Fundamentals of Metal Forming. 1. Overview 2. Material Behavior 3. Temperature Effects 4. Strain Rate Effect 5. Friction and Lubrication

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1 Fundamentals o Metal Forming. Overview. Material Behavior. Temperature Eects. Strain Rate Eect 5. Friction and Lubrication

2 ntroduction Metal Forming uses plastic deormation to change the shape o metal workpieces Materials (metals) plastic deormation External loads Tpicall compressive Sometimes Stretch the metal (tensile), bend the metal (tensile and compressive), shear stresses Shape - die and tools Classiication Bulk Deormation Sheet Metal Forming High surace area-to-volume ratio Parts are called stampings Usual tooling: punch and die

3 Bulk Deormation Process Rolling Forging Extrusion F Drawing Sheet metal orming Bending Drawing Shearing (Stamping) ntroduction RAM F v v F F v v F

4 Forging Sequence o Part and Die

5 Material Properties in Forming Desirable material properties: Yield strength? Ductilit? Second phase or nclusion? These properties are aected b temperature: When work temperature is raised, ductilit increases and ield strength decreases while loosing on surace inish and dimension accurac. Other actors: Strain rate and riction 5

6 Theories o Failure The limit o the stress state on a material Ductile Materials - Yielding Brittle Materials - Fracture n a tensile test, Yield or Failure Strength o a material. n a multiaxial state o stress, how do we use Yield or Failure Strength? 6

7 Yielding: Ductile Materials A. Maximum Shear Stress Theor (Tresca Criterion) n a tension specimen: max S max The diameter o the Mohr circle S For Plane Stress: ( ) S S S S, have same signs., have opposite signs. 7

stress ( ave ( )/) Now substitute, and with in an uniaxial

8 u Ductile Materials B. Maximum Distortion Energ Theor (von Mises Criterion) For the three principal stresses; u E [ ε ε ε ] ν ( ) For plane stress; [ ] ν E d S u d ν E [ ] Ater taken out the hdrostatic stress ( ave ( )/) Now substitute, and with in an uniaxial tension test. n a biaxial case, the same amount o distortion energ ν [ ] ν E E S S ( ), ( ) ( ) ave ave, A B ave 8

9 Plasticit Flow theor (Classical theor) The current strain rates depend on the stress. Deormation theor (Henck theor) The total strain is related to the stress. deal or nonlinear elasticit Still work or monotonicall increasing stresses everwhere in a bod Pressure-independent Hdrostatic pressure does not aect dislocation motion. Bauschinger eect The dierent behaviors in tension and compression 9

10 Stress Representation General -D Stress z Plane Stress z xz z x x x xz z Tensor Notation xz z z x

11 Stress Transormation (Plane Stress) x x x cosθ sinθ x sinθ cosθ x x cosθ sinθ x x x cosθ sinθ x x cosθ sinθ x x x x x x x cosθ sinθ x x cosθ sinθ x cosθ sinθ x cosθ sinθ x sinθ cosθ x sinθ cosθ x sinθ cosθ

12 Yield unction in D [] m δ [ ] where DeviatoricStress Tensor, m ( ) Hdrostatic Component o Stress δ ori j ori j ( ) ( ) where z xz z x xz z z x z x z x z x zx z ( ) ( ) ( ) ( ) C,,, Stress Tensor : Three Stress nvariants Yield unction: ( ) z x where Three Deviatoric Stress nvariants

13 Example: Stresses in D [ ] ( )( ) ( ) ( ) ( )( )( ) ( ) 7 and 5 8) (, 6MPa and MPa MPa, MPa 5 n n n To ind Principal Stresses: [ ] z z xz z zx x x x z

14 Example: Continue [ ] [ ] m δ [ ] & 6MPa, MPa 5 ( ) ( ) MPa. 6 5 m.7.; ;..7 9 ) (. ) (. ) (.67 ) ( Deviatoric Stress

15 5 Presentation o Yield Surace ( ) C S C,, ( ) ( ) ; 6, S C,, Max. Shear Stress Theor: ( ) ( ) C k k k C 6,, , ( ) ( ) ( ) ( ) ( ) [ ] C C k,, 6, Maximum Distortion Energ Theor: Based on uniaxial ielding:, ( ) ( ) ; 6, S C C Maximum Distortion Energ Theor Max. Shear Stress Theor For shear: ; ( ),, S S C S S S S

16 Plasticit S`Y S Y Bauschinger eect S Y sotropic hardening S<S Y Plastic State: Elastic State: mpossible: ( ) ( ) < ( ) > Loading: Unloading: Neutral: ( ) ( ) C, d C, d ( ) C, d > < 6

17 sotropic Hardening Strain Hardening Kinematics Hardening Convexit Normalit Rule: dε dλ Combined Hardening General Hardening 7

18 . Behavior in Metal Forming The stress-strain relationship beond elastic range assuming no unloading at antime and anwhere. n Kε Flow stress The instantaneous value o stress required n to continue deorming the material. Y Kε Average Flow Stress Y ε n d K d n ε ε ε Kε ε ε n For an metal, K and n in the low curve depend on temperature ε Y Y Y s ε 8

19 . Temperature in Metal Forming Cold Working Perormed at room temperature or slightl above Near net shape or net shape Better accurac, closer tolerances & surace inish Strain hardening increases strength and hardness Grain low during deormation can cause desirable directional properties in product No heating o work required Higher orces and power required Suraces must be ree o scale and dirt Ductilit and strain hardening limit the amount o orming 9

20 Warm Working Perormed at above room temperature but below recrstallization temperature.t m, where T m melting point (absolute temperature) Lower orces and power than in cold working More intricate work geometries possible Need or annealing ma be reduced or eliminated sothermal Forming eliminate surace cooling especiall highl alloed steels and Ti and Ni allos

21 Hot Working Deormation process at temperatures above recrstallization temperature (.5T m ) A perectl plastic material - Strain hardening exponent is zero (theoreticall) Lower orces and power required Metals become ductile Strength properties are generall isotropic No work hardening o part part can be subsequentl cold ormed Lower dimensional accurac Higher total energ required Poorer surace inish including oxidation (scale), Shorter tool lie

22 . Eect o Strain Rate Sensitive to strain-rate at elevated temperatures Strain rate: ε& m Relationship: Y Cε& A more complete relationship: Y v h Evaluation o strain rate is complicated b Workpart geometr Variations in strain rate on the part n m ε & ε Strain rate can reach s - or more or some metal orming operations A

23 Eect o temperature on low stress Y Y Y Room Temperature C C m C 8 C C... ε&. ε&. ε& ncreasing temperature decreases C & increases m At room temperature, eect o strain rate is almost negligible

24 5. Friction and Lubrication Friction retard metal low and increase power and wear Categories Temp. Range Strain-rate Sensitivit exponent Coeicient o Friction Cold Working <.T m <m<.5. Warm Working.T m -.5T m.5<m<.. Hot Working.5T m -.75T m.5<m<..-.5 Lubrication reduce riction & heat, improve surace inish Choosing a Lubricant Tpe o operation, reactivit, work materials, cost and ease o applications Cold working mineral oil, ats, att oils, water-based emulsions, soaps and coating Hot working mineral oil, graphite and glass

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