Microstructure and mechanical properties of a graded structural material

Based on the fact that the performance requirements are usually different from one part to another in one component, a novel graded structural material (GSM) of Ti-6Al-4V/Ti-6.5Al-3.5Mo-1.5Zr-0.3Si has been designed and successfully fabricated by laser melting deposition (LMD) manufacturing technology. Microstructure and chemical composition have been characterized by OM, SEM and EPMA. Micro-hardness and room temperature tensile property have been evaluated as well. Large columnar grains growing epitaxially and traversing multiple deposited layers occur both in Ti-6Al-4V part and Ti-6.5Al-3.5Mo-1.5Zr-0.3Si part. The columnar-to-equiaxed-to-columnar transition happens in the gradient zone because the thermal behavior in and around the molten pool undergoes a transition from unstable state to stable state at the beginning of the LMD process. The microstructure and micro-hardness in gradient zone vary gradually as a function of chemical composition. Both the chemical composition and micro-hardness in gradient zone change abruptly in interlaminations and exhibit a characteristic of steps. According to room temperature tensile results, the fracture takes place in Ti-6Al-4V part when the graded structural material is tested in the longitudinal direction, illustrating that the bonding strength in the gradient zone is stronger than the strength of Ti-6Al-4V in the longitudinal direction. The strength of the graded structural material in the transverse direction falls between that of Ti-6Al-4V in transverse direction and Ti-6.5Al-3.5Mo-1.5Zr-0.3Si in transverse direction. Mechanical behavior of the graded structural material during the room temperature tensile test is analyzed as well.