An RSM-Based Investigation on the Process-Performance Correlation and Microstructural Evolution of Friction Stir...

Area of Science:

• Materials Science
• Manufacturing Engineering
• Metallurgy

Background:

• Friction stir welding (FSW) is crucial for T-shaped thin-walled structures, but process-microstructure relationships are unclear.
• Fusion welding defects are a concern in T-joint manufacturing.
• Understanding FSW parameter effects on mechanical properties is vital for optimization.

Purpose of the Study:

• To establish quantitative regression models linking FSW process parameters (rotational speed, welding speed, plunge depth) to T-joint mechanical properties.
• To determine optimal FSW parameters for maximizing joint strength and hardness.
• To investigate the microstructural evolution and strengthening/softening mechanisms in FSW T-joints.

Main Methods:

• Response Surface Methodology (RSM) was employed to develop regression models.
• Tensile testing and hardness measurements were conducted to evaluate mechanical properties.
• Microstructural analysis using microscopy was performed to observe grain evolution and fracture mechanisms.

Main Results:

• Optimal parameters (400 rpm, 60 mm/min, 0.21 mm) achieved 74.1% UTS and 94.4% WNH compared to base material.
• Welding speed (v) significantly influenced joint mechanical properties.
• Microstructure analysis revealed dynamic recrystallization (DRX) in the stirring zone (SZ) leading to grain refinement, while the heat-affected zone (HAZ) showed softening and brittle fracture initiation.

Conclusions:

• DRX-driven grain refinement is the primary strengthening mechanism in FSW T-joints.
• Weld nugget hardness correlates with grain size.
• Softening in the HAZ due to thermal cycling and brittle compound precipitation leads to intergranular fracture, which can propagate into the ductile SZ fracture zone.