Fiber laser welding machine is used for welding difficult-to-weld materials

The fiber laser welding machine can adapt to various types of metal welding, and to a certain extent fills in welding processes that are difficult to achieve with traditional welding processes such as argon arc welding and gas shielded welding. For example: galvanized sheet welding, aluminum welding, copper welding, different types of metal splicing, etc.

Galvanized steel sheet welding

Various types of galvanized steel sheets are widely used in the automotive industry and other applications with anti-corrosion requirements, such as agricultural equipment and buildings. But in the past, because the boiling point of zinc is much lower than that of steel, the zero-gap lap welding of galvanized steel has posed challenges to laser welding. As a result, when laser energy is applied, zinc will vaporize first, and the generated air pressure is sufficient to blow away the molten steel, resulting in uneven welds and forming spatters that require subsequent cleaning. Because the laser power required to melt and maintain the weld hole will make the molten pool turbulent and unstable, it is difficult to control this dynamic behavior with a single focal spot.

This problem can be alleviated by denting the material to create a gap, or adding a gasket between the metal plates, so that there is enough space (about 0.1 ~ 0.5mm) for the vaporized zinc to flow around the keyhole in a controlled manner. Non-top discharge. One of the main difficulties of this method is that when encountering parts with complex three-dimensional shapes (such as car doors), it is difficult to maintain a uniform small gap between the thin plates. It is much easier to clamp the parts tightly together with a fixing device.

Aluminum welding

Nowadays, electric vehicles are becoming more and more popular with consumers. The lithium battery shells used in these vehicles need to be welded, so there is an increasing demand for aluminum welding. Specifically, the battery manufacturer must weld the top to the casing to maintain airtightness during the life of the component. Since water reacts strongly with lithium, generating gas and pressure, which may cause equipment damage, this seal can prevent moisture from penetrating, so it is very important. In addition, metal particles (and moisture) will generate internal leakage current and short-circuit the battery. Therefore, it is very important to avoid splashing during the welding process. Finally, the weld must have sufficient mechanical strength to withstand rough handling, even the impact of collision.

Because the battery wall is very thin (<1mm), the sealing of the aluminum battery shell is traditionally done by laser conduction welding. However, it is difficult to achieve a sufficient melting depth using conductive welding to produce a sufficiently strong weld, and to make the porosity low enough to prevent moisture intrusion. However, if a higher laser power is used to achieve a deeper penetration (melt hole) weld, there will be a risk of shell deformation and will always cause a certain degree of splashing.

FL-ARM technology

Some methods used in the past to eliminate the spatter problem when fiber laser processing certain materials include: laser welding in a process chamber whose pressure is much lower than atmospheric pressure (in the range of millibars), or drastically reducing the feed rate. However, these methods will eventually reduce processing capacity or cause practical difficulties in eliminating the inherent advantages of fiber lasers. Until recently, there has not been a technology that can deliver fiber laser energy in a very precise manner to control the dynamics of the molten pool, support the mass-production processing capabilities currently available, and be simple and easy to implement.

Figure 1: Simplified ARM fiber diagram and five basic power modes that may appear in a focused laser spot.

In the Coherent Application Laboratory in Tampere, Finland, an in-depth development work has verified a new solution that can achieve spatter-free high-speed metal processing by modifying the intensity of the laser focal spot on the workpiece Distribution, which significantly deviates from the traditional unimodal Gaussian distribution. This study shows that a beam consisting of a central Gaussian distributed focal spot surrounded by another concentric laser ring can often be an effective method.

The FL-ARM ring laser combiner and transmission fiber developed by Coherent's Finnish factory (formerly Corelase) make this unusual fiber laser focusing spot configuration a reality. The optical fiber adopts a traditional circular core and is covered with another layer of optical fiber core with an annular cross section.

FL-ARM can be integrated into four independent fiber laser modules, providing a maximum total output power of 2.5 to 10kW. Regardless of the specific configuration, in all cases, the overall beam distribution (that is, the power of the center and ring parts) can be adjusted independently as needed. In addition, the center and ring beams use independent closed-loop power control systems, which also ensure excellent stability within the overall power adjustment range (1% to 100% of the nominal maximum output power). The core and ring beam can even be independently modulated, with a repetition frequency of up to 5kHz.

In this layout (Figure 1), there are virtually an infinite number of combinations in the power ratio between the inner beam and the outer beam. Nevertheless, all these combinations can be roughly divided into several basic configurations. These basic modes can be adjusted to provide a wide range of processing characteristics to meet the needs of various applications in an optimal way.

Application result

The adjusted beam can output power in the center and ring position instead of forming a traditional single laser spot. The welding is mainly completed by the ring light spot, and the welding process is divided into two steps. First, the front edge of the outer ring preheats the workpiece, and the additional energy required for welding is provided by the rear edge of the ring spot. By dividing the provided laser energy into two parts and dispersing it over a larger area, a larger molten pool can be generated, thereby reducing the temperature gradient in the material. All these features help reduce splashing (Figure 2).

Figure 2: The cross section shows the weld on a 1.25mm thick galvanized steel sheet. The fiber laser is used, there is no gap between the plates, and the feed rate is 3.3 m/min. The weld formed by traditional laser focusing has gaps (a), and Using FL-ARM technology can form an excellent non-porous uniform weld (b).

At the same time, the central focal spot can maintain a deep penetration hole (at a lower temperature than the edge) to facilitate pushing the molten material to the side. In this way, the vaporized zinc can be easily discharged through the center, even if the parts are clamped together with zero gap, there will be no splashing.

In addition, because the ring beam is rotationally symmetric, there is no need to follow the direction of the welding seam to adjust the direction of the beam. On arc-shaped or other shaped workpieces, the direction of the welding seam often changes significantly. Therefore, this method can significantly simplify the process flow.

In this application, the FL-ARM laser is successfully used for deep penetration welding to achieve high-strength welding without material deformation. Similarly, the beam power of the center and the ring part can be configured.

Figure 3: The cross-section of the surfacing weld of two 1.6mm thick 5000 series aluminum parts shows deep penetration without porosity or spatter.

The method is effective because the leading edge of the ring beam raises the temperature of aluminum sufficiently high to increase its absorption capacity at the laser wavelength. Subsequently, a deep penetration hole is generated in the center of the beam, and the penetration hole is very stable due to preheating. The trailing edge of the ring beam provides sufficient support to the molten pool and allows gas to escape. Due to the stable melt hole, the material is not easy to solidify quickly, which promotes the whole process to be more consistent and the process window is also larger. The end result (Figure 3) is to achieve uniform material penetration and higher quality porosity and spatter-free welds.

Currently, although fiber lasers have been widely used in a variety of industrial processing fields, however, no single product can be the best choice for all use cases. This is why laser manufacturers such as Coherent-Rofin have developed a large number of different fiber lasers. Then on this basis, the company combines these products with a wealth of process knowledge to expand their utility and provide better results, such as reducing splashes, improving processing capabilities, and reducing users' production costs.