As one method for joining and bonding together a plurality of metallic plates, resistance welding has been known conventionally, by which a stacked body is formed by stacking the metallic plates, and after the stacked body is pressed and sandwiched between a pair of welding tips, an electric current is conducted between the pair of welding tips, whereby regions in the vicinity of contact surfaces on the metallic plates are melted and fused together. Under certain circumstances, three or more metallic plates are joined by resistance welding. Resistance welding may also be referred to as “spot welding”.
FIG. 53 is a front view of essential features in which there is shown schematically a case in which workpieces 1, 2, which are made from high tensile strength steel having a large thickness and thus exhibit a large electrical resistance, are joined together by spot welding. In this case, after a stacked body 3 has been formed by stacking the two high resistance workpieces 1, 2, the stacked body 3 is sandwiched and pressed between a first welding tip 4 and a second welding tip 5. Furthermore, an electrical current is conducted between the first welding tip 4 and the second welding tip 5, whereupon a region in the vicinity of a contact surface between the high resistance workpieces 1, 2 is heated to thereby create a melted portion 6. Thereafter, the melted portion 6 is solidified into a solid phase referred to as a nugget.
Since the electrical resistance of the high resistance workpieces 1, 2 is high, upon conduction of current therethrough, Joule heating in the vicinity of the contact surface is large. Owing thereto, as shown in FIG. 54, the melted portion 6 grows to a large size in a comparatively short time, and as a result, it is easy for the melted portion 6 to become scattered (spatter occurs easily). Accordingly, in the case that the high resistance workpieces 1, 2 are joined by spot welding, it is necessary for the welding current to be controlled with high precision in order to avoid the occurrence of spatter. However, such a control is not easy to perform. Such a problem also occurs in the case of small thickness high tensile strength steel.
Further, in the case of joining three or more workpieces, without any limitation being placed on the workpieces being made of the same materials and having the same thickness, for example, as shown in FIG. 55, a case also is known in which the thickness of the workpiece (low resistance workpiece 7) at the outermost position is the smallest. The stacked body 8 in FIG. 55 is formed by further stacking on the high resistance workpieces 1, 2 shown in FIGS. 53 and 54 a low resistance workpiece 7 having low electrical resistance and which is made from mild steel.
When spot welding is carried out on the stacked body 8, Joule heating generated in the vicinity of the contact surface between the high resistance workpieces 1, 2 is greater than Joule heating generated in the vicinity of the contact surface between the low resistance workpiece 7 and the high resistance workpiece 2. This is because contact resistance is larger in the vicinity of the contact surface between the high resistance workpieces 1, 2.
Accordingly, in the stacked body 8, at first, a melted portion 9 is formed at the contact surface between the high resistance workpieces 1, 2. In certain cases, as shown in FIG. 56, the melted portion 9 grows significantly before a melted portion is formed at the contact surface between the low resistance workpiece 7 and the high resistance workpiece 2. In such a situation, if current continues to be conducted for forming a melted portion at the contact surface between the low resistance workpiece 7 and the high resistance workpiece 2, melted material becomes scattered from the contact surface between the high resistance workpieces 1, 2, leading to the concern that so-called spatter may occur.
However, if the electrical current is stopped, a sufficient melted portion, and hence a nugget, will not be formed at the contact surface between the low resistance workpiece 7 and the high resistance workpiece 2, and therefore, it is difficult to ensure adequate bonding strength between the low resistance workpiece 7 and the high resistance workpiece 2.
In order that the nugget can be grown sufficiently large in the vicinity of the contact region between the low resistance workpiece 7 and the high resistance workpiece 2, it has been contemplated to increase the current value, whereby Joule heating generated in the low resistance workpiece 7 is increased. However, in this case, the large current also flows in the thick high resistance workpieces 1, 2, and as a result, it is easy for spatter to be induced from the contact surface between the high resistance workpieces 1, 2.
Apart therefrom, it can be considered to lengthen the time at which current is conducted. However, in this case as well, it is not a simple matter to generate a sufficient amount of Joule heating in the low resistance workpiece 7. Further, since the welding time increases, welding efficiency is lowered.
From these viewpoints, according to Japanese Laid-Open Patent Publication No. 2005-262259, when resistance welding is carried out on a stacked body made up from a stack of plural metallic plates on which a thinnest workpiece is arranged on the outermost side thereof, it has been proposed to carry out welding in two stages comprising a first stage, in which the pressure on the stacked body is small whereas a large welding current is applied for a short time period, and a second stage, in which the pressure on the stacked body is set to be greater than that during the first stage, and the current value is set to be lower than the current value of the first stage, and the energization time in which application of current is carried out is set to be longer than that of the first stage.
According to the disclosure of Japanese Laid-Open Patent Publication No. 2005-262259, unnecessary steps are not added, and a spot welded coupling can easily be manufactured having a nugget of adequate size and in which generation of spatter is eliminated.
Further, in Japanese Patent No. 3894545, the present applicants have proposed to set the pressing force of the first welding tip that abuts against a low resistance workpiece to be smaller in comparison with a pressing force of a second welding tip, when spot welding is carried out with respect to a stacked body. In this case, the contact pressure applied from the low resistance workpiece to a high resistance workpiece is small, and as a result, the contact resistance of the contact surface between the low resistance workpiece and the high resistance workpiece increases. Consequently, a sufficient amount of Joule heating is generated at the contact surface. Accordingly, it is possible for a nugget to be grown between the low resistance workpiece and the high resistance workpiece, which has a size substantially the same as that of a nugget formed between two high resistance workpieces, and a stacked body with superior joint strength can be obtained.