal diamond inserts for wire drawing (2m – 2mm).
Essential lubrication to improve die tool life, product surface finish and reducing drawing forces is achieved by.
Wet drawing – die and rod are immersed completely.
Dry drawing – rod is coated by passing through a stuffing box.
Metal coating – rod is coated with soft metal (tin, copper).
Ultrasonic vibration of die and mandrel to reduce forces.
Bar drawing
for large diameter bar and rod stock.
starting material is a straight cylindrical workpiece.
Wire drawing
small diameter stock
starting material is generally coiled
