ASM Metals HandBook Vol. 14 - Forming and Forging
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• Orientation of the orifices
• Modification of the shape of the orifices to correct for thermal shrinkage and die deflection under load
• Determination of bearing lengths for balancing metal flow
Fig. 12 Schematic of a flat die for extrusion of T-sections. Source: Ref 13
The details of a CAD technique for flat dies are given in Ref 13 and 15, in which the capabilities and application of an
interactive CAD program, called ALEXTR, are described. A flow chart of the operation of this program is shown in Fig.
13. The first input into the program is the cross section of the extrusion, expressed in terms of x,y coordinates and the
associated fillet or corner radii. These data are used to calculate such geometric parameters as cross-sectional area,
perimeter, shape factor (perimeter/weight/length), location of centroid, and size and location of the circumscribing circle.
The extrusion shape and the circumscribing circle are displayed on the graphics terminal with the calculated geometric
variables.
Fig. 13 Flow chart showing steps in the operation of ALEXTR. Source: Ref 13
ALEXTR then determines the number of extrusion orifices in the die (Ref 14). For this purpose, the user is asked to enter
the specific press number (among several available), the alloy to be used, and the extrusion temperature. For each
available press, characteristics such as press capacity, container diameter, maximum billet length, and runout length are
stored in a data table. Load and yield calculations are made with the use of this information. The yield Y, in percent, is
defined as:
(Eq 3)
Because the load decreases as the number of openings increases, ALEXTR makes the first load calculation based on a
single opening. If the result exceeds the press capacity, the calculation is repeated until the expected load is less than the
press capacity or until the number of openings required is greater than the number allowed by the defined press
characteristics. The number of openings is selected on the basis of:
• Operator specifications
• Maximum extrusion length
• Maximum material yield
The program also determines the optimal billet size for the number of openings to be used.
The next step in the design process is the layout of the extrusion orifices. Two considerations are involved. The first is
that certain minimum clearances have to be maintained between the die openings (so that the die and the backup tooling
have the strength to withstand the extrusion pressure) and between the die opening and the cylinder wall (to avoid flow of
metal from the outside surface of the billet into the product). The second consideration is that the metal flow must be
balanced by orienting the orifices such that a certain amount of symmetry is maintained, if possible.
The openings are positioned such that the center of gravity of an opening coincides with that of the billet segment feeding
that opening. The opening is also rotated such that its greatest dimension is parallel to and as close as possible to the
chord of the segment (Fig. 14a). If this layout is acceptable to the user, the single display is replaced with a display of the
full container circle and all openings (Fig. 14b). The layout can be modified by the designer, if desired, through the use of
the translate, rotate, and mirror image capabilities of the program. If unable to lay out the holes because of insufficient
clearances, the designer can go back and select a different number of holes or a different press.
Fig. 14
Placement of openings in the die by ALEXTR. (a) Extrusion section placed on one quadrant of the die.
(b) Arrangement of the entire die by repeating layout of the segment shown in (a)
After the layout is complete, ALEXTR corrects the openings for die cave and die deflection. A tool strength analysis is
then performed to determine the bending and shear stresses in the die, backer, and bolster due to the extrusion pressure.
The need for conforming tools to support the die is also determined by the tool strength analysis. These calculations allow
the user to evaluate the need for support tooling. Die deflection analysis also includes estimation of the bending of the die
at various locations. This information is helpful in predicting the dimensional changes in the die orifices under load,
during extrusion. Therefore, the dimensions of the die orifices can be modified to correct for these dimensional changes to
obtain the desired tolerances in the extruded shape.
The next step in the design process is the determination of die bearing lengths. The die bearing at any position is
dependent on the section thickness at that position and on its distance from the die center. The designer indicates, with the
light pen on the screen of the graphics terminal, the applicable thicknesses at various points on the extrusion perimeter.
These dimensions are used by the computer program to calculate the die bearing lengths.
Manufacture and Evaluation. After die design has been completed, the geometric design information can be used to
manufacture the dies by NC techniques. For manufacturing extrusion dies, either conventional electrical discharge
machining (EDM) or wire electrical discharge machining is used. In the first case, two EDM electrodes are machined by
numerical control; one electrode is used for machining the die opening(s) from the billet entry side, and the other is used
for machining the die bearings from the exit side of the die. In wire electrical discharge machining, a wire electrode can
be used for machining the die opening. However, the backside of the die, that is, the bearing areas, still must be machined
conventionally by milling or by conventional electrical discharge machining. Numerical control programs are also used
for preparing the templates for dimensional quality control of the dies and the extrusion.
In an example development study, the CAD/CAM technique was used to make flat-face dies (see Fig. 10b) and to extrude
a T-shape from aluminum alloy 7075 (Ref 13). This section was extruded using a 6.2 MN (700 tonf) hydraulic press
equipped with a 76 mm (3 in.) container. With an average bearing of 4.74 mm (0.187 in.), starting billet dimensions of
173 mm (2.875 in.) in diameter and 152 mm (6.0 in.) in length, and a flow stress of 52 MPa (7.5 ksi), a breakthrough load
of 2.34 MN (263 tonf) was calculated. When the average bearing was specified to be 6.35 mm (0.250 in.), the expected
load was 2.43 MN (273 tonf). The flow stress of 52 MPa (7.5 ksi) was estimated for the billet of aluminum alloy 7075
from data available in the literature and from preliminary extrusion tests with a round extrusion die. In these estimations,
calculations were made for the average strain (4.0 for the present extrusion ratio of approximately 15 to 1) and strain rate
(0.37/s for the ram speed of 406 mm/min, or 16 in./min).
The final step in the die design process was to indicate the section thicknesses and the bearing transition points. The die
design data were then saved on a disk file for subsequent access by NC programs. Therefore, this geometry was used for
NC machining of the EDM electrodes and the die bearings. The die opening, in H13 die steel with a hardness of 42 to 46
HRC, was machined through the die from the front. The die bearings were machined by numerical control from the back.
Computer-Aided Design and Manufacture of Dies for Lubricated Extrusion
Design. Lubrication in extrusion reduces load and energy requirements, reduces tool wear, improves surface finish, and
provides a product with nearly uniform properties. This technique is commonly used in the extrusion of shapes from
steels, titanium alloys, and nickel alloys. Proper die design is critical in lubricated extrusion, especially when noncircular
shapes are extruded. An effective die design must ensure smooth metal flow with consistent lubrication. It is desirable to
use shaped dies, which provide a smooth transition for the billet from the round or rectangular container to the shaped-die
exit.
Computer-aided design techniques have been developed for the design of dies for lubricated extrusion (Ref 14, 16). For
example, the design of a shaped die for extruding a T-shape from a round billet is illustrated schematically in Fig. 15. The
geometry of this die should be optimized to:
• Give a defect-free extrusion requiring minimum postextrusion treatment (twisting and straightening)
• Minimize load and energy requirements
• Yield maximum throughput at minimum cost
The design procedure for determining the optimal shape of the die involves the following three steps:
• Define die geometry in a general manner
• Calculate the extrusion load as a function of the die geometry
• Optimize and determine the die shape that requires the minimum extrusion load
Fig. 15 Schematic of a shaped die for extrusion of a T-shape. (a) Section through y-y'. (b) Section A-
A. (c)
Cross sections of the billet during extrusion. Source: Ref 16
To define the die geometry for a T-extrusion, first the position of the die opening with respect to the container axis is
determined. The initially circular cross section of the billet is then divided into a number of sectors. Starting from a plane
of symmetry, the final cross section is divided into the same number of segments. This is done while keeping the
extrusion ratios (ratio of the area of a sector in the billet to the area of the corresponding segment in the product) equal to
the overall extrusion ratio. Thus, the initial and final positions of the material flow lines along the die surface are
determined, and the path followed by any material point between the initial and final positions is determined by
calculating and optimizing extrusion pressure.
Manufacture and Evaluation. The surface of a shaped die is defined as an array of points. The practical method of
manufacturing this die is to NC machine a carbon electrode and then to electrical discharge machine the die. For this
purpose, the cutter paths for machining the electrode surface must be determined. In using a ball-end mill, the position of
the center of the spherical portion of the mill, with respect to any given point on the surface, can be determined by
constructing a vector normal to the surface at that given point. Typically, the normal vector is calculated from the cross
product of two vectors: one tangent to the surface along the material path line, and the other tangent to the cross-sectional
boundary. For a tool of given radius, the coordinates of the cutter paths are determined as the tool moves, in a
predetermined manner, over the array of points defining the die surface. The computer programs developed for
calculating the cutter paths contain special routines to check for undercutting and gouging. The calculated cutter center
points for extruding the T-section are plotted on the screen of a graphics terminal, as shown in Fig. 16.
Fig. 16 Cutter path for NC machining of the EDM electrode for the streamlined die shown in Fig. 15.
Source:
Ref 16
The CAD/CAM technique was used to design and manufacture shaped dies for a T-shape with a 51 mm (2 in.) wide web
and a 25.4 mm (1 in.) high rib. Billets 76 mm (3 in.) in diameter--from type 4340 steel and alloy Ti-6Al-4V--were
extruded in a 6.2 MN (700 tonf) hydraulic extrusion press. Comparison of experimental results with computer-aided
predictions showed that:
•
Shaped dies for lubricated extrusion of simple structural shapes in titanium and steel can be designed
and manufactured using computer-aided techniques
• Numerical control programs are adequate for machining three-dimensional stream
lined surfaces (die or
electrode) providing smooth transitions from round to structural sections. This was demonstrated by
machining the EDM electrode for a streamlined T-section
• Straight T-sections of titanium and steel can be extruded through a streamli
ned die without any die
modification
•
Extrusion loads can be predicted with reasonable accuracy if accurate flow stress and friction data are
available
•
In the extrusion of simple structural shapes, the product shape does not significantly influence the
extrusion load
Economic Aspects of Computer-Aided Design and Manufacture
The application of computer-aided design and manufacture in extrusion continues to increase. Several extrusion
companies are using computer-aided techniques for die making and process optimization (Ref 11, 17). In addition to cost
reductions, the implementation of computer-aided design and manufacture in extrusion provides the following potential
benefits:
• More precise estimation, and reductions in estimation costs
• Reduction in delivery schedules
• Less dependence on skilled diemakers
• Reductions in the number of die failures and in die-design and manufacturing costs
• Improved utilization of existing press capacity by reducing die trials
• Continuous improvement of die and process technology
• Increases in material yield and press productivity
References cited in this section
11.
C. Purnell and D. Males, Extrusion Die Design by Computer, Light Met. Age, April 1980, p 12
12.
C.T. Post, The Strong Link That Ties CAD to CAM, Iron Age, 22 Aug 1977, p 29
13.
C.F. Billhardt, V. Nagpal, and T. Altan, "A Computer Graphics System for CAD/CAM of Aluminum
Extrusion Dies," Paper MS78-957, Society for Manufacturing Engineers, 1978
14.
V. Nagpal and T. Altan, Design and Manufacturing of Dies for Lubricated Extrusion of Shapes,
J. Mech.
Work. Technol., Vol 1 (No. 2), Nov 1977, p 183
15.
V. Nagpal, C.F. Billhardt, R. Gagne, and T. Altan, "Automated Design of Extrusion Dies by Computer,"
Paper presented at the International Aluminum Extrusion Technology Seminar, Nov 1977
16.
V. Nagpal, C.F. Billhardt, and T. Altan, Lubricated Extrusion of "T" Sections From Aluminum, Titanium,
and Steel Using Computer-Aided Techniques, J. Eng. Ind. (Trans. ASME), Vol 101, Aug 1979, p 319
17.
R.A.P. Fielding, Computer Aided Design and Manufacture of Extrusion Dies, Light Met. Age,
April 1979, p
22
Conventional Hot Extrusion
References
1. K. Laue and H. Stenger, Extrusion: Processes, Machinery, Tooling,
American Society for Metals, 1981
(translated from the German)
2. J. Sejournet and J. Delcroix, Glass Lubricant in the Extrusion of Steel, Lubr. Eng., Vol 11, 1955, p 389-396
3.
R. Akeret and P.M. Stratman, Unconventional Extrusion Processes for the Harder Aluminum Alloys, Part I
and II, Light Met. Age, April 1987, p 6-10; June 1973, p 15-18
4.
A.R.E. Singer and J.W. Coakham, Temperature Changes Occurring During the Extrusion of Aluminum,
Tin and Lead, J. Inst. Met., Vol 89, 1961-1962, p 177
5. R. Akeret, A Numerical Analysis of Temperature Distribution in Extrusion, J. Inst. Met.,
Vol 95, 1967, p
204
6.
G.D. Lahoti and T. Altan, Prediction of Metal Flow and Temperatures in Asymmetric Deformation
Processes, in Proceedings of the 21st Sagamore Army Materials Research Conference, Aug 1974
7. G.D. La
hoti and T. Altan, Prediction of Temperature Distributions in Tube Extrusion Using a Velocity
Field Without Discontinuities, in
Proceedings of the Second North American Metalworking Research
Conference, May 1974, p 209-224
8. A.R.E. Singer and S.H.K. Al-S
amarrai, Temperature Changes Associated With Speed Variations During
Extrusion, J. Inst. Met., Vol 89, 1960-1961, p 225
9. K. Laue, Isothermal Extrusion, Z. Metallkd., Vol 51, 1960, p 491 (in German)
10.
R. Chadwick, Developments and Problems in Package Extrusion Press Design, Met. Mater.,
May 1969, p
162-170
11.
C. Purnell and D. Males, Extrusion Die Design by Computer, Light Met. Age, April 1980, p 12
12.
C.T. Post, The Strong Link That Ties CAD to CAM, Iron Age, 22 Aug 1977, p 29
13.
C.F. Billhardt,
V. Nagpal, and T. Altan, "A Computer Graphics System for CAD/CAM of Aluminum
Extrusion Dies," Paper MS78-957, Society for Manufacturing Engineers, 1978
14.
V. Nagpal and T. Altan, Design and Manufacturing of Dies for Lubricated Extrusion of Shapes, J. Me
ch.
Work. Technol., Vol 1 (No. 2), Nov 1977, p 183
15.
V. Nagpal, C.F. Billhardt, R. Gagne, and T. Altan, "Automated Design of Extrusion Dies by Computer,"
Paper presented at the International Aluminum Extrusion Technology Seminar, Nov 1977
16.
V. Nagpal
, C.F. Billhardt, and T. Altan, Lubricated Extrusion of "T" Sections From Aluminum, Titanium,
and Steel Using Computer-Aided Techniques, J. Eng. Ind. (Trans. ASME), Vol 101, Aug 1979, p 319
17.
R.A.P. Fielding, Computer Aided Design and Manufacture of Extrusion Dies, Light Met. Age,
April 1979, p
22
Hydrostatic Extrusion
Introduction
IN HYDROSTATIC EXTRUSION, the billet in the container is extruded through the die by the action of a liquid
pressure medium rather than by direct application of the load with a ram. The process of pure hydrostatic extrusion differs
from conventional extrusion processes (see the articles "Cold Extrusion" and "Conventional Hot Extrusion" in this
Volume) in that the billet is completely surrounded by a fluid, which is sealed off and is pressurized sufficiently to
extrude the billet through the die.
Hydrostatic extrusion can be done hot, warm, or cold and can be used to extrude brittle materials that cannot be processed
by conventional extrusion. Hydrostatic extrusion also allows greater reductions in area (higher extrusion ratios) than
either cold or conventional hot extrusion.
The primary advantages of simple hydrostatic extrusion over conventional hot or cold extrusion methods are:
• There is no fricti
on between the billet and the container. Therefore, the pressure at the beginning of
extrusion is much lower, and billets of any length can theoretically be extruded
• Friction at the die can be significantly reduced by a film of pressurized lubricant betwe
en the deforming
metal and the die surface
•
The lower extrusion pressures and the reduced die friction of hydrostatic extrusion allow the use of
either higher extrusion ratios or lower extrusion temperatures
• The uniform hydrostatic pressure in the contain
er means that billets do not have to be straight; coiled
wire can also be extruded
Limitations of the hydrostatic extrusion process include:
•
Containment of the fluid under high pressure (up to 2 GPa, or 290 ksi) requires reliable seals between
the conta
iner bore surface and both the ram and die. The technology required to achieve dependable
seals at these points is widely available, however. Also, sealing between the billet nose and the die can
easily be achieved by chamfering or tapering the billet nose to match the entry angle of the die
•
In addition to being tapered to match the die opening angle, the billet is also usually machined all over
to remove surface defects that would otherwise reappear on the extruded product. This is especially true
when cast billets are being used
Other limitations of the process arise when a relatively large volume of fluid is used compared to the billet volume to be
extruded. These include:
• Increased handling for injecting and removing the fluid for each extrusion cycle
• Reduced control of billet speed and stopping due to potential stick-
slip and excessive stored energy in
the compressed fluid
• Reduced process efficiency in terms of billet-to-container volume ratio
• Increased complications when extruding at elevated temperatures
The problems of billet speed and stopping control can be reduced by using viscous dampers and by improving lubrication
at the billet/die interface. Another approach to minimizing this and the other problems cited above is to keep the amount
of pressurizing fluid to an absolute minimum, as in the Hydrafilm process (see the section "The Hydrafilm Process" in
this article).
Hydrostatic Extrusion
Introduction
IN HYDROSTATIC EXTRUSION, the billet in the container is extruded through the die by the action of a liquid
pressure medium rather than by direct application of the load with a ram. The process of pure hydrostatic extrusion differs
from conventional extrusion processes (see the articles "Cold Extrusion" and "Conventional Hot Extrusion" in this
Volume) in that the billet is completely surrounded by a fluid, which is sealed off and is pressurized sufficiently to
extrude the billet through the die.
Hydrostatic extrusion can be done hot, warm, or cold and can be used to extrude brittle materials that cannot be processed
by conventional extrusion. Hydrostatic extrusion also allows greater reductions in area (higher extrusion ratios) than
either cold or conventional hot extrusion.
The primary advantages of simple hydrostatic extrusion over conventional hot or cold extrusion methods are:
• There is no friction between the billet and the container. T
herefore, the pressure at the beginning of
extrusion is much lower, and billets of any length can theoretically be extruded
•
Friction at the die can be significantly reduced by a film of pressurized lubricant between the deforming
metal and the die surface
•
The lower extrusion pressures and the reduced die friction of hydrostatic extrusion allow the use of
either higher extrusion ratios or lower extrusion temperatures
• The uniform hydrostatic pressure in the container means that billets do not have to be st
raight; coiled
wire can also be extruded
Limitations of the hydrostatic extrusion process include:
•
Containment of the fluid under high pressure (up to 2 GPa, or 290 ksi) requires reliable seals between
the container bore surface and both the ram and die
. The technology required to achieve dependable
seals at these points is widely available, however. Also, sealing between the billet nose and the die can
easily be achieved by chamfering or tapering the billet nose to match the entry angle of the die
• In a
ddition to being tapered to match the die opening angle, the billet is also usually machined all over
to remove surface defects that would otherwise reappear on the extruded product. This is especially true
when cast billets are being used
Other limitations of the process arise when a relatively large volume of fluid is used compared to the billet volume to be
extruded. These include:
• Increased handling for injecting and removing the fluid for each extrusion cycle
• Reduced control of billet speed and stopping due to potential stick-
slip and excessive stored energy in
the compressed fluid
• Reduced process efficiency in terms of billet-to-container volume ratio
• Increased complications when extruding at elevated temperatures
The problems of billet speed and stopping control can be reduced by using viscous dampers and by improving lubrication
at the billet/die interface. Another approach to minimizing this and the other problems cited above is to keep the amount
of pressurizing fluid to an absolute minimum, as in the Hydrafilm process (see the section "The Hydrafilm Process" in
this article).
Hydrostatic Extrusion
Simple Hydrostatic Extrusion