Laser powder bed fusion (LPBF) is an advanced manufacturing technology that involves the layer-by-layer deposition of metal powder, followed by rapid laser fusion. Unlike traditional methods for metal part fabrication, LPBF which does not require molds or tools can achieve near-net shaping of complex metal part, offering significant advantages. However, commercial LPBF systems generally use low-power (≤500 W), single-mode fiber lasers with Gaussian energy distribution. To fully melt the metal powder layer, key process parameters such as laser scanning speed, spacing, and layer thickness are constrained, resulting in low forming efficiency (1.8?16.2 cm³/h). This limitation hinders the use of LPBF in large-scale manufacturing. To address this, researchers have developed high-power LPBF (HP-LPBF) using lasers with ≥1 kW power. While studies have explored the formation of GH4169 alloys with HP-LPBF, most have used laser powers under 2 kW. Therefore, further investigation is needed to understand the densification behavior, microstructure, and mechanical properties of GH4169 alloys formed with laser powers exceeding 2 kW.

The HP-SLM320 laser additive manufacturing equipment, used as the HP-LPBF test platform, was developed in-house by our research team. The device was equipped with a multimode fiber laser that had a maximum output power of 4 kW and a near flat-top mode energy distribution. Metallurgical defects in the GH4169 alloys were observed using an optical microscope (OM). Phase composition was analyzed with a X-ray diffractometer (XRD). A scanning electron microscope (SEM) equipped with an electron backscattering diffractometer (EBSD) and an electron probe microanalyzer (EPMA) was used to characterize the microstructure, grain orientation, and composition distribution. Vickers hardness of the vertical section of the specimen was measured using a microhardness tester under a 500 g load and 15 s indentation time. Room temperature tensile tests were conducted on a high-precision electronic universal testing machine at a constant drawing rate of 2 mm/min.

In this study, a high-power, near-flat-top laser beam is used for the HP-LPBF of a GH4169 nickel-based superalloy. The metallurgical defects, microstructure, and mechanical properties of the as-formed HP-LPBF samples are studied. When the laser volumetric energy density is less than 65 J/mm³, unfused defects are observed in the specimens, as shown in Figs. 5(a1)?(c1). However, when the laser volumetric energy density exceeds 65 J/mm³, these defects disappear, and the specimen density exceeds 99.80%. Within the experimental range of this study, the samples prepared with different laser volumetric energy densities exhibit few pores, as shown in Figs. 5 (d1)?(f1). The microstructures of the HP-LPBF samples consist of columnar dendrites and cellular dendrites. Laves phases, which are harmful, are distributed along the dendrite boundaries, similar to what is observed with conventional LPBF technology. However, the primary dendrite arm spacing (PDAS, 1.8 μm) in HP-LPBF specimens is larger than that seen with conventional LPBF (0.5?1.0 μm). Based on the relationship between PDAS and the cooling rate of the molten pool, the average cooling rate in HP-LPBF (9.8×10⁴ K/s) is only 2.0%?16.9% of that (5.8×10⁵?4.8×10⁶ K/s) in conventional LPBF . The average grain size (331 μm) in the HP-LPBF specimen is an order of magnitude larger than that (8.6?32.0 μm) produced by conventional LPBF, as shown in Fig. 9. This is due to the slower cooling rate in HP-LPBF. Additionally, the HP-LPBF specimen exhibits stronger preferred orientation characteristics compared to those produced by conventional LPBF, as shown in Fig. 10, which is attributed to the more uniform temperature gradient achieved with the near-flat-top laser beam.

With a density exceeding 99.80%, the forming efficiency (118.8?166.3 cm³/h) of HP-LPBF specimens is more than 7 times that (1.8?16.2 cm³/h) of conventional LPBF technology. The elongation (34%) of a typical HP-LPBF specimen is comparable to the higher range (10%?40%) achieved with conventional LPBF. However, the ultimate tensile strength (895 MPa) falls within the middle to lower range of conventional LPBF results (845?1287 MPa), as shown in Fig. 13. The relatively coarse dendritic structure and relatively slow molten pool cooling rate are the main factors contributing to the elongation being on par with the higher levels observed in conventional LPBF. Meanwhile, the relatively larger solidified grains, coarse dendritic structure, and lower average geometrically necessary dislocation (GND) density are the primary reasons for the ultimate tensile strength aligning with the lower range of conventional LPBF results.

Samples with densities exceeding 99.80% can be obtained by adjusting the laser volumetric energy density to ≥65 J/mm³. The as-formed HP-LPBF sample exhibits a dendritic microstructure with a PDAS of 1.8 μm, which is larger than that of the GH4169 superalloy formed using conventional LPBF with a low-power Gaussian laser beam. The Laves phase volume fraction (9.5%) in the HP-LPBF sample is comparable to that obtained with conventional LPBF. The sample also shows a cubic crystallographic texture with a strong “<001> is parallel to the building direction” orientation, stronger than that observed in conventional LPBF. The average equivalent grain size (331 μm) is an order of magnitude larger than in conventional LPBF. The ultimate tensile strength (895 MPa) of the HP-LPBF sample is within the middle to lower range of conventional LPBF results, while its elongation (34%) is on par with the higher range. Additionally, the forming efficiency (118.8?166.3 cm³/h) is more than 7 times that of conventional LPBF, given a density above 99.80%.