GB2227198A - Resistance welding of clad metal sheets - Google Patents

Abstract

The method being constituted welding current is initially applied to the clad metal sheets and is then interrupted so as to allow the heat of the base metal to be transferred to the cladding materials so as to allow the temperature of the cladding materials to be elevated. The supply of current is then resumed to complete the welding. The disclosed device includes welding electrodes for pressing and supplying an electric current to the clad metal sheets; a welding machine circuit for supplying the power to the tips; and a power source control circuit to interrupt the supply. <IMAGE>

Description

RESISTANCE WELDING METHOD FOR CLAD METAL SHEET, AND DEVICE THEREOF The present invention relates to methods and apparatus for resistance welding clad metal sheets. More particularly, the invention relates to such a method and apparatus suitable for a resistance welding clad metal sheets such as thickly electro-plated metal sheets and metal sheets with claddings.

Generally, aluminum, zinc and copper are used as cladding materials for electro-plating or cladding metal sheets in order to improve the corrosion-resistant properties of structural stocks such as steel sheets.

Since metal corrosion is a kind of electro-chemical action, the electrical properties of cladding materials for preventing the corrosion of the base metal sheet is such that their resistance values are lower than that of the base metal.

However, in cases where two clad metal sheets S are to be resistance-welded by contacting under pressure the cladding materials C of the two sheets, as shown in Figure 1, the amounts of the released heat are different for the different material layers upon supplying the electric current through the tips W of a welding machine. This is due to the fact that the resistance values of the base metal P and the cladding material C are different each other. For example, the released heat here is represented by the formula, Q = 0.24 Rx i2 , where Q represents the amount of the released heat, R the resistance value, and i the current.

As can be seen in Figure 2, if the base metal P has a large resistance value, the heat release is correspondingly large, and therefore, the temperature is quickly elevated to reach the melting temperature Tp. The cladding material C, however, having a small resistance value, releases a small amount of heat, and therefore, its rise in temperature is slower than that of base metal P. Consequently, the cladding material C takes longer to reach its melting temperature than the base metal P. Reference signs tp' and tc' in the drawing respectively represent the periods of time which are required for the base metal and the cladding materials to reach their melting temperatures.

The above described trend does not pose any problem if the thickness of the cladding material is very thin, because in that case, the heat from the base metal P is readily conducted into the cladding material to raise its temperature. However, in the case where the electro-plated layer or the cladding material C is somewhat thicker, heat conduction is slower, and it takes longer for the cladding material C to melt than the base metal P, with the result that the welding penetration M is not sufficiently deep as shown in Figure 3. Accordingly, while there appears to be a solid weld, the welded portion can easily come apart upon application of an external force.However, if the welding time is extended until the cladding material C has sufficiently melted in order to overcome the above problem, then the portion of the base metal P which has already been melted suffers severe deformations due to the pressing of the tips W of the welding machine. Particularly, the heat affected zone (HAZ) is expanded, thereby forming a structurally weak section.

The present invention is intended to overcome the above described and other disadvantages of the prior art.

It is therefore an object of the present invention to provide a resistance-welding method and apparatus in which electro-piated metal sheets or clad metal sheets can be welded in a short period of time with a deep welding penetration by making the cladding materials melt before the base metals.

To achieve the above objects, the resistance welding method of the present invention is constituted such that, when a resistance-welding is to be performed on clad metal sheets consisting respectively of a base metal and a cladding material, first the clad metal sheets are pressed and supplied with an electric current, then the source of the electric current is disconnected to allow the temperature of the cladding materials to elevate through the conduction of heat from the base metals. Then, the source of the current is supplied again to complete the welding.

Further, in achieving the above objects, a resistance welding device according to the present invention for resistance-welding clad metal sheets includes tips for pressing the clad metal sheets and for supplying an electric current thereto, a welding circuit for supplying the electric current to the tips, and a power source control circuit which, after initiating the welding by supplying power to the tips, disconnects the power source for a predetermined waiting period. Once this waiting period has elapsed, power is again supplied to complete the welding.

The above objects and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention with reference to the attached drawings in which: Figure 1 is a schematical, partly sectional side view showing a resistance-welding of clad metal sheets; Figure 2 is a graphical illustration showing the temperature rise of the base metal sheets and the cladding materials, obtained from the conventional resistance-welding method and apparatus; Figure 3 is a sectional view of a welded portion of clad metal sheets, in which the resistance welding is completed by the conventional method and apparatus; Figure 4 is a block diagram showing the constitution of a resistance-welding device according to the present invention;; Figure 5 is a block diagram showing an examplary constitution of the power source control circuit of Figure 4; Figure 6 is a graphical illustration showing the temperature rise of the base metals sheets and the cladding materials, which are obtained from a method and device according to the present invention; and Figure 7 is a sectional view of a welded portion of clad metal sheets, in which the resistance welding is completed by a method and device of the present invention.

As shown in Figure 4, the resistance welding device according to the present invention includes tips W for pressing and supplying an electric current to the opposite sides of clad metal sheets S upon receipt of a voltage V; a welding machine circuit for supplying the electric current of a proper voltage to the tips W; and a power source control circuit which, after initiating the supplying of the power source to the welding machine circuit, disconnects the power source after a predetermined period of time, and which, after a second predetermined period of time, resumes the supply of power to complete the welding.

The power source control circuit can be constituted, for example, as shown in Figure 5 to include a pulse generating section CLOCK for generating pulse signals at intervals predetermined by a controller CONTR; a first counting section COUNT1 for turning off a switch section SW by detecting and counting a predetermined number of pulse signals generated from the pulse generating section CLOCK; and a second counting section COUNT2 for turning on the switch section SW by detecting and counting a predetermined number of pulse signals.

The difference between the number of pulse signals counted by the second counting section COUNT2 and the number counted by the first counting section COUNT1 constitutes the waiting period. In this example, quantitatively, the waiting period is represented by the formula, waiting period = difference between the numbers of the pulse signals x pulse interval.

The waiting period should be about 0.01 to 0.5 seconds, preferably 0.1 seconds, and this waiting period should be set differently depending on the material make-up of the clad metal sheets and the thicknesses thereof.

It may be desirable to configure the control system such that the number of pulse signals to be counted respectively by the first counting section COUNT1 and the second counting section COUNT2, and the intervals of the pulses generated by the pulse generating section CLOCK, may be adjusted by the controller CONTR.

The operation of a resistance-welding device of the present invention will now be explained with reference to Figure 6.

First, the clad metal sheets S are arranged between the tips W of the welding machine. Upon supplying of voltage V to the welding machine circuit, the tips W of the welding machine press and supply an electric current to the clad metal sheets S to be welded. Then, the base metals P and the cladding materials C of the clad metal sheet start the heat releasing and their temperatures are elevated governed by the relationship, Q = 0.24 Ri2 (Refer to Figure 6).

Upon supplying the voltage V, the pulse generating section CLOCK(Figures 5) generates pulse signals at preset intervals, and the pulse signals are supplied to the first and second counting sections COUNT1 and COUNT2 which count the pulse signals.

When the first counting section COUNT1 completes the counting of a predetermined number of the pulse signals (that is, when a predetermined period has elapsed after the start of the welding) the first counting section COUNT supplies a power source blocking signal to the switching section SW which is thereby turned off, so that the supply of the power to the tips W of the welding machine is terminated. This stops the releases of heat from the base metals P and the cladding materials C.

Upon reaching temperature T1 as shown in Figure 6, the base metal P transfers its heat during the waiting period A t to the cladding material C, which has been at a temperature T2, until the temperature of the base metal P falls to a temperature T3. As result of the heat transfer from the base metal P, the temperature of the cladding material C is elevated to temperature T3. At this point, the base metal P and the cladding material C are both at the same temperature T3. The equalization of temperatures at point T3 is illustrative only and shows the effect of sufficient conduction of heat from base metals P to the cladding material C. However, if there is sufficient heat transfer, the present invention will not be impeded if the base metal P and cladding material C do not attain the same temperature.

If the second counting section COUNT2, which has been counting the pulse signals from the start of the welding, completes the counting of a predetermined number of pulse signals, it supplies a power supply signal to the switching section SW, upon which the switching section SW is turned on. This results in power being once again supplied to the tips W of the welding machine, and that the base metal P and the cladding material C resume the release of heat.

The cladding material C, which has been at the temperature T3, reaches the melting temperature Tc and begins to melt. At a later point in time, the base metal P reaches its melting temperature Tp and begins to melt. In Figure 6, reference signs tp, tc represent respectively the time at which the base metal and the cladding material begin to melt.

The result of this process is that the cladding materials C, which are to be directly welded to each other, are melted first, and therefore, as shown in Figure 7, not only is the welding penetration M of a sufficient depth, but also the cladding materials C are completely melted at the relevant locations, thereby providing a perfect weldjoining.

Further, whereas conventional methods required an overall length of time tc' to complete the welding which was at least longer than the time required to melt the cladding materials, the present invention requires a shorter time tp which is the period of time required to melt the base metal, notwithstanding that there is a waiting period A t.

According to the resistance-welding method and device of the present invention, the welding penetration occurs to a sufficient depth, and improroved welding is realized through complete melting, with the result that not only is the welding quality improved, but also the welding time is reduced. Thus, the productivity is improved owing to the reduction of the cycle time, and reductions in defect rate can be achieved owing to the reduction of the heat input.

Although the invention has been described in detail above by way of reference to the disclosed embodiments, it should be understood that the invention is not limited to the disclosed embodiments but should be interpreted only in accordance with the claims which follow.

Claims (5)

1. A resistance-welding method for clad metal sheets including a base metal and a cladding material, comprising the steps of: releasing heat by pressing said sheets and supplying an electric current to said clad metal sheets; interrupting the supply of said current to allow heat of said base metal to be transferred to said cladding material to allow the temperature of said cladding materials to be elevated; and resuming the supply of said current to complete the welding.

2. A resistance-welding device for welding clad metal sheets, comprising: tips of welding machine for pressing said metal sheets and supplying electric current to said clad sheets; a welding machine circuit for supplying the power to said tips; and a power source control circuit which first initiates welding by supplying power to said tips, then shuts off said power for a predetermined waiting period, and then, after said period, again supplies the power to complete the welding.

3. The resistance-welding device for welding clad metal sheets as claimed in claim 2, wherein said power source control circuit comprises: a pulse generating section for generating pulse signals at intervals predetermined by a controller; first and second counting sections for receiving said pulse signals; and a switching section which is turned on or off upon receipt of signals generated by said first and second counting sections so as to allow voltage from the power source to be supplied or withheld.

4. A resistance welding device for welding clad metal sheets as hereinbefore described with reference to Figures 4 to 6 of the accompanying drawings.

5. A resistance-welding method for clad metal sheets substantially as hereinbefore described with reference to Figs; 1 and 4 to 7 of the accompanying drawings.