This study investigates the influence of the Nd–YAG laser power wave mode on the porosity and mechanical properties of SUS 304L and inconel 690 weldments. Initially, a rectangular laser power waveform is specified. The output is then progressively changed from a pulsed wave mode to a continuous wave mode by reducing the value of ΔP (ΔP = Pp−Pb, where Pp is the peak power and Pb is the base power) to zero. Bead-on-plate (BOP) and butt welding are performed at a constant mean output power (1.7 kW). The BOP results demonstrate that the depth/width (D/W) ratio of both materials increases with ΔP and attains a maximum value when full penetration just occurs. The D/W ratio and the travel speed for full penetration are higher for SUS 304L than for inconel 690. In butt-welds of inconel 690 and SUS 304L, the porosity ratio decreases from 7.1% to 0.5% and from 2.1% to 0.5%, respectively, as ΔP increases from 0 to 2780 W. Therefore, the tensile strength and percentage elongation are enhanced significantly in inconel 690. The degree of porosity reduction in inconel 690 exceeds that of SUS 304L. This suggests that the viscosity of the molten inconel 690 metal is higher than that of SUS 304L. Consequently, the effect of porosity reduction due to the increase in molten metal fluidity caused by increasing ΔP is greater for inconel 690 than for SUS 304L.

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This study investigates the microstructure and fracture behavior of dissimilar weldments of alloy 690 and SUS 304L for various additions of niobium (0.1, 1.03, 2.49, and 3.35 wt pct) in the flux. With identical parameters and procedures, weldments were butt welded by the shielding metal arc welding (SMAW) process using three layers, with each layer being deposited in a single pass. The results indicate that the microstructure of the fusion zone was primarily dendritic and that the contents of Ni, Cr, and Fe within this zone remain relatively constant and resemble alloy 690. With Nb addition, it is noted that the microstructure changes from a cellular to columnar dendrite and equiaxed dendrite. Meanwhile, the dendrite arm spacing reduces and the secondary arms grow longer. Moreover, the composition of the interdendritic phase, whose precipitate volume percentage increases from 5 to 25 pct, changes from Al-Ti-O to Nb rich. The spread of the interdendritic phase is less in the root bead than in the cap bead due to the greater influence of base metal dilution in this region. Mechanical tests indicate that Nb addition increases the average hardness of the weldment and reduces its elongation prior to rupture. However, the tensile strength is essentially unchanged by Nb addition. It is found that the average hardness of the root bead is generally lower than the cap bead, and that the tensile specimens all rupture in the fusion zone, with the fracture surfaces exhibiting ductile features. It is noted that the cap bead tends to rupture interdendritically with increasing Nb addition. Finally, fractography shows that the dimples in the root become larger and shallower with Nb addition and are rich with an interdendritic phase.