Tomas Forsman, Peter Nerman

Abstract

This study was performed with two goals:

• to validate a formability relationship by butt welding Dogal 800 DP (high strength, zinc coated, dual phase carbon steel), TRIP 700 (high strength, zinc coated, plasticity induced martensite transformation) and Docol 1200 (extreme high strength, martensitic carbon steel).

• to investigate the laser weldability during tailor blank production of steels by butt welding Rephos 260 (low strength, zinc coated carbon steel) to Dogal 600 DP (high strength, zinc coated, dual phase carbon steel) with the same thickness, a 1.0 mm sheet to a 1.8 mm sheet of Docol 600 DP (high strength, dual phase carbon steel) welded at seven different focal positions and 0.8 mm stainless steel 301 (two different extreme strength grades) to 1.6 mm DP 600 zinc coated, dual phase carbon steel (two different steel manufacturers).

In total, 15 combinations were laser welded and for each combination the full penetration weld produced at the highest speed was analysed by visual inspection, transverse cross-sections, X-ray, cross-sectional hardness, tensile test, and Erichsen cupping test.

Visually, the welds were free from defects. The cross-sections revealed minor
misalignment or root concavity for a couple of welds. The geometrical mixes had varying weld shapes but were without serious defects. The most narrow welds produced (Dogal 800 DP, TRIP 700 and Rephos 260 to Dogal 600 DP) had a top width of about 0.75 mm and a root width of about 0.50 mm. The X-ray test also revealed the small misalignment and root concavity defects. Single small pores were found in four of the weld combinations.

The hardness test showed that Dogal 800 DP had a hardness top in the HAZ, Docol 1200 had a hardness drop in the HAZ and 301 also a hardness drop in the HAZ. For Docol 1200 and the full-hard 301 grade the base material is as hard as the weld metal.

The tensile test yielded failure outside of the weld for the best performing weld
combinations (Dogal 800 DP, TRIP 700, Rephos 260 to Dogal 600 DP and Docol 600 DP welded at positions 1, 3 and 10. The remaining combinations failed in the HAZ of the thinner sheet.

The Erichsen cupping test showed that the best performing weld combinations (Dogal 800 DP and TRIP 700) failed by a crack developing transverse to and across the weld. The weld in Dogal 800 DP yielded an Erichsen number of 83% of that of the base material while TRIP 700 reached 60%. The best geometrical mix (focal position 1) reached 66% and the best stainless to carbon steel mix (full-hard 301 to Thyssen DP 600) reached 67%.

Dogal 800 DP and TRIP 700 were found to fit the formability relationship (a linear
relationship for the Erichsen number as a function of differential weld hardness times weld width) while Docol 1200 failed due to the hardness drop in the HAZ. Although highly questionable, the other weld combinations were tried in the model showing excellent agreement except for Docol 1200. A linear relationship was shown for the Erichsen number as a function of base material strength.