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)

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.

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

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

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° ).

Strain and strain rate obtained after single pass

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.

Distribution of Maximum Principal Stresses

Development of Damage

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

Development of damage using Normalized Cockcroft Criterion

(a) No back-pressure and (b) 80 MPa back-pressure

Mg sample

Significance of Back Pressure in ECAP

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

Fracture of ECAP sample

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

Fracture of ECAP sample

(a) No back pressure and (b) With back Pressure

Deformation behavior of consecutive Work-pieces

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.