Electric heating – Metal heating – For bonding with pressure
Reissue Patent
2000-08-11
2002-07-23
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
For bonding with pressure
C219S092000, C219S094000, C219S117100
Reissue Patent
active
RE037799
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an automatic assembling system of a galvanized steel sheet by spot welding where, in the field of automobiles and household electric appliances, spot welding is performed between the bonding or faying surfaces of members comprising at least two molded steel sheets through a galvanized layer to assemble these members as a structure.
2. Background Information
Galvanized steel sheets are being increasingly used in the field of automobiles and household electric appliances. However, problems of deterioration of the welding electrode of a welding machine have been occurring. Spot welding of a galvanized steel sheet requires a high weld current value and a long welding time (current conducting time), as compared to spot welding of a bare (non-coated) steel sheet, and consequently, deterioration of the welding electrode, that is, deformation of the welding electrode or formation of an alloy with zinc is promoted. If the welding electrode is deteriorated, the resulting weld will be unstable and, finally, a nugget will not be obtained. Accordingly, the welding electrode must be frequently replaced and the operating efficiency of the production line is reduced.
A small electric resistance value between bonding surfaces is considered to be a main cause for an early deterioration of the welding electrode. Noting this point, the inventors have recently proposed to interpose a resistance increasing material between the bonding surfaces of galvanized steel sheets which are to be jointed. Thus, the electric resistance between the bonding surfaces is increased to effect spot welding (See Japanese Unexamined Patent Publication Nos. 64-62284 and 64-62286. Japanese Patent Publication No. 5-85269 and U.S. Pat. Nos. 4,922,075 and 5,075,531). The result obtained by this attempt was an improved weld performance. That is, this welding method has the advantages that (1) the cost of electric power, (2) the chance of explosion, and (3) the welding stain are reduced, (4) marking is small, and (5) no burr occurs due to less consumption of energy in bonding. Also, with this method, melting of each welding electrode is significantly reduced, and thus smaller-sized spot welding machines can be used.
On the one hand, one problem with the spot welding of a galvanized steel sheet is that quality control is difficult. At present, an apparatus for monitoring a weld current and a voltage or resistance between electrodes is employed in quality control. Also, some nondestructive test methods have been proposed for welded portions. Furthermore, a peel test using a chisel has been widely carried out.
On the other hand, weld-bonding using bonding and welding together has recently attracted attention for the assembling of automobiles, and the range of its application is expanding gradually. While conventional spot welding results in point bonding, weld-bonding results in surface bonding. For this reason, weld-bonding can enhance bonding strength and rigidity and is effective in the weight reduction of the body of an automobile. Furthermore, the weld bonding method has advantages in that it results in an excellent vibration-impact characteristic, noise is reduced, and sealing performance is assured.
However, in the spot welding method where a resistance increasing material is interposed between bonding surfaces, apart from an experimental implementation or a case where the number of strike points or spots of welding is relatively small, when various kinds of galvanized steel sheets are used and parts with a complicated shape are welded many times for a short period of time, as in the case of automobiles, the method may be inefficient and unproductive.
More particularly, it is necessary in the aforementioned welding method that a resistance increasing material be accurately arranged at a fixed position on each bonding surface and also, just adjacent to the resistance increasing material, steel sheets electrically conduct current across the electrodes. In this case, however, the placement of the resistance increasing material cannot be confirmed from the outside. Furthermore, it is difficult in this welding method to maintain a good contact state between bonding surfaces because of the existence of the resistance increasing material. Particularly when the welding electrode is deteriorated, the current of the electrode tends to be unstable. There is the possibility that any of these disadvantages will lead to a reduction in productivity, and in spite of the remarkable improvement in welding performance, it was difficult to put this welding method to practical use in a mass production system.
Incidentally, with respect to quality control, spot welding is widely used in the assembly of automobile bodies, and it may be said that the quality of the automobile body is determined by whether the spot welding is good or bad. For example, the automobile body is constituted by 600 to 800 parts and most of them are assembled by spot welding. The number of welding strike points or spots reaches 3000 to 5000 per automobile. And, for various reasons, it is difficult to avoid occurrence of a welding defect in the manufacturing process. While the shape, marking, spatter, cracks, pit, and the like of a nugget are prescribed in judging the quality of the welded section of a galvanized steel sheet, in practice, it is considered most important to assure a proper nugget diameter. If the nugget diameter is insufficient, it will cause deterioration of an electrode or cable, reduction in the electric current value due to a fluctuation of the welding power supply voltage, and a misalignment between bonding surfaces. The fluctuation of the power supply voltage results from the simultaneous use of a plurality of spot welding machines, power use of other factories, and a difference between available day time and night time power supplies. And, in a galvanized steel sheet, the range of suitable electric current for spot welding is narrow, and a nugget may not be properly formed depending upon whether there is a variance in the electric current value. Therefore, there are good reasons why quality control is considered particularly important in the spot welding of galvanized steel sheets.
In the conventional monitoring apparatus described above, reliability is poor with respect to galvanized steel sheets, unlike the case of bare steel sheets, and consequently, there are many cases where welding lines are stopped due to problems. For this reason, the conventional monitoring apparatus is insufficient as far as improving the length of time a production line be continuously operated unmanned. In addition, in the aforementioned peel test method using a chisel, a sampling test is conducted, and if a defect is found, measures will be taken to check all previous products and carry out the spot welding again. Consequently, the labor costs of the test and the costs of abandoning the defective products have been excessive.
Under such circumstances, it is desirable that quality be guaranteed within a process, and the development of a monitor which checks all welded sections while they are welded is in demand. Furthermore, even in the case of the conventional weld bonding method, the aforementioned troubles resulting from deterioration of an electrode remain as they are.
Accordingly, it is a first objective of the present invention to improve the spot welding of a galvanized steel sheet using a resistance increasing material and to maintain excellent welding performance and high productivity under a mass production system.
It is a second objective of the present invention to solve problems in the quality control of welded sections and to overcome troubles associated with the quality of welding in advance under a mass production system. An in-process quality test is performed by checking all welded sections at the same time they are welded and also the troubles associated with the quality of welding are monitored in advance.
It is a third objective of the present invention, under a
Sofue Tadashi
Watanabe Toichi
Morgan & Finnegan , LLP
Shaw Clifford C.
Watanabe Toichi
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