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Metal Forming Processes-
Wire, rod, and tube drawing
Wire, rod, and tube drawing
Drawing is an operation in which the cross section of a bar, rod, or wire is reduced by
pulling it through a die opening
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Bar/rod drawing
• Pointed end of a rod passes through a die of smaller cross section, gets placed in grips and
pulled in tension, drawing the entire rod through the die.
• Hydraulic cylinders are used to provide pull for short-length products, while chain drives are
used for products up to 30m length.
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Bar/rod drawing
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Bar/rod drawing
• The reduction in area is usually restricted b/w 20 and 50%, as higher values require higher pulling
forces. Higher reductions also cause lubricant breakdown.
• For a desired size/shape, multiple draws are made through a series of progressively smaller dies.
• Intermediate anneals are provided to restore ductility after each pass.
Drawing
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Drawing
Wire drawing
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Wire drawing
• Wire drawing begins with large coils of hot-rolled rod stock approximately
9 mm in diameter. After descaling, one end of the coil is pointed, fed
through a die, gripped, and the drawing process begins.
• Lubrication boxes often precede the individual dies to help reduce friction
drag and wear of the dies.
• The material moves continuously from one station to another in a
synchronized manner that prevents any localized accumulation or tension
that might induce fracture.
• Intermediate anneals are properly placed so the final product has a
selected amount of cold work.
Wire drawing
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Wire drawing
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Drawing
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Drawing
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Tube drawing
Tubing used in applications as varied as aircraft hydraulic lines, diesel fuel lines,
thermocouple sheathing, chromatography, and semiconductor manufacture are prepared
by drawing process. Tubular products for these applications often require close
dimensional control, smooth and ultraclean ID surfaces.
Tube sinking is simply drawing the tube through a die to reduce the
outside and inside diameters.
Sinking does not use an internal support.
Commodity tubing for applications such as low-cost lawn furniture is
often produced by multiple sinking operations.
The typical die angle is 24 degrees. Lower angles tend to cause wall
Tube drawing with no mandrel thickening, whereas higher angles cause wall thinning.
(Tube Sinking) Sinking uses a long bearing to achieve the correct size and optimal
roundness, making this process suitable for a final sizing operation.
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Tube drawing
Tube drawing produces high-quality tubing with smooth surfaces, thin walls, accurate
dimensions, and added strength (from the strain hardening).
Internal mandrels are used to control the For a controlled internal diameter in a long-
inside diameter of tubes (12 to 250 mm length product, a floating plug is used. The
diameter). These mandrels are inserted plug must be designed for the specific
through the incoming stock and are held in material, reduction, and friction. A properly
place during the process. Products are designed floating plug will assume a stable
generally limited to lengths of 30m or less. position within the die.
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Drawing
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Drawing
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Drawing
Area reduction in drawing,
Draft,
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Drawing
Considering friction is present in drawing and the work metal
experiences inhomogeneous deformation,
Draw Stress
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Drawing- Problem 1
Wire is drawn through a draw die with entrance angle = 15°. Starting diameter is
2.5mm and final diameter is 2.0 mm. The coefficient of friction at the work–die
interface is 0.07. The metal has a strength coefficient K = 205 MPa and a strain-
hardening exponent n = 0.20. Determine the draw stress and draw force.
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Drawing- Problem 1
Wire is drawn through a draw die with entrance angle = 15°. Starting diameter is
2.5mm and final diameter is 2.0 mm. The coefficient of friction at the work–die
interface is 0.07. The metal has a strength coefficient K = 205 MPa and a strain-
hardening exponent n = 0.20. Determine the draw stress and draw force.
Do = 2.5 mm Df = 2.0 mm
2.25 mm
94.1 MPa 1.16
0.966 mm
295.5N 0.446 mm
145.4 MPa
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Drawing-Problem 2
Show that the theoretical maximum reduction possible in a single draw ignoring
friction and redundant work in wire drawing is 0.632.
In this ideal case, the maximum possible draw stress is equal to the yield strength of the
work material.
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Drawing-Problem 2
Show that the theoretical maximum reduction possible in a single draw ignoring
friction and redundant work in wire drawing is 0.632.
In this ideal case, the maximum possible draw stress is equal to the yield strength of the
work material.
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Wire drawing
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Drawing - Applications
• Rods for shafts
• Machine and structural components
• Electrical wiring
• Cables
• Tension-loaded structural members
• Welding electrodes
• Springs
• Paper clips and staples
• Spokes for bicycle wheels
• Stringed musical instruments
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Drawing defects
• Center cracking as in extrusion
• Seams - longitudinal scratches or folds in the material. Seams may open
up during subsequent forming operations (upsetting, heading, thread
rolling, or bending of the rod or wire)
• Scratches and die marks can result from improper selection of the process
parameters, poor lubrication, or poor die condition.
Roller leveling
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Superplasticity
The term superplasticity refers to the capability of some materials to undergo large
uniform elongation prior to necking and fracture in tension. The elongation ranges from
a few hundred percent to as much as 2000%.
Necessary conditions include: (a) a small grain size, typically smaller than 10 μm; (b) a
high deformation temperature, above 0.4 Tm; (c) a well-controlled slow strain rate,
typically below 3 x 10-4/s; and (d) a stable grain size at the deformation temperature.
For superplastic deformation, n~𝟎, m varies from 0.3 to 0.9
The mechanisms of superplasticity in metals are still under debate—many believe it
relies on atomic diffusion and the sliding of grains past each other.
Superplastic materials have the ability to form components with double curvature and
smooth contours from single sheet in one operation, with exceptional dimensional
accuracy and surface finish with no spring back.
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Superplasticity
Superplasticity has been observed in metals
(including titanium-, aluminum-, magnesium-,
and nickel-based alloys), intermetallics
(including iron, nickel, and titanium base), and
ceramics (including monoliths and composites).
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Superplasticity
Necessary conditions include:
(a) a small grain size, typically smaller than 10 μm;
(b) a high deformation temperature, above 0.4 Tm;
(c) a well-controlled slow strain rate, typically below 3 x 10-4/s; and
(d) a stable grain size at the deformation temperature.
For superplastic deformation, n~𝟎, m varies from 0.3 to 0.9
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Friction in metal forming
Friction in metal forming arises because of the close contact between the tool and work
surfaces and the high pressures that drive the surfaces together.
Friction is undesirable in metal forming as
• Retarded metal flow causing residual stresses and defects in the product
• Forces and power to perform the operation are increased
• Tool wear leading to loss of dimensional accuracy
• Metal forming features high pressures between a hardened tool and a soft workpart,
plastic deformation of the softer material, and high temperatures.
• These conditions result in high coefficients of friction, even in the presence of
lubricants.
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Friction in metal forming
Sticking in metalworking (also called sticking friction) is the tendency for the two surfaces
in relative motion to adhere to each other rather than slide.
If the friction stress between the surfaces exceeds the shear flow stress of the work metal,
the metal deforms by a shear process beneath the surface rather than slip at the surface.
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Friction in metal forming
Considerations in choosing an appropriate metalworking lubricant include
• Type of forming process
• Forming temperature
• Work material
• Chemical reactivity with the tool and work metals
• Ease of application
• Toxicity
• Flammability
• Cost.
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Friction in metal forming
• Lubricants for cold working include mineral oils, fats and fatty oils, water-based
emulsions, soaps, and other coatings.
• Hot working is sometimes performed dry for certain operations and materials
(e.g., hot rolling of steel and extrusion of aluminum).
• Lubricants for hot working include mineral oils, graphite, and glass.
• Molten glass is an effective lubricant for hot extrusion of steel alloys.
• Graphite contained in water or mineral oil is a common lubricant for hot forging.
• Solid lubricants
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