Erik Tolf, Joakim Hedegård, Maria Lundberg

Abstract

In this project at the Kimab Joining Technology Centre, the effects of resistance spotwelding parameters such as pulsed welding, pre- and post pulses, quenching time, and annealing pulse cycles have been examined. Welding trials have been performed with high strength- and ultra high strength C-Mn steels as well as ultra high strength stainless steel. Spot welding guns with both AC and DC power supplies and stationary welding machines have been used for the welding trials. In some cases, comparative trials were performed to study the influence of the equipment. In other, the welding equipment used was determined by the intended user (the customer).

The parameter studies were as follows:

• Effects of applying a pre-pulse prior to the main weld pulse for two- and three-sheet joints of galvanised thin sheet steels.
• Comparison between a multiple pulsed weld schedule and a single pulse weld schedule for a two-sheet joint of DP 600 (dual phase steel).
• Investigation of the weldability problems for a difficult three-sheet joint combination.
• Reducing the need of high electrode forces for a two-sheet joint combination of ultra high strength stainless steel by optimising the weld schedule.
• Annealing pulse cycle optimisation to improve the fracture behaviour of a mixed material combination.
• Study the influence of the surface condition on the weldability of pickled, blasted and Al-Si coated ultra high strength boron steel.
• Improving the fracture type and mechanical properties for the two-sheet joints of boron steel by applying different in-process heat treatments. A matrix of temper times, quench times and currents was set-up to define the effective range of tempering.

The results have been validated by comparing current ranges, nugget development and performing cross-, shear- and peel tensile testing, hardness measurements and metallographical examinations.

Introducing a pre-pulse or an up-slope prior to the main weld pulse for galvanised thin sheet steel was negative for the weldability. However, applying a pre-pulse was beneficial for improving the weldability at low electrode forces (≤ 3kN) for the two-sheet joints of ultra high strength boron and stainless steels. Using a pulsed weld schedule was beneficial for shifting the spatter limit towards higher currents but the pause time between the pulses, cooled down the weldment and thereby reduced the nugget growth compared to a single pulse. This was evident especially at lower current settings. With an optimised in-process heat treatment it was possible to substantially decrease the weld hardness in the boron steels. To achieve this tempering effect, an extended post heat cycles was required. The boron steels with the different surface conditions showed good weldability. Most of the trials were successful and the results are further illustrated and discussed in the report.