04 Forming4 Metal Extrusion

7/28/2019 04 Forming4 Metal Extrusion

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1stYear MechanicalDr. tayseer

Lecture Notes Metal Extrusion1

Metal Extrusion

Definition

Extrusion is the conversion of an ingot or billet into lengths of uniform cross section byforcing metal to flow plastically through a die.

Classification of Extrusion Processes

Direct Extrusion [Forward Extrusion]:

The metal billet is placed in a containerand driven through the die by the ram(extrusion ram and metal flow are in thesame direction); high forces are requiredto overcome the metal deformation andthe friction between the billet and containerduring process.

[Stationary container & die and moving ram]

Indirect Extrusion [Backward Extrusion]:

The extrusion die is fixed to a hollow ram(extrusion ram and metal flow are inopposite direction); the required extrusionforces are lower because of no relativemovement between billet and container.

[Stationary ram & die and moving container]


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1. Direct Extrusion

2. Indirect Extrusion

A. The billet is upset to fillthe container.

B. further compressionuntil the beginning of

metal flow through thedie.

C. Extrusion proceeds.

D. Piping defect occurs.

E. Formation of dead metalzone.


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Impact extrusion:

Impact extrusion is a special process toform hollow shapes with short lengths.Typical product is collapsible toothpastealuminum tube. As shown in figure, thedie has an accurate outside diameter of

the tube and a punch having an accurateinside diameter of the tube strike the billetat high velocity. It is a restricted processto soft metals such as aluminum, copper,tin and lead.

Extrusion Forging:

Extrusion forging is acombined process offorging and extrusion. Acommon example is theproduction of engine valves,where the billet is partiallyextruded to form the valveshank, and the rest of thebillet is forged to form thevalve head.


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Hydrostatic Extrusion:

In hydrostatic extrusion, a pressurizedfluid is used instead of extrusion ram toact on the billet sides and back face, theadvantages of this process are nofrictional forces, and the billet need not tobe of the same shape as the container.

Typical products are stepped shafts.


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Production of Hollow Shapes and Tubes:

Tube Extrusion:

Tubes and hollow sections can be produced by extrusion by attaching a mandrel, theclearance between the mandrel and die wall determines the wall thickness of the tube.

Fixed Mandrel:

Moving Mandrel:


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Porthole or Spider Die:

Porthole dies are suitable forvery long section of differenthollow shapes, and for thinwall thickness or smalldimensions.

The metal (Aluminum) flowthrough the holes and thenpressure welded duringdeformation at die outlet.


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Extrusion Die

Extrusion dies are made from different types of hot working tool steels. The figure belowshows schematic sketches for (a) Taper die for hot extrusion of steels with molten glass asa lubricant, and (b) flat die commonly used for non-ferrous materials.The cross section explains the entrance angle, die land and relief angle.


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Metal Flow in Extrusion:

The flow pattern in extrusion is important because of its effect on the quality andmechanical properties of the final product, where the improper metal flow during extrusioncan produce various defects.

In extrusion metal flow longitudinally like a fluid flow, so the extruded products have anelongated grain structure.

A common technique forinvestigating the flow patternis to section the billet intotwo longitudinal halves andmark the sectional face witha square grid pattern, thetwo halves are placed in thechamber together andpartially extruded, and then

they are taken apart andstudied.

Low friction High Friction Dead Metal zone


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Extrusion Defects:

Due to improper material conditions and process variables, several types of defects can bedeveloped in extruded products. There are three principle extrusion defects:

Surface cracks (also called speed cracks)

Cause: high speed or high speed and friction.These cracks are intergranular (along the grain boundaries) and are usually caused by hotshortness. Hot shortness is the tendency for some alloys to separate along grainboundaries when stresses of deformed at high temperatures due to the low melting pointconstituents segregated at grain boundaries.Solution: Reduce billet temperature and extrusion speed.

Piping (also called fish tailing)

Cause: Surface oxides and impurities.According to the metal flow pattern, surface oxides and impurities are drawn towards thecenter during deformation, as much as one third of the extruded product length maycontain this type of defect, and have to be cut off as scrap.Solution: Piping can be minimized by modifying the flow pattern and controlling friction, orby machining the billet outer surface before extrusion to remove surface scales.

Internal cracks

Cause: high internal stresses at the centerline.Internal cracks are due to high internal stresses near the centerline in the deformationzone of the extrusion die.The tendency y for center cracking:

Increases with the increase of die angle and amount of impurities.

Decreases with the increase of extrusion ratio and friction.Solution: Select proper die geometry and extrusion variables.


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Hot Extrusion & Cold Extrusion

Typical billet temperatures for hotextrusion

Material Billet Temp. CLead alloys 90-260

Magnesium alloys 340-430Aluminum alloys 340-510

Copper alloys 650-1100Titanium alloys 870-1040

Nickel alloys 1100-1260

HOT EXTRUSION is the process of forcing aheated billet to flow through a shaped die opening.The temperature at which extrusion is performeddepends on the material being extruded. Hotextrusion is used to produce long, straight metalproducts of constant cross section, such as bars,

solid and hollow sections, tubes, wires, and strips,from materials that cannot be formed by coldextrusion.

Steels 1100-1260

Cold Extrusion

Cold extrusion is a general termoften denoting a combination of

operations, such as direct orindirect extrusion and forging toproduce short solid or hollowshaped products.

Workpieces are often cup-shapedand have wall thicknesses equalto the clearance between thepunch and die.

Ductility must be restoredbetween operations by annealing,and any scale formed duringannealing must be removed byblasting or pickling beforesubsequent extrusion.

Cold-Extruded Metals in the orderof decreasing extrudability areAluminum and aluminum alloys,copper and copper alloys, low-carbon and medium-carbonsteels, modified carbon steels,

COLD EXTRUSION is so called because the slug orpreform enters the extrusion die at room temperature.Any subsequent increase in temperature, which mayamount to several hundred degrees, is caused by theconversion of deformation work into heat. Coldextrusion involves backward (indirect), forward(direct), or combined backward and forward (indirect-


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low-alloy steels, and stainlesssteels.

direct).


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Metal Drawing

This lecture is concerned with the plastic deformation of metals through conical taperedand circular dies. The forming process is called Drawing when the metal is forced throughthe die by tensile force applied to metal at the exit of the die, and it is called Extrusionwhen a compressive force is applied at the entry of the die.

Rod and shape Drawing:

In rod drawing operations, one end of the a bar isreduced or pointed, inserted through a die (withsmaller cross sectional area than the original bar),gripped and pulled in tension, and drawing theremainder of bar through the die.

The reduction in area per pass is usually restrictedto (20% - 50%) to avoid tensile fracture. Therefore,multiple draws through a series of dies may berequired to produce a desired final size.Intermediate annealing may be required to restoreductility and enable further working.

Tube Drawing:

Tube drawing is a reduction process in whichone end of a tube is grasped and pulledthrough a die that is smaller than the tubediameter. To obtain the desired size, a seriesof successive reductions, or passes, may benecessary. Because of its versatility, tubedrawing is suited for both small and largeproduction runs.

Fixed Mandrel Without mandrel

With floating Mandrel


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Wire Drawing:

Wire drawing is a metal-reducing process in which a wire rod is pulled or drawn through asingle die or a series of continuous dies, thereby reducing its diameter. Because thevolume of the wire remains the same, the length of the wire changes according to its newdiameter. Various wire tempers (hardness) can be produced by a series of drawing andannealing operations.

A wire drawing die is a tool that consists of a highly polished, shaped hole through whichwire is drawn to reduce its diameter. The choice of die material, viz. natural or syntheticsingle crystal diamond, polycrystalline diamond, carbide etc. depends on the material ofthe wire to be drawn and the operating parameters.

Extrusion Stresses & Drawing StressesExtrusion ratio R is determined by dividing the original area undergoing deformation by thefinal deformed area of the workpiece: [R= AO/AF]. Because volume remains constantduring extrusion, the extrusion ratio can also be estimated by increase in length. Anextrusion ratio of 4 to 1 indicates that the length has increased by approximately a factor offour. The metal being extruded has a large effect on the maximum ratio that is practical.Some typical approximate maximum extrusion ratios are 40 for (aluminum alloy 1100), 5for (1018 steel) and 3.5 for (type 305 stainless steel) and similar austenitic grades.

Extrusion

Reduction Ratio = (AO-AF)/AO

True Strain = Ln (LO/LF) = Ln (AF/AO)

AO = Original Billet Area

AF = Final Extruded Bar Area

Axial Stress = yield Ln (AF/AO)

Drawing

Reduction Ratio = (AO-AF)/AO

True Strain = Ln (LF/LO) = Ln (AO/AF)

AO = Original Billet Area

AF = Final Extruded Bar Area

Axial Stress = yield Ln (AO/AF)


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= yield Ln (dF/dO)2

=2 yield Ln (dF/dO)

Axial Force = Axial Stress * AO

= yield Ln (dO/dF)2

=2 yield Ln (dO/dF)

Axial Force = Axial Stress * AF

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04 Forming4 Metal Extrusion