Joakim Hedegård, Johan Andersson, Erik Tolf

Abstract

This is the final report of a second SIMR Joining Technology Centre project concerning resistance spot weldability of EHS and UHS steels. The report consists of two parts where the second part consists of enclosures (material and weld data).In this project, some interesting materials for the passenger car and commercial vehicle industry were evaluated in terms of spot weldability. Two- and three-sheet joints with both similar and mixed materials were studied. Weld data were developed on coupons and in some cases verified on formed profiles of complex shape.

Selected C-Mn steels for the resistance spot weldability tests:

• Thyssen DP600: 1.6 and 1.0 mm thickness, zinc coated (10 µm)
• SSAB DOGAL 800 DP: 1.5 mm thickness, zinc coated (10 µm)
• SSAB DOCOL 1400 DP: 1.5 mm thickness, uncoated
• Thyssen TRIP 700: 1.2 mm thickness, uncoated
• Usinor USIBOR 1500: 1.2 mm thickness, Al-Si coating
• Thyssen V-1436: 0.8 mm thickness, zinc coated (7 μm)

Selected stainless steels for the resistance spot weldability tests:

• AvestaPolarit 4310 (HyTens® 1200): 1.5 mm thickness
• AvestaPolarit 2205: 1.5 mm thickness
• AvestaPolarit LDX® 2101: 1.5 mm thickness

From the welding trials, the following main conclusions can be made:

• Good spot weldability results have been reached for Thyssen DP600, SSAB DOGAL 800 DP, SSAB DOCOL 1400 DP, Thyssen V-1436, AvestaPolarit 4310, AvestaPolarit 2205 and AvestaPolarit LDX® 2101. The exception is Thyssen TRIP 700 where difficulties remain. This material and Usinor USIBOR 1500 need further testing.
• The evaluated stainless steels have excellent spot weldability and mechanical properties.
• For a mixed material joint with stainless steel and C-Mn steel, a brittle behaviour was encountered due to the high contents of carbon and nitrogen. This was partly compensated by pulsed welding.
• Two three-sheet joints have been evaluated. Good results were achieved for both joints after process optimisation. In the case where different materials were present, the choice of electrode polarity influenced the current ranges significantly.
• The welding tests on formed specimens (profiles) with SSAB DOGAL 800 DP showed good results. The current ranges for the profiles were slightly shifted towards higher welding current, but still of approximately the same magnitude as for the coupons.
• Many of the spot-welded UHS materials encountered interfacial failures during shear tensile testing, although the rupture forces were high. This fracture mode is not only influenced by the brittleness of the weldment, but also by the material thickness and tensile strength (i.e. joint stiffness during testing).
• The use of large electrode contact diameters is beneficial for spot welding of EHS/UHS steels. Good results were achieved with both 8.0 mm electrodes and dressed 6.5 mm electrodes (contact surface diameter).