US3156763A

US3156763A - Electrical conductor joining practices

Info

Publication number: US3156763A
Application number: US117775A
Authority: US
Inventors: Walter G Zelley
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1961-06-19
Filing date: 1961-06-19
Publication date: 1964-11-10
Anticipated expiration: 1981-11-10

Description

United States Patent This invention relates in general to improvements in mechanically joining electrical conductors, as distin guished from welded and soldered joining practices. More specifically, the invention is addressed to mechanical pressure-restrained rigid joints between electrical 0011- I ductors at least one of which conductors presents an aluminum surface for pressure-restrained interfacial contacting assembly with another metallic conductor surface constituting a mechanical joint therebetween.

The term aluminum as used herein and inthe appended claims encompasses commerciallyvpure electrical conductor (EC) grade aluminum and commercially employed aluminum base alloys in which the elemental metal aluminum is present in amounts of at least 50 percent by weight.

The as-fabricated'surfaces of aluminumconductor, as

exhibit foreign materials adhered thereto during their manufacture, as well as being characterized by a tenacious natural oxide surface film, all of which contaminants are productive of increased electrical contact resistance of joints made therebetween, as compared to direct metal-to metal joints. e I

The primary object of the invention is therefore to provide an electrically :conductive aluminum surface for incorporation in pressure-restrained interfacial contacting mechanical joining with other metallic surfaces which insures a low electrical contact resistance joint.

Another object of the invention is to condition the surface of an aluminum conductor in such a manner as to impart passive properties thereto against its subsequent natural oxidation in storage as Well as in subsequent rigid interfacial'contacting'joined assembly with another conductor. 1

A further object of the invention is to provide a conductor having an exposed surface of aluminum characterized by its ability to retain a protective film of conventional unctuous joint compound. 4

These and other objects will be more fully understood and appreciated from the following detailed description based on an extended development and test program establishing consistently superior results through the'practice of the invention. I

The chief problem which had to be'overcome in reach ing the present invention was to remove adhering and included surface contaminants from as-fabricatcd aluminum conductor surfaces. Such contaminants have been found to prevail in the form of incrusted and ingrained surface lubricants conventionally employed in the fabrication of wrought aluminum conductor and wrought aluminum fittings used therewith, as well as surface adhered mold wash components and foreign materials employed in casting aluminum conductor fittings. in addition, the natural oxide surface films have had to be removed, as a further recognized contaminant conducive to increasing electrical contact resistance of mechanical pressurerestrained joints incorporating at least one interracial joint contacting aluminum surface I have discovered that as-fabricated aluminum conductor surfaces can be completely cleaned of foreign surface contamination and passively conditioned against further natural oxidation by first chemically coating the as-fabricated surfaces and thereafter stripping the so- 3,156,763 Patented Nov. 10, 1964 ice coated surfaces. I have further determined that aluminum surfaces thus treated exhibits the property of high initial electrical conductivity and retain this characteristic over long periods of storage prior to and following subsequent use in the environment of pressure-restrained interfacial contacting surface joints incorporating at least one of such cleaned and passivated surfaces.

Preparation or conditioning of aluminum surfaces in accordance with the practice of the invention preferably comprises performance of the following preliminary cleaning and conditioning steps:

Step1 .-Preclean as-fa-bricated conductors, and/orfittin s emolo ed therewith in mechanical 'oinin ractices,

l y l g to remove surface contaminants in the form of excess surface lubricants and residues thereof, as Well as molding or casting contaminants.

Step 2.Chemical zincate dip to substantially completely dissolve the natural oxide surface film and provide a replacement zinc immersion coating on the exposed surfaces of the conductors and or fittings, followed by water rinse. 1

Step 3.Acid strip the chemically produced zinc immersion coating, followed by water rinse. I

it has been found in practice that aluminum surfaces for intended environment in mechanical pressurerestrained interfacial contacting joining practices of the invention can in many instances be conditioned in their as-fabricated form by following steps two and three, with- .out, preliminary precleaning under step one. However,

more consistently improved results have been observed by following the three-step process asserted above.

The preliminary cleaning of step onecan follow any one of several established practices, as follows A. Solvent cleaning comprising exposing the surfaces to be preliminarily cleaned to non-emulsion solutions such as trichlorethylene, carbon tetrachloride and high-flash point petroleum solvents.

B. Alkaline cleaning comprising exposing the surfaces to be cleaned to an aqueous caustic soda solution, held at to 5., for a relatively short period of time, 5 to 30 seconds, followed by a Water rinse to remove residual caustic soda.

C. Sulphuric-chromic cleaning comprising exposing the surfaces to be cleaned to an aqueous solution made up of 10 percent by volume concentrated sulphuric acid and 4.5 ounces per gallon chromic acid anhydride, held at 110 to F. for a period of 2 to 10 minutes, followed by a water rinse. I p

D. Combination of A and B preliminary cleaning treatments.

E. Combination of B and C preliminary cleaning treatmerits.

--The second processing step, or step 2, in the practice of the invention, following any one 'of the preliminary cleaning procedures of A, B, C, D and E, entails providing the intended contacting surface of the electrical conductor, and/or fitting to be employed in a mechanical interfacial contacting joint, with a chemically produced zinc immersion coating. This has been best and preferably accomplished by immersion or exposure of the aluminum surface to be zinc coated in an aqueous solution made up from 120 to 500 grams per liter of sodium hydroxide and 20 to 100 grams per liter of zinc oxide, held at a temperature of 60 to 100 F., for a period of 10 to 60 seconds, followed by a water rinse. This second or zinc immersion step results in dissolving the natural oxide surface film and replacing the same with a thin adherent coating of metallic zinc, without exposing the surface so treated to oxidation.

The third processing step, or-step 3, of the invention comprises an acid strip to remove the chemically produced zinc coating of step 2. This third or acid strip ping step preferably comprises exposing the zinc coated material by immersion in the same sulphuric-chromic solution defined above under the preliminary cleaning step C, for a period of from 10 to 40 seconds, followed by thorough rinsing in water. Alternatives of the preferred sulphuric-chromic solution for stripping the zinc coating is a 50 percent by weight aqueous nitric acid solution at room temperature, with an immersion time of 5 to 30 ported in the table were calculated from Kelvin bridge measured electrical resistances and dimensional measurements for each individual test assembly or mechanical joint, each resistance ratio value reported being the appar' ent resistance of a jointassembly exclusive of the con- Parallel Groove Connector Clamp Alloy Composition ductors outside the connection or joint assembly, divided by the resistance of an equal length of the same conductor orconductors incorporated in the connection or joint assembly.

Table Resistance Ratio EC Aluminum Conductor As- Clamping Connector Clamp Surface Treatment Fabricated Force 7 Initial After 500 Heat Cycles 195 Alloy-Nominal composition by weight: 4.5% Cu, 0.8% Si, balance aluminum.

356 Alloy-Nominal composition by weight: 7.0% Si, 0.3% Mg, baalnce aluminum.

6062 Alloy-Nominal composition by weight: 0.25% Cu, 0.6% Si, 1.0% Mg, 0.06% Cr, balance aluminum.

6062 Alloy Nominal composition by weight: 0.25% Cu, 0.6% Si, 1.0% Mg, 0.06% Or, balance aluminum.

6062 Ally-Nominal composition by weight: 0.25% Cu, 0.6% Si, 1.0% Mg, 0.06% Cr, balance aluminum.

As-Iabricated casting Zinc Immersion-l-HNOa acid strip,

in accordance with invention.

Zinc Immersion+HzSO, acid strip,

in accordance with invention.

As-fabricatcd casting Zinc Immersion-l-HNOa acid strip,

in accordance with invention.

Zinc Immersion+HzS04 acid strip,

in accordance with invention.

As-fabricated extrusion Chemically cleaned in accordance with preliminary cleaning treatment E.

Zinc Immersion+HNOa acid strip,

in accordance with invention.

As-fabricated extrusion-wire scratch brushed.-

Zinc Immersion+Sulphuric-Chromic strip, in accordance with invention.

As-fabricated extrusion-wire scratch brushed.

Zinc Immersion-FSulphuric-Chromic strip, in accordance with invention.

aluminum strands on one do o No. 4 ACSR, 6/1 Cable, six {3,20O(I1JOl1IidS.

central steel strand O.D. =0.250".

No. 1/0 ACSR Cable, six alumig num strands on one central steel strand 0.D.=0.398.

No. 1 0 ACSR Cable, six alumif g 1 num strands on one central 0 i 1 W 1 69 steel strand O.D.=0.398

#2 Solid, 1 wire O.D.=0.257- pound foot 0.75 0.82

torque. do do 0.81 0.74

seconds, or a to percent by weight aqueous sulphuric acidsolution at room temperature for 5 to 15 seconds.

Electrical conductors, and pressure-attachable fittings employed therewith, for subsequent use in mechanical interfacial contacting pressure-restrained electrical joining are preferably, but not necessarily, provided with a thin coating of aconventional unctuous joint compound, such as a protective mineral grease, to insure long storage life for the conditioned surfaces thereof. With and without the aforesaid grease film, as-fabricated aluminum surfaced electrical conductors and fittings, treated and conditioned in accordance with the invention, have exhibited passive properties inert to the usual formation of non-conductive natural oxide surface films over periods of extended storage and following their assembly in the formation of low initial and extended electrical contact resistance mechanical joints.

Aluminum conductor surfaces conditioned in accordance with the practice of the invention, as above described, have also been observed to be more receptive to surface retention of conventional unctuous electrical joint compounds, as compared to as-fabr cated aluminum surfaces.

The following table reports the results of laboratory controlled tests simulating actual installations of conventional electrical conductor grade (EC) stranded aluminum steel reinforced (ACSR) and solid aluminum cables, to which conventional extruded and cast aluminum connector clamps were applied in pressure-restrained assembly thereon to constitute rigid electrically conductive interfacial contacting mechanical joints. Two methods of applying the assembly pressure are indicated under the column heading Clamping Force, the reported pounds clamping force representing total pressures applied by direct reading hydraulic laboratory testing equipment, and the reported pound foot torque identifying torque wrench applied loading through the conventional bolts of the parallel groove connector clamps employed in the tests,

The resistance ratio values in every instance re-' energization of the joint assemblies identifiedtherewith,

interrupted at the end of each one-hour periodof energization and cooled to ambient temperature, employing an energizing current of 178 and 227 amperes,'respectively in the order of last two reported 6062 parallel groove connector clamp joint assemblies.

The resistance ratio values reported in the aforesaid table establish lower electrical contact resistance for joints in which one of the interfacial contacting surfaces has been conditioned in accordance with the chemically produced z-inc immersion-acid strip'treatments of the invention, as compared to joints incorporating as-fabricated aluminum surfaces and conventionally chemically cleaned aluminum surfaces. The table also includes resistance ratio determinations for joint assemblies prepared in accordance with the invention which are quite comparable to the initial resistance ratio values reported therein for wire scratch brushed connector clamps, the latter treatment having been accomplished by a time consuming and anything but economical laboratory conducted mechanical scratch brushing treatment to actually remove a surface layer of aluminum and its contaminants before immediate assembly in a pressureerestrained interfacial contacting mechanical joint incorporating the scratch brushed surface protected against oxidation by a thin coating of conventional unctuous joint compound. In

respect of the wire scratch brushed joint assemblies re-,

ported in the table, it is significant to observe that both examples thereof have revealed higher'resistance ratio values after 500 repeated heat cycles, whereas theidentical joint assemblies conditioned in accordance with the invention are characterized by lower resistance ratio values.

Following the a-foredescribed chemically produced zinc immersion coating and acid strip treatments of the invention, electrical conductor contacting surfaces for interfacial contacting joints secured by pressure alone, have been regularly produced in an economical manner with attendant consistent improvement in electrical contact resistance of joints incorporating at least one of such conditioned surfaces.

What I claim is:

1. The method of joining an aluminum member to another electrically conductive member in the production of a mechanical pressure-restrained interfacial electrical joint assembly exhibiting low initial and extended electrical contact resistance, the steps comprising; conditioning and passivating an as-fabricated surface of the aluminum member of the joint assembly against substantial natural reoxidation by exposure to an aqueous solution of an alkali metal zincate to deposite a coating of zinc thereon, removing the deposited zinc coating by exposure to an oxy-acid stripping solution, and thereafter securing the conditioned and passivated stripped aluminum surface in pressure-restrained coextensive surface contact with the other electrically conductive member.

2. In a mechanical pressure-restrained interfacial electrical joint assembly between coextensive contacting surfaces of two electrically conductive metallic members, at least one member of which is aluminum, said joint assembly exhibiting low initial and extended electrical contact resistance as compared to the same joint assembly incorporating the aluminum member in as-fabricated condition, said aluminum member having its as-fabricated surface before incorporation in coextensive interfacial surface contact with the surface of the other electrically conductive member in the joint assembly conditioned and passivated against substantial natural reoxidation by exposure to an aqueous solution or" an alkali metal zincate to deposit a coating of zinc thereon followed by removal of the deposited zinc coating by exposure to an oxy-acid stripping solution, and clamping means securing the pressure-restrained coextensive interfacial surface contacting joint assembly between the members.

References Qited in the file of this patent UNITED STATES PATENTS 2,190,824 Cook Feb. 20, 1940 2,423,290 Bonwitt July 1, 1947 2,650,886 Zelley Sept. 1, 1953 2,650,901 Van Der Horst Sept. 1, 1953 2,662,831 Culverhouse Dec. 15, 1953 2,739,932 Forestek Mar. 27, 1956 2,890,943 Schwarz June 16, 1959 2,901,722 Arnott Aug. 25, 1959 2,906,987 Fox Sept. 29, 1959 2,995,814 Chamness Aug. 15, 1961 FOREIGN PATENTS 632,487 Great Britain Nov. 28, 1949 776,149 Great Britain June 5, 1954 776,836 Great Britain June 12, 1957 796,650 Great Britain June 18, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 156,763 November 10, 1964 Walter G. Zelley It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 2, for "exhibits" read exhibit columns 3 and 4, in the Table, first column, line 6 thereof,

for "baalnce" read balance same Table, second column,

line 4 thereof, for H 30, read H 80 same Table,

fourth column, line 4 thereof, for "3, 100" read 3,200

Signed and sealed this 30th day of March 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 156 T63 November 10, 1964 Walter G. Zelley It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 2, for "exhibits" read exhibit columns 3 and 4, in the Table, first column, line 6 thereof, for "baalnce" read balance same Table, second column,

line 4 thereof, for H 50, read H 50 same Table, fourth column, line 4 thereof, for "3, 100" read 3,200

Signed and sealed this 30th day of March 1965.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ERNEST W. SWIDEIR- Attesting Officer

Claims

2. IN A MECHANICAL PRESSURE-RESTRAINED INTERFACIAL ELECTRICAL JOINT ASSSEMBLY BETWEEN COEXTENSIVE CONTACTING SURFACES OF TWO ELECTRICALLY CONDUCTIVE METALLIC MEMBERS, AT LEAST ONE MEMBER OF WHICH IS ALUMINUM, SAID JOINT ASSEMBLY EXHIBITING LOW INITIAL AND EXTENDED ELECTRICAL CONTACT RESISTANCE AS COMPARED TO THE SAME JOINT ASSEMBLY INCORPORATING THE ALUMINUM MEMBER IN AS-FABRICATED CONDITION, SAID ALUMINUM MEMBER HAVING ITS AS-FABRICATED SURFACE BEFORE INCORPORATION IN COEXTENSIVE INTERFACIAL SURFACE CONTACT WITH THE SURFACE OF THE OTHER ELECTRICALLY CONDUCTIVE MEMBER IN THE JOINT ASSEMBLY CONDITIONED AND PASSIVATED AGAINST SUBSTANTIAL NATURAL REOXIDATION BY EXPOSURE TO ANAQUEOUS SOLUTION OF AN ALKALI METAL ZINCATE TO DEPOSIT A COATING OF ZINC THEREON FOLLOWED BY REMOVAL OF THE DEPOSITED ZINC COATING BY EXPOSURE TO AN OXY-ACID STRIPPING SOLUTION, AND CLAMPING MEANS SECURING THE PRESSURE-RESTRAINED COEXTENSIVE INTERFACIAL SURFACE CONTACTING JOINT ASSEMBLY BETWEEN THE MEMBERS.