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Untersuchungen zum Widerstandspunktschweißen von hoch- und höchstfesten Feinblechen

Investigations to resistance spot welding of high- and ultra high strength steels
 
: Marx, R.

Senftenberg, 2008, 84 pp.
Senftenberg, FH Lausitz, Dipl.-Arb., 2008
German
Thesis
Fraunhofer IWU ()
Widerstandspunktschweißung; hochfester Stahl; Schweißbarkeit; Schweißparameter; Leistungsregelung; servomechanischer Kraftantrieb; TRIP; HT700T; 22MnB5; HSS; UHSS; resistance spot welding; high strength steel; ultra high strength steel; weldability; welding parameter; welding power control; servomechanical drive for electrode force

Abstract
Today spot welding is still the most applied joining technique in the automotive industry. But new developed materials require the adaption and enhancement of this technology for a robust joining process. Investigations were focused on weld-ability of high- and ultra high strength steels like HT700T (TRIP steel) or 22MnB5 (press hardening steel) by using a servo-mechanical drive for electrode force as well as power control instead of conventional welding current controller.
The results of the experiments showed that the TRIP-steel as well as the press hardening steel can be safely welded with an electrode force of more than 5 kN and an welding current of about 8 kA. Best results were achieved with a process control using multiple current impulses. By using an after-heat impulse hardness of spot welds is decreased and the failure mode of the specimen during the chisel test could be improved. The course of hardness across the spot weld lens as well as inside of the base material is already on a high level but drops significantly steep inside of heat affected zone (HAZ).
The advantage of the servo mechanical electrode drive is the backlash-free threaded spindle which leads to an increase of electrode force because of the thermal expansion of material during heating up to melting temperature. This effect improves the weldability of 22MnB5 and leads to an increase in tool-life time of the spot welding electrodes.
The use of process control with controlled welding power in conjunction with a multi impulse process have favorable effects on the results of the tensile tests and ensures a robust welding process.
It could be proven that, if the appropriate welding parameters are chosen, joining of HT700T as well as 22MnB5 by resistance spot welding is possible where the welded connections fulfill the requirements of quality in respect of lens diameter, connection strength and failure mode.

: http://publica.fraunhofer.de/documents/N-74307.html