Laser-powder-filling welding is an important method for joining aluminum matrix composites owing to its high processing speed, minimal thermal impact on the matrix, and excellent controllability. However, in practical applications of laser-powder-filling welding, the intense Marangoni convection in the aluminum-alloy molten pool results in undesired weld morphologies such as humps and surface instability, thus deteriorating the mechanical properties and stability of the joints. Magnetic-assistance technology, owing to its advantages of high flexibility, high efficiency, low cost, and non-contact nature, has become a potentially effective method to control the flow of molten metals, improve welding controllability, and enhance product quality. Recent studies that improve welding quality using steady magnetic fields primarily focus on the effect of magnetic fields on arc welding or deep-penetration laser welding. Meanwhile, most studies examine the effects of transverse steady magnetic fields on aluminum-alloy welding, whereas few studies investigate the powder or wire-filling welding of aluminum matrix composites. Thus, the effect of reinforcements on the physical properties of the base metal is yet to be elucidated. Furthermore, owing to the difficulty in capturing the flow behavior of molten pools and the complex effect of external magnetic fields in the molten pool, the mechanisms of magnetic-field distributions and the Lorentz force in the molten-pool flow field under magnetic-field assistance remain ambiguous. Therefore, welding simulation studies should be conducted on magnetic-field-assisted aluminum matrix composites.

A SiC_p/2009Al composite matrix and AlSi12 filler powder are used in this study, with the magnetic field directed vertically perpendicular to the bottom surface of the matrix, and the magnetic-field intensity reaching 1.0 T. First, a three-dimensional transient numerical model is established using the COMSOL Multiphysics simulation software, which considers the variations in the material physical properties and couples fluid heat transfer with the magnetic field. Meanwhile, thermal buoyancy, surface tension, the Lorentz force, and other forces are applied to the molten pool. Subsequently, the Lorentz-force distribution in the steady magnetic field, as well as the fluid flow, heat transfer, and cooling behaviors in the central region of the molten pool are investigated. Finally, the profile morphology of the molten pool is verified through experimental observations.

The functional relationship between the longitudinal section height of the weld and the magnetic flux density was obtained via fitting, which provides clear understanding regarding the decay law of the magnetic field in space. Under the action of the longitudinal magnetic field, the molten pool is primarily affected by transverse electromagnetic forces, which can generate a shear effect on the interface (Fig. 4). As the steady magnetic flux density increases, the Marangoni convective motion in the molten pool weakens gradually, whereas the vortex rings on the front and rear sides of the molten-pool center diminish gradually until they disappear, thus decreasing the convective heat-transfer intensity in the molten pool, reducing the temperature gradients, and resulting in a more uniform distribution of the laser heat input in the longitudinal direction (Figs. 5, 6, and 7). Additionally, the cooling rate in the molten pool decreases significantly with the magnetic flux density, and the cooling and heat dissipation mechanisms of the molten material primarily involve thermal conduction and radiative heat loss (Fig. 8). At any position of the molten pool in the steady magnetic field, the induced Lorentz force and convective flow are in opposite directions, thus compensating for the Marangoni shear force and yielding an electromagnetic braking effect on the molten pool, which is the primary contributor to the weakening of convective heat transfer in the molten pool (Figs. 9 and 10). Meanwhile, the simulated molten-pool contour undulation and the mushy zone decrease. Additionally, a uniform microstructure distribution, good formability, and welding joints with reduced porosity are achieved from the welding processes (Fig. 11). The simulation results of temperature-field distribution and geometry agree well with the experimental results.

In the laser-powder-filling welding of SiC_p/2009Al composites under the assistance of a steady magnetic field, the magnetic field alters the energy distribution in the molten pool, which minimizes the thermal gradients and consequently reduces the heat-affected zone of the welding joint, thus resulting in a more uniform microstructure distribution. Meanwhile, the steady magnetic field reduces the cooling rate of the molten pool, favors an extended period for allowing bubbles to escape the solidification interface, and decreases the number of pores in the welded joint. Increasing the magnetic flux density will further weaken the Marangoni convection in the molten pool and reduce the heat-transfer intensity of convection in the molten pool. The convective vortices gradually diminish until they disappear, thus resulting in a more stable flow in the molten pool. Furthermore, the transverse Lorentz force generated by the longitudinal magnetic field exerts a significant braking effect on the lateral fluid flow in the molten pool, thus reducing the cross-sectional size of the molten pool. Solutes are primarily transported toward the bottom of the molten pool under the action of natural convection, thereby improving weld formation.