January_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 8 1 9 AM PARTS INCLUDE MILLIONS OF WELDS The most common form of met- al AM uses pre-alloyed powder or wire and a directed energy source to local- ly melt small volumes of material, se- rially building a final part [2] . This differs significantly from conventional produc- tion routes that rely on monolithic ma- terial sources machined or forged to a final geometry. The advantage of ad- ditive compared to subtractive manu- facturing is clearly visible in the ability to produce highly complex geometries with non-line-of-site features. However, the processing route to achieve these geometries severely limits the types or quality of material. Historically, alloys fall into two main categories, either cast or wrought, which have relative- ly predictable process-structure rela- tionships where material properties are dictated by the microstructure that de- velops subject to the processing condi- tions applied. Additive manufacturing adds a new complexity to traditional cast and wrought alloys by imposing multiple sequential melting and solidi- fication steps (Fig. 2). Thus, an AM com- ponent may best be characterized as comprised of thousands, if not millions, of individual welds. This welding analogy is important, as it points to several known metallur- gical issues. Typically, alloys amena- ble to AM are easily welded in bulk form without the aid of a filler metal. While several “unweldable” alloys can be joined through the addition of filler metals, thus altering the composition of the weld towards a more amenable so- lidification condition [3] , the use of a filler metal ultimately leads to a fundamen- tally different material and microstruc- ture at the weld. As a result, welds are generally weaker than the base materi- al being joined. In additive manufactur- ing, because the entirety of the part is effectively welded, additive processes typically require a single alloy system, limited to an alloy whose solidifica- tion behavior is similar to welding filler metals. Of the greater than 5000 alloys Fig. 2 — Laser powder bed additive manu- facturing. Long exposure (10 s) optical pho- tograph of build volume. Laser path (glow) indicates sequential melting protocols, where contours are first melted followed by a hatched fill, highlighting the dynamics of the additive process. A dditive manufacturing (AM) has the potential to transform design and fabrication, disrupting multi- ple industries. Due to the unique process- ing conditions in powder-based metal AM, only a handful of alloys can currently be additively manufactured, limiting the impact of this technology. The authors re- cently reported a metallurgical approach todrastically expanding the number of al- loys amenable to processing with exist- ing AM hardware, based on manipulating solidification mechanisms via nanofunc- tionalization of feedstock powders with carefully chosen lattice-matching nucle- ants [1] (Fig. 1). They demonstrated this approach by selective laser melting of alumi- num alloy Al7075 and Al6061 powders functionalized with zirconium hydride nanoparticles, resulting in a crack-free, equiaxed, fine-grained microstructure with yield strengths in excess of 370 MPa, a 15X improvement over un- modified powders. Through additional process optimization and post-treat- ment, further increases in strength (>415 MPa yield) and ductility (>15% elongation) can be realized in AM Al7075 alloys. The nanofunctionaliza- tion approach is both alloy and ma- chine agnostic, providing a foundation for broad industrial applicability. This new approach is expected to displace current additive alloys with higher per- formance, lower cost, and applica- tion-specific alloy systems. Fig. 1 — Comprehensive approach to additive manufacturing with nanofunctionalized alloys. Left, crystallographic and thermodynamic selection of ideal nucleants and assembly on base alloy powders; middle, processing using conventional additive manufacturing; right, production of high strength, complex geometry components with 2X increase in tensile strength.