2008

Direct Metal Laser-Sintering of Titanium

DMLS titanium parts can be used in aerospace and medical applications.

During the first decade of direct metal laser-sintering (DMLS), the metals employed were generally ones developed specifically for DMLS, rather than those used in traditional metalforming methods. But in recent years, the range of available powder metals and the production quality of DMLS parts have advanced considerably, driving new interest in rapid manufacturing.

Greater acceptance of rapid manufacturing has, in turn, encouraged further development of metals for DMLS that are analogous to existing, conventional materials. For instance, titanium-based alloys are commonly used in applications with demanding requirements, but parts are typically expensive to manufacture conventionally, because titanium is generally difficult to cast and to machine. For this reason, titanium alloys for DMLS are well worth exploring.

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Figure 1. (A) Optical micrograph of Laser-Sintered Titanium Ti64, showing fully dense structure (only single pores), and (B) dendritic, martensitic structure with preferential orientation.

EOS Titanium Ti64 is a pre-alloyed Ti6AlV4 alloy, characterized by excellent mechanical properties and corrosion resistance combined with low specific weight and biocompatibility. Parts built from this alloy can be machined, spark-eroded, welded, micro-shot-peened, polished, and coated, if required. Typical uses include aerospace and medical applications.

To create Ti64 parts, DMLS equipment deposits a layer of Ti64 powder on top of a titanium build platform. A focused, 200-W ytterbium-fiber laser then melts a selected area of the powder. The machine builds parts in cross-sections, layer by layer, fusing them into solids of approximately 100% density (Figure 1A). The DMLS process for titanium takes place in an inert argon atmosphere to ensure that the final part is free of impurities. Laser-sintered parts made from Ti64 meet industry requirements regarding the maximum concentration of oxygen and nitrogen impurities.

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Figure 2. Properties of Ti64 Compared to Conventional Manufacturing.

The mechanical properties of Ti64 are comparable to or better than the properties of the same material processed with conventional manufacturing; for instance, powder metallurgy combined with hot iso-static pressing (HIP), or parts forged to the German industry standard (see Figure 2). It should be noted that the material structure and properties vary according to the build strategies (e.g. laser exposure patterns) and parameters used.

Moreover, DMLS-manufactured parts do not always have exactly the same properties as those made from conventional materials. A unique characteristic of the laser-sintered titanium alloy is that it develops a typically dendritic, martensitic structure with grains growing perpendicularly from layer to layer. The grain structure is highly uniform (Figure 1B). This structure is created by the laser energy of each vector partially re-melting the previously solidified layer below and removing the existing layer boundary. The metal subsequently re-crystallizes, growing through the layers.

As DMLS technology matures and is more widely deployed, the range of usable materials will grow. These materials will probably include additional “tailored” versions of commercially available metals such as other titanium alloys, Inconel, precious metals, and additional stainless steel.

This article was written by Dr. Michael Shellabear, vice president of EOS GmbH Electro Optical Systems, Krailling, Germany. For more information, click here.

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