DOU Zheng, CAO Li, HAO Yu, ZHANG Li, SU Hui, FENG Yinghao, LI Xiaofeng
【Objective】Powder bed fusion-laser beam (PBF-LB) can efficiently fabricate alloy parts with high relative density and complex structure, presenting good application potentials in the aerospace, biomedicine and transportation fields. Al-series alloy is one of the commonly-applied materials in PBF-LB, such as Al-Cu alloy with good mechanical strength and heat resistance. However, during nonequilibrium solidification of PBF-LB, the insufficient element diffusion and abundant defects easily result in the brittle intermetallic compounds (IMCs) precipitated at grain boundaries continuously. It would weaken the interface bonding strength, thus decreasing the plastic deformability and inducing the brittle fracture. Nowadays, heat treatment is a key approach ameliorating the PBF-LB-formed microstructure segregation, by utilizing IMCs dissolution-precipitation behaviors and grain recrystallization-growth mechanisms. Commonly, the strength and ductility properties show the opposite evolution laws along the varied microstructure. Therefore, studying the appropriate treatment regime for the specific-series alloy and its influence on the microstructure is essential for achieving mechanical properties trade-off. This work attempted to improve the strength-ductility properties of a novel PBF-LB-fabricated Al-Cu-Ni-Mg-Si alloy synchronously, by validating the influence of heat treatment on the microstructure. Herein, only the solid solution approach was utilized to coordinate the relationship between the strengthening effect and plastic deformability.
【Method】The gas-atomized Al-Cu-Ni-Mg-Si alloying powders were served as raw materials, and the PBF-LB equipment (EP-M150) was employed to fabricate alloy specimen layer by layer (Fig.1). The key parameters included the laser power of 240 W, scanning speed of 600 mm/s, hatch space of 180 μm, layer thickness of 30 μm and rotation angle of 67°. After then, the as-fabricated specimens were treated in an electric furnace (SX-B01123), and the related solid-solution temperature was designed based on the Differential Scanning Calorimetry (DSC) thermal analysis. The defect morphology was examined on the Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) instruments. And, the microstructure and element distribution were characterized by SEM and Transmission Electron Microscopy (TEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS). Besides, the grain information was obtained by the Electron Backscatter Diffraction (EBSD) detection. To evaluate the mechanical properties, the tensile tests and Vickers hardness tests were conducted. Through the synthetic analysis of microstructure and mechanical properties, their relationships were validated to select the appropriate treatment regime and optimize the strength-ductility properties.
【Result】DSC curves of the PBF-LB-fabricated Al-Cu-Ni-Mg-Si alloy present three endothermic peaks (533 ℃, 587 ℃ and 639 ℃), representing the melting behaviors of different IMCs (Fig.2). On the basis, the heat treatment temperature was designed. When the temperature is over 510 ℃, the serious pore defects are generated due to the overburning effect of IMCs (Fig.3). Under 480-495 ℃, the microstructure homogeneity and relative density are obviously improved, primarily contributed by the promoted diffusion of atoms and vacancies under high temperature. The grain-boundary IMCs including Al2Cu, Al7Cu4Ni and Mg2Si, undergo the nodulizing, dissolving and coarsening process during treatment (Figs.4,5). The partially-dissolved submicron particles were still present at 480 ℃, whereas most of them were dissolved and only a few particles with higher melting point were coarsened. Newly-formed nano-sized particles were also observed, mainly induced by the precipitation reaction of the supersaturated solid solution during nonideal heating or cooling process. Besides, the columnar grains in the as-printed alloy become much coarser which grow up to over 40 μm due to the recrystallization effects (Figs.6). Under the microstructure features above, the mechanical properties and failure modes were regulated (Figs.7,8). The as-printed alloy shows the ultimate tensile strength (UTS), yield strength (YS) and elongation rate (EL) of 438.5 MPa, 340.4 MPa and 10.4%, respectively (Table 3). Due to the continuous aggregation of IMCs phase at grain boundary, the brittle fracture morphology is presented under as-printed state. After solid solution, the failure mode transforms into the ductile fracture completely. Particularly, the alloy treated at 480 ℃-1 h can maintain the YS at 340.3 MPa, meanwhile shows the improved UTS of 485.3 MPa and EL of 15.0%. The ductility is significantly improved by the reason that the precipitate density decreased and the matrix grains coarsened. Whereas, the partially-dissolved IMCs can enhance the solid-solution strengthening effect, and the nano-sized phase can pin up dislocations to promote working hardening rates, thus avoiding the degradation of mechanical strength.
【Conclusion】The strength-ductility properties of the PBF-LB-fabricated Al-Cu-Ni-Mg-Si alloy were synergically optimized through solid solution treatment. The ultimate tensile strength and elongation rate were improved from 438.5 MPa and 10.4% to 485.3 MPa and 15.0% under the treatment regime of 480 ℃-1 h, respectively. Meanwhile, the yield strength shows no obvious degradation (340.3 MPa). The optimization was primarily determined by the microstructure features of IMCs precipitates and matrix grains. As validated by the DSC tests and physical characterizations, it can be known that high-temperature (≥510 ℃) treatment would bring out the overburning phenomenon, thus causing the high-proportion pore defects. Decreasing the temperature to 480-495 ℃ can improve relative density of the as-printed alloy, meanwhile improve the distribution homogeneity of precipitate phase and restrain its coarsening degree. Under the synergic effects of the microstructure features including the partially-dissolved precipitates, nano-sized precipitates and recrystallized columnar grain, the balance between the strengthening mechanism (solid solution, precipitate and grain boundary strengthening) and plastic deformability was achieved.
