corso di studi di fabbricazione - unibg - fundamental of metal... · corso di studi di...

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Università di Bergamo – DIGIPTecnologie e Sistemi di Lavorazione

Studi di Fabbricazione – 2018 – 3a

Corso diStudi di Fabbricazione

3a

Richiami dei processi tecnologici di trasformazione

FUNDAMENTAL OF METAL FORMING

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METAL FORMING

• Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces

• The tool, usually called die, applies stresses that exceed the yield strength of the metal

• The metal takes a shape determined by the geometry of the die

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Metal forming dominates the class ofshaping operations identified as thedeformation process

METAL FORMING

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ROLLING

This is a compressive deformation process in which thethickness of a slab or plate is reduced by two opposingcylindrical tools called rolls. The rolls rotate so as to draw theworkpiece into the gap between them and squeeze it

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ROLLED PRODUCTS MADE OF STEEL

150 mm x150 mm

250 mm x40 mm

40 mm x40 mm

From soaking, the ingot is moved to the rolling mill, where itis rolled into one of three intermediate shapes called blooms,billets or slabs.

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Blooms are rolled into structural shapes and rails for railroad tracks.


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Slabs are rolled into plates, sheets and strips. Hot-rolledplates are used in shipbuilding, bridges, boilers, weldedstructures for various heavy machines, tubes pipes, andmany other products.


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Billets are rolled into bars and rods. These shapes are theraw materials for machining, wire drawing, forging andother metalworking processes


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(a) 2-high rolling mill.

TWO-HIGH ROLLING MILL

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(b) 3-high rolling mill.

THREE-HIGH ROLLING MILL

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(c) four-high rolling mill.

FOUR-HIGH ROLLING MILL

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• Multiple backing rolls allow even smaller roll diameters

(d) cluster mill

CLUSTER MILL

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• A series of rolling stands in sequence

(e) tandem rolling mill.

TANDEM ROLLING MILL

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• A series of rolling stands in sequence

(e) tandem rolling mill.

TANDEM ROLLING MILL

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In forging, a workpiece is compressed between two opposing dies, sothat the die shapes are imparted to the work. Forging is traditionally ahot working process, but many types of forging are performed cold.

FORGING

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FORGING

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• In open die forging, the workpiece is compressed between twoflat (or almost flat) dies, thus allowing the metal to flow withoutconstraint in a lateral direction relative to the slide surface.

OPEN-DIE FORGING

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• In impression die forging, the die surfaces contain a shape orimpression that is imparted to the workpiece during compression,thus constraining metal flow to a significant degree.

IMPRESSION-DIE FORGING

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• In flashless forging, the workpiece is completely constrained withinthe die and no excess flash is produced. The volume of the startingworkpiece must be controlled very closely so that it matches thevolume of die cavity.

FLASHLESS FORGING

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• Sequence in impression-die forging: (1) just prior to initialcontact with raw workpiece, (2) partial compression, and (3)final die closure, causing flash to form in gap between dieplates.

IMPRESSION-DIE FORGING

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• Trimming operation (shearing process) to remove the flashafter impression-die forging.

TRIMMING AFTER IMPRESSION-DIE FORGING

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This is a compression process in which the work metal is forced to flowthrough a die opening, thereby taking the shape of the opening as its owncross section

EXTRUSION

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EXTRUSION

• Compression forming process in which work metal is forced to flow through a die opening to produce a desired cross-sectional shape

• similar to squeezing toothpaste out of a toothpaste tube

• In general, extrusion is used to produce long parts of uniform cross sections

• Two basic types:• Direct extrusion

• Indirect extrusion

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INDIRECT EXTRUSION

In indirect extrusion process the die is mounted to the ram rather thanat the opposite end of the container. As the ram penetrates into theworkpiece, the metal is forced to flow through the clearance in adirection opposite to the motion of the ram.

Since the billet is not forced to move relative to the container, there isno friction at the container walls, and the ram force is therefore lowerthan in direct extrusion

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EXTRUSION

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• A complex extruded cross section for a heat sink (photo courtesyof Aluminum Company of America)

COMPLEX CROSS SECTION

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In this forming process, the diameter of a round wire or bar is reducedby pulling it through a die opening

WIRE AND BAR DRAWING

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WIRE AND BAR DRAWING

• Cross-section of a bar, rod, or wire is reduced by pulling it through a die opening

• Similar to extrusion except work is pulled through die in drawing (it is pushed through in extrusion)

• Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening

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CONTINUOUS WIRE DRAWING

• Continuous drawing machines consisting of multiple draw dies (typically 4to 12) separated by accumulating drums

‒ Each drum (capstan) provides proper force to draw wire stock throughupstream die

‒ Each die provides a small reduction, so desired total reduction isachieved by the series

‒ Annealing sometimes required between dies to relieve workhardening

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CONTINUOUS TUBE DRAWING

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Bending involves straining of a metal sheet or plate totake an angle along (usually) straight axis

SHEET METAL BENDING

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SPRINGBACK• Increase in included angle of bent part relative to included angle of forming

tool after tool is removed

• Reason for springback:

‒ When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape

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• Continuous bending process in which opposing rolls produce longsections of formed shapes from coil or strip stock

Roll forming of a continuous channel section: (1) straight rolls, (2) partial form, (3) final form.

ROLL FORMING

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• Large metal sheets and plates are formed into curved sections using rolls

ROLL BENDING

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In sheet metalworking, drawing refers to the forming of a flatmetal sheet into a hollow or concave shape, such as a cup, bystretching the metal. A blank holder is used to hold down theblank while the punch pushes into the sheet metal.

DRAWING

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DRAWING

Drawing of a cup-shaped is the basic drawing operation.

A blank of diameter Db is drawn into a die by means of a punch of diameterDp. The punch and die must have a corner radii given by Rp and Rd. If thepunch and die were to have sharp corner a hole punching operation wouldbe accomplished rather than a drawing operation.

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DEFECTS IN DRAWING

Wrinkling in the flange, that consists of a series ofridges that form radially in the undrawn flange of theworkpart due to compressive buckling

Wrinkling in the wall, if and when the flange is drawninto the cup, these ridges appear in the vertical wall.

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DEFECTS IN DRAWING

Tearing is an open crack in the vertical wall due tohigh stresses that cause thinning and failure of themetal in this location

Earing that is caused by an anisotropy of the material

Surface scratches that can occur if the punch and dieare not smooth or if lubrication is insufficient

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This process is somewhat out-of-place in our list of deformation processes,because it involves cutting rather than forming. A shearing operation cutsthe workpiece using a punch and a die. Although it is not a forming process,it is included here because it is necessary and very common operation insheet metalworking

SHEARING OF SHEET METAL

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