Hydrostatic Extrusion
3.3 Conventional Hydrostatic ExtrusionExtrusion
3.3.2 Process Sequence
Figure 3.62 shows the process sequence in hy-drostatic extrusion. In principle, the billet can be loaded between the stem and the container or
Fig. 3.63 Fundamental variation of the hydrostatic pressure in the container over the stem displacement for hydrostatic extrusion
between the die and the container. The container volume is filled with the hydrostatic medium (usually castor oil) through a central hole in the stem. When the billet has been extruded to the discard length, the stem and the container travel back far enough to allow the discard to be pulled from the die with a manipulator and the section cut with a saw. Both the seal between the die and the container and the seal between the stem and the container are usually made from an elas-tomer with a copper beryllium support ring.
These seals have a very limited life.
3.3.3 Axial Loads in Hydrostatic Extrusion The variation of the hydrostatic pressure in the container with the stem displacement is usu-ally studied in hydrostatic extrusion (Fig. 3.63).
If it is assumed that the frictional losses in the stem/container seals are negligible, then the hy-drostatic pressure p in the container is equal to the stem load FSt divided by the stem cross-sectional area ASt:
FSt
p⳱ (Eq 3.62)
ASt
The stem load is, therefore, obtained from the product of the stem cross-sectional area and the pressure. The fact that the hydrostatic medium has to be initially compressed must be taken into account when considering the variation in the hydrostatic pressure p over the stem displace-ment. The actual deformation process com-mences after the start pressure pˆ needed for the deformation has been reached. The pressure then reduces to the constant value p¯ required for the
semistationary deformation process (Fig. 3.63).
The characteristic curve of the pressure increase against the stem displacement depends on the compressibility and volume of the hydrostatic medium, the elastic behavior of the pipe con-nections, the tooling, and the press frame. (Fig.
3.64).
The gradient of the characteristic curve in-creases as the volume of the hydrostatic medium decreases. The compressibility of the hydrostatic medium also influences the profile of the pres-sure increase characteristic curve.
Figure 3.64 shows the variation of the hydro-static pressure p with the stem displacement for different hydrostatic media and billet lubricants.
The curve with the highest pressures exhibits severe oscillations at the start of the process.
These are associated with the erratic transition from sticking to slipping in the tribological sys-tem extruded material/lubricant/die known as the stick slip phenomena. The friction in the tri-bological system billet/lubricant/die is also very important. Castor oil is usually used as the hy-drostatic medium.
If the hydrostatic medium dos not have suf-ficient lubrication properties, it is possible to add lubricating additions to the hydrostatic medium or to lubricate the billet before it is loaded into the press (see Fig. 3.64).
Figure 3.65 shows in principle:
● The variation of the stem load FStover the stem displacement w
● The variation of the hydrostatic pressure p over the stem displacement w
● The variation of the axial load FMexerted by the billet onto the die over the billet displace-ment s.
The stem load and the hydrostatic pressure over the stem displacement have to follow the same principle curve because, according to Eq 3.61, the stem load is derived from the product of the hydrostatic pressure and the stem cross-sectional area, which is a constant.
The axial load on the die FM behaves differ-ently because it is the product of the billet cross-sectional area and the hydrostatic pressure:
FM ⳱ p • ABillet (Eq 3.63)
During the semistatic deformation process:
F¯M ⳱ p¯ • ABillet (Eq 3.64)
For the unsteady upsetting process:
Fig. 3.64 Variation of the hydrostatic pressure over the stem displacement for different hydrostatic media and billet lubrication [Fio 67]
FˆM ⳱ pˆ • ABillet (Eq 3.65)
The maximum value FˆMis approximately 15%
higher than the value F¯M:
ˆ ¯
FM ⳱ 1.15 • FM (Eq 3.66)
If the axial load FM applied by the billet onto the die is plotted against the billet displacement s, the curve FM⳱ f(s) is comparable to the vari-ation FM⳱ f(s) for other extrusion processes.
For approximately the same frictional condi-tions in the interaction between the extruded
product and the die, the same die aperture angle and the same extrusion ratio, therefore: The ax-ial load F¯Mon the die for the semistatic defor-mation process is approximately the same as for direct cold extrusion with lubrication, indirect cold extrusion with lubrication, and hydrostatic cold extrusion.
3.3.4 Products
Figure 3.66 shows interrupted extrusions of copper wire from copper billets with extrusion ratios of V⳱ 50 and an initial billet temperature
Fig. 3.65 Variation of the stem load FStand the hydrostatic pressure p over the stem displacement w as well as the variation of the axial force FMover the billet displacement s.
of 20C and V ⳱ 800, and an initial billet tem-perature of 300C and a hydrostatic pressure of 16 kbar.
With pressures up to 20 kbar, hydrostatic ex-trusion can be used for the cold exex-trusion of alu-minum alloys, copper and copper clad alumi-num billets and, to a limited degree, even steel billets to sections (see Fig. 3.67).
It therefore competes with indirect cold extru-sion with lubrication and without a shell and, to some degree, also with direct cold extrusion
with lubrication and no shell. Hydrostatic extru-sion appears to be particularly suitable for the manufacture of helical sections (see Fig. 3.68).
In this case, the billet rotates in the container during extrusion.
Figure 3.68 shows helical gear bars extruded from case-hardening steel cylindrical billets.
These helical gear bars can be produced by ex-trusion only if the billet can rotate in the con-tainer completely free of any frictional restraint.
This requirement is met in hydrostatic extrusion.
Fig. 3.66 Stages in the extrusion of copper billets to copper wire (Source: ASEA)
Fig. 3.67 Hydrostatic pressure against the extrusion ratio for different materials (Source: ASEA). 1, high speed steel; 2, mild steel;
3, soft unalloyed mild steel; 4, commercial copper (Cu 99.5%); 5, aluminum alloy 7075; 6, pure aluminum (Al 99.5)
It is also possible to extrude tubes with a mov-ing mandrel as shown in Fig. 3.69. This shows a mandrel fitted in an end piece and pressed into a prebored billet. The hole in the billet is slightly smaller than the mandrel diameter.
In the extrusion of copper clad aluminum sec-tions, an aluminum billet is first pressed into a copper tube. The tube is sealed at one end by a pressed-in closure piece and tapered at the other
so that it can seal against the die when the con-tainer volume is pressurized. Figure 3.70 shows the operating sequence during extrusion.
Figure 3.71 shows the hydrostatic pressure needed plotted against the extrusion ratio for dif-ferent copper contents. It is clear that the hydro-static pressure needed increases both with the increasing copper content and increasing extru-sion ratio.