Frequently Asked Questions
Aluminium and its alloys have high thermal and electrical conductivity compared with steel and, as the process depends on resistance (Joule) heating, they require much higher welding currents. In addition, the surface contact resistance plays a major part in heat generation. Short weld times are employed to generate the heat quickly and thus minimise the heat lost by conduction. The harder alloys are generally more easy to weld, conversely pure aluminium is not recommended for spot welding.
Aluminium oxide, which forms naturally on the aluminium surface, has a very high electrical resistance. Therefore, the surface condition of the material has a strong effect on the heat generated, the settings required and also on the weldability of the material.
||Generally weldable but heavy or variable oxide can give inconsistency.
|Chemically deoxidised or abraded sheet:
||The removal of oxide just prior to welding can impart a consistent but very low surface resistance, so a much higher welding current is required. Such controlled procedures are used for aerospace quality welds.
|Surface modification treatments:
||Some specially designed chemical surface treatments, often applied by the material supplier, give a medium to high surface resistance and can be consistent. Good weldability can be achieved.
|Anodised, heavy chromate or other passivation treatments, or dry-lubricated sheet:
||These treatments usually give very high resistance, and sometimes completely insulating, surface layers and are usually unweldable.
Electrodes for sheet material in the range 1 to 3mm thick have a dome face with a radius from 50 to 100mm, preferably, increasing with sheet thickness. Larger radii may be used for high quality welding to minimise indentation. High conductivity electrodes (Class 1 types) are usually recommended but Class 2 (e.g. Cu/Cr/Zr) can be used for commercial quality applications.
As copper readily forms an alloy with aluminium, and the contact heating is high because of the high welding currents, electrode wear is very rapid. Attention to the electrodes is usually required after making a few hundred welds, but some of the special surface treatments give reduced wear. More frequent electrode dressing with automated electrode dressing tools is a recommended strategy to control the electrode condition and minimise the effect on weldquality. Electrodes are dressed very frequently when making aerospace quality welds, to avoid any contamination of the contact faces. Very frequent, light abrading, or buffing, of the electrodes, which prevents build-up of a visible alloy layer on the electrode, has been shown to give excellent reproducibility of weld quality and long electrode life.
Detailed weld settings are available in standard text books or from suppliers of the material or equipment, and may depend on the detailed quality requirements and materials being used. The standard 'BS EN ISO 18595 Resistance welding - Spot welding of aluminium and aluminium alloys - Weldability, welding and testing', provides useful guidelines.
For commercial quality welds on 1.2mm mill-finished sheets, weld settings are typically in the region of -
|3 to 5 kN
2 to 5 cycles (40-100ms)
22 to 28kA
While ac power supplies can be used for welding aluminium alloys, inverter DC power supplies have been suggested as providing greater control of the short time welding pulse, and thus improved weldability. It should be noted that with DC supplies, that there is a marked polarity effect. Nugget growth is offset slightly towards the positive electrode and this can be used to advantage when welding dissimilar thicknesses, by having the thinner sheet on the positive side. In addition, the positive electrode is subjected to faster wear than the negative one. DC power supplies generally provide a more efficient use of the mains power at the high welding currents required.
Weld splash can be detrimental to weld quality and should be avoided. Some nugget porosity or cracking can occur, especially in sensitive alloy types. These flaws do not normally influence weld strength significantly, provided that normal plug (button) fracture occurs on testing. Dual force schedules are needed for high quality welds where control of nugget flaws is required.
When comparing similar thickness materials, the static and fatigue shear strengths of aluminium alloy spot welds are about one third the value of steel welds (depending on the alloy type). Where aluminium is used instead of steel, its gauge is normally increased, giving weld strengths which compare more favourably with those of steel - but still enabling weight savings to be made.