Abstract
A comprehensive mathematical model of the hot extrusion process for aluminum alloys has been developed and validated. The plasticity module was developed using a commercial finite element package, DEFORM-2D, a transient Lagrangian model which couples the thermal and deformation phenomena. Validation of the model against industrial data indicated that it gave excellent predictions of the pressure during extrusion. The finite element predictions of the velocity fields were post-processed to calculate the thickness of the surface cladding as one billet is fed in after another through the die (i.e., the transverse weld). The mathematical model was then used to assess the effect a change in feeder dimensions would have on the shape, thickness and extent of the transverse weld during extrusion. Experimental measurements for different combinations of billet materials show that the model is able to accurately predict the transverse weld shape as well as the clad surface layer to thicknesses of 50 μm. The transverse weld is significantly affected by the feeder geometry shape, but the effects of ram speed, billet material and temperature on the transverse weld dimensions are negligible.
Highlights
Billet-on-billet extrusion is utilised in the vast majority of commercial aluminum extrusion operations
The previous billet material left inside the regular feeder geometry is much larger than that of the tapered feeder
At the end of the extrusion, there is no prior billet material left in the tapered feeder but there is some prior billet material left inside the regular feeder
Summary
Billet-on-billet extrusion is utilised in the vast majority of commercial aluminum extrusion operations. On the other hand there are unwanted consequences of extruding in this manner including the development of a transverse weld which will develop between the individual billets as they are extruded into the final extrudate. The surface of the second billet is clad with the first billet with remnants of the oxide films from the billet end and feeder face on the interface This effect can extend for many metres into the second billet and the main objectives in continuous billet-on-billet extrusion is to minimize the transverse weld interface length and the associated scrap allowance, while at the same time providing a weld strong enough to withstand the stretching process after extrusion. It is recognised [3]
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