US3731050A

US3731050A - Method of making article from metallic powders

Info

Publication number: US3731050A
Application number: US00156651A
Authority: US
Inventors: Tour H La
Original assignee: Armco Inc
Current assignee: Armco Inc
Priority date: 1971-06-25
Filing date: 1971-06-25
Publication date: 1973-05-01
Anticipated expiration: 1990-05-01

Abstract

A method of making a continuous strip at rapid rates from metallic powders, which powders are devoid of non-metallic binders. This is accomplished by the simultaneous compacting and resistance-heating welding of the powders being fed between electrically energized rolls. The roll load coupled with a low voltage - high amperage current therebetween assures a highly dense metallic strip suitable for further metallurgical processing. Means are also provided to assure a full width of uniformly dense metallic strip substantially equal to the width of the fed metallic powders.

Description

o Muted States Patent 1 1 [111 fifiamse LaTour 5] May 1, 1973 METHOD OF MAKING ARTICLE 3,359,100 12/1967 Claus et a1. ..75 200 x FROM METALLIC POWDERS 3,290,145 12/1966 Daugherty ..75/200 X 3,250,892 5 1966 I ..219 149 [75] Inventor: Harry LaTWr Middletown Ohio 3 194 858 7/1965 s i i'fhheim .15 260 x [73] Assignee: Armco Steel Corporation,

Middl t Ohi I Primary Examiner-J. V. Truhe Assistant ExaminerL. A. Schutzman [22] Flled' June 1971 Attorney-John W. Melville et a1. [21] Appl. No.: 156,651

[57] ABSTRACT [52] US. Cl. ..219/149, 75/200, 75/214, A method of making a continuous strip at rapid rates 75/226, 29/1822, 219/76 from metallic powders, which powders are devoid of [51] Int. Cl. ..B21j 1/06 non-metallic binders. This is accomplished by the [58] Field of Search ..2l9/l49, 148, 91, imultaneous compacting and resistance-heating weld- 9/ 6, ing of the powders being fed between electrically energized rolls. The roll load coupled with a low voltage high amperage current therebetween assures a 1 References Clted highly dense metallic strip suitable for further metallurgical processing. Means are also provided to assure UNITED STATES PATENTS a full width of uniformly dense metallic strip substan- 3,567,903 3/1971 Parker ..75/226 X tially equal to the width of the fed metallic powders. 2,906,596 9/1959 Ballhauser..... ....75/226 X 3,340,052 9/1967 lnoue ..75/200 19 Claims, 11 Drawing Figures Patented May 1, 1973 5 Sheets-Sheet l INVENTOR/S AMRRY ATTORNEYS 5 Sheets Sheet 2 INVENTOR/S Mill? Wm ATTORNEYS Patgnted- May 1 1973 Y 7 3,731,050

5 Sheets-Sheet 4 Fin: :1

INVENTOR/S BY M VM, 2;; nu/25 ATTORNEYS Patented Ma 1, 1973 5 Sheets-Sheet 5 NVENTOR/S x/mfy Aral/e ATTORNEYS METHOD OF MAKING ARTICLE FROM METALLIC POWDERS BACKGROUND OF THE INVENTION This invention relates to a method for the rapid production of a continuous strip of metal from metallic powders. I-Ieretofor the subject of powder metallurgy was used in conjunction with the production of single items, or at most a slow speed continuous process. Further, said processes were of the type known as oven-sintering or induction sintering.

Briefly, each of the aforementioned process are ones involving at least two operations, namely, compaction followed by sintering. That is, the powders are compacted at a first location and subsequently treated or transferred to a second location in the green condition where the product is sintered. A third location is also utilized for further hot or cold working to increase density. Such procedures have been widely used since it was readily observed that such products exhibited an excellent finish with excellent tolerances. This represented a savings over using a forged product which requires cutting and/or machining to achieve the final product. Also, by the elimination of the step of having to remove metal, crack propagation was substantially reduced.

While these procedures represented some savings over products produced by other processes, there were factors in the operations which tended to add to other costs. For example, the two procedures require expensive dies and equipment along with operating times up in the hours. Another drawback to these procedures is the fact that it was often necessary to incorporate a binding agent along with the metallic powders in order to give the product some green strength sufficient to subsequently treat or transfer the item to the sintering station. It was necessary then during the sintering operation to burn off the binding agent from the compacted powders. As a consequence, it was often difficult to obtain the desired density in the sintered product.

Quite recently, a new procedure was developed to overcome several of these disadvantages and to produce a rather highly dense sintered product. This procedure has been defined in the art as spark sintering" or spark discharge". Spark sintering, for example, has been defined as a process for making powder metallurgy parts by the simultaneous application of a moderate amount of force on said powder while subjecting the powder to a combination of DC and AC current. While the precise theory is not known at this time, it is believed that while the metallic powders are under the limited load, the electric current which is being passed therethrough results in a spark discharge between adjacent surfaces of metallic particles such that fusion between particles occurs. Such a procedure is characterized by a combination of low pressure and electrical characteristics of high voltage and low amperage. Like the earlier procedures, the latter one is a relatively slow operation making it unsuitable for a continuous operation. For additional information reference may be made to the five INOUE patents, namely, U.S. Pats. Nos. 3,250,892; 3,241,956; 3,317,705; 3,340,052 and 3,387,972.

In contrast to the procedure just described, the present invention contemplates a process whereby continuous strips can be produced at a rapid rate from metallic powders by means of a combination of relatively high pressure and low voltage high amperage electricity. The resulting product approaches the theoretical density and is accomplished without the use of binding agents admixed with the metallic powders.

SUMMARY OF THE INVENTION Briefly in the practice of this invention, a metallic powder, such as low carbon 18Cr-l 2Ni-2.5Mo stainless steel, of a size passing through a 325 mesh screen, is subjected to a simultaneous compaction and welding of the metallic powders between electrically energized rolls. The latter operation is characterized by the application of a heavy roll load and the electrical characteristics of low voltage and high amperage.

By utilizing loads well in excess of those contemplated in similar prior art procedures, more intimate contact is achieved between the particles such that with the application of the electrical current it is possible to rapidly heat the entire mass of the particles through resistance heating up to the welding temperature needed to fuse the product between the rolls.

The further advantages of this operation will become apparent in the detailed description thereinafter.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic representation of the process of this invention, including the subsequent operations of rolling and annealing.

FIG. 2 is a sectional view of a second embodiment of this invention, said embodiment utilizing a traveling mold to contain the metallic powders during compaction.

FIG. 3 is an enlarged sectional view taken through the electrode rolls and traveling mold of FIG. 2.

FIG. 4 is a schematic representation of a third embodiment wherein a plurality of opposing rolls are arranged in tandem relationship.

FIG. 5 is a schematic representation of a pre-compaction system for use in the practice of this invention.

FIG. 6 is a simplified sectional view of an alternative manner of electrifying the compaction welding rolls which are segmented.

FIGS. 7a and 7b are simplified perspective views showing two metallic powder feeding systems to vary the depth of metallic powder being fed into the compaction welding rolls.

FIG. 8 is a sectional view showing the variation in depth of metallic powder, which variation may be achieved by the systems shown in FIGS. 7a and 7b.

FIG. 9 is a front elevational view of opposing contoured compacting welding rolls, the counter being exaggerated for illustration purposes, for use in the practice of this invention.

FIG. 10 is a perspective view, a portion of which is cutaway, of the compacting welding rolls, such as in the system illustrated in FIG. 2, but incorporating powder dividers preceeding said rolls.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS This invention relates in general to the production of a continuous-uninterrupted strip of metal produced from metallic powders, devoid of non-metallic binders; more particularly, it relates to the production of said strip having a uniform density throughout its width.

The foregoing is accomplished by a unique process involving the simultaneous compacting and resistance heating to weld the powder into strip form. It will be appreciated that the compacting pressures and electrical characteristics will vary over a range of values depending on the powder being processed. Nevertheless, since this is in effect a continuous welding operation, wherein the heating is effected by resistance heating of the metallic particles, it was determined that with increasing compacting pressures the resistivity of the particles decreased thereby requiring a lower voltage across the welding zone. However, it is believed a person skilled in the art can readily select the parameters necessary to achieve the results herein in accordance with this invention.

While the further description will be primarily directed to the production of a strip of low carbon l8Cr-l 2Ni-2.5Mo stainless steel or Ti-6Al-4V titanium alloy, it should be understood that other metallic powders may also be processed by the procedures to be described. Therefore, no limitation is intended to be imposed on this invention, except as set forth in the appended claims.

Referring now to FIG. 1 on the accompanying drawings, there is shown a hopper 10 adapted to continuously feed metallic powder vertically between the pair of electrode rolls 12, 12a ofa compacting mill. The pressure exerted thereby is represented by the opposing arrows and P Supplied to said hopper 10 is a metallic powder free of non-metallic binders.

The rolls 12, 12a are energized by the electrical circuitry shown so as to heat the powder between the nip of the rolls by resistance heating to the welding temperature. The circuit may be AC. or D.C., and is characterized as low voltage high amperage.

Since this invention is dealing with the simultaneous compaction and welding of metallic powders, devoid of non-metallic binders, it may be desirable to include some edge restraining means on each side of the strip. While a specific means is not contemplated as a limitation on the invention, effective results have been achieved with side rails or guards, and rolls, the latter being free wheeling as opposed to driven.

While the type and thickness of the compacted powder strip will affect the speed of the process, it is contemplated that rates over a broad range may be effectively followed. Thus, the roll speed of rolls 12, 12a, which are driven, shall vary.

As the metallic powders pass between the rolls, they are compacted and heated to the welding temperature to emerge as a highly dense strip 14. While this procedure is geared to produce a high density product, there are practices which can be followed to improve or increase said density. For example, the use of powders of a relatively uniform size, smaller diameter rolls in relation to the strip thickness, and a thinner strip are factors which can be used to increase density.

On materials where oxidation may be a problem at the welding temperation, gas purging nozzles 16, 16a may be provided. In other words, by the use of said nozzles, or other suitable means, it is possible to control the environment about the compacting welding operation. Dry hydrogen, having a dew point no greater than 40C., is suitable for the continuous welding of stainless steel.

The hot emerging strip IQ is conveyed away from the compacting welding operation and may be directly coiled for use in the as welded condition, or processed further to effect additional metallurgical changes in the strip. For instance, the strip may be passed through a cooling station 18 to be quenched, or merely gas cooled. In any case, controlled cooling may be effected in a variety of mediums. If greater densities, and/or smoother finishes are desired on the strip, a cold rolling station 20 may be employed in the system. Finally, said rolling may be followed by a further heating or annealing at a heat treating station 22 prior to the coiling of the strip.

While the preceding are typical operations in the processing of metallic strip, it should be understood that sequence changes, additions or variations may be made without departing from the scope of the invention.

FIG. 2 represents a second embodiment to the compacting welding operation of FIG. 1. In contrast to the system of FIG. 1, the rolls 30, 300 are disposed in a vertical plane in spaced relationship. Between the rolls a continuous traveling mold 32, or a mold of predetermined length if discrete strip lengths are required, is disposed to receive the powdered metal from a hopper 34.

The mold 32 is U-shaped such that the opening 36 therein is substantially equal to the width of the proposed strip 14a as seen in FIG. 3. To insure full width compaction of the powder within the mold, the upper roll 30 should be designed to substantially engage the opening 36. Thus, by this embodiment, i.e., feeding the powdered metal into the opening 36 or trough of the mold 32, one can eliminate many of the problems which may be encountered with the flow of powder into the nip of the rolls of the horizontal mill shown in FIG. 1.

FIG. 4 is a schematic representation of a modified procedure for effecting the compacting and welding in stages. This system may be used with either the horizontal or vertical mills of the embodiments shown in FIGS. 1 and 2.

It will be observed that in this latest system a plurality of pairs of rolls are disposed in tandem relationship. For convenience, three sets of rolls are illustrated and have been designated 40, 4011;42, 42a; and 44, 44a. As explained previously, since the operation herein is welding, and it is by means of resistance heating of the particles, it would follow that with increasing pressures or loads (P, P P;,), the resistivity of the particles to be welded decreases. That is, the particles become more compact with fewer and smaller voids therebetween and therefore a more direct contact between adjacent particles results. As a consequence, the voltage requirements decrease across

rolls

40, 40a; 42, 42a; and 44, 44a, respectively (V, V V

To help demonstrate the effectiveness of this invention, a strip of low carbon l8Cr-l2Ni-2.5 Mo stainless steel was produced by a system similar to that illustrated in FIG. 2.

EXAMPLE 1 Low carbon l8Cr-l2Ni-2.5Mo stainless powder (100 percent at 325 screen size, 7.25 microns) was secured and the analysis was as follows:

Cr 17.55% Ni l 1.35% C 0.030% Mn 0.10% Mo 2.20% Si 0.98% Fe balance.

emerge from between the rolls having a thickness of about 3.0mm, with a width of about 7.3mm. The strip width was less than the top roll width due to the loss of pressure at the outside edges of the convex top roll.

In the as welded condition, the strip was tested for tensile properties. These results may be seen in the TABLE I along with a duplicate sample annealed in dry hydrogen and slowly cooled.

TABLE I Strip Condition YS TS Hardness Elongof Sample MN/m MN/ RB As welded 209 Dry H, Anneal 305 459 81 7 Without any cold reduction of either sample, densities on the order of about 94 percent of the solid density were achieved. Increasing the roll load and/or the addition of a cold rolling stage should substantially increase the density such as to approach the theoretical density.

While some edge restraining means could have been used effectively to improve the lateral dimension and character of the strip, other means acting normal to the strip have been found to produce uniformly dense strips from edge to edge. However, said means will be discussed hereinafter.

Turning now to FIG. 5, there is shown a horizontal system wherein a precompaction mechanism 50 is employed preceding the compacting welding rolls 52,

52a. Such a system permits higher feed rates, hence, 55

faster mill speeds.

The precompaction mechanism 50 comprises a pair of opposing metallic belts 54, 540, each of which incorporates one of said rolls 52, 52a. The belts 54, 54a, are

driven by rolls 56, 56a, respectively and are arranged in a common vertical plane, but skewed as to their horizontal relationship. In other words, said belts are tapered at an angle, which may vary from several degrees up to about 8 to converge at the roll bite of rolls 52, 52a. The upper roll 56 is displaced from roll 56a so as to permit the metallic powder feed from hopper 57. Finally, disposed within each said belt,

there are provided a plurality of pairs of opposed pres sure rolls 58a 53m acting to precompact the metallic powders prior to their entry between rolls 52, 52a.

It was noted earlier that some difficulty was encountered in achieving a fully compacted strip from edge to edge. This has been overcome by regulating the temperature across the strip in the welding zone.

The first of several such means forming a part of this invention is illustrated in FIG. 6. Each of the compacting welding rolls 60, 60a are comprised of

segments 62a

62n, and 64a 6411, respectively, with opposing pairs of segments individually energized. In other words, each said pair, i.e.,

roll segments

62a, 64a, constitute a set of compacting welding rolls, which when employed with other pairs of like character can be used to cover the entire width of a proposed strip 66. Adjacent roll pairs are insulated from each other by insulation spacers 68a 68m and from the central shafts 70, 70a. This will insure that the individual controls are effective.

The electrical powder leads 72a 72n may be secured to the individual roll segments in any well known manner, and energized by means of a primary of a power transformer '74. Means such as a rheostat may be employed in the several circuits to control the power therethrough.

It was found that in the typical single pair rolls, hot spots developed at the center between the said rolls. For example, a temperature profile took on the appearance of an hour glass. But with the present individually controlled segmented rolls, it is possible to avoid said hot spots by reducing the power to the middle segments. In other words, temperature uniformity can be obtained by regulating the power level across the width.

FIGS. 7a and 7b represent a different approach to ef fect density uniformity in the welded and compacted strip from edge to edge. For example, it has been found that in resistance welding of metallic powder there is an inverse relationship between resistance and density. That is, resistance decreases with an increase in density. Further, density is directly related to the feed rate of the metallic powder. Accordingly, resistance decreases with an increase of the feed rate. It is upon this latter principle that the systems disclosed in FIGS. 7a and 7b operate to insure density uniformity throughout the strip width.

In FIG. 7a, a single feed hopper is positioned over the conveyor belt 82 delivering the metallic powder to the compacting welding rolls. Disposed beneath said hopper 80 are a plurality of strips 34, whose width may be broad or narrow, such as plastic or Teflon, the latter being a registered trademark, which may be moved in or out to expose a greater or lesser area of the underlying conveyor. This naturally results in a variable amount of powder being deposited onto the conveyor. FIG. 8 shows a preferred profile of metallic powder as it is positioned prior to its entry into the roll bite of the compacting welding rolls. it is obvious that the powder depth profile can change markedly; however, by the general configuration of FIG. 8 one can compensate for edge density losses, or increase the edge temperature.

FIG. 7b represents a variation to FIG. 7a but which produces the same result. Here, a plurality of feed belts 86a-86 with complimentary hoppers 88a 88m are each arranged so as to deposit metallic powder on only a limited portion of conveyor 90. By the simple procedure of adjusting the feed and speed of belts 36a Son, it is possible to vary the depth of metallic power from edge to edge. For example, by operating the outermost feed belts at a higher speed than the innermost, a configuration can be developed similar to that shown in FIG. 8.

FIG. 9 not only depicts another means for achieving density uniformity, it represents a departure from normal rolling practice. Typically, a rolling mill for reducing conventional strip material comprises a pair of rolls which are center crowned or possess a slightly convex surface. In contrast, to this, the compacting welding rolls 92, 92a of FIG. 9 are each provided with a convex surface 94 or negative contour. That is, the diameter through the center is less than the diameter at each edge. Thus, by allowing the powder 96 to compress more on the edges than in the center, the temperature and pressure will allow full density compaction up to the edge of the

rolls

92, 92a. This may be demonstrated in the following example.

EXAMPLE II A titanium alloy (Ti-6Al-4V) with 6 percent aluminum and 4 percent vanadium (l+400 mesh) was selected for use on contoured rolls such as shown in FIG. 9. Using a 12.7mm width roll with 0.127mm concavity (circular segment), a full 12.7mm width strip was produced. On the other hand, under substantially identical conditions, in using a flat contour on 12.7mm rolls, a strip width of only approximately 9.5mm resulted.

While the use of contoured rolls resulted in a 33 percent improvement over the flat rolls, it should be made clear that such increases can not be attributed automatically to a given set of rolls irrespective of roll size. Nevertheless, this demonstrates that improvements of a major order are possible.

A final but different mechanism for achieving uniform density from edge-to-edge is illustrated in FIG. 10. Such a system may be used with the traveling mold concept described above with respect to FIG. 2. It will be recalled that the metallic powder is deposited on a mold 100 at a location preceding the compacting welding rolls 102, 102a, where the said compacting and welding takes place. However, it has been found that with such a mechanism, there is a tendency for the loose metallic powder to move in a circular pattern, i.e., follow the flux path between the compacting welding rolls I02, 102a, in the traveling mold 100. The powder tends to move around the edge of roll 102 to the exit side. This loss of powder results in a width reduction.

To prevent said movement dividers or baffles 104 are provided to isolate this effect and allow the full strip width to develop. While experience will dictate the number of dividers needed, it is contemplated that a plurality will be required to effect the isolation. Finally, the dividers 104 are curved at the roll bite end 106 to insure a satisfactory channeling of the metallic powders into the compacting and welding zone. The curved end 106 is such as to compliment the curvature of roll 102, but without interfering with the operation thereof.

While the invention has been described in relation to its most preferred embodiments, it should be recognized that numerous variations may be followed in the system without departing from the spirit and scope thereof. Accordingly, no limitation is intended to be imposed on this invention except as set forth in the following claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

l. A method of producing a continuous metallic strip from powdered metal comprising the steps of selecting a metallic power free of non-metallic binding agents, feeding said powder between opposed compacting electrode rolls, simultaneously applying pressure by means of said rolls to compact said powder and a low voltage high amperage current thereacross to resistance weld said powder into a highly dense continuous strip.

2. The method claimed in claim 1, wherein an open sided traveling mold is interposed between said rolls to receive said metallic powder.

3. The method claimed in claim 2, wherein said mold is U-shaped and one of said rolls is adapted to ride within said mold.

4. The method claimed in claim 1, including the steps of cooling said strip and winding said welded strip into coil form.

5. The method claimed in claim 1, wherein said rolls are preceded by at least one pair of opposed strip compacting electrode rolls whose voltage is greater than said first noted voltage, said rolls effecting a pre-welding of said metallic strip.

6. The method claimed in claim 5, wherein there are a plurality of pairs of initial rolls, each pair of which is characterized by a voltage decreasing in the direction of feed of said metallic strip.

7. The method claimed in claim 6, wherein one roll from each said pair is disposed within an encircling belt, and said belt engages the pre-welded metallic strip.

8. The method claimed in claim I, wherein said welding-compacting is conducted in a controlled environment substantially free of oxide promoting conditions.

9. The method claimed in claim 8, wherein said environment is substantially a dry hydrogen atmosphere.

It). The method claimed in claim 1, including the step of varying the gap between said rolls, whereby to insure a uniform and fully dense strip from edge to edge.

11. The method claimed in claim 10, wherein said variation is achieved by providing said electrode rolls with a uniform negative crown from one end to the other.

12. The method claimed in claim ll, including the step of depositing said powder metal on a conveyor, whereby said conveyor is positioned to feed the powdered metal directly between said compacting electrode rolls.

13. The method claimed in claim 12, including the step of varying the depth of said powdered metal on said conveyor from one edge thereof to the opposite edge.

14. The method claimed in claim 13, wherein said variation is achieved by means of a plurality of feeder hoppers, each said hopper depositing said powdered metal at a predetermined rate on a preselected portion of said conveyor.

15. The method claimed in claim 13, wherein said variation is achieved by interposing flat strips between the feeding source and said conveyor.

16. The method claimed in claim 12, including the step of subjecting said powdered metal to a compacting operation preceding said compacting-welding.

17. The method claimed in claim 1, wherein said welding is accomplished in a plurality of zones transverse to said strip, and that each said zone is individually controlled so as to vary the current passing through said strip from one edge to the opposite edge.

18. The method claimed in claim 2, including the steps of isolating said powdered metal into a plurality of narrow channels from a location preceding up to the said compacting-welding.

19. The method of producing a metallic article from powdered metal comprising the steps of selecting a metallic powder free of non-metallic binding agents, placing said powder between opposed compacting electrodes, each said electrode characterized by a plurality of individual zones, each zone connected in electric circuit with one zone of the opposing electrode, simultaneously applying pressure by means of said electrodes to compact said powder and a low voltage high amperage current thereacross to resistance weld said powder into a highly dense article, with said current selectively controlled in each pair of electrically connected zones so as to vary the current flow between adjacent zones in a given electrode.

Claims

1. A method of producing a continuous metallic strip from powdered metal comprising the steps of selecting a metallic power free of non-metallic binding agents, feeding said powder between opposed compacting electrode rolls, simultaneously applying pressure by means of said rolls to compact said powder and a low voltage - high amperage current thereacross to resistance weld said powder into a highly dense continuous strip.

8. The method claimed in claim 1, wherein said welding-compacting is conducted in a controlled environment substantially free of oxide promoting conditions.

10. The method claimed in claim 1, including the step of varying the gap between said rolls, whereby to insure a uniform and fully dense strip from edge to edge.

12. The method claimed in claim 1, including the step of depositing said powder metal on a conveyor, whereby said conveyor is positioned to feed the powdered metal directly between said compacting electrode rolls.

19. The method of producing a metallic article from powdered metal comprising the steps of selecting a metallic powder free of non-metallic binding agents, placing said powder between opposed compacting electrodes, each said electrode characterized by a plurality of individual zones, each zone connected in electric circuit with one zone of the opposing electrode, simultaneously applying pressure by means of said electrodes to compact said powder and a low voltage - high amperage current thereacross to resistance weld said powder into a highly dense article, with said current selectively controlled in each pair of electrically connected zones so as to vary the current flow between adjacent zones in a given electrode.