The Thermal Effects of Laser Cutting

reprinted from The Fabricator, January 1997

Originally presented at the Alternative Methods for Precision Sheet Metal Fabricating Conference, June 1996, San Jose, CA

by Leonard Migliore




Laser cutting has become a popular method of producing short runs of sheet metal parts. Current laser systems are able to easily cut out any shape presented to them; their computers can generate tool paths, set cutting parameters and optimize nesting so that the manufacturing process is highly automated. It's getting so that you show the machine a picture of what you want and tell it how many of them you need; a pallet full of parts appears on your shop floor shortly thereafter. There is, however, a danger associated with this degree of automation: it allows us to forget that laser cutting is a thermal process, and metal is being melted and otherwise altered so we can get our parts cut. We can get away with this pretty often, but for critical work, it is important to be aware of the effects of the process on the product.


Principles
Metals, and most other materials, are laser-cut using one of two quite similar methods. The first, gas-assisted fusion cutting, is performed by concentrating the light from a laser onto a surface so that the material melts. The melted material is then removed by flowing gas. The second method, reactive gas-assisted cutting, is the same except that the gas reacts with the material, providing more energy for the process. The typical arrangement of equipment is shown in Figure 1. The laser beam, aimed vertically down in the drawing, is focused by a lens onto the surface of the material to be cut. The space between the lens and the work is enclosed by a chamber. This chamber is pressurized by incoming assist gas which exits through a nozzle onto the workpiece. This arrangement is sometimes called a "coaxial gas jet cutting nozzle".

The important characteristics of this process are:
Metals are melted, not vaporized. This means that laser cutting only works well when it goes completely through the workpiece so that the molten material can be ejected. It also means that the process is subject to recast material adhering to the parts.

Metal is removed by the gas, not by the laser. The laser only serves as a heat source to melt the metal. Many process variables in laser cutting relate to gas flow rather than to laser settings.

The front edge of the cut is slightly inclined from the vertical. This means that the process is very sensitive to the polarization of the laser and that, as material gets thicker, the beam does not reach the bottom edge of the cut.


Carbon Steel
The most common application of lasers in fabrication is the cutting of carbon steel sheet. Oxygen is generally used as an assist gas. The energy liberated by the reaction 2Fe + O2 yield 2FeO is roughly equivalent to the laser power. (footnote
) With oxygen, then, the power available for melting is doubled compared to that available using inert gas. The cutting region is hotter, making the molten material more fluid and easier for the gas to eject. The cut edge, though, is covered with a thin layer of iron oxide. This adversely affects subsequent welding or coating processes.

The purity of the oxygen assist gas has a dramatic effect on cutting speed and quality. Tests show that lowering oxygen purity from 99.998% to 99.5% drops the cutting speed by over 20%. (footnote 2) It is not immediately obvious why this should occur. The tremendously exothermic reaction between steel and oxygen is not significantly slowed down by such a small loss in O2 concentration. Surprisingly, the very strength of this reaction is the reason for its sensitivity to contamination.

Figure 2, a schematic cross-section of the cutting process, shows why small amounts of inert impurities have such a great effect on the reaction. The molten steel has a very great affinity for oxygen and so removes it from the assist gas. It does not react with impurities such as nitrogen, so they remain. As the assist gas flows down the cut edge, an aerodynamic boundary layer forms where gas velocity is low and there is little mixing. The concentration of impurity atoms rises significantly in this boundary layer because oxygen is being extracted at the liquid metal interface. The metal, then is not being cut with 99.5% oxygen but with a far lower concentration, resulting in a significantly lower reaction rate, surface temperature, and cutting speed.

A similar effect is seen when cutting thick (.38" or greater) steel. Even if the assist gas is pure coming out of the nozzle, it entrains air and has an appreciable nitrogen content at the bottom of the kerf. Cutting performance with thick steel can be considerably improved by using a compound nozzle that surrounds the central coaxial oxygen jet with a lower-velocity annular oxygen flow. The outer gas ring insures that the gas entrained by the cutting jet will still be oxygen.

The striations generally seen on laser-cut edges are affected by the exothermic reaction of iron and oxygen. In reactive gas-assisted laser cutting of steel, the melt front moves more rapidly than the laser spot until it is quenched by the bulk material. The spot then catches up with the front and the cycle repeats. Appropriate combinations of laser power, travel speed and oxygen flow damp the oscillations and produce quite smooth edges. If the oxygen flow is increased, the oscillations become stronger and increase the surface roughness.


Thermal effects on steel
Laser cutting is done by melting metal. The edge of a laser-cut part, then, has been raised to its melting point. All sorts of effects are caused by this.

For cold-rolled low-carbon steels, the edge is softened. There is a small annealed zone extending a few thousandths of an inch in from the cut surface. It is not generally a problem in subsequent finishing operations or in the function of the completed articles.

If the steel's carbon level is above 0.3%, however, martensitic transformations occur which create a hardened edge. Since the cut edge had to have been heated to the melting point of steel (steel melts around 2800 °F), the material adjacent to the cut must also have been very hot. Steel whose temperature is raised above 1650°F transforms to austenite; rapid cooling of austenite forms martensite, which is what "hard" steels are made of.

This effect can, in some cases, be beneficial. Certain tools, such as parts for pancake dies, can be shaped and hardened in the same operation; in other parts, the hardened edge confers improved properties such as wear resistance. In other situations, it can be a problem. Mounting holes in hardenable steel are subject to cracking; if a part with hard edges is formed after cutting, the edge may not form properly or even crack.

The high process rates typical of laser cutting generate some peculiar microstructures in as-rolled steel. While it is true that steel turns into austenite above 1650 °F, the resulting material is not homogeneous. The carbon, which is usually in the form of iron carbide, must diffuse into the low-carbon ferrite areas. If the original structure was coarse, the time required for diffusion will be greater than the duration of heating. When the steel is cooled, the high-carbon regions will be hard and the low-carbon regions will be soft. Such mixed microstructures generally have poor mechanical properties. If it is desired to produce a hardened edge by laser cutting, it is best to heat treat the sheet prior to cutting so that it has a uniform microstructure. A good way to do this is to harden the material and then temper it down to the desired bulk hardness. The fine microstructure of hardened and tempered steel responds quickly and uniformly to the rapid heating and cooling cycles experienced during laser cutting.


Alloy steel
Low-alloy steels such as 4140 and 8620 respond very well to laser cutting. Steels with greater levels of alloy additions, such as many tool steels, do not cut as well. Many alloying elements, such as chromium, increase the viscosity of the melt and form adherent oxides. This leaves a dark, adherent scale on the surface and produces rough edges on thick (over .25") workpieces. Any alloy steel with an appreciable carbon level will form martensite on the cut edge.


Stainless steel
Since all stainless steels are high-chromium alloy steels, they behave as described above when cut using oxygen assist: the edges have a heavy chromium oxide layer. This is, in many cases, detrimental to the subsequent processing or use of the parts. The substitution of nitrogen, which does not react in any appreciable way with stainless steel, as the assist gas, prevents this layer from forming.

An inert assist gas requires that all the power for melting come from the laser. For any given laser, the process speed and thickness capability will be less for inert gas than for oxygen. The viscosity of the molten material is quite high, and dross adhesion to the bottom edge of the material is a problem. This is generally addressed by using very high assist gas pressures.

Very little happens metallurgically to stainless steel as a result of laser cutting. For austenitic steels, some softening of the edge occurs. Ferritic and martensitic stainless have edge hardening.


Aluminum
Aluminum and its alloys are characterized by high reflectivity and thermal conductivity. This makes them relatively difficult to cut with lasers. Modern units with high power, well-focused beams can, however, cut the material reliably in thin sections. On many aluminum alloys, especially the high-strength heat-treatable 2000 and 7000 series, there is a strong tendency to create microcracks on the cut edge. (footnote 3) In aerospace applications, which are the primary market for these alloys, such an edge condition is unfavorable because it reduces the fatigue life of the part. It is generally allowable to use such parts only after the microcracked edge is mechanically removed. The cost of this removal operation usually makes laser cutting impractical. Abrasive water jet cutters, which do not create an observable heat affected zone, are much more useful for cutting aluminum for aerospace applications.


Titanium
Titanium and its alloys react strongly with oxygen and nitrogen. If titanium is cut with oxygen as an assist gas, it burns quite vigorously. If air is used, the cut edges have a thick layer of hard and brittle oxides and nitrides; such parts are useless for most purposes. Nitrogen produces similar results. The only assist gases appropriate for cutting titanium are completely inert ones such as argon and helium.
While argon, which is much less expensive than helium, seems to be a logical choice here, it is possible to encounter metallurgical problems when using it. Overheating of the cut edge can alter the phase balance of Ti 6Al 4V and generate an "alpha case" on the edge. This condition is unacceptable for many aerospace applications because it reduces the fatigue strength of the component. Helium, which has a very high heat capacity and thermal conductivity, is often mixed with argon to provide cooling during laser cutting. A 25% helium-argon mix is usually satisfactory, although 50% helium is sometimes needed.


Effects on Non-metals
Lasers can cut an extremely wide range of materials without regard to their hardness or electrical conductivity. While the majority of laser cutting is performed on metal, the process has the great value that it is not limited to hard, conductive materials.


Inorganic
Non-metallic inorganic materials, as a class, have low vapor pressures and poor thermal conductivities. These characteristics, combined with their generally high absorption of 10.6 mm light, should make them good candidates for laser cutting. Unfortunately, many common ceramics and glasses have very high melting points and poor thermal shock resistance. This tends to make them harder to process than metals.

Alumina
Fired alumina (Al2O3) is often cut or scribed by lasers. Cutting is performed using high power pulses to vaporize the material, since recast material is a problem. The high melting point of alumina, coupled with the low average power of lasers operating in the enhanced pulse mode, results in low cutting speeds. The process of scribing is the standard method of preparing alumina substrates for hybrid microcircuits. Scribing is performed by drilling rows of holes partially though the material. These perforations make it possible to snap the ceramic apart along the lines. For typical 0.6 mm thick alumina substrates, holes are drilled 0.2 mm deep and .0.18 mm apart. This allows a laser pulsing at 1000 Hz to scribe at 10 meters/min.

Quartz
The high thermal shock resistance of quartz allows it to be processed much like metal. Continuous CO2 radiation is used since quartz is quite transparent to the 1.06 mm light emitted by Nd:YAG lasers. Strains caused by thermal stresses must often be relieved by annealing of the parts after cutting.

Glass
The laser cutting of glass is limited by the poor thermal shock resistance of most compositions. This causes complex glass parts to crack apart after cutting. Glass also tends to have a lot of recast material on the cut edge because it does not have a well-defined melting point.

Organic
Organic materials are generally decomposed by laser light. The energy required to do this is usually much lower than that required to melt inorganic substances, so cutting can often be done at high speeds or with lower power lasers. The large volume of the decomposition products causes some problems: Gases in the kerf have trouble escaping, limiting process speeds and degrading edge quality. In addition, many organic materials evolve toxic compounds during laser cutting. These effluents must be handled in a manner to eliminate hazards to operators and to the environment.

Wood
The most common application of laser cutting of wood is the fabrication of steel rule dies. The material is typically birch plywood about 20 mm thick, and the kerfs must be accurately controlled so that steel blades can be inserted in them.

Cloth

Since it is so thin, cloth presents few problems for laser cutting. Most of the difficulties are related to the construction of systems capable of moving fast enough to keep up with the laser. Scanning systems, where computer controlled mirrors are used to steer the focused beam over the material being cut, allow much higher speeds than conventional systems which move the cutting head. Scanning systems cannot use assist gas and work by vaporizing the cloth.

Plastic
A wide variety of polymers are cut with lasers. The beam causes melting, vaporization, or decomposition of the material. Thermoplastics such as polypropylene and polystyrene are cut by shearing of molten material, while thermosets such as phenolics or epoxies are cut by decomposition. Materials which decompose in the beam leave carbon residues on the cut edge. This must often be removed by some operation such as bead blasting before the parts may be used. Decomposition products of laser cut polymers have been found to be quite hazardous.

Composites
Composites are materials consisting of two or more distinct constituents. Usually, one component is fibrous while the other forms a surrounding matrix. By selecting appropriate matrices and reinforcing elements, the material can be engineered to have properties optimized for a specific use. From the standpoint of laser cutting, the main differences between composites are whether the matrix, the fibers, or both are organic.

Organic
If organic fibers are set in an organic matrix, the laser has little difficulty cutting. Kevlar (aramid) fibers in an epoxy matrix, a common high-performance composite, is readily laser-cut in thicknesses up to 6 mm. Thicker sections exhibit considerable charring of the cut edge.

Organic–inorganic
The presence of inorganic materials changes the response of composites to laser heating. To cut fiberglass-epoxy, the laser must melt the glass. This takes much more energy than decomposing the epoxy, and so controls the processing rate. Graphite-epoxy is a very difficult material to cut because graphite must be heated to 3600 °C to vaporize it. Since graphite has fairly good thermal conductivity, the epoxy near the cutting zone is exposed to high temperatures which decompose it for a significant distance from the cut edge. Conventional lasers produce heavily charred edges when cutting this material. Excellent results have recently been obtained using a diode-pumped, Q-switched, frequency doubled Nd:YAG laser. This device produces short (~100 ns) pulses of green light which can be focused to a very small spot. The extremely high irradiance of the beam vaporizes graphite, while the short pulse duration decreases the amount of heat conducted by the remaining fibers.

Inorganic
Some of the highest-performance materials available today are metal-matrix composites. The addition of refractory fibers to a superalloy matrix produces tremendous strength at high temperatures combined with high toughness. Unfortunately, these characteristics also make it very difficult to machine such materials. Lasers have successfully cut several types of metal-matrix composites, and should see increasing use in this application. One effect that must be controlled is the melting back of the matrix from the cut edge, leaving exposed fibers. The use of high energy pulses, as produced by Nd:YAG lasers, minimizes this problem.


Conclusion
A laser cutter is not a magic wand. While it works on a wide variety of materials, laser cutting is a thermal process and the effects of that heat can often be detrimental to the parts being cut.

References

1. A. Ivarson, "On the Physics and Chemical Thermodynamics of Laser cutting", Thecknska Hogskolan Lulea, 1993, p 69.

2.A. Ivarson, p132.

3. N. Kar and L. Migliore, Metallurgical Characteristics of Laser Cut Aerospace Alloys, "ICALEO '90, Vol.71, Laser Material Processing (Laser Institute of America, 1991), P167-168.

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