Job knowledge for welders - Laser cutting: process variants

There are two commonly used types of industrial cutting laser, CO₂ and Nd:YAG. These differ in that the wavelength of infrared light produced is 10.6µm for CO₂ lasers and 1.06µm for Nd:YAG lasers. Both these types of lasers produce the cut by focusing a beam of monochromatic light to a very small spot size by lenses and mirrors giving power densities in the up to 10⁵ W/mm². This power density is sufficient to melt locally or even vaporise most materials. Once a through thickness zone of molten or vaporised material is generated (a keyhole), a jet of assist gas, delivered co-axially through the cutting nozzle, is used to eject this material from the kerf. (Fig 1).

The characteristics of the laser cutting process relate to the fact that the beam can be focused to a spot of less than 0.5mm diameter to achieve these very high power densities. The resulting cut edge is very square and the process is capable of cutting at very high speeds. The combination of an intensely concentrated heat source moving at high speeds also results in very little heat being transmitted to the surrounding material and, therefore, very little thermal distortion of parts.

The difference in wavelength between the two types of lasers is significant in that the shorter wavelength of the Nd:YAG laser enables the light to be transmitted to the workpiece by fibre optics allowing three dimensional cutting or trimming of parts.

Light from CO₂ lasers on the other hand are transmitted to the workpiece by mirrors or transmissive optics. Although three dimensional cutting systems are available for CO₂ lasers they are relatively cumbersome compared to fibre delivered Nd:YAG lasers and CO₂ lasers are more commonly used for two dimensional flat bed cutting.

Assist gases

The types of assist gases used to eject the material from the kerf can be classified as either reactive or inert. The CO₂ gas used in CO₂ lasers is not the assist gas, but one of the gases excited to produce the laser light in the lasing cavity, usually quite a distance from the cutting process head. The most commonly used reactive assist gases are oxygen or air. Oxygen is used primarily for cutting low alloy steels and readily reacts with iron at high temperatures producing additional heat energy which enables thicker parts to be cut or greater speeds to be achieved. This gas is delivered at relatively low pressures and flow rates and the process is referred to as 'low pressure oxygen cutting'. 

Inert assist gases commonly used are either nitrogen or argon. These provide no thermal assistance to the cutting process and are used simply to blow the molten material out of the kerf. They are used at pressures of around 10 bar and the process is referred to as 'high pressure inert gas cutting'. Inert gases can be used for alloys which readily oxidise in the presence of oxygen such as stainless steel, aluminium or titanium to give a very bright and clean cut edge. Occasionally, inert gases are recommended for cutting low alloy steels where the edges are to be subsequently laser welded. This reduces the formation of an oxidised layer on the face of the cut edge and will reduce porosity in the resulting weld.

Cut quality

The precision or dimensional accuracy of a cut is important as it helps to ensure correct part tolerances and fit-up, thus eliminating rework or secondary processing operations further down the production line. The main criteria used to assess the quality of a cut, together with typical values for lasers areas follows.

Kerf

Defined as the width of the cut at its widest point in millimetres, the kerf gives an indication of the minimum internal radius or feature that can be cut. Laser cuts possess a narrow to very narrow kerf width (0.5-1.0mm) for CO₂ and Nd:YAG lasers respectively.

Cut edge roughness, Rz,mm

Cut edge roughness is used to define the cosmetic appearance of a cut and can give an indication of whether subsequent machining operations are necessary. It is determined by an Rz value in microns (also known as the ISO 10 point height parameter). This is a measure of the surface roughness transverse to the cut edge produced by traversing at 2/3 depth with a stylus and taking an average value. Both CO₂ and Nd:YAG processes produce cuts with a low edge roughness (<50µm).

Cut edge squareness, U

Edge squareness is of interest because it gives an indication of the fit-up between two components and whether any post cutting machining operations will be necessary. It is defined in terms of the Perpendicularity and Angularity tolerance, U (mm). This is a measure, in millimetres, of how much the cut edge deviates from a perfect square edge. CO₂ and Nd:YAG lasers are capable of producing cuts with good edge squareness (<0.5mm). This is highlighted in Fig. 2.

Heat Affected Zone (HAZ) width

HAZ width is defined as the width of a detectable microstructural change measured perpendicular to the cut edge face. This is only applicable to alloys that are hardenable or heat treatable. The width of the HAZ is of interest because, due to the potential degradation of properties and this material may have to be removed before final assembly of the product. The concentrated heat source produced by both CO₂ and Nd:YAG lasers produces a very narrow HAZ (<0.5mm).

Dross

Dross describes the resolidified material that adheres to the bottom edge of a cut produced by a thermal process. Levels of dross are quantified subjectively, with none, light, medium and heavy being the terms used most commonly. For laser cutting, dross is light provided the cutting parameters are optimised.

Economics of laser cutting processes

Whilst most suited for precision cutting of thin sheet in the 1-15mm thickness range, both CO₂ and Nd:YAG laser cutting systems require high capital investment. The cost of purchasing laser equipment can range from £50k-£250k depending on the output power requirements of the system. Precision work handling equipment is required if a laser is to be used to its full potential, in terms of cutting speed and quality. When combined with a chiller unit, this can add a further £100k to the cost of implementing a laser system.

As a result, laser cutting systems typically are used where high cut quality requirements make their application essential, or where the initial investment is offset by the high production rates that can be reached as a result of their high cutting speeds on thin sheet materials as illustrated in Fig.3.

For low production volumes, sub-contracting work to laser job shops can offer an attractive alternative to such an investment.

This article was prepared by David Howse and Andrew Woloszyn