Influence of the Pulse Duration onto the Material Removal Rate and Machining Quality for Different Types of Steel

AbstractWhen high requirements concerning machining quality are demanded, ultra short pulsed lasers with pulse durations from a few 100fs to 10ps may be the tool of choice. For these pulses it is known that the removal rate and machining quality slightly increases with shorter pulse duration. But as cost-effectiveness is also a key factor for a successful transfer of a technology to industrial applications, these systems compete against more cost effective systems with pulse durations from several 10ps to a few ns. It was found in previous work that the removal rate for metals strongly depends on the pulse duration. For steel also the composition and microstructure will influence the ablation processes. A systematic study of the removal rate and the machining quality for different types of steel and for pulse durations of several 100 fs to few ns will be presented

Measurement of the maximum specific removal rate : unexpected influence of the experimental method and the spot size

Ultra short laser pulses often are the tool of choice when high requirements concerning machining quality are demanded. But for industrial use the process has also to be efficient, meaning that the removal rate (ablated volume per time and average power) should be as high as possible. Many publications deal with the threshold fluence and the removal rate for various materials but often use different methods and beam parameters to determine these values. To demonstrate the influence of the different methods, the removal rate for steel and copper was determined for different pulse durations and different spot sizes using the following three different methods: With the first method the removal rate is calculated from the threshold fluence and the energy penetration depth deduced by machining craters at low repetition rates, measuring its depths and using the logarithmic ablation law. With the second method the removal rates were directly determined by measuring the volume of these craters and with the third method they were determined by measuring the volume of squares machined with a pulse overlap and higher repetition rates. This systematic study shows differences between the investigated methods themselves. Additionally it reveals for all three methods an unexpected influence of the spot size which is much more pronounced in the case of steel

Review on Experimental and Theoretical Investigations of Ultra-Short Pulsed Laser Ablation of Metals with Burst Pulses

Laser processing with ultra-short double pulses has gained attraction since the beginning of the 2000s. In the last decade, pulse bursts consisting of multiple pulses with a delay of several 10 ns and less found their way into the area of micromachining of metals, opening up completely new process regimes and allowing an increase in the structuring rates and surface quality of machined samples. Several physical effects such as shielding or re-deposition of material have led to a new understanding of the related machining strategies and processing regimes. Results of both experimental and numerical investigations are placed into context for different time scales during laser processing. This review is dedicated to the fundamental physical phenomena taking place during burst processing and their respective effects on machining results of metals in the ultra-short pulse regime for delays ranging from several 100 fs to several microseconds. Furthermore, technical applications based on these effects are reviewed

Improvements in ultra-high precision surface structuring using synchronized galvo or polygon scanner with a laser system in MOPA arrangement

n earlier work the capabilities of synchronizing a galvo scanner or a polygon line scanner with a picosecond laser system in MOPA arrangement were presented. However these systems only enabled precise positioning of laser pulses on the target relatively to each other. Since then a novel approach to increase the absolute precision in positioning has been developed. This improvement enables new and more efficient process strategies such as bidirectional processing or high precision structuring of large areas in combination with additional mechanical axes. These improvements represent a major step towards large scale industrial applications in laser based micromachining

High throughput surface structuring with ultrashort pulses in synchronized mode with fast polygon line scanner

High precision laser micromachining requires an exact synchronization of the laser pulse train with the mechanical axes of the motion system to ensure for each single pulse a precise control of the laser spot position - on the target. For ultra short pulsed laser systems this was already demonstrated with a conventional two-axis galvanometer scanner. But this solution is limited by the scanner architecture to a marking speed of about 10m/s with a maximum scan line length of about 100mm. It is therefore not suited for average powers far beyond 10W when working at the optimum point with highest removal rate and machining quality is desired. A way to overcome this limitation is offered by polygon line scanners which are able to realize much higher lateral speeds at large scan line lengths. In this work we will report on the results with a polygon line scanner having a maximum moving spot velocity of 100m/s, a scan line length of 170mm, spot diameters of 45µm (1064nm) and 22µm (532nm) together with a 50W, 10-ps laser system. The precise control of the laser spot position i.e. the synchronization is realized via the new SuperSyncTM technology. Decoating, perforation and 3D patterning will act as benchmark processes to evaluate this scanning technology

Random Surface Texturing of mc-Silicon for Solar Cells with Picosecond Lasers; a Comparison between 1064 nm, 532 nm and 355 nm Laser Emission Wavelengths

Multicrystalline Silicon was textured with picosecond laser. Different laser wavelengths (λ = 1064, 532, 355 nm) where compared regarding laser-induced damage. We found that λ = 355 nm picosecond radiation resulted in shallower defect-reach region

Random Surface Texturing of mc-Silicon for Solar Cells with Picosecond Lasers; a Comparison between 1064 nm, 532 nm and 355 nm Laser Emission Wavelengths

Multicrystalline Silicon was textured with picosecond laser. Different laser wavelengths (λ = 1064, 532, 355 nm) where compared regarding laser-induced damage. We found that λ = 355 nm picosecond radiation resulted in shallower defect-reach region