Synergistic Regulation Mechanism of Oscillating Laser and Solution Treatment on Microstructure and Properties of Laser-Arc Hybrid Welded Joints of 2507 Super Duplex Stainless Steel

Technical Summary: Synergistic Regulation Mechanism of Oscillating Laser and Solution Treatment on 2507 SDSS Welded Joints

Problem Statement
2507 Super Duplex Stainless Steel (2507 SDSS) is a high-performance alloy widely used in marine and petrochemical industries due to its excellent strength and corrosion resistance. However, welding this material presents significant challenges. Conventional welding thermal cycles often disrupt the ideal 1:1 ferrite-austenite phase balance, leading to excessive ferrite content (often exceeding 70%), grain coarsening, and the precipitation of detrimental intermetallic phases. These microstructural defects result in degraded toughness, reduced corrosion resistance, and compromised long-term service safety. While Laser-Arc Hybrid Welding (LAHW) offers high efficiency, it frequently produces non-uniform heat input along the plate thickness, creating "funnel-shaped" welds with severe microstructural gradients. Furthermore, post-weld heat treatments must be carefully controlled to avoid inducing secondary phase precipitation while attempting to restore phase balance.

Methodology
This study investigated a synergistic approach combining Oscillating Laser-Arc Hybrid Welding (OLAHW) with post-weld solution treatment to optimize the microstructure and properties of 5 mm thick 2507 SDSS butt joints.

• Welding Process: The researchers compared conventional LAHW with OLAHW. The OLAHW process utilized a continuous-wave fiber laser (2.5 kW) coupled with a MIG arc (250 A) in an arc-leading/laser-trailing configuration. The laser beam underwent transverse linear oscillation (1 mm amplitude, 300 Hz frequency) to enhance melt pool stirring.
• Post-Weld Treatment: All welded joints underwent solution treatment at 1050 °C for 15 minutes, followed by water quenching.
• Characterization: The study employed comprehensive characterization techniques, including X-ray diffraction (XRD) for phase analysis, Optical Microscopy (OM), and Electron Backscatter Diffraction (EBSD) for grain size and texture analysis. Mechanical properties were evaluated via tensile testing, Charpy V-notch impact testing at multiple temperatures (20 °C, -40 °C, -80 °C), and microhardness profiling. Corrosion resistance was assessed through potentiodynamic polarization in a 3.5 wt.% NaCl solution.

Key Contributions and Results

1. Weld Formation and Geometric Uniformity:
   Oscillating laser welding significantly altered the weld cross-section from a "funnel shape" (typical of LAHW) to a "wine-glass shape." This change reduced the width ratio between the top and bottom of the weld from 10:1 (LAHW) to 6.3:1 (OLAHW), indicating a more uniform heat distribution and reduced geometric gradients along the plate thickness.

2. Microstructural Evolution and Phase Balance:

   • Phase Ratio: The OLAHW process increased the fraction of intragranular austenite (IGA) and reduced Widmanstätten austenite (WA), achieving a ferrite-austenite ratio closer to the ideal 1:1. In the Heat-Affected Zone (HAZ), austenite content increased from 30% (LAHW) to 42% (OLAHW).
   • Grain Refinement: The oscillating laser effectively suppressed ferrite grain coarsening in the HAZ, refining the average grain size from 26.4 μm (LAHW) to 13.6 μm (OLAHW).
   • Solution Treatment: Post-weld solution treatment further promoted ferrite-to-austenite transformation and microstructural homogenization. While it slightly shifted the phase balance in some regions (ferrite dropping to ~27%), it significantly enhanced overall uniformity.

3. Mechanical Properties:

   • Tensile Strength and Ductility: The as-welded OLAHW joint achieved an optimal balance, with a tensile strength of 920.4 MPa and elongation of 25.5%. Crucially, fracture occurred in the base metal rather than the weld metal, indicating the weld zone was no longer the mechanical weak link. In contrast, the as-welded LAHW joint fractured in the weld metal with lower ductility (21.7%).
   • Impact Toughness: OLAHW significantly improved low-temperature impact toughness. The solution-treated OLAHW joint exhibited the highest toughness, with HAZ values at -80 °C reaching 129 J/cm² (matching the base metal), compared to 93 J/cm² for the as-welded LAHW HAZ.
   • Hardness: The OLAHW process reduced hardness gradients across the weld thickness. Solution treatment further homogenized hardness, bringing the weld metal values closer to the base metal.

4. Corrosion Resistance:
   The solution-treated OLAHW joint demonstrated the best corrosion resistance among all samples, with a corrosion current density of 353.8 nA/cm², nearly matching the base metal (327.6 nA/cm²). This improvement is attributed to the homogenization of the microstructure and the restoration of the optimal phase ratio, which minimizes local depletion of corrosion-resistant elements like Cr and Mo.

Significance and Claims
The paper claims that the synergistic combination of oscillating laser welding and solution treatment provides a precise mechanism for regulating the microstructure and properties of 2507 SDSS joints. The study demonstrates that this approach effectively mitigates the microstructural imbalance and property degradation typically associated with welding super duplex stainless steels. By enhancing melt pool convection and optimizing the thermal cycle, the process yields joints with superior geometric uniformity, refined grain structures, and a balanced dual-phase microstructure. The authors conclude that this methodology offers a robust theoretical basis and technical support for the high-quality joining of 2507 SDSS in harsh service environments, such as marine engineering and petrochemical applications, where both mechanical integrity and corrosion resistance are critical.