October_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 | O C T O B E R 2 0 1 7 1 7 A dditive manufacturing (AM) is an exceptionally flexible process- ing technology, allowing users to quickly create complex parts that target specific applications. By itself, AM pro- vides many benefits, but by combining different materials processing techniques like traditional castingwith additiveman- ufacturing to create hybrid processes, custommaterials can be tailor-made and mass produced for applications with spe- cific performance needs. Recent work at Oak Ridge National Laboratory and Rice University shows that such hybrid pro- cessing routes can yield unique materials with attractive thermal and mechanical properties. In engineering applications, per- formance demands place considerable constraints on materials selection. A metaphor often encountered in deci- sion making is the cost/quality/time triangle, where only two of the three constraints can be optimized for. This metaphor is ever-present in materials selection. Optimizing a material for a specific application involves consid- erable effort to balance base material properties, production methods, end- use demands, environmental concerns, and total cost. Even with diligent engi- neering, attention to detail, and a rig- orous selection process, materials have limitations. Recent advances in ad- ditive manufacturing offer engineers new tools with which to select materi- als and geometries for specific appli- cations, through the ability to create unique geometries that are unable to be machined or cast, exploiting novel material properties, and enabling new composite materials to add flexibility to their designs. ADDITIVE MANUFACTURING PROGRESS Since its inception in the late 1960s, additive manufacturing has kept a strong foothold in certain niche appli- cations, finally entering the commercial mainstream in 1987 when 3D Systems released the first commercially avail- able additivemanufacturingmachine [1] . Recent developments in AM technology have reduced total cost, enabling in- creased adoption of AM and fabrication of complex components [2] . Most cur- rent metal AM techniques manufacture components via a layer-by-layer depo- sition process. Slight modifications to this approach enable the introduction of new materials or changes in material composition and properties with each successive build layer. Such techniques have been used to fabricate function- ally graded materials (FGMs) [3-5] . FGMs feature gradual changes in materi- al properties across a component, al- lowing different areas of the geometry to meet specific requirements. While most research on additively manufac- tured FGMs has focused on metallic sys- tems, these processes can be extended to many other materials systems in- cluding ceramic-metal composites like WC-Co [6] . However, a major drawback with conventional fusion-based ap- proaches to additive manufacturing of FGMs is that deleterious intermetallic phases can form as layers are fused to- gether [3-5] and these intermetallics can crack during the build or degrade the mechanical properties of the final part. COMBINING AM WITH CASTING Recent attempts to avoid interme- tallic phase formation [3] have kept close control over the composition gradient between layers during the build process to produce a gradient consisting sol- ely of desirable phases. Although this approach has been demonstrated in certain materials systems [3] , its success depends on maintaining precise con- trol over the mixing of separate pow- ders, and precisely controlling the melt pool temperature in order to minimize interactions between build layers. Even when this technique is executed in a highly con- trolled fashion, the build process is at its core only a slight modification of traditional AM and retains many of the current disad- vantages of this process, namely build speeds that are too slow and costs that are too high for large scale production [7] . While these problems may be resolved in the future, for near-term mass production, it is desirable to combine the possibil- ities and flexibility of AM with faster and scalable production processes like casting. Recent work performed at Oak Ridge National Laboratory (ORNL) and Rice University demonstrates that such hybrid processing schemes are pos- sible and can achieve unique hybrid materials [8] . Rather than deposit multiple dis- tinct materials during a single build, the ORNL/Rice team printed a lattice out of stainless steel, and then filled the voids in the lattice with an aluminum cast- ing alloy, a material with a significantly lower melting point than stainless steel. The result of this two-step approach is an interpenetrating phase composite (IPC), where each component forms a continuous network. Research begin- ning in the 1990s has shown that IPCs exhibit a unique range of properties that cannot be achieved in a single material or traditional dispersed composites [9] . Most of the prior research involving IPCs has focused on two classes of compos- ites: polymer composites and metal-ce- ramic composites [9–12] . These typically take the form of fiber-reinforced com- posites, where fiber reinforcements run continuously throughout the ma- terial and can be prepared using a va- riety of methods, including AM. Several studies on the properties of interpene- trating structures [13–15] have shown that IPCs can blend together the best prop- erties of both constituents. The ORNL/ Rice team’s approach is an alternative technique for creating IPCs, employing Fig. 1 — Lattice preform used in the synthesis of the additively manufactured interpenetrating phase composites (AMIPCs).