This is part one of the final report of a SIMR Joining Technology Centre project concerning resistance spot weldability of EHS and UHS steels.
Today, there are many extra- and ultra high strength (EHS and UHS) steels on the market. How many of these are evaluated in terms of spot weldability and reported in literature? To evaluate the state of the art of 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 spot weldability of EHS and UHS steels were found, but no material with tensile strength above 600 MPa was found reported. Therefore it was decided that welding trials should be performed on some interesting materials for the car and vehicle industry.
Selected C-Mn steels for the resistance spot weldability tests: (each steel welded to itself)
- Dogal 600 DP: 1.0 mm material thickness (zinc coated, hot dip galvanised on both sides, 7 µm coating thickness)
- Docol 800 DP: 1.0 mm material thickness (uncoated) Docol 1000 DP: 0.8 mm material thickness (uncoated)
- Boloc-02: 1.2 mm material thickness (uncoated)
- V-1437 (“Rephos 260"): 1.0 mm material thickness - reference material (zinc coated, hot dip galvanised on both sides, 7 µm coating thickness)
From the welding trials on C-Mn steels, the following conclusions can be made:
- Good spot weldability results have been reached for most of the evaluated EHS/UHS steels. The exception is Boloc-02 where some difficulties remain. Boloc-02 needs further testing with pulsed welding.
- Large weld lobes and spot weld sizes were easily reached for materials with up to 1000 MPa tensile strength. Above this level, more process optimisation work can be necessary when spot-welding the materials.
- Tests with formed specimens (profiles) were made to examine the usability of weld data developed on smaller coupons. The results showed that a larger deviation from the coupon data could occur when the material exceeded 600 MPa in tensile strength. (For DP 600, weld data based on 3 kN electrode force resulted in a strongly reduced current range for the formed specimens, while the change of the data based on 4 kN electrode force was limited). Comparative spot welding tests on profiles are therefore recommended when developing data for UHS-steels.
- The use of large electrode contact diameters is beneficial for spot welding of EHS/UHS steels. The exception is when the electrode force is too low (3 kN is not safe, for instance - partial plugs can occur). Perhaps should the selection of the electrode contact diameter size not only be guided by the material thickness, but also by the electrode force used. (The latter refers to how the electrode diameters are selected in the Volvo standards.)
- Truncated radius electrodes (Cap B, SS-ISO 5182 type) gave better results than flat electrodes (a reduced risk for miss-alignment).
- Adaptive control for spot welding appears to be promising as a tool to improved quality. The scatter of weld sizes was reduced and the welds became slightly larger in average.