Microstructure and mechanical properties of friction stir welded AISI321 stainless steel

Abstract

In this investigation, Friction stir welding (FSW) was performed to join 3 mm-thick AISI 321 stainless steel sheets. The rectangular sheets of size 100 mm × 50 mm were butt joined to contemplate the impact of tool rotational speeds for the steady welding traverse speed of 55 mm/min, axial force of 13 kN and tool tilt angle of 1.5°. The present assessment aims to correlate the relationship between the weld joint properties and important FSW parameter, the tool rotational speed. Microstructural investigation on the FS welded joints was performed using optical microscope and scanning electron microscopy (SEM). Transverse tensile test was undertaken to ascertain the quality and strength of the welded joints such as yield strength, ultimate tensile strength, etc. The microstructural features reveal evolution of fine equiaxed austenite grain structures in the weld stir zone dominated by discontinuous dynamic recrystallization mechanism. The weld joints fabricated at 500 rpm and 700 rpm had significantly better strength about 88% and 93% compared to the base metal. Subsequently, the hardness of the weld stir zones was significantly greater than the hardness of the base metal. All the welds fractured at a distance to that of the weld stir zone and dimple structures were observed at the fracture locations.