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Defects in Metal Forming Processes

 Prepared by:- Amitkumar R. Shelar

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OverviewProcess Classification

◦Bulk Deformation Process◦Sheet Metalworking

Material Behaviour in Metal Forming◦Flow Stress◦Average Flow Stress

Temperature in Metal FormingEffect of Strain RateFriction & Lubrication

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Bulk Metal FormingRolling - compression process to reduce

the thickness of a slab by a pair of rolls.Forging - compression process performing

between a set of opposing dies.Extrusion - compression process sqeezing

metal flow a die opening.Drawing - pulling a wire or bar through a

die opening.

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Bulk Metal Forming

Rolling Forging

Extrusion Drawing

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Sheet MetalworkingForming on metal sheets, strips, and coils.

The process is normally a cold working process using a set of punch and die.

Bending - straining of a metal sheet to form an angle bend.

Drawing - forming a sheet into a hollow or concave shape.

Shearing - not a forming process but a cutting process.

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Sheet Metalworking

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Material Behavior in Metal Forming n

f KY

nKY

n

f

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Yf Flow Stress Maximum strain

for forming process

K Strength coefficientAverage flow stress

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Temperature in MetalworkingCold working

◦Pros better accuracy better surface finish strain hardening increases strength and hardness grain flow during deformation provides

directional properties no heating is needed

◦Cons higher forces and power are required surface should be cleansed ductility and strain-hardening limits the extent of

forming

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Temperature in MetalworkingWarm working - temperature between

room temperature and recrystallization temperature, roughly about 0.3 Tm◦Pros against cold working

Lower forces and power more intricate work geometries possible need for annealing may be reduced/eliminated.

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Temperature in Metalworking

Hot working - Deformation at temperature above recrystallization temperature typically between 0.5Tm to 0.75Tm◦Pros

larger deformation possible lower forces and power forming of room temperature low ductility

material is possible isotropic properties resulted from process no work hardening

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Temperature in MetalworkingIsothermal Forming - preheating the tools

to the same temperature as the work metal. This eliminates the surface cooling and the resulting thermal gradient in the work part.

Normally applies to highly alloyed steels, titanium alloys and high-temperature nickel alloys.

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Effect of Strain Ratem

f CY

strain rateThe strain rate is strongly affected by the temperature.

mnf AY

A = a strength coefficient

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Friction and LubricationFriction is undesirable:

◦retard metal flow causing residual stress◦increase forces and power◦rapid wear of tooling

Lubrication is used to reduce friction at the workpiece-tool interface

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Defects in Metal FormingSpringback Effect-In Bending ,after plastic deformation there is an elastic

recovery this recovery is called spring back.Spring back can be calculated approximately in terms if

radii Ri and RfRi/Rf = 4 ( Ri Y / ET )3 – 3 (Ri Y /ET) + 1 Spring back Increases as (R/T ratio & yield stress of

material ) increases as elastic modulus E decreases

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Compensation for spring backOver bending of part Bottoming the punch – coin the bend area by subjecting it to

high localized compressive between the technique tip of the punch and the die surface.

Stretch bending – Part is subjected to tension while being bent.In order to reduce spring back bending may also be carried to reduce spring back bending may also be carried out at elevated temperatures

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Springback cannot be avoided but can be minimized by several methods such as

Applying tension,Overbending, Warm and hot forming . Finite element method

(FEM) is widely used in industry to predict metal flow and springback.

Based on the springback predictions obtained from FEM, the tool geometries are virtually modified to compensate for the springback before the tool is manufactured. Thus, tool manufacturing time and cost are significantly reduced.

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Maximum bending force, P = KYLT2

W

K – constant ranges from 0.3(wiping die) – 0.7(u-die)-1.3(V-die)

Y – yield stressL- length of the bendT- thickness of sheet

For a V-dieMax bending force, P = (UTS)LT 2

WUTS – Ultimate tensile strength

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Wrinkles in Deep Drawing operationWrinkles can be prevented by using a blank holder,

the function of which is to facilitate controlled material flow into the die radius.

The main geometric parameters of the die which influence the wrinkling are:

diameter of the punch and punch edge radiuses.In the case of friction between the piece and the tool, the increase of the coefficient of friction determines the wrinkling to reduce, but high

values of the coefficient can cause cracks and material breakage

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Variables in Deep drawing operations:Many variables affect the deep drawing

process, these include Material properties, die design, friction

conditions.Drawing ratio , punch corner radius,

punch speed etc. A properly chosen BHF can prevent

wrinkling.

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Figure shows the Wrinkle Defects

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Thank You