Objective High strength and high conductivity copper alloy has high thermal conductivity and high strength simultaneously. Its complex parts are widely used in aerospace, petrochemical engineering, weaponry, oceanographic ship, and other fields. In the past decades, additive manufacturing such as selective laser melting (SLM) has gained increasing attention to fabricate metal parts due to the abilities of design freedom, near-net or net shape production, efficient use of materials, and fabrication of complex geometries. Due to the extremely high laser reflectivity and thermal conductivity, there are a few researches on high strength and high conductivity copper alloy fabricated by SLM. In this paper, a high power fiber laser with the maximum power of 2000 W is used to the SLM QCr0. 8 copper alloy. The influence of process parameters on the densification behavior of the SLM QCr0. 8 alloy is first studied. Then, the optimal process parameters are obtained. Finally, the microstructure and properties of the SLM QCr0. 8 alloy are investigated. Methods Gas atomized QCr0. 8 copper alloy powders with spherical shape are chosen as the starting material. The SLM system at Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology (HUST) is used to manufacture the QCr0. 8 alloy samples. The post-heat-treatment condition of the samples is annealing at 480 degrees C for 4 h followed by furnace cooling. The samples are made by the standard metallographic process and their densities are measured by the image method. The microstructures of the samples are characterized by the optical microscope, the scanning electron microscope, and the transmission electron microscope. The phase structures are tested using X-ray diffraction. The samples are examined using the tester to evaluate the tensile properties at room temperature. After the tensile test, the fracture morphologies are characterized by the scanning electron microscope. The electrical resistances are taken using the resistance tester. Results and Discussions With the increase of scanning speed, the density of samples first increases slightly and then decreases (Fig. 2). When scanning speed is too low, the defect type is round pores. When scanning speed is too high, the defect type is irregular pores (Fig. 3). With the increase of hatching space, the density of samples increases and then decreases (Fig 4). The optimal process parameters are laser power of 2000 W, scanning speed of 600 mm/s, hatching space of 0.20 mm, and layer thickness of 0.05 mm. By using these optimal process parameters, a density of 99.9% can be achieved for the SLM QCr0. 8 alloy. The microstructure in the XOZ plane is columnar crystal growing along the building direction. The microstructure in the XOY plane can be divided into a fine grain region and a coarse grain region (Fig. 7). The fine cellular crystal structure can be observed by scanning electron microscope (SEM). The main phase observed by X-Ray diffractometer(XRD) is alpha-Cu (Fig. 6). However, Cr phase and Cr2O3 phase can be observed by the TEM micrographs. The average size of precipitates is 30 nm (Fig. 10). For the annealed samples after forging, the microstructures are equiaxed crystals with a grain size of 40-60 mu m. A large number of granular or long strip precipitates can also be observed in the microstructures, and the size of precipitates is 2-30 mu m (Fig. 8). The tensile strength, yield strength, elongation, and electrical conductivity of the SLM QCr0. 8 samples are 234.7 MPa, 173.9 MPa, 26.0%, and 37.8% of international annealed copper standard (IACS), respectively. After the aging treatment, the tensile strength, yield strength, elongation, and electrical conductivity are 468. 0 MPa, 377. 3 MPa, 19. 2%, and 98. 3% of IACS, respectively (Table 2). The fracture morphologies show that the tensile samples are ductile fractures (Fig. 12). Conclusions When the process parameters are not suitable, the round pores and irregular pores coexist. The former appears when the input laser energy is too high, while the latter appears when the input laser energy is insufficient or the hatching space is too small. After optimizing the process parameters, the optimal density is 99.9 %. Because the forming process and the cooling rate of SLM are different from those of forging and annealing. The microstructures of the SLM samples are different from those of the annealed ones after forging. The microstructures of the SLM samples are columnar crystals growing along the building direction, while those of the annealed samples after forging are equiaxed crystals. For the SLM samples, the size of precipitates is about 30 nm. For the annealed samples after forging, the size of precipitates is 2-30 mu m. The tensile strength and yield strength of the SLM samples are higher than those of the annealed samples after forging. However the elongation and electrical conductivity of the SLM samples are lower than those of the annealed samples after forging. After the aging treatment, Cr phases precipitate from the Cu matrix and the strength and conductivity are improved, however the elongation is reduced. This study can promote the development of the SLM copper alloys with high strength and high thermal conductivity. A new method to fabricate the high strength and high thermal conductivity copper alloy parts with complex shapes can be developed.
