In our series “diode lasers in their application”, we regularly present the most important applications based on diode lasers. One particular application field should not be forgotten: copper welding.
But why is this special application so important? Copper is one of the most important raw materials for electrical signal transmission and thus a key component of many modern technologies. In battery cells of mobile devices, in induction coils or in accumulators of electric cars, copper segments can today be found everywhere. Often the components are exposed to a high current, high operating temperatures and strong vibrations. This significantly affects the production process: When components are made of several parts, the joining seams must be highly solid and must not create additional resistances in the signal flow to prevent an increased temperature in the component. As brazed joints often lack the required head resistance, weld seams created by lasers are the best option here.
However, not every diode laser is the same nor is every copper. What sounds trivial at first is, however, decisive. Because, when quickly taking a closer look at the copper components, the wide spectrum of the components becomes clear: Wafer-thin wires are used as well as sheets with thicknesses of several millimeters. This affects what kind of welding tool has to be used. When thin foils have to be joined, low-impact depths and moderate-energy inputs are essential. High intensities would separate the copper foil immediately, but since you need high intensities to fuse copper with the infrared laser, you are faced with a dilemma. Thus, the infrared diode laser is not suitable for processing filigree copper components; rather, the ideal is to weld with blue diode lasers, as with Laserline’s LDMblue, the world's first blue industrial laser in the multi-kilowatt range. The reason? Wavelengths in the blue spectrum can be absorbed outstandingly well by copper. With the 450 nm of the new LDMblue, an absorption rate of way above 50 percent is reached, and when talking about laser welding of thin non-ferrous metals this is a quantum leap. A direct comparison with this industry’s typical infrared lasers makes this strikingly clear: With identical process conditions at copper welding via IR radiation, only an absorption rate of 5 percent is attained. Therefore, infrared-based copper welding is only possible with high laser output powers. Additionally, the process is often unstable hence welding mistakes occur, mainly spatters. With the blue diode laser, these problems belong to the past. Thanks to clearly improved absorption with the LDMblue, a moderate energy input and thus a controlled heat conduction welding of copper can be realized for the first time. Low impact depths and low material vaporizations create homogeneous weld seams without spatters or pore formation.
However, as soon as thicker copper components are to be joined, the blue diode laser finds its limit because joining thick sheets could only be realized with the deep welding process, a method that is only possible with very high radiation intensity due to the good thermal conductivity of copper. Here, the high absorption of the LDMblue did not offer much help: At 1,000 Watt it reaches typical welding penetration depths of 0.6 to 0.7 mm. Higher output powers — currently LDMblue lasers with up to 2 kW are available — improve the welding penetration depth. Since blue diode lasers are more expensive to produce than infrared systems, keyhole welding of copper is currently facing the question of economic efficiency. The use of a classic infrared laser would be much cheaper in this application, but the already mentioned process disadvantages - instability and welding defects - stand in the way of an efficient welding process here as well with the world's first blue industrial laser in the multi-kilowatt range.
So, Laserline has developed alternative approaches. The result was a completely new hybrid concept that combines both laser versions. Over a special focusing optics, the beam of a blue diode laser is joined with the beam of a classical infrared laser. The blue spot with a diameter of 0.6 - 1 mm is overlaid by a smaller infrared spot, on account of which, the advantages of both methods can be combined: To fuse the workpiece surface, the high absorption rate of the blue diode laser is used at the beginning. The connected infrared laser now opens the vapor capillary and realizes the actual deep welding process. During the whole process, the LDMblue stays connected and guarantees a calm and stable molten pool, hence an innovative approach with astounding success. In terms of welding penetration depth, seam quality and process calmness, the results are overall fairly convincing. At welding penetration depths of up to 3 mm, calm molten pools without visible spatters in the seam area and usually without pores as well can be fully realized. At the lap welding of two copper sheets as well as at the butt-joint, extraordinarily smooth joining seams are created and impurities are nowhere to be found.
Besides, in terms of energy efficiency the hybrid concept is convincing: While at the blue laser 500 W to 1 kW are mostly sufficient, the infrared laser requires two to five times more output powers. At previous welding processes via the hybrid concept, powers between 1 to 5 kW were thus used. Only infrared-based copper welding is based on far higher intensity beams; here, output powers of 6 kW or more are normal.
Only at welding penetration depths of more than 3 mm does the new approach have to be further optimized. Here, the method currently leads to instable processes, spatter creation and sometimes to the collapse of vapor capillaries. Thus, Laserline carries out field studies with particular focus on spot size, protective gas and feed rate. The goal: making copper deep welding also possible at welding penetration depths of more than 3 mm via the hybrid approach. Beneath the 3 mm border, there are today no reservations anymore as the process is fully stable and suitable for series production. Once more, it shows how much potential the development of diode laser technology has, and there is no end in sight.