Investigation of bypass-coupled double-pulsed directed energy deposition of Al–Mg alloys

Abstract

Wire and arc additive manufacturing (WAAM) is a promising directed energy deposition (DED) technique for Al–Mg alloy parts. However, excessive workpiece heat input may limit its high productivity application. Hence, a bypass-coupled double-pulsed WAAM (BCDP-WAAM) method is developed to control the workpiece heat input without deteriorating productivity. High-speed camera images demonstrated that the arc and droplet trajectory were deflected towards the molten pool side in BCDP-WAAM. The observed strong-to-weak change in the arc necessitated a periodic insertion of the bypass-cathode wire into the molten pool for heat absorption. Spray and bridging transfers occurred in the anode and bypass-cathode wires, respectively. Current–voltage waveforms indicated that a dynamic current distribution between the workpiece and bypass-cathode wire was achieved in BCDP-WAAM, decreasing the workpiece current. Consequently, the workpiece heat input was decreased by 38.2 %. Owing to the twin-melted wires, the height and effective area were increased by 134.1 % and 149.0 % in BCDP-WAAM, respectively. The obtained electron back-scattered diffraction (EBSD) results indicated that the average grain size was reduced to 21 µm in BCDP-WAAM compared to a grain size of 36 µm in double-pulsed WAAM. This is attributed to the reduction in the workpiece heat input and the change in solidification induced by the bypass-cathode wire. The BCDP-WAAM enhanced the mechanical strength of the part. Furthermore, the ultimate tensile strength (UTS) and yield strength (YS) of the optimized part increased to 256 and 124 MPa in BCDP-WAAM, representing increments of 7.1 % and 14.8 %, respectively. The average micro-hardness was enhanced to 88.2 HV, representing an increase of 16.1 %. • The arc and droplet trajectory were deflected towards the molten pool side. • Bypass-cathode wire periodically inserted into the molten pool for absorbing heat. • A dynamic current distribution between workpiece and bypass-cathode wire occurred. • Reduced workpiece heat input and improved productivity simultaneously occurred.