Impact of liquid nitrogen cooling on the microstructural and mechanical properties of CMT-welded joints of AA2099-T86 alloy

Aluminum-lithium (Al-Li) alloys, particularly the 2xxx series, are challenging to join due to precipitate dissolution-induced softening. However, their exceptional mechanical properties, including high toughness, stiffness and specific strength, make them an attractive choice for aerospace, automotive, and construction applications. This study investigates the effects of liquid nitrogen (LN) cooling on the microstructural and mechanical properties of cold metal transfer (CMT)-welded joints. The welding process employed ER4047 filler wire with a diameter of 1.2 mm. LN-assisted CMT welding (LN-CMT) was utilised to fabricate butt joints of identical thickness, and the resulting microstructural and mechanical behaviour were analysed. The process parameters varied, including current, welding speed and contact tip-to-work distance, with a constant gas flow rate of 16 L/min. The microstructural analysis utilised a field emission scanning electron microscope. LN facilitated grain refinement, reducing grain size due to effective cooling and restricted grain growth. The results revealed that applying LN during welding yielded a 4.49% increase in microhardness and an 8.33% enhancement in ultimate tensile strength compared to welds produced without LN. With 21.59% elongation, the LN-CMT welded junction was more ductile than the CMT (20.6%) and source metal (14.9%). This percentage is 4.8% higher than CMT and 44.89% higher than base metal elongation.