Bylines

Direct Metal Laser-Sintering of Titanium
By Dr. Michael Shellabear, EOS GmbH

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.

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, 200W 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.

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 isostatic pressing (HIP), or parts forged to the German industry standard (Table 1). It should be noted that the material structure and properties vary according to the build strategies (e.g. laser exposure patterns) and parameters used.

Table 1. Properties of Ti64 Compared to Conventional Manufacturing

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 (see Figure 1B). This structure is created by the laser energy of each vector partially remelting the previously solidified layer below and removing the existing layer boundary. The metal subsequently recrystallizes, 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.