JPS5843192B2 - spot welding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in the spot welding or lap resistance welding process.

In spot welding, scattering tends to occur between the plates to be welded.

As shown in Fig. 1, the plates 2, 2' to be welded are
1, when a welding current as shown in Fig. 2a is applied under pressure, a molten part, that is, a nugget 4 is generated at the resistance contact surface between the plates 2 and 2', and the thermal expansion there is absorbed by the unmelted part (as shown in Fig. 1). It is generally understood that the break-up 3 occurs because the part indicated by A) is no longer supported.

That is, it is considered that an excessive welding current after the peak of the inter-electrode voltage has passed (that is, after nuggets start to form) is the cause of the occurrence of chipping.

When such scattering occurs, the tensile shear strength of the welded portion decreases and large variations occur.

An object of the present invention is to eliminate the above-mentioned scattering in the conventional method.

That is, according to the present invention, before the voltage between the electrodes passes the peak, that is, while the contact resistance continues, a higher current (a current value higher than the scattering generation limit) is applied as an initial current, and when the passing of the voltage peak is detected, the scattering occurs. The current is lowered to a lower current below the generation limit, that is, the nugget generation current value, thereby gradually saturating the nuggets and performing welding with stable quality without causing expulsion.

By this method, a galvanized steel plate with a thickness of 10 mm is held with the plated surfaces aligned and a pressure of 225 kg is applied between the electrode tips, and as shown in Figure 2b, an initial current value of 10,000 A is first applied, and from the fourth cycle a nugget generation current of 880 OA is generated. When the current was applied for up to 10 cycles, the tensile shear strength was concentrated in a small range around 66 kg, as shown by curve B in FIG.

On the other hand, when the conventional method shown in Fig. 2a is applied to the same specimen, that is, when a welding current of 100OOA is applied for 10 cycles, expulsion occurs at point a (i.e., at the 4th cycle), and the resulting weld is As shown by curve A in FIG. 3, the tensile shear strength was widely distributed around 63 kg.

In the above example, the inter-electrode voltage was detected, and since it had passed its peak, the current was lowered to detect the initial stage of nugget formation.However, this current can be switched as desired to reduce the current smoothly in consideration of the load balance on the welding equipment. You can also do that.

In addition, there are other methods that do not rely on detecting the interelectrode voltage, such as a method of detecting the nugget formation process using an expansion monitor (monitoring the phenomenon in which the electrode is pushed up due to the volumetric expansion of the welded part during spot welding), an ultrasonic monitor ( A method that uses an ultrasonic transducer attached to an electrode to oscillate ultrasonic waves and monitors the amount of transmission that passes through the joint), or an acoustic emission method (
It is also possible to determine the current switching timing by using a method using a monitor that uses acoustic emission signals accompanying melting and solidification during welding.

Although the present invention has been described above in detail with reference to its preferred embodiment, it is to be understood that the present invention is not limited to this specific embodiment, and that many changes and modifications can be made without departing from the spirit of the present invention. .

[Brief explanation of drawings]

Fig. 1 is a model diagram explaining nugget formation and chipping occurrence in spot welding, Fig. 2a is a graph showing the relationship between energization time and energization current according to the conventional method, Fig. 2b is a similar graph according to the present invention, and Fig. 3 The figure is a graph showing the tensile shear strength of the conventional method and the method of the present invention. 1... Electrode tip, 2. z・・・・・・Steel plate,
3... Nakasaki, 4... Nuggets.

Claims (1)

[Claims] 1. A spot welding method characterized in that an initial current, which is passed as a welding current between two plate members to be welded and is above the spatter generation limit, is reduced to a current value below the spatter generation limit after nuggets are generated.