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

• Force increases gradually at first

• Force increases rapidly when flash forms

• Final steep force is applied to achieve complete filling

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

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.

Isothermal Forging

• (Hot die forging)

• Die heated to temperature of forging

• Good dimensional accuracy

• Die is made of nickel alloy

Coining operation• Force or pressure applied is 5 to 6 times the

flow stress of the material

Heading• Upsetting operation at the end of the rod

• Can be done in highly automatic machines called Headers.

• 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

Die Allowances

• Shrinkage allowances

• Machining allowances

• Work holding allowance

• 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

• 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

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

Type of Forging Process

• Hydraulic press:– Constant low speed– Ram speed can be varied– Large amount of energy can be applied (75,000

tons)

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

• Screw Press– Energy from flywheel– Load transmitted through vertical screw– High precision path– Extremely accurate alignment top and bottom

halves– 32,000 tons

• 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

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

• 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

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

Anisotropic Behavior

• Not always considered as a defect

• Metal flows in different directions

• Thus we obtain different strength at different points of a forging

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”

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

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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.

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