4. Forge ® validation using both original observations and results from the
4.2 Extrusion experiments for validation
4.2.1 Extrusion press
Figure 4.1 General layout of the extrusion press and the direct and indirect tooling (Subramaniyan 1989, p.49)
The layout of the extrusion press and the direct and indirect extrusion tooling are shown in Figure 4.1.
Experimental data was taken from Subramanian (1989)’s experiments. Extrusion
was performed on a 5MN press (at Imperial College, London) operating with tooling set up for direct and indirect extrusions. Both extrusion ratios are 40:1, the ram speed is 5mm/s and 3mm/s for direct and indirect extrusions respectively. The initial billet temperature was 400ºC and the temperature for tools is 350ºC. The billets were 75mm in diameter and 95mm long and were heated in an induction heater.
The load was measured by a Mayes load cell situated directly above the ram, the output from the cell being recorded on a Labmaster. Output from a pressure transducer situated at the inlet to the main cylinder was also recorded in order to check load measurements. Ram speed and displacement were measured by a rectilinear potentiometer fixed between the moving crossheads and the press bolster which transmitted to the Labmaster.
The container was hydraulically lowered into position and the ram removed to its highest point. Two semi circular rings were placed on top of the container to prevent any damage to the main ram.
The hot billet was transferred from the induction heater into the container. A pressure pad was dropped on top of the billet. The ram was then lowered, initially under a fast approach and then at a predetermined speed during the extrusion cycle.
The ram, followed by the container, was then raised allowing the extrudate to be cut and pushed into the quench tank. The discard was then removed by raising the container, and pushing it out slowly using a tight fitting scrapper pad in front of the main ram.
The procedure for the indirect extrusion was essentially the same except that the 75mm ram was removed from the main ram and immediately prior to extrusion the container was raised such that the die assemble at the top of the mandrel was positioned in the bottom 50mm of the container. Upon transferring the preheated
billet, the dummy block was placed in the container and the extrusion cycle was initiated as for the direct extrusion. When the main ram hit the dummy block both the billet and container were pushed down onto the mandrel and moved simultaneously at the predetermined speed during extrusion.
4.2.2 Material compositions
The material used in the experiment was supplied by Alcan Labs, Banbury, in the form of semi continuous logs of 86mm in diameter. The quoted composition is given in Table 4.1.
Cu Mn Mg Fe Si Zn Ti Al
4.66 0.69 1.35 0.19 0.08 0.02 0.01 Balance
Table 4.1 Chemical compositions (wt%) of cast alloy AA2024
4.2.3 Tooling
Figure 4.2 The die used for rod extrusion (in mm)
The geometry of the die used in the extrusion for validation is shown in plane and
section view in Figure 4.2, in which for an extrusion with extrusion ratio of 40:1, a is 11.54 mm, b, 15.63 mm.
4.2.4 FEM model setup
For hot extrusion, the elasticity effect can be ignored and hence the most economical constitutive laws are purely viscoplastic approximations. The above-mentioned equations (2.8) and (2.9) were used to describe the flow stress.
The aluminium alloy AA2024 was chosen as the material for all direct and indirect simulations. For the aluminium alloy AA2024, ∆H=148880 J/mol, A=3.252x108, =0.016, =4.27 (Sheppard and Jackson 1997; Sheppard and Wright 1979b).
Figure 4.3 FEM model of the rod extrusion
In direct extrusion, the ram pushes the billet towards the die orifice to obtain the desired shape and properties. The container and die are fixed in this case (Figure
Ram
Die
Container
Axis
Billet
4.3). For indirect extrusion in an industrial environment the container and ram are fixed and the die moves towards the billet. However for the purpose of these simulations the die was fixed and both the container and ram were allowed to move together towards the die.
To reduce the computer analysis time, axisymmetric FEM model was used and ram, container and die are assumed to be rigid, which means there is no deformations considered for the tools and a single temperature value is assigned to each component during thermomechanical coupled computation. The radius of the die entrance is 1mm. The mesh size is a set value of 4mm with a meshing option of ‘fine front’ value of 2mm. This allows finer meshes near the surface of the billet or at the die corner (as shown in Figure 4.3). Six-node triangle elements are adopted to discretise the billet. Each element side is described by a second order curve. The heat transfer coefficient between the billet and tools (die, ram and container) is set as 20000Wm-1 K-1. The convective heat transfer coefficient is 10Wm-1K-1. The emissivity is chosen as 0.05. The Tresca friction law is adopted.
The friction factor (0≤m≤1) on the ram/billet is 0.4, and 0.85 for the container/billet. For the die, according to Paterson’s study (1981), the friction factor on the die land contact region is much lower than that in other contact regions, in this study, the friction factor within the die land/billet interface is 0.1, and 0.8 for the remaining part of the die.