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What is surface preparation for joining?

 

Surface preparation can be defined as steps and/or procedures to be followed prior to welding aimed at ensuring (in conjunction with all the elements of a qualified welding procedure) sound welds.

Surface preparation consists of two main steps:

A) Joint geometry, which involves three different aspects:

  • Joint type: butt, tee, lap, edge.
  • Edge preparation: U, J, V, etc.
  • Joint dimensions.

B) Surface cleaning

Preparation of surfaces before joining is a critical operation in ensuring the integrity of a joint. 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; austenitic stainless steel is very sensitive to solidification cracking arising among other factors from contaminants in grease.

Processes which are autogenous (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.

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.

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 be noted in the welding procedures or work instructions that are required by most welding application specifications.

Further information

Surface preparation on resistance spot welds.

Surface preparation on adhesive bonding.

Cleaning of aluminium.

For further advice on surface preparation for the joining of different materials please refer to their respective sources of information.

For more information please email:


contactus@twi.co.uk