This invention relates to resistance welding machines in general, and specifically to capacitive discharge resistance welders.
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 xe2x80x9cthermal runawayxe2x80x9d 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 xc2xd 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 usage. In fact, mains current requirements may preclude the use of many prior art welding machines.
Accordingly, it is an object of the invention to provide an improved resistance welding apparatus.
Another object is to provide an improved welding apparatus characterized by relatively small size, weight power consumption and cost.
Yet another object is to provide an improved welding apparatus characterized by high efficiency.
Yet another object of this invention is to provide an improved welding apparatus that will weld relatively long continuous perimeters with low current demand from power mains.
The invention is a resistance welding apparatus that supplies essentially high current, constant voltage pulses to very low impedance weld structures. The high current pulses of substantially constant voltage are provided with substantially smaller, less massive, less expensive equipment requiring less energy storage for equivalent welds, than the prior art devices.
The thermal runaway condition that is caused by the use of constant current sources of the prior art is largely mitigated by the use of welding power supplies of the present invention that behave more like constant voltage sources. When a low impedance voltage source is used for welding, the magnitude of the weld current is modulated by the dynamically changing resistance of the weld to change the magnitude of the weld current, thereby minimizing the occurrence of particle expulsion. Expulsion can also be exacerbated by the use of shorter than conventional current rise times.
Moreover, with the invention, the low impedance voltage pulses can be selectively varied in duration. When used in conjunction with a dynamically applied force to the weldment, the particle expulsion effect encountered by prior art systems is eliminated by selecting an appropriate pulse width for the welding application. Also some weld configurations require longer pulse times to reach the required thermal distribution.
The welding apparatus according to the invention, provides substantially higher output current per unit weight and or per dollar cost, than prior art devices. Further the invention effects substantial reduction in bulk and cost for its power supply. Further, the invention permits usage in locations where current mains are insufficient to power prior art devices. As another feature the invention is that it provides a means for high current welding which does not require penetration of a high current welding bus through the wall of an environmental chamber.
The invention describes means applicable to portable welders. Such welders are required for the assembly of motor vehicles, ships, aircraft, and the like. The features which provide the high efficiency of the welding apparatus of the invention are particularly useful for welders intended for these industries. Handheld units may be made considerably smaller and lighter than conventional portable welders. Welders that are positioned by robots are generally much lighter and cost less.
The welding apparatus of the present invention in one form utilizes energy from ordinary low current mains, 120 volt, typically 15 ampere power sources. The apparatus stores the energy in one or more banks of capacitors, each connected to a switch for power control, and producing therefrom, low voltage, current pulses of adjustable width and amplitudes between 1000 amperes and 5,000,000 amperes. Associated with the invention is a welding head and integral pulse transformer, power supply, energy storage, energy discharge means, and mechanical force and guidance means. In a preferred form, the apparatus includes consists of a weld head with an integral pulse transformer, and an associated driver for the pulse transformer. The transformer portion of the apparatus includes a core of magnetically permeable material surrounded by a relatively wide set of two or more alternating insulated layers of high conductivity material which constitute primary and secondary conductive windings respectively. The relatively high conductance secondary layer immediately adjacent to said core combined with a low reactance secondary circuit acts as a shield to the core for the magnetic field generated by the peripheral primary winding.
In operation, immediately prior to a main discharge pulse, the core is magnetically saturated in the direction opposite (xe2x80x9creverse flux saturatedxe2x80x9d) to that produced by the primary winding voltage. The combination of low inductance and reverse flux saturation reduces the required weight of the transformer to a level typically ten times lower than would otherwise be required in accordance with the prior art. In contrast, prior art devices allow the magnetic field of the core to return to its remnant state. The apparatus of the invention reverses the flux state of the core to be at or near reverse saturation. One or more low inductance bus configuration output conductors corresponding to the number of paired primary and secondary windings are used to deliver current through the welding head to the parts-to-be-joined. A high efficiency power supply is used to minimize the cost, weight, and power usage of the apparatus. In a preferred form, the apparatus controls the force applied to the parts-to-be-joined before, during, and after the high current welding pulse is applied.
In a preferred form of the invention, the transformer and welding head functions are performed in combination in a single unit, providing a size reduction and increase in efficiency compared to devices of the prior art. This combination of functionality eliminates losses incurred by transferring the welding current from a transformer to the load or part being welded. In prior art welders, in contrast, the electrical conductors that connect the welding head to the transformer cause resistive losses, and reactive storage of energy in the fields surrounding the conductors decreases the available energy to transfer to the weld joint.