Effect of Build Orientation on Thermal Expansion of LPBF Printed Ti-6Al-4V

Abstract

Abstract Ti-6Al-4V is a widely adopted alloy for components produced by Laser Powder Bed Fusion (LPBF). Due to its high strength-to-weight ratio, its excellent corrosion resistance, and biocompatibility, it is used in a wide range of applications in the aerospace, chemical and process engineering, energy, and biomedical sector. While conventionally produced Ti-6Al-4V typically features an $$\alpha +\beta $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>α</mml:mi> <mml:mo>+</mml:mo> <mml:mi>β</mml:mi> </mml:mrow> </mml:math> microstructure, additively manufactured Ti-6Al-4V is well known to develop an acicular martensitic $$\alpha^{\prime}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>α</mml:mi> <mml:mo>′</mml:mo> </mml:msup> </mml:math> structure with often pronounced anisotropic properties resulting from the hexagonally closest packed (hcp) structure of $$\alpha^{\prime}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>α</mml:mi> <mml:mo>′</mml:mo> </mml:msup> </mml:math> . This study investigates the effect of the build direction of an LPBF printed Ti-6Al-4V (Grade 23) alloy on the resulting thermal expansion behaviour up to 1100 °C. Vertically built samples show a distinctly positive step in the apparent thermal expansion between approximately 850 °C and 1000 °C, while that of horizontally built specimen exhibit a dip. This is shown to be associated with (1) the alignment of prior $$\beta $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>β</mml:mi> </mml:math> grains growing epitaxially with $${\langle 001\rangle }_{\beta }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mo>⟨</mml:mo> <mml:mn>001</mml:mn> <mml:mo>⟩</mml:mo> </mml:mrow> <mml:mi>β</mml:mi> </mml:msub> </mml:math> along the direction of the thermal gradient that is related (but not parallel) to the build direction, (2) the Burgers orientation relationship and crystallographic variant selection causing acicular $$\alpha^{\prime}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>α</mml:mi> <mml:mo>′</mml:mo> </mml:msup> </mml:math> laths to grow at approximately 40 deg to the primary axis of prior $$\beta $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>β</mml:mi> </mml:math> grains, and (3) the strain occurring during the $$\alpha^{\prime}\to\, \beta $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:mi>α</mml:mi> <mml:mo>′</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:mspace/> <mml:mi>β</mml:mi> </mml:mrow> </mml:math> transformation. The apparent anisotropy of the thermal expansion of the as-printed Ti-6Al-4V is expected to have a relevant impact on the residual stress state after heat-treatment of LPBF components.

Topics & Concepts