FIELD OF THE INVENTION This invention relates to el ectr i c—resi stance welding machines or welders, particularly to roller electrodes used in such, the electrodes being of the type having stationary and rotatably mounted components separated by clearance gapsj and an electrically and thermally conductive l iquid contained by the components to bridge the gaps to conduct a welding current between the two components, and in certain instances also, to cool one of the components via a coolant flowing through the other component.

BACKGROUND OF THE INVENTION

Metal cans are fabricated by forming a flat metal bl nk, usually rectangular in shape, into a tubular configuration with the lateral ends or edges being lapped and welded together, de ining a longitudinal seam. End closures are then secured across the open ends of the tubular configuration to complete the can formation. The term "tubular" is not restricted to a circular cross-section, as square or other shaped cans may be fabricated with this same approach. Also, similar continuously welded seams may be used for fabricating structural components other than cans.

One form of seam weld is made as opposed roller electrodes, one on the inside and one on the outside of the tubular configuration, continuously track along in the direction of the lapped ends. A large welding current is transmitted between the rol ler electrodes, via the smal l regions of contact defined between the opposing rol ler electrodes, between and through the lapped

ends. The current is pulsed, to provide that the welded seam is actually comprised of a series of "spot welds", made closely adjacent one another.

With a tin—plated steel blank, a copper wire intermediate electrode, typically of rectangular cross—section , is fitted in a circumferenti l groove on each roller electrode, and pressed then against the opposite sides of the lapped ends. T is serves to help carry the melted tin away from the welded seam region; although the tin does tend to sol idif nd build up on the roller electrode, and in the groove. Tin buildup increases the electrical resistance (compared to a new roller electrode), and reduces the effective welding current ...resul t ing in erractic or even poor welds. This tin buildup periodically can be removed, by machining or "reprof i 1 ing" the periphery and groove of the roller electrode. As the roller electrodes must have specific minimum overall diameters and contact angles, reprof i l ing may be done only a l imited number -of times; thereafter, the roller electrode must be replaced. The steel blank could alternatively be zinc-coated, but the same buildup problems exist.

One form of inner roller electrode of this type commonly has a stator supported by the welding machine, and a rotor rotatably carried on the stator and having a circumferential guide groove for the copper wire. The rotor and stator are separated from one another by very small radial and circum erential, clearance gaps (some possibly only 0.6 of a mill imeter wide), across which the relative movement of the rotor and stator takes place. An electrically conductive l iquid is sealed in the roller electrode, spanning substantial portions of the gaps, to conduct the welding

cυrrent between the stator and rotor components. Appropriate bearings and surrounding insulators support relative rotation of the stator and rotor components, but otherwise electrically insulate these components from one another.

One form of roller electrode cool ing provides for circulating a coolant l iquid, commonly water charged with an anti—freeze, through passages defined in the stator. Thi cools the stator surfaces exposed to the electrically conductive l iquid, and the electrically conductive l iquid in turn then also serves to thermally cool the rotor.

The stator and rotor components of the roller electrode are commonly formed of a copper alloy having a h i gh content (possibly 98 of copper, for yielding high electrical and thermal conductivity. Such an alloy also structurally resists deformation under the welding temperatures and pressures.

Modern welding equipment may weld with 6000 amperes of current at up to 40-50 kilowatts of power, giving a l inear welding speed of 70 meters per minute, and yielding a production up to 600 cans per mi nute .

The electrically and thermally conductive l iquid almost universally used in commerical roller electrodes has been mercury. Mercury remains a l iquid to approximately -38 degrees C, "unequal ed by any other conductive metal or eutectic mixture of metals, that is also stable at room tempertures. Mercury can carry the high welding currents needed in the roller electrode, and mercury can also provides sufficient cool ing for the rotor.

Despite its wide use, mercury is not a will ing first choice; in fact i t has many very poor if not outright dangerous

characteri sti cs.

For example, mercury has electrical and thermal conductiviti s of approximately only 2% that of cooper. The l imi ed wetting abil ity of mercury adds to the reduced e fec iveness of both ^' electrical and thermal transmission between the stator and rotor components; Consequently, its presence: adds appreciably to the electrical current needed to generate the welding temperatures, which raises both the operating temperatures and cool ing requirements; and gives off heat to the stator so poorly that the coolant may be heated only a few degrees in passing through the roller electrode, despite being chilled to below room temperature, possibly between 5-15 degrees C. Moreover, the thermal expansion of mercury in the anticipated temperature range of use, 5-50 degrees C, is very large, so that compl icated seals and/or overflow devices must be associated with the roller electrodes to accomodate this expansion.

Mercury is also very corrosive to the copper alloy stator and rotor components, producing an amalgam that l imits both the shelf and operating l ives of the roller electrode, to perhaps only several weeks. The amalgam, in its initial stage is gelatinous or paste-l ike, to increase drag against electrode rotation; whereas in its more advanced stages, it solidifies rock-hard to bind the components together completely. Once sol idified, it is typically impossible to dislodge the amalgam and disassemble the electrode components, such as for rebuilding and salvaging them for a second work cycle. The amalgam has poorer electrical and thermal conductivities than fresh mercury, correspondingly imposing ever higher welding currents and cool ing demands.

Attempts to extend the shelf l ife provide that the mercury rol ler electrode may be maintained under re rigeration and/or repositioned frequently. Also, manufacturers may s ip the roller electrodes empty, wi h a separate supply of the mercury the user must pour into the roller electrode and seal , when the need arises.

To i c i y of mercury however, remains probably i s most significant drawback, from a l iabil ity standpoint. Mercury is frequently looked upon as a substance requiring special standards of care, and government approvals for its wide scale use and disposal . Such restrains detract from the appeal of having the user fill the roller electrode with the mercury and/or add appreciably to the overall costs associated with its use in roller electrodes. Mercury leaks to the environment, or even the threat of it, can be unsettl ing.

Other electrically conductive l iquids have been proposed, to avoid the above-mentioned problems of mercury. However, such generally have not found commerical appl ication, because the rol ler electrode had to be modified so much- that it would not work in conventional welding machines; or the current carrying capacity of the l iquid was inadequate for the high output welding demands.

A roller electrode is disclosed in Patent No. 3,501,611; non-mercury electrically conductive l iquids are disclosed in Patents Nos. 4,188,523 (6?.5 +or- 5 Atomic '/. of gall ium, 15.2 +or- 1.0 Atomic V. indium, 6.1 +or- 1.0 Atomic V. tin, ^' 4.5 +oι—0.8 Atomic '/. zinc, 3.2 +or- 0.5 Atomic '/. si lver, and 1.5 +or- 0.5 Atomic '/. aluminum; 4,433,22? (pure gall ium, or bianary metals of gall ium including gall ium/indium and gall ium/tin) ; and Patent No. 4,642,437 discloses a colbal t coating on the roller electrode. Foreign

patents of interest might include West Germany patents Nos. 2,351,534 (Beck); 2,805,345 (Janitzka) and 3,432,49? (Lorenz); Swiss No. 597,971; and Japanese No. 0001583, disclosing different conductive l iquids of gall ium and/or different overlying coatings on the roller electrode.

SUMMARY OF THE INVENTION

The present invention provides improved mercury-free roller electrodes for use in electric-resistance welders, the electrodes having a substantially nontoxic highly conductive (both electrically and thermally) l iquid contained by the components across the very small rotational gaps between the components. Moreover, a protective plating on the hotter of the components exposed to the conductive liquid, further inhibits corrosion and lengthens the operating l ife. Low thermal expansion of the contained liquid reduces the demands of the containing seals; and the electrode has greatly extended shelf and operating l ives.

The present invention provides a roller electrode having electrically and thermally conductive liquid of a composite eutectic mixture of gall ium (Ga) , indium (In), tin (Sn) and zinc (Zn) . The specific composi tion , _α by weight, is approximatel 61% Ga, 25 In, 13% Sn , and 1% Zn .

The present invention also provides having ^" the rotor surfaces plated wi h a very thin la r of: (1) gold (Au) ; (2) rhodium (Rh); or ⁽ 3 ⁾ gold first and rhodium over the gold. This plated layer may be on: (1) the hotter rotor surfaces that will be exposed to the conductive l iquid, specifically on the inside of the rotor opposite

the copper wire electrode, serving to resist corrosion of the rotor; or (2) the hotter exterior rotor surfaces, specifically at the formed groove or trough that would normal ly guide the copper wire electrode, or at a circumferenti l rib, formed in place of the groove or trough, that would replace the copper wire electrode, each serving to resist wear of the rotor. Instead of rhodium, other members of the platinum family may be used, including platinum (Pt), iridium (Ir>, palladium (Pd) , ruthenium (Ru ⁾ or osmium (Os); but reduced conductivity and increased costs may make these alternatives more academic than practical .

The present invention also provides improved controls in the coolant system, including fine filters to clean the coolant, and temperature sensors and/or flow valves to provide for the operation of the rol ler electrodes only in the proper temperature ranges.

The present invention also provides a roller electrode having a rotor formed in part of a composite sintered mixture of copper (Cu) and tungsten (W) , being in the range of 60-70% tungsten and 40-30% copper by weight. This provides increased rotor strength against deformation and mechanical wear, resistance to corrosive attack by the contained conductive l iquid, and increased resistance against bonding of tin to said rotor, to minimize detrimental tin buildup on the exposed surface of the rol ler electrode. Other refractory metals, such as molybdenum (Mo) having good electrical conducti i ty, may also be used instead of tungsten in a composi te sintered mixture of copper (Cu) and molybdenum (Mo), particularly when balancing the .durabi 1 i ty aoainst the costs.

BRIEF DISCRIPTION OF THE DRAWINGS

The disclosure of the present invention includes the accompanying drawings, in w ich:

Fig. 1 is an e1 evat i onal-type sectional view of el c ri c—resi stance welding equipment, taken generally in a direction axially in l ine with the formation of a tubular can, and illustrating a can bl nk In phantom and the inner and outer roller electrodes cooperating therewith;

^• Fig. 2 is a top plan—type vi w of Fig. 1;

Fig. 3 is an enlarged sectional view, as seen from the same l ine of sight as Fig. 1, except through the center of an inner roller electrode used on the electric-resistance welding equipment of Figs. 1 and 2;

Fig. 4 is a reduced sectional view taken generally from line 4-4 in Fig. 3;

Fig. 5 i s an enlarged sectional view, as seen from the same line of sight as Fig. 1, except through the interior of an outer roller electrode used on the el ctric-resi tance welding equipment of Figs. 1 and 2;

Fig. 6 is a greatly enlarged sectional view, similar to Fig. 1, illustrating the seam weld formed in the mated edges of the can blank;

Fig. 7 is a transverse sectional view of a modified roller electrode; and

Figs. 8 and 9 are greatly enlarged sectional views, similar respectively to portions of Figs. 3 and 7, illustrating alternative construc ions of the rotor.

DET I ED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

In Figs. 1 and 2, appropriate rol ler electrode components of an electric-resistance welder 10 are illustrated, adapted to weld can blanks 12 along longitudinal seams 14, formed where opposite ends 16a and 16b (see Fig. 6) of each blank are overlapped sl ightly. The can blanks 12 typical ly wi ll start out flat (as illustrated as 12f in Fig. 2), and will be moved in somewhat spaced edge-to-edge relation, in the direction of the seams 14, and will be reshaped by conventional reforming structure (not shown) to a tubular configuration 12c (shown in phantom only in Fig. 1) with the blank ends overlapped. Outer and inner roller electrodes 20 and 22, respectively identified relative to the tubular configuration 12c of the formed can blank 12, bear against the blank ends at the seam 14.

The outer rol ler electrode 20 has a disc-l ike member 23 keyed to an elongated shaft 24, at one end thereof; and the shaft at its opposi te end is rotatably mounted at. bearing housing 26. The bearing housing 26 is adjustably secured to frame 27a of the welding equipment 10, to orient the shaft 24 substantially perpendicular to the movement of the blanks 12 and the formation of the seams 14. The inner rol ler electrode 22 is mounted at one end of an elongated arm 30 that extends axially of the movement of the can blanks 12; and the arm 30 at the opposite end is adjustably supported relative to frame 27b of the welding equipment 10, where the blanks are flat or just beginning to be reshaped to be tubular. The inner rol ler electrode 22 wi ll thus be located at the downstream end of the arm 30, relative to the direction of movement of the blanks 12.

Generally, a copper wire "32 is fitted in annular grooves 33 and 34, respectively in the roller electrodes 20 and 22, to bear directly against the overlapped blank ends 16a and 16b; but onl the inner electrode wire 32 received in electrode groove 34 is illustrated in Figs. 3 and 4.' A plused welding current is carried between appropriate conductors 36 (Fig. 1), via the outer roller 20, the sandwiching copper wires 32 and the overlapped can blank ends 16a and 16b, the inner roller 22, and the support arm 30. Figs. 3 and 4 illustrate the inner roller electrode 22 in greater detail , the electrode having a stator 38,. and a rotor 40 supported by sealed bearings 39 to rotate on the stator 38 about an axis disposed transverse to the arms 30. The stator 38 may be of a unitary construc ion, including extended ends 42 keyed nonrotatably to the inner roller electrode support arm 30, and a central .disc 44 between the extended ends 42. The rotor 40 typically has two adjacent sections 46 and 47 press-fit or otherwise secured together and sealed by 0-ring 48 at the overlapping joint.

The joined rotor sections 46 and 47 define a cavity 50 larger than the stator disc 44; and the generally concentric stepped adjacent faces of each are closely spaced from one another across peripherial gap 51 and opposed side gaps 52. -Seals 53 operate between the stator 38 and rotor 40 to seal and electrica ly insulate these components relative to one another. An electricall and thermally conductive l iquid (not shown) fills approximatel 80-90% of this sealed roller interior or cavity 50, tap 54 being used for this and then closed wi h plug 56. The liquid bridges the gaps 51 and 52 between the stator and rotor; electrically and thermally connecting the stator 42 and rotor 44 together, w ile

al lowing relative rotation of these components.

Axially extended coolant passages 55a and 55b formed in the arm 30 communicate with one another via axial , radial and peripherial passages formed in the stator 38. In the roller electrode 22 illustrated, the stator ends 42 have axial passages 56a and 56b extended to opposite sides of a partition 57 fitted in the stator disc, and four radial passages 58a and 58b respectively on opposite sides of the partition extend between the central passages 56a and 56b, and annular passages 59a and 59b on opposite sides of the partition. As illustrated, the four radial passages 58a and 58b on each side of the partition are angled approximatel 90 degrees apart; and the passages on one side are angled to l ie approximately midway between the passages on the other side. Through openings 61 formed in the partition 57 connect the two annular passages 5?a and 5?b together, at locations appro imately midway between the eig t radial passages 58a and 58b.

The housing 26 illustrated in Fig. 5 has sealed bearings 64 and rotary seals 65 that cooperate to rotate the shaft about its longitudinal axis, whil seal ing the components together against l iquid leakage and whi le electrical ly insulating the components from one another. The housing cavity 66 is larger than the shaft 24, and generally concentric adjacent faces of each are closely spaced from one another across a peripheral gap 68. An electrically conductive l iquid (not shown) ills approximately 80-90% of this sealed housing interior 66 , fill cap 69 being used for this and closed by bol s 70. The l iquid bridges the peripheral gap 68 between the shaft 24 and housing 26; electrically connecting the components together, while allowing

relative rotation between them.

Collar manifolds 71a and 71b are sealed around the shaft 24, at one end thereof adjacent the housing 26, communicating via radial passages 72a and 72b, and axi lly extended passages 73a and 73b. formed in the shaft 24, with passages 74 formed in the disc 23 of the outer roller electrode.

Coolant may be circulated through the outer and inner roller electrodes, for cool ing them. As is illustrated in Figs. 1 and 2, the coolant may be passed through a chiller 77 and a il er 79, and then in parallel through the outer and inner roller electrodes 20 and 22, respectively. In circulating through the stator 38 the coolant may enter from one passage 55a of the arm 30, pass outwardly in one set of four radial passages 58a to the one annular passage 59a, cross over via the openings 61 to the other annular passage 59b, move inwardly in the other set of four radial passages 58b, and exit via the other passage 55b of the arm 30.

The rotor 40 is cooled by the thermal conductivi y of the conductive l iquid, across the gaps 51 and 52 between the stator disc 44 and rotor . In modern practi-ce, some of the coolant passages in the stator may be of very small cross-section, as small as 1.0 millimeter across; and up to possibly 19 liters per minute of the coolant may be circulated through these flow passages, representing very hi h coolant flow velocity.

As will be appreciated, the outer roller electrode may have both the stator and rotor components cooled by the circulating coolant, so that the conductive l iquid need only axrry the electric current between these components and not cool one from the other. Moreover, the stator of the outer roller electrode surrounds part

of the associated rotor in supporting the rotor, instead of the rotor surrounding the stator as in the inner roller electrode.

The invention provides for the use of an improved electrically and thermally conductive l iquid, having many advantages over the most conventional l iquid commerical ly used, namely mercury. The improved conductive l iquid is a composite eutectic mixture of gal l ium (Ga), indium (In),, tin (Sn) and zinc (Zn). The approximate composition, by weight, is 61% Ga, 25% In, 13% Sn , and 1% Zn.

Some important, unusual and benefical properties of using this conductive l iquid in the roller electrodes, particularly when compared to mercury, are:

1. The conductive l iquid is essentially nontoxic, and provides roller electrodes safe even for forming cans intended for foodstuff .

2. The thermal expansion of the conductive l iquid is relatively low, in the anticipated operating temperature range between possibly 5 and 50 degrees C. This reduces problems of seal damage and/or overfill ing the roller electrode, even when fi ll ing up to 85-95% maximum capacity, and also allows the roller electrode, filled to this greater percentage, to provide a l rger percentage of the components'" surfaces wetted by and exposed to the l iquid, reducing the unit density of current.

3. The corrosive reaction of the conductive l iquid against copper and/or copper alloys is very low at temperatures below 50 degrees C, which may be obtainable wi th proper roller electrode cool ing, anticipating very long expected shelf l ife and operating l ife for such rol ler electrodes.

4. The conductive l iquid has electrical and thermal conductivities almost four times better than mercury, and has much better "wetting" characteri tic than mercury; providing that such roller electrodes produce equivalent or even better welds, compared to mercury roller electrodes, at less electrical current and at reduced overall operating temperatures.

5. The conductive l iquid deteriorates to a putty-l ike consi stency...not a rigid sol id, allowing for stator and rotor component disassembly and rebuilding, for greater overall economy.

As the disclosed conductive l iquid provides overall less resistance to the welding current, less operating heat will be built up in the roller electrodes. Moreover, the disclosed conductive liquid provides overall good wetting of the interior stator and rotor surfaces, to offer good thermal cool ing and electrical conductivity across the gaps between the stator and rotor. Also, an equivalent welding temperature and weld can be obtained wi h less overall welding current, compared to a weld made with a conventional mercury roller electrode.

Another aspect of thi invention provides that inner roller electrode 22 is coated, on the inside rotor surface that is to be exposed to the conductive l iquid, as- a means to reduce corrosive reaction between the conductive l iquid and the copper and/or copper alloy structural material of the rotor. Because of the intense heat of the rotor in the rim region opposite the actual welding contact, this region is most critical to corrosive attack of the conductive l iquid and should be coated. Because the stator surfaces exposed to the conductive l iquid are generally much coole

than any of the rotor surfaces exposed to the conductive l iquid, and b low the temperatures where the conductive l iquid would attack the copper alloy stator, the stator surfaces generally should not be coated.

In one embodiment, gold (Au) is used, elec roplated to a thickness of between 0.0025 and 0.025 of a millimeter. The gold has electrical conductivity about 60% that of copper; but the minimum thickness does not appreciable add to the overall resistance of the welding current. Gold improves resistance against corrosion of the disclosed conductive l iquid against the copper and/or copper alloy of the rotor; at most operating temperatures of the roller electrode.

In another embodiment, rhodium (Rh> is used, electroplated to a thickness of between 0.0025 and 0.025 of a mill imeter. The rhodium has electrical conductivity about one-third that of copper; but the minimum thickness does not appreciably add to the overall resistance of the welding current. Rhodium improves resistance against corrosion of the disclosed conductive l iquid against the copper and/or copper alloy of the rotor; at even elevated operating temperatures. The rhodium coating also reduces corrosion of conventional mercury against the copper and/or copper alloy of the rotor, making it advantageous to use this rhodium coating even in a conventional mercury- ill d roller electrode.

In ye another and preferred embodiment, gold may be el ro lated onto the rotor surface, and the rhodium may be electropl ted onto the gold. This gold-rhodium combination will typically be more economical than plating with just gold and/or rhodium, particularly when compared to the overall improved

performances. The thickness of gold may be between 0.001 and 0.0125 of a mill imeter, and the thickness of rhodium may be sl ightly less; so that the overall plating may thus possibly be of the order of 0.0015-0.025 of a mill imeter thick. As noted, rhodium provides good resistance against any corrosive reaction of the disclosed conductive l iquid and/or mercury against the copper alloy rotor.

The roller electrode may be coated also, on the outside rotor surface, as a means to reduce wear of the copper alloy structural material , and/or to reduce buildup of tin or zinc on the rotor per iphery.

Fig. 8 shows an enlarged illustration of inside roller electrode and the rotor groove 34a, sized and shaped to receive and guide the copper wire electrode 32. The coating 34c would be on the bottom and adjacent sides of the groove 34a. Fig. 9 shows an enlarged illustration of the inside roller electrode, where the rotor groove would be replaced wi h an annular continuous raised rib 132a. With this embodiment, no copper wire electrode 32 woul be used, but the rib 132a itself would ride against the overlapped edges of the can blanks. The coating 132c would be on the top and adjacent sides of the rib 132a.

A preferred coating would be of gold elec roplated onto the rotor surface, and rhodium el ctropl ted onto the gold. The thickness of gold may be between 0.001 and 0.0125 of a mill imeter, and the thickness of rhodium may be more; so that the overall plating may thus possibly be of the order of 0.0025-0.040 of a mill imeter thick. Rhodium may be used alone as an alternative, being electroplated directly onto the rotor surface. Rhodium

provides good resistance against mechanical wear of the groove, to increase the operating l ife of the rol ler electrode before having to reprofile the rotor periphery, specifically at the annular groove. Rhodium also resists tin and zinc bui ldup in the groove and/or on the raised rib.

Instead of rhodium, other members of the platinum family may be used, including platinum (Pt), iridium (Ir), palladium (Pd) , ruthenium (Ru) or osmium (Os), particularly when balancing conductivity against costs.

Because the coating, regardless of its composi tion, has greater resistivity than the copper alloy of the rotor and/or stator component, the coating should be made only where needed. Also, the rotor of outer roller electrode 20 may advantageously be coated in the region of the copper wire lectrode groove 33 and/or in the region of the continuous raised rib 132a used instead of the copper wire electrode, for increasing resistance against mechanical wear of the groove or raised rib.

To have any disclosed improved roller electrode operate properly for its expected long l ife, the temperature and qual ity of the coolant must be extensively regulated. This includes protection against impurities in the coolant, or buildup of slag, oxidation or the l ike on the wal ls of the coolant passages...al 1 of which lead to reduced cool ing, and the possible resul ant overheating and greater corrosive activi ty of the l iquid against the components. Thus, a thin coating of silver may be electroplated on or otherwise appl ied to the wal ls of the coolant passages, which inhibi ts the oxidation of the passage walls,

particularly where the roller electrode is formed of a copper .alloy. The thickness of the silver coating may be between 0.001 and 0.0125 of a millimeter.

On the other hand, excessive cool ing can also be very damaging to the operating l ife of the improved electrode rollers, causing the l iquid to change phase at approximately 3 degrees C. Excessive cool ing is possible if welding may be discontinued over any length of time, while the chiller operation and coolant flow continued, a if there was no stoppage.

As thus illustrated in Fig. 1, the coolant flow to the roller electrodes is from chiller 77 through a filter 79, of possibly 10-50 micron size, to trap out all but the smallest particles that may otherwise block the very small coolant passages of the roller electrodes. Inlet and outlet temperature sensors 81 and 82 are in the coolant flow circuit of the welding equipment, to sense both the safe low inlet and high outlet operating temperatures. As the conductive liquid may change phase at temperatures below approximatel 3 degrees C, the low safe inlet temperature range ma be set some several degrees above this. The safe outlet temperatures normally would be in the range of 5-20 degrees C, and " the low and high safe l imits may be selected as such. The sensors 81 and 82 may activate an audible and/or visual al rm 84 to advise of either excessive heating or cooling.

The sensor 82 may also activate a variable flow valve 83, to close- it and circulate the coolant via l ine 85 around the roll r electrode, responsive to a sensed coolant temperature that may be too cold, or in the event of excessive cool ing. A bleed valve 86 may bypass this valve 83, to allow l imited flow through the roller

electrode even when the valve 83 is closed.

It should be remembered that even wi th the best known protective coatings on the copper alloy stator and/or rotor components, the conductive l iquid wil l become pol luted over time and usage due to the corrosion of these components, and the electrical and thermal conductive capaci ties of the conductive l iquid accordingly then deteriorate T is means that the welding current must be increased to accomplish the same weld, and the operator of the welder periodically does increase the welding current to help keep the welds somewhat uniform over time. T is increased current inputs more heat to the roller electrode, but unfortunately when the conductive l iquid simul taneously provides reduced cool ing effectiveness, thereby allowing the overall roller electrode temperature to rise, which accelerates even more the corrosive activity of the conductive l iquid.

To help compensate for this, the valve 83 would also have first and second opened posi tions, allowing different rates of coolant flow through i t in such posi tions. The valve would be initially in the first opened position, which would be sized to allow sufficient coolant flow through i t to cool the roller electrode when the conductive l iquid is fresh and at both its best thermal and electrical conductivity ra.tes. This flow rate might be between two and three l iters per minute, for example. The sensor 82 would be set to shift the valve 83 to i ts second opened posi tion, responsive to a predetermined increase in the sensed coolant temperature. The valve in i ts second opened position would increase the coolant flow rate, to perhaps up to even five l i ers per minute, for increased cool ing capacity; and this would occur

- 20-

automatϊcal 1y when and as the conductive liquid becomes polluted and its conductivity rates drop.

The inner and outer electrode rollers can have parallel coolant flow hoo ups, or separate controls and flow l ines can be provided for each; although for simpl icity, only the flow for the inner roller electrode 22 is illustrated.

Some degree of care is also needed for shipping and/or storing the disclosed roller electrodes, for maintaining them above the lower phase change temperature. Once done, the disclosed roller electrode will otherwise have a very extended, almost unl imited, shelf life. Even should the disclosed conductive l iquid thicken because of being excessively cooled, it will only be gelatinous or slushy, not sol id, and it may be usable after warming again and returning to the liquid phase.

The rotor may be made of a copper alloy having approximatel 0.3-.7% beryll ium (Be), 1.5-2.0% nickel (Ni), with the balance copper (Cu); or up to possibly 0.3% cobalt (Co) may be added too. This material has sufficient hardness to withstand the temperatures and pressures associated with the rotor; and adequate conductivity. The stator is not subjected to the same temperature and pressure extremes as the rotor, and may be made up of a copper alloy having approximatel 1% chromium (Cr) and the balance copper (Cu); being much more conductive than, but not as hard as, the rotor material . The rotor and stator materials, and the more conductive l iquid, provide less resistance to the electrical current through the roller electrode, to produce less heat and otherwise a better weld, than obtained with a mercury roller electrode and/or an electrode

having the stator made of the same material as the rotor.

The rotor section 47 may alternatively be made of a sintered mixture of tungsten and copper, of the order of 60-70% tungsten (W) and 40-30% copper (Cu). Of great si nificance, tin does not bond to or bui ldup on this sintered material , greatly extending the period between, or possibly even el iminating completely the need for, reprof il ing the rotor periphery. The sintered rotor section provides uniform thermal expansion and contraction. The rotor section 46 may be formed of the same sintered material , or may be formed of the copper/alloy.

The sintered tungsten and copper rotor has great resistance against corrosion, up to temperatures of the order of 600 degrees C. It also is structurally durable against mechanical wear, approximately four times better than the typical copper alloy rotor. This reduces the wear of the rotor periphery, including wear of the groove 33, where wear changes of the exterior rotor shape or groove depth adversely reduce the performance and/or operating l ife of the roller electrode. Al though the conductivity of the sintered tungsten and copper rotor is perhaps one—third less than the conventional copper alloy rotor, the improved structural properties and the improved conductivity of the disclosed conductive l iquid tend to compensate one another. The overall combination is l ikely to achieve welding performances comparable to those of the conventional mercury roller electrodes.

The rotor section 147 (Fig. 7) of the sintered tungsten-copper mixture may have a c i rcu erenfl al 1 y continuous raised rib 132 formed on the rotor periphery at a location where the intermediate wire electrode groove (l ike 34) would be; although this rib need

not have any coating. As noted above, during use then the rotor rib would ride against the overlapped edges of the can blank; and no intermediate electrode wire (like 32) would be used between the rotor and the can blank 12.

Other refractory metals, such as molybdenum (Mo), having good electrical conductivity may also be used instead of tungsten, In a sintered molybdenum-copper mixture of the order of 55-75% molybdenum (Mo) and 45-25% copper (Cu), particularly when balancing the durabil ity against the costs.

Another aspect of the disclosed roller electrodes is the rebuilding program possible with them, providing for greatly increased overall electrode roller economy. Thus, after the roll r electrode has failed, because of a bad bearing or even after the conductive liquid being polluted to the stage that it is gelatinous or slushy, the components may still be easily disassembled.

Once separated, the stator and rotor components may be cleaned inside and outside, as needed. The component surfaces that were exposed to the conductive l iquid may be blasted with gl ss beads, to clean them without removing component material , or with aluminum o ide for removing the more difficult surface impurities. It has been observed that the conductive liquid does not permanently contaminate the -exposed componen surfaces, as mercury tends to do, so that they can be cleaned up real by good. The coolant passages may be cleaned by flushing wi h an acetic acid, including with abrasive such as glass beads or aluminum o ide. Generally, the components are not recoated during this rebuilding program. The cleaned components may be reassembled with new or rebuilt bearings and

seals, and recharged with fresh conductive l iquid. The rebuilt roller electrode may be reprof i led as needed before being reinstal led on the welder.

While the disclosure has been directed more to the inner rol ler electrode, it should be appreciated that many of the same inventive aspects will apply equally well to the outer roller el ec trode .

Some inner roller electrodes 22 of the disclosed design, have operated in-the-field to weld in excess of 34 mill ion cans, on conventional electric resistance welders.