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 8 both laser AM and conventional cast- ing techniques to produce metal-metal AM interpenetrating phase composites (AMIPCs) that allow application-specif- ic combinations of the properties of two dissimilar metals. By introducing each phase into the composite separate- ly, this approach minimizes tempera- ture-induced chemical interactions between the dissimilar metals, re- ducing the potential to form brittle intermetallics. As an initial proof of concept, the ORNL/Rice team first designed and printed a repeating lattice structure (Fig. 1) that is described by two pa- rameters: the lattice strut diameter (D) and the lattice spacing (a). This ini- tial step builds the AMIPC’s underlying mesh structure, and other useful fea- tures, such as integrated sprues and a skin that directs the flow of alumi- num into the lattice during infiltration. Builds were fabricated using a Ren- ishaw AM250 metal AM system, depow- dered, and then invested in plaster of Paris (Fig 2). To form the IPC, the emp- ty lattice was filled with molten materi- al using centrifugal casting, which could be replaced by other pressure-assisted industrial casting processes such as die casting that would also overcome sur- face tension between the steel and alu- minum and promote full infiltration. This hybrid processing approach yields metal-metal AMIPC materials, combining the flexibility and unique op- portunities for complex geometry pres- ent in AM with the speed and robust- ness of traditional high-volume casting techniques. This processing technique is especially useful because it offers de- signers the ability to freely adjust lo- cal material properties by varying the composite structure throughout the component. HYBRID MATERIAL PROCESSING RESULTS The ORNL/Rice team has demon- strated the ability to produce com- posites using both fixed and spatially varying lattice geometries. In this initial effort, the team produced composites consisting of an AM lattice made of 316L stainless steel and filled with cast A356 aluminum. The team produced several different volume fractions of 316L stain- less steel and tested the thermal con- ductivity, compressive strength, and tensile strength of each. The thermal conductivity and strength data shown in Fig. 3 demonstrates how the ther- mal conductivity of the composites de- creases with increasing stainless steel volume fraction, while the compressive flow stress increases [16] . Like thermal conductivity and strength, strain to failure is another ma- terial property of special industrial im- portance. Figures 4a-b illustrate the unique behavior observed during the tensile loading of this composite. The AMIPC exhibited an order of magnitude increase in strain to failure compared to the matrix material (A356) alone. The significant increase in strain to failure is evident in Fig. 4a, which shows tension stress-strain curves for the A356 and the composite specimens. The composite’s stress-strain curve fea- tures large serrations that correspond Fig. 2 — Process for synthesizing AMIPCs. The model is printed using a selective laser melting process, filled with liquidmetal using centrifugal casting, and then post-processed to optimize the properties. Fig. 3 — Thermal conductivity and flow stress at 10% strain of A356/316L AMIPCs as a function of the volume fraction of 316L stainless steel. to microcracking events. These microc- racks can be seen in the computed to- mography reconstructions in Fig. 4b, where several cracks perpendicular to the tensile specimen axis are evident. Fig. 4 — a) Stress-strain curves from tension tests on a 39 vol% stainless steel AMIPC and the A356 material; b) x-ray computed tomography reconstructions of the A356 and 316L tensile specimens. A356 shown in red and 316L in blue. The higher magnification image shows a 316L ligament bridging a crack in the A356. (b) (a)