Revealing the mechanism of laser welded aluminum alloy joints in-situ reinforced by carbon nanotubes based on microstructural evolution

In-situ addition of carbon nanotubes (CNTs) was innovatively applied to laser welding Al-Cu-Mg alloys to form composite joints, and the mechanism of CNTs strengthening joints was revealed from the viewpoint of microstructure evolution. Tensile test showed that the average tensile strength of the CNTs-reinforced joint increased by 37.3% to 306 MPa. Initially, microscopic observation and elemental analysis found that part of the CNTs were combined with the precipitate, and part of the CNTs were dispersed in the matrix, which provided conditions for the transfer of the load from the matrix to the CNTs. In addition, the fusion zone (FZ) grains of the joints in-situ strengthened by CNTs are finer and isotropic compared to the joints without CNTs. Since CNTs and aluminum alloys have remarkable differences in thermal expansion coefficients to form thermal mismatches, and part of CNTs may act as grain boundaries, resulting in dislocations cannot cut through them to form Orowan strengthening. As a result, CNTs-reinforced joints achieved significant improvements thanks to these primary strengthening mechanisms. The mechanism of CNTs-strengthened aluminum alloy joints is comprehensively and deeply revealed from the microscopic level, which provides a reference for the design of Al/CNTs composite joints and performance regulation.