Direct observation of the joint interface between Al₂O₃ plates with nickel foil and carbon nanotube sheets as insert materials during laser welding

Abstract

Manufacturing processes using 3D additive manufacturing of ceramics have become a rapidly growing area of research. The development of a laser welding technology for thick ceramics would improve productivity and reduce manufacturing costs. In this work, 3-mm-thick Al₂O₃ plates were laser butt joined with Ni foil at different powers and with or without a carbon nanotube sheet, and the mechanical properties and microstructures of the joints were investigated. Plate edge surfaces were metallized with Mo by friction stirring as a pre-treatment to improve wettability and reduce thermal expansion differences by gradually changing the thermal expansion coefficient of the material. The 3-mm-thick laser butt joints obtained at 65 W and with a carbon nanotube sheet showed a deep reaction from the surface towards the interior under laser keyhole welding conditions, resulting in a tensile strength at 0.64 MPa. Tensile strength was insufficient because the weld bead was concave owing to spatter and there were cracks within the weld and heat-affected zone from the surface towards the interior. Crack initiation mechanisms and improvement methods were discussed by direct observation of the joint interface during laser welding.