The integrated laser powder bed fusion (LPBF) forming of complex components of lightweight and high-strength aluminum alloys provides a new impetus for the development of lightweighting in the aerospace field. However, the control of the forming quality and performance of LPBF-fabricated high-strength aluminum alloys components and their applications still face bottleneck problems. On the one hand, the strengthening mechanism of Al-Si alloys is relatively simple, and the tensile strength of LPBF-fabricated Al-Si alloys is usually lower than 400 MPa, which hardly meets the high-performance requirements in the aerospace field. On the other hand, Al-Cu 2xxx alloys and Al-Zn 7xxx alloys, which are often used in conventional processing, can be heat-treated to achieve a large number of diffusely distributed nano-precipitated phases, providing an effective diffusion strengthening effect and realizing excellent mechanical properties in excess of 500 MPa. However, LPBF is a non-equilibrium melting process with cooling rates of up to 105?107 K/s owing to the transient interaction of the high-energy laser beam with the metal powder. Due to the wide solidification interval of 2xxx and 7xxx aluminum alloys, the solid-liquid coexistence time is long, and it is difficult to fill the liquid phase in time during the laser fast melting and solidification process. This easily results in solidification cracks along the grain boundaries, which greatly limits the application and development of LPBF forming with high-strength aluminum alloys. In this work, we achieved a densification of 99.5%, a tensile strength of 512.4 MPa, and an elongation of 13.3% for the specimens by optimizing the laser process parameters for LPBF forming of Al-Mg alloys modified with rare-earth elements, namely, Sc and traces of Zr. The research results can provide a reference for the integrated LPBF molding of aerospace lightweight components.
Atomized prepared Al-4.2Mg-0.4Sc-0.2Zr alloy powder was used for LPBF process. First, the samples were manufactured using scanning speeds of 400, 800, 1200, and 1600 mm/s, maintaining other parameters consistent. Then, the densification behavior, metallurgical defects, and microstructure of the samples were analyzed using optical microscopy and scanning electron microscopy, respectively. The grain structure and nano-precipitated phases inside the specimens were characterized using backscattered scanning electron (BSE) microscopy. Subsequently, the samples were aging treated at a temperature of 325 °C for 4 h, and tensile tests were performed. Meanwhile, FLUENT was used to carry out the simulation of heat transfer behavior in the molten pool of the LPBF-formed rare-earth-modified high-strength aluminum alloy. Finally, laser additive manufacturing forming tests of typical components in the aerospace field were conducted.
After process optimization, the specimens show excellent forming quality and mechanical properties, with an optimal densification of 99.5%, tensile strength of 512.4 MPa, and elongation of 13.3%. The surface smoothness and relative density of the samples increase and then decrease with increasing scanning speed, and the samples fabricated using a scanning speed of 800 mm/s exhibit satisfactory forming quality (Figs.3?4). BSE images of the microstructure of LPBF-fabricated high strength aluminum alloy samples show that the solidification organization presents equiaxial crystal features at the melt pool boundary, whereas coarsened columnar crystals are formed in the middle of the molten pool (Fig.6). White precipitated particles with sizes ranging from 200 nm to 1 μm can be observed, and these precipitated phases are Al3(Sc,Zr) particles. The optimal tensile strength and elongation of the specimen, 512.4 MPa and 13.3%, respectively, are obtained using a laser scanning speed of 800 mm/s (Fig.7). Metallurgical defects (e.g., porosity) due to improper laser energy input are the main cause of degraded mechanical properties. Finally, two types of aerospace typical parts fabricated using the optimized process parameters of laser power of 300 W and laser scanning speed of 800 mm/s exhibit low surface roughness, of no more than 7.3 μm, and high dimensional accuracy, of less than 0.1 mm/100 mm (Table 2).
In this work, the laser processing parameters for LPBF of rare-earth modified high-strength aluminum alloys Al-4.2Mg-0.4Sc-0.2Zr are optimized. Combining experiments and numerical simulations, the influence mechanism of laser scanning speed on the surface quality, internal metallurgical defects, heat and mass transfer behavior of the molten pool, and distribution of nano-precipitation in the formed specimens is revealed. When the optimized laser power is 300 W and the laser scanning speed is 800 mm/s, coupled with a 325 °C/4 h aging heat treatment, the formed specimens show an optimal densification of 99.5%, tensile strength of 512.4 MPa, and elongation of 13.3%. Two types of typical complex components in the aerospace field were manufactured using the optimized processing parameters, and the maximum external size of the formed specimens reached 570 mm, with surface roughness Ra≤7.3 μm, and dimensional accuracy of less than 0.1 mm/100 mm.
