With new fields of application, component geometries and materials, it can happen that conventional laser optics with restricted movement radii and limited beam shaping options are no longer sufficient to guarantee a flawless welding result. These include the joining of thermoplastics and metal as structural components in vehicle construction and the welding of hairpins in electric motors. The laser specialists at LMB have therefore developed two new concepts together with partners in order to be able to positively influence the melt pool through more flexible control and more precise power distribution.
“The laser is an established tool in manufacturing,” says Dipl.-Ing. Peter Schlüter, Managing Director of LMB Automation GmbH. “Despite its versatile possibilities and characteristics, there are a few stumbling blocks to its use that can be overcome through adaptation and continuous development.” Not all laser beams are the same: the shape influences the product quality by determining the power density, cutting width, melt pool dynamics and heat-affected zone. Depending on the application, the beam can be focused, widened or split. Particular attention should be paid to the weld pool, which affects the geometry, strength and microstructure of the joint and therefore the quality of the weld seam. The better these dynamics can be controlled with optimized optics independent of location and time, the easier it is to avoid typical problems such as cracks, pores, spatter, undercuts or insufficient penetration in the keyhole.
© LMB
Design of an optical system with rotating components
A typical case in which the combination of a demanding material and insufficient flexibility of the optics can lead to problems is the welding of copper connections, which are used for hairpins in stator production, for example. The viscous molten copper leads to a pronounced process dynamic, as a result of which the material starts to spatter very quickly. However, a process with as little splashing as possible is necessary for production, as no ejected material may enter the stator. To ensure this, LMB uses an adapted bifocal optic in the Twin-Weld process, which additionally divides the beam in the forward movement compared to conventional, fixed scanner optics and allows it to rotate in small circles around the optical axis.
“The two partial jets use two keyholes in the melting area to ensure that the melted material is mixed better and pressed into the gap by the rotary movement,” explains Schlüter. This increases the strength of the weld seam. At the same time, the division and rotation ensure that the time taken to pass over a reference point is halved compared to just one focus point, thereby speeding up the entire welding process.
With the combination of high rotation speed and flexible optics, the Twin-Weld process makes it possible for the first time to process all contours with these optics. This would not even be possible with a fixed bifocal lens. In addition, the higher rotation speed significantly reduces the number of pores that occur in the upper area of the weld seam. “As CU components in particular are very prone to spattering, we wanted to prevent this at all costs. We achieve this by quickly melting and pressing the heated material in so that it cannot escape upwards out of the keyhole in the first place,” says Schlüter.
© LMB
Laser optics for joining plastics with a wide seam
Thermoplastics represent a new challenge for laser welding. They are increasingly being used as structural components for the realization of lightweight construction concepts. Although laser welding of plastics is not new, it is mainly used to create relatively narrow weld seams, for example in microfluid applications. Structural components, on the other hand, require large connecting surfaces between the individual components in order to ensure sufficient force transmission. In order to achieve higher strength and tightness when joining different plastics and plastics with metals, wider weld seams are required than can be created with standard optics. “In principle, homogenized laser focuses can be used to create wide weld seams,” Schlüter adds. “However, studies have shown that the maximum process temperatures when joining with a large laser focus at small radii lead to partial overheating of the material in the inner area.” The main reason for this is the severely limited or non-existent possibility of varying the intensity distribution within the required large laser beam spot during the joining process in terms of location and time.
This is where the new multi-spot welding head comes in, which LMB has developed together with six partners as part of an “KMU-innovativ” project. The optics use a matrix of 3×3 laser diodes, which can be controlled individually, allowing the intensity distribution in the laser focus to be determined very flexibly. For this purpose, each of the individual optics is supplied by its own laser diode. This has the advantage that, for example, the outer area of a contour can be exposed to a higher beam power and the inner area to a reduced beam power. This allows the welding process to be adapted much more precisely to the welding geometry and material properties.
Interaction of plastic and metal in lightweight construction
This look has proven to be particularly promising when welding plastic to metal, which is increasingly being used in vehicle construction for doors or interiors, for example. “In the tests, the load-bearing element was a sheet metal frame to which a plastic cladding was to be attached for protection and, above all, to reduce weight,” reports Schlüter. The weld must achieve a high level of strength and be tight so that no moisture can reach the frame. In preparation for the process, the joining surfaces of the metal part were first roughened using a different laser system. The two components were then pressed together and the metal heated with the new multi-spot welding head. With an input power of 100 W per diode, a total of 900 W could be used to melt the pressed-on plastic at the contact surface to the hot metal. Thanks to the adjustable intensity distribution, the melt flowed evenly into the structured areas without heat loss, resulting in a particularly homogeneous and stable seam.
Both optics are currently being used for in-depth application tests in prototype structures to find out how the intensity distribution and the melting result behave with different geometries and materials. The aim is to gain important insights into how the structure and operating costs can be made even more efficient in the future through possible adjustments.
For LMB, both projects showed that constantly questioning and further developing established concepts pays off: “It all started with a simple question: How can the efficiency of the laser be improved when it is introduced into the workpiece? In the end, we now have two practicable and future-oriented optical concepts that can make the laser tool a little more versatile and ultimately more sustainable in lightweight construction,” summarizes Schlüter.
Project Multispot
In order to implement the complex tasks, it was necessary to work together with specialists in the individual departments. In the Multispot project funded by the Federal Ministry of Education and Research, implementation was achieved with four SMEs, a research institute and two associated partners. The tasks were distributed as follows: The LZH (Laserzentrum Hannover) was responsible for the process and software development; the development of the diode unit was implemented by NeoLase GmbH in cooperation with Coherent; the construction of the special optics was carried out by Sill Optics GmbH&Co. KG; a special measuring device was developed by Primes GmbH to carry out the necessary measurements with regard to intensity distribution; the task of LMB Automation GmbH was to integrate all the necessary components, including cooling, into an optical head and prepare it for use on a robot; application samples for testing were provided by VW AG.
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