Laser Blank Welding High-Strength Steels

Abstract

Four representative high-strength steels (HSS) with nominal thicknesses of 0.76 mm were selected to be laser welded to similar-gage mild steels (MS). The formability of laser-welded blanks was investigated. The HSS studied included two transformation-hardened steels, one interstitial-free (IF) rephosphorized steel, and one steel strengthened by both niobium grain refinement and transformation hardening. Metallurgical examination and mechanical testing were performed to characterize the welded blanks to establish correlations based on the processing-structure-formability behavior of laser-welded blank sheets. Laser welding produced narrow welds with increased hardness. In transverse tensile strain testing, the elongation transverse to the weld line depended on the strength ratio of the two base metals. When the ratio of the yield strength of the HSS to the tensile strength of MS was less than the limiting strength ratio (LSR), both MS and HSS were deformed plastically. Otherwise, deformation was restricted to the MS, greatly reducing the ductility of the welded blank sheets in transverse tensile strain testing. In longitudinal tensile strain testing, both ductility and strength were largely controlled by the attached high-strength sheet. The increased level of weld hardness reduced the strain to failure along the weld line, but the small width of the welds minimized this negative effect. The equiaxial stretch formability was evaluated by standard ball-punch testing. For the welded blanks composed of two parent steel sheets of large strength difference, the movement of weld lines during ball-punch testing restricted the strain in the direction perpendicular to the weld line to the MS side of the welded blanks. The fractures initiated in MS and propagated parallel to the weld lines during ball-punch testing. The stretch formability of welded blanks was 12 pct lower that that of the parent steel sheets. The formability of these welded blanks was primarily determined by the ductility of the MS, regardless of the hardness of the weld lines or the ductility of HSS. For welded blanks composed of materials of similar strength, the weld remained at the center of the cup during stretch formability testing. Fracture initiated in the weld line and propagated across the weld during ball-punch testing. This only slightly reduced the stretch formability of welded blanks compared with that of the parent steel sheets.