Dr. Michael Shellabear, vice president of Metal Technology, EOS GmbH, took time to answer a selection of questions compiled by the editors of Aerospace Manufacturing and Design magazine regarding DMLS and it pertinent applications within the aerospace industry.
Q: What is direct metal laser-sintering (DMLS), and does titanium pose any unique technical challenges for this technology?
A: DMLS is an emerging technology for rapid, additive manufacturing of metal parts. Among other applications, it is well suited to production of high-strength, low-weight components at low manufacturing volumes. As such, it has exciting potential for aerospace and often involves less cost than conventional manufacturing processes.
DMLS starts with a 3D digital model divided into cross-sections by control software. A thin layer of powdered metal is deposited on a build platform in a sealed chamber. The software then guides a laser on a path dictated by the cross-section, melting the metal powder. The process repeats, layer-by-layer, with a high degree of precision, until the part is complete.
Because titanium is highly reactive in its molten state, it requires a low, controlled oxygen content in the metal powder lots and an inert gas (argon) for a processing atmosphere. Apart from these considerations, titanium is as easy to laser-sinter as other proven metals.
Q: What special benefits does laser-sintering a titanium alloy present over traditional molding and machining processes?
A: Since DMLS is an additive technology, it dramatically reduces material waste in comparison with traditional processes. Investment casting of titanium, for example, is difficult and often has a high scrap rate. Currently, many titanium aerospace components are machined from solid stock, often cutting away 90% or more of the original material – a time-consuming, costly operation that is completely eliminated with DMLS.
DMLS also has lower labor costs since it is a nearly automatic process that involves minimal programming and no “hands-on work” or supervision during part manufacture.
Finally, some of the characteristics that make titanium ideal for aerospace applications also make it difficult to machine. Its hardness and low heat conductivity reduce tool speeds and life, require a great deal of liquid cooling during machining, and limit the producibility of certain shapes, such as thin walls. Laser-sintered titanium, however, retains the beneficial properties of the metal and involves no tool-wear or coolant costs. In addition, nearly any geometry, including thin walls, can be created with laser-sintering.
Q: What makes laser-sintered titanium especially suitable for aerospace?
A: As we know, titanium is a material of choice in aerospace because of its extremely high strength-to-weight ratio. DMLS enhances that performance ratio by enabling the building of ultra-light parts with thin walls, hollow sections, and intelligent fill structures that stand up to the rigorous demands of aerospace applications.
Tests by EOS customers have compared the properties of laser-sintered titanium parts to those of cast or wrought titanium parts, and found that the DMLS parts can have significantly better mechanical properties. Typically, titanium parts made with DMLS have an ultimate tensile strength of 1,200Mpa + 30Mpa (175ksi + 4ksi), comparable to or stronger than conventionally manufactured titanium components.
Q: What are some examples of titanium aerospace components that are presently being manufactured with DMLS? A: Most of our customers in the aerospace field want to keep confidential what they are manufacturing with DMLS, so it is difficult to give specific examples. However, in regards to other industries, the types of DMLS applications include blades and hubs for turbines, housing parts, fixtures, and parts for scaled-down wind-tunnel testing and other tests, just to name a few. DMLS is actively used in prototyping and product development in many industries, and is also being qualified for series production.
Q: Do some of these components have advantages that would be difficult (or impossible) to gain with traditional processes, and in what ways is DMLS a landscape-changing technology for aerospace?
A: Laser-sintering prototypes and fixtures already achieves significant cost and time savings over other processes, but more importantly, DMLS offers a new age of innovative design freedom. Engineers no longer need to be concerned with draft angles, parting lines, corner radii, and the minutiae of turning complex models into matter.
Laser-sintering offers the potential for design-driven manufacturing – the creation of a component based solely on a vision of its ultimate function, without the compromises imposed by process limitations. Instead, designers can focus on creating products that most efficiently and effectively meet the utmost performance goals.
Q: What quality assurance, especially materials and process tracking, are in place for DMLS?
A: All powder batches from EOS are subject to batch-specific quality assurance and traceability. When the DMLS system is in production, monitoring software logs all outputs from the DMLS process monitoring – for example, confirming that the oxygen level stays within its specified range. The software can also produce quality reports and processing statistics for every part produced. DMLS systems are now available with an integrated process chain management system that includes controlled, semi-automatic powder delivery, unpacking, and sieving for uniformity. The system ensures the highest quality of the laser-sintered parts with the least wasted material.
Q: How do you see the use of DMLS for titanium parts in aerospace expanding, and what will drive the expansion?
A: The expanded use of laser-sintered titanium parts will follow the same route as composites are traveling now, and as aluminum did before. Aerospace companies are mainly appraising manufacturing applications for DMLS through proprietary projects and small studies. These early adopters often begin by re-creating titanium part designs previously made by machining or casting. They then discover cost reductions from the elimination of tooling, scrap, and waste. The success of these early projects will promote a gradual review of other existing parts.
At the next stage, as engineers become familiar with laser-sintering technology, they can fully implement design-driven manufacturing to achieve more innovation in new part designs. In particular, the complex geometries possible with DMLS enable the engineer to integrate several parts from a previous design generation into one, eliminating manufacturing and assembly operations and attaining still greater weight and cost reductions. Such breakthroughs will naturally lead to greater reliance on DMLS.
Q: Are there other laser-sintered metals that you see having a strong role in aerospace going forward?
A: EOS CobaltChrome MP1, which is a CoCrMo alloy, is already in use at several aerospace companies. Recently introduced nickel-based superalloys such as EOS NickelAlloy IN718 (which corresponds to Inconel 718) and NickelAlloy IN625 (which corresponds to Inconel 625) will also have a growing presence in aerospace. The recently introduced EOS Aluminum AlSi10Mg found good response-specifically in the satellite and space industry. In the long term, DMLS will likely support manufacturing with novel alloys that cannot be effectively used in conventional processes. These will open up new applications in aerospace, and in other fields as well.
Photo courtesy of Morris Technologies
EOS North America