The influence of reactive process atmospheres on the microstructure evolution of additively manufactured CP titanium

Laser Powder Bed Fusion of titanium alloys typically requires processing atmospheres with low oxygen and nitrogen contents in the build chamber as titanium is highly reactive and tends to form oxides and nitrides. However, the high reactivity can be exploited to investigate the occurring interaction between process atmosphere and material during additive manufacturing processes. In this work, commercially pure Ti is first processed in an Ar atmosphere to produce cubic samples. Subsequently, sample surfaces are re-melted with variable heat input in three different atmospheres (Ar, CO 2 , N 2 ). The surface layer formation and microstructure of the re-melted layer are investigated by optical and electron microscopy as well as energy dispersive x-ray spectroscopy and electron backscatter diffraction analysis. The formation of a complex surface layer consisting of oxides, carbides, or nitrides is observed in CO 2 and N 2 as a result of process gas uptake. It is revealed that the layer thickness depends on the heat input during re-melting. Increased levels of O, C and N also induce a martensitic transformation in the re-melted material during cooling. Printing multiple Ti layers under Ar on top of the CO 2 re-melted surface leads to a dissolution of the O rich surface layer and a re-distribution of O in the following layers inducing the formation of fine martensite. • Carbide and nitride rich layers form after re-melting CP Ti in CO 2 and N 2 . • The process gas uptake (O, C, N) induces martensite formation in CP Ti upon cooling. • The thickness of the surface layer increases with heat input. • Oxide and carbide surface layers dissolve upon further printing in Ar.