Variations in Fusion Zone Geometry, Cooling Rates, and Solidification Parameters During Adjustable Mode Beam Laser Powder Bed Fusion

Abstract In laser powder bed fusion (LPBF) additive manufacturing, a focused laser beam selectively melts thin layers of metallic powders in a layer-by-layer manner to print parts in 3D. Traditional LPBF uses a laser beam that generally follows Gaussian power distribution. In contrast, in recent years, adjustable mode beam (AMB) lasers have gained popularity in LPBF. AMB lasers consist of two laser beams: one core beam that generally follows Gaussian power distribution and a ring beam concentric to the core beam that follows a donut-shaped power distribution. The total power can be distributed between the two laser beams. Temperature fields, fusion zone geometry, cooling rates, solidification parameters, and grain structure can be controlled by adjusting the power of the two laser beams. Since adjustable mode beam laser powder bed fusion (AMB-LPBF) is an emerging field, there is a lack of understanding of the effects of core and ring beam power, beam size, and power density distribution on fusion zone geometry, cooling rates, and solidification parameters. In this work, we developed, tested, and used numerical models of AMB-LPBF to provide a better understanding of the variations in fusion zone geometry, cooling rates, and solidification parameters. A finite element method-based 3D transient numerical heat transfer model was developed for AMB-LPBF. A rectangular solution domain consisting of the powder layers and deposits was considered. Temperature-dependent thermophysical properties of materials were considered. For the powder bed, effective properties were taken into account depending on the powder packing density and shielding gas entrapped among the powders. The energy inputs from the core and ring beams were applied on the top surface of the powder layers as boundary conditions. Convective and radiative heat losses were implemented on the surfaces. The outputs from the model included 3D transient temperature fields, fusion zone geometry, temperature gradients, cooling rates, and solidification parameters. The model was rigorously tested against independent experiments for AMB-LPBF of an aluminum alloy, Scalmalloy. We found that the effects of the ring beam were the most pronounced when its power was between 80 to 100 percent of the total power. We constructed several power density distribution graphs using the model that will serve as a basis for process parameter selection. LPBF with AMB laser exhibited a wider fusion zone than LPBF using a single Gaussian beam of the same power. In addition, AMB-LPBF exhibited a low temperature gradient to the solidification growth rate ratio, resulting in the formation of an equiaxed grain structure, which was consistent with the experimental observations. Using the well-tested model, we studied the variations in the pool dimensions, peak temperature, cooling rates, and solidification parameters with the ring beam power percentage. The pool width was found to increase significantly as the ring beam power percentage rose. In addition, as the power percentage increases, the peak temperature decreases because more heat is distributed over a larger area. Both the cooling rate during solidification and the ratio of temperature gradient to solidification growth rate were reduced at high power percentages. The results from the research are expected to reduce costly and time-consuming trial-and-error experiments to select an appropriate set of processing conditions to manufacture defect-free and structurally sound parts using AMB-LPBF.