Dual beam laser fusion-brazed Ti6Al4V/AA7075 dissimilar lap joint: Crack inhibition via inoculation with TiC nanoparticles

Severe hot cracks defects formed in the aluminum weld metal during laser fusion brazing (LFB) Ti6Al4V/AA7075 dissimilar lap joint. Herein, TiC nanoparticles were used to optimize the microstructure of aluminum weld metal. The wettability of molten aluminum alloy on the titanium was improved due to the increased laser absorptivity of aluminum with the addition of TiC nanoparticles. In contrast to the aluminum weld metal of the LFB joint with coarse cellular and dendrite grains, the LFB-TiC joint had a refined microstructure, consisting of equiaxed grains with segmented secondary phase, both of which intrinsically inhibit the intergranular hot cracking. Most of the TiC nanoparticles are dispersed in the precipitated phase or adsorbed on the boundary surface between the matrix phase and the Mg(Zn,Cu,Al)2 precipitated phase. The average thickness of TiAl3 IMC (intermetallic compound) increased to a maximum of 1.73 μm, ensuring interfacial bonding reliability. Benefiting from the inhibition of cracks and the synergy effect of grain refinement strengthening and load-bearing strengthening, the LFB-TiC joint exhibits elevated mechanical performance, the equivalent tensile strength and displacement increased from 289.8 MPa and 0.35 mm of the LFB joint to 487.7 MPa and 0.58 mm of the LFB-TiC joint. The enhancement in ductility is mainly attributed to the grain boundary modification caused by the dispersed TiC nanoparticles. The results show that Ti/Al lap joint with excellent mechanical properties can be obtained by laser fusion brazing method with proper microstructure control.