Control of gravity effects on weld porosity distribution during variable polarity plasma arc welding of aluminum alloys

Stability of Weld Pool and Elimination of Weld Defects in Aluminum Alloy Plasma Arc Keyhole Welding at Continuously Varying Positions

“…In addition, the larger thermal gradient on the solidification side makes the metal easy to solidify and form a weld, which provides an ideal heat source for variable position welding [ 25 , 26 ]. Porosity is easily formed in the solidification process of non-vertical up welding, this kind of pores are mainly hydrogen pores, the key reason for the formation of hydrogen pores is the difference in the solubility of hydrogen in the solid and liquid metal [ 13 ]. Previous studies have proposed the idea of an asymmetric heat source for solving asymmetric flow and reduced porosity, which provides a theoretical basis for improving variable position welding without changing the welding parameters [ 2 , 13 , 17 ].…”

“…Porosity is easily formed in the solidification process of non-vertical up welding, this kind of pores are mainly hydrogen pores, the key reason for the formation of hydrogen pores is the difference in the solubility of hydrogen in the solid and liquid metal [ 13 ]. Previous studies have proposed the idea of an asymmetric heat source for solving asymmetric flow and reduced porosity, which provides a theoretical basis for improving variable position welding without changing the welding parameters [ 2 , 13 , 17 ]. The weld formation during variable position welding is the premise of practical applications, and elimination of the welding defects caused by asymmetric flow is the goal.…”

“…VPPA vertical-up welding has several advantages: (1) the plasma jet creates a keyhole through the entire work-piece that increases the gas-liquid interface of the weld pool, thereby greatly increasing the possibility of gas escaping; (2) the keyhole weld pool avoids the high temperature zone of the heat source when the plasma jet pushes away the metal, and the gas-liquid interface of the weld pool remains within a stable temperature zone, which is favorable for ensuring that the liquid at the rear side of the weld pool integrates steadily and forms a weld seam [ 7 , 8 , 9 , 10 ]. In nonvertical-up welding, the lack of gravity-assisted molten pool flow can cause asymmetric metal flows [ 11 , 12 , 13 , 14 ], resulting in asymmetric welds and a decreased weld quality.…”

“…Studies have shown that by reasonably adjusting and matching the thermal output of the VPPA, the requirements of welding assemblies in multiple fixed positions (e.g., horizontal welding, vertical-up welding, and flat welding) can be realized, which greatly broadens the application scope of this type of welding [ 11 , 12 , 13 , 15 , 16 ]. However, when the position is continuously changed during welding, gravity dynamically influences the keyhole weld pool and makes it difficult to maintain the quasi-stable state of fixed position welding, which becomes unstable during welding, and the metal melt behind the weld pool fluctuates greatly, which can cause intermittent closure or even a lack of closure, thus resulting in uneven weld morphologies and continuous cutting [ 17 ].…”

“…Previous studies have investigated the stability of the keyhole weld pool, the flow behavior of liquid metal and the stagnation point, the mechanisms of the formation and elimination of porosity defects, and the optimization of welded joint asymmetry and have revealed the mechanism of defect formation in variable-position welding and weld-seam optimization [ 2 , 13 , 17 ]. However, welding in which the position is continuously varied has not been studied.…”