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.