Every process has its process parameters, if you set the correct parameters then only you will get the desire output. And if you set parameters and one of these is not correct then it affect the others to, therefore the determination of appropriate spot welding parameters for spot welding is a very complex issue. A small change of one parameter will effect all the other parameters. This, and the fact that the contact surface of the electrode is gradually increasing, makes it difficult to design a welding parameter table, which shows the optimum welding parameters for different circumstances.
In the types of welding all processes covered in two basic categories.
1. Pressure welding or Plastic welding or Solid state welding – Metal parts are heated in plastic state and join together with the help of external pressure. E.g. Spot / Resistance welding
2. Fusion welding or Non-pressure welding -The metal parts are heated at jointed state at molten temperature and get solidify. E.g. Arc welding
Spot welding parameters include:
- Electrode force
- Diameter of the electrode contact surface
- Squeeze time
- Weld time
- Hold time
- Weld current
The purpose of the electrode force is to squeeze the metal sheets to be joined together. This requires a large electrode force because else the weld quality will not be good enough. However, the force must not be too large as it might cause other problems. When the electrode force is increased the heat energy will decrease. This means that the higher electrode force requires a higher weld current. When weld current becomes too high spatter will occur between electrodes and sheets. This will cause the electrodes to get stuck to the sheet.
An adequate target value for the electrode force is 90 N per mm2. One problem, though, is that the size of the contact surface will increase during welding. To keep the same conditions during the whole welding process, the electrode force needs to be gradually increased. As it is rather difficult to change the electrode force in the same rate as the electrodes are “mushroomed”, usually an average value is chosen.
Diameter of the electrode contact surface
One general criterion of resistance spot-welding is that the weld shall have a nugget diameter of 5*√t, “t” being the thickness of the steel sheet. Thus, a spot weld made in two sheets, each 1 mm in thickness, would generate a nugget 5 mm in diameter according to the 5*√t-rule. Diameter of the electrode contact surface should be slightly larger than the nugget diameter. For example, spot welding two sheets of 1 mm thickness would require an electrode with a contact diameter of 6 mm. In practice, an electrode with a contact diameter of 6 mm is standard for sheet thickness of 0.5 to 1.25 mm. This contact diameter of 6 mm conforms to the ISO standard for new electrodes.
The squeeze Time is a time interval between the initial application of the electrode force on the work and the first application of current. Squeeze time is necessary to delay the weld current until the electrode force has attained the desired level.
Weld time is the time during which welding current is applied to the metal sheets. The weld time is measured and adjusted in cycles of line voltage as are all timing functions. One cycle is 1/50 of a second in a 50 Hz power system. (When the weld time is taken from American literature, the number of cycles has to be reduced due to the higher frequency (60Hz) that is used in the USA.)
As the weld time is, more or less, related to what is required for the weld spot, it is difficult to give an exact value of the optimum weld time.
- Weld time should be as short as possible.
- The weld current should give the best weld quality as possible.
- The weld parameters should be chosen to give as little wearing of the electrodes as possible. (Often this means a short weld time.)
- The weld time shall cause the nugget diameter to be big when welding thick sheets.
- The weld time might have to be adjusted to fit the welding equipment in case it does not fulfill the requirements for the weld current and the electrode force. (This means that a longer weld time may be needed.)
- The weld time shall cause the indentation due to the electrode to be as small as possible. (This is achieved by using a short weld time.)
- The weld time shall be adjusted to welding with automatic tip-dressing, where the size of the electrode contact surface can be kept at a constant value. (This means a shorter welding time.)
When welding sheets with a thickness greater than 2 mm it might be appropriate to divide the weld time into a number of impulses to avoid the heat energy to increase. This method will give good-looking spot welds but the strength of the weld might be poor.
By multiplying the thickness of the sheet by ten, a good target value for the weld time can be reached. When welding two sheets with the thickness 1 mm each, an appropriate weld time is 10 periods (50Hz).
Hold time (cooling-time)
Hold time is the time, after the welding, when the electrodes are still applied to the sheet to chill the weld. Considered from a welding technical point of view, the hold time is the most interesting welding parameter. Hold time is necessary to allow the weld nugget to solidify before releasing the welded parts, but it must not be too long as this may cause the heat in the weld spot to spread to the electrode and heat it. The electrode will then get more exposed to wear. Further, if the hold time is too long and the carbon content of the material is high (more than 0.1%), there is a risk the weld will become brittle. When welding galvanized carbon steel a longer hold time is recommended.
Weld current is base spot welding parameters. The weld current is the current in the welding circuit during the making of a weld. The amount of weld current is controlled by two things; first, the setting of the transformer tap switch determines the maximum amount of weld current available; second the percent of current control determines the percent of the available current to be used for making the weld. Low percent current settings are not normally recommended as this may impair the quality of the weld. Adjust the tap switch so that proper welding current can be obtained with the percent current set between seventy and ninety percent.
The weld current should be kept as low as possible. When determining the current to be used, the current is gradually increased until weld spatter occurs between the metal sheets. This indicates that the correct weld current has been reached.