Comparison of defect formation, mechanical strength and corrosion resistance of GTAW and LBW joints performed in pre-strained sheets of galvanized dual-phase steel

Technical Summary: Comparison of Defect Formation, Mechanical Strength, and Corrosion Resistance of GTAW and LBW Joints in Pre-Strained Galvanized Dual-Phase Steel

Problem Statement
The automotive industry increasingly utilizes Tailor-Welded Blanks (TWBs) to optimize vehicle weight and manufacturing costs. These components often involve joining sheets with varying thicknesses and pre-strain levels. While Dual-Phase (DP) steels offer an excellent strength-ductility balance, their application is complicated by the presence of zinc coatings (galvanization) and the pre-straining required for forming. The combination of pre-straining, low-melting zinc coatings, and thermal cycles during welding creates a high risk for defects such as liquid metal embrittlement (LME), gas porosity, and hot cracking. Furthermore, the heat-affected zone (HAZ) in DP steels is prone to softening due to martensite tempering. Although Gas Tungsten Arc Welding (GTAW) and Laser Beam Welding (LBW) are common joining methods, the specific influence of pre-strain levels on the thermal fields, defect formation, microstructural evolution, and corrosion resistance of galvanized DP590 steel remains insufficiently understood.

Methodology
This study employed a combined numerical-experimental approach to evaluate GTAW and LBW processes on pre-strained galvanized DP590 steel sheets (0.89 mm and 1.34 mm thickness).

• Experimental Design: A full-factorial $2^3$ Design of Experiments (DoE) was utilized, with welding process (GTAW vs. LBW), welding current/laser power, and pre-strain level as control factors.
• Characterization: Macrographs and micrographs were analyzed to assess Fusion Zone (FZ) geometry, aspect ratios, and internal defects (porosity, cracking). Microstructural analysis was conducted using Optical Microscopy, SEM, and EPMA to quantify phase fractions and zinc coating degradation. Microhardness profiles were measured across the BM, HAZ, and FZ.
• Corrosion Testing: Potentiodynamic polarization tests were performed on isolated FZ and HAZ regions in a 3.5 wt.% NaCl solution to determine corrosion rates and mechanisms.
• Numerical Simulation: A Finite Element (FE) thermal model was developed to predict transient temperature distributions, cooling rates, and heat accumulation. Different heat source models (double-ellipsoidal for GTAW and elliptical paraboloid for LBW) were calibrated to account for the modified FZ morphology caused by sheet deflection.

Key Results

• Defect Formation: Pre-straining significantly altered FZ morphology, transitioning it from elliptical to trapezoidal or rectangular shapes. This widening facilitated the escape of Zn vapor bubbles, effectively suppressing porosity. High cooling rates accelerated solidification, preventing hot cracking and minimizing the interaction time between solidified metal and residual liquid Zn, thereby mitigating LME. No solidification cracks were detected in any welds.
• Microstructure and Hardness:
  • GTAW: The wider FZs led to heat accumulation and prolonged residence in the intercritical temperature range, promoting ferrite formation. This resulted in a heterogeneous microstructure (predominantly Widmanstätten and polygonal ferrite) with lower and non-uniform microhardness (average ~326 HV) and localized softening.
  • LBW: The concentrated heat source and higher cooling rates suppressed ferrite nucleation, promoting a refined, homogeneous bainitic–ferritic microstructure with a higher martensite fraction. This yielded higher and more uniform microhardness values (average ~398 HV) throughout the FZ.
• Corrosion Resistance:
  • GTAW: The heterogeneous microstructure (ferrite coupled with bainite/martensite) created numerous micro-galvanic couples, leading to higher galvanic corrosion rates. The loss of the Zn coating resulted in uniform corrosion.
  • LBW: While the FZ was susceptible to chloride-induced pitting due to the absence of a stable passive film, the higher fractions of martensite and upper bainite (which exhibit more noble electrochemical behavior) resulted in lower overall corrosion rates compared to GTAW joints. The HAZ in LBW was negligible, preserving the base material's properties.

Significance and Conclusions
The study concludes that Laser Beam Welding (LBW) is a more suitable joining process than GTAW for pre-strained galvanized DP590 steel sheets of low thickness. The primary advantages of LBW stem from its ability to generate high cooling rates, which refine the microstructure, homogenize mechanical properties, and reduce susceptibility to galvanic corrosion. While pre-straining significantly influences FZ geometry and heat accumulation, the LBW process effectively manages these thermal effects to produce defect-free joints with superior mechanical and corrosion performance. The research highlights that the specific thermal cycles inherent to each welding process, rather than just the input energy, dictate the phase transformation and subsequent performance of welded galvanized DP steels.