The burst mode for ps and fs pulses for steel and copper is investigated. It is found that the reduction of the energy in a single pulse (in the burst) represents the main factor for the often reported gain in the removal rate using the burst mode e.g. for steel no investigated burst sequence lead to a higher removal rate compared to single pulses at higher repetition rate. But for copper a situation was found where the burst mode leads to a real increase of the removal rate in the range of 20%. Further the burst mode offers the possibility to generate slightly melted flat surfaces with good optical properties in the case of steel. Temperature simulations indicate that the surface state during the burst mode could be responsible for the melting effect or the formation of cavities in clusters which reduces the surface quality.
When 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.
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.
Picosecond laser systems have been widely used in industrial microprocessing applications since they are a cost-effective tool to achieve high throughput. To better understand the ablation process, firstly the dependence of the ablation depth and the threshold fluence on the laser spot size were determined experimentally by performing ablation with a 10ps pulsed laser system. Further, a 2D axisymmetric model was established to demonstrate the possible mechanism of the phenomena. Three sets of spot radii, namely 15.5μm, 31.5μm and 49.6μm, respectively with equal laser peak fluences ranging from 0.6J/cm 2 to 4.5J/cm 2 were applied on copper. It was found that the laser ablation depth increases while the threshold fluence decreases with decreasing spot size at identical peak fluence. A 2D axisymmetric thermomechanical model was developed to qualitatively illustrate the mechanism behind these phenomena. The numerical results of the position where the tensile stress exceed to ultimate tensile strength (UTS) of copper show the same trend as the experimental ones. The longitudinal tensile stress was seen to play a more crucial role than the radial tensile / compressive stress on laser ablation process. The impact of the thermal stress on the ablation depth and threshold fluence is derived from the lattice temperature gradient along the surface of the material, leading to spallation and possible modifications of the mechanical properties already at lower laser peak fluences. This is elucidated numerically and analytically. The deviation of the experimental results from the simulation might be attributed to the fact that this simulation model is static. Nevertheless, at low laser fluences, this static approach can provide good explanations of the cold ablation with ultrashort pulsed laser. The limitation of this model is also illustrated.
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