Objective The rapid development of selective laser melting (SLM) technology provides an excellent solution for the rapid manufacturing of new complex aluminum alloy parts. Most studies on SLM of aluminum alloy remain in the stage of optimizing the processing parameters. Through continuous optimization of processing parameters, aluminum alloy samples with a density of over 99.00% and good tensile properties can be obtained. However, forming efficiency should also be considered in the actual forming process. To improve the forming efficiency, the most direct solution is to increase the layer thickness. The layer thickness determines the selection of other parameters, such as laser power, and the size of a single molten track and heat dissipation rate, which further determines the microstructure and properties of forming parts. Although properly increasing layer thickness is essential to improve the forming efficiency, if the layer thickness is excessively increased, the surface quality of formed parts will be severely reduced, and the metallurgical defects will also be increased, decreasing the mechanical properties. There are few reports on the effect of different layer thickness on microstructure, properties, and forming efficiency of SLM of aluminum alloy. In this study, the forming technology of AlSi10Mg alloy is investigated using optimized process parameters under different layer thickness. Besides, the influence of layer thickness on density, microstructure and properties, defects, and forming efficiency is analyzed, which provided a reference for further application of laser selective melted AlSi10Mg alloy in engineering.
Methods AlSi10Mg powder with good appearance quality is selected. First, the aluminum alloy substrate is preheated to 150 ℃, and the oxygen content in the forming chamber is kept below 0.1%. Concept laser X Line 1000R is selected as a SLM equipment. The high layer thickness of 60 μm is compared with the low layer thickness of 30 μm. Other processing parameters are designed based on the layer thickness, and a series of square blocks and bars are formed. After forming, the samples are annealed at 260 ℃ for 2 h. The densities of the samples are measured using the Archimedes method. Then, the microstructure and internal defects of the samples are observed through the metallographic microscope and scanning electron microscope. The size of the samples’ defects is counted using Image-Pro Plus. The formed bars are used to test the room-temperature tensile properties. Finally, the fracture morphology is observed and analyzed using a scanning electron microscope.
Results and Discussions The AlSi10Mg alloy with high density (Fig. 4 and Fig. 5) and good tensile properties (Fig. 6) can be formed under 30 μm lower layer thickness and 60 μm higher layer thickness. There are still differences as follows: the strength of AlSi10Mg alloy formed at 30 μm layer thickness is slightly higher than that of 60 μm layer thickness. This is attributed to the fine grain strengthening effect caused by smaller eutectic Si size in the 30 μm lower layer thickness samples (Fig. 9). Besides, the Z-direction elongation of the samples formed at 30 μm lower layer thickness is significantly higher than that at 60 μm higher layer thickness. This is because the molten pool at 30 μm lower layer thickness is smaller and densely arranged, leading to more zigzag crack propagation path and increased the difficulty of crack propagation; thus, resulting in a higher elongation (Fig. 7). The results showed that the defects with 30 μm lower layer thickness are more distributed in the molten pool boundary, while the defects with 60 μm higher layer thickness are more distributed in the molten pool. The Z-direction fracture surface with different layer thicknesses is perpendicular to each other (Fig. 10) since the eutectic Si in the boundary is relatively coarse, which becomes the weak area of crack propagation. When the AlSi10Mg alloy samples are with a density of over 99.00% and similar tensile properties, the forming efficiency with 60 μm higher layer thickness is about 2.7 times higher than that of 30 μm lower layer thickness.
Conclusions The layer thickness effect on relative density, microstructure, tensile properties, and forming efficiency of AlSi10Mg alloy fabricated by SLM investigated. The results showed that within the optimized laser energy density range, the relative density of the samples fabricated at 30 μm lower layer thickness and 60 μm higher layer thickness reached over 99.00% and possessed good tensile properties. The tensile strength of the 30 μm lower layer thickness sample is slightly higher than that of the 60 μm higher layer thickness sample, which is attributed to the fine grain strengthening effect caused by the finer eutectic Si in the 30 μm lower layer thickness sample. The Z-direction elongation of the 30 μm lower layer thickness sample is significantly larger than that of the 60 μm higher layer thickness sample since it is not easy for cracks to propagate along the smaller and more densely arranged molten pool boundaries in the 30 μm lower layer thickness samples. The defects in the 30 μm lower layer thickness sample are distributed along the molten pool boundaries, while the defects in the 60 μm higher layer thickness samples are distributed inside the molten pool. Besides, the forming efficiency of 60 μm higher layer thickness is about 2.7 times higher than that of 30 μm lower layer thickness with similar forming quality.
