By: Klaus Loeffler, Dr. Tim Hesse, Peter Kaupp

TRUMPF Laser und Systemtechnik GmbH, Ditzingen, Germany
TRUMPF Werkzeugmaschinen GmbH, Ditzingen, Germany

The existing problems of weld defects in welding zinc coated steels in a overlap situation are present in the automotive industry. Overlap joints are the most common type of joint geometry used in the automotive industry. In this case there are 2 layer of zinc coatings in between the two steel sheets. The boiling point of Zinc is at 906C and the melting point of steel at 1536C. The different boiling and melting points causes Zn outgassing. Welding with a technically zero gap creates weld defects by material ejection. The consequences are reduced strength of the weld, requirement of additional sealer and additional maintenance of the equipment. The technical work around are designed gaps between the sheet to allow controlled outgassing Extensive research has been taken on with highspeed cameras to evaluate the root cause. Two mechanisms have been found. The first one is called “balloon effect”. This effect describes an Zn-jet that is initiated on the key hole front and acts focused on the molten material of the capillary back wall. The result is weld sputter at the capillary back wall and loss of molten material. A quick forward movement of the key hole back wall boosts the balloon effect. Right at this moment the interface between the sheets will be disrupted. The second mechanism has been called “lateral Zn-outgassing”. The Zn-layer “wings” lateral to the molten pool area are impacted by the heat conduction. The width of the impacted Zn-layer increases along the molten pool. The molten pool allows the high pressure vaporized zinc from the Zn-layer “wings” to outgas. Explosive ejections of the material happens in an area of the molten pool where the top material layer is already solidified. There are 3 possible solutions to reduce / eliminate welding defects. The first solution is a defined gap with local distance holder. These distance holders can get applied by a laser pulse. While using remote laser welding the same equipment can get used to apply laser pulses on to the material. A certain parameter set creates reliable dimples on the material of 0,1 to 0,2 mm. Another reliable method is the use of glass powder. The second solution is the use of special Zn coatings. The development together with steel producers works in the direction to reduce the zinc coating and to apply a secondary coating for corrosion resistance. The research has shown that a 0,2µm Zn coating almost eliminates the weld defects. The third solution with the least implication is he so called K-joint design as shown in figure 1. The K-joint (patend pending) includes several advances. First, the joint is designed to weld no. First, the joint is designed to weld no longer in overlap configuration through both layers. The k-joint only welds through one layer in an area where zinc is not present. This way there are none of the mechanisms as described in this paper present. The design allows to weld with half the laser power or twice as fast. Based on the result the new joint design is the most advanced solution. Additional developments on the material / coating side are under way.

Figure 1: K-Joint