More efficient laser micromachining

More efficient laser micromachining thanks to flexible burst pulses
Burst enables high quality with short processing times

GFH has now integrated the option for burst mode operation as a standard feature into the control system of its laser micromachining centers, allowing all new systems with a suitable beam source to be switched as needed.
Lasers with ultrashort pulses have so far been the method of choice in micro machining to push the boundaries of what is technically feasible. However, this process typically requires very long processing times, which makes it economically viable for series production only with products that have a very high added value. There is now an alternative: laser systems with flexible burst pulses can remove large volumes of material while maintaining excellent quality. The processing efficiency is sometimes up to 10 times higher than that of conventional manufacturing methods. At the same time, material damage from excessive energy input is prevented by splitting the energy into multiple burst pulses. GFH GmbH has now integrated this new technology into its laser micro machining systems, combining ultrashort pulse (USP) lasers with software-supported process know-how into a complete package that enables cost-effective mass production of components ranging from toolmaking to medical technology and automotive parts. To efficiently process materials with laser pulses, the goal should be to convert as much of the available laser energy as possible into material removal. Depending on the material, processing is most efficient when the material is treated with laser pulses at an energy density specifically tailored to it. For example, the best removal rate for carbide is achieved at energy densities around 1 J/cm². Industrial laser systems today deliver average powers of up to 150 W with pulse lengths of just a few picoseconds. The maximum energy of a single pulse currently reaches around 500 µJ, generating energy densities of up to 900 J/cm². While this regime is particularly well suited for laser drilling and cutting, it exceeds the energy density at which the removal process reaches its efficiency peak by a factor of 1,000. The excess energy in such applications leads to unwanted effects, such as melting, burrs, or cavity formation.

At excessively high fluence, cavities and melting occur on the material surface.

At too low laser fluence, spikes form on the material due to unstable material removal.
To implement material removal processes efficiently, laser systems are required that can deliver high average power at the ideal energy density for each material due to high repetition rates. Theoretically, this could scale the removal rate; however, in practice, both system and process-related limitations restrict such scaling. For instance, when increasing the pulse repetition frequency, the scanner speed must also be increased accordingly to maintain constant pulse overlap and ensure uniform material removal. Current scanner systems for universal material processing, however, cannot operate at speeds of several tens of meters per second. Furthermore, doubling the scanning speed does not necessarily result in halving the process time. Downtimes caused by acceleration and deceleration distances must also be taken into account, significantly limiting possible scaling. Additionally, process-related factors play an equally important role. For example, when processing tool steels with short laser pulses of a few picoseconds, the most efficient fluence is around 100 mJ/cm². This energy density is close to the removal threshold, which can lead to instabilities during melting and vaporization of the material, manifesting as unwanted deformations. Therefore, achieving high material removal rates while maintaining high surface quality is particularly challenging for materials whose threshold fluence is near the most efficient fluence using conventional strategies.

In burst mode, the high energy density is distributed across individual pulses. The average power remains constant overall.

The relationship between laser fluence and machinability was investigated on a carbide workpiece with a processing area of 1 x 1 mm. A maximum removal rate of 2 mm³/min was achieved at 4.2 MHz, with a laser power of 36 W. Laser source: Time-Bandwidth.
One way to overcome these system- and process-related limitations in laser micromachining is through flexible pulse trains, also known as bursts. This refers to the splitting of a single laser pulse into multiple short, successive burst pulses. Each individual pulse can be divided into up to eight consecutive segments, with the original energy density either distributed evenly or configured by the system in steps from 0 to 255 for each individual burst pulse. The sequence of individual pulses is in the range of 12.5 ns. GFH GmbH has now integrated the necessary regulatory functionalities into its control system, GL.control, as a standard feature. This enables all new laser micromachining centers, with suitable beam sources, to switch to burst mode as needed. To assist the user, all relevant process parameters are presented in an easily understandable graphical user interface, facilitating individual adjustments. Additionally, various predefined standard parameter sets for different materials are available, already tailored to specific targets such as removal rate or surface roughness, which can be directly adopted.

The burst process enables an optimal removal rate through multiple laser pulses. In this case, a volume of 180 mm³ was removed within one hour (left: microscope image of the removal result, right: negative volume).

The resulting separation of laser energy shifts the process-related issues with certain material groups into a range that allows for reliable and precise machining with high material removal rates. Additionally, the pulse pauses in burst mode correspond to those in standard operation without burst, enabling the use of commercially available scanners. Generally, this machining strategy allows for removal rates of up to 3 mm³/min with a surface roughness of less than 0.5 µm Ra. Even more complex geometries with curves and varying depths, totaling a volume of 180 mm³, can be produced within one hour while maintaining consistently high quality and without unwanted material changes.