TWI Frequently asked questions
by John Weston
Preparation of surfaces before joining is often a critical operation in ensuring the integrity of a joint. In almost all cases it is necessary to ensure that the surfaces in and around the joint are clean, free from scale and other heavy oxide coatings, dry and free from organic materials.
The degree and extent of surface cleaning will be dependent on the materials being joined and the joining process being used. For example, when welding aluminium and its alloys, surface preparation is very important as this material is extremely sensitive to the pick-up of hydrogen (from grease or moisture) which gives rise to weld porosity. Standards of cleanliness when welding sheet steels do not need to be as stringent as when welding aluminium and its alloys.
Processes which are autogeneous (i.e. no filler metal is added), for example electron beam, TIG or plasma welding, need to be free from surface coatings which will out-gas or contaminate the weld pool. These processes tend to have small or deep and narrow weld pools which freeze rapidly and are more sensitive to contamination compared with open preparation welds that require filler metals.
Where the materials are to be bonded with adhesives, it may be necessary to follow a strict regime of chemical treatments
Surface preparation might also be required to make the joint geometry suitable for the joining process. For example, when butt-welding material over about 3mm thick using an arc welding process, it is generally necessary to cut or machine a bevel on the plate edges. The resulting groove is filled with weld metal to achieve correct weld penetration and a joint with the required properties.
It may also be necessary to ensure that the edges or surfaces to be joined are free of defects. This may require the use of non-destructive examination techniques such as ultrasonic testing.
It is important to remember that filler materials also have surfaces that can be contaminated. Surface cleanliness of welding consumables that will be incorporated into the weld pool is therefore important. It is normally necessary to clean and degrease aluminium and titanium welding wires just before use.
The Standards or guidance documents used as the basis for fabricating a component or structure often give guidance on requirements for surface preparation. Specific requirements for surface and filler wire preparation should benoted in the Welding Procedures or Work Instructions that are required by most welding application specifications.
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