Metal Powder 3D-Printing, Well Explained

 

If your boss told you it was your job to explain metal additive manufacturing (AM/3D printing) to a new mechanical designer, test engineer or material scientist, where would you go for resources? Vendor websites have in-depth information but with an understandably narrow focus. Given that more than 25 system manufacturers offer slightly, or greatly differing ways to convert powdered metals into layer-built parts, it’s hard to get a handle on them all. Beyond that, it’s helpful to understand how each approach compares to traditional processes such as metal injection molding (MIMing) and hot isostatic pressing (HIPing), and how post-processing plays a role.

YouTube clips are great for showing 3D printers in action, but a well-crafted summary serves as a starting-point, too. Striking a good balance of overview and technical details is the recently issued publication, “Introduction to Additive Manufacturing Technology: A Guide for Designers and Engineers.” [Link to https://www.epma.com/epma-free-publications ]

Now in its second edition, this independent report comes from the European Powder Metallurgy Association (EPMA); members of an EPMA working group, the European Addition Manufacturing Group (EuroAM), have condensed information and images from an impressive ninety companies and organizations into this succinct 56-page document. The content is so fresh, even experienced AM engineers will probably learn something new.

Metal AM Processes

Addressing metal-only technologies, the authors do a good job presenting the essential similarities and differences between laser-beam melting, electron-beam melting, binder-jetting and directed energy deposition systems, citing examples from such manufacturers as Arcam, Concept Laser, Digital Metal, EOS, ExOne, Hybrid Manufacturing Technologies, Renishaw, SLM Solutions, MAZAK, Optomec, Trumpf and more.

The report also presents the bigger picture that newcomers may not immediately consider: all the steps involved in the full manufacturing process, many of which are the same steps as in classic MIM and HIP productions. Successful metal AM requires 3D-CAD modeling, the creation of 3D-printable files (the typical format still being STL), the repair of files to close holes and clarify facet layouts, preparation of the files (part orientation, support design, file slicing), the actual layered-manufacturing process, and a variety of possible post-build steps.

Post-processing in particular can be costly and time-consuming, and each of the previous steps greatly influences this final phase of production. It may include manual or automated removal of excess powder, removal of supports, heat treatment and surface finishing. Again, some of these steps would be needed even for traditional metal-powder part production; however, with AM, there may also be unexpected benefits.

For example, studies have shown that HIPing laser-melted parts can not only make their fatigue characteristics out-perform those for similar cast parts but also put them on a par with those of wrought/annealed parts.

Powder Production: Processes and Characteristics

Given the focus and expertise of the report’s authors, it’s no surprise the Guide includes ten pages covering powder manufacturing processes (gas atomization, vacuum-based gas atomization, and plasma and other approaches) and powder characterization. Since each AM process depends on having the right type of powder, understanding how to specify such parameters as maximum particle-diameter, particle size distribution (PSD), chemical composition and trace elements is important.

The guide notes how chemical composition will influence various mechanical properties of the final part, and explains how PSD influences powder flowability, surface roughness and more. There’s a short section about typical defects to be controlled and minimized, and a list of the applicable standards for determining density and flow rate of powders and granules.

Be sure to note the diagram plotting hardness and yield strength for a number of commonly used AM metal powders, and the discussion of defects that may occur in the final part.

Design Guidelines and Case Studies

EuroAM chose to focus on laser-beam melting AM for the section on design guidelines, since this technology is both widely used and has been employed for more than 20 years. The report explains and illustrates the importance of such attributes as minimum wall thickness, minimum hole-diameter, maximum arch radius and minimum gap distance, as well as the influence of part orientation and support structures. It then examines the larger yet related topic of functional design optimization for AM parts, with its associated trade-offs.

Since case studies always add spice to generalized technical discussions, you’ll be pleased to browse the 55 short summaries (with images) highlighting parts produced for the aerospace, energy, medical, industrial, automotive/car racing and consumer fields.

EuroAM states its objectives are fourfold:

• To increase the awareness of Additive Manufacturing (AM) technology, with a special focus on metal powder based products.
• To enable the benefits of joint action, for example through research programs, workshops, benchmarking and knowledge exchange.
• To improve understanding of the benefits of metal-based AM technology by end-users, designers, mechanical engineers, metallurgists and students.
• To assist in the development of International standards for the AM sector.

This highly readable Guide goes a long way toward supporting this mission.