1. Field of the Invention
This invention relates to electrodes for electric arc welding and in particular to a non-fusible electrode having a working tip which has an in situ formed plug centrally disposed in the end face thereof.
2. The Prior Art
Tungsten electrodes are well known and have mainly been used for TIG(tungsten inert gas) arc welding in a gas shield atmosphere utilizing a non-fusible electrode. In order to increase the emission force of the electrode while maintaining a stable arc, it has been known to manufacture such electrodes from a mixture of powdered tungsten and a metal oxide by methods such as sintering. Electrodes of this type may contain up to approximately 2% thorium dioxide and may be referred to as thoriated tungsten electrodes. If the thorium dioxide additive is increased above the 2% level, the emission force of the electrode slowly increases but at the same time the brittleness of the resultant electrode also increases considerably. Apart from the use of thorium dioxide, additives of zirconium, rhenium and other high melting point metals and their oxides are also known. However of the aforementioned additions, thorium dioxide is preferably used inasmuch as it withstands high temperatures and possesses a low tendency to evaporate.
Although such prior art electrodes have desirable characteristics, it has been proven that they do not operate satisfactorily for automatic microfusion welding, that is for the welding of thin metal sheets having a thickness of at least 1 mm. In microfusion welding the welding current strength is less than 12 amperes. Additionally, ignition must take place safely, i.e. in a reliably contactfree fashion in a helium shielded atmosphere over a long period of time. However, a series of experiments has proven that even thoriated tungsten electrodes do not exhibit a precise ignitability and, on the average, they have substantially lost their ignitability after approximately 60 ignitions. More precise observations have shown that, at the beginning of the experiment, the ignition takes place from the tip of the electrode outwards, and that within increasing number of ignitions, a consistently increasing area of the surface of the electrode participates in the ignition. After an average of 60 ignitions (calculated from a widely dispersed range), an "arc-back" takes place from the outer surface of the electrode to the burner sleeve of the welding burner. This eventually results in failure and after further ignition experiments, ignition could no longer be achieved. Although in using thoriated tungsten electrodes, the ignitability may be re-established by regrinding the electrode tip, the drastic decrease in the varying behavior of the ignitability in dependence upon the number of ignitions makes it difficult to use tungsten shield gas welding and plasma processes in mechanized production.
It has been proposed (See German Pat. No. 1,806,856) to improve the ignition reliability of such electrodes by applying a coating layer in the vicinity of the electrode tip, which layer consists of a metal-nonmetal compound having a relatively high melting point such as, for example, metal oxides, metal carbides or metal nitrides, or mixtures of several such substances. With an electrode of this kind, a stable arc may be ignited in an exact sequence, with the electrode exhibiting a relatively high stability. The stability achieved results from the application of a mass composed of powdered thorium dioxide with a binding agent, to the electrode tip. Thereafter the electrode is heated until the thorium dioxide melts. An arc is then ignited at a low current strength. The electrode distance is thereafter enlarged and the current strength further increased, in fact to beyond the normal operative range of the electrode. Thus, the thorium dioxide is fuse-sintered on the end of the electrode. With an electrode of this type, produced by the known process, ignition takes place uniformly from the tip of the electrode outwards.
However, it has been observed, that over a long period of burning of an electrode of this type, an "arc-back" will take place from the shaft of the electrode to the burner in the area adjacent the electrode tip which has been provided with the coating layer. This is due to the fact that the mass which has been applied to the electrode tip by fuse-sintering, encourages the arc to burn from the outer peripheral surface of the electrode, so that following a drop in the emissive capacity of the coating mass, "arc-back" occurs.