Selecting the right mix of assist gases for the material to be cut is a key decision that should not be taken lightly. Here we see the resuts of different gases on different types of steel. The type of gas and material on each photo highlight the differences.Click image to enlargeby Noelle Stapinsky

Fabricators can optimize fiber laser cutting processes by improving assist gas management

With the rapidly growing popularity of fiber laser cutting technology, increased power and speeds means that laser cutting assist gas has become a very important subject for many fabricators. Although termed as an assist gas, it’s definitely more of star player because how it’s used and the prescribed pressures required all effect the overall cut quality. And too often, when troubleshooting a not-so-clean cut, the assist gas is often overlooked. 

SHOP Metalworking Technology asked industry professionals to weigh in on the importance of assist gas, trends and troubleshooting.

 

What are the recent changes and trends for laser cutting assist gas?

Allen Vines, Applications Manager for BLM Group: There have been some real improvements in both speed and quality using high pressure, zero grade air. This change has been occurring on flat lasers for the last few years and now BLM GROUP is leading the way in processing tube with air as well. When cutting mild steel materials, we are seeing a high reduction in costs up to 75 per cent, with speeds matching or exceeding nitrogen by up to 15 per cent. Where dry air cutting is really leading the way is with aluminum processing. With greatly reduced burr on the material, smoother edge quality and up to 150 per cent increases in speed (on 1/8-inch thick aluminum), zero grade air is definitely a process that any aluminum producer would want to add to their shop.

Stefan Colle, laser product sales manager for LVD North America: With a growing population of higher-powered fiber lasers that use nitrogen as the preferred assist gas and can require a large volume of this inert gas, this has become a very important subject. 

The purpose of the assist gas is to evacuate the molten material from the cut. That’s done using a fairly high pressure of assist gas. With nitrogen as the assist gas, you are cutting at 300 PSI. Using oxygen for thicker materials (1-inch or greater), you’re typically cutting with low pressure at a maximum of 10 PSI. So, the consumption level for nitrogen assist gas can be substantial.

Laser manufacturers like LVD are looking at various ways to reduce cutting gas consumption. For instance, we introduced a different type of nozzle—a chamber nozzle—to deliver the assist gas to the material. This unique nozzle better directs the assist gas and, because of this, offers a significant improvement in efficiency. Instead of cutting at 300 PSI, we can cut at 100 PSI and, as a result, are consuming a lot less nitrogen. The chamber nozzle isn’t suitable for all material thicknesses and it comes with a certain trade-off in edge quality—not a sub-standard edge quality, just a different edge quality. The technology tables incorporated into our laser cutting machines are pre-loaded with the settings for this nozzle technology. The operator can choose the conventional method or the chamber nozzle method depending on the job. 

We are also investigating more efficient assist gas mixtures that lower the amount of nitrogen needed while improving cut speeds and maintaining edge quality. 

With high-powered lasers we’ve seen advantages when you mix the nitrogen assist gas with a certain amount of oxygen to produce what is known as “dirty nitrogen.” It’s a bit of a misnomer, because this is clean nitrogen (99.9 per cent purity) with a mixture of some oxygen (say 10- or 20-per cent). Advancements in gas mixing—combining nitrogen and oxygen—offer a solution to lower the volume of nitrogen that’s needed while producing a clean, oxide-free cut, which means there’s no need for a secondary operation, you can take the part straight to welding or painting.

Brett Thompson, TruLaser technology and sales consultancy manager for TRUMPF: There have been many trends. As power goes up so do speeds and therefore the need for large amounts of assist gas. With large nozzle diameters nitrogen consumption can be well north of 3500 cubic feet per hour unless you have a low-pressure technology like Highspeed/Highspeed Eco (around 1200 cubic feet per hour). In situations where Highspeed or Highspeed Eco is not present some are opting for high-pressure compressed air systems to combat the substantial nitrogen costs.

 

Can you provide examples of assist gas use with your laser systems, and what is best for certain materials or thicknesses?

Vines: The power level really controls which gases can be used with different material types. 

Zero grade dry air is by far the best way to go with aluminum. With virtually dross free cuts and smooth edges up to 3/8-inch at 3kW and 1-inch at 8kW and higher, the low cost and speed of operation is hard to beat. Nitrogen can be used as well but would result in a higher cost with no speed benefits. 

Nitrogen continues to be the best method for processing stainless with gas purity dictating the cleanness of the edge quality. Though air will cut stainless, it leaves a blackened edge due to the 21 per cent oxygen content in air, so the use of air would be depend on the desired end result.  As for mild steel, this is where power and gases have the most variety of choice. Air is the lowest cost and provides comparable speeds to nitrogen. However, power levels will limit the thickness of which air can be used (up to 1/8-inch at 3kW or ¼-inch at 6kW and above). Additionally, air leaves an impregnated oxygen layer in the material that may affect some powder coating treatments. Oxygen is low cost and can be used on virtually any thickness level your laser power dictates (up to ¾-inch thick at 3kW and 1-inch at 4kW and higher). Unfortunately the exothermic reaction of oxygen results in much slower speeds than nitrogen on thinner materials. Additionally, all oxygen cuts leave an oxidized edge that normally needs to be cleaned off before powder coating to prevent paint chipping. Nitrogen is used for faster speeds in ¼-inch and under and leaves a completely clean edge that has no reaction to paint or powder coating so it is the universal “best case” gas.

Colle: To make a decision on what type of assist gas to use, you need to weigh processing speed, edge quality requirements, and cost of operation. There are a number of variables to consider—too many, to quickly and/or easily address this question. It’s best for the fabricator to consult with the machine manufacturer for guidance.

Thompson: Generally there are two cutting processes: exothermic and fusion. Oxygen cutting is exothermic whereby the oxygen burns, using the heat from the flame to cut. Fusion cutting uses the laser beam to melt the material and an inert gas (e.g. nitrogen) to eject the material from the kerf and to shield the material. It’s becoming more apparent that there are situations where mixing the gases can be helpful. Cutting thick steel with a mixed gas rather than straight nitrogen produces a cleaner bottom condition and typically offers greater feed rates.

Vines: The power level really controls which gases can be used with different material types. 

Zero grade dry air is by far the best way to go with aluminum. With virtually dross free cuts and smooth edges up to 3/8-inch at 3kW and 1-inch at 8kW and higher, the low cost and speed of operation is hard to beat. Nitrogen can be used as well but would result in a higher cost with no speed benefits. 

Nitrogen continues to be the best method for processing stainless with gas purity dictating the cleanness of the edge quality. Though air will cut stainless, it leaves a blackened edge due to the 21 per cent oxygen content in air, so the use of air would be depend on the desired end result.  As for mild steel, this is where power and gases have the most variety of choice. Air is the lowest cost and provides comparable speeds to nitrogen. However, power levels will limit the thickness of which air can be used (up to 1/8-inch at 3kW or ¼-inch at 6kW and above). Additionally, air leaves an impregnated oxygen layer in the material that may affect some powder coating treatments. Oxygen is low cost and can be used on virtually any thickness level your laser power dictates (up to ¾-inch thick at 3kW and 1-inch at 4kW and higher). Unfortunately the exothermic reaction of oxygen results in much slower speeds than nitrogen on thinner materials. Additionally, all oxygen cuts leave an oxidized edge that normally needs to be cleaned off before powder coating to prevent paint chipping. Nitrogen is used for faster speeds in ¼-inch and under and leaves a completely clean edge that has no reaction to paint or powder coating so it is the universal “best case” gas. 

Colle: To make a decision on what type of assist gas to use, you need to weigh processing speed, edge quality requirements, and cost of operation. There are a number of variables to consider—too many, to quickly and/or easily address this question. It’s best for the fabricator to consult with the machine manufacturer for guidance.

Thompson: Generally there are two cutting processes: exothermic and fusion. Oxygen cutting is exothermic whereby the oxygen burns, using the heat from the flame to cut. Fusion cutting uses the laser beam to melt the material and an inert gas (e.g. nitrogen) to eject the material from the kerf and to shield the material. It’s becoming more apparent that there are situations where mixing the gases can be helpful. Cutting thick steel with a mixed gas rather than straight nitrogen produces a cleaner bottom condition and typically offers greater feed rates.

What are some common troubleshooting tips for fabricators?
Vines: Focus, gas and speed are the big three when troubleshooting cut conditions with air or nitrogen. The way gas can affect the cut would be the following: too little gas will not push the molten material out of the cut and cause the cut to not go all the way through the material (or possibly refill the bottom of the cut with oxygen). Too much gas can cause excess dross (slag) to reconnect to the material since the process is cooled too fast before the molten material can separate from the cut, or will cause excessive gouging in the cut surface in the case of oxygen.

Colle: A lot of advanced technology is built into today’s laser cutting machine, so there’s not much in the way of troubleshooting required by the operator. Over the years, cutting technology has evolved so that if there’s a drop in assist gas pressure—there isn’t enough gas pressure—the machine will automatically stop cutting. It won’t continue to cut and produce bad parts. This is a safeguard that’s found in all modern laser machines because cutting without assist gas is both dangerous (it could cause a potential fire) and expensive. Doing so can immediately damage the laser lens and the cutting head window. 

Is air becoming a big thing and why?

Thompson: Air is becoming a big thing. As laser powers go up so does gas consumption to accommodate the increased feed rate. Innovative nozzle technologies such as Highspeed Eco can harness the speeds of high power laser systems without the need to increase gas usage because all the gas is going through the kerf and not being wasted. Air cutting is used because it’s a less expensive way to cut. Even with low-pressure technologies like Highspeed Eco it can be helpful, but on lasers where Highspeed Eco isn’t present it’s really a must; the increase in nitrogen consumption could nearly offset the speed gains on these high power systems if nitrogen is always the main assist gas.

Colle: When people talk about “air” they are referring to air coming from their compressor. What they don’t understand is that air is actually nitrogen, but it’s just not pure nitrogen, as I explained before. Some fabricators are trying to use their compressor to deliver this gas mixture, but a compressor can’t provide the volume that’s needed. That’s why an increasing number of laser users are turning to a nitrogen generator. The nitrogen generators allows the user to vary the purity of the gas and is a way for them to reliably and cost-effectively have more control over their gas consumption.

We’ve recommended nitrogen generators to users who want a high level of flexibility.  

What are the most efficient delivery systems and why?

Vines: Gas generation systems allow the company to control how much gas they have at any point and time. Over time they also provide the best ROI and avoid being tied into long term contracts with gas suppliers. 

Bulk systems work well but require long-term contracts and high gas usage to get the best rates, and having a quality supplier that can come fill your tank when needed. 

Colle: For some laser users, a nitrogen generator can be a highly efficient choice. There are two types of these systems—a membrane system that requires a certain level of maintenance and a PSA generator that’s less maintenance intensive but more expensive. These systems are gaining in popularity. They offer the advantage of having nitrogen available when it’s needed, provide a degree of cost savings and can oftentimes be customized to the user’s needs.  SMT

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