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 Force increases rapidly when flash forms Final steep force is applied to achieve complete filling

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

Types of Dies

Dies for Hydraulic Press

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

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