Strengthening mechanism of Y2O3 nanoparticles on microstructure and mechanical properties of the laser additive manufacturing joint for large thickness TC4 titanium alloy

The laser additive manufacturing technology with laser melting deposition as the core technology presents superior efficient connectivity of titanium alloy components with large thickness and complex structure in the aerospace field. The content of nanoparticles affects the microstructure and properties of TC4 large thickness titanium alloy joint when it is manufactured with laser additive manufacturing. A pair of TC4 titanium alloy test plates with a thickness of 80 mm were connected by laser additive manufacturing technology with the mixed powder as filler material which was produced by adding different content Y2O3 nanoparticles into TC4 powder. Optical microscopy, scanning electron microscopy, micro-hardness test and tensile test were conducted to study the effect of Y2O3 content on the microstructure and mechanical properties of the TC4 titanium alloy joint. The strengthening mechanism and agglomeration weakening mechanism of Y2O3 nanoparticles on microstructure and mechanical properties was investigated based on the experimental results. It was found that the addition of more Y2O3 nanoparticles resulted in partial powder agglomeration and degradation of tensile strength. The results indicated that the columnar grains in the middle of the deposition zone with Y2O3 nanoparticles were finer than those without Y2O3 nanoparticles. The joint fracture mode gradually changed from plastic fracture to quasi-cleavage fracture with the increase of Y2O3 content. The average hardness values of the joint with various Y2O3 contents are all higher than that of the TC4 titanium alloy substrate.