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–Fe1.8Al7.2Si 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.