Residual stresses in longitudinal and transverse directions were analysed in the weld metal (WM), heat-affected zone (HAZ) and base metal (BM) in each sample after welding. The results are shown in Figure 3.
The longitudinal residual stresses at the joints of each welding condition had a homogeneous and tensile behaviour in the HAZ and BM, while the WM showed compressive stresses with the new mixture (Ar + O2 + N2O) and tensile ones with Ar + He and pure Ar. The high longitudinal tensile residual stresses in the HAZ may have contributed to the reduction of the mechanical strength of the welded joint, because all specimens broke in this region.
In the transverse direction, the behaviour of residual stresses in welded specimens with the Ar + He and the new gas mixture (Ar + O2 + N2O) was very similar with high compressive values in the BM, low tensile values in the HAZ and compressive in WM. As occurred in the longitudinal direction, the welded joint with the protection of pure Ar can be considered the most critical condition with tensile residual stresses around 50% of the yield strength initially reported by the manufacturer in Table 2.
Therefore, the new mixture showed a beneficial residual stresses state in relation to the other welded joints. However, the tensile residual stresses in the HAZ made this region of the joint critical and with a tendency to fail.
Figure 4 shows the presence of pores in WM, especially in welded joints with pure Ar and Ar + He. Porosity is always present in aluminium samples welded by GTAW process with pure Ar, as reported by Prakash et al. [12], and the addition of He to the blend can significantly reduce this defect due to the higher thermal conductivity of He, which results in greater energy transferred to the weld pool.
In Figure 4c, which corresponds to the welded joint using the new mixture, it can be observed more refined grains and that the formation of porosity was much lower compared to welded joint with previous shielding gases. Additionally, it was verified the presence of a crack, which may have arisen due to the alignment of the pores.
Pores form due to the sharp decrease in hydrogen solubility during the solidification process, since the solubility of hydrogen in the molten aluminium is about 20 times higher than in the solid aluminium. The formation of pores can be reduced by proper joint preparation, use of high purity shielding gas with low-dew-point and careful storage of the filler metal. However, the 5XXX series filler alloys, as used in this research, are particularly susceptible to surface oxide hydration, which promotes the porosity formation [13, 14].
In Figure 5, the joint welded with pure Ar had a more critical porosity profile, with deeper pores compared to joints welded using the other shielding gases, being this result consistent with Prakash et al. [12]. Additionally, Arana et al. [15] also observed that argon generated greater porosity area percentage compared to a three-phase mixture Ar +O2 + N2O mixture considering the same shielding gas flow rate and deposition strategy in the wire-arc additive manufacturing (WAAM) process with ER5356 filler metal. Figure 5c showed the presence of dispersoids, particles of the Al(Fe)MnSi phase, which inhibit grain growth in aluminium alloys by anchoring the movement of grain boundaries [16, 17].
The joint welded with Ar + He (Figure 6) presented an intermediate porosity condition, confirming what was noticed in the micrographs by optical microscopy. The pore alignments showed in Figure 6a can make the joint susceptible to crack formation.
Viskoc et al. [18], evaluating the effect of shielding gases on the properties of AW 5083 aluminium alloy laser weld joints with 5087 filler metal, found that, compared to welding process with pure argon, there was a 50% reduction in porosity formation using Ar + 5% He and about 30% using Ar + 30% He. The extra heat potential of He can reduce gas entrapment and thus porosity by widening the weld fusion and penetration in Ar + He blends.
The joint welded with the new mixture (Figure 7) showed lower porosity, also corroborating what was observed previously by optical microscopy. Although Miller et al. [10] did not perform a specific porosity analysis when using different shielding gases for aluminium alloy welding, the mixture with Ar + 200 ppm N2O + 200 ppm O2 showed an excellent bead appearance and generally better characteristics when compared with use of only one active gas at the same total concentration.
Figure 8 shows the average values of yield strength (σYS) and ultimate tensile strength (σUTS) obtained in the tensile tests carried out on the base metal and for each welding condition.
The welded joints presented, in general, yield strength and ultimate tensile strength equivalent. Vyskoc et al. [18] have shown that the shielding gas does not cause any change in the yield strength and ultimate tensile strength, which can also be seen in Figure 8. Therefore, it can be concluded from these results that shielding gas was not a significant factor on the tensile mechanical properties.
For all welding conditions, the ultimate tensile strength was about 12% lower compared to the base metal and the yield strength was reduced by 38% (for samples welded Ar and Ar + He) and 33% (for the sample welded with the new mixture). Srivatsava et al. [19] obtained a yield strength of 150 MPa and a ultimate tensile strength of about 290 MPa in samples of AA5083 welded by the non-pulsed GTAW process, being these values lower than those indicated for the base metal. This result is in agreement with Vasu et al. [20], who stated that the tensile properties of welded joints are significantly affected by the loss of alloying elements caused by evaporation.
The weld performed with the new mixture showed better yield strength when compared with other weld joints, especially when compared to the sample welded with Ar, where the difference was 10 MPa. This result was consistent with the microstructural analysis, where the new mixture presented less porous formation and, consequently, less stress concentration. Additionally, the tensile residual stresses in this joint had smaller magnitudes, compared to the other joints, in the longitudinal direction.
In both welding conditions, all specimens fractured in the HAZ, which is consistent with the microstructural analysis, since the grains are coarser in this region, reducing the resistance of the material. The influence of tensile residual stresses in this region may also have occurred.
Vickers microhardness results at weld metal (WM), heataffected zone left (HAZL) and right (HAZR) and base metal left (BML) and right (BMR) are presented in Figure 9.
The Vickers microhardness of the samples welded with Ar and Ar + He were similar. The base metal presented a higher value in relation to the heat-affected zone and weld metal in both conditions.
Vasu et al. [20] also observed higher hardness values in the base metal compared to the weld metal in AA5059-H136 aluminium alloy welded joints by GTAW and GMAW processes.
Vyskoc et al. [18] also found that the lowest Vickers microhardness values in the weld bead were obtained when welding with Ar and, thus, the use of He in the mixture caused an increase in this mechanical property. They reported that the Vickers microhardness in the centre of the weld bead was 59 HV for Ar and 60 HV for Ar + 30% He, and these values are close to those observed in Figure 9.
The new mixture (Ar + N2O + O2) provided a joint with average microhardness values in HAZ and WM close to BM and with greater magnitude in relation to the other welded joints. These results of the new mixture are consistent with the microstructure analysis, which showed refined grains in the WM.