Fluid Dynamics Simulation Of Laser-welded Cu/Al Molten Pool And Researches On Joint Microstructure Characteristics

Copper and aluminum have been applied widely in the fields of electronic,electrical and electric. It not only saves cost, reduces weight, but also optimizesproduct design for the copper replaced by aluminum, which could be achieved by thejoining of Cu and Al. However, there are many differences in the thermal physicalproperties between them, and the risk of forming brittle intermetallic compoundsduring welding process. Laser welding with the features of the rapid solidification andnarrow heat affect zone could reduce the formation of intermetallic compounds.However, the welding pool characterizations during laser welding of Cu and Al,including temperature distribution, velocity field, composition distribution, weldingpool morphology and the formation mechanism of Cu/Al intermediate layer, have notbeen investigated in details. Moreover, the effect of laser welding parameters onmicrostructure, and the effect of microstructure on the shear strength need to befurther explored to control the microstructure in fusion zone by appropriate processingparameters.Computational Fluid Dynamics (CFD) method was applied to investigate thewelding pool characterization including temperature distribution, velocity field,composition distribution and welding pool morphology. Melting and solidificationwas simulated not by tracking the solidus-liquidus interface but using theEnthalpy-Porosity model to save calculation time. Welding pool of different Cu/Al lapjoint was simulated to compare the morphology, temperature, velocity, andcomposition field between them and validate the simulation with experimental results.SEM&EDS analysis on microstructure of fusion zone and tensile shear testingof the joint was conducted to investigate the effects of welding parameters on themicrostructure, the effect of microstructure on the shear strength, and the fracturemechanism of the joint.Results show that a fusion zone with bawl shape and a Cu/Al intermediate layerwith hill shape were formed during laser lap welding with Cu on top under theinteraction of gravity, buoyancy and surface tension. When aluminum is on top, thefusion zone on top of Al plate is wide but it is much less on top of Cu, especially thevelocity nearby the Cu/Al interface is almost zero. Thus it is very difficult to form theCu/Al joint when aluminum is on top.The averaged cooling rate in the solidification temperature range decreases with increasing laser power and decrease with increasing welding speed. The thickness ofCu/Al intermediate layer and velocity of molten metals increases with increasing laserpower and decreases with increasing welding speed.The laser-welded lap joint is comprised of aluminum fusion zone, copper fusionzone and the intermediate layer between aluminum and copper. Fusions of aluminumand copper presents homogeneous morphology while the intermediate layer exhibitsdiverse microstructure and heterogeneous composition distribution.Intermediate layer with diverse morphology and composition was formed due tothe high temperature and composition gradient during laser welding of Cu and Al withcopper on top. Banded structure was observed in the hypereutectic region; banded orcellular structure was found in the mixed zone of hypereutectic and eutectic region;lamellar structure was observed in the eutectic region; dendritic structure wasobserved in the hypoeutectic region. Keep the velocity100mm/s as a constant, theprimary dendrite arm spacing increased with increasing laser power from1450to1850W, but the shear strength of the joint decreased when the joint faulted in thedendrite zone; keep the laser power1650W as a constant, the morphology in themixed zone between hypereutectic and eutectic transform from cellular structure tobanded structure, and shear strength increased when the joint faulted in the mixedzone.