Steinbeis experts enhance developments in laser beam microjoining used in medical technology
Demand for laser welding in the manufacture of medical components continues to rise. These are components in sometimes highly complex shapes and sizes, sometimes also with seam contours and these can be subject to extremely tight tolerance requirements. Undertaking a task on something like ultrathin capillary tubes (which sometimes have walls less than 200μm thick and have to be welded onto increasingly complex components) soon becomes impossible using traditional laser welding techniques, especially without additional filler metals. The section thicknesses suffer and the capillary tubes become significantly thinner. It is simply not possible to guarantee that tubes will be joined together as required. This was the starting point for a collaborative project initiated as part of the Central Innovation Program for SMEs (ZIM). The project is called “Laser Beam Microjoining of Cannula Tubes for Medical Products using filler metals.” Both of the project partners from Chemnitz – the welding production company STF GmbH and the Joining Technology Steinbeis Innovation Center – are working on new ways to insert wire-shaped materials into a pulsed laser process in order to deliver reproducible welds.
As medical instruments become smaller and smaller, it is becoming more and more challenging to produce the right welds and join materials in new ways. There are also commercial considerations. Not only is important to build on existing joining technology, which can be adapted to meet these new challenges; new methods and techniques also have to be developed. Finally, the project partner needs to keep costs under control.
The experts working on the development project decided to use a wire feeder made by a laser application specialist called L&A. This can be automated and it has an open interface allowing the user to integrate the technology into existing laser systems and processes. The wire feeder makes it possible to feed wires with a diameter of up to 300μm into a component, without affecting the process, so it could be ideal for many other joining applications in the future. The system also includes an adaptable fixation device consisting of two hydraulic clamping arms and an ultra-precise positioning device on three axes. This makes it possible to adjust the inserted wire as required and reproduce configurations to match certain shapes and seam contours. The seam welds are made along thin-walled cannula tubes using stainless steel (type 1.4301) with diameters of between 1.5 and 4.05 millimeters and wall thicknesses of 140 – 200μm.
To work out the different combinations of parameters that will be relevant to this laser beam welding technique, the project team decided to use a special material that is known to be suited to medical technology: QuaLAs QL Med 4430. The experts at the Steinbeis Innovation Center and STF conducted a series of experiments to test how to match the laser beam welding parameters (focus diameter, pulse energy, welding speed) to processes when the filler material is added. They also determined suitable parameter ranges for inserting the filler material (wire diameter, wire insertion rate, length of protruding wire ends, the angle of attack of the wire, etc.). Further work carried out for the project included a comparison of the properties of welds without the filler material.
After comparing their results with the diameters on the original parts, which had been joined without inserted fillers, it was found that the diameters with additional fillers had a wider join between the cannulas. Accordingly, using wire-shaped fillers made it possible to raise the maximum tensile load of seams by up to 30 percent (maximum possible load without additional filler 1400N; with additional filler 2000N). The experts therefore concluded that welding the parts without additional fillers did indeed result in extremely thin walls on the tubes as a result of the welding process. If there were faults with welded seams using additional fillers, these were generally along the pipe walls because the diameters were too low for the weld. The dimensions of the different elements along the welded joint – where it may not be possible to fill the welding area right up to the point where the two tubes with additional fillers meet – have no negative impact on component strength.
When curved cannulas needed to be welded, a small gap developed during the bending process due to different tolerances. This gap ran between the tubes in the area around the bend and this could be safely connected by using molten filler material. The seam properties were no different to straight welds.
The parameter list provided by the Steinbeis Innovation Center for Joining Technology covers connecting welds with additional fillers for a variety of semi-finished capillary tubes. The list has allowed STF to optimize its existing welding process and extend its systems to cover a wider spectrum of variants. This has of course also allowed the firm to expand its portfolio of medical components. Because the welding process is now also more automated, production has become more efficient and this will help safeguard the competitiveness of the Steinbeis partner into the future.