Laser powder bed fusion (LPBF) technology, a laser additive manufacturing (AM) technology based on powder bed melting, slices a three-dimensional model layer-by-layer through a computer. The laser scans the powder surface according to the two-dimensional slice profile and then the material stacks layer by layer to fabricate three-dimensional metal parts. Parts fabricated by LPBF technology have high dimensional accuracy and surface quality and can obtain nearly 100% relative density. In this study, we fabricate Sn-3.0Ag-0.5Cu (SAC305) material using LPBF technology without a protective atmosphere, which reduces the volume of equipment, improves the portability of equipment, and verifies the improvement of the mechanical properties of the LPBF fabricated alloy, providing the possibility for parts processing and printing under special conditions.
Different process parameters for LPBF printing of SAC305 cube samples and tensile specimens are designed. Following this design phase, an analytical balance is employed to measure the density of the sample. Subsequently, a conversion to calculate the relative density of the sample is performed. To support this calculation, the conductivity of the samples is measured using an eddy current conductivity tester. A three-dimensional microscopic system is used to observe the surface topography and to measure the roughness. After polishing the sample, a metallographic microscope is used to observe pores, cracks, and other defects. X-ray diffraction (XRD) is used to analyze the phase compositions of SAC305 powder and fabricated samples. An electronic testing machine is used to test the tensile properties of tensile specimens, and the fracture morphology is observed.
SAC305 samples are successfully printed by LPBF in an air environment without a protective atmosphere. The top surface of the fabricated sample is yellow-brown, and the surface is free of cracks, obvious holes, warpage, collapse, and other defects; furthermore, the formability is good. When the laser energy density is high, owing to the excessive local instantaneous energy input, the resulting spatter particles fall onto the surface, causing internal defects or spheroidized particles. When the laser energy density decreases, the spheroidization phenomenon decreases, and the amount of unmelted powder increases, as shown in Fig.3. When the laser energy density reaches approximately 20 J/mm3, the density of the sample is high, and only a few tiny holes are present inside, as shown in Figs.4 and 5. The tensile strength reaches 85.29 MPa when the scanning speed is 700 mm/s, and the laser power is 30 W, as shown in Fig.6. The powder, along with the formed structures, primarily comprises of the β-Sn phase and Ag3Sn phase. The oxygen inside the fabricated sample is randomly distributed in the form of tin oxide, and no obvious aggregation occurs, as illustrated in Fig.9. According to the scanning electron microscope (SEM) diagram of the longitudinal profile of the fabricated sample, there is no obvious oxide film layer inside the sample, and oxygen is evenly distributed. The energy dispersive spectrometer (EDS) line scanning is performed over a range of multiple layers along the built direction, as shown in Fig.10. The oxides, rather than being enriched at the molten pool boundary, are distributed in the molten pool stage owing to the influence of heat input from subsequent layers. During the fabrication of the SAC305 alloy using LPBF without a protective atmosphere, less spatter is generated because of the low laser power, and most of the spatter is powder spatter, as shown in Fig.11.
Under an unprotected atmosphere, a violent oxidation reaction gradually occurs when the laser energy density exceeds 20 J/mm3, resulting in an increase in the surface roughness of the sample and even fabrication failure. When the laser energy input is lower than 15 J/mm3, the internal manifestation of the fabricated sample is loose and the powder in the scanning area does not fully melt, resulting in low relative density and poor mechanical properties of the sample. The best process parameters are a laser power of 30 W and a laser scanning speed of 700 mm/s.
The relative density of the LPBF fabricated SAC305 sample reaches 98%, while its tensile strength and elongation are 85.29 MPa and 15.37%, respectively. The mechanical properties are better than those of the cast-fabricated samples, in which the tensile strength increases by 108%. The conductivity of the fabricated sample reaches 14.99% of international annealing copper standard (IACS), which meets the general conductivity requirements for this material.
In the process of LPBF fabricating SAC305 alloy without a protective atmosphere, less spatter is generated owing to the low laser power, and most of the spatter is powder. The interior of the sample is composed of β-Sn, oxide particles, and a small amount of Ag3Sn. Owing to the high cooling rate of the LPBF printing process, the microstructural grains of the fabricated SAC305 alloy are smaller than those by the casting process, which improves the tensile strength of the material.
