Microstructure and mechanical properties in additive manufacturing by friction surfacing of AA6061 alloy

This present study uses a novel solid-state additive manufacturing method based on friction surfacing as a mechanism for layer-upon-layer deposition. This utilizes severe plastic deformation to create a homogeneous fine-grain microstructure without melting to avoid solidification-related defects. The deposition of AA6061-T6 aluminum alloy was achieved by rotating a 20 mm diameter rod on top of a substrate at 1000 RPM and applying a 300 mm/min transverse speed with an axial feed rate of 50 mm/min. Advanced macro/microstructural techniques characterized the grain and precipitate structures in the deposited material, i.e., electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) analyses, along with micro hardness and tensile testing of the mechanical properties. The multi-layer deposition contains a refined grain size in the range of 5-10 & mu;m and sub-grains less than micron size, while a reasonable reduction in mechanical properties compared to the as-received rod was observed; in which hardness was reduced from 114 HV to 49 HV, and the tensile strength was reduced from 333 MPa to 161 MPa. These reductions are largely attributed to the dissolution of precipitates in the original rod due to the high temperatures imposed by friction and deformation. Achieving metallurgical bonding between the layers while suppressing porosity is critical to additive manufacturing, and this feasibility study of friction surfacing demonstrates this can be achieved along with superior isotropic mechanical properties. Based on the results, it was observed that additive manufacturing by friction surfacing is particularly suitable for metals that are sensitive to heat during deposition, though parameter optimization and secondary treatment may be required to enhance properties further.