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Resistance spot welding of some EHS and UHS steels: part 2, stainless steels and mixed materials

Joakim Hedegård, Johan Andersson

Abstract

This is a second part of the final report of a SIMR Joining Technology Centre project concerning resistance spot weldability of EHS and UHS steels.

There are many EHS and UHS steels, both C-Mn and stainless, on the market today and the development of new and improved high strength steels continues. How many of these have been studied in terms of resistance spot weldability and reported in literature?

To evaluate the state of the art of resistance spot welding of EHS and UHS steels, a project was formed at the SIMR Joining Technology Centre. First, a literature study was performed. Some reports and articles concerning resistance spot weldability of EHS and UHS steels were found, but no material with tensile strength above 600 MPa was found reported. Also, little data concerning stainless steels were found. Therefore, it was decided that spot welding trials should be performed on test coupons with some materials of interest. (Results for the C-Mn steels are presented in part one of the report.)

Selected stainless steels for resistance spot weldability tests: (each steel welded to itself)

  • SS 2377 (AvestaPolarit 2205, ASTM S32205); t = 1.5 mm
  • SS 2378 (AvestaPolarit 254 SMO, ASTM S31254); t = 1.5 mm
  • SS 2331 (AvestaPolarit 4310, HyTense 17-7, ASTM 301); t = 1.5 mm
  • SS 2333 (AvestaPolarit 4301, ASTM 304); t = 1.5 mm - reference material Selected mixed materials for resistance spot weldability tests:
  • Docol 350YP; t = 1.5 mm (C-Mn, uncoated) + SS 2333 (ASTM 304); t = 1.5 mm
  • B500; t = 1.5 mm (C-Mn, 20 µm Zn-coating) + SS 2333 (ASTM 304); t = 1.5 mm

In general, good spot weldability results were achieved for all the evaluated materials and combinations. However, the examined stainless steels behaved quite differently during the spot welding trials. Two of the four tested stainless steels achieved a fairly good current range for 6 mm weld (plug) sizes at the fairly low 4 kN electrode force level. The other two stainless steels needed 6 and 9 kN respectively to reach a sufficient current range.

Some of the evaluated materials had a tendency for developing a porosity or shrink cavity in the centre of the weldment at lower electrode forces. The seriousness of these discontinuities is not high as long as the geometry is round and the size is limited. To avoid these types of discontinuities, an increased electrode force should be used.

During shear tensile testing of the stainless steels, it was common with the interfacial failure fracture mode. The fracture forces were, however, quite high. For the mixed material joints, interfacial failures were rare. During the cross-tension testing, full plugs were achieved in almost every case (for all material combinations).

As future research, pulsed welding is suggested at low electrode forces in an attempt to avoid centre porosities at these force levels. Also, verifying tests is suggested where spot welding is performed on larger profiles with the developed coupon data from this project. (Robustness control of data.) A further investigation of the interfacial failures at shear tensile testing is also of interest.

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Senast ändrad av: Peter Hjertsson
26 mars 2015

 


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