Abstract
The influences of ultrasonic vibration during cold metal transfer welding process on the microstructure, element distribution and pitting resistance of AISI 430 ferritic stainless steel joints with ER 308L as filler metal were investigated. The combined effects of mechanical vibration, acoustic streaming and cavitation of ultrasonic vibration significantly refined the primary ferrite grain in the weld metal and then impacted the subsequent solid-phase transition process, leading to the ~45% reduction of ferrite content in the weld metal. Moreover, these effects also resulted in the homogenization of alloying elements in the weld metal. The pitting corrosion resistance of the welded joints with ultrasonic vibration was increased compared with that of without ultrasonic vibration, but lower than the base metal. The pitting resistance of the weld metal with ultrasonic vibration was higher than that of the weld metal without ultrasonic vibration and base metal, while that of the one without ultrasonic vibration was lower than the base metal.
Highlights
Ferritic stainless steel (FSS) has the advantages of less linear expansion, better resistance to chlorine-stress corrosion cracking and atmospheric corrosion in comparison to austenitic stainless steel (ASS) [2], and it has been widely applied in the automotive exhaust system, home appliances and building industries because of its good corrosion resistance and low cost [3,4]
As noticed by Lippold and Kotecki [25], the ferrite and austenite dual-phase structure is the typical weld microstructure formed by FA solidification mode, and the transformation path of FA solidification mode can be obtained from the pseudo-binary phase diagram
When the temperature of the weld metal (WM) dropped to the ferrite–austenite dual-phase zone, the stability of ferrite deteriorated gradually to some extent and transformed into austenite through the diffusion-controlled reaction, and the secondary austenite grains nucleated and grew at the grain boundaries of the primary ferrite
Summary
There are some problems occurring in the WM, such as chrome-rich carbon nitride precipitates and uneven distribution of elements These disadvantages will exert a detrimental effect on corrosion resistance and impact toughness of the welded joints [9]. The influence of ultrasonic vibration during the CMT welding process of FSS with ASS wire as filler metal on the WM solidification, solid phase transition and element distribution, and, on corrosion resistance of the welded joints is rarely reported. In order to improve the corrosion resistance of FSS welded joints, a welding technology based on CMT process with synchronous ultrasonic vibration was proposed and applied. AISI 430 FSS joints were fabricated by ultrasonic-vibration-assisted CMT welding with ER 308L ASS as filler metal.
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