Same Quality at a Third of the Machining Time

Removal of stainless steel at 1.1 MHz, 20-times magnification. The removal quality hardly changes with variations in the pulse duration: 240 fs (left column), 900 fs and 10 ps, with 0.1 J/cm² (top row), 0.2 J/cm² and 0.5 J/cm², respectively.(Source: GFH)

Removal of stainless steel at 1.1 MHz, 20-times magnification. The removal quality hardly changes with variations in the pulse duration: 240 fs (left column), 900 fs and 10 ps, with 0.1 J/cm² (top row), 0.2 J/cm² and 0.5 J/cm², respectively.(Source: GFH)

In comparison with conventional fabrication methods such as milling, turning and grinding, laser machining allows for a much higher removal precision. But precision is not the only concern in fabrication processes – economic feasibility also plays a role: The removal requirements should be achieved in the shortest possible time, with good quality results and cost-effectively as well. Tests by the laser machining experts at GFH GmbH have shown that in order to achieve these goals, it is essential to adjust the pulse duration according to the material and application.
The pulse duration is a key parameter in fabrication processes. Nanosecond pulses, for example, offer a higher removal rate with limited quality, since removal in this case primarily takes place by way of the liquid phase when the material melts due to the long application time of the laser. The quality achieved with removal processes can be increased significantly by using short-pulse laser systems operating in the picosecond range – but at the expense of a considerably longer machining time, which has made it harder for this technology to gain a foothold in the market. Thanks to the continuous advances made in beam source technology, femtosecond lasers which are suitable for industrial use are now creating the opportunity to combine a high removal rate with good quality. So GFH performed studies in order to analyse the efficiency and quality of femtosecond-range pulses on a variety of materials.
The tests looked at different parameter settings for the pulse duration, pulse energy and repetition rate (the length of the pause between two pulses in Hertz). The beam source was a laser with a maximum average power of 15 W, capable of generating pulse durations in the range of 240 fs to 10 ps. It was used to machine samples out of 1.4301 stainless steel, VGH2 carbide and the ceramic aluminium nitride. Then the removal rate in units of cubic millimetres per minute was calculated and the quality was analysed under a microscope.
The results of the tests showed that the removal rate for stainless steel can be increased by a factor of 3 by reducing the pulse duration from 10 ps to 900 fs. The opposite was found for ceramics, aluminium nitride in this case. Here the removal rate was increased by raising the pulse duration from 900 fs to 10 ps. This means that metals and dielectric materials have different optimal pulse duration values. Thus cycle times can be reduced by up to 70 percent by selecting the respective appropriate duration: 900 fs for metals, 10 ps for dielectric materials (Fig.).
In addition to the removal rate, the quality and surface structure were also examined since these parameters are of great significance for some functional requirements of components. In the course of the testing, the quality as a factor of pulse duration was examined and compared to a repetition rate of 1.1 MHz for a range of fluence (pulse energy per unit surface area) values.
It was found that the quality remains virtually constant even as the pulse duration is varied. Only stainless steel exhibited a poorer surface quality when the pulse duration was increased to 10 ps. In addition, increasing the fluence resulted in a higher removal rate for all studied materials. However, the fluence value cannot be set to an arbitrary high value since the quality drops as this value is increased. Good settings were found to be 0.2 J/cm² for stainless steel, 0.5 J/cm² for carbide and aluminium nitride. It may be possible to use even higher fluence values for these materials, since 0.5 J/cm² is the highest value used during the tests and still produced good quality results. Another effect which was observed for all of the tested materials is that the removal rate increased with the repetition rate. But there is also an optimal value for this parameter, and an excessively high repetition rate reduces the quality on account of heat build-up. A rate of 1.1 MHz provided good results in the tests.
The effect of the pulse duration on the efficiency of laser micro machining is due to the different properties of the materials: For metals, the lattice heats up approx. 900 fs to 1 ps after the start of the laser pulse, before that only the electron system gets hot. If a pulse duration greater than 900 fs is selected, then additional energy is introduced into the material by the laser beam after this time even though the lattice is warm already. This leads to energy losses to the surrounding material, causing it to melt. With shorter pulse durations, the entire energy of the laser pulse is used for removal since the lattice is not warm yet, only the electron system. In this case, the material transitions to the gaseous state directly without melting. However, selecting a short pulse duration with a high fluence value produces the same effect observed with longer pulses. Due to the large amounts of applied energy, the surrounding material starts to melt.
But a different effect is responsible for the optimal pulse duration of 10 ps for ceramics: Since no material is removed here at lower fluence values, a lot of energy and thus higher fluence is required for machining in this case. Ceramics are also good at retaining heat, which means that the material heats up more and stores the heat when the pulses are longer. This results in a higher amount of energy in the material during the next pulse, since heat from the first pulse adds to the energy of the second pulse.
Selecting the right laser for the material and machining requirements
Tests with different materials have shown that it is certainly possible to achieve a high level of efficiency with simultaneous good quality through laser machining – if the right pulse duration for the material is selected. This means that the choices for the most suitable laser can be narrowed down on the basis of the material being machined and the required machining process – drilling, cutting or removal. The tests also showed that a pulse duration below 900 fs is not required for any of the studied materials, since the removal rate is constant below this value. This also translates into an economical removal rate in this regard, since lasers with a longer pulse duration are also more cost-effective. Longer pulse durations are also less susceptible with regard to stability.
In addition to the pulse duration, pulse energy and repetition rate parameters, however, there are additional criteria which have to be considered when selecting a beam source. Among other things, they include the power output stability, pulse stability, robustness with regard to fluctuations in the environment, in-field service, the mean time before failure (MTBF), the mean time to repair (MTTR) and other so-called soft specs. In order to select the most suitable laser system for the respective application despite the wide range of requirements, GFH draws on an extensive knowledge base and the experience gained from several kilowatts of installed short pulse laser power. Since the company’s system technology is not limited with regard to the laser source used, the same system solution can be used to provide the customer with different laser systems and configurations according to the application. (Source: GFH GmbH)
Links: GFH GmbH

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