Using lasers to cut thick plate
July 12, 2001
The most common power levels ranged between 1,500 and 2,000 watts. However, a statistical survey conducted by the AMT Laser System Product Group indicates a steady increase during the last 12 months of installations for high-power 3,000- to 4,000-watt laser systems and a decline in sales of lasers with power levels less than 2,000 watts.
Industry trends indicate that metal fabricators increasingly are selecting higher-power lasers for cutting applications. Two years ago, the most common power levels ranged between 1,500 and 2,000 watts. However, a statistical survey conducted by the AMT Laser System Product Group indicates a steady increase during the last 12 months of installations for high-power 3,000- to 4,000-watt laser systems and a decline in sales of lasers with power levels less than 2,000 watts. Most recently, the North American market has experienced an interest in laser power levels from 4,000 to 6,000 watts.
Today, some experienced fabricators are moving toward purchasing higher power levels for their second systems to expand the laser cutting portion of their businesses into thicker material ranges. Some first-time laser users are buying higher-power systems to differentiate themselves from their competitors. These users may not have an application requiring high power, but they are willing to cultivate the market opportunities that exist for this work.
This article presents some of the technical aspects involved in high-power, thick-section laser cutting and addresses applications and market segments for this technology.
High-power lasers can process thick stainless steel with an oxide-free edge. A 6,000-watt laser can process up to 11/4-inch stainless steel in this fashion. In this application, high-pressure nitrogen commonly is used as an assist gas. In high-pressure or inert-gas cutting, the primary function of the assist gas is to shield the cut edge from oxide buildup and to blow the molten material quickly and cleanly through the kerf before it sticks to the edge and forms a burr.
The inert nitrogen gas serves little function in aiding the burning process of the material. This is different in reactive-gas cutting, for which the assist gas—usually oxygen—provides a fair amount of the energy used to burn the material. In inert-gas cutting, the laser provides almost all of the energy required to burn the material.
The nitrogen assist gas is fed into the nozzle coaxial to the focused laser beam. The gas flows through the nozzle orifice and down into the kerf at a high speed. The focus of the beam—the most powerful portion—is set deep into the material. This allows the upper part of the kerf to become wider, increasing the flow of laser gas down into the cut.
A high-pressure tip is used for nitrogen assist gas cutting. This device is designed to maximize the speed and force at which the assist gas exits the nozzle. Assist gas pressures usually range between 300 and 400 pounds per square inch (PSI) when cutting thick stainless steel. Thinner stainless can be cut with pressures in the lower ranges of 100 to 200 PSI.
The absence of an oxide layer, or blackening, on the laser cut edge means that stainless parts can be welded without extensive grinding. The oxide layer must be removed from parts that are cut using oxygen assist gas to ensure weld integrity.
Secondary grinding can be time-consuming, especially on thicker stainless steel from which a significant amount of the oxide-contaminated edge must be removed. Because most of this grinding is done by hand, parts can be difficult to fit up and fixture for welding. As a result of this inconsistency, these parts often can be difficult to weld manually and nearly impossible to weld robotically.
Parts cut using nitrogen have a shiny bright edge and can move directly to welding as soon as they are removed from the machine. The finished parts have squared edges with no taper and are free of burrs. No rework of the parts is required; they are simply removed from the laser cutting machine and passed on to the next cell for value-added operations.
The high-pressure gas flow also helps keep the part cool. This, combined with the very small but concentrated spot size of the laser, results in the smaller heat-affected zone (HAZ) (see Figure 1).
Capacity and Productivity. Using a 4,000- to 6,000-watt laser, 3/4- and 1.0-inch-thick stainless steel can be processed at competitive feed rates. Although the feed rates often are not as fast as those of plasma, secondary operations associated with plasma are reduced or eliminated.
For cases in which the laser's small HAZ may still pose a problem, some fabricators find it cost-effective to cut the blank on the laser first and then trim the part using a mill. The laser is used to rough out the part at a higher feed rate than a mill or electrical discharge machining (EDM) could achieve, freeing up these machines for value-added work.
A 6,000-watt laser also cuts thinner material quickly and with good edge quality. For example, 0.160-inch-thick stainless steel can be processed three times faster with a 6,000-watt laser as with a 2,000-watt laser. The same material can be cut more than twice as fast with a 6,000-watt laser as with a 3,000-watt laser.
Operating Costs. The operating costs for processing thick stainless must be examined closely. Hourly costs for processing 1/2- to 1-1/4-inch stainless steel with a 6,000-watt laser can be as high as $120 per hour. When processing stainless with nitrogen, large volumes of nitrogen assist gas are used, so more than 90 percent of the hourly operating costs are associated with the use of the assist gas. Less than 10 percent of the operating costs are absorbed by the actual running and maintenance cost of the laser.
Costs initially may seem high, but a big-picture approach to examining the cost of this type of work is required to justify the laser's use. While the cost of processing thick stainless parts with a laser tends to be higher than laser processing of carbon steel, the thick stainless parts cut by the laser usually are parts that can be cut only by higher-cost processes such as machining and wire EDM. The elimination of most secondary and nonvalue-added operations such as grinding and deburring usually offsets this cost.
Further time savings can be found downstream in the form of reduced fixturing time and improved fit-up after laser processing. In many cases, the feed rates achieved with the laser also allow the part to be produced faster, so the overall cycle time is reduced.
For example, a 3/4-inch-thick stainless steel part that used to be processed on a milling machine in 100 minutes can be processed on a 6,000-watt cutting system in 18 minutes. In addition, the part produced on the laser requires no secondary operations. Because the part spends less time on the machine, it is produced at a lower cost, and the machine is freed up to take on additional jobs.
Another application for high-power lasers is the cutting of carbon steel. A 4,000-watt laser can process up to 1.0-inch-thick carbon steel, and a 6,000-watt laser can process 1-5/8-inch-thick carbon steel.
Oxygen always is used to process plate in this material thickness range. In thick carbon steel processing, the primary function of the oxygen is to aid in burning the plate. It also helps eject the molten material. Typically, assist gas pressure and volume are very low. For instance, between 6 and 8 PSI of oxygen typically are used to process 1-5/8-inch-thick plate.
As in inert-gas cutting, the assist gas (oxygen) flows through the nozzle orifice and down into the kerf. While a solid flow of assist gas is required to eject the molten material from the kerf, assist gas pressures that are too high tend to cause uncontrollable burning. Once the burning process is started, very little assist gas is required to sustain the combustion process.
However, the molten material still must be cleared by a solid flow of the assist gas. If a standard nozzle is used, shock waves in the assist gas column will cause the cut edge to appear very striated and gouged. Annular flow nozzles can prevent this problem, and these are discussed in more detail later in this article.
Capacity and Productivity. Higher laser power is useful primarily in increasing capacity in carbon steel. In general, gains in feed rates are nominal as power is increased. There is a power/feed rate threshold at which a given material thickness can be cut using oxygen. Pouring more power into the material does not necessarily increase the rate at which the material is processed, as there comes a point at which the process has to wait for the exothermic reaction of the oxygen to take hold. The material can be processed only as fast as the oxygen will allow it to burn, which is why the productivity curve levels off in the higher powers when processing thinner materials.
Some original equipment manufacturers (OEMs) currently are experimenting with nitrogen cutting of thinner carbon steel. Because nitrogen is an inert gas, additional power can be used to increase the feed rate without waiting for the exothermic reaction of the oxygen to take place. The result is an edge with no oxide layer or burr.
However, this process is somewhat limited in the material thickness it can cut. With a 4,000-watt laser, 3/8-inch-thick carbon steel usually is the maximum practical thickness; with a 6,000-watt laser, material thickness can be increased to 1/2 inch. In thicker materials, a small but very hard and difficult-to-remove burr is formed. Processing carbon steel with nitrogen tends to solve some of the problems associated with adherence of paint to cut edges because the oxide layer is not an issue.
Operating Costs. Typical operating costs for processing carbon steel with oxygen range from $9 to $14 per hour. The cost of laser cutting carbon steel plate is lower than that for stainless and does not vary as greatly for different material thicknesses. This is primarily because lower assist gas pressure and volume are used when processing carbon steel. The typical oxygen assist gas pressure range for carbon steel is between 5 and 25 PSI. Nitrogen pressures often can vary between 100 and 400 PSI.
Because of the pressure differences, and because oxygen is less expensive than nitrogen, the assist gas component makes up a considerably smaller percentage of the overall cost of carbon steel cutting as opposed to stainless steel cutting. In fact, less than 10 percent of the cost of carbon steel processing is for assist gas.
The most important factor in laser cutting of thick-section plate is plate quality. Plate quality can vary drastically, and obtaining consistent results with the laser depends solely on obtaining good-quality plate. Inclusions or high silica content will cause disruption or eruptions when cutting. Scale adherence, carbon content, and residual element ratios also are factors in producing consistent, high-quality results. Feed rates may be affected drastically by the chemical makeup of the plate.
Users often wrongly assume that they will be forced to pay a high premium for plate that cuts well on a laser. Actually, good-quality plate is available at prices comparable to those of standard plate. Recently, many plate suppliers have been experimenting and developing plate grades specifically designed for laser cutting.
In both Europe and Asia, steel manufacturers have been providing users with laser-grade plate for some time now. This is due in part to the early acceptance of high-powered lasers for processing thick plate in those areas. In the U.S., leading plate manufacturers are working with high-power laser suppliers and large manufacturing companies that laser-cut thick plate to develop a better understanding of what makes good laser-grade material.
For laser processing of very thick plate, it is important to contact a steel manufacturer first. To get the quality and thickness required, fabricators may get the best results by dealing directly with the plate mills.
It is not difficult to obtain laser-friendly grades of plate. These grades are not significantly more expensive than the standard grades and are less expensive than paying to have standard grades of thick plate pickled and then struggling with inconsistent results (see Figure 2).
The cutting nozzle design is a concern with stainless but is a bigger issue for thick carbon steel processing. In thick plate material, ejection is key to achieving a quality cut, as a tremendous amount of molten material must be ejected when cutting 1-5/8-inch-thick carbon steel.
It is important to get as much oxygen as possible into the kerf to aid in burning the material and to ensure that it is ejected cleanly from the kerf. To optimize this process, an annular nozzle may be used.
Like a standard cutting nozzle, the annular nozzle has a single orifice through which the laser is centered and through which the assist gas column flows. Unlike a standard nozzle, the annular nozzle has several other orifices circling the main orifice. These additional orifices are used to get more assist gas flowing down into the kerf. Because more assist gas jets surround the main orifice, the higher volumes of gas move into the kerf at lower pressures and without causing the turbulence or shock waves associated with a single, more powerful jet of assist gas.
If a standard single orifice nozzle is used, too much pressure is needed to force the gas into the kerf. The increased force of the assist gas jet creates turbulence or shock waves in the kerf at the cutting front. The waves cause the cut to burn uncontrollably and gouge out the material rather than cut it cleanly and smoothly.
One of the most important aspects of beam delivery for a high-power laser system is the beam purge. Beam purge can be defined as a low flow of clean, dry air or nitrogen that is introduced into the enclosed laser beam path. This flow of gas keeps a positive pressure inside the enclosed beam path to keep contaminants (particulate or vapors) out of the path of the laser beam.
A good beam purge is required to obtain consistent performance from any laser system, regardless of the power level; at power levels higher than 3,000 watts, it is absolutely essential because problems created by poor-quality beam purge often are magnified at these power levels.
On any CO2laser system, a number of external optics are used to transport the unfocused raw beam from the laser source to the workpiece. The path that runs between each external optic is enclosed in a hard conduit or collapsible bellows. This covering provides protection from accidental exposure to the beam and protects the beam from exposure to contamination.
The external beam path is subject to contaminants such as dirt, grinding dust, and other large particulate that can distort the shape of the raw beam if introduced into the beam path. Less obvious contaminants such as paint and chemical fumes, oil mist, water vapor, and other sources of hydrocarbons also can cause distortion of the raw beam, creating disastrous results. This distortion is known as thermal blooming, or abnormal widening of the laser beam. The raw beam focus capability is changed as the focal point shifts in and out of the material, creating inconsistent and unacceptable cutting results.
To prevent this occurrence and keep the focal point constant, a proper beam purge must be used. Most manufacturers of laser systems usually purge the external beam path with a clean, dry flow of gas or air. This keeps a positive pressure of flowing gas within the beam delivery path and prevents shop contamination from entering.
Laser users often mistakenly assume that a system can operate with beam purges turned down and still maintain peak performance. As an alternative for saving money, air can be used in place of nitrogen as the beam purge gas. However, the air must be clean, dry, and oil-free. It is not adequate to use standard air from a shop compressor to purge the beam path. Oil mist, water vapor, and hydrocarbons can be found in the cleanest shop air, even when a compressor with an air dryer is used.
The best solution is to obtain a secondary air dryer and locate it close to the laser machine. The air dryer should filter the air to 1 micron and regulate the dew point to 40 degrees Fahrenheit or lower. Using a dedicated air dryer and leaving the purge turned up at all times will help prevent countless hours of troubleshooting poor cut quality.
Recently, optics manufacturers have been promoting low-absorption coatings on optics used in conjunction with high-power laser sources. These coatings are applied specifically to the output coupler and focal lens and are designed to prolong the life of the optics for high-power applications.
In higher-power applications, both the output coupler and focus lens are exposed to tremendous amounts of concentrated energy and heat. Low-absorption coatings work to minimize the amount of optic distortion from this effect and, in doing so, reduce premature thermal lensing of the output coupler and focal point shift of the lens.
While low-absorption optics are relatively new to the industry, they offer some theoretical advantages over standard optics. The benefits of their use depend on the application. Low-absorption optics cost 30 to 50 percent more than the uncoated optics, but if focal point shifting or premature lensing appears to be causing problems, the low-absorption optics may be worth consideration.
Another recent development is the use of constant-beam-path devices for high-power systems. These devices keep the beam path at a constant or near constant length in an attempt to ensure uniform cut quality in all quadrants of the machine table.
High-power systems usually have large tables, and most are flying-optic machines. The optical path of these systems can vary, on average, from 7 meters in the near field (where the cutting head is positioned closest to the laser source) up to 12 meters in the far field (where the cutting head is farthest from the laser source). This large distance may cause the beam to change slightly in diameter in the near and far fields, which in turn changes the laser's cutting capabilities in those fields.
Because high-power lasers are designed to allow users to maximize cutting performance by cutting thick material, most of these systems use a constant-beam-path device of some type. These devices fall into several categories, including:
1. Articulated arm—This type of device is essentially a jointed, segmented tubular arm. One end of the arm is connected to the output of the laser; the other end is attached to the cutting head. Each joint in the arm has a mirror in it to direct the beam from the output of the laser to the cutting head. The diameter of the tube is sized to allow the unfocused beam to fit without any interference. The articulated arm also should be equipped with an air or nitrogen purge.
Using the arm ensures that the beam path is kept at a constant distance no matter where the cutting head is positioned. This ensures that the laser will cut the maximum thickness of material consistently, regardless of the table quadrant in which cutting occurs.
2. Optic slide—Another kind of constant-beam-path device is the optic slide, often referred to as a trombone style. The device functions similarly to the articulated arm but uses a reflective optic mounted on a slide. The slide is controlled by a servomotor and drive system that is synchronized to move with the X and Y axes to maintain a predetermined optical path distance.
Both the articulated arm and the optic slide device are equally effective in obtaining consistent cut quality in both near and far fields. They should be considered essential if a laser user plans on operating a laser at maximum cutting capacity and achieving consistent cutting results.
High-power laser sources are providing metal fabricators with expanded processing capabilities for thick-section cutting, as well as the ability to process thin materials faster. They are allowing these users to expand into applications previously reserved for computer numerically controlled (CNC) milling and wire EDM equipment, while achieving a narrow kerf, no bevel, small HAZ, and reduced secondary operations and scrap.
Of course, fabricators have different cutting requirements, and not all would benefit from using a high-power laser. The best way to determine whether a high-power laser is right for a specific application is to consult a laser source supplier.