Diffusion-Limited Nonequilibrium Phase Transformation of Nickel/Aluminum Dissimilar Materials During Laser Welding Part II: Magnesium-Strengthened Polycrystalline Aluminum Alloy

In order to mitigate porosity, eutectic β phase and surface defects to increase strength and ductility, weld geometry, metallurgical defects and mechanical properties of deep and narrow keyhole weld are correlated for weld quality control during laser welding high-strength 5083H116 polycrystalline aluminum-magnesium alloy. The effect of microstructure heterogeneity and nonequilibrium phase transformation on mechanical properties in the isotropic material is discussed in more detail for comprehensive understanding of physical and chemical welding phenomena. Excessive porosity and tenacious nonequilibrium eutectic reaction are predominantly detrimental to tensile properties and limit tensile stress and strain, which also bring about significant fluctuation of mechanical properties. In the case of using high-energy laser welding, the balance between porosity diminishment, Mg-rich eutectic β phase Al3Mg2 reduction, alloying magnesium vaporization mitigation and weld surface formation amelioration of both-sided keyhole weld is required for large proportion of α-Al solid solution to substantially improve weld quality. Low thermal input of fast cooling rates is beneficial to decrease of eutectic β phase in the supersaturation solid solution, diminution of alloying magnesium vaporization, smooth weld surface formation and grain refinement, etc. However, high thermal input of slow cooling rates expediently lessens porosity within the fusion zone. When impinging on the surface with laser incident angle to reduce optics damage and improve absorptivity, laser energy is split into two inequivalent parts on the material to modify heat distribution, thereby developing asymmetrical weld pool shape, producing irregular surface, and thus deteriorating keyhole instability and hydrodynamic behavior during laser welding. Diffusion-driven eutectic β phase is inevitable as a consequence of nonequilibrium solidification. Because of kinetics and thermodynamics for nucleation and growth, brittle eutectic β phase is inhomogeneously distributed at top, middle and bottom parts inside an overheating weld pool during solidification. Problematically, there is a significant difference in eutectic β phase morphology, size and distribution among these locations, e.g., left side, center and right side of weld regions, indicating that these locations easily worsen crack nucleation, growth and propagation during tensile loading. This is of particular importance when asymmetrical weld is metallurgically heterogeneous. Although the base material is strengthened by alloying magnesium in α-Al solid solution, the weld is separately softened and strengthened, such as vaporization-related softening and grain refinement strengthening. Fracture surface unequivocally elucidates dimple ductile fracture and brittle intergranular fracture. Notwithstanding aluminum-magnesium alloy is susceptible to porosity and eutectic β phase to increase metallurgical discontinuity, it is possible to simultaneously suppress porosity and eutectic β phase and minimize weld surface defects through straightforward multiple welding conditions optimization to acquire desirable metallurgical and mechanical properties. To reduce vulnerability to mechanical performance deterioration and acquire well-developed keyhole weld, in-depth optimization of welding conditions for disadvantageous factors limitation is unavoidable to incrementally balance between adequate mechanical properties and heat input, and improve fracture resistance to meet requirement of high-energy laser welding industry applications.