Materials & Design (Aug 2023)

Microstructure formation during laser powder bed fusion of Ti-22Al-25Nb with low and high pre-heating temperatures

  • Joachim Gussone,
  • Marcus Willi Rackel,
  • Silvana Tumminello,
  • Pere Barriobero-Vila,
  • Frederic Kreps,
  • Klemens Kelm,
  • Andreas Stark,
  • Norbert Schell,
  • Florian Pyczak,
  • Jan Haubrich,
  • Guillermo Requena

Journal volume & issue
Vol. 232
p. 112154

Abstract

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We compare microstructure formation during laser powder bed fusion (LPBF) of a Ti-22Al-25Nb alloy applying low and high pre-heating build plate temperatures. Fast cooling rates during low-temperature LPBF lead to metastable weakly ordered β phase, i.e., bcc–(Ti,Al,Nb) with pronounced 〈100〉 texture in build direction and nanosized segregations within grains elongated in the build direction. For high-temperature LPBF a Widmanstätten microstructure was observed with lenticular O phase precipitates within the β matrix. Microscopical and in situ high-energy synchrotron diffraction investigations demonstrate that the microstructure formation can be widely explained by the precipitation of O phase from supersaturated β rather than by a sequence of solid-state phase transformations, as it would be expected under thermodynamic equilibrium conditions. A detailed analysis of the microstructural gradient in the subsurface region, however, demonstrates that precipitation from metastable β cannot fully explain the observed microstructures, and that the process energy density, i.e., the intensity of the intrinsic heat treatment of LPBF, plays a relevant role. The investigation of the graded area near the surface of the LPBF materials produced with high pre-heating temperature is particularly interesting as it reveals preferred nucleation of the O phase at subgrain boundaries and dislocations.

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