Frequently Asked Questions
The wear of electrodes is an inevitable result of the thermal and mechanical damage caused during spot welding. It is worse on coated materials where the coating has a low melting point and alloying of the tip face occurs.
The main factors to consider are as follows:
This is most the important factor and inadequate cooling is the most frequent cause of electrode life problems. It must be attended to first. Typically 4 l/min of water flow through the electrodes is recommended for uncoated, thin steel sheet and 6 or more l/min for coated and thicker steels. An inlet temperature not exceeding 20°C is recommended and outlet temperature not exceeding 30°C. The electrode diameter should be a minimum of 2.5 to 3 times the tip diameter to increase the effectiveness of cooling. The cooling tube should be cut at 45° and directed on the inside face of the electrode. It is important to check that blockages or back pressure on the outlet do not affect the flow rate. When using female caps, the holder should be drilled through to direct water onto the electrode itself.
The electrode should be the correct material and shape for the job. Truncated cone electrodes normally give the longest electrode life but need to be well aligned. Copper/chromium/zirconium electrodes are traditionally used for spot welding coated steels but the higher conductivity copper/zirconium electrodes are considered to give less sticking and longer lives. Aluminium oxide dispersion strengthened copper electrodes are also found to give some advantage but are much more expensive.
The welding machine or gun should be sufficiently strong and the electrode holders of sufficient diameter to avoid electrode skidding under load. Good electrode alignment is also crucial to avoid more localised heating of the electrode tip. Excessive impacting of the electrodes onto the sheet surface should be avoided where possible, although reduction of the electrode approach rate may increase the weld cycle time. Servo guns enable variable electrode approach speed to be programmed, and reduced speed may be set just before electrode contact. This is claimed to give improved electrode lives.
Ensure a suitable welding condition is set for the job, avoiding excessively high welding current or excessively long times. The use of high electrode forces is not necessarily a problem if they are required to provide a tolerant welding condition with sufficient safety margin between the minimum acceptable weld size and weld splash. A well set-up condition provides greater tolerance to electrode wear before quality is affected.
In order to correct for electrode wear, electrode dressing and/or current stepping (programmed progressive increase in welding current) can be applied. Dressing can be conducted frequently, with little material removal, as a means of maintaining the tips in perfect condition, or by allowing substantial wear within the life of the tip, then dressing substantially back to the original shape. In the latter case, a current stepping operation may accompany the dressing procedure. A range of commercial electrode dressing tools is available and additional features can include automatic tip changers, tip size and alignment checking and electrode force and current measuring devices. Such accessories offer the possibility of providing routine quality checks.
Resistance welding of sheet metals - a guide to best practice