1. Field of the Invention
The present invention relates to a non-consumable arc welding (hereinafter referred to as TIG welding) process. More particularly, the present invention relates to a process for the narrow gap butt welding of aluminum and aluminum alloy plates wherein a filling material is inserted into a welding groove and direct welding current with straight polarity is applied between a non-consumable electrode which does not have a sharp end and the plates to be welded. Further, the welding is performed under a shielding gas of an inert gas comprising at least 50% helium. The welding is accomplished with high efficiency and a sound welding bead having a bead width along the inner portion larger than the bead width along the surface of the bead, can be obtained.
2. Description of the Prior Art
Recently, various high efficiency welding processes have been proposed because of the increasing demand for welding structures from thick plate materials. However, most of these processes adopt the MIG (metal inert gas arc) welding technique in the weld butt joints of I-grooves or narrow V-grooves. In this technique, a small weaving motion within the welding groove, or two run multipass welding or control of the welding current is practically used in order to prevent the formation of defects such as the lack of fusion at both groove faces or at the boundaries of the respective welding passes. If these processes are used, welding defects often occur and the welding apparatus is very complicated. Thus, it has been difficult to practically apply these processes for welding which requires high quality. This is especially true for the welding of aluminum alloys which are widely used as low temperature materials for use in the transportation of LNG (liquefied natural gas) or in storage tanks for LNG. The materials which are welded are thick plates and therefore in order to weld these materials, high welding efficiency is desired in all welding positions and such a welding technique is urgently required.
In the fabrication of welding structures of aluminum alloys, many difficulties are encountered because of the physical and metallurgical properties of aluminum alloys in contrast to the fabrication of welding structure from ferrous materials.
Generally speaking, both TIG welding processes using non-consumable electrodes and MIG welding processes using consumable electrodes can be employed in the welding of aluminum alloys. Although TIG welding is far superior to MIG welding in arc stability and in the soundness of welds, a number of difficulties still remain with the TIG process. For instance, when TIG welding of aluminum alloys is done, an alternating welding current upon which a high frequency current is impressed is used. This is because if direct current is used with the electrode negative, it is impossible to clean the oxide film formed on the plate surface. On the other hand, if the electrode is positive, the electrode is apt to be consumed. Therefore, the welding current must be greatly decreased. If a high alternating current is used, the electrode can also be consumed. Thus, conventional TIG welding must be done with low electric currents and consequently efficient welding with deep penetration has not been attained.
In TIG welding using straight polarity, wherein the electrode is negative, and the plate is of positive polarity, deep penetration can be obtained and very little of the electrode is consumed. However, as mentioned above, cleaning of the oxide film cannot be expected, and consequently, puckering of the beads easily occurs. In attempts to overcome these problems, it has now been discovered that when the electrode has a round or flat end and it is positioned very close to the plates to be welded and excessive distribution of the arc is suppressed while the arc concentration in the central position of the arc column is prevented, and consequently the arc distribution on the plate is kept approximately the same as the cross sectional area of the electrode end, the molten pool is not agitated even if the welding current is high, the melting of the plate proceeds parallel to the arc column and the configuration of the weld metal cross section is such that the depth of penetration is sufficiently large relative to the bead width. Because of this effect, when a straight polarity is used, it is possible to conduct welding with a stable arc free from puckering of the plate and thus obtain deep penetration. Nevertheless, it is difficult to feed deposited metal into a groove having a certain width. Because cleaning of the oxide film normally cannot be expected in the situation when a current of straight polarity is applied, inclusion of the oxide film into the deposited metal and insufficient penetration between the deposited metal and the plates to be welded is apt to occur if the filler wire is fed into the welding region by the conventional TIG welding process. Further, because of the short arc length, it has been difficult to feed filler wire into the crater formed by the welding arc of short length.
On the other hand, the MIG welding process is inferior to the TIG welding process in arc stability and soundness of weld. But the MIG process has the advantage of being a faster welding process than the TIG process. In practice, MIG welding is used. However, it is indeed possible to use a high electric current in MIG welding compared to the currents used in TIG welding, but there is the disadvantage that deep penetration into the plates to be welded cannot be obtained even with high welding currents, especially in vertical welding, overhead welding and horizontal welding because of the particular characteristics of the aluminum alloy. In other words, because the melting point of pure aluminum is about 660.degree. C while the melting point of iron is about 1530.degree. C, and the density of aluminum is one-third that of iron, the amount of deposited metal when aluminum alloys are welded is much larger than the amount of metal deposited in the welding of iron with respect to equal welding currents. The large amount of deposited metal prevents the arc plasma from reaching the plate to be welded and consequently the base plate is not heated enough. Furthermore, the heat conductivity of aluminum is too high, and consequently, the heat input introduced into the welding zone is diffused therefrom which adversely affects the fusion of the base metal. If the welding current is increased, the amount of deposited metal is increased instead of increasing the penetration of the electrode into the plate to be welded, thus causing insufficient penetration and overlapping. Moreover, aluminum itself is a very reactive element, and thus perfect gas shielding is required to complete the welding process.
In view of these problems, the shape which the welding grooves can assume are limited, i.e., the angle of the Vee must be large enough when welding is accomplished by the MIG technique, which inevitably increases the cross-sectional area of the groove, thus requiring an increased amount of deposited metal. When aluminum alloys are welded, a wide weaving action of the electrode results in insufficient penetration into the alloy plate as well as adverse effects on the joint strength of the alloy because of the micro cracks which seem to occur as a result of eutectic melting of the aluminum alloy. Accordingly, a weaving motion of the electrode has been avoided. Accordingly, up to the present, welding has been accomplished as a straight bead or as a narrow bead resulting from limited weaving motion of the electrode. Because of all of the factors enumerated above, a high efficiency welding technique for the welding of thick aluminum and aluminum alloy members has not yet been developed.
As previously described, the conventional TIG and MIG welding processes present numerous difficulties. By reference to FIG. 1 it can be appreciated that in conventional welding the bead width at the surface of the base metal is large, while the bead width steadily narrows as the depths of the groove are reached. On the other hand, as shown in FIG. 2 for the narrow gap welding of aluminum alloys, filling metal is first deposited in the groove and then the groove is subjected to TIG welding. However, in this technique, the lower corners of the groove are not filled with metal and a fully fused junction is not achieved. In order to overcome this defect, the voltage or current can be increased to completely fuse all portions of the welding joint. But under these conditions, an undesirable undercut will form as shown in FIG. 3.
Although TIG welding with a straight polarity is widely known as described in "Welding Journal," May 1971, page 332-341, such TIG welding which is effected after insertion of the filling metal into the narrow groove between very thick plates is not described anywhere.
A technique is described in U.S. Pat. No. 3,825,712 which relates to narrow groove DCSP-TIG welding. In this technique much difficulty is encountered in placing the filling wire under the welding arc as described above especially for all position welding.
A need therefore continues to exist for a method of accomplishing the narrow butt welding of aluminum materials by a technique which overcomes the deficiencies of the conventional methods.