Production Technology IV

1. Production Technology IV (Metal Forming)

2. INTRODUCTION TO METAL FORMING • METAL FORMINGincludes all manufacturing processes by which the shape of a material is changed without removal or addition of any excess metal by using mechanical force (plastic deformation). • Advantages of products manufactured by metal forming: • Higher strength due to work hardening • Reduction in material cost • Closing up and welding of cracks, blow holes, and cavities • Superior surface finish • Types of metal forming: • Cold forming • Hot forming • Warm forming • Isothermalforming

3. COLD FORMING VS. HOT FORMING

4. Warm forming 0.3 Tm 0.5 Tm 0.75 Tm

5. Metal forming operations Forces Work done Forces

6. Plastic deformation curve (flow curve) • The flow curve determine the strain required to cause plastic deformation of the metal at a given strain (Ø). σf Ø

7. Generalized strain measure: • ε*= • : dimension before deformation • at m= -1,ε* = (Engineering strain) • at m= 1 , ε* = (true strain) • at m=0 , ε* = (log. strain) (Ø)

8. εOr Ø • Imagine a 3 step elongation process, • The first step by 1.25 • The second step by 1.5 • The third step by 2 Therefore we use Ø instead of ε

9. Strain rate (Ø*) • Ø* = = x v = (s-1) Where: v: tool velocity h: instantaneous height of specimen

10. Temperature and Strain-Rate Dependence • Flow stress depends on strain rate and temperature, usually increasing with strain rate and decreasing with temperature. • The strain-rate effect at constant strain can be approximated by • Where: • C is a strength constant that depends upon strain, temperature, and material • m is the strain-rate sensitivity of the flow stress. For most metals at room temperature, the magnitude of m is quite low (between 0 and 0.03).

11. Determination of the flow curve • Progressive plastic deformation of a metal at a given temp ( T ), and strain rate ( Ø*). • Determination of the instantaneous value of equivalent stress ( σe) causing plastic deformation ( Øe ). σe= σ1 - σ3 or σe= Øe = Øe = • Plastic deformation condition: Øe Øf • To simplify the calculations and reduce the error , the metal is tested under uni-axial state of strain

12. Determination of the flow curve by compression test Stress at no friction Stress at friction • Cook and Larke method • σ = σf (1 + μ ()) • The volume is constant • Aoho = Ah de=do • σ = Ø = ln()

13. Determination of the flow curve by compression test • Method: • Compression test on cylindrical specimen with different ratios (d/h) • Measurements of F & h • Calculations of σ for barrel shape specimen by using the equivalent diameter (de) • The strain is calculated from Ø = ln() • Graphical representation of σ as a function of Ø • Interpolation of σ to σf at (d/h)=0

14. Determination of the flow curve by tension test • Advantages over compression: • No friction • Availability of tension test machines • σf = k Øn • K , n : material constants • K: strength coefficient • n: strain hardening exponent • This relation is valid for Ø in the range between 0.1 - 1

15. Determination of the flow curve by tension test • Determination of n: • At fmax : df=0 , d(σA) = 0 = Adσ + σdA • dσ = -σ , dØ= • ( = σ)f=fmax • kØn = nkØn-1 • n = Ø :uniform strain

16. Determination of the flow curve by tension test • Determination of k: • σut = = • σut= -Ø , Ø = ln() • = σ-Ø= k Øn-Ø, at max load Ø=n • σut= k nn-Ø • k = σut ()n