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Laser engraving and Laser milling have become widely established commercial activities and have been applied to many different types of metals. As a non contact process, Laser micro-machining offers many advantages over conventional mechanical processing technologies. Although compared with conventional milling methods material removal rates of a few 10s of mm3/min may seem low, their use is targeted at micro-machining where the feature size and process control offered by Laser sources are not achievable with conventional tool based machining methods.
Nanosecond pulse duration Lasers have proven capabilities in this area due to the combination of high average power and high peak power. Pulsed Fiber Lasers may not have the peak power and pulse energy of some q-switched diode pumped solid state Lasers, but have nevertheless proven to be capable of engraving many types of metals.
The compact high efficiency fiber sources give users flexible materials processing tools that offer low maintenance and operational costs and are displacing lamp and diode pumped solid state Lasers. One of the key differences is the wide range of temporal pulse shapes available from our MOPA Fiber Lasers.
Whilst other types of ns pulse duration Laser sources typically have a Gaussian pulse shape, our redENERGY pulsed Laser series technology have a different kind of temporal profile with a fast pulse rise ensuring that energy absorption thresholds are rapidly overcome. With PulseTune technology these Fiber Lasers are able to produce a range of pulse shapes with varying pulse duration giving finer control enabling a wide range of processing capabilities.
There are many different grades of aluminium alloys that are routinely micro-machined by Lasers. Although there are some minor processing differences between alloys they generally behave in the same way, but process optimisation is required.
In order to maximise material removal rates careful consideration needs to be given to the incident power density and the pulsed spot overlap machining strategies employed.
The incident power density is a function of the optical arrangement such as the beam diameter and the focal length of the processing optics, however, it should be noted that the beam quality also plays a significant part. In engraving, a balance needs to be achieved between incident power density and spot size in order to maximise material removal whilst maintaining surface quality.
Our range of pulsed Lasers offers different beam qualities ranging from M2 1.2-3.7 which can give a range of materials processing options.
Several different removal mechanisms occur during the aluminium engraving process including thermal vaporisation and melt ejection. Experimental results show that a larger incident spot can be more effective than a smaller spot so it is likely that a significant amount of material removal is via melt ejection than vaporisation.
The melt ejection process involves the generation of a molten pool from which material is removed by the reactive vapour pressure which occurs as a result of the rapid expansion caused by the incident nanosecond Laser pulses.
Photo 1 showing effects of Laser parameters using a simple repeated processing pattern.
Laser engraving is a layer based process, in that multiple passes are used to achieve the required material removal depths. Typically a few 10s of microns are removed in a single pass. Complex machining strategies are often employed to achieve optimal removal rates and generate the smoothest surfaces. Laser processing parameters such as pulse to pulse overlap and line spacing are critical variables and need careful optimisation. Layer strategies are also an important factor, in that the same pattern should not always be repeated as this generates very rough surfaces. In order to create smooth surfaces the processing angle needs to be changed between passes, this and the occasional use of high rep rate short pulse cleaning passes helps ensure smooth processed surfaces.
There are also some limitations of depths that can be achieved. Studies show that the position of the focus relative to the machining surface is quite critical and typically the depth of field is in the order of 250-500microns. In order to successfully machine greater depths periodic adjustment of the focal position is required.
Finally the finish of the machined surface can be controlled by the laser. Some applications require a high level of contrast and so a dark matt finish can be achieved, however, if a bright shiny effect is required then a simple cleaning pass can leave the surface with a more polished finish.
Photo 2 showing the effects of changes of focal position on engraving quality each character was machined with a 250micron change in focal position.
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