Influence Mechanism of the Heat Input on the Element Distribution and Mechanical Behavior of Beads Welded by Pulsed GMAW

Similarly well-formed welds can differ markedly in service performance, yet the metallurgical origin of this difference has rarely been quantified. Bead-on-plate welds were prepared on AA6061 with an ER4047 filler by pulsed gas metal arc welding (P-GMAW); with the pulse waveform kept constant, the base current was raised from 30 A to 110 A, which varied the heat input from 122 J/mm to 235 J/mm. The grain boundary (GB) morphology and the Al, Mg, and Si contents at the GBs and in the grain interior were quantified by SEM/EDS, together with tensile, bending, and impact tests. With increasing heat input, the GBs changed from convex to concave; Mg and Si segregated at the GBs to 8.5 and 31 times their contents in the grain interior; Al was depleted to 66.68% of its interior content; and solidification cracks multiplied. The tensile strength fell from 276.13 MPa to 252.92 MPa and the impact energy from 2.02 J to 1.57 J; the elongation rose from 4.21% to 4.99%, and the fracture mode changed from dimple–cleavage composite fracture to ductile dimple fracture and, at the highest heat input, to cleavage fracture. The deepening GB depressions reduced the load-bearing area, whereas the transition to ductile fracture governed the gain in elongation. The results therefore establish a quantitative link between the heat input, the GB microsegregation and morphology, and the mechanical behavior of well-formed P-GMAW welds.