In the joining processes group, basic as well as application-oriented projects particularly in the areas of dissimilar materials, novel joining processes, brazing and micro welding are dealt with. Common bracket of the work is the holistic consideration of the complex interactions between process and material. This is done in order to realise joint properties meeting the requirements by developing new and optimising existing joining processes. A thematic relationship exists with the BIAS core competences fibre laser applications, joining in lightweight construction, joining of dissimilar materials and system technology for joining.

Current work in the areas:

Dissimilar Materials

In vehicle construction the use of hybrid structural elements from dissimilar materials is currently intensely discussed. The aim is to achieve weight reduction whilst maintaining or improving the mechanical properties. The joining of aluminium to steel alloys is of special interest for car construction, but also for shipbuilding. Laser-based processes for these tasks, such as e.g. laser beam welding with filler wire, are investigated at BIAS for more than 10 years. They provide a high welding speed (typically up to 8 m/min) and are fit e.g. for tailored blank applications. However, zero gap conditions are a prerequisite. In order to enable also the joining of more complex parts, a laser-MIG hybrid joining process was developed to combine high productivity and gap tolerance.

The figure (left side) exemplarily displays a demonstrator made from a steel and an aluminium tube joined by this process with a girth weld. A possible application for such joints is to be found e.g. in car axles.

The application of this process is also conceivable e.g. for side wall elements of deck houses in yacht construction, where a sheet thickness of 3 mm is common. A corresponding demonstrator profile (aluminium + steel, thickness 3 mm each, 1 m seam length) was produced together with BLS Lasertechnologie (figure, right), in order to verify the principle feasibility of joining aluminium to steel for ship structures.

Novel Joining Processes

The development of novel joining processes often is a decisive step towards solving quality problems. As an example, a hybrid laser-magnetic field welding process was developed for aluminium alloys sensitive to hot-cracking. With this process, an improved dilution of the weld metal with filler materials is enabled, thus significantly lowering the danger of hot-cracking. For the aircraft industries in special, where laser beam welding is increasingly used for such alloys, this process is of considerable interest.

Another example is the development of welding processes with low distortion. Thus, laser GMA welding with single-mode fibre laser was developed specially for joining in the thin sheet range with filler material and at low heat input. By combining the processes, it becomes on the one hand feasible to weld butt joints with the single-mode fibre laser and on the other hand to significantly increase welding speed compared to a pure GMA process. Consequently, total heat input can be reduced whilst productivity is improved.
 
The figure shows a butt joint from DC04 (thickness 1.4 mm), where a gap of approx. 0.7 mm (approx. 50% of sheet thickness) was bridged at sufficient top bead and excellent root quality at a welding speed of 4.5 m/min.

This is a significant improvement compared to the state-of-the-art. In combination with the significantly lower investment cost of single-mode fibre lasers  of lower power (compared to high power lasers), it is expected that numerous interesting areas of application in the sheet metal industry will open for this process.

Brazing

A wide variety of materials for metallurgical reasons requires joining at an exceptionally low heat input. Here, e.g. laser or plasma brazing processes may be a choice, as the joint is formed by brazing materials with a lower melting temperature than the base materials. Thus, melting of the base materials is not required, and the heat affected zones can be minimized. Currently, mainly fundamental investigations on laser beam brazing of aluminium alloys and high-strength steels are performed (figure), in which, aside from a holistic understanding of joint properties, the avoidance of fluxes is aimed at.

Micro Welding

The increasing trend towards miniaturisation of components in the fine mechanical and electro industries requires precise production and, in special, joining processes enabling the compliance with part tolerance requirements in the µm range. For such processes, single-mode fibre lasers with high beam quality or pulsed Nd:YAG lasers are available. However, working ranges for process parameters can not be scaled down easily to a material thickness of several 10 µm. As an example, stiffness is disproportionally reduced with sheet thickness, consequently increasing the relevance of distortion.  In spite of these challenges in welding of micro parts, there are already very promising results for joining of special materials, but also for material combinations (figure, stainless steel + copper).

Publicly funded researche projects:

• Hybrid
  
• Magnetic Stirring
  
• Mikroverzug
  
• ProReMix
  

Contact:

Dipl.-Ing. Claus Thomy
Group Manager Joining Processes

Tel.: +49-421-218-5037
thomy@bias.de