US3180023A - Method of joining an electrically conductive metal to a refractory hard metal - Google Patents

Description

April 27, 1965 L. TITUS 3,180,023

METHOD OF JOINING AN ELECTRICALLY CONDUCTIVE METAL TO A REFRACTORY HARD METAL 2 Sheets-Sheet 1 Filed Feb. 2, 1961 IN V EN TOR. ZESZZt? Tz'ias ATTORNEY rrrus 3'180'023 AN ELECTRICALLY CONDUC'I'IVE A ril 27, 1965 METHOD OF JOINING METAL TO A REFRACTORY HARD METAL 2 Sheets-Sheet 2 Filed Feb. 2, 1961 -INTERFACE E m F R E T W INVENTOR. LESLIE TITUS ArroRlvEy 3,180,023 METHQD F JUlNlNG AN ELECTRICALLY CUN- DUCTIVE METAL T0 A REFRACTURY HARD METAL Leslie Titus, Campbell, Califi, assignor to Kaiser Aluminurn & Chemical Corporation, Ualrland, Calif., a corporation of Delaware Filed Feb. 2, 1961, Ser. No. 86,806 it Claims. (Cl. 29--527) This invention relates to a method of joining electrically conductive metals to refractory hard metal bodies and to composite structures produced thereby. More particularly, it relates to a novel method of joining refractory hard metal current-conducting elements to metal cap members wherein the joint has high strength and electrical conductivity and to the novel capped elements so made.

Refractory hard metal elements are finding increasing use as current-conducting elements, e.g., cathodic elements, in aluminum reduction cells. Use of refractory hard metal elements or bars, as current-conducting elements in aluminum reduction cells, requires a metallic cap or other suitable joining device whereby an electrical connection can be made to the bus system. The cap must be bonded to the refractory hard metal material by a joint which is characterized by low electrical resistance and high strength.

Various methods of capping with an electrically conductive metal, such as aluminum, have been used. One such method is to electroplate the refractory hard metal surface with another metal, e.g., nickel, and then join the refractory member to the aluminum cap by soldering or brazing also with the use of various fluxes. Another method of capping has been to cast the molten aluminum onto the refractory hard metal in a vacuum without a flux at about 945 C.

In practice neither of the above methods has shown satisfactory reliability with respect to complete bonding of electrically conductive metal to the refractory bar, and the mortality of cap joints in service has been unduly high. Cap joints made by the above methods often result in incomplete or no bonding or in bonds which are mechanically Weak and have relatively low electrical conductivity. Capped refractory hard metal elements which have failed have been characterized, metallographically at the joint interface, by extensive intermetallic compound formation and by segregation of intermetallics at the grain boundaries resulting in embrittlement. These characteristics seriously affect the joints and cause excessive cap failures which in turn involves excessive shutdown time and undesirable production losses in the aluminum reduction cells.

According to the present invention there is provided an improved method for obtaining a joint between the refractory hard metal member and an electrically conductive metal cap for effecting the current connection to the refractory hard metal member having both superior mechanical and electrical characteristics. Capped refractory hard metal elements or members, according to this invention, are made by placing the refractory hard metal element in a mold in such a manner to expose one end portion of the refractory hard metal element and casting the capping metal in contact with the exposed portion of the refractory hard metal member at a temperature United States Patent Ofiice 3,l8d,023 Patented Apr. 27, 1965 to cause substantially complete wetting of the refractory hard metal member by the capping metal. The preferred electrically conductive metals for use in the invention are aluminum, copper, and iron. When the terms alurninum, copper, and iron are used hereinafter in the specification and the claims, they are intended to include additions of other elements to these metals which do not deleteriously affect the cap-bar joint and/or use of the capped element in aluminum reduction cells.

Various objects and advantages of the instant invention will be apparent from the ensuing description thereof.

The refractory hard metal material used for cathodes possesses a low electrical resistivity, a low solubility in molten aluminum and molten electrolyte under cell operating conditions, wettability by molten aluminum under cell operating conditions, and good stability under the conditions existing at the cathode of a reduction cell. The expression refractory hard metal, as used in the disclosure and claims, means a refractory and hard chemical compound included among the nitrides, carbides, borides, and silicides of the transition elements of the fourth, fifth, and sixth groups of the Periodic Table such as tungsten, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, and molybdenum. Reference is made to the text by Dr. Paul Schwarzkopf and Dr. Richard Kieffer, Refractory Hard Metals (Mac- Millan Company, New York, 1953). The preferred refractory hard metal material for the practice of the invention is that which consists essentially of at least one of the materials selected from the group consisting of the carbides and borides of titanium, tantalum, niobium and zirconium and mixtures thereof, with or without additions. Such materials are found to exhibit all, or substantially all, of the above properties.

It has been found that when the molten electrically conductive metal is cast about the refractory hard metal bar, without the use of a fiux, at high temperatures above a critical minimum temperature, the consistency and quality of wetting and intermetallic bonding between the metal cap and the refractory elements is greatly increased and a capped joint is produced which is capable of service throughout the life of the refractory metal cathode element.

In the accompanying drawings there is illustrated by way of example suitable apparatus for carrying out the method of the invention as well as several embodiments of the connection.

FIGURE 1 is a vertical elevation view in section showing a casting assembly.

FIGURE 2 is a perspective view showing a refractory hard metal member and a metal cap member joined according to the invention and wherein the cross-sectional area of the cap member is substantially the same as the cross-sectional area of the refractory hard metal member.

FIGURE 3 is a perspective view partly in section showing a refractory hard metal member and a metal cap member joined according to the invention and wherein the metal cap forms a sheath over the extremity of the refractory hard metal member.

FIGURE 4 illustrates a connection between a flexible metal member and a refractory hard metal member.

FIGURE 5 is'a photornicrograph of a section through the cap-element interface made according to the invention.

.with graphite mold material.

Although no part of the invention, the furnace (hereafter called the capping furnace) is shown in the drawing to simplify explanation of the capping method. The essential part of the capping furnace is the heater element 1 which is simply a graphite tube of smaller cross section through the midsection to concentrate the heating Zone. The tube is fixed in a vertical position.

FIGURE 1 shows a hot capping furnace including water-cooled aluminum terminal block bus connections 5 which are electrically connected to graphite terminal blocks 3. Graphite terminal blocks 3 have a 3 taper joint to a graphite heater tube 1. Within graphite heater tube 1 is located a graphite capping mold 9 which contains the refractory hard metal bar 11 to be capped. Lampblack insulation 2 surrounds the heater tube 1 and is itself contained within a steel drum 4. Granular alumina 6 is used to insulate the lower end of the furnace. A thermocouple well 8 containing a PT/PT 13% Rh thermocouple is inserted through the furnace into the heater tube to control the temperature during the casting operation so as to maintain the critical minimum temperature necessary. Sight tube 7 may be employed for surveillance. Removable graphite cover 12 is provided to seal the heater tube during heating. A slow argon purge is started through purge tube 10 for the protection of the furnace. After the desired temperature is reached the power is turned off and the furnace is allowed to cool. When cooled to a suitable temperature above the freezing point of the metal, for example, about 800 C., the mold is removed from the capping furnace and placed in the cooling furnace, not shown.

The cooling furnace is a Nichrome resistor cylindrical type which surrounds the top part of the mold containing the casting. Its temperature is brought to about 800 C. at the time the capping mold is inserted and then its power supply cut off. The bottom part of the capping mold is surrounded by an iron pipe forming an air jacket which permits rapid but not sudden cooling. Thus, with the retarded cooling of the top of the mold and the relatively rapid cooling of the bottom a chilled casting is obtained with a minimum of piping in the metal.

FIGURE 2 shows a refractory hard metal member 11 joined to -a metal cap member 13. The cross-sectional area of the metal cap member is substantially the same as that of the refractory hard metal member 11.

In FIGURE 3 there is depicted a refractory hard metal member 11 joined to a metal cap member 14 wherein the metal cap member forms a sheath over the extremity of the refractory hard metal member 11. The crosssectional area of member 14 is greater than that of member 11 and can be formed by employing a mold of suitable inner cross-sectional area in relation to the crosssectional area of the refractory hard metal member.

The temperature of the cap metal at casting must be controlled within relatively narrow limits and is critical to the successful operation of the invention. These critical temperature limits will vary according to the particular capping metal employed. For example, when aluminum metal was used it was found that at temperatures below 1300 C. wetting of the. refractory hard metal bar was not complete under usual operationand at temperatures below 1200 C. practically no wetting occurs. At temperatures above 1500 C. the aluminum reacts In the range of 1250 C.- 1500 C., and in particular between 13001500 C., the certainty of complete wetting is very high, although, as indicated above, some bonding may be obtained at lower temperatures above-1200 C. using longer holding periods. However, in order to insure relatively rapid and substantially complete wetting, temperatures above 1500 C. should be used. At 1250 C. the bond is fairly strong but wetting is spotty. It should be also noted that the upper limit is governed by the mold material and the highest temperature should be such that no significant 5 reaction between the mold and the molten capping metal occurs.

The following examples are illustrative of the method of the present invention and also serve to indicate that the composite products formed by the practice of the present invention are an enormous improvement over the prior art and of great importance in industry.

EXAMPLE I In this example of the method of capping according to the invention, a refractory hard metal bar of titanium boride (TiB three inches in diameter is placed in a capping mold and then both are placed in a furnace similar to the one illustrated in FIGURE 1. Small aluminum pieces (or a precast slug) suflicient to make a cap one inch in depth above the bar are placed in the mold around and above the end of the bar. The furnace is closed with a graphite cover and slow argon purge started through a purge tube. Power is applied at a rate to bring the temperature between 1400 C.l450 C., as measured by a PT/ PT 13% Rh thermocouple, in 70-90 minutes. After the maximum temperature of 1450 C. is reached, the power is turned off and the furnace is allowed to cool. When cooled to about 800 C. the mold is removed from the capping furnace and placed in a cooling furnace.

Micrographic examination of interfaces prepared from bars whose caps have failed show an intergranular precipitate of TiAl which is seen about 1 mm. from the interface in the aluminum. Such a precipitate embrittles the aluminum and promotes failure at higher temperatures under applied stress. The majority of cap failures have exhibited hot tears in the aluminum. When the capping operation is carried out at temperatures between 1300 C. and 1500 C. according to the instant invention secondary phases of a different type precipitate from the aluminum melt in the form shown in FIGURE 5. This occurs because growth of the precipitate crystallites can proceed over an extended period prior to solidification. Gravity segregation accounts for the random orientation they assume at the interface. Little or no evidence of precipitation of TiAl at the grain boundaries has been found in high temperature caps and accordingly the tendency to hot tear is greatly reduced and/or completely ehminated. In the high temperature cast cap the intermetallic compounds are present as well-formed angular crystallites, many exceeding 50 microns in length.

The aluminum used in capping refractory hard metal bars includes high purity aluminum metal and aluminum alloys wherein the amount of aluminum is 99% by weight or better; for example, Alloy 1100 wherein the minimum aluminum content is 99% by weight, E.C. (electrical conductor) alloy wherein the minimum aluminum content is 99.45% by weight, and high purity aluminum of 99.99% purity.

EXAMPLE H A refractory hard metal bar of high TiB and low TiC content was capped with high purity aluminum (99.99% A1) at 1310 C. The capped bar was examined and the results showed good wetting of the refractory hard metal.

EXAMPLE III Another refractory hard metal bar of high TiB content was capped with molten high purity aluminum as in Examples I and II above but at a maximum temperature of 1200 C. The results showed poor wetting on the sides and hardly any wetting on top.

EXAMPLES IV-VII One E.C. grade copper capped bar was prepared according to the procedure of Example I at a maximum temperature of 1600 C., two at 1660 C., and one at 1850 C. At 1600 C. there was no wetting of the bar. At 1660 C. there was about 20% wetting in one case but very little in the other. At 1850 C. there was complete wetting and an excellent cap joint was obtained. The

other cap joints were not secure and the caps and bars fell apart. Bars capped at temperatures between 1650 C. and 1900 C. will have good bonds.

EXAMPLE VHI Another bar was capped with EC. grade copper using a maximum temperature of 1855 C. The heat-up time was one hour, fifty-four minutes. The furnace was purged with argon as usual, but in addition hydrogen was bubbled through the copper when it melted and until just before it froze. The joint was examined and found to be sound. Thereafter, the capped bar was put into operation in an aluminum reduction cell.

EXAMPLES IX-X Two cappings were made with iron. At 18% C. the iron reacted extensively with the carbon of the capping mold, carburizing all the iron to a brittle mass. The iron also reacted with the bar as well as reducing the diameter at the end of the bar by about /2 inch. A second iron capping was made at 1576 C. maximum temperature. The reaction with carbon was minimized and the bar was completely wetted and a sound bond effected. Although cappings may be performed in carbon molds, it is preferable that refractory molds which are non-reactive should be used with iron capping operations, for example, fused alumina, alumina cements, etc. While these two examples refer to iron, iron alloys and steels can also be used very effectively.

' EXAMPLE XI A minibar containing 26% TiC, balance TiB was placed upright within alumina shields in a high frequency induction furnace. A small piece of low carbon iron rod inch diameter by A inch long was placed on the flat upper end of the bar. The system was evacuated, flooded with pure argon, and the bar and iron heated to above the melting point of iron to l575 C.- C. The capped bar was allowed to cool to room temperature in about one hour. The capper bar was examined and the iron was found to be bonded to the bar. The cap-bar joint was tested for strength and a force was applied. The break occurred within the bar, not at the interface, showing definite wetting and excellent capping.

As indicated by the above examples, refractory hard metal members of titanium boride (TiB with additions of titanium carbide (TiC) ranging from 10-40% can be joined to aluminum, copper, iron and steel caps in the manner described in the above examples. In all cases the joints possess superior mechanical strength and electrical conductivity characteristic of the invention.

Under aluminum reduction cell conditions, the performance of caps produced according to this invention has been excellent. No cap failures ha e occurred which were due to deterioration in the quality of the metal-to-refractory hard metal joint. The only failures that have occurred have been on aluminum caps due to abnormal circumstan es where very high bath temperatures and large currents through the refractory hard metal bars resulted in melting of the metal caps. The performance of some of the high temperature caps since they were initiated is shown in Table l. Sen/ice age accumulated on the high temperature caps is tabulated and shown for illustration. However, in practice there has not been a single cap failure due to cap-joint bond deterioration since the high temperature capping method of this invention was adopted. Since that time over 1200 bar-test days have been accumulated without a cap failure. In contrast to this, the average number of bar-test days between cap failures was about before this high temperature capping process was adopted.

Cap-bar joint quality has now been improved by the high temperature process to the point they will last as long as the bars except, of course, in those cases where there is extreme abnormal overheating of the cell, high temperatures, and high overloads result in melting caps off.

6 Table I SERVICE HISTORY OF HIGH TEMPERATURE CAPS TESTED Cap Service Age-De s Bar Number y Sound Caps Failed Caps 2 1 These cups retired from service at indicated age due to bar failure. 2 All cap failures have been actual melting of caps due to abnormal heating in test pot or cell.

It should be noted that the capping metal may be applied to the refractory hard metal bar in any convenient manner. The casting may be effected in situ, as described above, wherein metal pieces are placed on top of the refractory bar and the assembly heated to the required temperature, or other methods, such as melting the capping metal in a separate crucible and pouring the metal over the heated refractory bar, may be used.

In practice, after the refractory hard metal bars are capped they must be secured, at the cap end, to rods for connection to the bus system which supplies the power for the reduction cell. This operation is referred to as rodding the bars. The rods may be of any suitable electrically conductive material but are typically of aluminum or the same metal as the cap. The attachment of the rod to the capped bar may be effected in a number of Ways. One procedure that has proved quite effective and inexpensive is to cast the rods onto the caps while the caps are cooling during the capping operation. The bars to be used in tie reduction pots may be rodded routinely by simultaneous casting of the rod onto the cap before the cap metal has solidified. When the bar is to be used in top entry cathode pots the rod is cast onto the cap at to the axis of the bar. As another illustration, in the case of copper caps and copper rods a Si ma welder can be used to secure the rod to the cap.

An alternative method of connecting the refractory hard metal bar and cap to a bus system is to use a flexible con necting member. A flexible connection of this type may at times be more desirable. Such a connection is shown in the drawings as numeral 15 in FIGURE 4 wherein ll. designates the refractory hard metal member; 16, the cast aluminum metal cap member; and 17 a flex comprising multiple leaves of aluminum. One presently used method of connecting a flex to a refractory hard metal bar is by welding the end of the flex to the metal cap which has been cast onto the end of the bar. In another procedure designed to do away with the welding step, the almninum cap is cast onto the refractory hard metal member as described above; however, before the aluminum metal has solidified one end of the hex which has been preheated is set into the molten aluminum. The connection is then allowed to cool in the same manner as described previously.

in mechanical testing of joints made according to this invention refractory hard metal and cast metal cap memher was placed in a testing frame wherein the entire composite was supported near the end and loaded at the joint by means of a hydraulic ram. From the force required to break the joint and the length of the span the modulus of rupture was calculated. In all the cases it has been found that the bar system will break in the refractory hard metal body, clearly indicating that the joint is stronger than the refractory hard metal phase adjacent to the joint.

In electrical testing the voltage drop across the joint with a current of 700 amperes .wasmeasured by suitable apparatus, e.g., a potentiometer. From the joint voltage drop in the current the joint resistance can be calculated. In determining the joint voltage drop a potential traverse method was employed wherein voltage drops are measured at increasing one-half inch intervals while traversing the refractory hard metal member and metal cap casting from each extremity thereof across the joint location. The voltage drop of each traverse is then plotted against the distance from the joint and the difference or distances (joint voltage drop) between the two traverses at the joint location is measured. By employing the potential traverse method for determining joint voltage drop it was found that the joints produced by the method of the invention have no significant joint resistances. These results indicate complete or substantially complete wetted contact between the refractory hard metal material and the metal cap.

The foregoing examples of the method of the present invention and the articles produced thereby are intended for illustrative purposes only. It is apparent that various changes and modifications of the instant invention may be made without departing from the principles and spirit of the invention. For these reasons the scope of the pres ent invention should not be limited by the foregoing dis closure but rather only by the appended claims.

What is claimed is:

1. A method of joining electrically conductive metal with a refractory hard metal member comprising contact ing molten metal from the group consisting of aluminum, copper and iron, with the refractory hard metal member at a temperature sufiicient to cause substantially complete wetting of the refractory hard metal member by the molten metal and solidifying said molten metal.

2. A method of joining an electrically conductive metal from the group consisting of aluminum, copper and iron, with a refractory hard metal member comprising casting said metal in contact with the refractory hard metal member at a temperature sufficient to cause substantially complete wetting of the refractory hard metal member by said electrically conductive metal and solidifying said molten metal.

3. A method of joining a metal from the group consisting of aluminum, copper, and iron, with refractory hard metal members comprising placing the refractory hard metal member in a mold so as to expose at least a portion of the refractory hard metal to be joined, and casting the joining metal in contact with the exposed portion of the refractory hard metal at a temperature suflicient to cause substantially complete wetting of the refractory hard 8 metal member by the joining metal and solidifying said molten metal.

4. A method of capping a refractory hard metal bar for use as a cathodic element in an electrolytic reduction cell for the production of aluminum comprising placing a refractory hard metal bar in a mold in such a manner to expose one end portion of the bar, contacting molten capping metal selecter from the group consisting of aluminum, copper and iron, with the exposed end portion of the refractory hard metal bar at a temperature sufiicient to effect substantially complete wetting of the refractory hard metal bar, and allowing the resulting capped refractory hard metal bar to cool.

5. A method of capping a refractory hard metal bar for use as a cathode in an electrolytic reduction cell for the production of aluminum comprising placing a refractory hard metal bar in a mold in such a manner to expose one end portion of the bar, disposing capping metal selected from the group consisting of aluminum, copper and iron in the mold in contact with the exposed end portion, heating the mold assembly to a high temperature sufiicient to melt the capping metal and to effect substantially complete wetting of the refractory hard metal bar, and allowing the resulting capped refractory hard metal bar to cool.

6. A method according to claim 5 wherein the capping metal is aluminum and the mold assembly is heated to a temperature between 1300 to 1500 C.

7. A method according to claim 5 wherein the capping metal is copper and the mold assembly is heated to a temperature between 1650" to 1900 C.

8. A method according to claim 5 wherein the capping metal is iron and the mold assembly is heated to a temperature between 1535 to 1600 C.

9. A method according to claim 2 wherein the capping metal is aluminum and the mold assembly is heated to a temperature between 1400 to 1450 C.

10. A method according to claim 1 wherein said refractory hard metal consists essentially of at least one of the materials selected from the group consisting of carbides and borides of titanium, zirconium, tantalum and niobium.

References Cited by the Examiner UNITED STATES PATENTS 1,359,719 11/20 Mead 22204 X 2,013,411 9/35 Hummel 29155.55 2,019,599 11/35 Driggo 22204 2,396,730 3/46 Whitfield. 2,506,326 5/50 Adams. 2,544,671 3/51 Grange et al. 2,785,451 3/57 Hanink 22204 2,836,885 6/58 Macdonald et al 29472.9 2,970,065 1/61 Greene et a1. 3,091,027 5/63 Clair 29472.7 3,100,338 8/63 Henry 29473.1

WHITMORE A. WILTZ, Primary Examiner.

Claims (1)

1. A METHOD OF JOINING ELECTRICALLY CONDUCTIVE METAL WITH A REFRACTORY HARD METAL MEMBERS COMPRISING CONTACTING MOLTEN METAL FROM THE GROUP CONSISTING OF ALUMINUM, COPPER AND IRON, WITH THE REFRACTORY HARD METAL MEMBER AT A TEMPERATURE SUFFICIENT TO CAUSE SUBSTANTIALLY COMPLETE WETTING OF THE REFRACTORY HARD METAL MEMBER BY THE MOLTEN METAL AND SOLIDIFYING SAID MOLTEN METAL.

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