This article will review those areas and the role of metal powders and industrial gases in several AM processes that involve laser metal fusion and laser metal deposition. It will also discuss how industrial gas technologies are helping to address the challenges ahead.
The popularity of 3D printing with powder metals has risen rapidly over the past five years because it provides many benefits, including shorter lead times, fewer process steps, less labor and reduced waste. Most notably, however, it is AM’s ability to produce new designs that are not possible with conventional metal fabrication methods such as casting and machining. With no need for this tooling, parts can be manufactured faster and on demand, reducing the need for extensive machine infrastructure and inventory. Quality advances in powder metal (PM) processes aided by industrial gas technology are helping to push AM and the automotive industry into fast forward.
There is a logical industry progression from prototyping to limited-scale production to mass production. Industrial gases such as nitrogen and argon and related control technology continue to help at every step. This includes improving the batch-to-batch quality of powder metals, the quality of metal 3D-printing processes and finishing operations.
Additive Manufacturing and Automotive
3D printing has been used in the automotive industry for prototyping for at least two decades. AM giant SLM Solutions announced in February that it is now producing limited quantities of spare parts for Audi’s W12 engine using its selective laser melting process. 3D printing is also now actively used in Formula 1 racing, allowing teams to test different concepts in just weeks. Last year, McLaren Racing even started producing 3D-printed spare parts on demand at trackside. So how will the automotive industry move 3D printing into mass production?
There are already signs that automotive manufacturers and suppliers are buying multiple machines to increase 3D-printing production capacity with an aim to accelerate rapid prototyping, tooling and some pre-serial parts manufacturing. Using multiple machines responds to one major drawback of AM: speed. It may take 48 hours (or longer) to make a specialized metal part. But what if that part is needed for 10,000 cars a year?
Scaling from simple prototyping to mass production is formidable for an entire passenger car, but it is not a stretch for any single part. The simple answer is to 3D print 10,000 parts using 55 to 60 printers. Advances in AM technology, of course, can further improve 3D production rates. Also, when designing for manufacturing, a key goal is to reduce assembly. A single AM part can easily replace several parts, leading to further improvements in production rates and reductions in assembly costs.
FIGURE 1. 3D-printed wing from a powdered-metal titanium alloy. Freedom of design, lightweight construction, structural stability and surface detailing are primary advantages of additive manufacturing with powder metals (courtesy of Linde).
3D printing an entire vehicle has, in fact, already been accomplished. In 2014, Local Motors of Phoenix, Ariz., produced a small electric two seater known as the Strati in collaboration with Oak Ridge National Laboratory. The company is focusing on low-volume manufacturing and now markets Olli, a co-created, self-driving shuttle.
For major automakers to mass produce standard passenger cars by 3D methods, it will require multiple design cycles and an industry transition of a decade or more to overcome practical challenges. For example, producing multiple designs from the same machine does reduce capital investment, but changeouts can add undesired process variability, and any variability can add to manufacturing downtime.
Dedicated AM processes can address this with machine redundancy at scale, as mentioned previously, but will require a substantial investment in new AM tooling at each phase. There are faster approaches and shortcuts such as high repeatability, simplified design that reduces assembly and labor costs, or perfecting an end-to-end design and then mass manufacturing the entire car by duplicating dedicated factories.
Manufacturers are already pursuing these and other paths, and the economics for more 3D-printed automotive parts and assemblies appear to be falling into place. The value of the automotive market for 3D printing was estimated at $600 million in 2016. The global market for 3D-printing technology (all markets) is expected to reach $16.8 billion by 2022, according to Global Industry Analysts.
It should be noted that this market estimate includes AM materials other than metals. Processes for powder metals typically require more energy and are more complex than for thermoplastics, but metal parts can be essential to meet structural requirements and to withstand high temperatures under the hood.
AM, of course, eliminates waste, which is important since the materials are more expensive than steel. The price of Ti-6-4, one of the most widely used grades of titanium due to its strength-to-weight ratio, stiffness and corrosion resistance, could be over $25/pound. Even aluminum, the most widely used metal for lightweighting and alloying, has climbed from a low of about $0.65/pound in the U.S. in mid-2015 to about $1/pound in 2018. The value of waste reduction climbs as AM quality improves. Zero defects would mean no rework, no recycling and no associated labor.
Technologies that Turn Powder into Repeatable Parts Quality
The ultimate goal for any 3D process is reproducibility, which drives down the cost of production and boosts reliability. In many cases, AM metal parts can exceed the performance of a cast metal part, for example, or replace multiple machined or brazed metal or plastic parts. The early targets for production in military and aerospace industries have been high-performance parts and applications that can meet multiple goals. For mass production, automotive designers must take into account the overall impact of AM processes on material and assembly costs as well as overall value, including potential for branding and customization.
PM press-and-sinter and metal injection molding (MIM) have already gained ground in the value chain of automotive component manufacturing. As mentioned, AM has already moved from rapid prototyping to an early mass-production method for special high-end car designs. AM technology has also found its way back to support the PM and MIM industries by providing easy and quick tooling and rapid prototyping to eliminate costly tool design failures. All these technologies still need to meet the demanding quality requirements of automotive manufacturers.
The PM value chain where industrial gases play a role encompasses: powder production, PM or MIM sintering, or AM followed by post heat treatment. All of these processes use industrial gases, which can be utilized for process control to achieve consistent, high-quality parts.
Like many of the components produced by PM or MIM sintering, 3D-printed parts also require some form of finishing process. The mechanical properties of parts produced by AM printers are highly dependent on an exacting manufacturing process and how the physical properties of the metal powder are impacted.
Even before the AM process, industrial gases and their enabling technologies play a fundamental role in the production and handling of the metal. A high-pressure stream of argon (Ar) or nitrogen (N2) moving at Mach speeds atomizes molten highly alloyed metals into small, uniform spheres of powder particles. This creates the “raw” material from which finished parts will be formed.
Metal powders are susceptible to oxygen and moisture, which can impact their performance during printing. Powder storage cabinets are now available that not only control the temperature and humidity with an inert gas but will purge the atmosphere whenever a cabinet door is opened and closed to return it to normal levels.
Gases are also vital for the various laser-based layering processes to “bind” the powdered-metal alloys. One such method is laser metal fusion (LMF). A high-power laser beam scans over a bed of metal powder to form the required shape. After each layer is scanned by the laser, the bed is lowered a short distance and a new layer of powder is applied.
The process takes place in a sealed chamber with a controlled-gas atmosphere, using an inert gas such as Ar or N2. Typical parts made by this method include combustor and fuel-injector prototype parts for aerospace, wheel suspension and drive-shaft fittings, and prototypes for medical devices. Active gas mixtures are currently being developed to fine-tune the properties of the manufactured part.
FIGURE 2. Lattice structure in a 3D-printed metal part can achieve multiple design goals, including lightweighting and material savings.
To further monitor and protect the metals in the printing chamber, one company developed a measuring and analysis technology developed in direct response to needs at Airbus Group Innovations. The portable unit can measure and more precisely control the level of oxygen and humidity within the printer chamber to help prevent unwanted reactions.
Another AM process is laser metal deposition (LMD). LMD uses a high-power laser beam connected to a robot or gantry system to form a melt pool on a metallic substrate fed layer-by-layer with powder. The metal powder is contained in a carrier gas and directed to the substrate through a concentric nozzle around the laser.
This solution uses Ar, helium (He) and nitrogen gases in the design, provision and installation. An active gas supply can also be mixed to satisfy project demands. LMD is used for a range of applications, including cladding and repair as well as mold-to-surface application for high-value parts such as aerospace engine components and military equipment.
FIGURE 3. Pure gaseous or liquid argon systems create the appropriate inert atmospheres for laser metal fusion (LMF), which is known by many as metal selective laser sintering, metal laser melting, direct-metal printing and direct-metal laser sintering (courtesy of the MTC Catapult, U.K.).
Finishing Treatments for Perfect Parts
Hot isostatic pressing (HIP) is an advanced material heat-treatment process that utilizes high-temperature and high-pressure conditions to eliminate internal porosity and voids within cast-metal materials and components. This helps ensure the integrity of manufactured parts by improving mechanical properties and fatigue performance. This process optimizes reliability and service life of critical high-performance products, including automotive engine parts, components for gas turbines, turbo charter wheels, aerospace structural parts, medical implants and prosthetics. High-purity argon is typically used to provide the inert atmosphere necessary to prevent chemical reactions that might adversely affect the materials being treated.
FIGURE 4. Aluminum- and titanium-alloy powders are highly sensitive to oxygen and humidity. The measuring and analysis technology precisely monitors and controls these levels during 3D printing for reproducible quality.
Once a component has been produced, it must go through a final step of cleaning to ensure it is ready for market. Parts that have been printed have some rough surfacing and flashing that require smoothing before they are ready for use, and technology to solve this is integral to the entire AM process.
A water-free cleaning technology has been specifically designed for industrial surface finishing. The technology produces dry-ice particles on demand for cleaning. By feeding liquid carbon dioxide (CO2) into a specially designed snow chamber, solid dry-ice particles are created and shot onto the surface of the component using compressed air for an effective cleaning process to smooth out edges of a finished piece.
For 3D metal-printed parts, another cleaning system adds abrasives to the high-pressure flow of CO2 particles. The blasting process is ideal for removing surface oxides and unfused metal powders.
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4. “Autonomous for all of us,” Local Motors, https://localmotors.com/, Accessed 2/23/2018
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7. Aluminum pricing, http://www.infomine.com/investment/metal-prices/aluminum/, Accessed 2/21/18
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