Central Metallurgical Research & Development Institute, Egypt
Graduate School of Engineering, Osaka University, Japan
Joining and Welding Research Institute, Osaka University, Japan
Currently, the automotive industry is facing an increasing demand to increase fuel economy and reduce greenhouse gas emissions. Therefore, the trend is moving toward an increase in the percentage of components made of lightweight structural materials such as magnesium alloys. In order to implement these materials, it’s critical to have the ability to produce defect-free welds with reproducible high quality. Laser welding, owing to its versatile advantages, is very promising in this regard. Moreover, the recent development of fiber and disc laser sources with excellent beam quality has increased the capability and enhanced the performance of the laser welding process. Compared to Nd:YAG and CO2 laser sources, the superior beam quality can be utilized to obtain higher welding speeds, and deeper penetration, longer distance to the work piece and lower cost can be realized.
Fiber lasers with its high beam quality (M2~ 1.10 are routinely being used for a welding and cutting for a rage of industrial applications. These fiber lasers are very compact and robust and have an edge over lamp pumped Nd: YAG lasers in terms of beam quality and wall plug efficiency (approx 20%).
To date majority of the laser material processing work with these high beam quality lasers has been carried out either with a continuous wave (CW) or modulated output because currently conventional CW fibre lasers have no peak power over maximum average power capability. This is due to the peak power limitations of the diode pump sources used. Significant lifetime degradation occurs if the junction temperature of a laser diode is increased during operation for any significant length of time which would normally be needed for percussion drilling of aerospace alloys (i.e. milliseconds and above). However it may be possible to use these high beam quality lasers to trepan various sizes holes. Unlike percussion drilling where high pulse energies (up to 20 joules) and high peak powers (up 20kW) are needed to drill holes of 0.3-0.75mm diameter. For trepanning applications the main laser requirements are good beam quality with CW/high frequency modulated output to drill holes at reasonable drilling speeds.
First part of the work describes laser trepanning of aerospace alloys with a 400W single mode (SM) fiber. The second part of the work describes laser ablation of aerospace alloys with the same laser. Normally laser ablation of variety of materials is Q-switched lamp pumped or diode pumped solid state lasers. Here we show that it is possible to the CW fiber for ablation work. Turning the SM fiber laser on and off typically produces a relaxation pulse which is 4-5x the CW power as shown in Figure 1, which is very useful for ablating a range of materials.
Figure 1: Typical output waveform with relaxation pulse, when turning the SM fiber laser on and off.
During drilling primary concern to the component designer is achieving adequate airflow through the holes so that the appropriate cooling is provided. Airflow is governed principally by the size and shape of the hole and hence the need for tight control of size, roundness and taper. There are other factors also to consider; holes are often very closely positioned to one another on a component and any deviation in size may adversely encroach on other holes or even weaken the component locally. Excessive bell- mouthing or barreling is therefore undesirable in addition to recast layer and heat-affected zone.
The geometrical features and the metallurgical characteristics of each laser drilled hole generated during the present study were investigated. The cross sections of some of holes drilled with 400W SM fiber laser are highlighted in Figures 2-3.
Figure 2: 1.25mm thick HastalloyX, 0.5mm dia hole very parallel hole with low recast layer (<25µm thick)
Figure 3: 4mm thick Haynes alloy, 3.6mm dia hole, average recast layer < 40µm
The above holes were drilled with SM fiber laser with CW output, however it possible to use the same laser to remove thermal barrier coatings (TBC) prior to laser drilling. The ablation tests were carried out by optimizing the modulation frequency and the peak power in the initial spike of the relaxation pulse. Some of the ablation results are highlighted in Figures 4-5.
Figure 4: HastalloyX alloy with 0.4mm thick TBC coating; material removal rate 13.8mm3/min; 15 kHz, 40MW/cm2
Figure 5: 2mm thick Haynes alloy with 0.5mm thick TBC coating, 15 kHz, 40MW/cm3 material removal rate 14mm3/min; 1mm dia trepanned hole with the same laser after removing the TBC.
The work reported here show that by optimizing laser and processing parameters it is possible to produce good drilling and ablation results (dual processing) with the same SM fiber laser.
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here
Beam deflection confronting again? Take care of media bias during laser material processing.
By: Masami Mizutani and Seiji Katayama
Joining and Welding Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka, Japan
It has been generally accepted that the laser beams of about 1 mm in wavelength, such as fiber, disk and YAG lasers have smaller interaction to the media such as plume and plasma between the optics system and the specimen than CO2 laser beam of 10.6 mm.
However, we have recently found that weld bead formation is greatly affected by the state of the gas flow, the atmosphere, the ultrafine particles or the plasma plume above the specimen during fiber laser remote welding. The result showed that a laser beam tended to be deflected and focused on the far position without the positive elimination of the media above the specimen. Meanwhile, mirage effects and heat haze effects are seen as natural phenomena by varied temperature of the air, which suggest that the air possessing the temperature gradient enable the ray to deflect.
Therefore, in order to understand what extent the temperature distribution of the media between the optics and the specimen affects the laser beam path, a theoretical calculation of the laser beam trajectory in the assumptive media, which possesses refractive index gradients led by temperature gradients, was carried out. As one of the important results showing in the figure, the beam trajectories propagating through a high-temperature media below, show that the beam diameter at the original focal point (200 mm) is broadened and the concentration area of the beams shifts downward. This result may interpret why the laser beam focused on the far position without the positive elimination of the media above the specimen during fiber laser remote welding, which has been experienced.
It should be emphasized that the trajectory of the laser beam, the wavelength of which is about 1 mm, is deflected to a certain extent, by the media possessing the refractive index gradient led by the thermal gradient, which is presumably attributed to the thermal and plume-jet-induced updraft, and the reheat of ultrafine particles. Therefore, it is concluded that the beam profile of the laser, which has propagated through the gaseous matter possessing the thermal gradient, is no longer the same as originally intended.
Computing results of focused beams propagating from entrance to focal point of 200 mm, assumed for original rays without deflection to be focused at, showing differences in beam trajectories between with and without refractive index gradient.
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here
