3D printing is becoming ubiquitous and is poised to affect flow control products. 3D printing, or additive manufacturing (AM), traces its roots back to 2D inkjet printing in the 1970s. Polymer 3D printing began in the late 1980s, with metal 3D printing coming into play in the late 1990s. By 2006, both aluminum and titanium could be printed with stainless steel, cobalt and nickel alloys, as well as tungsten coming online a few years later.1
Various types of 3D metal printing are available and include selective laser melting (SLM) or direct metal laser sintering (DMLS), which uses metal powder as a starting material. Essentially, a laser selectively melts a 2D design onto a flattened bed of powder before a new powder layer is pushed on top and the process is repeated. Directed energy deposition (DED) or laser metal deposition (LMD) is a powder-fed system that sends a highly concentrated metal powder stream through an extruder, which is immediately met with a laser at the surface of the part. DED is a highly accurate metal 3D printing process, which can also be used to repair broken parts. Electron-beam melting (EBM) is also used for 3D printing in which the raw material can be either metal powder, filaments, rod or wire. These printing methods can be done under vacuum to reduce porosity, oxidation and other defects. Emerging metal printing methods include the use of controlled electroplating and the printing of metal powder in a liquid polymer suspension.
3D metal printing has several advantages over traditional machining. Material scrap is lower since it is a direct print additive process versus a subtractive machining or etching process. By printing a single piece instead of using assembly steps, the manufacturing process can be simplified. Metal and plastic 3D printing is ideal for fast prototyping and low-volume, high-cost products or for the replacement of legacy products. Features such as weight-reducing internal lattices can also be formed using AM techniques.
The aerospace industry has trailblazed the tooling, materials, metrology and quality standards for 3D-printed metal that can be leveraged by the chemical processing and automotive industries. Many of the additive manufacturing alloys developed for aerospace applications are corrosion-resistant and are already in use in the chemical processing markets.
Metal additive manufacturing does not stop after the 3D printing step. For applications demanding reliability and high quality, post-processing will be required. Post-processing of printed metal parts can start immediately after the printing step. For laser and e-beam fabrication metal, the product is in a state similar to a welded metal part. Significant stress is found in the metal structure. As in the case of welding, an anneal step can reduce some of this built-in stress, which reduces the likelihood of warpage and cracking. The anneal, often in an inert atmosphere or vacuum, can be done before or after the part is removed from the build plate. Hot isostatic pressure is also used in some aerospace parts to improve fatigue life of the AM metal parts. A band saw or electrical discharge machining (EDM) can be used to remove the part from the build plate. Trapped metal powder may need to be shaken out of cavities.
Next, the relatively, as-printed matte surface finish can be improved with machining, bead blasting or tumbling. Machining and surface treatments are also common to ensure dimensional specification and smooth outer surfaces. After the preceding machining and anneal steps, a cleaning step is used to remove any powder or residue.
Finally, like conventional cast and machined parts, metrology is important to ensure quality. Metrology is required to make sure the part meets the specifications and is free of voids and cracks. Quality tests for aerospace and medical devices are preferably nondestructive and can include dye-penetrating tests, ultrasonic scanning, X-ray and computed tomography (CT) scanning. For some applications, witness coupons, powder chemistry and microstructure analysis are required. Internal cavities, channels and lattice structures can be a complicating factor in final inspection.
Additive manufacturing versus casting
What differentiates AM from machining and casting of metals? One benefit of 3D printing, like investment casting, is the ability to combine multiple product elements together into a single piece with minimal wasted materials. 3D printing is ideal for complex parts, low-volume applications and where shorter lead times are required. New methods of printing metal are also adding minimum feature size as well as internal lattices and offer advantages for additive manufacturing over other forming methods like casting and machining.
3D printing standards for quality
To enter mainstream chemical processing applications, AM products and processes need to pass the same quality certification processes that other metal-forming methods have in the past. The aerospace industry has pushed this technology into compliance with existing standards and is creating new AM-specific standards. Both the International Organization for Standardization (ISO) and ASTM have generated metal 3D printing standards. Additive manufacture is defined by ASTM: F2792 – 12a (2012) as the “process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining.” ASTM F3301 – 18 is a post-AM processing standard.
Table 1 shows examples of AM standards for titanium and nickel alloys fabricated using the DMLS powder-bed laser method. While various corrosion-resistant stainless steels can be 3D printed with laser and e-beam systems, the same approach to the standards and certification process will be required for the adoption of AM-manufactured flow and pressure meters and related corrosion-resistant alloys.
Pressure and flow sensors
Figure 1a shows how a stainless steel pressure sensor combining the sensor element and fluidic package with its male pipe thread fitting and hex nut can be 3D-printed. After post-print polishing of the diaphragm, the device can be used for fiber optic pressure sensing. Figure 1b shows that a patterned thin or thick film circuit can be fabricated on a flat, polished steel surface, in this case, a piezoresistive Wheatstone bridge for a pressure sensor capable of operating in chemically aggressive applications.2 The fluid under test is exposed only to the opposite side of the metal diaphragm, keeping the electrical sensing circuits safe.
Industrial resonant Coriolis mass flow sensors are traditionally made from stainless steel or titanium tubing/pipe and require multiple welding steps to fully assemble the final product. 3D printing offers a method of reducing the part count of stainless or titanium tube-based sensors by combining the resonant tube with some of the packaging elements. A wide array of small (millimeter-long tubes) or larger macro size (meter-long tubes) resonating flow sensors can be fabricated using this technique.
Figure 2, on the left side, shows a small titanium 3D-printed resonant sensor, which combines the two resonating tubes with the package frame, buried U-shaped return flow channel and female threaded fluid fittings in a single piece. These resonant devices can measure mass flow rate, fluid density and binary chemical concentration via density. Figure 2, on the right side, shows a single tube torsional mode balance Coriolis mass flowmeter prototype made with laser-based AM. Figure 2 demonstrates that flange or threaded fittings can be combined along with the resonating tubes and other structural housing and mounting elements into a single printed piece.
3D printing allows new concepts to be quickly evaluated without the delays and costs often encountered in molded products for both plastic molding and metal casting. The size of a 3D-printed Coriolis mass flowmeter would only be limited by the printer, and a wide variety of large and micro printing systems have been and are being developed and commercialized.
Size capabilities of 3D metal printing
While most 3D-printed metal devices are in the 3 to 15 centimeter range, on the large end of the scale, Lockheed-Martin recently printed the domed ends of a titanium satellite fuel tank with e-beam printing.3 The metal domes had a diameter of 1.16 meter and reduced metal scrap by 80 percent more than machining. Demonstrations of metal printing of 2-meter-long parts have also been demonstrated using the powder-fed laser melt EFESTO printer.
The smaller 3D-printed metal systems under development are leveraging years of work in the 1980s on metal inkjet printing for hybrid ceramic circuits4, metal printing on porcelain-coated steel substrates5, atomic force microscopy (AFM) and electroplating-based micromachining, often called LIGA or Lithographie, Galvanoformung, Abformung in German.6 LIGA involves the electroplating of metal structures in a photoresist mold and has been used to make fuel injector nozzles and sensors such as accelerometers and gyroscopes in the automotive field. The smallest 3D-printed metal structures use metal-containing solutions such as metallo-organics in a solvent or metal sulfates in an acidic plating solution. Caltech has printed submicron-sized nickel lattices7, and the FluidFM system has been able to print copper cantilevers and probes that are 1 to 100 microns in width.8 Further work is needed before these methods are ready to print micro flowmeters or pressure sensors with corrosion-resistant alloys. A wide AM metal feature size range of 1 micrometer to 2 meters allows the majority of chemical process and automotive flow sensor sizes and flow rates to be covered with this promising manufacturing method.
Additive manufacturing of corrosion-resistant metals has developed to the point where pressure and flow sensors for process control can begin to use this new technique. The materials used for 3D printing are only limited by what metal or alloy can be either powdered or formed into wire, which opens up for chemical process and automotive applications in addition to aerospace and medical applications already in use. Quality standards have been developed for many of the 3D-metal printing alloys and processes, and emerging methods have demonstrated printed metal parts in sizes from microns to meters, enabling a range of products.
1. E. Matias, B. Rao, “3D Printing: On its Historical Evolution and the Implications for Business,” 2015 Proceedings PICMET, pp. 551-558, (2015).
2. H. Shioiri et al., Method for Production of Silicon Thin Film Piezoresistive Devices,” US Patent 4,657,775, (1987).
3. T. Vialva,” Lockheed Martin produces its largest 3D printed parts for space,” 3D Printing Industry, July (2018). https://3dprintingindustry.com/news/lockheed-martin-produces-its-largest-3d-printed-parts-for-space-136207/.
4. R. Vest, E. Tweedle, R. Buchanan, “Ink Jet Printing of Hybrid Circuits,” International Journal of Microcircuits and Electronic Packaging 6(1), pp. 261-267, January (1983).
5. D. Sparks and G. Vest, “Copper Films from Aqueous Solutions of Copper Nitrate Trihydrate,” Thin Solid Films, vol. 200, p. 77, (1991).
6. D. Sparks, M. Chia, G.Q. Jiang, “Cyclic Fatigue and Creep of Electroformed Micromachines,” Sensors and Actuators A, 95(1), pp. 61-68, (2001).
7. R. Perkins, “New Process allows 3-D Printing of Nanoscale Metal Structures,” Caltech, Feb. (2018). http://www.caltech.edu/news/new-process-allows-3-d-printing-nanoscale-metal-structures-81373.
8. L. Hirt et al., “Template-Free 3D Microprinting of Metals Using a Force-Controlled Nanopipette for Layer-by-Layer Electrodeposition,” Adv Mater. 28(12), pp. 2311-2315, March (2016).
Doug Sparks is the president of M2N Technologies LLC and has worked for many years with pressure and Coriolis mass flow sensors as well as MEMS sensors such as gyroscopes and accelerometers. Most recently, he was the CTO at Hanking Electronics, founded a microsensor packaging company called NanoGetters and worked in automotive sensors with Delphi. Sparks holds a Ph.D. in materials engineering from Purdue University.