Big Parts, Big Energy Goals

Additive manufacturing (AM) offers fresh approaches that could put wind in the sails of America's clean energy sector, which must boost the production of very large components for wind, nuclear, and hydro-power equipment.

Aggressive renewable energy targets are expanding clean energy markets, but currently the demand outstrips domestic manufacturing capabilities.

Addressing these gaps gains new urgency as the United States seeks to meet 35% of its electricity needs through wind energy by 2050—over three and a half times what wind contributes today.

Tackling this daunting supply chain challenge requires capabilities that currently don't exist, and AM techniques—most often considered for small, intricate parts—may be the key to unlocking U.S. manufacturing potential for these massive metal components. In order to make this a reality for wind turbines, further strides are needed to increase AM metal deposition rates and reduce the printed material cost.

Industrial-scale castings, such as steel castings exceeding 10 tons, are a choke point in scaling up production of wind turbine parts including rotor hubs, bed plates, and support frames. The scale is vast and growing in both size and weight as the industry continues to expand into offshore wind turbines.

The labor costs involved with sand-casting large metal components drove U.S. manufacturers to start purchasing them from foreign sources years ago.

Only one American foundry remains capable of handling the larger parts needed for offshore wind, with limited U.S. capacity to machine them into their final shape. Currently, the lead time is six months to more than a year for procuring critical large metallic components. Shipping them from abroad creates a large carbon footprint, in addition to being expensive and slow. Reliance on foreign components also creates the potential for a single point of failure in the American wind-energy supply chain.

An alternative is to additively manufacture these large parts, then finish them using automated machine tools guided by computerized manufacturing software. The benefits are clear: AM offers more design flexibility and complexity than traditional casting, and topology optimization strategies enabled by 3D printing can offer significant weight reduction.

Multiaxis printers can rotate a part to print different portions and reach different angles, avoiding gravity distortion problems that have limited designs in the past. By combining multiaxis, out-of-plane printing with multiple robotic deposition heads, the range of geometries that can be produced is dramatically expanded.

Unlike conventional casting, this type of 3D printing allows the creation of complex internal features such as lattice structures, integrated hydraulic lines, and electrical wire pathways. It also reduces printing time by breaking up fabrication among multiple systems working simultaneously on the same object.

An AM system called MedUSA at the Department of Energy's Manufacturing Demonstration Facility (MDF) at Oak Ridge National Laboratory (ORNL) utilizes three robots, each with six degrees of freedom of movement.

MedUSA can print more than 54 pounds of metal an hour or print with different materials simultaneously. The parts it produces require minimal custom tooling compared with traditionally manufactured parts, although finishing is usually still needed.

A large machine tool is being installed soon to work in tandem with MedUSA, finishing the printed parts to fit exact design tolerances. Very few facilities in the world can offer this combination of capabilities for large components. Because so much of the process is automated, the labor costs are far less than for traditional casting.

"Our research is looking at how you mesh additive manufacturing with the metal finishing step, so they work together in an efficient way," said ORNL researcher Joshua Vaughan. "How do you print parts so they are easily finished for an end application, and what advantages does that offer?"

Several teams are working to answer these questions, and they are designing flexible control systems that enable the process to work at multiple scales and in different settings. They are also examining how scale-up affects the properties of the printed material. And experiments are underway to demonstrate that combining AM to near-final shape with machining can produce a component that is the same quality as a traditionally cast and finished part.

Other researchers have printed smaller metal parts such as a skeleton node, which serves as a load-bearing joint between the structural beams inside the nacelle of a wind turbine. A new internal truss structure promises to reduce the node's mass and print time while maintaining the same material strength.

For AM methods to compete with foreign casting, both component quality and price must be comparable. AM costs are somewhat offset by the savings in labor and transportation. Although the U.S. has domestic sources for AM equipment, metal powders, and printing wire, the metals used in 3D printing are expensive compared to those used in the wind industry.

Because of this, research needs to target lower-cost materials. A promising new technique being explored by ORNL is the use of an electroslag system with an affordable, commercial metal strip feedstock to print large renewable energy castings. This system can achieve a build rate approaching 110 lbs an hour per print head.

For large wind components other than metal castings, transportation remains a challenge that can be offset by automated printing onsite. ORNL, in partnership with General Electric, demonstrated the feasibility of 3D printing large concrete towers in the field. Adding height to these structures provides greater wind access but requires an even larger base. These pieces are not only heavy, but logistically difficult to move under bridges or through tunnels. AM offers mobile, onsite possibilities even in far-flung locations.

While AM printing of large, heavy metal castings and concrete towers is at an earlier stage of development, more strides have been made in creating lighter composite parts for the wind sector. Large vacuum infusion molds were printed at the MDF five years ago for producing turbine blades.

A more recent innovation uses AM to improve the molds by coextruding wire with the polymer matrix. Electric current passing through the wire generates resistive heat, allowing resin to cure inside the blade mold. This approach could replace a labor-intensive step in today's production process: A team of technicians manually wrapping and attaching wire woven into a pattern over the back surface of a mold that is more than 160-ft long.

AM can be used to improve both blade production and design. Novel approaches offer the potential to improve structural performance and reduce weight in some components. For example, ORNL demonstrated additive printing of a 10-ft, honeycomb-shaped interior structure for a small wind blade.

The next generation of those efforts, underway through a partnership between ORNL and GE, is creating a highly automated process using fully recyclable thermoplastic composites. It can print a structural reinforcement coupled to a thermoplastic composite skin to produce wind turbine blade tips that are 40-ft long. Materials, speed, and size are being constantly improved in the lab.

"We want things to be precise, fast, and reliable," said Dan Coughlin, leader of industrial collaborations in the Manufacturing Science Division at ORNL.

"We’re taking AM out of its usual box and turning it into something that can print parts larger than the printer." ORNL researchers will continue to work on these challenges with industrial partners in the MDF, a 100,000-sq-ft (9,290-sq-m) facility for the development of integrated capabilities in materials, software, and systems. The Department of Energy user facility is a springboard for moving manufacturing innovations from development to deployment.

Many new AM technologies developed for wind, particularly related to hybrid AM/casting and finishing, could also be applied to large metal parts for nuclear reactors or hydro-power plants. Strides in research are vital for AM to seize this historic opportunity to bring more manufacturing back to the U.S. while helping to slow climate change.

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