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Impression-Die Forging (cont’d)

Impression-Die Forging (cont’d). F=K p Y f A K p – presure multiplying factor Simple shapes (without flash): 3-5 Simple shapes (with flash): 5-8 Complex shapes (with flash): 8-12 Y f – flow stress for strain rate A – projected area. Force increases gradually at first

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Impression-Die Forging (cont’d)

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  1. Impression-Die Forging (cont’d) • F=KpYfA • Kp – presure multiplying factor • Simple shapes (without flash): 3-5 • Simple shapes (with flash): 5-8 • Complex shapes (with flash): 8-12 • Yf – flow stress for strain rate • A – projected area

  2. Force increases gradually at first • Force increases rapidly when flash forms • Final steep force is applied to achieve complete filling

  3. http://www.mscsoftware.com.au/products/software/msc/superforge/http://www.mscsoftware.com.au/products/software/msc/superforge/

  4. Closed Die Forging • Forging created without flash • Proper volume of die is required • Precise control of parameters • Near net shape forging • Very precise machines • Special die designs • Aluminum, magnesium, etc.

  5. Isothermal Forging • (Hot die forging) • Die heated to temperature of forging • Good dimensional accuracy • Die is made of nickel alloy

  6. Coining operation • Force or pressure applied is 5 to 6 times the flow stress of the material

  7. Heading • Upsetting operation at the end of the rod • Can be done in highly automatic machines called Headers.

  8. Piercing • We can create holes or cavities on a part • Hubbing • A hardened puch with a particular tip geometry is pressed into the surface of a blcok. Produce a die cavity • Cogging • Progressive pressing of a arge part

  9. Die Allowances • Shrinkage allowances • Machining allowances • Work holding allowance

  10. Material Used • Tool Steel, Die steel • High carbon alloy steel (Cr, Ni, vanadium) • Hardness: Rc=45-60 • Material should have strength, toughness, at high temp. • Hardenability, mechanical and thermal shock resistance, wear resistance

  11. Dies are the most critical part of the forging operation • Proper fillet radius should be provided • Draft angle for all vertical faces for ease of removal • Flash should be allowed to form • Flash = 3% of max. thickness

  12. Roll forging • Cross sectional area of a bar is reduced and altered in shape by passing through a pair of rollers. • Tapers shaft • Pre stages of a crank shaft • Skew rolling • Used for making ball bearings

  13. Type of Forging Process • Hydraulic press: • Constant low speed • Ram speed can be varied • Large amount of energy can be applied (75,000 tons)

  14. Mechanical Press • Usually uses crank or eccentric • Force depends on the stroke position • Proper setting of the position is important • 12,000 tons

  15. Screw Press • Energy from flywheel • Load transmitted through vertical screw • High precision path • Extremely accurate alignment top and bottom halves • 32,000 tons

  16. Hammers • Potential energy of the ram • Speed can be high • Sometimes steam or air is used to aid the die • Multiple blows may be needed

  17. Selection of Press depends on • Precision • Strain rate sensitivity • Amount of deformation • Size of forging • Production rate

  18. Metal flows in the direction of least resistance • Distribute material so that it can properly fill die cavity • Several “Intermediate stage” Dies are used for obtaining final forging • E.G. connecting rod, crankshaft

  19. Defects in Forging • Fatigue resistance is reduced • Corrosion, stress raisers • In complete metal for machining • Mismatch of halves of the pierce • Poor strength in the direction of principle stresses

  20. Anisotropic Behavior • Not always considered as a defect • Metal flows in different directions • Thus we obtain different strength at different points of a forging

  21. Effect of corner radii • Metal flows better as a larger radius than in a smaller radius • For smaller radius, the metal can fold over itself to cause “cold shuts”

  22. Defects • Surface cracking • Improper temperature, strain rate, design of dies • Buckling • Lap formation can occur – importance of forming distribution • Solution – increase the thickness of the part • Internal Defects • Improper filling of the die • Larger than required billet can cause it

  23. http://www.transvalor.com/forge2/demo1.htm

  24. Grain Flow Pattern • Grains flow is exposed: end grains • Can be avoided by intermediate steps in forging and proper orientation of workpieces • Stress raiser • Corrosion, etc.

  25. Summary • True Strain/True Stress, Strain rate, strain rate sensitivity • Forging – Forces involved in rectangular and cylindrical work pieces • Different types of forging – open, closed, impression etc • Multi stage forging • Cogging, coining etc • Roll forging • Types of presses • Defects

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