Deformation behavior of consecutive workpieces and Stable -Unstable Flow in Materials Processed in equal channel angular pressing and grain refinement.

1. Deformation behavior of consecutive workpieces and
Stable -Unstable Flow in Materials Processed in equal
channel angular pressing of solid dies
Chandrakesh Prasad
(IIT Kharagpur ,India)
Reference
1. Joo S.H., Yoon S.C., Jeong H.G., Lee S. and Lee H.S., Deformation behavior of
consecutive workpieces in equal channel angular pressing of solid dies,
J.Mater. Sci.,Vol.47,pp.7877–7882 (2012)
2. Figueiredo R.B., Cetlin P.R. and Langdon T.G., Stable and Unstable Flow in
Materials Processed by Equal-Channel Angular Pressing with an Emphasis on
Magnesium Alloys, Int. J.Miner.Met.Mater.Soci.,Vol.41(A), pp.778-786(2010)
3. Lapovok R.Y., The role of back-pressure in equal channel angular
extrusion,J.Mater.Scie.,Vol.40,pp.341-346(2005)

2. ECAP
The technique is able to refine the microstructure of metals and
alloys, thereby improving their strength according to the Hall-Petch
relationship [1].
 Cold work can be accomplished without reduction in the cross
sectional area of the deformed work piece.

3. WHY ECAP ?
 Grain Refinement
Hardness improvement
Toughness
Yield strength increased
Conductivity (Cu) improvement
Ref.*Afsari A. ,Int. J. Nanosci. Nanotechnol.,
Vol. 10(4), pp. 215-222 (2014)
Problems With ECAP
 Defects in pressured samples
 Fracture after some passes
 Low Productivity
 Time Taking Process

4. Schematic illustration of ECAP showing the channel angle Φ
and the corner angle Ψ

5. Strains obtained during ECAP
εN =(N/ ) [ 2cot {( φ/2)+( Ψ/2) }+ Ψ cosec{( φ/2)+( Ψ/2) }]…(1)
Where,
εN = Strains obtained during ECAP
Ψ = angle of the arc of curvature at the outer
point of intersection of the two channels(20°).
N= Number of pass through die.
Φ = channel angle of die (90° ).

6. Strain and strain rate obtained after single pass

7. Flow Characteristics
Unstable flow at strain-rate
sensitivities of (a) 0 and (b) 0.01
Stable flow at strain rate sensitivities
of (c) 0.05 and (d) 0.1.

8. Distribution of Maximum Principal Stresses

9. Development of Damage

10. Prevention of fracture of low ductile
materials during ECAP
Cockcroft–Latham criterion for damage evaluation
The fracture criterion for monotonic deformation
Critical strain for fracture

11. Development of damage using Normalized
Cockcroft Criterion

12. (a) No back-pressure and (b) 80 MPa back-pressure
Mg sample
Significance of Back Pressure in ECAP

13. (a) No back pressure and (b) With back-pressure.

14. Fracture of ECAP sample
(a) No back pressure and (b) With back-pressure.
X-ray CT image of a sample

15. Fracture of ECAP sample
(a) No back pressure and (b) With back Pressure

16. Deformation behavior of consecutive Work-pieces

18. CONCLUSION
 In consecutive work-piece ECAP ,no splitting of deformation zones in
second work-piece and lower strain rate observed.
 Accumulated damage is significantly reduced in the second work piece.
 The folding defect was less pronounced in the second work-piece because of
the back slant head shape.
 Plastic instability causes an expansion of the area of the tensile principal
stresses in ECAP and there is a large overlapping of this area with the
deformation zone, giving rise to a large accumulated damage.
 The flow-softening effect leads to a displacement of the deformation zone,
hence an enhanced accumulation of damage at the upper surface.
 The imposition of a back pressure increases the ability of the billet to fill the
exit channel but does not remove any plastic instabilities such as shear
concentrations.
 An imposed back pressure significantly reduces the level of the maximum
principal stresses in the area in which deformation takes place and it leads
to a reduction in the tendency for billet cracking.
 The distribution of strain-stress becomes uniform and the low ductile
materials can be extruded without failure.