Densification of a powder-metal skeleton by transient liquid-phase infiltration
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20/1/04
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Densification of a Powder-Metal Skeleton by Transient Liquid-Phase Infiltration ADAM LORENZ, EMANUEL SACHS, SAMUEL ALLEN, LUKAS RAFFLENBEUL, and BRIAN KERNAN 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.
I. INTRODUCTION
TRANSIENT liquid-phase infiltration (TLI) provides a new alternative for densification of a powder-metal skeleton that achieves a homogeneous final-part composition without significant dimensional change. The process offers advantages over the existing densification processes of full-density sintering, which requires shrinkage and leads to distortion in large parts, and traditional liquid metal infiltration, which typically results in inferior material properties. The TLI method uses an infiltrant that is similar in composition to the skeleton material and also contains a melting-point depressant (MPD). The initial skeleton filled with liquid infiltrant is not at equilibrium, and diffusion of the MPD results in isothermal solidification and eventual homogenization. When used in conjunction with solid freeform fabrication (SFF) processes such as three-dimensional printing,[1] TLI makes possible the direct fabrication of large, homogeneous, net-shape, metal parts. It further offers the potential to match the final composition of parts to existing commercial alloys that are familiar to designers. The generic phase diagram in Figure 1 corresponds to the simplest example of TLI, in which the powder skeleton material is pure metal A, and the infiltrant is a binary alloy of metal A and an MPD. The final-part composition is based ADAM LORENZ, Po
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