Background of the Invention

1. Field of the Invention

This invention relates to arc welding, more specifically to inert gas shielded tungsten arc welding and means for starting arcs in such devices.

2.. Description of Related Art

Tungsten inert gas welding is also known as Heliarc* welding. Metal fusion is created by the heat of the arc formed between a nonconsummable electrode and the workpiece being welded.

A shielding blanket of inert gas such as argon or helium protects the metal surface from oxidation or contamination due to contact with atmospheric oxygen or other reactive gases.

The inert gas shielded power source can be DC or AC. Reverse polarity DC is less commonly used due to arc instability, however, it has the advantage of oxide clea¬ ning action when welding oxidizable metals such as alu- minium or magnesium. AC is more common being a combination of straight and reverse polarity direct current. AC has a half-cycle oxide cleaning action.

AC welding power sources typically do not pro¬ duce a voltage that is high enough to reestablish the arc in an inert atmosphere when the voltage goes through the nodal point of the AC cycle. In order to start the arc, produce a stable arc, and eliminate the troublesome zero point of the AC cycle, a high frequency current is often superimposed across the arc. "Tungsten electrode" is to be understood herein and throughout this application as also referring to alloyed electrodes such as zirconated or thoriated tung¬ sten electrodes.

In gas tungsten arc welding, the tungsten elec¬ trode is typically thoriated or alloyed with Th(_>2 ty¬ pically at 1 or 2% for improved D.C. arc striking charac¬ teristics. Tungsten electrodes are often zirconated for less contamination when working with aluminum.

For ignition, a high frequency, high voltage spark gap oscillator is often used to enable the arc to be ignited without touching down the electrode on the work. This helps prevent electrode contamination. The oscillator typically consists of an iron core transformer with high voltage secondary winding, a compacitor, a spark gap and an air core transformer, one coil of which is in the high voltage circuit and the other in the welding circuit. The compacitor is charged every half cycle to 3000-5000 V and discharges across the spark gap. The discharge is oscillatory. The spark discharged is phased to occur at the beginning of each 10 millesecond half cycle. To initiate the arc, the electrode is brought to within 6 mm of the work with the high frequency unit and welding current switched on. Groups of sparks pass across the gap ionizing it and the welding current flows in the form of an arc without contamination of the electrode by touching down, unfortunately, considerable radio and TV interference results including potentially damaging cur- rent induction in associated delicate circuits and chips. Gas tungsten arc welding systems could become more sophisticated if additional data monitoring, re¬ cording, and analyzing systems and instruments could be added to the basic welding system. A long standing problem preventing the enhancement with sophisicated instruments of basic welding systems, particularly gas tungsten arc welding systems in that the use of a high frequency current source or a capacitor discharge source for initiating the gas tungsten arc radiates energy that is potentially destructive of sensitive circuits and can cause losses of

electronically recorded test data often destroying long period test specimens.

In order for welding oriented computer appli¬ cations to increase, it is becoming increasingly neces- sary to develop methods for making the welding equipment compatible with the computer and computer controlled equipment. Such an environment presents high current transients from which computers must be protected.

The operation of gas tungsten arc welding at arc starting and systems requiring continuous high frequency for arc stabilization produce interference in communi¬ cations involving telephone, radio, and television. In¬ terference problems during arc starting affect computers and other related electrical systems. These interference problems occur with both mechanized and manual gas tun¬ gsten arc welding systems.

Electrolytic compacitors in series with the welding circuit have been able to reduce needs for high frequency currents to uses for starting only, however have not been able to eliminate the need.

It is an object of the present invention to disclose an apparatus overcoming the delicate circuit, primarily computer, interference problems that result from the high frequency current used to initiate gas tungsten arc welding.

It is an object of the present invention to disclose an alternative in place of high frequency ig¬ nition systems for inert gas tungsten arc welding.

Brief Description of the Drawin

Fig. 1 depicts a laser-started inert gas tung¬ sten welder according to the invention.

Figs. 2 and 3 present a graph of the laser spot diameter plotted against the energy in Joules transferred to the tungsten electrode.

Description of the Invention The present invention discloses an improved inert gas shielded tungsten arc welder which has an arc starter that does not produce interference with assoc¬ iated delicate electronics such as chips and micropro¬ cessors. The present invention is an improvement over gas tungsten arc welders using a high frequency current source or a compacitor discharge source for initiating the weld¬ ing arc.

The present invention comprises a gas tungsten arc welder having a low output laser to ignite the arc. Arcing is initiated by heating the negative polarity tun¬ gsten electrode to produce a hot spot which supplies enough electrons to permit the formation of a thermionic arc across the gap between the electrode and workpiece. The laser is focused upon the electrode, ad- vantageously near the tip. The laser is turned on when the electrode is energized so as to aid the formation of the thermionic arc without need for striking with the elec¬ trode of the workpiece surface. Preferably the laser is pulsed. Measured per second, the laser should have an output of 50-60 watts, though, larger output lasers can be used with shorter "on" times. The more important aspect is that the spot on the electrode struck by the laser is heated to approximately 1000°K. Appropriate lasers to accomplish this requirement can readily be selected by those skilled in the art. This hot spot sufficiently ionizes the immediate air gap and supplies sufficient electrons from the surface at the finite open circuit voltage enabling the formation of a thermionic arc.

The arc usually starts at the shank of the negative electrode and immediately transfers to the tip. The arc positions itself in a manner producing the minimum arc voltage. Looking now at Fig. 1, gas shielded electrode 2 is positioned near workpiece 1. An inert shielding gas such as argon is fed from source or cylinder 7 via gas transport tube 7A. The gas is routed around the electrode tip 2A. Water cooling is optionally provided from water source 6 through inlet tubing 6A and return tubing 5A to water return reservoir 5. The water return reservoir if large enough to dissipate acquired heat during operation can also function as the water source.

The electrode control station 10 with on-off buttons illustrated is connected to welding power supply 9 energizes electrode 2 when the on switch is engaged at control station 10. Laser 8 is simultaneously turned on. The laser beam is guided by optical fiber 4 to a point near electrode tip 2A. The optical fiber is held in place by two retaining clips. The laser beam heats a spot on the electrode causing thermionic arcing to be initiated. The laser which can be pulsed or continuous can be timed to turn off after a few fractions of a second of on time. Alternatively, circuitry can be provided to turn off the arc following starting in response to the voltage drop across the gap between the electrode. Such voltage responsive circuitry for example is illustrated in U.S. Patent 4,559,206 to Treharne. Other voltage response circuit variations are known to those skilled in the art.

Example

A 650 watt source continuous laser was directed at a tungsten electrode of an inert gas tungsten welder. The laser was operated for .25 to .50 seconds and instantly initiated an arc. The 650 watt emitted pulse of 0.25 to

0.50 seconds conveyed a laser energy of from 187 to 325

Joules to the electrode.

A minimum laser pulse or heating time (i.e. on time) of 0.1 seconds appears needed to initiate arcing 05 with a laser initiator. This threshhold time is also associated with the time for cumulative ionization, and related to the time constants for electrical circuits.

Laser bursts can reduce the on time but our experiments have shown .005 seconds to be too short of a time to heat 10. the electrode surface.

To ascertain the functional range of initiating an arc with a laser, the laser spot size was varied and the conditions of fusing were measured (See Figures 1 and 2).

By using empirical data together with heat

15 transfer solutions, a table was prepared relating laser pulse magnitudes and pulse length to achieve different temperatures over different areas.

Thus, by doing "just" melting experiments, we were able to relate energy from the laser to that absorbed 20 in the tungsten.

To scale to temperature of interest, we used equation 7.1.5 and Figure 7.7 from ϋCRL-5263, "Conduction

Heat Transfer Solutions" by James H. Van Sant, published by Lawrence Livermore. 25 For surface temperatures at x = 0, the equations reduced to:

(t - tj)K __ 2

From this we concluded that the temperature

30 reached is proportional to the square root of the pulse time and the temperature needed is directly proportional to the pulse height. We extrapolate back to when melting just started. This is at about 45 Joules.

It is seen from Fig.2 that melting just started at a computed 47.5 Joules with a fused diameter of .026 inches. The Joules were computed from the electrical condenser storage capacity and the voltage applied to it. While this is not the actual laser energy emitted from the laser it provides a useful estimate even though the energy from the laser is not all absorbed in the tungsten and under certain conditions 20 to 70 percent may be reflected or reradiated depending on specific pulse height, spec- tral conditions, and surface temperature. The "just melted" spot diamter from Fig. 3 is .021 inches at 47.5 Joules. The laser spot size was set for maximum for Fig. 3 and minimum or Fig. 2. Thus, a "just melted" point to use for an initiation appears to be with a laser spot of .020 inches diameter or .5 ms. The area is .197 mm ² for which the energy stored for the laser charging was comput¬ ed to be 47.5 Joules. This works out to be 241 Joules for laser charging per square mm of "just melted" surface. By using this emperical data together with the heat transfer solution of Van Sant we were able to relate energy from the laser to that absorbed in the tungsten.

To estimate laser charging magnitude and pulse lengths to achieve surface temperatures of interest, the pulse necessary to achieve a given temperature rise is inversely proportional to the square root of the pulse length and temperature achieved for a given pulse length is proportional to the energy of the pulse. The pulses are assumed to be square waves.

Computed Charging Energy (Joules) to Laser/mm ² Heated Thoriated Tungsten Area for Various Pulse Lengths in Milliseconds

Maximum Pulse Length Milliseconds

Surface Temp°C 5 20 80 320

3410 241 120 60 15

2500 171 85 42. 5 10.6

2000 132 66 33 8.3

1500 94 47 33. .5 5.8

1000 55 22.5 11. .25 2.4

Achieving a surface temperature of at least 1000°K appeared to be a minimal for arc initiation.

The principles, preferred embodiments, and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms dis¬ closed, since these are to be regarded as illustrative rather than restrictive variations and changes can be made by those skilled in the art without departing from the spirit and scope of the invention.