Spot welding the latest car steels
Welding Equipment
Martin Strasser, head of training at leading welder supplier, Wielander+Schill, explains some of the complexities facing repairers having to weld cars built from many different types of steel.
THERE ARE considerable differences in the types of steel found in today’s car bodies. Although they may have the same trade names, they have different qualities because of varying production techniques.
For example, under the category name of boron we could have BTR made in Germany with an average hardness of 1480MPa, or BOR made in Sweden with an average hardness 1500MPa or USIBOR made in France with an average hardness 1600MPa. It used to be so simple. The criteria in the year 2000 were: high welding power of 8000amps, a short weld time of 0.5sec, high clamping force of 300daN, very flat profile electrode tips (not domed or pointed) and 1000Hz (sometimes coupled up to 2000Hz).
The cars incorporated a few different types of steel, all fairly easy to weld. There was some high strength steel and some boron but you only needed one setting for the welder and one press of the button for good results every time.
In the last three years there have been major changes in construction of car bodies, producing a colourful mixture of different steels with various qualities as the above Mercedes diagram illustrates.
The first problem to solve is where to find the different types of steel.
Normally you will find the extremely hard steel (boron) at the side impact protection area. Beyond that, each car maker tries to optimise the shell in terms of weight, vibration control, noise control and crash-absorption.
COATINGS ‘ ANOTHER PROBLEM
The different sheet metal coatings also have a deep influence on the weld process. The additional layers alter the electrical conductivity and resistance as well as interfering with heat transfer inside the material itself. It is not always possible to determine the coatings when the new replacement part is usually covered with black factory primer. Therefore, before spot welding, always grind the sheet metal back to a clean metal surface free of primer.
If it is not clean metal you will have to adjust the weld parameters. There are numerous coatings used on automotive sheet steels and it pays to check on them before commencing to weld.
EQUIPMENT CHANGES For the manufacturer of welders changes in car build materials have meant a major change in direction for welders and welding techniques. The size of the nugget has to be bigger because of the new materials and the use of glue. To produce a bigger nugget, a wider electrode is required.
As the steels have become lighter they have also become harder. Harder steels need a greater clamping force. 500daN is often needed as opposed to 350daN previously.
Greater clamping force requires increased amps. Increasing the size of the electrodes also means still more amps are required.
The melting point of the different steels has meant that a greater control over the power is needed. To give full impedance measuring the use of machines running at 10,000Hz is now required.
By exerting more force onto the steel, the resistance decreases. To overcome this, more current is required. Conversely if more current is required, more clamping force is needed to hold the molten nugget in place The two elements daN and Amps are linked.
Gun force is controlled by air pressure. Too much air pressure is as bad as too little air pressure. An EU directive comes into force in May 2008 limiting the magnetic field when welding equipment is in operation. This is for the health and safety of staff and will be mandatory in Europe. All new welders coming onto the market this year will be trying to tackle all these problems. They won’t all succeed.
SPARKS, THE ENEMY OF THE SPOT WELD During the weld process, the molten steel becomes very elastic and the energy build up is huge. It is the job of the electrode tips and the clamping force of the gun to hold this molten metal in place. the metal escapes it does so in a shower of sparks that look very spectacular but it is also absolutely fatal for the spot weld in terms of quality and strength. The whole energy explodes into the air. There is no lens and no proper fusing of the metal.
The perfect weld is created by good preparation together with current and time.
SPOT WELDING BRIEFLY EXPLAINED The metal sheets are pressed together by the two electrodes. The welding current starts. The current (e.g.10,000amps) has to pass the narrow diameter of the electrodes. This ‘blockage’, together with the resistance of the steel, causes the sheet metal to get extremely hot between the electrodes. The clamping force (daN) of the electrodes forges the molten material together. At this time the current is shut off. The electrodes open and the material is welded together.
PROBLEMS OF COMBINATIONS As new cars come to the market, the different steels are becoming more difficult to weld. Here are some challenging sheet metal combinations: ‘ 2 x BTR (Boron) + 1 x normal. The problem is the connecting of the thin sheet. Solution: high gun force, high welding current, post warming. ‘ 2 x ZSTE 340 verz x 0,7 mm The problem here is the zinc coating on the steel coupled with the very thin metal thickness. Solution: high gun force, high welding current, post warming. ‘ Outside 2 sheets of DP 600 x 0,7 mm, inside Usibor 2,0 mm. Solution: high gun force, high welding current, post warming.
W+S SPECIAL MODE Invented by W+S, a world-wide patent covering this ‘special mode’ technique has been applied for. The new targets are: OEMs to reduce their effort and overhead costs with regard to supplying repair manuals; to give information to every workshop about the kind of material that is being welded (you need only the GT welder to get to this information); to improve the quality of the spot welds by precise setting of the parameters.