Strengthening Porous Skeletons by Metal Deposition from a Nanoparticle Suspension

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Strengthening Porous Skeletons by Metal Deposition from a Nanoparticle Suspension Nathan B. Crane, Emanuel M. Sachs, Andreas Frank Department of Mechanical Engineering, Massachusetts Institute of Technology 77 Massachusetts Ave Cambridge, MA 02139 ABSTRACT Solid freeform fabrication (SFF) processes such as three-dimensional printing (3DP) and selective laser sintering (SLS) produce porous bodies that must be densified for many applications. New homogenous infiltration techniques can produce dense, homogenous parts of selected standard alloys, but the increased infiltration temperature dramatically increases creep deflection under self-weight. This paper reports on a method that improves dimensional stability by reducing creep deflection rates at high temperature. This method is applicable to all metal skeletons that must be strengthened without increasing shrinkage. In this method, the skeletons are reinforced by the addition of nanometer-sized particles dispersed in a liquid. The liquid is applied to the structure either during 3DP printing or after forming (3DP, SLS, pressing). The liquid is then evaporated, depositing the metal in the skeleton. The metal nanoparticles are sintered to density below the sintering temperature of the micron-scale skeleton particles. This concept is demonstrated using a suspension of 8-10 nm iron particles infiltrated into lightly sintered porous steel skeletons. When heated with an unsupported overhang to a typical infiltration temperature, creep deflection was reduced 50-80% with 0.5-1 wt% added metal. INTRODUCTION Solid freeform fabrication (SFF) processes such as three-dimensional printing (3DP) and selective-laser sintering (SLS) produce porous bodies. For most applications, these must be densified to achieve the final desired properties. Densification may be achieved by sintering with large shrinkage or by infiltration. Studies of steel parts infiltrated with bronze- show that the 3DP printing process introduces approximately 0.1 mm of error in the green body. Post processing causes linear shrinkage of 1.5±(0.25)%. While the average shrinkage can be offset, the variation represents an accuracy error that scales with part geometry. Additional error is introduced by part warping from differential shrinkage rates and creep. [1] The traditional porous steel parts that are densified by infiltration with a bronze alloy produce a heterogeneous dense part. The final material properties are degraded relative to pure steel and designers are often uncomfortable using this composite. Lorenz et al [2,3] developed an infiltration method that produces a homogenous final composition through the use of a diffusing melting point depressant in the infiltrant. Kernan et al [4] have further developed this method to produce homogenous final parts of several standard steel alloys including stainless and tool steels. These new homogenous infiltration techniques can produce dense, homogenous parts of selected standard alloys with excellent material properties, but the increased infiltration tem