Method of positive flow extrusion

[57] ABSTRACT

This disclosure is directed to a method for extruding a material to form a product, e.g., wire, wherein deformation of the material is accomplished by passing the material within a controlled flow of pressurized fluid, the pressure of which fluid deforms said material. Also disclosed, with respect to the extrusion of materials by exertion of pressures thereon, is a method for automatically varying the pressures in response to the resistance of the material to extrusion so as to maintain extrusion at a desired product discharge rate.

Claims, 7 Drawing Figures

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PATENTED JUL 181972

3.677,048

SHEET 1 OF 5

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FIG. 2

VOLUME OF PRODUCT EXTRUDED

INVENTOR

F J. FUCHS, JR.

BY

MARN &amp; JANGARATH/S

ATTORNEYS

PATENTED Ml 81972

3,677.048

SHEET 2 OF 5

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Nj

PATENTED JUL 18197?

3,677,048

SHEET 3 OF 5

Z&gt;ZEO#/W770AS ZO/W

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PATENTED Ml 81972

3,677,048

SHEET 4 OF 5

STRESS

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POSITIONS OF ADVANCING FLUID CONTROL ELEMENT

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PATENTED Ml 8 is??

3.677.04Θ

SHEET 5 OF 5

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S? 69

3,677,048

METHOD OF POSITIVE FLOW EXTRUSION

BACKGROUND OF THE INVENTION

This invention is related to the extrusion of a material to form a product.

Considering briefly some of the prior art methods of extruding material, one of the earliest methods of extrusion comprises positioning a billet within a billet container having one end thereof closed by an extrusion die, and exerting a force on the billet, e.g. by an advancing ram, so as to force the billet through the extrusion die. The dimensions of the inner surface of the billet container are substantially identical to the dimension of the outer surface of the billet so that during extrusion, deformation of the billet material other than through the extrusion die is precluded. In such extrusion, the force required to extrude derives from the force exerted by the ram on the billet, and the work done by this force is made up of four parts:

1. The ideal work per unit volume required to change the shape of the billet into that of the product (work of homogeneous deformation) Ρ_Λ.

2. The work per unit volume expended in abruptly changing the direction of flow of the billet, first as it starts to deform and again as it reaches the exit of the die. (redundant work) P_r.

3. The work per unit volume required to overcome the interface friction between the billet and the die, Ρ_Λ.

4. The work per unit volume required to overcome the interface friction between the billet and the billet container (Pre)·

An improvement in extrusion processes was the development of “hydrostatic” extrusion. In hydrostatic extrusion, pressurized fluid surrounds all but the die-adjacent surface of a billet of material, exerting a high pressure on billet material to be extruded, separating the surface of the billet material from the surface of the billet container, and thereby eliminating interface friction between billet and container. Thus, the work per unit volume required to overcome the interface friction between the billet and the billet container (P_/c) is virtually eliminated because of the presence of fluid between the billet and the billet container surfaces. This extrusion method is discussed in greater detail in Canadian Pat. No. 476,793 to P.W. Bridgman which issued on Sept. 11, 1951.

Notwithstanding the reduction in necessary extrusion force resulting from the elimination of billet-billet container friction, the amount of force required for extrusion has continued to be undesirably high because of the effects of other components, viz. P_r and P_/d. Continuing attempts to eliminate or reduce the remaining forces have been made, but for the most part they have been unsuccessful.

In addition to the undesirably high extrusion forces required for known extrusion techniques, difficulties in achieving an acceptable product quality have been experienced. Considering for example the known hydrostatic extrusion methods, high hydrostatic pressures has been employed to extrude material by the process of subjecting such material uniformly, i.e., both the top and the sides of a discrete billet, to a high hydrostatic pressure, and forcing the material through an extrusion die by means of such uniformly applied hydrostatic pressure. In some hydrostatic extrusion processes, an additional longitudinal force, either tensional if applied to the product or compressive if applied to the billet, is employed to assist the longitudinal component of the hydrostatic pressure in forcing material through the extrusion die. For reasons which will be discussed in more detail infra, the products which have been made by the known hydrostatic extrusion methods have been made by the known hydrostatic extrusion methods have been unsatisfactory for many purposes. Undesirable chip formations have been experienced, the products have been often of non-uniform diameter with a poor surface finish, and the physical characteristics of the product material have been inconsistent, thus increasing the likelihood of post extrusion fracture.

Although there are many extrusion products wherein nonuniformity of shape or physical characteristics can be tolerated, for example, rods or certain types of wire, there are many others where uniformity of shape and physical characteristics is of prime importance. In the electronic art, for example, switching circuitry requires fine wire which exhibits uniform electrical characteristics which, in turn, are dependent, among other things, upon the uniformity of the wire diameter and grain structure.

Thus, known methods of extrusion such as those described above are inadequate for the production of many extruded products, particularly those which require uniformity of shape and physical characteristics.

SUMMARY OF THE INVENTION

The above-mentioned problems, viz. undesirably high forces necessary for accomplishing extrusion and nonuniformity of shape or physical characteristics in the products of extrusion, have been overcome by the method of the present invention, one mode of which may include the steps of establishing a controlled flow of pressurized fluid and passing material to be extruded within the flowing fluid to be deformed by the effect of fluid pressures exerted thereagainst.

Another aspect of practicing the method of the present invention may include the steps of pressurizing material to be extruded to generate radial and axial stresses therein, maintaining the radial stresses at a magnitude in excess of the axial stresses by an amount equal to at least the yield stress of the material, and deforming the material to form a product by passing the material through a controlled flow of pressurized fluid which exerts deforming pressures thereagainst.

Yet another aspect of the present invention comprises a method for extruding material to form a product, which method may include the steps of pressurizing material to be extruded to generate radial and axial stresses therein such as to cause the extrusion of said material to form a product, and automatically varying the magnitude of pressurization of the material in response to variations in the resistance of the material to extrusion so as to maintain extrusion at a desired product extrusion rate.

The methods of the present invention, constitute a novel approach to extruding materials in which deformation of the material is accomplished by the extrusion of pressures thereon by a flowing fluid, and in which non-uniformities in the characteristics of material to be extruded may be compensated for automatically by the substantially instantaneous variation of the pressures causing extrusion in response to the variations in the characteristics of the material. Deformation of the material by a pressurized flowing fluid eliminates friction-generating, material-to-material contact, such as the billet-die-interface friction experienced in the prior art, and reduces the total amount of work necessary to accomplish extrusion. This mode of deformation, particularly when considered with the capability of the present invention to compensate for non-uniformities in the physical characteristics of the material, enables the production of characteristically uniform product at a desired product extrusion rate.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be had from the following detailed description, read in the light of the accompanying drawings wherein:

FIG. 1 is a front elevational, cross-sectional view of a typical prior art hydrostatic extrusion apparatus;

FIG. 2 is a plot of cylinder extrusion pressure versus volume of product extruded for the prior art apparatus of FIG. 1;

FIG. 3 is a front elevational, cross-sectional view showing one apparatus for practicing the method of the present invention;

FIG. 4 is a schematic view of a billet being extruded by the apparatus of FIG. 3, showing the pressures and forces acting thereon;

3,677,048

FIG. 5 is a plot of material stress versus fluid control element advance during extrusion by the method of the invention;

FIG. 6 is a partial, cross-sectional view of a billet being extruded according to the method of the invention; and

FIG. 7 is a front elevational, cross-sectional view of an alternative apparatus for practicing the method of the present invention.

The full significance of the method of the present invention is best seen in the perspective of a prior art hydrostatic extrusion apparatus. In FIG. 1, there can be seen a typical hydrostatic pressure extruding apparatus of the type known to those skilled in this art. This apparatus, designated generally by the reference numeral 11, comprises a pressure vessel 12 having a closed end 13 and an open end for mounting a die 14 therein. The die 14 comprises a force transmission element 15 having a die plate 17 mounted therein, the die opening 16 of which is in communication with a bore 18 in die 14 for allowing the passage of the extruded product. The work material is shown in billet form 19 and product form 20 above and below the extrusion die plate 17, respectively.

In the operation of the apparatus of FIG. 1, a billet is positioned in pressure vessel 12 which is thereafter filled with fluid through a fluid line 21 and valve 22 from a high pressure fluid source (not shown). The fluid pressure is increased to a predetermined amount as dictated by the particular billet material, and the billet begins to flow through die opening 16 and is discharged as a product into bore 18.

As is evident from FIG. 1, the billet 19 is in contact with die plate 17 during extrusion. Thus, in order to extrude the billet 19, the pressure of the fluid acting on billet 19 must be sufficiently great to accomplish the work of homogenous deformation P„, redundant work P_r, and the working required to overcome the interface friction between the billet and the die P_fd. As was noted above, the effect of these three work loads is to necessitate that the hydrostatic pressure extruded on the billet be undesirably high.

Also, as mentioned above, the product quality is not satisfactory. For example, there has been a marked variation in product diameter causing longitudinally spaced circumferential ridges 23 which give the product a bamboo-like appearance. Further, the physical characteristics of the product, e.g. degree of work hardening and grain structure, have been unpredictably inconsistent.

In this regard, it has been found that ridges in the product, and variations in the work hardening and grain structure of the product result, at least in part, from the frictional forces generated by the physical engagement of the billet material with the die surface during extrusion. More particularly, during the extrusion of billets by an apparatus such as that of FIG. 1, the billet material has exhibited a tendency to lunge through the die rather than to flow through at a substantially constant rate, as is desirable in order to form physically uniform extrusion product. The lunging of the billet material has been found to be caused by either one or both of the following:

1. friction-generated extrusion rate cycling which occurs as a result of frictional forces caused by the substantially surfaceto-surface engagement of the billet material with the die plate (as noted above); and

2. non-uniformity in the physical characteristics of the billet material causing some portions of the billet material to be more difficult than others to extrude, i.e., variations in the physical characteristics of the billet material which vary the resistance of the material to extrusion.

The first condition, friction-generated extrusion rate cycling, has been found to be a result of the substantially surface-to-surface frictional engagement of the billet 19 with die plate 17 when the billet is in the extrusion position. Generally, when two bodies are in surface-to-surface contact more force is required to overcome friction therebetween and achieve an initial relative interface movement than is required to sustain an existing relative interface movement. Thus, when the billet 19 is initially subjected to pressure and force against die plate

17, a frictional force is generated therebetween. The amount of force against the billet which is necessary to overcome this frictional force requires an extrusion pressure which is greater than that extrusion pressure which is required to sustain an extrusion of the billet through the die at a constant product discharge rate. Thus, as additional fluid is introduced into vessel 12 at a relatively constant rate, the pressure of the fluid in vessel 21 increases, since no extrusion occurs until the friction between the die plate and the billet is overcome. Once the extrusion pressure increases sufficiently to generate a force adequate to overcome the interface friction and cause extrusion to begin, the rate of extrusion is greater than the ideal rate because the pressure in vessel 12 is sufficient to overcome the frictional load which, as was noted above, is greater than that necessary to maintain a constant extrusion rate.

The problem caused by billet-die interface friction can be explained more clearly by referring to FIG. 2 which is a plot of extrusion cylinder pressure versus volume of product extruded from the apparatus of FIG. 1. The desired extrusion pressurevolume relationship is indicated by the dotted line which shows a cylinder pressure increase until the ideal extrusion pressure level B is reached, whereafter extrusion occurs at a constant pressure and rate. The solid line, on the other hand, shows what actually happens because of billet-die interface friction.

As was noted above, the amount of force necessary to overcome the billet-die interface friction when there is no relative surface movement therebetween is greater than that necessary to maintain movement once commenced. Thus, in FIG. 2, the amount of fluid pressure necessary to generate sufficient force to overcome static, billet-die friction is shown as A, and as noted above, the ideal extrusion pressure is shown as B. At the beginning of the extrusion cycle, pressure builds up with no appreciable extrusion until pressure level A is reached, at which point the billet material breaks loose from the die surface and extrusion occurs. Because the extrusion begins under an extrusion pressure in excess of that necessary under the ideal conditions (by an amount equal to the difference between pressure level A and pressure level B), the extrusion rate is in excess of the ideal and the extrusion pressure thus drops rapidly since extruded product is being discharged from the vessel faster than pressurized fluid is entering. At some point, however, the fluid pressure in the cylinder is too little to continue to overcome the die-billet interface friction (pressure level C in FIG. 2) and extrusion stops. The pressurizing fluid continues to be supplied, however, pressure again builds up in the cylinder and the cycle repeats itself, causing the sawtooth effect of the plot of FIG. 2. Thus, the product is extruded only intermittently and the effect, as extrusion occurs, is one of lunging.

The second cause of lunging, inconsistencies in the physical characteristics of the billet material, is somewhat similar to the interface friction problem. If one portion of a billet is harder than another, i.e. the one portion has a higher yield strength and exhibits greater resistance to extrusion, it takes more force to push the harder portion through an extrusion die than is necessary for softer portions. Thus, when an area of billet material exhibiting increased resistance to extrusion enters the die, a force in excess of that necessary to extrude a less resistant portion of the billet is built up. Thereafter, when the hard material portion is extruded and softer billet material follows through the die, the excessive extrusion force generated to extrude the hard portion causes the billet to lunge through the die with extrusion rate cycling similar to that described above, but subject to a substantial dampening effect because of irregular occurrence of the problem.

In addition to a lack of product uniformity, other disadvantages result from variations in the rate of extrusion through the die. For any particular rate of extrusion, for example, an operating temperature is established in the zone in which material deformation occurs. Factors contributing to the deformation zone temperature are the environmental temperature, e.g. room temperature for many applications, the

3,677,048 amount of heat generated by the billet deformation and the frictional between the die and the billet, and the rate at which the die and vessel materials are capable of conducting generated heat away from the deformation zone. The environmental temperature can be regarded as a constant and, for present purposes, can be disregarded. Generally, as the extrusion rate increases heat is generated by deformation and friction faster than it can be carried away by conduction. Accordingly, the temperature in the deformation zone rises. Similarly, if the rate of extrusion decreases, the deformation zone temperature falls. Since work hardening rates, for example, vary with temperature, the amount of work hardening in the extruded product varies with the extrusion rate and, similarly, variations in the extrusion rate cause inconsistencies in the physical characteristics of the product. Further, variations in the work hardening alter the forces which are required to accomplish extrusion, thus causing additional potential sources of inconsistency in the characteristics of the extruded product.

Thus, it can be seen that prior art extrusion methods, such as the method practiced by the apparatus of FIG. 1, require undesirably high pressure in order to accomplish extrusion and often produce extrusion products which are geometrically non-uniform and non-uniform as to their physical charac- 25 teristics as a result of the tendency of the billet material to lunge through the die during extrusion.

Detailed Description of Apparatus For Practicing The Method Of The Present Invention

Referring to FIG. 3 extrusion apparatus designated 30 generally by reference numeral 50 comprises a pressure vessel 51 having an inner bore 52 which is closed at one end 53. The closed end 53 of vessel 51 is rigidly mounted on a base 54 by suitable means (not shown) such as bolts or the like. Telescopically received within bore 52 through the open end of vessel 35 51 is a generally cylindrical ram 58. Advancement and retraction of ram 58, schematically indicated by arrows 60, is accomplished by a press or other suitable mechanism (not shown) known to those in the art.

Threadedly mounted in a counterbore in the vessel adjacent end of ram 58 is a fluid control element 62 which is provided with a generally convergent, axially extending flow control surface 63. Fluid control element 62 cooperates with inner bore 52 to define a fluid chamber 64 in which a billet of material 65 is positioned for extrusion. Control surface 63 cooperates with an axially extending bore 66 in fluid control element 62 to define a passage 67 which communicates inner bore 52 of pressure vessel 51 with an axially extending bore 68 formed in ram 58. That portion of passage 67 bounded by flow control surface 63 defines a zone of deformation 69 in which deformation of the material of billet 65 occurs during extrusion.

The outer surface of fluid control element 62 is relieved and cooperates with the vessel adjacent end of ram 58 to define an annular channel for receiving a high pressure seal 71. Seal 71 includes an annular cup shaped soft seal 72 and an annular beryllium copper anti-extrusion ring 73; however, any of the high pressure seal structures known in the art and suitable for use in sealing the space between movable surfaces may be utilized.

Apparatus 50 is shown during the extrusion of a billet 65 to form wire 74, the extrusion being accomplished by the effect of a pressurized fluid passing through passage 67 of fluid control element 62 with the material of billet 65. In this regard, it should be noted that the billet material is not in contact with flow control surface 63 of fluid control element 62 at any time during extrusion. Rather, pressurized fluid flowing through passage 67 maintains the surface of the billet material spaced from the flow control surface 63 of flow control element 62 at all times, thereby eliminating the generation of surface-to-surface frictional forces such as those generated between the billet and die surfaces in conventional extrusion. Although surface-to-surface frictional forces are eliminated by the flow of fluid through passage 67 at all times during extrusion, it should be recognized that the fluid flowing through passage 67 is not a simple lubricant film. Specifically, in addition to the friction eliminating function, and as discussed in detail below, the flowing fluid of the present invention acts as the deforming 5 agent in accomplishing extrusion.

Operation of the apparatus 50 of FIG. 3 involves an extrusion cycle having three basic phases; start-up, extrusion and termination. Considering each phase briefly, the start-up phase includes the steps of positioning a billet to be extruded ¹⁰ in changer 64 which has been prefilled with a pressure transmitting fluid. Thereafter, ram 58 and fluid control element 62 are advanced within bore 52 into chamber 64 to establish a flow of fluid through fluid control element 62 and to establish ₁₅ a pressure against the head end of the billet. Force generated by fluid pressure being exerted against the head end of the billet is transmitted by the billet to pressurize the fluid in chamber 64 behind the butt end 70 of billet 65. As is discussed in detail below, the pressure in the fluid bearing against the butt 70 of billet 65 increases in magnitude at a slower rate than the pressure in the fluid bearing against the head end of the billet. The result, therefore, is disparate pressures acting on the billet which generate disparate stresses in the billet material. As the pressures increase in value, so also does the magnitude of the disparity therebetween and between the stresses generated thereby. This disparity increase continues until the stress disparity is equal to the yield stress of the material at which time deformation, viz. extrusion, commences and an extrusion equilibrium is established.

During the extrusion phase, the above-mentioned extrusion equilibrium is maintained. The term “extrusion equilibrium” is used herein to describe the situation after extrusion start up when (a) fluid control element 62 is advancing into chamber 64 at a constant rate, (b) a controlled flow of fluid is passing through fluid control element 62 and (c) fluid pressures in the system are of sufficient magnitude to generate stresses in the material capable of causing deformation and deformation is occurring with product 74 being discharged through passages 66 and 68 at a desired rate.

Upon completion of the formation of product 74, the termination phase of the cycle occurs. Ram 58 is withdrawn from chamber 64 thereby relieving the pressure in the fluid acting against the billet. Any unextruded residue of billet material may then be separated from product 74 and removed from chamber 64. Alternatively, the entire billet may be caused to be extruded. In either event, however, when chamber 64 is clear, it can be refilled with fluid, a fresh billet can be positioned in the vessel and the operation repeated.

Having described the operation of the apparatus in a general manner, the respective phases will now be considered in detail. Considering therefore the start-up phase, ram 58 and fluid control element 62 are retracted fully from bore 52. Fluid chamber 64 is filled with pressure transmitting fluid 66, 55 e.g., castor oil, and billet 65 is positioned therein with its butt end 70 spaced from the closed end 53 of vessel 51. Such positioning, is made possible by sizing billet 65 with respect to inner bore 52, to provide a narrow annular space 76 between the sides of billet 65 and the surface of bore 52. As is discussed in detail below, the narrow annular space 76 provides such a great resistance to the flow of fluid 66 therethrough that such flow is substantially eliminated. Thus, when billet 65 is positioned with its butt end spaced from the closed end 53 of vessel 51, it appears to be supported by the fluid 66. In fact, how65 ever, it is approaching closed end 53 but at such a slow rate as to be virtually unnoticeable.

In order to facilitate start-up of the operation the head end of billet 65 may be preshaped, e.g., in the same manner disclosed in copending application for METHOD OF PER70 FORMING MATERIALS WHICH WORK HARDEN, Ser. No. 758,732 filed on Sept. 10, 1968 and assigned to the same assignee as the present application.

With fluid 66 in changer 64 and billet 65 positioned therein, ram 58 is advanced into inner bore 52. The initial advance75 ment of ram 58 causes a flow of fluid out of chamber 64,

3,677,048 through· passage 67 and out bore 68 in ram 58. As the flow control surface 63 of fluid control element 62 approaches the head end of billet 65, the space between the head end of billet 65 and the flow control surface 63 decreases in size and progressively restricts the flow of fluid therethrough. The flow 5 restriction, which in effect is a valving action, causes an increase in the pressure of the fluid in the vicinity of the head end of the billet. Further, in passing between the head of billet 65 and flow control surface 63, the fluid from chamber 64 experiences a pressure drop as it flows out of chamber 64, ¹⁰through the zone of deformation 69, and into fluid passage 67.

The pressure of the fluid flowing between flow control surface 63 and the head end of billet 65 exerts pressures against the head end of the billet, which pressure can be considered to ] $ have both axial and radial components. The axial components of the pressures generate a force which displaces the billet 65 away from fluid control element 62 deeper into fluid chamber 64. Thus, the fluid flowing through the zone of deformation 69, by displacing billet 65 into chamber 64, causes the billet to 20 act as a piston thereby pressurizing fluid behind the butt end 70 thereof.

The magnitude of the pressure generated in the fluid bearing against the butt end 70 of billet 65 is less than either the effective axial component of pressure of the fluid bearing against the head end of the billet or the pressure generated in the fluid at the entrance to the zone of deformation 69 by the advance of fluid control element 62. More particularly, it was noted above that the pressure of fluid 66 bearing against the butt end 70 of billet 65 is caused by the fluid passing between flow control surface 63 and the head end of billet 65, displacing the billet into chamber 64, and that the fluid pressure at the butt end is increasing with the head end fluid pressure, but at a slower relative rate. It was also noted that the fluid flowing between surface 63 and the head end of billet 65 experiences a pressure drop as a result of, inter alia the restriction of flow caused by the valving action between the billet and the flow control surface 63. These pressure relationships are shown in FIG. 4 which is a force and pressure diagram showing the force and pressure components acting upon a billet during the extrusion thereof in the apparatus of FIG. 3.

Referring therefore to FIG. 4, billet 65 is shown as having three basic points of reference, viz. the point of entry into the zone of deformation, point A, the point of discharge from the zone of deformation, point B, and the point at which the butt end of the billet is positioned, point C. Considering point A, this point is the approximate point of maximum pressure in the system in that fluid flows away from this point in both directions, i.e., from points A to B through the zone of deformation 69 and from A to C between the surface of billet 65 and the surface of bore 52. It will be recalled, however, that the annular space 76 between billet 65 and bore 52 is narrow and presents a high resistance to the flow of fluid therethrough. Thus, the primary flow of fluid is through fluid ^5 control element 62.

Fluid flowing through the zone of deformation between points A and B exerts, because of the restriction to flow between flow control surface 63 and the head of billet 65, decreasing pressures on the surface of the head end of the billet. The pressures exerted thereagainst generate both axial and radial pressure components. In this regard, the axial components of the pressures exerted against the tapered end of billet 65 may be said to cooperate to exert an effective pressure 65 P_e„ against a projected transverse area A! of the tapered head end of the billet. Area A, is equal to the transverse area A₂ of the butt end 70 of billet 65 minus the transverse area A₃ of product 74, each of the areas being shown by dotted line in FIG. 4. 70

Considering the axial forces acting on billet 65, pressure P_eobearing against projected area A„ generates a force F, which tends to displace billet 65 away from flow control surface 63. Thus:

(1) 75

Discounting any viscous drag effect between the surface of the billet, the flow control surface 63, and the surface of bore 52, force fi is transmitted undiminished by billet 65 to the fluid bearing against the butt end 70 of billet 65. The force F₂ of the butt end of the billet is equal to the area A₂ of the butt end of the billet multiplied by the pressure P_c of the fluid bearing thereagainst. Thus:

F₂ = P&lt;./1₂ (2)

Since force F, is transmitted undiminished by billet 65, then is equal to F₂ and equations (1) and (2) can be combined as:

PPcA₂ (3)

Since A₂ is greater than A_lt equation (3) is satisfied only when P_c is less than P_KJ.

In this regard, it was also noted above that P„, is the axial component of the effective pressure of the fluid passing between flow control surface 63 and the head end of billet 65. By necessity, therefore, the pressure P_a of fluid at the entrance to the zone of deformation is greater than the effective pressure P_e and the following relationship can be noted:

P„ &gt; P_e„ &gt; P_c (4)

Relating the foregoing to start-up, the continuous advance of ram 58 and fluid control element 62 progressively restricts the flow of fluid out of chamber 64. The progressive flow restriction generates pressure increase in the fluid at the head end of the billet which, in turn, generates forces which are transmitted by the billet to generate pressure in the fluid bearing against the butt end of the billet. As noted above the magnitudes of the pressures acting against the butt and head ends of the billet are proportional to the transverse areas of the butt end A₂ and the projected head end A_t. Thus, as the fluid control element 62 continues to advance within bore 52 into chamber 64, the pressures of the fluids at both the head end and the butt end increase, but the pressure of fluid at the head end of the billet increases at a faster rate than the pressure at the butt end of the billet.

The effect of the exertion of pressures of varying magnitudes in billet 65 is the generation of disparate stresses in the billet material. Referring again to FIG. 4, therefore, an increment of billet material at point A is shown as being subjected to a plurality of stresses: viz. S_r, a radial stress generated by the effect of fluid at pressure P„ bearing against the surface of billet 65 at point A; S_ea, an axial stress generated by the effect of the summation of the axial components of the pressures exerted against the head end of the billet; and S_c, axial stress generated by the effect of fluid at pressure P_c bearing against the surface of the butt end 70 of billet 65. Since the magnitudes of the stresses are directly related to the pressures from which they are generated, and since, as noted above, the pressure at the head end of the billet increases at a rate faster than that at the butt end of the billet, the radially directed stresses increase at a rate faster than the rate of increase of the axially directed stresses. This relative rate of stress increase is shown in FIG. 5 which is a plot of stress magnitude at some point such as point A in the billet material, with respect to the continued advance of fluid control element 62.

More particularly, FIG. 5 shows the disparity in the rates of increase of radial stress S_r and axial stress S_o during start-up as well as during other phases of the operation. The increases in the magnitudes of the stresses continue during start-up until, at some point in the cycle when the stress generating pressures are of sufficient magnitude, the disparity in the magnitudes of the radial and axial stresses S_r and S„ is equal to the yield stress S„ of the billet material. At this point the material begins to deform and extrusion commences.

Considering not only the material at Point A, the entrance to the zone of deformation, but all the material in the zone of deformation, it will be recognized by those skilled in the art that deformation of billet material through the entire length of the zone of deformation requires that the material in the zone of deformation be maintained in a state of deformation, i.e., subject to stresses such that throughout the length of the zone of deformation the radial stress in the material be maintained in excess of the axial stress in the material by an amount at

3,677,048 least equal to the yield stress of the material. This is accomplished in the present method by maintaining a controlled flow of pressurized fluid between flow control surface 63 and the head end of billet 65 at such pressures and flow rates as to generate the state of deformation in the billet material at any point in the zone of deformation.

Considering therefore the extrusion of billet 65 in the apparatus of FIG. 3, the billet material can be described generally as being in three states of stress during extrusion according to the method of the invention; (a) stressed but not in the state of deformation while in chamber 64 prior to entrance into the zone of deformation; (b) stressed and in a state of deformation throughout the zone of deformation 69, and (c) unstressed (or stressed but not in the state of deformation) upon discharge from the zone of deformation. The stresses generated at any point in the billet material are radial stresses generated as a result of the radial pressure being exerted against the surface of the material at that point and axial stresses generated by the combined effect of fluid pressure against the butt end 70 of the billet and the axial components of the pressures bearing against the head end of the billet in the zone of deformation.

As noted above, the material passing within the flowing fluid through fluid control element 62 in the zone of deformation 69 is stressed and in a state of deformation. In order to maintain such a state of deformation, the stresses occurring in a particular increment of material must be disparate by an amount equal to or greater than the yield stress of the material. Relating this to the deformation of the billet 65 of FIG. 3, the radial stress on any increment of material within the zone of deformation must exceed the axial stress by an amount equal to at least the yield stress of the material. The manner in which this stress relationship is maintained according to the present invention, i.e., by passing the billet material within a controlled flow of pressurized fluid, may be better understood by again referring to FIG. 3, and in particular to three points, viz. X, Y, and Z, which correspond to points in the zone of deformation which are adjacent the point of entrance to the zone of deformation (point A), intermediate the points of initial and final deformation and adjacent the point of exit from the deformation (point B), respectively.

Thus considering initially the billet at point X, the pressures acting thereon which generate stresses therein are Ρ„ (the radial component of the pressure of the fluid flowing in the zone of deformation at point X), P_c from the fluid in chamber 64 behind the butt end 70 of billet 65, and P_la (the summation of the axial components of the pressure exerted against the head end of the billet in the zone of deformation). The increment of billet material at point X is thus subjected to stresses S_xr from pressure Pj., S_c from pressure P_c, and S_M from P_s„. In order that deformation be maintained at point X, having begun at point A as noted above, the disparity between radial stress Sj._r and axial stress S_M and S_c, continues to be equal to or greater than the yield stress of the material being deformed.

At a point Y ( FIG. 3) intermediate the points of initial and final deformation, the pressure P„ exerted by the flowing fluid, which has increased in velocity and decreased in thickness, i.e., radial dimension against the surface of the deforming material has decreased as a result of an increase in velocity, work expended in accomplishing deformation and the viscous drag of the flowing fluid. Thus, the pressures acting on the billet at point Y which generate stresses therein are P„ (the radial component of the pressure of the fluid flowing in the zone of deformation at point Y), P_s„ (the summation of the axial components of the pressure exerted against the head end of the billet downstream of point Y in the zone of deformation), and P_M2 (the pressure P_c minus the summation of the axial components of the pressure exerted against the head end of the billet upstream of point Y in the zone of deformation). The increment of billet material at point Y is thus subjected to stress from pressure P_ur, S,„ from pressure P_ral, and S_ra2 from pressure P,_o2. Again, in order to maintain deformation in the billet material, the disparity between the radial stresses S_ur and axial stress S_Mi and S„_a continues to be equal to or greater than the yield stress of the material being deformed.

Finally, at point Z, the pressure of the fluid flowing has been reduced to virtually its minimum for the same reasons noted above with respect to point Y, and at the same time has further increased its velocity and decreased in thickness. The pressures acting on the material at point Z are P_tr (the radial component of the measure of the fluid flowing in the zone of deformation at point Z), P_IO (the axial component of the pressure of the fluid flowing in the zone of deformation at point Z) and P,„₃ (the pressure P_r minus the summation of the axial components of the pressure exerted against the head end of the billet material upstream of point Z in the zone of deformation). An increment of billet material at point Z is thus subjected to stresses S_lr from pressure P_lr, S_IO from pressure P_z„, and S,„3 from pressure P_M3. Yet again, even at this stage of the deformation just prior to the discharge of the material from the zone of deformation, the disparity between the radial stress S„ and axial stress S„, and S_ra3 continues to be equal to or greater than the yield stress of the material being deformed.

It can be seen from the foregoing that material deformation, in the practice of the present invention, is accomplished by the exertion of pressure against the material being deformed by fluid flowing through fluid control element 62. Further, the pressure of the fluid throughout the zone of deformation, while decreasing in the direction of flow, must be of sufficient magnitude to maintain radial stress at any point greater than the axial stress at that point by an amount equal to or greater than the yield stress of the material. Control of the fluid pressure to achieve the desired stress relationship is accomplished by providing fluid control element 62 with a flow control surface 63 which is shaped to define a controlled path to achieve the desired pressure regulation. In this regard, the shape of the 35 flow control surface may be linear (as illustrated) or nonlinear and, as was done in practicing the present method to extrude as discussed below with reference to the example, may be determined empirically. Further, it has been found that the angle of the surface of the billet material in the zone of defor40 mation and the angle of flow control surface 63 differ with respect to the central longitudinal axis of the billet such as to approach convergence at the exit of the zone of deformation. It should also be noted, however, that for every particular extrusion situation there has been found to be a particular path 45 defined by the shape of flow control surface which enables extrusion at maximum efficiency. Flow control surfaces with this capability form the subject matter of my copending U.S. application Ser. No. 862,674, filed Oct. 1, 1969 for FLUID CONTROL ELEMENT FOR AND METHOD OF POSITIVE 50 FLUID FLOW EXTRUSION, and the copending application of N. Ahmed, Ser. No. 862,765, filed Oct. 1, 1969 for METHOD OF POSITIVE FLUID FLOW EXTRUSION AND OPTIMUM FLUID CONTROL ELEMENT THEREFOR, both of which are assigned to the same assignee as the present 55 application.

Considering that portion of billet 65 in chamber 64 upstream of the entrance to the zone of deformation 69, it was noted above that the maximum fluid pressure P_a in the fluid system occurs at the entrance to the zone of deformation, 60 point A. It was also noted in equation (4) that P„ is greater than P_c notwithstanding that the fluids which are exerting pressures P_a and P_c are in communication through space 76. That P_c can be less than P_o when, for all purposes they are distinct pressures in the same body of fluid, appears to be con65 trary to the principle that pressure in a body of fluid is transmitted undiminished in all directions. This is not the case in the practice of the invention by the apparatus of FIG. 3 since the width of narrow annular space 76 is provided to be sufficiently small to restrict the flow of fluid along billet 65 from 70 the head end to the butt end. The restriction of flow in this manner enables the pressure differential between P„ and P_c to be maintained. The effect of the restricted flow of fluid in narrow annular space 76 is to generate a substantially linear pressure drop from the pressure P„ at point A to the pressure P_c at 75 point C.

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A determination of the dimensions of narrow annular space 76 for any particular extrusion situation can be made by those skilled in the art based upon the pressure differential required between P„ and P_c, the viscosity of the pressure transmitting fluid at the environmental pressures, and the permissible rate of flow of fluid along the billet surface which may be an arbitrary value approaching a no-flow condition.

The pressure reducing effect of the flow restriction established in narrow annular space 76 (FIG. 3) also accomplishes the useful function of precluding sinkage of the billet material during extrusion, i.e. premature deformation of the billet material prior to entry into the zone of deformation. Specifically, at point A (FIG. 4), the stresses on an increment of material are S_r, the radial stress generated by pressure P„ against the surface of the material, S_c from the pressure P&lt;. of the fluid in chamber 64 bearing against the butt end 70 of the billet, and S_ea from the pressure P_e„ which is the summation of the axial components of the pressure exerted against the head end of the billet in the zone of deformation. As noted above, initial deformation occurs substantially at point A, thus the magnitude of the radial stress S„ is greater than the axial stress S_c and S_Ja by an amount equal to the yield stress of the material. Since the surface of the billet 65 at any point between points A and C is not inclined with respect to the longitudinal axis of the billet 65, no pressures are exerted against the billet material upstream of fluid control element 62 which cause a variation in the axial stress in the material. The radial pressures between points A and C, however, diminish linearly thereby reducing the radial stresses generated in the billet material. Thus, upstream from point A, the disparity in the axial and radial stresses experienced in the billet material is, at every point in the billet, less than the yield stress of the material. For this reason, no deformation can occur in the billet material upstream of point A and thus the possibility that “- ₃₅sinking” may occur os obviated.

With respect to the pressures exerted on and stress experienced in product 74 after discharge of the material from the zone of deformation, it was noted above that the environmental pressure at the discharge of the zone of deformation ₄θ may be atmospheric or otherwise. In either event, once deformation has been completed, any pressure exerted on product 74 should not be such as to create disparate stress in the material in excess of its yield stress. Clearly under such conditions further deformation would occur. 45

Having considered the start-up phase of an extrusion cycle and the overall operating conditions in the system as extrusion commences, the extrusion or product producing phase of the cycle will now be discussed.

As was noted above, the completion of the start-up phase of 50 the cycle is marked by the establishment of an extrusion equilibrium. Such extrusion equilibrium occurs with respect to the apparatus of FIG. 3 when (a) fluid control element 62 is advancing into chamber 64 at a constant rate, (b) fluid pressures in the system are of sufficient magnitude to generate 55 deforming stresses in the material, (c) a controlled flow of fluid is passing through fluid control element 62, and (d) deformation is occurring with product 74 being discharged through passages 66 and 68 at a desired velocity. Referring to FIG. 5, the establishment of an extrusion equilibrium is seen to occur at those times when the magnitudes of stresses S_r and S„ remain constant.

Considering initially the extrusion equilibrium established at the completion of the start-up phase of the cycle, the continuing advance of fluid control element 62 toward billet 65 has generated pressures against the billet material which cause stresses therein and deformation to occur. As noted above, pressure generated in the system fluid occurs as a result of valving action on the flow of fluid through the zone of defor- ₇₀mation between the head of the billet and flow control surface 63. Once the pressures necessary for deformation occurs, and assuming that the resistance of the billet material to deformation remains substantially constant so as to provide a material yield stress, e.g. S„, in FIG. 5, which is substantially constant, 75 each increment of advance of ram 58 and fluid control ele ment 62 displaces from chamber 64 volumes of billet material and fluid in constant proportion and equal in volume to the incremental displacement of fluid control element 62. In other words, the advance of fluid control element 62 into chamber 64 causes the displacement of volumes of billet material and fluid which, because of the extrusion equilibrium established in the system, are maintained in constant proportion throughout extrusion.

Since the magnitude of the pressures generated to establish and support extrusion is determinable from the yield stress of the material and reflected in the spacing between the flow control surface 63 and the head of billet 65, the extrusion of billet material having a uniform yield stress, e.g., S_u, (FIG. 5) is characterized by a constant pressure profile acting on the billet material and a constant spacing between the surface of the billet and flow control surface 63 along the zone of deformation, it being recognized that the thickness of the fluid between the flow control surface and the billet head varies from point to point along the flow control surface.

It is to be recognized, however, that the resistance of billet material to deformation varies with variations in hardness and like characteristics of the material. Further, the ordinary commercially available billet material does not have uniform characteristics. Accordingly, another aspect of the present invention provides for the capability to compensate automatically for non-uniformity in the billet material and, in particular, by way of example, for variations in the hardness of the billet material.

FIG. 6 shows a fluid control element 62 of the type utilized in the apparatus of FIG. 3 controlling a flow of fluid therethrough to deform a billet, various positions of the billet head with respect to the flow control surface 63 of the fluid control element are shown in broken line with billet 65 shown in a position spaced from the flow control surface 63 by a distance E. This distance E corresponds to the position of the billet when extrusion equilibrium initially is achieved, i.e., extrusion is proceeding at a constant product discharge rate, the pressures in chamber 64 are maintained constant since the resistance to deformation S_ul of the billet material remains constant (FIG. 5), and the billet is maintained in a constant position with respect to flow control surface 63.

Assuming now that a hard spot in the billet material is encountered, i.e. material having a higher yield strength than was previously being deformed, e.g. material with a yield strength S_Bo2 of FIG. 5, the following occurs:

a. fluid control element 62 continues to advance in a constant rate in the direction shown by arrows 60;

b. because of the hard spot, billet 65 offers increased resistance to deformation and tends to assume position F closer to flow control surface 63, thereby further restricting the flow of fluid through the flow control element 62.

c. the further restrictions in the flow of fluid between the flow control surface and the billet increases the effective pressure of the fluid flowing between the head end of the billet and flow control surface 63, thereby increasing the axial force tending to displace the billet away from the flow control surface;

d. the increase in the axial force generated in the zone of deformation displaces the billet further into chamber 64 thereby increasing the pressure of the fluid bearing on the butt end 70 of billet 65; and

e. the pressure increase continues until radial and axial stresses are sufficiently unequal to exceed the yield stress Sje (FIG. 5) of the hardened material thereby causing it to be deformed.

At this point, extrusion equilibrium is reestablished at the higher pressure. Thus, the net result of the above-listed steps is that all the pressures in the extrusion system, i.e., P„, P_ea, P_c(FIG. 4), are increased in response to the increased resistance of the billet material to extrusion. The increased pressures generate increased stresses capable of deforming the relatively harder material at the same rate at which extrusion was proceeding prior to the entrance of the hard material into the

3,677,048 zone of deformation. In this regard, although the adjustment of the fluid pressures in response to the entry of the hard billet material into the zone of deformation is described above as a step by step evolution, the actual adjustment occurs virtually instantaneously so as to insure that extrusion occurs at a con- 5 stant product discharge rate.

Assuming now that the hard portion of billet material has passed through the zone of deformation and a portion of billet material which is softer is experienced. The softer material has a lower yield strength S_u3 (FIG. 5) and as such requires lower 10 radial and axial stresses to maintain extrusion at a constant rate. In the absence of pressure adjustment, extrusion of relatively soft material after relatively hard material causes the billet to be deformed through the zone of deformation more quickly, thereby effectively increasing the spacing between 15 flow control surface 63 and the head end of the billet. The increased deformation rate causes the billet to assume a spacing with respect to flow control surface 63 such as that shown at G on FIG. 6, and the increased spacing between the flow control surface and the head end of the billet decreases the pressure ³θ drop in the fluid passing between the head end of the billet and the flow control surface, thereby decreasing the axial force tending to displace billet 65 into chamber 64. Such a reduction in the axial force generated at the head end of the billet causes the billet to be displaced toward flow control surface ^z:&gt;63 by the action of the pressure of the fluid in chamber 64 bearing against the butt end 70 of the billet. This displacement toward fluid control element 62 causes the billet to move from relative position G to relative position E in FIG. 6 until the pressure in chamber 64 bearing against the butt end 70 of the billet and the pressures in the zone of deformation bearing against the head end of the billet are reduced by an amount sufficient to re-establish an extrusion equilibrium. The reduction in pressure level is such as to maintain extrusion at a constant product discharge rate in that the material passing through the zone of deformation is softer, i.e., has a lower yield strength S_u3 (FIG. 5) and thus requires the exertion of less pressure to generate adequate forces of extrusion.

Thus it can be seen that during the extrusion phase of the operating cycle of the apparatus of FIG. 3, a positive flow of fluid is established through the zone of deformation of a fluid control element, which flow of fluid exerts pressure on the billet material to accomplish the deformation thereof. The material is extruded at a desired product discharge rate, which 45 rate is maintained notwithstanding variations in the resistance of the billet material to deformation. Further, since the billet material does not contact flow control surface 63, no surfaceto-surface friction is generated, the amount of work required to accomplish extrusion is thereby reduced, and product hav- 50 ing uniform characteristics is produced.

Upon completion of the extrusion phase of the cycle, termination occurs as noted above either by complete extrusion of the billet material through fluid control element 62 or by the withdrawal of fluid control element 62 prior to Complete 55 extrusion of the billet.

ram 106 is schematically indicated by arrows 107.

Fluid control element 101 is threadedly secured to the chamber-adjacent end of ram 106 and is provided with a 60 frusto-conically shaped passage 109 the surface 110 of which defines a flow control surface for controlling the passage of fluid from chamber 98 through passage 109 to an axially extending passage 111 formed in ram 106 to accommodate the discharge of product 115 after extrusion.

A seal 112 is provided in an annular groove formed by the cooperation of fluid control element 101 and ram 106 to prevent the escape of fluid from first chamber 98 around ram 106 during the operation of the extrusion apparatus; the seal 112 may also be of the same type as aforedescribed seal 71.

In practicing the method of the invention with the apparatus of FIG. 7, chambers 98 and 99 are filled with a suitable pressure transmitting medium, e.g., a hydraulic fluid such as silicone oil or castor oil. A billet 100 is inserted through bore 96 in sleeve 95 sufficiently far to expose a portion of its 75 peripheral surface as well as its butt end 116 to the fluid in

The apparatus of FIG. 3 has been used, e.g., to extrude approximately 240 feet of wire from a billet 43 inches long at a product discharge velocity of 4,000 feet per minute. The following information relates to this extrusion of wire:

BILLET - length - 43 inches, O.D. - 0.360 inches, nose taper - 30° included angle, material - 99.5 percent aluminum.

BILLET CONTAINER (Vessel 51)- length - 46.5 inches (entrance to zone of deformation to end at beginning of 65 operation), bore - 0.376 inches.

FLUID CONTROL ELEMENT - exit orifice - 0.0450 inches, flow control surface shape - 40° included angle, straight (linear) taper, speed of advance - 60 feet per minute.

FLUID - Cindol 4683 (aluminum drawing lubricant,)

FLUID PRESSURES - at butt end P_c - 110,000 psi (approx.), at entrance to zone of deformation P„ - 120,000 psi (approx.) at exit of zone of deformation P₆ - atmospheric.

EXTRUDANT- O.D. - 0.0448 inches, length - 240 feet, discharge velocity - 4,000 feet per minute.

As discussed above, the method produced by the apparatus of FIG. 3 relies on a narrow annular space 76 for restricting the flow of fluid along billet 65 so as to maintain the pressure differential between fluid at the entrance to the zone of deformation and fluid bearing against the butt end of the billet.

Another apparatus for practicing the method of the invention is shown in FIG. 7 wherein the fluid at the entrance to the zone of deformation and the fluid bearing against the butt end of the billet are separated by a physical seal so as to establish distinct bodies of fluids at different pressures. More specifically, there is shown in FIG. 7 an apparatus designated generally by the reference numeral 80 which comprises a pressure vessel 81 having an inner bore 82 for receiving securely, such as by press or shrink fit, a cylindrical casing 83 therein. The interior cylindrical casing 83 comprises an axial bore 84 having an internally threaded portion 85 at one end and an inwardly extending annular shoulder 86 disposed at its other end, and an axial counterbore 87 extending from shoulder 86 through the remainder of casing 83.

A plug 89 is threaded into casing 83, and abuts the closed end 91 of a generally cylindrical inner liner 90 which is securedly fitted within bore 84. The opposite end 92 of liner 90 is open, the liner extending axially within bore 84 from plug 89 to a position intermediate plug 89 and shoulder 86. An annular seal 94, which may be of the same type as seal 71 described above, is mounted in bore 84 between plug 89 and liner 90 and prevents leakage of fluid around plug 89.

A sleeve 95 is slidably mounted in bore 84 between the open end 92 of liner 90 and shoulder 86. An axial bore 96 is provided within sleeve 95 to allow the insertion of a billet 100 therethrough. Further, a suitable annular seal 97 is mounted on the shoulder-adjacent end of sleeve 95 to preclude fluid flow either between sleeve 95 and/or bore 84, or between sleeve 95 and billet 100 at bore 96 when the fluid in chamber 98 above shoulder 86 is pressurized as is discussed below. Thus, it can be seen that when a billet 100 is positioned in bore 96 through sleeve 95 prior to extrusion, the volume within vessel 81 is divided into a first chamber 98 defined by counterbore 87, sleeve 95 and a fluid control element 101, and a second chamber 99 defined by bore 84, sleeve 95 and the closed end 91 of liner 90.

A passage 102 is formed in casing 83 and has a first opening 103 communicating with first chamber 98 and a second opening 104 communicating with second chamber 99. Second opening 104 is located between the open end 92 of liner 90 and shoulder 86 so that when slideably mounted sleeve 95 engages the end 92, opening 104 is covered to prevent the passage of fluid therethrough between chambers 99 and 98.

Telescopically received within first chamber 98 is a generally cylindrical ram 106 in sliding engagement with the surface of counterbore 87. Advancement and retraction of ram 106 is accomplished by press or other suitable mechanism (not shown) known to those in the art. The advancement of

3,677,048

16 chamber 99. The billet 100 cooperates with the inner surface of seal 97 and the bore 96 to form a seal for preventing any unwanted communication between fluid in chamber 99 and fluid in chamber 98. As the billet 100 is being inserted into chamber 99, fluid displaced thereby passes through opening 5 104, which is open at this time, into passage 102 and thereafter out of opening 103 into chamber 98. This flow can occur since the insertion of billet 100 into chamber 99 tends to pressurize the fluid in chamber 99 which is thereby caused to flow into chamber 98 which has not, as yet, been subjected to pressure.

With the billet 100 in position, ram 106 is advanced within counterbore 87 into chamber 98. As the ram 106 is advanced, the fluid in chamber 98 becomes pressurized due to the valving action discussed above with respect to the embodiment of ¹⁵FIG. 3. The pressurized fluid causes sleeve 95 to be displaced downwardly within bore 84 so as to engage the end 92 of liner 90 and cover opening 104 thereby isolating chamber 99 from chamber 98. Continued advance of ram 106 generates increasing pressures in the fluid in chamber 98 which, as discussed above with respect to the operation of the apparatus of FIG. 3, generate axially directed pressures against the head end of billet 100 which displace billet 100 toward and further into chamber 99. The pressure of the fluid in chamber 99 is ₂j thereby increased, but because of the difference in the area of the butt end of billet 100 and the projected radial area of the head end of billet 100, the pressure of the fluid in chamber 99 is less than the effective pressure of the fluid generating the axial force component. 3θ

Continued advance of ram 106 further increases the pressures in chambers 98 and 99, which pressures generate radial and axial stresses in the material of billet 100. As was also noted above with respect to the apparatus of FIG. 3, the pressures generated by the advance of ram 106 are greatest at the 35 head end of fluid control element 101. Thus, as before, the pressures in fluid chamber 98 exceed those in fluid chamber 99.

The pressure of the fluid in chamber 98 generates radial stress in the material of billet 100 and the pressure of the fluid 40 in chamber 99 bearing against the butt end 116 of billet 100 generates an axial stress in the material of billet 100. With the radial stress generating pressure being higher than the axial stress generating pressure, it can be seen that as the pressure in the system generated by the advance of ram 106 increases, a condition of deformation will occur when the radial stress generated by the pressure of the fluid in chamber 98 exceeds the axial stress generated by the pressure of the fluid in chamber 99 by an amount equal to the yield stress of the material of billet 100 (see generally FIG. 5). Once deforma- ⁵⁰tion has been initiated, an extrusion equilibrium is established in the manner discussed above with respect to the operation of FIG. 3, and the material of billet 100 is deformed by the pressure exerted by fluid flowing through passage 109 of fluid con- - &lt; trol element 101 so as to form a product 115.

With the commencement of extrusion and the establishment of an extrusion equilibrium, extrusion continues by the advancement of ram 106 into chamber 98 until the fluid control element 101 is adjacent shoulder 86 in counterbore 87. At this stage, the extrusion of billet 100 is complete, the force on ram 106 is reversed and the ram is withdrawn from fluid chamber 98. The withdrawal of ram 106 from chamber 98 reduces the pressure of the fluid therein below the pressure of the fluid in chamber 99 thereby causing sleeve 95 to be dis- 65 placed axially so as to uncover opening 104 to reestablish fluid communication between chambers 99 and 98. In this manner, the pressure of the fluid in chamber 99 is relieved without forcing the head end of billet 100 into the flow control surface 110 of fluid control element 101. Finally, with ram 106 fully 70 withdrawn, the butt end of billet 100 is removed from the apparatus after which the extrusion operation can be repeated.

The primary difference between the structure and operation of the apparatus of FIG. 7 in the practice of the method of the present invention, and the structure and operation of the ap- 75 paratus of FIG. 3 in the practice of the present invention, is that the embodiment of FIG. 3 maintains a disparity between the pressure of fluid at the head end of the billet and the pressure of fluid at the butt end of the billet by providing a substantially linear pressure drop in a narrow annular space along the length of the billet, whereas the embodiment of FIG. 7, the fluid acting against the butt end 116 of billet 100 is physically isolated from the fluid acting against the head end of the billet during operation. The physical separation as provided in the embodiment of FIG. 7 precludes a flow of fluid along the billet between the surface of the billet and the surface of the counterbore 87. Thus, in the absence of such a possibility for flow, a pressure drop along the length of undeformed billet cannot be established. Accordingly, the entire surface of the billet in chamber 98 is subjected to the same fluid pressure as is experienced at the entrance to the zone of deformation, viz., pressure of an amount sufficiently great to generate a radial stress which is in excess of the axial stress by an amount equal to or greater than the yield stress of the material being deformed, and all of the material in chamber 98 is in a deformable state thereby raising the possibility that sinkage might be experienced.

In this regard, sinkage is what occurs when a portion of the billet not in the zone of deformation deforms radially inwardly thereby causing a void into which extrusion fluid 98 can flow rather than flowing, in a controlled manner, out of chamber 98 through the zone of deformation as is desired. If such sinkage occurs, the likelihood is great that the pressurized fluid 98 will flow into the hole rather than out of the chamber thus causing the head of the billet to come in physical contact with flow control surface 110 thereby causing extrusion to occur mechanically rather than as a result of pressurized fluid flow.

Such sinkage, being undesirable, is obviated in the apparatus of FIG. 7 by sizing billet 100 and counterbore 87 correspondingly so as to render the fluid containing portion of chamber 98 a narrow annular space. This annular space would be chosen in the same manner as discussed above with respect to the embodiment of FIG. 3, i.e., by providing a high resistance to any flow of fluid therethrough. Thus, assuming that a portion of billet 100 in FIG. 7 starts to deform other than in zone of deformation, its initial deformation will cause an increase in the volume of the adjacent fluid which would tend to relieve the pressure in this fluid thus tending to render the pressure levels too low to support deformation. Secondly, fluid in the surrounding areas still at a high pressure would tend to flow into the zone of deformation but this flow would be restricted by the narrow annular channel so as to render such flow sufficiently slow so as to not interfere with the flow of fluid through the zone of deformation 109.

As the relatively small flow of fluid does regenerate pressure in the area of sinkage, further sinkage may or may not occur depending upon the degree of work hardening experienced by the material during the initial singkage. More specifically, as is known in the art, any deformation of material causes work hardening which in turn increases the yield strength of the hardened material. Thus, once initial sinkage has occurred, further sinkage becomes more difficult in that the material deformed during sinkage has, by reason of the deformation, become work hardened. Thus, to some extent, the sinkage problem is self correcting. In any event, the provision of a flow restricting annular space between billet 100 and bore 84 minimizes the effect of possible sinkage during the operation of the apparatus of FIG. 7 to such an extent that its effect becomes negligible.

In summary, a positive flow of pressurized fluid is established through the zone of deformation of a fluid control element, the rate of flow of the pressurized fluid is controlled by relative axial displacement of the billet toward and away from the flow control surface of an advancing fluid control element in response to variations in the resistance of the billet material to extrusion, and upon the occurrence of appropriate disparate stress conditions in the billet material, extrusion commences and an extrusion equilibrium is established.

3,677,048

Changes in the resistance to deformation of the material being extruded cause compensating variations in the fluid pressures acting on the material. The variations occur virtually instantaneously and in such a manner so as to maintain the extrusion of the material at the desired product discharge rate.

Further, the method practiced by the apparatus of FIGS. 3 and 7 eliminates the occurrence of the bamboo effect and lunging discussed above and reduces the total amount of the work of extrusion required to form the extrusion product in that deformation of the material being extruded is accomplished by the exertion of fluid pressures thereagainst rather than by a mechanical surface contact as occurs in known extrusion apparatus. Thus, the work of extrusion is reduced by the substantial elimination of billet die interface in friction as occurs in prior art apparatus such as the apparatus of FIG. 1.

It should be recognized that although each of the embodiments disclosed for practicing the method of the present invention is described as discharging the extrusion product into some pressure, e.g. atmospheric pressure, the method of the present invention contemplates positive fluid flow extrusion into an environment, the pressure of which may be higher or lower than atmospheric.

The practice of extrusion wherein material deformation is accomplished by a controlled positive flow of pressurized fluid acting against the material to be deformed, has resulted in the extrusion of products which are uniform in diameter, hardness and other physical characteristics and which products have been deformed without the occurrence of deforming contact between the material being deformed and anything other than the pressurized fluid flowing between the material and the fluid control element. Positive fluid flow extrusion according to the present invention can be accomplished without lunging and with far less work than has been required by prior art processes. Thus, the present invention constitutes a significant step forward in the state of the extrusion art.

Many modifications and variations of the method of the present invention are possible light of the above teachings. It is therefore to be understood, that the invention may be practiced otherwise than as specifically described.