Extrusion of Sections with Varying Thickness Through Pocket Dies

Authors: D.Lesniak, W.Libura

Received: 31 August 2006

Published: November 2007

Presented By: Brad ElmerDate: 19 September 2007

Background

Al market based on:Complicated shapesHighest possible quality

Appropriate shape Dimensional tolerances Surface quality Homogenous structure Uniform mechanical properties

Background

Metal flow through the die must be as uniform as possible

Especially a problem on profiles with varying thickness Non-uniform metal flow High velocity gradient in die opening

Solution

Correctly designed pocket dieControls metal flow Enhances product quality

Purpose

“…to determine the influence of the pocket die geometry on the metal flow during extrusion of sections with varying thickness and consequently on its geometrical stability and surface quality”

Mechanical properties of cross sections Extrusion force measured for all dies

Method

Profile 1 is wall thickness ratio g1/g2=6:2

Profile 2 is wall thickness ratio g1/g2=12:2

Metal Flow Test

Goal: “…check pocket dies geometry on the formation of cracks and geometrical stability of extrudate.”

Metal Flow Test

Flat die and different pocket dies W2 was varied

1mm pocket A 3mm pocket B 7mm pocket C

Results Profile 1

Results Profile 2

Results

Lessens dead zones Symmetry zones result in uniform flow

Results

Shape stability Test

Results

Flat die: material flows faster at thick part “…application of two large pockets at the

thin profile part leads to faster metal exit speed…”

Can bend the other way

Results

Results

Results

Flat die: inhomogeneous material flowCreates velocity gradientUnbeneficial state of stresses in orificeTensile stresses responsible

B good qualityMost beneficial state of stressesHomogeneous meterial flow

Extrusion force Test

Indirect extrusion ram pressure = p = Yf x (x is from Johnson eqn)

and where Yf is found using the the ideal true strain = ln rx

In direct extrusion, difficult to predict the chamber/billet interactive friction, so use the shear yield strength ( about Yf /2 ) to estimate the chamber wall shear force as

pf Do2/4 = Yf Do L/2

giving

pf = 2 Yf L Do

and where pf = additional pressure to overcome wall friction force

Total ram pressure becomes

p = Yf (x + 2L Do )

Results Profile 1

Results Profile 2

Results

Results

Micro properties Test

Conclusions

References