Black deposit on and besides the weld
With many applications (pallets for the chemical and food industry, ladders, scaffoldings) the black deposit is rather disturbing. Simple brushing removes it, however, requires additional operations which can often be done only manually at areas of limited access.
The deposit results from evaporation and following fallout of Magnesium oxide. Magnesium is an alloying element of Aluminium which substantially increases its strength and can not be left out in most cases.
Magnesium oxide is usually known with white colour. An EDS-analysis eliminates every doubt of not being MgO which is known to show colours of grey, yellow, brown or black besides its white form of existence.
The following possibilities exist to prevent MgO-formation:
- Use of wire electrodes with low or no Mg content (AlMg3, AlSi5)
- Optimised pulse parameters for the lowest possible formation of metal vapour
- Avoid poor accessibility and resulting unfavourable torch position
- Sufficient shielding-gas protection to keep the intake of Oxygen as low as possible