Homogeneous steel infiltration
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I. INTRODUCTION
WITHIN the field of rapid prototyping (RP) or solid freeform fabrication (SFF), one goal is the direct manufacture of functional metal parts that generally utilizes a powdered material as the feedstock. Several SFF methods are capable of making a porous metal green part, such as 3D Printing (3DP)[1,2,3] or selective laser sintering (SLS).[4,5,6] The production of metal tooling directly is one application of SFF.[7–10] High-density parts are obtained by either sintering or infiltration of the green part. Sintering can produce high-quality small parts; however, when applied to larger parts, differential shrinkage can cause warping. Infiltration of steel powder with a copper or copper-based infiltrant has a long history in the traditional powder metallurgy industry, and these methods have been adapted by the RP industry.[11,12] Such infiltration with a bronze alloy often produces a useful part; however, the resultant nonhomogeneous microstructure may result in properties that are inferior to those desired. Certainly, rapid manufacturing and rapid tooling would be more useful and widespread if the parts or tools could possess the chemistry and properties of conventional steel alloys. In the past, attempts have been made to infiltrate one steel alloy with another steel alloy that contains a higher percentage of a melting point depressant (MPD), such as carbon. An example is infiltrating nearly pure iron with cast iron (4.2 wt pct C) in order to from an eutectoid steel (0.8 wt pct C).[13,14] Because of the high diffusivity and solubility of carbon in BRIAN D. KERNAN, Senior Metallurgist, is with The ExOne Co., Irwin, PA 15642. Contact e-mail: [email protected] EMANUEL M. SACHS and SAMUEL M. ALLEN, Professors, are with the Massachusetts Institute of Technology, Cambridge, MA 02139. ADAM LORENZ, Postdoctoral Researcher, formerly with Massachusetts Institute of Technology, is with Evergreen Solar, Marlboro, MA 01752. CHRISTOPH SACHS, Researcher, is with the Max Planck Institute for Iron Research, Dusseldorf, Germany. LUKAS RAFFENBEUL, Researcher, is with Volkswagen, Germany. ALBERTO PETTAVINO, Mechanical Engineer, is with Giugiaro Design, Moncalieri, Italy. Manuscript submitted July 15, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
iron, full infiltration is difficult to achieve, as the liquid infiltrant can freeze off during infiltration as carbon diffuses into the skeleton. During isothermal solidification, a race occurs between infiltration of the liquid into the skeleton and diffusion of carbon into the powder. Carbon diffusion nearly always wins this race because the homogenization diffusion time for reasonable powder sizes is less than 1 second, while the time to infiltrate is several seconds.[15,16] The use of additional pressure to force the liquid into the skeleton faster than capillarity would draw it in is possible;[17,18] however, this approach is not very attractive for making complexly shaped objects. Other MPD elements, such as boron and phosphorous, have been used along with or instea
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