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Material processing applications for multiple laser beams

DATE:2016-10-27READ:


 New technology
Laser welding process is mainly focused on the optimization of the process and improve the quality of processing. We will explain the use of multi-beam laser processing with different parameters in body brazing, silicon aluminum alloy coating stainless steel plate welding and polymer and metal welding materials.
       From the heavy industry to the consumer electronics industry, lasers are an indispensable material processing tool. Laser processing is generally regarded as a mature, can improve the productivity of the processing technology, and continue to seek to expand to new applications. More recently, this exploration has generated an interesting tendency to use multiple beams at the same time on a single workpiece and each beam is optimized for the entire process.
 
This paper will focus on three kinds of multi-beam laser processing applications
First of all, we introduce how the use of three-spot brazing with the whole beam with the high-strength automotive materials, welding and beautiful appearance
Next, the laser cleaning in the high-strength steel two-step welding provides good strength and uniformity in laser welding
Finally, we will see how laser metal surface textures enable high-strength, high-sealing polymers to be welded to metal. This means that multiple beams with different core diameters, pulse widths, or even wavelengths can be combined with one another to achieve unprecedented processing efficiencies.
Three - spot brazing
The characteristics of lasers are important for the automotive industry because laser welding results in a higher weld strength with the least amount of solder, while improving safety and economy. This is mainly due to its high welding strength, the use of less material, while also improving the safety of the fuel reduction. When laser welding is widely used in automobile manufacturing, people are more inclined to use a more beautiful process to meet along the roof and car interior visible weld production needs.

Compared to conventional welding, brazing is a process that does not require melting of the substrate and soldering. For automotive applications, the laser energy melts the wire to weld two steel or aluminum alloy surfaces together seamlessly. Automotive manufacturers need a simple clean-up before painting to achieve a truly seamless welding of the brazing process.
Brazing of low carbon steel electroplating research focuses on welding quality and weld appearance. Especially the oxides and contaminants left on the galvanized layer, which are the main cause of splash and edge roughness. Research in this area has spawned a new three-spot brazing system, along the steel edge of the two guide beams, used to clean up contaminants and preheat galvanized layer to improve the infiltration. The subsequent main beam provides energy to melt the Cu / Si solder, and the two steel plates, which have just been cleaned of the edge surface, are welded seamlessly as shown in Fig.

Fig.1 Schematic diagram of three-spot brazing. Two red German guided beams clean and preheat the edges of the steel plate to improve wetting. The subsequent main beam provides energy to melt the Cu / Si solder and seamlessly welds the two steel plates that have just been cleaned of the edge surface. After painting, these welds are not visible to the naked eye.
The three spot brazing system relies on the flexibility of the fiber optic technology shown in Fig. The fiber laser is coupled with three optical fibers with different core diameters and is transmitted through one optical fiber cable. Near the work piece, the transmission fiber will produce a 3-spot spot as designed so that the smaller guide beam is pre-cleaned before the main beam finishes splashless brazing.

Figure 2 three-core three-point brazing light path. A specially designed optical path allows optical fibers with different core diameters to be transmitted through a single operating fiber, producing different sizes of spots in the brazing area.
To more intuitively evaluate the advantages of three-spot brazing, a 1.6-mm CuSi3 alloy wire and 3.5-kW fiber laser were used to braze the main beam, and the sample was hot-dipped at a speed of 4.5 m / min. Brazing. Better edge uniformity and surface finish are evident after pre-cleaning the material with a 350 W pilot beam prior to soldering (Figure 3).
Figure 3 Comparison of single spot and three-spot brazing. Single-point (a) and three-spot (b) brazing are used Cu / Si solder brazing. The improvement of the surface finish and the reduction of the edge roughness are evident in the three-spot brazing. Through the cross-section (c) can see the consistency of three-spot brazing and welding quality.
Three-spot brazing in a process will be cleaned before welding and brazing combined, greatly reducing the painting before the process after the requirements of the brazing process. Three-spot brazing enables high-speed automated, high-strength welding and repeatability on flat or curved surfaces. Automotive manufacturers are increasingly using their preferred three-spot brazing for higher-profile sheet metal welding to achieve the highest efficiency and best appearance.
Two - step laser welding of high strength steel.
Automotive manufacturers are looking for new materials and welding processes that can produce safer, more efficient vehicles. High-strength boron-containing high-strength steels (HSS) have therefore entered the field of automotive innovation, the use of life-saving tools in North America "life jaw" and even the standard of life-saving tools redefined. Assuming no change in the welding technique, higher strength means that the weight of the body can be reduced by using less material. In the welding of high-strength steel, the car manufacturers are more inclined to laser welding process. But the early attempt to use laser welding but because of the use of heat in the process of forming AlSi protective coating problems blocked. Because the use of laser welding of AlSi-coated high-strength steel may lead to internal iron-aluminum metal layer embrittlement.
No need to worry about the brittle fracture of the aluminum-aluminum layer, the welding area of the anti-corrosion coating will be cleaned up after a significant increase in high-strength steel welding quality. Figure 4 shows a complete cleaning of the AlSi coating using a 1 kW pulsed fiber laser with a pulse width of 70 ns. The laser provides up to 100 mJ of pulse energy (7-10 J / cm2 per mm2) with a new square-shaped fiber to clean AlSi with a thickness of 30 μm at an accurate and economical rate of 10 m / min coating. And then use the near-infrared kilowatt continuous (CW) fiber laser to complete the welding process, so that high-strength, lightweight tailor-welded blanks can be supplied to the automotive manufacturing industry.

Figure 4 high-strength steel welding AlSi coating clean-up. High pulse energy fiber laser technology through a new square fiber transmission, efficient cleaning AlSi coating, access to the steel body surface to strengthen the welding quality
Unlike three-spot brazing using two continuous laser beams with different core diameters, HSS two-step welding is first laser ablated with a high-pulse energy nanosecond laser followed by a high power continuous laser. The application example we follow is a two-step process, but we extend it to the sub-nanosecond range and use two different-wavelength lasers.
Polymer and metal welding
Welding requires that both sides of the substrate in the weld near the area of melting, so as to be stable welding of the two together. Welding between metal and metal, polymer and polymer used between the most extensive. As the melting point of polymers and metals is very different, welding between the two materials is unlikely. Finding the welding method between polymer and metal is still a hot technology to explore from many industries, such as consumer electronics to medical equipment. The recent two-step process using fiber laser technology provides a solution for this problem, great prospects for development.
In the first step, metal surface texture was performed using a near-infrared fiber laser with an average power of 30 W and a peak power of 400 kW with a pulse width of 150 ps (Fig. 5a). Microscopic studies have shown that laser irradiation melts the nanoscale surface and quickly forms a fine, large surface area that fits the knot structure of the posterior process. The importance of these textured surfaces is that they can be formed on highly ferrous metals, even copper, etc. (Figure 5b). The senior welder knows that the uniform dark-colored surface has the largest process space because the change in reflectivity affects the energy threshold of the laser on the high antimetal.
Figure 5 Copper surface texture by subnanosecond fiber lasers. (A) Example (magnified 10,000 times) having a refined structure obtained using a subnanosecond near-infrared fiber laser. (B) After the texture of the copper surface becomes perfect black, making it the ideal laser processing after the absorption of the body.
Polymer and metal materials, the use of welding wavelengths of 1.9 μm doped thulium continuous fiber laser. Mid-infrared wavelengths are much more likely to be absorbed by conventional transparent polymers than near-infrared or direct-diode lasers. The traditional 1 μm laser beam through the polymer, only heated to the metal surface, the heat transfer within the polymer, the final melting to the metal, welding strength is poor.
We found that the surface texture was first performed and the surface of the metal was darkened, and then the heat of the 1.9 μm fiber laser significantly improved the weld strength of the polymer to the metal. Longer wavelengths can transfer heat directly to the polymer and from the polymer to the metal surface. The direct thermal conductivity of the polymer in combination with the dark junction of the metal surface provides the ideal welding conditions. We have a polymer and titanium welding, sealing, welding strength, when subjected to shear force in the polymer layer failure.
In contrast, when the surface texture is omitted, the shear force test fails at the polymer-to-metal surface, demonstrating that the strength of the weld is not as high as the surface texture before re-welding. Polymer-to-metal rugged yet sealed soldering has opened up a larger space for design and manufacturing and has generated interest in a wide range of customers from medical devices to consumer electronics and low-cost consumer products.