Matters of material
- Category: Laser Cutting
- Created: 2017-10-03
Overcoming metal cutting challenges, part 1: Steel and aluminum
by Patrick Medlin, president, Advanced Technology Sales & Service
In today’s cutting and fabricating environment, flexibility is paramount when it comes to finding success. Customer requests come in all shapes and sizes, testing fabricators’ mettle on a daily basis. Fortunately, today’s machines are equipped to handle a broad range of material types and thicknesses. The trick is understanding the nuances of the material and then leveraging your existing equipment for the best outcome.
Imagine this scenario: You’ve been asked to bid a substantial job, one that will keep your laser cutting machines operating at full capacity and provide a much needed boost to the company’s bottom line. However, winning the job could be a curse to your company, your customer and, ultimately, your career if you don’t have the necessary experience cutting the type of metal needed to fill the order.
Metal cutting can be a minefield of hidden obstacles. But when you identify the challenges associated with various metals and thicknesses, it will be easier to take on special customer requests.
To address the myriad of materials that fabricators are faced with cutting, Shop Floor Lasers will publish a series of articles, focusing on specific material types. This first installment examines some common steel and aluminum cutting challenges and what you should know to help make your fabrication life easier when it comes to cutting these materials.
There are more challenges involved when cutting material that has a higher rate of thermal conductivity, such as aluminum.
Right tool for the job
Obviously, you didn’t invest in your laser not to use it, but in that same breath, it’s important to understand a laser’s limits. Although lasers are versatile, there are instances when waterjet or plasma is a better option. Lasers are negatively affected by several factors, including material thickness and quality, alloy type, heat conductivity, metallurgy and even part geometry. In some cases, waterjet is a better choice simply because it doesn’t emit the high temperatures associated with lasers. Conversely, plasma or oxyfuel is a better choice for thick parts where edge quality and taper are less important.
To ensure that fabricators are getting the most out of their laser investments, many OEMs are introducing higher power machines to help address thick cutting issues. As an example, Mitsubishi now offers its fiber lasers in 4-kW, 6-kW and 8-kW iterations. The company also introduced its patented Zoom head cutting technology to expand the machine’s capabilities. Mitsubishi’s Zoom head can manipulate focal lengths to 3.75 in., 5 in., 7.5 in. and 10 in. as well as the spot size with no operator intervention and without changing optics. These features are paramount for fabricators cutting stainless steel, aluminum and carbon steel – thick and thin.
The Zoom head can manipulate not just the focal length, but also the shape of the mode. In essence, the Zoom head can transform the beam product parameter (mode) from looking like a TEM00 mode on a CO2 laser to looking like a CO2 TEM01 mode. (See Figures 1 and 2, which expand on the idea of beam mode.) The results are a smaller, conical shaped mode for fast thin material cutting and a doughnut shaped mode for better edge quality in thicker plate cutting.
With various power levels and technologies available, it’s key during the due diligence phase of a new fiber laser purchase to research and understand the differences and features of the cutting head when considering resonator wattage. When choosing a fiber machine, a cutting head capable of manipulating the beam product parameter is ideal.
Choosing CO2 or fiber
When considering a laser purchase, business owners must choose between CO2 and fiber. Assuming a laser is a viable option for future work, it’s essential to understand the cutting applications for which a specific laser was designed. Undoubtedly, fiber and CO2 lasers each have domains in which they excel. Whether the job is best suited for CO2 or fiber technology hinges on the application.
For cutting stainless steel sheet or processing parts for medical, aerospace or other tight-tolerance industries, you’ll need the precision of a fiber laser. The concentrated laser beam of a fiber laser – when compared to CO2 – is, in essence, a sharper knife that delivers noticeable edge quality improvement. Because of their short wavelength, smaller kerf size, beam density and high beam quality, fiber lasers are measurably faster for high-volume and intricate geometry applications.
Fiber lasers have been shown to cut anywhere from 5 to 500 percent faster than CO2 and at a higher wall efficiency, depending on material thickness. Likewise, they excel in cutting and etching reflective materials such as aluminum, brass and copper. Fiber lasers also enjoy smaller pierce mounds, a 70 percent or higher reduction in consumables and an elimination of laser gases for the resonator and beam path bellows.
Thick material and other conditions, however, can contribute to fiber laser processing problems. For example, when cutting a material like aluminum, which has a high thermal conductivity rate compared to steel, it’s possible for much of the energy to be laterally transferred into the material. When energy is transferred into a material inefficiently, cutting process speed and efficiency suffer.
For decades, CO2 lasers have served as the workhorse of the industry due to their flexibility and adaptability on the most common types of materials, such as steel, stainless steel and aluminum. CO2 lasers have been processing the full range of metals for more than 25 years.
Because they can handle a wider range of material thicknesses, many job shops serving a diverse customer base do well with their trusted CO2 laser.
As it turns out, laser cutting is more sensitive to material quality than waterjet, plasma and other cutting processes. This sensitivity, however, was more apparent in the past. Ten to 15 years ago, procuring quality steel plate thicker than 0.5 in. was quite difficult. Because surface finish can dramatically impact the quality of laser cutting and negatively affect cutting speeds, waterjet and plasma cutting technologies were heavily relied on. But, times have changed.
Today, most steel suppliers offer laser-quality plate up to 1 in. thick. Laser-quality thick steel plate has a very tight, smooth mill scale; tighter chemical consistency; and typically has lower silicon and manganese content. While it’s true that these premium products come with a slightly higher price, the added expense is more than made up for with increased feed rates, higher quality parts, less scrapped parts and less head crashes due to part tip-ups and blowouts.
Because of the shift to laser-quality thick plate, waterjet and plasma technology is losing market share to laser cutters. But, of course, there’s more to the equation than just the availability of quality materials. The base characteristics of the material are also important to consider when cutting with lasers, be it CO2 or fiber.
As an example, when comparing steel to aluminum, fabricators must keep in mind that aluminum exhibits about a third of the density of steel. While waterjet may have a much slower feed rate on mild steel and stainless steel, the lower density aluminum can be processed much faster with waterjet, especially in thicknesses greater than 0.25 in. Feed rates that closely compete with laser are quite commonplace. That fact, among other factors, will come into play when cutting aluminum.
In addition to the lower density of aluminum, two primary challenges are associated with cutting the material: loss of the cut due to reflectivity and re-adherence of the melted material onto the bottom of the part (also known as dross or slag). Furthermore, different grades of aluminum, primarily 3003, 5052 and 6061 introduce additional variables into the cutting process.
Simply put, the softer the aluminum, the larger the burr. Although harder aluminums result in less burr on the bottom of the part, it’s often more difficult to remove than the burr created with softer grades. Additional considerations, such as feed rate, come into play when comparing softer grades of aluminum to harder grades. As an example, 3003 aluminum requires a slightly slower feed rate to achieve optimum results when compared to 5052 and 6061 in the same thicknesses.
For thin aluminum, it’s advised to consider using oxygen as the laser assist gas instead of nitrogen. The edge will be dull and grainy, but for most applications the parts produced are still acceptable and the increases in feed rates for materials 0.125 in. and thinner can be as high as 25 percent faster.
For these applications, be certain your oxygen source is capable of supplying higher delivery pressure and volume than you would experience when processing mild steel with oxygen. Typically, a minimum of 230 psi at the tank is required and a minimum volume of 20 scfm is recommended.
In many instances, such as in the use of oxygen when cutting aluminum, the laser's ability to cut can be further improved by selecting the proper assist gas. Oxygen assist gas serves two purposes: to assist in combustion and blow the debris or molten metal away from the kerf.
Nitrogen, on the other hand, is an inert gas that serves to discharge the molten metal away from the kerf. When cutting with nitrogen, the energy from the laser is the only tool used in the thermal process of melting/cutting the material. In addition to nitrogen and oxygen, there are certain applications specific to exotic metals, such as titanium and nickel, where argon can achieve greater speeds and improved edge quality while simultaneously eliminating dross/slag. Any type of material, however, can benefit from the use of nitrogen, air, or a blend of oxygen and nitrogen.
Generally, compressed air is also an option. It produces faster cuts than nitrogen in mild steel, stainless steel and aluminum. To realize even higher gains of 20 to 40 percent, a blend of nitrogen and oxygen can be considered – as can proprietary mixes, such as Mitsubishi’s ABR option. Mitsubishi’s blended gas mixer has yielded substantial increases in feed rates and reduced dross/slag across all material types.
In the end, selection of assist gas is predicated less on material type and more on other factors, such as desired edge quality, speed, part geometry, paint adherence and whether the part is to be MIG or TIG welded. There are no “dyed in the wool” absolutes – it’s more about which outcome is the most important to the fabricator.
A common pitfall is to assume that any part can be cut on any machine. The truth, however, is that some parts are simply not compatible with certain materials or machines. For example, some part geometries are affected more than others by the thermal process. Corners or smaller areas of a part absorb more heat and consequently, the probability of thermal runaways or violent reactions like blowouts increase. These instances are more predominant in carbon steel, but are also seen in thicker stainless steel when oxygen is used as the assist gas. As mentioned earlier, waterjet cutting can be a better option in such cases.
Typically, the more complicated the part geometry, the more difficult it is to maintain constant cutting speeds. Often, speed and productivity are compromised when cutting shapes with varying curves and angles.
It is generally more efficient to speed up a laser when cutting curves in thinner materials of all types to prevent overheating the part and deteriorating edge quality. Pulsing the laser rather than using a continuous wave to pop or drill holes is one method that is used for avoiding thermal problems. This is particularly true in thicker materials.
While lasers are powerful and versatile, they have their limits and are affected by a number of variables. Everything from the type of laser to metal type and thickness down to the parts themselves must be considered to avoid poor cut quality, extended machine run times, cost overruns and lost contracts. By better understanding metals and your laser’s parameters, you can more fully leverage your company’s laser cutting capabilities and reduce risks.