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Metal forming defects


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Defects in metal forming operations

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Metal forming defects

  1. 1. Defects in MetalForming Processes Prepared by:- Amitkumar R. Shelar 1
  2. 2. Overview Process Classification ◦ Bulk Deformation Process ◦ Sheet Metalworking Material Behaviour in Metal Forming ◦ Flow Stress ◦ Average Flow Stress Temperature in Metal Forming Effect of Strain Rate Friction & Lubrication 2
  3. 3. Bulk Metal Forming Rolling - 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. 3
  4. 4. Bulk Metal Forming Rolling Forging Extrusion Drawing 4
  5. 5. 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. 5
  6. 6. Sheet Metalworking 6
  7. 7. Material Behavior in Metal Forming Yf  K n K n Yf  1 nYf Flow Stress Maximum strain for forming processK Strength coefficient Average flow stress 7
  8. 8. Temperature in Metalworking  Cold 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 8
  9. 9. Temperature in Metalworking Warm 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. 9
  10. 10. 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 10
  11. 11. Temperature in Metalworking Isothermal 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. 11
  12. 12. Effect of Strain RateY f  C m  strain rateThe strain rate is stronglyaffected by the temperature. Y f  A n m A = a strength coefficient 12
  13. 13. Friction and Lubrication  Friction 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 13
  14. 14. Defects in Metal Forming Springback 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 Rf Ri/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 14
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  16. 16. Compensation for spring back Over 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 16
  17. 17.  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. 17
  18. 18. Maximum bending force, P = KYLT2 WK – constant ranges from 0.3(wiping die) – 0.7(u-die)- 1.3(V-die)Y – yield stressL- length of the bendT- thickness of sheetFor a V-dieMax bending force, P = (UTS)LT 2 WUTS – Ultimate tensile strength 18
  19. 19. Wrinkles in Deep Drawing operation Wrinkles 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 19
  20. 20. Variables in Deep drawingoperations: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. 20
  21. 21. Figure shows the Wrinkle Defects 21
  22. 22. Thank You 22