Laser deposition of Ti6Al4V-316L composition gradient structure: Challenge on intermetallics

Functional gradient materials (FGM), with their composition gradually varied in different positions, have attracted much attention for research and development during the years. Normally there are two types of functional gradient materials: one without the formation of intermetallics which is easier for fabrication due to less scientific and technological challenge; the other one with the formation of intermetallics in some part of the gradient changed composition range such as Ti-Al, Ti-Fe, Ni-Al gradient materials, which still exists big challenge on intermetallic, resulting in high brittleness and unavailability.

Laser deposition is a competitive candidate for FGM due to its capability on accurate composition and precise dimension control. According to an aeronautical application requirement to gradually change the composition from Ti6Al4V to 316L, this paper reports on laser deposition of Ti6Al4V to 316L gradient composition structure. A big challenge is the formation of the TiFe intermetallic especially near the composition of Ti₅₀Fe50, which results in very high cracking tendency. Many efforts have been made to optimize the Ti-Fe composition ratio, the deposition process and the deposition parameters, not a single Ti6Al4V-316L composition gradient structure was successful due to the formation of the very brittle TiFe and TiFe₂ intermetallics. This indicates that it is almost impossible to direct deposit Ti6Al4V to 316L without cracking and separation. We used Inconel 625 powder as an interlayer between the Ti6Al4V and the 316L, thus a sound Ti6Al4V-316L composition gradient structure was successfully achieved by laser deposition. The brittle TiFe and TiFe₂ intermetallics were replaced by a series of phase evolutions: α+β → β+Ti₂Ni→Ti₂Ni+TiNi→TiNi→TiNi+TiNi₃→Ni₃Fe→ Ni₂Fe₃→γFe. The brittleness was dramatically decreased or diluted and crack-free composition gradient structure was available. The gradient structure was further analyzed on hardness, microstructural evolution, the Young’s modulus and Poisson’s ratio.