In the past few decades, Nd:YAG pulsed lasers have been a major force in material processing, and many of the machines have been in use for more than 30 years. Among them, pulsed lasers with wavelength of 1070nm are most widely used, such as medical devices, aviation, electronics and so on. However, the laser still needs to be improved in some aspects, such as high peak power, low average power, low electrical efficiency, unstable beam quality when power is increased, focusing spot size is similar to gaussian beam, and several rounds of pulse preheating are required before achieving stable output. However, despite various drawbacks, such lasers have been playing an important role for quite some time. In the field of aviation, Nd:YAG pulse laser is the dominant position, widely used in various devices cooling hole processing.
At the beginning of 2009, people in the material processing industry began to look at pulsed lasers that can provide peak power, and continuous lasers with high power levels, which generally reach a peak power of 3kW and an average power of 300W. Technological leaps have led to higher peak and average power. Today, the peak power is up to 20kW, the average power is 2kW, and ultra-high power continuous laser has come out. With the continuous upgrading of power, fiber lasaerpointer has been put on the stage of aviation device processing.
Compared to the traditional Nd: YAG laser, fiber laser in the electro-optical conversion efficiency and beam intensity (single mode or low operation) were improved significantly, without preheating, power change, whether it is a flat pattern (as shown in figure 1), or gaussian mode, spot diameter remain stable, at the same time, the pulse frequency is higher, parameters real-time regulation performance is stronger. Because fiber lasers use a single emitter, they have a qualitative leap in reliability, power stability and flexibility compared with strobe pumped lasers.
Compared with Nd:YAG pulsed laser, the advantages of fiber laser are obvious. First, the pump source of a fiber laser is a diode, not a flash, so it can form a perfect square wave. Second, Nd:YAG lasers pumped with flash bulbs rise and fall slowly, so there is always a portion of the laser energy below the evaporation threshold in the target area, which melts the material and causes the thermal barrier coating to peel off. The pulse period must be less than 1ms to meet the recast layer specifications. At this point, the fiber laser has an absolute advantage because of its ability to generate square waves, so it can meet the requirements of recasting layers and cracking requirements of aviation devices using 10ms pulses.
Fiber lasers are characterized by flat top mode output, while Nd:YAG lasers are approximately gaussian mode. Therefore, thanks to the flat-top model, all the energy of the former exceeds the evaporation threshold, while a considerable part of the latter is below the threshold. The research shows that the fiber laser needs less energy to achieve the same drilling effect under the same conditions. Because of this property, fiber lasers are more efficient at drilling holes with less thermal damage. With less thermal damage, the coating peeling and recasting layer will be improved.