Microstructural characteristics and mechanical properties of additively manufactured Cu–10Sn alloys by laser powder bed fusion

Cu – 10 wt% Sn (Cu–10Sn) alloy specimens were additively manufactured by varying the laser powder bed fusion (LBPF) processing parameters, e.g., laser power (200–350 W) and laser scan speed (100–1400 mm/s) at a fixed hatch spacing (0.12 mm) and slice thickness (0.03 mm), to examine their effects on the density, melt pool structure, and microstructure. Microstructural characteristics of the most dense sample were examined using electron microscopy (SEM, TEM) and X-ray Diffraction. The grains in the top-most melt pool layer showed a fan-like structure, whereas all other areas showed small columnar structure aligned nearly parallel to the build direction due to multiple melt-solidification cycles. The high cooling rates associated with LPBF yielded a fine dendritic microstructure, which consisted of primary α₁(Cu) dendrites and a mixture of α₂(Cu) and δ (Cu₄₁Sn₁₁) phases in the interdendritic region. In addition, formation of thin laths (∼120 nm) was observed by TEM presumably due to diffusionless martensitic transformation from the high temperature β phase associated with fast cooling rates. The primary α₁(Cu) dendrite and α₂(Cu) within the interdendritic region contained low (∼6.6 wt%) and high (∼11.9 wt%) Sn concentration, respectively. Mechanical properties in tension of the dense Cu–10Sn alloy was measured in as-built condition, i.e., σ_UTS = 564 MPa, σ_Y = 387 MPa, El = 26%; these are significantly superior to the properties determined in previous LPBF investigations and as-cast Cu–10Sn alloy, most likely due to contributions from Hall-Petch and dislocation strengthening mechanisms. Solidification path in LPBF fabricated Cu–10Sn alloy was also revealed from an extensive TEM investigation.