Electric heating – Metal heating – For bonding with pressure
Reexamination Certificate
2002-05-31
2004-06-29
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S116000, C307S419000
Reexamination Certificate
active
06756558
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to resistance welding machines in general, and specifically to capacitive discharge resistance welders.
BACKGROUND OF THE INVENTION
In resistance welding, fusion of two or more electrically conductive materials is effected by resistance heating caused by the passage of high current pulses through the conductive members being joined. The temperature rise effecting the weld is substantially proportional to the current squared times the resistance in the weld region, but is somewhat increased by the duration of the applied current.
Prior art resistance welding equipment has been built using principles developed by Fruengel. See High Speed Pulse Technology Vol. 1 by Frank Fruengel, Academic Press 1965. Transformed Capacitor Discharge in Welding pp345-347. In volume 3, pp310-311, Fruengel describes the resistance welding equipment that was available as of the publication date.
Another form of prior art welder utilizes transformed mains power to provide weld current pulses thereby eliminating the requirement for energy storage. The limitations of this technology can be illustrated by the realization that if 100,000 amperes at 5 volts is required to make a weld, then at 100 percent efficiency, a 220 volt main would be required to supply over 2000 amperes of current. The use of 3 phase mains can reduce the current requirement, however, direct transformation machines that are capable of producing more than 100,000 amperes of weld current are very expensive, massive, and require very large mains capacity.
Weldments produced by resistance welding find applications in many fields including the hermetic sealing of optoelectronic components, semiconductor and hybrid circuit packages, packaging of micro electro mechanical systems (MEMS), surface acoustic wave devices (SAW), hermetic feed-thrus, diaphragms for transducers, rupture discs, automotive, aircraft, and the like, or other applications requiring a continuous weld. Similar power supplies to those used in resistance welding systems are used in both magnetizers for permanent magnets and in devices that magnetically form metallic sheets.
Continuous welds are often required to prevent the passage of fluids and gasses across the weld boundary, in addition to providing required mechanical integrity. For this reason, continuous welds are often used for hermetic sealing. In previous welding equipment, only relatively short weld perimeters could be continuously welded with a single discharge pulse. As weld perimeters exceeded the capability of available equipment to produce satisfactory welds, industry turned to other methods, such as seam welding, in which a series of small overlapping welds provide the required hermeticity. Seam welding is considerably slower than projection welding which provides a continuous weld in a single discharge. Also, the seam welding process dissipates considerably more heat into the part being welded than projection welding, thereby raising its bulk temperature well above what is required with projection welding. In addition, the strength of seam welds tends to be lower than projection welds.
Conventional resistance welding equipment is designed to supply a relatively high welding current into a variety of weldment metals and geometries. The impedance of structures-to-be-joined of may be from a few micro-ohms to over 100 micro-ohms. This variation in weld impedance is caused by the composition of the weld metals, the thickness of the weld metals, and the geometry of the structures-to-be-joined. A prior art welding machine that is designed to supply a constant high weld current to a low impedance (e.g. a 100 micro-ohm) weldment will not perform in an optimum manner. This is because the welding supply acts as a constant current source when welding very low impedance welds.
Although prior art system are capable producing welding currents of 60,000 amperes, such systems typically waste 90% of the energy stored in the capacitors, and encounter both reactive and resistive losses associated with bus bars, transformers, and the weld head. To overcome this waste, relatively large sizes and expenses are associated with these components. Mains power connections are required that are similar to a commercial substation. As a result, the prior art systems require considerable effort to install and move.
In resistance welding, a weldment is always comprised of metal parts which possess a coefficient of resistance that varies with temperature, as well as geometrical, compositional, and surface variations. These factors result in both static and dynamic variations in the resistance of the weldments, and can lead to local overheating during the welding process. The impedance of the weld undergoes dynamic resistance changes caused, in part, by weld current-induced joule heating. Because the coefficient of resistance change with temperature is positive for commonly welded metals, the resistance change during welding is in a positive direction. When welding is performed by a constant current source, as with conventional welders, the dynamically increasing resistance of the weld results in a rapid increase in power dissipation during the weld process. This can result in a local overheating condition which may adversely affect the reliability of the welded components. This so-called “thermal runaway” condition, due to high constant current, cannot occur when using constant voltage or proper impedance source regardless of risetime. This eliminates expulsion of particles from the melting material in the weld region. Expelled particles may cool and become a source of internal contamination which may be injurious to the reliability of the component.
Another limiting factor in conventional welding machines, is the ability to maintain mechanical force on a rapidly melting weld, and the requirement to accelerate the welding electrodes to maintain force on the contours of the melting structures. To do so effectively, it is essential to reduce the mass of the inertial components of the welding machine. Yet, in conventional welders after reducing mass has been accomplished, it is still possible to generate welding current pulses that melt the weldment more rapidly than mechanical components have the ability to maintain force, again resulting in particle expulsion.
In an exemplary conventional pulsed high current welder, a large bank (typically 4 feet high by 3 feet wide by 3 feet deep) of high energy storage capacitors are charged by a power supply requiring a 208-440 volt 20-100 ampere source of alternating voltage. High current electronic switches discharge the energy storage capacitors into a transformer, which can weigh 400 to 2000 pounds or more. The massive copper secondary connects by means of relatively massive, for example 4 inch by ½ inch, copper bus bars which may be up to several feet in length, to connect the output of the transformer to the input of the welding head.
The welding head may be enclosed in a chamber which provides proper environmental gas mixtures. The two large copper bus bars and an insulating spacer must penetrate the wall of the environmental chamber in order to conduct the entire welding current through the chamber wall. The function of the welding head is to conduct the weld current through suitable electrodes to the weldment, to hold the electrodes in proper alignment during the welding process, and to provide proper static and dynamic clamping force to effect the weld.
Thus, in the generation of high current pulses, prior art welding devices require expensive, relatively massive transformers, and large amounts of energy storage (due to the inefficiency of the conversion of energy stored in the capacitors) for power to be delivered to the weld load. In addition, large expensive power supplies are used due to the inefficiency of prior art devices. Those prior art weld power supplies consequently also require high power (heat) dissipation, and access to high current mains. Relatively high expense is incurred for wiring, heat dissipation, and operating energy usa
Becker George
Briefer Dennis K.
Salzer Thomas E.
AKC Patents
Centaur, Inc.
Collins Aliki K.
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