Encuesta 2

The following processes fall into the heading indirect cold extrusion:

● Indirect cold extrusion without lubrication and without a shell

● Indirect cold extrusion without lubrication with a shell

● Indirect cold extrusion with lubrication and without a shell

The first two processes are indeed theoretically possible but technically unimportant. In these two processes the temperature increase in the de-formation zone from the dede-formation process can produce similar thermal conditions to indi-rect hot extrusion. The material flow when ex-truding through flat dies is approximately the same as in indirect hot extrusion, when fast ex-trusion speeds (sealing stem speeds) and high extrusion ratios are used.

A quasi-adiabatic deformation process, there-fore, takes place. The temperature increase in the material being deformed in the deformation zone is calculated from Eq 3.26.

Section exit temperatures of approximately 300
C can be achieved in the indirect cold ex-trusion of Al99.5 with exex-trusion ratios over V⳱ 100 and high sealing stem loads.

The industrial application for the two pro-cesses, indirect cold extrusion without lubrica-tion and without a shell, and indirect cold extru-sion without lubrication and with a shell, is restricted by the relatively high sealing stem load F_VSt on the container cross-sectional area A0 required. This relative load is referred to as the sealing stem pressure p_VSt:

F_VSt

p_VSt⳱ (Eq 3.56)

A₀

Indirect cold extrusion with lubrication and without a shell is more important for the extru-sion of composite materials, solders, powder metal (P/M) alloys and materials with relatively low flow stresses.

3.2.7.1 Indirect Cold Extrusion with Lubrication and without a Shell Process Sequence in Indirect Cold Extru-sion with Lubrication and without a Shell.

The process was first reported in [Sie 77]. In indirect cold extrusion with lubrication and without a shell, a conical die as shown in Fig.

3.55 is used. The billet with a tapered end is coated with the lubricant and loaded into the container along with the die. Although, it is true that in indirect extrusion a reduction of the fric-tion between the billet and the container cannot be obtained because there is no friction, it is pos-sible to reduce the friction between the die and the container and between the die and the ma-terial being extruded [Sie 77].

With suitable design of the die and the sealing stem and the correct selection of a lubricant that usually has high thermal viscosity and high re-sistance to mechanical stresses, a lubricant buf-fer forms at the die entry. Lubricant flows from this into the gap between the die external surface and the container as well as into the working area between the die and the material being ex-truded (see Fig. 3.55).

Introducing seals between the sealing stem and the container and between the die and the container gives a smooth transition between this process to hydrostatic extrusion and the thick film process.

Figure 3.56 shows the sealing stem and the hollow stem as well as the container for a con-tainer diameter of 85 mm and a sealing stem load of 8000 kN for indirect cold extrusion with lu-brication. The sealing stem load force acting over the cross-sectional area of the container (sealing stem pressure) is 1400 N/mm²(see Eq 3.56).

Figure 3.57 shows the process sequence of indirect cold extrusion with lubrication without a shell. It is recommended that loose dies be used because of the extremely high sealing stem pressures usually used.

Material Flow in Indirect Cold Extrusion with Lubrication and without a Shell. Figure 3.58 shows stages in indirect cold extrusion with lubrication. It can be seen that a quasi-stationary deformation process forms within the die cone.

The material flow is approximately the same in all stages.

Figure 3.59 shows for different die opening angles that the deformation zone extends into the adjacent billet volume with larger opening an-gles. However, it can be clearly seen that the billet volume outside the deformation zone is

Fig. 3.56 Sealing stem, hollow stem, and container for a container diam of 85 mm and a sealing stem load of 8000 kN

Fig. 3.55 Principal depiction of indirect cold extrusion with lubrication and without a shell [Sie 77]

completely unaffected by the deformation pro-cess. The billet length is, as in all indirect extru-sion processes, not limited by the material flow but merely by the maximum possible length of the hollow stem calculated to resist buckling.

Axial Forces in Indirect Cold Extrusion with Lubrication and without a Shell.

Be-cause there is no friction between the billet and the container to be taken into account in any indirect extrusion process, then, according to Eq 3.52:

FVSt艑 FHSt艑 F ⳱ FM U

Fig. 3.57 Process sequence for indirect cold extrusion with lubrication and without a shell. a, platen; b, die holder; c, hollow stem;

d, die; e, billet; f, container; g, sealing stem; h, lubricant film; i, separator; j, manipulator

As in direct cold extrusion in the die entry plane the ideal axial compressive stress is, according to Eq 3.40:

¯ ¯

pXEid ⳱ k • uf gges ⳱ k • lnVf

The mean flow stress for all the material being deformed in the deformation zone is:

¯ ¯˙

kf⳱ f( ¯u , u , ␽)g g

and can be obtained from the quasi-adiabatic flow curves for the initial temperature␽E⳱ 20

C and the mean principal strain (Eq 3.15):

¯ 3

ug⳱ 2 • ln D_S D₀ Ⳮ 2

In cold working where the deformation tem-perature is well below the material recrystalli-zation temperature, the influence of the mean logarithmic principal strain rate ¯˙u_g is usually negligible. If this is not the case, then ¯˙u_gis ob-tained from Eq 3.18.

The ideal axial compressive stress p_XEid needed for perfect deformation in the die entry plane multiplied by a factor C gives the actual axial compressive stress pXE acting in the die entry plane.

According to Eq 3.41:

pXE ⳱ C • pXEid

where the factor C, according to Eq 3.21, con-tains a profile factor f_pand a shape change effi-ciency factor g_U(see section 3.1.1.4).

Fig. 3.58 Stages in indirect cold extrusion with lubrication and without a shell of Al99.5

Fig. 3.59 Development of the deformation zone with differ-ent die opening angles for cold indirect extrusion with lubrication and without a shell of Al99.5

In indirect cold extrusion with lubrication and without a shell of round bars through conical dies, then, from Eq 3.42 with fp⳱ 1:

C⳱ 1 ⳱ 1.1 g_U

Accordingly, from Eq 3.43 as with direct cold extrusion with lubrication and without a shell:

¯ ¯

pXE ⳱ C • pXEid⳱ 1.1 • k • uf gges⳱ 1.1 • k • lnVf

This stress is constant in indirect cold extru-sion between the die entry plane and the billet end because there is no friction to overcome be-tween the billet and the container. Thus, the seal-ing stem pressure is given by:

F_VSt

p_VSt⳱ ⳱ p_XE (Eq 3.57)

A₀

Figure 3.60 shows the variation of the radial and axial compressive stresses over the billet length for indirect cold extrusion.

The sealing stem force referred to the con-tainer cross-sectional area at the start of extru-sion is obtained from:

ˆ ˆ

F_VSt F_VSt

pˆ_VSt⳱ ⳱ ₂ (Eq 3.58)

A₀ D₀

p • 4

The relative sealing stem force during the quasi-stationary deformation process is:

¯ ¯

F_VSt F_VSt

p¯_VSt⳱ ⳱ ₂ (Eq 3.59)

A₀ D₀

p • 4

The relative sealing stem force pˆ_VStat the start of extrusion is approximately 20% higher than the sealing stem force p¯_VSt:

pˆ_VSt艑 1.2 • p¯_VSt (Eq 3.60)

Fig. 3.60 Fundamental variation in the compressive stress pxand the radial compressive stress prover the sealing stem displacement sVStin cold indirect extrusion with lubrication (l0, upset billet length in the container without the billet tip; D0, container diam; lU, length of the deformation zone