Site specific control of crystallographic grain orientation through electron beam additive manufacturing

Abstract

Site specific control of the crystallographic orientation of grains within metal components has been unachievable before the advent of metals additive manufacturing (AM) technologies. To demonstrate the capability, the growth of highly misoriented micron scale grains outlining the letters D, O and E, through the thickness of a 25·4 mm tall bulk block comprised of primarily columnar [001] oriented grains made of the nickel base superalloy Inconel 718 was promoted. To accomplish this, electron beam scan strategies were developed based on principles of columnar to equiaxed transitions during solidification. Through changes in scan strategy, the electron beam heat source can rapidly change between point and line heat source modes to promote steady state and/or transient thermal gradients and liquid/solid interface velocity. With this approach, an equiaxed solidification in the regions bounding the letters D, O and E was achieved. The through thickness existence of the equiaxed grain structure outlining the letters within a highly columnar [001] oriented bulk was confirmed through characterizing the bulk specimen with energy selective neutron radiography and confirming with an electron backscatter detection. Ultimately, this demonstration promotes the ability to build metal components with site specific control on crystallographic orientation of grains using the electron beam melting process.