Wavelength Dependency in High Power Laser Cutting

By Stephan Ziermann and David Havrilla

Laser cutting and welding have been around for more than 30 years. Within those three decades there has never been a greater variety of high power laser types and wavelengths to choose from than there is today. There are many considerations when choosing the right laser for any given application. However, one of the most fundamental questions that must be asked and answered is – “what type of laser is best suited for the application?”

Manufacturers and users alike are realizing what, in retrospect, may seem obvious – there is no such thing as a universal laser. This article will examine the application fields of high power, high brightness 10.6 and 1 micron laser 3D cutting, and will provide general guidelines for selecting the laser that is best suited for each application. Processing speed and edge quality serve as the key criteria.

Before investigating the cutting performance on the most common industrial metals, it is important to note that both the 10.6 and 1 micron lasers have their niches. The CO2 laser continues to reign over several applications. We can start with the obvious niches – polymers, glass, wood, paper, fabric and other non-metallic materials. The longer wavelength of the carbon dioxide laser simply absorbs much better into these materials compared with the disk or fiber lasers. Of course, the 1 micron lasers have their absorption niches as well. These lasers open the door to processing highly reflective materials such as copper, brass and gold.

The direct comparisons presented below, however, will consider the most common industrial materials – mild steel and stainless steel. Speed and cut quality (roughness, dross and burr) will be compared using a 3 kilowatt disk laser (TruDisk 3001) with 1.03 mm wavelength and a 5 kilowatt CO2 laser (TruFlow 5000) with 10.6 mm wavelength. The focus conditions for both lasers are comparable.

As a rule of thumb it can be said that the cutting of mild steel with oxygen assist gas is very similar for 1 and 10.6 micron lasers at the same laser power. Surface roughness for material thicknesses exceeding 4 mm are comparable, but for thinner materials, the 1 micron cutting quality is even higher than the already excellent CO2results (see Figure 1). Since the cutting speed in oxygen cutting of mild steel is driven by the exothermic process, the laser power used at each thickness is optimized for speed and quality, while averting excessive burning. Due to this phenomenon, the cutting speeds with oxygen are somewhat limited.

Fig. 1, left – Cutting of mild steel with oxygen assist.
Fig. 2, right – Cutting of stainless steel with nitrogen assist (fusion cutting).

For cutting stainless steel with thicknesses up to about 3 mm with nitrogen assist gas, roughness values are quite similar, but the 1 micron wavelength provides greater cutting speeds at lower laser power (i.e. 3 kW disk or fiber is comparable to 4 kW CO2). However, for thicknesses over 4 mm, although speeds are very similar, the cutting quality isn’t. With 1 micron some burr and a rougher surface compared to CO2laser cutting results cannot be avoided (see Figure 2). Table 1 gives a simplified summary of cutting stainless.

Table 1: Relative ratings, nitrogen cutting of stainless steel.

Thin thicknesses of mild steels can be cut with nitrogen, too – at much higher speeds than is possible with oxygen. For up to about 2.5 mm thickness, cutting quality, and for some thicknesses even speeds of the 1 micron fiber delivered lasers, are superior to 10.6 micron CO2 laser results (at 5 kW). Cutting speeds of up to 30 m/min (for 1 mm material) make the fiber delivered lasers the perfect tool for cutting hot formed 3D parts, which is a booming market (Figure 3).

Fig. 3: High speed cutting of a hot formed automotive component.

With all the excitement about the newer, high brightness 1 micron lasers like disk, fiber, or in the not too distant future, direct diode, some thought it sounded the death toll for the older, well established lasers like the 10.6 micron CO2 laser. However, there are niche applications and application regimes where each laser type excels. In fact, there is no one type of laser that can accomplish all tasks equally well. There is strength in diversity.

Stephan Ziermann is an engineer for TRUMPF in laser applications both in Germany and the U.S. and David Havrilla is the manager of the Product & Applications Group at TRUMPF.

About the Author
Steven Glover is a proud member of the LIA staff. When he is not at work he is actively involved in several charitable efforts.
Posted in Laser Editorials