Influence of shielding gas on microstructure and mechanical properties of laser welded–brazed Al/steel lapped joint

Laser welded–brazed Al/steel lapped joint was successfully obtained under various shielding gas contents (pure Ar, CO₂+Ar and pure CO₂) with laser power of 2000W. Influence of shielding gas contents on weld formation, interfacial microstructure and tensile–shear strength was investigated. The addition of CO₂ in the shielding gas would not only enhance the total fusion volume but also improve the wettability of molten filler. Interfacial microstructure observations showed that addition of CO₂ in the shielding gas would thicken the interfacial IMC. Continuous layered η–Fe₂(Al,Si)₅ and serration shaped θ–Fe(Al,Si)₃+τ₅–Fe_1.8Al_7.2Si was newly formed. In addition, higher peak temperatures were produced with the increase of CO₂ volumes, which was proven by numerical simulation results. Higher laser energy absorption for CO₂, alternation of melt flow pattern and smaller thermal conductivity of CO₂ were responsible for these changes. Highest average tensile–shear strength of 163MPa was obtained under the shielding gas content of 50 %Ar+50 %CO₂ when laser power was 2000W.