GB2160460A - Method of preparing duplex-spot welded joints, especially of producing high-melt rod-foil-rod type current conductors press-sealed into hard glass or quartz glass. - Google Patents

Abstract

A process for preparing duplex-spot resistance-welded joints especially of high-melt, rod-foil-rod type (1,2,6,) current conductors press-sealed into hard glass or quartz glass. A duplex-spot joint (7) is made in a single operation by using a tubular electrode (8), at least on one side. By sensing the infrared radiation of the foil section lying between the two welded spots by means of a suitable photodetector (9), the rejection of unsatisfactory welds regulation of the welding process are rendered possible. <IMAGE>

Description

SPECIFICATION Method of preparing duplex-spot welded joints, especially of producing high-melt rod-foil-rod type current conductors press-sealed into hard glass or quartz glass The present invention relates to a method of preparing duplex-spot welded joints, which is intended especially for use in producing, by means of resistance spot welding, high-melt rod-foil-rod type current conductors press-sealed into hard glass or quartz glass.

Vacuum-tight inclusion of high-melt current conductors made of molybdenum or tungsten, presssealed into quartz glass or hard glass, is carried out by the generally known method of using edged molybdenum foils. Wires are attached to the edges of molybdenum foils in some electrically conductive way, usually by means of resistance spot welding.

The intricate structural configuration of the conductive path causes serious difficulties in joining the base materials which lend themselves to welding anyway only under specified conditions. The great dimensional differences between parts to be bonded together by welding/wires of 0.4 to 1 mm diameter are to be joined with 0.02 to 0.03 mm thick foils/and the contact surfaces being undefined/contact areas built up along the generatrix of conductor rods tends to spread in their width during the welding process/are of the main concern.A further problem is caused by the considerable difference between thermal coefficients of expansion of molybdenum and tungsten, giving rise to high mechanical stresses in the compressed sections of press-sealed joints in normal service of the lamp, mainly during switching-on and -off operations, leading to fracture of the foil having become brittle due to recrystallization taking place in the zone affected by the thermal effect of welding.

A known method of preventing recrystallization within the thermal zone is to interpose between wire and foil pieces of some suitable metal/e.g. Pt, Ta, etc./, whereby the temperature required for making satisfactory welded joints is considerably reduced, overheating of the foil in the thermal zone is prevented, and recrystallization is avoided. Good results can be obtained by adding suitable alloying elements for raising the temperature of recrystallization.

According to another method used in the manufacture of long-life light sources, pre-glassed conductors are employed within the press-sealed section instead of using rod-foil combinations. The glass/es/ applied for bonding ensure/s/ sealings between quartz glass and high-melt current conductors, without harmful residual stresses.

Drawback of the above methods in excess work associated with the application of glass coatings or fill pieces, apart from considerably increasing the material costs.

Particular object of our invention is to provide a mass-production welding method applicable mainly for manufacturing rod/foil type current-carrying fittings press-sealed into glass, whereby high-quality welded joints can be obtained for lamps of improved reliability, without interposing extra metal piece & coatings or foils, to the foil or without pre-glassing the current conductors.

Because of the unfavourable load-carrying properties of traditional - seam-like - spot welds, modification of the seam geometry is justified, since with linear seam shapes the enhenced introduction of heat required for fusing high-melt metals, such as molybdenum and such tungsten leads to recrystallization within the zone of thermal effects developing during the welding process.

When preparing large-size spot welds the thin foil tends to recrystallize within its entire cross-section, whereby cracks are formed along the grain boundaries, rendering any further processing impossible.

Spot weld shapes exhibiting favourable properties both statically and dynamically can be determined by investigating the stresses arising under a given load; since the ratio of tensile stresses to shearing stresses permissible in a molybdenum foil is equal to the ratio of the respective moduli of elasticity, the loadability under a combined shear ingltensile stress may be almost three-times higher than that caused by pure shear'values of elasticity modulus for molybdenum being E = 3.4 x 105 Ni mm2 and G = 1.28 x 105 Nlmm21. This means that by increasing the longitudinal dimension of a seam mainly the risk of recrystallization will become higher, due to the increased rate of heat input required for the welding, while the static loadability of the joint will hardly change, whereas its dynamic loadability will decrease considerably, as a result of restrained thermal expansion.

The consideration outlined above may be proved from many sides through technological experiments and tensile tests, on the basis of which the introduction of joints prepared by the method of duplex-spot welding appears to be preferable for obtaining cross-sections of spot-welds required for current-carrying welded joints. The dynamic and static loadability of such joints is very good; the shape of the spot weld is ideal - its cross-section being very close to circular - and the length of seam sections in shear and tension will be doubled, whereby the permissible load per unit length of seam will reach very favourable values. Strength of the seam will approach that of the base material, because this way the occurrence of recrystallization can safely be avoided.

According to an arrangement proposed in the German Pat. No. DE-26 04 696, for the duplex-spot welding of wires to foils, the foil is incised crosswise, and the wire is passed through the incised slot so that one part of the wires lies over the foil and the other under it, then the wire is joined to the foil by resistance welding. Thus, one spot-weld will form at the lower spot-welding electrode, which the other at the upper one, between the respective current-carrying wire and the foil.

Drawback of this method is the weakening of the foil by the incision and the reduced load-carrying capacity of the suplex-spot weld resulting from the two spot-welds being located in different planes with respect to the foil. A further problem is associated with the difficulty of making the current-carrying rod pass through the incised slot of the foil under conditions of mass production, as well as that no perfect symmetry of welded parts can be achieved, since their exact positioning along the foil is rather intricate.

In the Swiss-Patent Specification No. CH 545 163 a contact-free monitoring system is provided for checking the formation of the spot weld in the course of the resistance welding process, by utilizing the signal issued by the generally known infrared radiation detector/phototransistor/: during the welding process, attainment of welding temperature is observed by measuring the intensity of infrared radiation in immediate vicinity of the welding electrode. The signal proportional to the superficial temperature radiation of the weld is required to reach a predetermined minimum to obtain "go" qualification.

This method has the drawback of requiring precise colour filtering by its optical system, beside the necessity of solving the tasks of eliminating a number of other interfering signals and noises.

The object of the present invention is to improve the method of duplex-spot welding by eliminating the above-mentioned drawbacks of the known methods and to improve the conditions of quality control during the production process.

According to the proposed duplex-spot welding method, beside automatic feeding of the foil, the latter is welded to one side of the current-carrying rod by means of a specially designed resistancewelding electrode.

In summing up, the invention relates to a resistance-welding method especially for making duplex-spot welded joints of foil-to-rod type, current conductors press-sealed into hard glass or quartz glass, the process being characterized in that the duplex-spot welded joint is produced in a single operation by using a tubular electrode at least on one side of the foil.

In an advantageous embodiment of the method according to the invention for use in the technology lamp production the bore diameter of the tubular electrode intended carrying out resistance welding is equal to 0.5 to 1.5-times the diameter of a current inlet or a discharge electrode or a filament of the lamp.

According to another advantageous embodiment of the proposed method a very important feature is the utilization - for indirect checking of the quality of seam - of the superficial temperature radiation observed over the section of surface lying between the duplex-spot welded joints by applying a test pulse/reheating/ after the welding process, and employing a photodetector accommodated in the bore of the tubular electrode for the purpose of indication.

The invention will be described in more detail by way of example and with reference to a preferred embodiment illustrated in the attached drawing, wherein: Figure 1 is a side view of a current-carrying conductor press-sealed into quartz glass, Figure 2 is a partial top view of a halogen floodlight lamp showing one end portion of the presssealed part and Figure 3 shows an implementation of a duplexspot current-carrying joint, illustrated in Figure 2, by means of the process according to the invention.

In Figure 1 the current-carrying fitting presssealed in flattened quartz glass of a high-pressure glow discharge lamp is shown, where a lead-in wire 1 and a discharge electrode 3 are joined to the two ends of a foil 2 at welded spots 5. Vacuum-tight joint along the section situated between welded spots 5 of the foil 2 is ensured by a quartzglass press-seal 4.

As it is shown in Figure 2 the lead-in wire 1 and a coiled filament 6 are joined to the foil 2 by duplex-spot welds 7. The portion of the seating area of the current-carrying lead-in wire 1 and the coiled filament 6 on foil 2 is not increased by the duplexspot weld 7, since the required cross-sectional area of the joint can be provided with respect both to mechanical strength and current-carrying capacity, as a result of the favourable geometry of the Iclose to circular shape of the weld. In fact, the duplexspot weld 7 is made in a single operation, so that no shunting effect occurs which would otherwise impose a limit on reducing the distance between the spots.

Figure 3 shows the duplex-spot resistance welding process developed according to the invention for the purpose of duplex-spot current-conducting joints performed by the resistance welding electrode.

During the resistance welding process, thermal energy develops along the contact resistances defined by the thrust acting on the electrodes. This thrust must be equal at both spots of welding to obtain equal strength of the two welded joints.

This can be ensured by the symmetry of a tubular electrode 8 permitting, at the same time, the achievement of approximately circular weld shapes exhibiting favourable strength characteristics. By ensuring good self-aligning properties for the tubular electrode 8, and a welding electrode 10, equalsize joints of equally good quality are obtained for both spot welds.

In order to permit the performance of resistance welding of the lead-in wire 1 to the foil 2 at two separate points, the bore diameter of the tubular electrode 8 should be larger than the width of the welded spot 5.

The present invention permits application of a simple checking procedure to observe in an indirect way the extension and bond strength of the welded spot.

By utilizing the possibilities offered by the rodfoil-rod type current-carrying fitting and the advantages of the process developed by us as described in the foregoing, the application of dual-channel resistance spot welding machine has been proposed for preparing current-carrying joints according to the duplex-spot resistance welding method, where, by observing the temperature change tak ing place in the course of reheating after the welding cycle and carried out by detecting the superficial temperature radiation of the foil section lying between the two welded spots,the formation of the proper-size spot weld can be easily verified. If the contact spot is sufficiently large, the contact resistance will become so low that no perceivable heating in the vicinity of the spot weld will be caused by the reheating pulse/lasting no longer than a half cycle/.

The superficial temperature radiation of the foil is detected by a photodetector 9 built into the bore of the tubular electrode 8, and by comparing the result with the experimentally determined signal amplitude level, defective joints can be pinpointed or the technological parameters can be controlled for improving the conditions of producing. By this method the quality of the seam may be eliminated, and it may be ensured that only reliable-quality joints will remain in subsequent phases of production, In the following, a few practical examples will be presented to demonstrate the application of the duplex spot resistance welding process detailed in the description.

Example 1 A lead-in wire 1 of 0.6 mm dia. for a high-pressre discharge lamp was welded to a foil 2 of 22 lim thickness by means of a pair of tubular electrodes 8 of outer diameter 3 mm. Each electrode was provided with a sintered tungsten lining bush of d, = 1 mm bore dia. Welding time was 1 cycle, secondary voltage 2 volts A.C., 50 Hz, electrode thrust 100 N.

No recrystallization was observed in the heated zone of the weld, and with joints 7 made by duplex-spot welding and checked by tensile tests, 40% higher shearing-tensile strength values were obtained than those measured at rupture of samples prepared by traditional welding method.

Example 2 A discharge electrode 3 of 1.2 mm dia. of a highpressure discharge lamp was welded to a foil 2 of 25 Fm thickness by means of a tubular electrode 8 of D = 3 mm outer dia. provided with a sintered tungsten lining of bore diameter df = 1.5 mm and by means of a welding electrode 10 of D = 3 mm diameter.

It was found sufficient to use a tubular electrode 8 as welding electrode on the side of foil 2 only, whereas the welding electrode 10 bearing on the discharge electrode 3 and on the lead-in wire 1 may be a simple roll-ground tungsten pin.

Example 3 A thermal radiation detector/photodiode, phototransistor/9 located in a tubular electrode 8 was for indicating the intensity of thermal radiation prevailing in the vicinity of the foil spot lying in the bore axis of the tubular electrode 8.

The photodiode is sensitive to wavelengths of temperature radiation above 100 C, providing steeply rising photo- currents proportional to temperature radiation There are various generally-known connection schemes suitable for recording signals obtained through amplification of photocurrents.

The photocurrent of an OS 13 type phototransistor was used to detect thermal radiation, and the signal of the pulse/reheatingiproportional to the photocurrent, applied after the welding process, was recorded on the screen of a storing oscilloscope TEKTRONIX 7000. By this method the variation of temperature radiation of the foil section observed under reheating applied after the welding process was recorded in the function of time. The magnitude of signals proportional to the reheating photocurrents pertaining to joints of satisfactory seam strengths was thus determined experimentally. E.g., with the measuring setup used, the output signal proportional to the photocurrent obtainable after a good-quality welding did not exceed the value of 0.2 volt.On this basis, it was obvious to come to the conclusion that good correlation exists between magnitude of the photocurrent signal and the shearing-breaking force measurable on the duplex-spot weld 17.

If the maximum photocurrent value of the test pulsefre-heating/ is specified and the method of scrap sorting based on the measurement of this value is introduced, the reliability of duplex-spot resistance welding method developed according to the invention can be further enhanced.

With the aim of supporting the above, welds were performed on two sets of samples each comprising 20 pieces, where two pieces in each set exceeded the maximum values specified for the photocurrent signals. All welds were subjected to tensile testing, then, excluding those found unsatisfactory, the average and the standard deviation of the shearing-tensile loads were determined.

With elimination of the relatively few unsatisfactory joints/two in each seti, the average of breaking loads was found to increase but slightly!by about 6%/, whereas the standard deviation of shearingtensile loads measured on the joints improved considerably: the corrected empirical standard deviation for the two sets decreased from 5.1 N to 2.56, and from 5.2 N to 2.54, respectively.

It can be stated also from the measuring results that, with good-quality welding, by introducing the selection based on specifying the magnitude of photocurrent signal of the test pulse applied after the welding process, good-quality joints are obtained, utilizing the possibility offered by proposed arrangement of current-carrying fittings. With the duplex-spot joints made by the duplex-spot resistance welding process developed by us, the extension of the seam formed in the vicinity of the duplex-spot joint 7 during the welding process is checked by means of observing through reheating the superficial temperature radiation of the foil section lying in the bore axis of the tubular electrode 8.

From the above description, it should be understood that methods equivalent to those given above will be within the scope of the claimed invention and such methods will depend on the field of application and the given circumstances.

Claims (8)

1. A method of preparing duplex-spot welded joints especially for making rod-foil-rod type joints of current conductors press-sealed into hard glass or quartz glass by means of resistance welding, characterised by producing the duplex-spot joint in a single operation using a tubular electrode at least one one side.

2. A method as claimed in claim 1, wherein the bore diameter of the tubular electrode used for resistance welding is equal to 0.5 to 1.5-times the diameter of a current inlet or a discharge electrode or a coiled filament of a lamp.

3. A method as claimed in claim 1 or 2, including the utilisation - for indirect checking of the quality of seam - of the superficial temperature radiation observed over the section of surface lying between the duplex-spot welded joints by applying a test pulse (reheating) after the welding process, and employing a photodetector accommodated in the bore of the tubular electrode for the purpose of indication.

4. A method as claimed in claim 3, including using the signal of the photodetector as control signal for rejecting unsatisfactory welds.

5. A method as claimed in claim 3 or 4, including using the indication of the photodetector for controlling the welding process.

6. A method according to claim 1 substantially as herein described with reference to and as shown in the accompanying drawing.

7. Current conductors including joints made by the method claimed in any preceding claim.

8. Electric discharge lamps including a lead-in conductor and a discharge electrode connected by the method claimed in any preceding claim.