"3D printing" or the more descriptive term - additive manufacturing (AM) - has received much publicity in recent years. What is this technology and what are some of its practical features? This blog is a summary of AM and its on-going development to make actual service components using metal alloys. Presented are selected portions of information provided in the two references cited.

Conventional manufacturing methods such as machining derive the desired, final product by removing initial material, either externally or internally, from an initial generally shapeless mass of metal. Often multiple manufacturing and assembly steps are required. This is a subtraction process in which there may be considerable material waste often with multiple follow-on steps after the initial process. Further it may not be possible to create desired internal features using external tools.

AM, just as the name says, is an additive manufacturing process. The raw material used is built up layer by layer to form the finished or almost finished component. Raw material is melted using heat energy generated by a laser, a beam of electrons or a plasma arc as either the energy source or the raw material moves inside a specialized AM machine. First a three-dimensional drawing, via computer aided design (CAD), of the desired component is created. This is then used to generate a computer program that provides precise instructions for automated spatial movements between the melted material being deposited and the inner surface of the AM machine on which the component is being built.

The traditional raw materials used for AM were plastics. This form of AM has been on-going for over twenty-five years and continues. One of the primary uses of the resulting components is to quickly provide 3D prototypes of small devices or parts that provide relatively inexpensive models (relative to prototypes made by conventional manufacturing methods) for study and analysis during design. Mechanical properties of these plastic models and the rate at which they can be built are irrelevant to their intended use. However, over approximately the last seven to five years there have been considerable high-profile efforts to apply AM using metallic feed materials. In 2012 the Obama administration formed a research and educational organization funded by industrial memberships to promote AM in the U.S. That organization, now known as America Makes, can provide the reader with more information through its website, www.americamakes.us.

AM metallic feed materials typically are powders that have specific requirements in terms of not just chemical composition but also in precise granular sizes and consistency. Some metal alloys in wire form are also used but currently this form is less widely employed worldwide than powders. The number of suitable metallic alloys for AM is limited but presently includes particular titanium, tool steels, stainless steels and nickel-based alloys. A specific titanium alloy, i.e., Ti -6Al-4V, has been most widely studied in the development of AM using metals. This is because there are strong economic incentives to use this alloy in specific components, e.g., fuel nozzles for gas turbine engines in aircraft. The incentives include the high direct material cost of the alloy, the need for making relatively small numbers of components and the high cost of generating needed geometric features with conventional manufacturing methods.

The benefits of using AM with a metal alloy as the feed material versus manufacture by conventional methods include the following: 1) AM creates little or no material waste, especially important when expensive alloys are required, 2) It can generate intricate geometric features and internal passages in a single component that are impossible or would require making separate parts that must be made and then assembled via traditional methods, 3) AM often can generate parts that may not require any further processing such as final machining, 4) It often can generate components with mechanical properties similar to or better than those found in products made by the traditional manufacturing methods, 5) AM provides a significant economic advantage when just a small number of very specialized parts are needed whereas conventional methods are usually viable only for large runs of simpler parts.

However, the competitive disadvantages of AM with metallic raw materials include the following: 1) AM alloy raw materials are presently very expensive - powders and wire - and few alloys are available, 2) AM processes and machines presently are not qualified and standardized and thus consistency in the mechanical properties of components produced even by the same process or machine can vary. ASTM Committee F42 is working to address these issues, 3) AM presently is limited in the maximum size of components that can be made, 4) AM produces anisotropy (values of mechanical properties differ depending on direction in the finished part) because of thermally generated metallurgical variations that occur during the melting and solidifying of layer upon layer of the alloy in the build direction. Specifically mechanical properties (including fatigue strength) of the finished product are inferior in the "Z" direction, i.e., parallel to the build direction as melted alloy is deposited vertically, 5) AM will generate the best economic advantage over traditional methods when its rate of production, called build volume per unit of time increases considerably from the currently available levels. However, as the build rate increases the quality control on required features currently becomes worse.

Thus as with most developing technologies, AM using metallic raw materials has both advantages and shortcomings. Presently AM machines that can apply metallic alloys have only about 10% of the total machine tool market worldwide. Technical challenges include decreasing the cost and increasing the number of suitable metallic alloys available, increasing rates of production (build rate) while maintaining suitable quality control, defining verified standards for the different machines and processes, increasing the sizes of parts that can be made and providing clear economic rationales that show metallic AM is competitive with traditional manufacturing methods.

References

Frazier, William E., "Metal Additive Manufacturing: A Review", Journal of Materials Engineering and Performance, June 2014, pp. 1917-1928

Roland Berger Strategy Consultants, "Additive Manufacturing - A Game Changer for the Manufacturing Industry?" Munich, November 2013 (find this via Google search at Roland Berger additive manufacturing)