Laser welding enables the creation of neat, narrow and repeatable joints with a relatively low heat input into the workpiece. However, to achieve a uniform weld, simply selecting the correct power setting, head feed rate and shielding gas is not enough. The condition of the surface and the precision with which the components to be joined are prepared are of great importance.
Laser welding enables the creation of neat, narrow and repeatable joints with a relatively low heat input into the workpiece. However, to achieve a uniform weld, simply selecting the correct power setting, head feed rate and shielding gas is not enough. The condition of the surface and the precision with which the components to be joined are prepared are of paramount importance.
Rust, oil, moisture, paint, scale, coolant and cutting residues can cause porosity, spatter, unstable formation of the weld pool and problems in achieving consistent penetration. Therefore, before starting work, care must be taken not only with the welder’s settings, but also with cleanliness, edge geometry and the secure clamping of the workpieces.
Why is surface cleanliness so important?
The laser beam concentrates a large amount of energy into a small area. Contaminants present at the joint heat up very quickly, evaporate or react with the molten metal. This can disrupt the welding process and leave gas bubbles, inclusions or discolouration in the weld.
Sources of problems can include oils, greases, oxides, moisture, residues of cooling emulsions and dirt transferred from tools. Even a thin layer of greasy residue can impair process stability, particularly with thin components and joints requiring a high standard of finish.
The surfaces around the future joint should be dry, clean and free from thick layers of oxides and organic materials. Not only the edges themselves but also the area on both sides of the weld, over which the laser head or shielding nozzle will pass, must be cleaned.
When should cleaning be carried out before welding?
Surface preparation should ideally be carried out as close as possible to the start of welding. Cleaned metal left for a prolonged period in a damp, dusty or greasy environment may become recontaminated with a film or get soiled during transport and storage at the workstation.
In mass production, it is advisable to establish a standardised procedure for preparing components. This should specify the sequence of steps, the cleaning agents used, the type of cleaning cloth, how components are to be stored after cleaning, and the rules for pre-welding inspection. This ensures that every component arrives at the workstation in a consistent condition, and makes the process easier to replicate.
Remove oil, grease and coolant
The first stage should be degreasing. Components following cutting, turning, milling, drilling or grinding may be coated with machining oil, cooling emulsion, preservatives or greasy marks left by hands.
Use a degreasing agent suitable for the metal in question and compliant with the process requirements. After cleaning, the surface must be allowed to dry completely. Do not commence welding if any liquid, moisture or a dissolved layer of dirt remains on the workpiece.
It is not enough to simply wipe the component with any old cloth. A soiled cleaning cloth may simply spread the oil over a larger area. It is best to use clean, lint-free materials and avoid touching the prepared area with bare hands. When working on components with high quality requirements, it is advisable to wear clean gloves.
Remove rust, paint and scale
Before laser welding, black steel should be free of loose rust, paint, thick scale, protective coatings and residues from previous processing. Contaminants must be removed from both the joint line and the area that will be exposed to heat and the shielding gas.
Surface preparation can be carried out by grinding, brushing, abrasive blasting, industrial washing or laser cleaning. The choice of method depends on the type of contamination, the material and the required surface quality. Laser cleaning allows for the precise removal of rust, paint, oil and process coatings without mechanical contact with the material.
After grinding or brushing, dust, swarf and abrasive particles must be thoroughly removed. Care must also be taken not to make scratches that are too deep, not to overheat thin edges and not to alter the geometry of the joint. With thin sheets, even minor damage to the edges can make it difficult to achieve a smooth weld.
How should aluminium be prepared for laser welding?
Aluminium requires special attention due to its oxide layer and the process’s sensitivity to moisture and contamination. Aluminium oxides have a different melting point to the base material, and hydrogen – which can originate from moisture, lubricants and surface contamination, amongst other sources – may increase the risk of porosity in the weld.
First, the surface must be degreased, and then the oxides removed using a suitable method. A stainless steel brush designed exclusively for aluminium may be used. It should not have been previously used to clean steel, as it could transfer foreign particles to the workpiece.
For demanding joints, it is also worth paying attention to the condition of the filler wire, if used. The wire should be clean, dry and protected from contamination. Contaminants on the filler material can end up directly in the weld pool.
Ensure the components fit together precisely
A clean surface is not enough if the components are poorly fitted. The edges should fit together evenly, and the gap along the entire length of the joint must be as consistent as possible. Sudden changes in the gap width can cause uneven fusion, weld collapse, lack of fill, or the need to constantly adjust the feed rate.
The components must be securely clamped in the fixture or pressed down so that they do not shift under the influence of heat, vibrations or the operator’s movements. In series production, well-made fixtures are one of the fundamental prerequisites for repeatable laser welding.
For wider gaps, filler wire may be used, as laser welding can be carried out both with and without filler material. However, this does not replace proper edge preparation, alignment of the workpieces and dimensional checks before starting work.
Remove burrs and check the edges
After laser, plasma or mechanical cutting, or sawing, burrs, slag, sharp irregularities and burn-through may remain on the edges. These must be removed before positioning the components. The workpiece should abut against the other part across the entire intended joint surface.
Before welding, it is advisable to check:
- that the material grade and thickness match,
- that the edges are straight and clean,
- that the gap is uniform along the entire length of the joint,
- that the workpiece is securely clamped in the fixture,
- the absence of oil, dust, moisture, paint and scale,
- cleanliness of the nozzle, protective glass and head components,
- correct supply of shielding gas.
Effective gas shielding reduces oxidation of the weld and is crucial for achieving a high-quality joint. If the gas flow is unstable or the nozzle is dirty, even a well-prepared workpiece may produce a poorer result.
The most common mistakes before laser welding
One common mistake is welding components that have only been superficially cleaned, even though the gaps still contain oil, coolant or machining dust. Other problems include leaving paint on the underside of the joint, using dirty brushes and mixing tools intended for steel with those for aluminium.
Do not touch the cleaned surface with bare hands. Greasy fingerprints can be a source of localised contamination. Once prepared, components should ideally be handled with clean gloves and placed in the fixture as quickly as possible.
It is also a mistake to neglect checking the gap and the stability of the clamping. If a workpiece shifts during welding, the process may become unstable and the weld will require correction or reworking. In laser welding, the repeatability of workpiece preparation is just as important as the repeatability of the machine settings.
Summary
Proper preparation of the metal for laser welding involves removing oil, moisture, rust, paint, scale, oxides and residues from previous machining operations. Equally important are smooth edges, a narrow and uniform gap, and stable clamping of the components.
A clean and well-fitted workpiece ensures a more stable process, minimises porosity and reduces the number of reworks. In mass production, it is advisable to establish a standardised procedure covering degreasing, cleaning, edge inspection and protecting the surface against re-contamination.
