The Effect of Die Geometry on Forging Conditions in Axisymmetric Hot Aluminium Forging. An Example
Filling of the die impression in hot closed-die forging depends strongly on the geometry of the impression and the flash gap. The stage before the workpiece material reaches the side walls of the impression might be critical with respect to eventual cracking if a material with low ductility is forged. At later stages, die cavity filling is controlled by flash gap design. The present investigation deals with effects regarding changes in material flow of alloy AA6082 as a result of small changes in die geometry in centre of an axisymmetric die. Furthermore, effect of flash land design on stresses inside the impression is investigated. The early open-die forging stages, and the later closed-die stages of filling of the die impression, are analyzed for two different impression geometries in centre of the die, and four different flash land designs. Experiments and FEM-simulations using the code DEFORMTM 2D are performed. A die with a small cavity in centre resulted in ~14% lower hoop stress during early forging before flash is formed, compared to a corresponding die without such a centre cavity. Approximately 4% lower die load is predicted at the stroke length at which the cavity forming the rim of the forging is filled. Die design characterised by decreasing the height of the flash gap resulted in reduced volume of flash, but also in ~14% higher radial stress at centre of the forging, compared to original design. A tapered flash gap with same volume reduction showed ~8% increase in radial stress. A parallel flash gap with grooves in the flash land, caused a higher volume of flash, and increased radial stress at the centre of the forging of ~7%. The effect of grooves on impression filling - by increased flow restriction - is mainly dependent on width of grooves, and shear behaviour of workpiece material.
Dirk NOLTE, Henry VALBERG
Aluminium Forging, Open and Closed-die Forging, Die Geometry, Flash Gap Geometry, Cavity Filling, Excess Material Volume, Cracking.