FIELD OF INVENTION:
The invention generally relates to a device and method to carry out resistance spot welding (RSW) of corrugated sheets with plain sheets and to develop a special electrode for spot welding and fixture for holding the job.
BACKGROUND OF INVENTION:
Conventionally, the corrugated thin stainless steel sheets are joined with plain thin stainless steel sheets by gas tungsten are TIG welding process only. These sheets find wide application in power, energy and heavy engineering sectors. Since, the conventional method is less productive and time consuming, it was proposed to develop resistance spot welding process for joining corrugated thin stainless steel sheets with plain thin stainless steel sheets.
Thin sheet industry has relied on RSW as a principal joining method for many decades. Advantages of RSW in high-volume thin sheet manufacturing are as follows.
• Rapid process (< 1 second per weld)
• Ease of automation (flexible process)
• No requirement of welding consumables (i.e. no issues of piece-cost, inventory, additional weight or recycling difficulties)
Resistance Spot Welding (RSW) is widely used in high volume manufacture of sheet metal products. Thin sheet industry carries out on RSW as a principal joining method since decades. Resistance welding is accomplished when current is caused to flow through electrode tips and the separate pieces of metal to be joined. The resistance of the base metal to electrical current flow causes localised heating in the joint, and the weld is made. The resistance spot weld is unique because the actual weld nugget is formed internally in relation to the surface of the base metal. Figure 1 shows a resistance spot weld nugget compared to a Gas Tungsten arc (TIG) spot weld.

3
From Figure 1 it is known that gas tungsten-arc spot is made from one side only. The resistance spot weld is generally made with electrodes on each side of the work piece. Resistance spot welds may be made with the work piece in any position. The resistance spot weld nugget is formed when the interface of the weld joint is heated due to the resistance of the joint surfaces to electrical current flow. The pressure of the electrode tips on the work piece holds the part in close contact during the operation of the weld. Mostly, only flat sheets or sheets with flat surface are welded by resistance spot welding.
Resistance spot welding depends on the resistance of the base metal the amount of current flowing to produce the heat necessary to make the spot weld and time. In most cases several thousand amperes are used in making the spot weld. Such amperage values, flowing through a relatively high resistance, creates abundant heat in a short time. To make good resistance spot welds, it is necessary to have close control of the time the current is flowing. Time is the only controllable variable in most single impulse resistance spot welding applications. Current is very often economically impractical to control and unpredictable.
Since RSW is used only for flat surfaced sheets, for joining the corrugated sheets with plain sheets, the TIG spot weld is generally used. The absence of appropriate spot welding tool, fixture for holding the job as well procedure and optimised parameters are the impediment to weld the corrugates sheets with plain sheets by resistance welding.
FR2429641A1 1980-01-25 titled: APPARATUS FOR THE AUTOMATIC WELDING OF CORRUGATED PLATES discloses an invention that relates to an automatic apparatus of welding of corrugated plates.

Il includes/understands a blowtorch assembled on a head, the latter being fixed on the apparatus by means of a segment in the shape of arc trained in rotation movement via a pinion, to move the head along this segment and to control the position of the blowtorch. The segment is involved moving angular by an Engine assembled in the apparatus and it comprises, at its ends, two small revolving wheels on which a belt passes or chains which moves the teeth of the segment parallel to; the head

4
of blowtorch is fixed at the median part of the chain, so that, when the segment is involved on a given angle, this head is involved simultaneously on an angle twice larger.

Application: welding of sheets corrugated for the construction of gas liquéfie.

tanks.
GB1188681A (1970-04-22) titled: Improvements in or relating to a Method of Joining Metal-Sheets by Automatic Welding and Apparatus for Carrying Out Said Method teaches about a welding apparatus for seam welding a lap joint between sheets, the welding torch is guided by means engaging the surface of one sheet to maintain the torch at a constant distance from such surface and by means engaging the edge of that sheet to align the torch with the edge. Metal sheets are lap welded by first tack-welding and then continuously seam-welding without the use of filler metal. Sheets of thickness 0À012-0À158 in. may be so welded using a torch of the plasma arc or inert gas shielded non-consumable electrode type. The continuous welding of the tack-welded sheets 26, 27, Fig. 2, is effected by a torch mounted on a carriage 3 slidable on rods 8, 9 of a frame having four feet 21, 22, the front pair of which abut against the edge 43 of the upper sheet 26. A pinion 25 driven by a motor 4 on the carriage 3 engages a rack 16 on the frame to traverse the welding torch. At the end of the weld the rack 16 is rotated out of engagement with the pinion 25 and the carriage 3 is returned manually. The torch is carried in a holder 48 adjustably mounted on a bracket 5 pivotable about a shaft 28 mounted in the carriage 3 so as to be axially slidable and non-rotatable. A spring 38 presses the bracket 5 against a shoulder on the shaft 28. Rollers 56 or a slider carried by the lower end of the shaft engage the surface of the upper sheet 26 and so maintain the torch at a constant vertical distance from the sheet 26. The bracket 5 supports rigidly a rod 41 carrying a spring-loaded feeler 42 engaging the edge 43 and pivoting the bracket against the action of a spring 46b so that the torch follows a path corresponding to irregularities in the edge 43.

5
This invention is for using for a torch of the plasma arc or inert gas shielded non-consumable electrode type welding without filler wire and not for resistance welding.
US3016450A 1962-01-09 titled: welding method teaches about a welding stacked metallic sheets at spaced points of welding the sheets that are stacked with stop weld between adjacent sheets in areas where no welding is to occur and leaving void areas of bare metal contact, the stack is brought to welding temperature and pressure is applied uniformly and simultaneously over substantially the entire area of the stack. A stack 15, Fig. 1, of stainless steel sheets is sandwiched between sheets 21, 22 of asbestos or mica in a press 25, 26 and electrodes 28, 29 from a transformer 30 contact the ends of the stack. Direct current electromagnets 35 are arranged on both sides of the stack. The current supplied to the stack may be of 60 cycles per second frequency and the input current to the transformer primary may be passed through an arc formed between a conductive rod in sodium hydroxide solution and a rod at the surface of the solution. Alternatively, direct current may be supplied to the stack and alternating current of 60 cycles per second to the electromagnets. In making honeycomb material areas of the plates not to be welded are masked, e.g. by applying to the upper face of each plate aluminium oxide in ethylsilicate as bands 40, 40a, Fig. 3, the bands being staggered between the sheets so that metallic contact occurs only where the uncoated areas are. A rib 45, 45a of the masking material may be provided opposite the uncoated areas to induce buckling of the sheets. A stack so coated is welded and the block expanded to form honeycomb. For welding a stack of aluminium sheets 50, Fig. 5, the sheets are sandwiched between mica layers 54 and the composite stack is sandwiched between stainless steel plates 56, 57 insulated from the press by sheets 60, 61. Electrodes 65, 66 supply current from a transformer 68 to the aluminium sheets and current from a transformer 72 is supplied by electrodes 69, 70 to the plates 56, 57. Radiator cores may be produced using the method and the layers to be welded may be of foil. The applied current may be of higher frequency.

6
This invention is for welding of stacked metallic sheets.
OBJECT OF THE INVENTION:
An object of the invention is to develop a resistance spot welding method, spot welding tool and fixture for joining corrugated sheets with plain sheets.
Another object of the invention is used to develop weld lobe for resistance spot welding of corrugated sheets.
SUMMARY OF THE INVENTION:
A fixture for resistance spot welding of a plain stainless-steel sheet and corrugated strip comprising: bottom plate on which a column is mounted; a circular disc mounted above the said column; said circular disc comprising of plurality of guide rollers; wherein the plain stainless-steel sheet and corrugated strip are placed in between the said guide rollers and wherein an attachment drive is provided to rotate the guide rollers to weld the stainless-steel sheet and corrugated strip at required positions
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig 1 shows tungsten arc spot and resistance made on a metal sheet in accordance to the prior art.
Fig 2 shows electrodes used during trials in accordance to the invention.
Fig 3 shows design of corrugated strip on the sheet in accordance to the invention.
Fig 4 shows fixture developed for Resistance spot welding of the strip to the sheet in accordance to the invention.
Fig 5 shows welding spots to be made in accordance to the invention.
Fig 6 shows fixture developed for moving the strip along the sheet in accordance to the invention.

7
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
Figure 1 shows gas tungsten arc spot made from one side only whereas the resistance spot is made from both sides of the sheets and the nugget is formed to make the joint at the interface of both the sheets. Most resistance spot welds are made in very short time periods. Since alternating current is normally used for the welding process, procedures may be based on a 60 cycle time (sixty cycles = 1 second). With the addition of the time element, the heat generation could be computed as follows:
H = I2RT
H = Heat
I2 = Current Square
R = Resistance
T = Time
Here, Control of time and current are important. If the time taken is too long and the current is not optimum, there is the possibility of expulsion of metal from the weld joint, which could decrease the cross section of the joint and weaken the weld. Shorter weld times also decrease the possibility of excessive heat transfer in the base metal. Distortion of the welded parts is minimized, and the heat affected zone around the weld nugget is substantially smaller.
The size of the material for the electrode and the design of electrode tip point controls the size of the resistance spot weld. Generally, the weld nugget diameter should be slightly less than the diameter of the electrode tip point. If the electrode tip diameter is too small for the application, the weld nugget will be small and weak. If, however, the electrode tip diameter is too large, there is danger of overheating the base metal and developing voids and gas pockets. In either instance, the appearance and quality of the finished weld would not be acceptable.

8
Determining electrode tip diameter requires some decisions on the part of the weldment designer. The resistance factors involved for different materials will certainly have some bearing on electrode tip diameter determination. The tip diameter discussed here refers to the electrode tip diameter at the point of contact with the work piece.
RSW process is still extensively used for joining plain stainless steel sheets. Melting is induced by resistance heating due to the passage of an electric current through the work piece materials at the joint. The chromium copper can be used for the electrode. The electrode tip to be made in the form of a truncated cone with an open angle of 128 deg. Formation of weld nugget depends on the welding current and its duration and on the clamping force applied by the electrodes. A set of electrodes have been designed and developed to suit the actual product joint design as shows in Figure 2. After experimental works, the tool with oval shape top (1) has been used for welding.
Figure 3 and 5 show the design of the component to be welded on the SS sheet. It can be seen that the plain sheet (9) has to be joined with the corrugated sheet (10) as shown in Figure 3.
A fixture as shown in Figure 4 has been made to spot weld the corrugated strip to sheet to avoid the manual alignment. 4a shows the bottom part of welding fixture. 4b is the design view of top part of fixture. 4c is another view of bottom fixture.
The bottom plate (11) is designed to have a slit of 1 mm and top (2) and a circle
(3) for positioning the bottom electrode. Similarly the top part of welding fixture
(4) is designed to provide space for positioning the top electrode.
Figure 5 shows the schematic diagram of spot welding. The fixture is fixed between top (7) and bottom (8) spot welding electrodes. The plain steel sheet (9) shall be placed in the bottom fixture which has slit (2) and the corrugated steel strip (10) shall be placed on the plain steel sheet to perform welding.

9
In each set multiple number of spot welds are to be made in the pre-determined positions. Hence to feed the corrugated strip and SS sheet and to make the spot weld in the predetermined position a fixture had been developed as shown in Figure 6 to have accurate spots and this made the fitting process easier and speedy. This fixture has a bottom plate (11) and column like structure (12). The top of the stand has a circular plate (13) on which four guide rollers (14) are fixed. The plain SS sheet and the corrugated strip are placed in between the roller and they are made to move longitudinally using the guide rollers. The guide roller shall be made to move by the special attachment drive (15). The drive has been chosen in a way that every time the roller drive is made to rotate it will help the SS strip and the corrugated strip to move ahead and place the materials in the exact position where the spots have to be made. Thus, the fixture is useful to ensure right location of spot welding continually and offer the following advantages.
• No marking on the sheets for spot location.
• Fixture ensures dimensional accuracy
• Easy set up for welding spots
• Reduced distortion
• Reduce set up time between spots
• Enhanced productivity and Assured quality.
After welding the welded piece continue to move out using the rollers provided in the other side of the arrangement.
Considering the above factors RSW trials have been conducted with the newly designed copper electrodes for developing Welding Procedure for RSW of SS corrugated strips with plain SS sheets conforming to ASME or any other equivalent Code.

10
PQR and WPS as per ASME IX have been developed for different combinations of parameters like weld current, time and force to carry out qualification tests viz. Peel, Shear tensile and macro analysis.

WE CLAIM:

1. A fixture for resistance spot welding of a plain stainless-steel sheet and
corrugated strip comprising¬
- bottom plate (11) on which a column (12) is mounted;
- a circular disc (13) mounted above the said column (12);
- said circular disc (13) comprising of plurality of guide rollers (14);
wherein the plain stainless-steel sheet and corrugated strip are placed in between the said guide rollers and
wherein an attachment drive (15) is provided to rotate the guide rollers to weld the stainless-steel sheet and corrugated strip at required positions.
2. The fixture for resistance spot welding of a plain stainless-steel sheet and corrugated strip as claimed in claim 1, wherein multiple spot welds are made at predefined position.
3. A method for resistance spot welding of a plain stainless-steel sheet and corrugated strip by the fixture as claimed in claim 1, wherein the fixture is disposed between a spot welding electrode and the corrugated strip mounted on the plain stainless-steel sheet is positioned at slit (2) of the fixture.
4. A method for resistance spot welding of a plain stainless-steel sheet and corrugated strip by the fixture, as substantially described and illustrated herein with reference to the accompanying drawings.