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