Transient liquid-phase infiltration (TLI) involves a powder-metal skeleton and an infiltrant with similar composition containing a melting-point depressant (MPD). Upon infiltration, the MPD diffuses into the skeleton, causing isothermal solidification and allowing a homogeneous final-part composition. Diffusional solidification of the infiltrant can restrict liquid flow and result in premature freeze-off if the liquid solidifies before filling the entire part. A capillary-driven fluid-flow model was developed to predict the infiltration rate and freeze-off limit using a variable skeleton permeability. Diffusional solidification was measured via quenching experiments, compared to theory, and used to define the change in permeability of the skeleton. The infiltration rate was measured via mass increase and compared to the flow model for various skeletons with powder sizes ranging from 60 to 300 m. The predicted horizontal infiltration freeze-off limits were proportional to the square root of d 3 ␥/D 2 , where d is the average powder diameter, ␥ and are the infiltrant surface tension and viscosity, respectively, D is the solid diffusivity, and  is a function of the solidus and liquidus concentrations. These relations can be used for selection of processing parameters and for evaluation of new material systems.
Transient liquid-phase infiltration (TLI) is a new method for densifying a powder-metal skeleton that produces a final part of homogeneous composition without significant dimensional change, offering advantages over traditional infiltration and full-density sintering. Fabrication of direct metal parts with complex geometry is possible using TLI in conjunction with solid freeform fabrication (SFF) processes such as three-dimensional printing, which produce net-shape powder-metal skeletons directly from computer-aided design models. The TLI method uses an infiltrant material similar in composition to the skeleton, but also containing a melting-point depressant (MPD), which allows the liquid metal to fill the skeleton void space and later facilitates homogenization. The materials requirements for such a system are discussed, and four experimental material systems were developed with final compositions of approximately Ni-40 wt pct Cu, Ni-4 wt pct Si, Fe-3 wt pct Si, and Fe-12 wt pct Cr-1 wt pct C, with copper, silicon, and carbon serving as the MPDs. Infiltration techniques include gating the introduction of liquid, saturating the melt to prevent erosion, and controlling variations in bulk composition along the infiltration path. Infiltration lengths exceeded 200 mm in the two nickel systems and exceeded 100 mm in the two iron systems. After infiltration, various heat treatments were conducted and mechanical properties were tested, including the tensile, hardness, and impact strength.
The direct manufacture of metal parts by rapid prototyping (RP) often involves using a metal powder. Densification of the powder can be done either by sintering or infiltration. Infiltration avoids the shrinkage and distortion that typically accompanies sintering. However, in steels, the use of copper or bronze infiltrants limits the usefulness of parts because of the nonhomogeneous structure and properties. In this work, a conventional tool steel alloy has been made via homogeneous steel infiltration (HSI), a gated infiltration process that uses as the infiltrant a steel alloy with a lower melting point than the base powder. The infiltrant liquid uses carbon or silicon as a melting point depressant (MPD). Freeze-off of the steel infiltrant is avoided by infiltrating at a temperature at which some liquid is stable at chemical equilibrium. Examples of successful infiltrations using D2 tool steel as a target composition are shown. Mechanical properties (hardness and impact strength) following various heat treatments of the infiltrated D2 are compared with conventional wrought D2 tool steel and found to be very similar. The HSI is believed to be suitable to a wide variety of tool steels and stainless steels. Further, the process is applicable to the postprocessing of any suitable steel skeleton made by RP, as well as other powder metallurgy processes, such as metal injection molding.
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