Gases for laser material processing, commodity or optimising tool?

LASERS ensure highly reliable processes across a wide variety of industrial and engineering sectors for welding, cutting and drilling parts during manufacturing or for repair. They are also useful for marking or etching serial numbers or part numbers during manufacturing.

LASERS ensure highly reliable processes across a wide variety of industrial and engineering sectors for welding, cutting and drilling parts during manufacturing or for repair. They are also useful for marking or etching serial numbers or part numbers during manufacturing.

These applications are used in automotive, consumer goods manufacture, defence and aerospace, general fabrication and shipbuilding industries.

Lasers have proved themselves as the tool of choice for flexible, rapid and cost-effective metal manufacturing of components — from one-offs to large batches.

Compared to other power beam processes, such as electron beam welding which is done in vacuum, processing by laser beam takes place with the aid of a gas atmosphere.

In laser material processing, the influence of many different parameters (such as beam energy, beam quality, focussing etc) is well researched and understood. However, the influence of the different parameters of a gas atmosphere (such as composition, pressure or flow) can not always be explained in a rational way.

Gas is often not proactively employed to improve the processing of metals. However, the careful use of process and shielding gases can lead to improved quality and profitability.

Laser beam cutting

When considering metal cutting and metal welding, 85 per cent of laser usage takes place in the field of cutting. Much research has been carried out to investigate why there is a large drop in laser cutting speed as the plate thickness increases.

In oxygen-assisted laser cutting of mild steel of plate thicknesses above 8mm, a rapid (mechanism 1) and then slow (mechanism 2) mechanism occurs.

Mechanism 1 is only fast in the upper part of thick plates, and in thin plates, because it can proceed there via a gas/gas reaction (oxygen-iron vapour). Mechanism 2 is slow because the gaseous oxygen first has to diffuse through the liquid iron oxide before it can react with the iron.

Laser beam welding

A plasma plume can occur in the gas above the laser weld, which impairs welding performance because the plume absorbs the laser beam – particularly so with CO2 lasers.

The formation of a plasma plume can be controlled by using a shielding gas that is less ionisable, such as helium.

However, if the laser beam power is not too high then part of the helium can be replaced by more readily ionisable gases such as argon and nitrogen, that is, helium can be replaced partly with argon or nitrogen without disrupting the welding process.

Depending on the laser power, the mixture ratios for helium/argon are approximately as follows: 4kW, 40/60; 6kW, 50/50; 8kW, 60/40. Care must be taken when admixing N2 because nitrogen can dissolve in iron, then leading to the formation of pores in areas where the weld overlaps (e.g. the start-stop region in circular welds).

Helium can transmit heat from hot to cold places much better than argon, air or nitrogen can. Therefore, helium is very effective at transmitting energy from the focus area where there are particularly high temperatures, to the environs. If the welding process shows uncontrolled ejection of melt or humping, the weld process can be improved by replacing some of the argon process gas with helium.

Fusion penetration can be controlled with active gas components (for example O2, CO2 in the process gas) by deliberately influencing the surface tension of weld pools, especially at lower welding speeds.

At these relatively slow welding speeds the Marangoni flows excited by this can change the geometry of the weld pool fundamentally (Marangoni flow is the circular flow of molten metal in the weld pool).

Welding with diode lasers normally takes place in the ‘thermal conduction’ regime (that is, no keyhole). However, the influence of active gas components in the weld process gas enables them to pass from thermal conduction welding to deep penetration welding (that is, with keyhole formation, and therefore capable of welding thicker section material) without changing the welding parameters. This occurs by merely changing from inert gas to active gas.

The influence of gas in laser beam brazing

Production processes involving laser beam brazing of zinc coated steel plate with CuSi3 usually works in air without shielding gas.

This braze is relatively viscous and relatively easily traps air mechanically when it spills out of the melt pool. Only part of this trapped air can escape before the braze solidifies and the rest then remains in the braze joint as pores.

If the oxygen content is raised above the 20 per cent level contained in air by admixing a suitable process gas, the surface tension of the braze is reduced and the air can escape more easily, thereby reducing the number of pores.

In laser beam brazing of a zinc coated plate with aluminium, a perfect bond is obtained on the zinc coat side of the plate. On the aluminium side, the aluminium oxide skin that is always present there prevents good wetting by the braze.

For this reason the oxide skin is removed in-situ by means of an upstream plasma torch, or gaseous flux in the shielding gas.

Is it possible to conduct laser beam welding without shielding gas?

When welding under an inert gas, the product is free of oxides. This enables good wetting.

The negative effect of oxygen or air humidity is not manifest in normal tensile tests, but in bending stress fatigue tests, especially on higher-strength steels, endurance can be reduced a factor of ten.

* Dr. Wolfgang Danzer is head of The Linde Group Laser Application Development Department, based in Munich. He has a degree in welding engineering, and a Doctorate in Chemistry, from Munich University. Dr. Danzer has been with the Linde Group since 1976.

BOC -a member of the Linde Group, 131 262.