Why does a laser weld seam become porous?

Some of them lie on or in the component: oil, coatings, zinc, moisture, the oxide layer on aluminium. Others arise in the process itself, for example from inadequate gas shielding or an unstable vapour capillary. Anyone trying to fix a porous weld seam solely through parameters can easily overlook the first group.

This article sets out causes, testing and countermeasures and shows what role the preparation of the joint can play. It replaces neither the qualification of a welding procedure nor a risk assessment.

What a pore in a weld seam is

A pore is a gas-filled cavity in the solidified weld metal, in a laser weld seam just as in other fusion weld seams. A technical article based on research by Fraunhofer IPK names the main cause as gases trapped in the melt pool that can no longer escape during solidification.

What matters is how much gas is produced and how long the melt pool stays liquid so that the gas can escape.

Pores can occur individually, in clusters or finely distributed, open at the surface or entirely inside the seam.

Material-related pores and process pores

A distinction made in a University of Stuttgart dissertation on laser beam welding of aluminium die castings is helpful:

• material-related pores: gases that are trapped in the material and released on melting
• process pores: pores that can arise from fluctuations of the vapour capillary in deep penetration welding

Applied analogously to the surface, the distinction leads to different countermeasures. For oil at the joint, cleaning is a more obvious choice than changing parameters. An unstable vapour capillary, on the other hand, cannot be calmed by cleaning alone.

Oil, grease and residues at the joint

Sheets from previous forming processes are coated with oils. In addition, there are emulsions, dry lubricants and preservation layers, some of which are barely visible.

These substances vaporise under the laser beam. The vapour enters the melt pool, and some of it can become trapped during solidification. Schweißaufsicht im Betrieb explicitly names oil residues and other contamination on the workpiece surface or at the joint edge as a cause of pores, and cleaning the workpieces as a countermeasure.

The joint edge also matters here: contamination in the joint lies exactly where the melt pool forms and is not always covered by cleaning the top surface.

Paint, primer and organic coatings

Paints, primers and other organic coatings can release gases and fumes during the welding process and act in the seam area much like contamination.

Manufacturers of laser cleaning systems describe the removal of shop primer on steel sheet, of preservation layers and of forming lubricants as typical preparation before welding.

A coating that is needed later as corrosion protection should only be removed where this is necessary for the seam. How wide this area needs to be depends on the seam geometry, heat input and the requirements for the component, and cannot be stated in general terms.

Galvanised sheet and zinc vapour

With galvanised steel, the cause lies in the physics of the materials. According to Fronius, zinc melts at around 420 °C and vaporises at 900 °C, while the melting point of steel is above 1,500 °C. So when the steel melts, the zinc in the immediate vicinity is already gaseous.

The lap joint is particularly critical because there the zinc layer lies in the joint plane between the sheets. Schweißaufsicht im Betrieb describes how, in lap joints, the zinc vapour has to escape through the melt pool. The Institut für Füge- und Schweißtechnik at TU Braunschweig names the consequence as a disturbance of the vapour capillary with melt pool ejections, blow-throughs and weld spatter.

According to the same source, the state of the art is setting degassing gaps that allow directed vaporisation of the zinc in the interlayer. Research shows that bifocal or ring-shaped spot arrangements can, under certain conditions, also enable reliable welding without a preset gap.

If the zinc layer is removed in the seam area, there is no corrosion protection there. Whether this is permissible depends on the requirements for the component.

Aluminium, oxide layer and hydrogen

With aluminium, hydrogen is a key source of pores. According to the welding technology company ERL, the solubility of hydrogen in aluminium drops abruptly at around 600 °C during cooling, roughly in a ratio of 1:20. Whatever was dissolved in the melt has to escape during solidification. If it does not, it remains behind as a pore.

Hydrogen can originate, for example, from moisture or from oils and greases on the surface. Aluminium immediately forms an oxide layer in air. Oxides can additionally lead to oxide inclusions — a separate imperfection alongside the pore.

As preparation, ERL names working with stainless steel brushes and chemical cleaning shortly before welding. ERL gives preference to pickling in alkaline solutions; in many cases ERL considers solvents questionable for occupational safety reasons, because residues can be converted by the arc into gases and fumes that are harmful to health. The addition “shortly before” is essential, because the surface reacts again after cleaning.

With aluminium die castings there is an additional source that no surface treatment reaches: the Stuttgart dissertation describes gases from the casting process trapped in the material, including hydrogen and nitrogen. Fraunhofer IWS additionally names mould release agents as unfavourable for seam formation.

Moisture and condensation

Water on the surface is a direct source of hydrogen. For aluminium, ERL states that condensation must be avoided at all costs and gives an example: at 70 per cent humidity, water can already condense if the metal is around 5 °C colder than the ambient air.

This can affect, for example, components that come from a cold store into a warm workshop. It can then make sense to let them reach workshop temperature before welding.

Shielding gas: type, quantity and delivery

The shielding gas is intended to separate the melt pool from the ambient air. The technical article on the work of Fraunhofer IPK states that the choice of shielding gas type, the correct setting of the flow rate and optimised gas delivery contribute significantly to a stable process.

The study examined hand-held laser beam welding on T-joints made of the low-alloy steel HX340LAD with a sheet thickness of 1.5 mm, at a gas flow rate of 14 l/min. The results differed markedly:

• Argon: maximum porosity 4.43 per cent, quality level D
• Nitrogen: maximum porosity 1.35 per cent, quality level B
• CO₂: maximum porosity 0.14 per cent, quality level B

The classification was made in accordance with DIN EN ISO 13919-1. As drawbacks, the article cites more spatter with CO₂, possible slight surface oxidation and, where applicable, a moderately higher laser power. The values apply to this set-up and cannot readily be transferred to other materials.

More gas is not automatically better; flow rate and delivery have to suit the application.

Unstable vapour capillary in deep penetration welding

In deep penetration welding, the laser beam penetrates deep into the material and creates a thin channel filled with metal vapour — the vapour capillary, known in English as the keyhole. Researchers at Empa describe how this capillary can become unstable, collapse and leave a pore in the weld seam.

Studies on laser hybrid welding of aluminium describe the sequence in more detail: the capillary pinches off, a gas bubble forms at the bottom of the melt pool, and the advancing solidification front can capture it. A larger volume of liquid metal gives the bubble more time to escape there.

Which parameters help depends heavily on the material:

• Schweißaufsicht im Betrieb names a reduced feed rate combined with a larger focal spot, so that the melt pool can degas for longer.
• For aluminium die castings, the Stuttgart dissertation found a critical speed range with maximum pore area. As the speed increased further, finely distributed seam porosity resulted there.
• For aluminium die castings, Fraunhofer IWS reports a considerably improved seam quality through high beam quality and high-frequency beam oscillation.

A rule such as “slower is always better” cannot be derived from this; parameters have to be tested on the specific material and joint.

Detecting and assessing pores

Visual inspection only covers what lies at the surface. Porous areas that extend to the surface are visible; enclosed ones are not. Other methods are used for the inside of the seam:

• Radiographic testing: according to SLV Nord, differences in density inside the workpiece become visible, so that pores and inclusions can be detected and distinguished. Schweißaufsicht im Betrieb cites DIN EN ISO 17636-1 and -2 for this and points out that on beam-welded seams the top and bottom sides have to be machined down.
• Ultrasonic testing in accordance with DIN EN ISO 17640.
• Destructive tests such as fracture tests or metallographic sections, which give a random-sample insight into the seam.

For the assessment, DIN EN ISO 13919-1 provides requirements and recommendations for quality levels for imperfections in electron and laser beam welded joints in steel, nickel, titanium and their alloys. Which level is required for a component follows from its requirements, not from the standard alone.

Narrowing down countermeasures systematically

Why a seam becomes porous can rarely be resolved with a single adjustment. A structured approach helps more than changing many variables at the same time:

1. Document the pore pattern: location, size, frequency, affected batches or components.
2. Clarify the material and surface: coating, galvanising, die casting, oil, oxide.
3. Clean the joint and keep the time until welding short.
4. Rule out moisture and condensation.
5. Check the shielding gas: gas type, flow rate, delivery.
6. Change parameters in a targeted way: feed rate, focal spot, where applicable beam guidance.
7. Test each change individually and check the result.

The pore pattern can provide clues: if pores only occur in certain batches, it makes sense to look at the material and the surface. This does not replace an investigation of the cause.

Joint preparation and laser cleaning

Many of the causes described lie before the first weld spot. A clean, dry joint that is welded promptly removes some of the process’s gas sources.

Common methods include solvents, brushing and pickling. Laser cleaning can be a further preparation step. Clean-Lasersysteme, as a manufacturer, describes the removal of forming lubricants, natural oxides, oils, greases, hydrates and shop primer before welding; on galvanised sheet, zinc hydrates near the surface are to be removed without damaging the zinc layer. These are manufacturer’s statements that should be confirmed for your own component in trial welds.

The limitations are just as important:

• No surface cleaning reaches gases inside the material, for example in die castings.
• An unstable vapour capillary or inadequate gas shielding remains unaffected by this.
• The inner surfaces of a lap joint are no longer accessible after joining and have to be prepared beforehand.
• After cleaning, aluminium reacts with its surroundings again.

For preparing the joint, both the cleaning machines in the Beamlux range and the laser welding machine may be considered; in addition to welding, the latter is designed for weld seam cleaning and rust removal. Whether a machine fulfils a specific preparation task can only be assessed on the real component.

Occupational safety

In hand-held laser cleaning and hand-held laser welding, the laser radiation is at least partly openly accessible during operation. DGUV Information 203-093 supports the risk assessment for such laser equipment and names damage to the eyes and skin as possible consequences. Welding galvanised components also produces zinc oxide fume, which according to Fronius can cause flu-like symptoms; Fronius names effective extraction as a measure. This article does not replace a risk assessment.

Sources

• Schweißaufsicht im Betrieb (WEKA) — Laserstrahlschweißen: Fehler und wie man sie vermeidet — causes of pores from contamination, zinc vapour and an unstable vapour channel; cleaning and longer degassing as countermeasures.
• Schweißaufsicht im Betrieb (WEKA) — Elektronenschweißnähte und Laserstrahlschweißnähte prüfen — non-destructive testing methods for beam-welded seams with the associated standards.
• DER PRAKTIKER / Home of Welding — Mit Schutzgas die Porosität reduzieren — study by Fraunhofer IPK on the influence of the shielding gas in hand-held laser beam welding.
• Home of Welding — Update für die DIN EN ISO 13919-1 — scope of the standard on quality levels for imperfections in electron and laser beam welded joints.
• Reinhard Winkler — Porenbildung beim Laserstrahlschweissen von Aluminium-Druckguss — dissertation, University of Stuttgart, Herbert Utz Verlag; material-related pores, process pores and the influence of welding speed.
• Fraunhofer IWS — Neue Perspektiven für das Laserstrahlschweißen von Bauteilen aus Aluminium-Druckguss — trapped gases, mould release agents and high-frequency beam oscillation.
• all-electronics — Laserschweißen: Mess-System erkennt unerwünschte Poren automatisch in Echtzeit — report on research by Empa; collapse of the vapour capillary as a cause of pores.
• Xu et al., Materials — Influence of Arc Power on Keyhole-Induced Porosity in Laser + GMAW Hybrid Welding of Aluminum Alloy — pinching off of the capillary, gas bubbles at the bottom of the melt pool and capture by the solidification front.
• TU Braunschweig, Institut für Füge- und Schweißtechnik — Projekt MultiZink — disturbance of the vapour capillary by zinc, and degassing gaps in lap joints.
• Schweißen und Schneiden — Strahlschweißen von verzinkten Stahlblechen im Überlappstoß ohne voreingestellten Entgasungsspalt — beam shaping with bifocal and ring-shaped spot arrangements.
• Fronius — Verzinktes Blech schweißen oder löten? — melting and vaporisation temperature of zinc, porosity and zinc oxide fume.
• ERL GmbH Schweissen + Schneiden — Schweißen von Aluminium — hydrogen solubility, oxide layer, condensation, cleaning before welding and occupational safety when using solvents.
• SLV Nord — Durchstrahlungsprüfung — detecting pores and inclusions via differences in density.
• Clean-Lasersysteme GmbH — Vorbehandeln zum Löten und Schweißen — manufacturer’s documentation on the laser pre-treatment of joints.
• DGUV Information 203-093 — Handlungshilfe für die Gefährdungsbeurteilung beim Betrieb von offenen Laser-Einrichtungen zur Materialbearbeitung mit Handführung oder Handpositionierung — scope and hazards from openly accessible laser radiation.

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