A steel fabrication shop, whose main product was railway tanker wagons, received an order for aluminium alloy tankers and set itself up for both MIG and TIG welding. Production started, but the company was soon in deep trouble and approached TWI with the following cry for urgent assistance
'We have carried out welding procedure tests to BS 4870: Part 2:1982 (Approval testing of welding procedures) and all our welders have passed the tests in BS 4871:Part 2:1982 (Approval testing of welders working to approved welding procedures, parts of BS 4870 have since been superseded by more recent standards). Our first production welds are full of porosity and we have stopped all aluminium welding until the problem is solved'.
A visit by a TWI welding engineer soon revealed the cause of the trouble. Procedure testing and approval testing of the welders had been carried out in a training school where small quantities of materials were stored and handled under clean and dry conditions; Aluminium was the only metal being welded in the training school and the components were cleaned immediately before welding. The situation on the shop floor was, however, completely different: sheets of aluminium alloy were stacked horizontally in piles, with the top sheet covered by a layer of dust and dirt originating from an adjacent mild steel fabrication area where welding and grinding operations were carried out.
Cleaning edge preparations (degreasing and wire brushing) had apparently been carried out efficiently, but there was a delay of several hours before welding during which time the surfaces of the aluminium could have become contaminated.
The problem was solved by good housekeeping, the rules of which can be summarised as follows:
- Keep aluminium and steel fabrication separate - preferably in different workshops; at least in different areas.
- Store aluminium indoors in a dry, clean and well-ventilated area to avoid the surface becoming stained with a combination of dirt and condensation. If humid conditions cannot be avoided, it is important to remove porous outer wrappings and interleaving from the aluminium.
- Store plates on edge, spaced by strips of plastic or other inert material to allow good air circulation.
- After cutting to size and forming edge preparations by mechanical methods or by plasma cutting, degrease the prepared edges and adjacent surfaces with a commercial solvent by wiping, dipping, spraying or vapour degreasing. The most convenient method of degreasing for the majority of aluminium fabrications is local swabbing with a lint-free cloth impregnated with an organic solvent such as:
- Methyl alcohol
- Petroleum ether
- White spirit
- (Warning: all these solvents are highly flammable!)
- Degreasing, as well as the subsequent wire brushing should, whenever possible, be carried out before fit-up of the parts because of the difficulty of removing solvents from assembled joints.
- Wire brush the surface of the edge preparation for butt welds and the plate surfaces for fillet welds, including the region a short distance (5mm) beyond where the welds are to be deposited. Wire brushing may be carried out manually or with a motor driven wire brush with stainless steel bristles 0.15-0.40mm diameter. The bristles should be degreased periodically to avoid contaminating the aluminium surface. Excessive pressure should be avoided when using motor-driven brushes to prevent burnishing the surface - which can entrain aluminium oxide and give poor quality welds.
- If welding is not to be carried out within four hours of cleaning, it is advisable to prevent atmospheric contamination by covering the joint areas with strips of strong paper 50-75mm wide, attached by adhesive tape.