In model-based development of vehicle powertrains, through hardware-in-the-loop (HIL) to mule integration, a new enabling design tool is emerging from recent advances in large-scale additive manufacturing (AM) that has become known as big area additive manufacturing (BAAM). AM creates components directly from a computer model and is well-suited for rapid prototyping as it is extremely flexible and enables the rapid creation of very complex geometries with minimal waste. This technology could be transformative for many sectors including automotive.
Until recently, AM processes were constrained to relatively small scales for both polymers and metals. The polymer AM processes used in these applications and studies have been limited in scale due to the constraint of needing reduced oxygen and constant heat environments. In addition, there are some issues with residual stresses during the AM process with both metals and polymers that have made larger-scale printed parts difficult to produce with precision and dimensions needed for automotive applications.
Recent advances in BAAM with polymers and composites have enabled larger scales. Conventional polymer additive systems are capable of producing workpieces in the size range of less than a few cubic feet in volume. The BAAM systems from Cincinnati Inc. now have the ability to print pieces on the order of 1000 ft3 (28.3 m3). The initial BAAM systems were the result of co-development by Oak Ridge National Laboratory (ORNL) and Lockheed Martin.
The ability to directly print a large, complex working part directly from a CAD file with these BAAM systems allows for direct generation of parts such as welded tube frames and body-in-white as opposed to conventional mule manufacturing processes. To date, however, there have only been limited efforts to use large-scale AM for vehicles. For example, in 2014, ORNL, Cincinnati and Local Motors printed a vehicle named the Strati at a trade show using a BAAM system.
This article focuses on the combined use of a BAAM system and HIL for rapid vehicle prototyping. HIL is deployed to develop the powertrain and its controls with the same accelerated time frame achieved by BAAM to create the vehicle chassis. Opportunities and challenges associated with the use of BAAM for rapid prototyping of vehicles are documented, using a printed Shelby Cobra replica as the case study.
BAAM for printed vehicle prototype
The new BAAM system is able to print polymer components at speeds 500 to 1000 times faster and 10 times larger than is possible with current industrial additive machines. These systems are able to produce very large components, including those on the order of a vehicle frame, from pellets and provide a unique resource for rapid vehicle prototyping.
The BAAM system used for this project was a polymer-extruded nozzle fitted to a multi-axis computer-aided servo system. Other key features include the use of a 0.2-in (5-mm) diameter nozzle, resulting in a 0.03-in (0.76-mm) surface variation. The BAAM system was capable of deposition rates of about 20 lb/h. In addition, the pellets used for printing were relatively inexpensive, typically under $5/lb.
A carbon-fiber-reinforced ABS plastic was used. Previous experiments had determined that a blend of carbon fiber higher than 15-20% led to significant reduction in warping out of the oven. This behavior makes the addition of carbon fiber an enabling technology for large printed workpieces and can eliminate the need for additional ovens to prevent curling.
The mechanical design of the vehicle body and frame was driven by the drivetrain, suspension and battery components already selected and by the special requirements of 3D printing with the carbon-fiber-reinforced ABS polymer. A number of different body styles were considered before settling on the classic Shelby Cobra roadster shape. Dassault Systèmes SolidWorks design software was used for all mechanical modeling.
The vehicle frame was designed specifically for 3D printing that keeps stresses below 1000 psi (6.9 MPa) and provides fasteners to prevent possible delamination between the horizontally printed layers. FEA shows that with the frame loaded to 2000 lbf (1000 lbf per side), near mid-span the stress is under 600 psi (4.1 MPa), and maximum deflection is under 0.35 in (8.9 mm). The polymer frame mass is about 260 lb (118 kg).
The polymer frame could not be designed with sufficient torsional stiffness within the envelope restrictions, so a metal torsion bar was added between the firewall and rear of the cockpit. The vehicle body consists of front, middle, and rear deck single bead (0.22-in thick) sections with multiple bonded stiffeners and internal supports. The front and rear sections are removable for maintenance access. All of these body panels were printed using the BAAM system and have a combined mass of about 430 lb (195 kg).
CAD files were transformed into STL files and input into a slicing program that transformed the 3D geometry to machine tool path commands. Printing the frame was completed in one process taking approximately 12 hours. This rate is 500 times above that normally associated with polymer AM processes. It should be noted that a newer BAAM co-developed by ORNL and Cincinnati is capable of printing larger pieces at a maximum deposition rate of 100 lb/h.
The skins took about 8 hours, and the supports 4 hours to print.
As with any layered process, BAAM exhibits anisotropic mechanical properties. The carbon fiber aligns with the tool path direction, providing manufacturing-controlled strength and stiffness. The weakest direction of the parts was between layers. To help with the integrity of the frame, drivetrain components were attached with threaded rods that put the layers in compression.
Additive Manufacturing webinar
Additive manufacturing is gaining steam in the automotive industry, and not just for prototyping parts. 3D printing processes increasingly are being evaluated for production components, with their promise of shorter development times, lower tooling costs, parts consolidation, more dramatic part shapes and sizes, among other benefits. During this free one-hour SAE Technical Webinar, scheduled for late September, experts will discuss these benefits as well as implementation challenges, detail current additive-manufacturing technologies and applications, and offer a vision for what the future could hold for 3D-printed parts in production vehicles. To register, visit www.sae.org/webcasts.
3D printing machines can't be built fast enough
As Deposition Science and Technology Group Leader at Oak Ridge National Laboratory (ORNL), Ryan Dehoff facilitates the development of additive manufacturing of components, utilizing various techniques including electron beam melting, laser metal deposition and ultrasonic additive manufacturing. He is developing processing techniques and exploring new materials via additive manufacturing to improve energy efficiency during component production, decrease material waste and improve material performance. We recently spoke with Dehoff to learn more about these innovations and industry trends.
What are some of the new applications where additive manufacturing could potentially be used?
A couple of examples I've seen in the automotive industry are things like utilization of metal powder bed systems to make injection mold tooling. That's a really big application for additive manufacturing because you don't necessarily have to certify and qualify an end-use part, but it can dramatically increase the cycle time of injection molded components and therefore lead to decreased cost of producing that component. People are also looking at utilizing additive technologies to build prototype engines that they might want to go into production in the future. So they're trying to make those engines more efficient and more cost-effective through design optimization, and additive gives them a valuable tool to be able to go through and look at those designs prior to going into the casting or production process.
Where is the 3D printing standards discussion at currently?
The standards that are being developed, I think, are a good first step in implementation of additive into different industrial applications. But I think the big challenge with additive is it may be difficult to actually qualify and certify parts with a conventional mind-set. There are a lot of different groups; I know there are several different standards organizations and they all have efforts in additive manufacturing ongoing. Some of the government standards organizations also have some fairly large efforts going on in how to certify and qualify additive. It would be good to make sure as we go through and start trying to develop those standards that it's not only the aerospace community that's involved in standards development, but it's also automotive and other industrial sectors that are also involved with that development work.
How do you see the automotive industry embracing additive manufacturing?
There's a lot going on behind the scenes that a lot of people aren't necessarily talking about. Because it does have the potential to revolutionize people's business cases. Right now, most of the additive manufacturing are niche applications, especially in the automotive industry. We have a tendency for additive parts to focus on customization. An example of the potential for customization is something like Jay Rogers from Local Motors, and what he's trying to do is make a micro-factory where you may come in and design your car. At the same time, we see that going down into mass customization for the tool and die industry where you can start getting into very low-volume production as well, which is a little bit unique and a niche market. Eventually it may be adopted well beyond that also.
Is the aerospace industry much farther along in terms of adopting additive manufacturing?
The general trend that I've seen in the industry over the past decade is that aerospace seemed to be the main driver because it had huge payoffs associated with making components lighter and making components more efficient. What we're starting to see in the additive world is that the costs of components are dropping, the technology is becoming more reliable, you can get parts fabricated faster and that's allowing different industries to adopt additive technologies like the auto industry. There are some unique things that I know Cummins has done where they've been able to increase the efficiency of their engine through additive technologies. I don't know if it's being bulk adopted for 3D printing of car frames or bumpers; that's probably not where we're going to be any time soon, but on specific applications in turbochargers, water pumps and engine housings, those types of things may be a reality sooner than we think.
What are some of the materials being considered for additive?
Holistically, most of the materials that are being developed are materials that we currently use today in castings or machine forms. I think we're limiting ourselves a little bit when we do that. What we're starting to see a general trend in is the development of new materials specifically designed with the very harsh thermal environment during the processing condition. We get a lot of thermal transients during building. Those thermal transients can be very hard on conventional materials, but if we're developing materials specifically in mind of being processed with additive we can actually make better material than we can today with other processes. In the next 10 years you'll start to see customized materials specifically for additive manufacturing.
What are some of the challenges yet to be overcome?
One of the things that I see as a unique challenge in additive manufacturing is as these technologies show promise the additive manufacturing community is growing at a tremendous rate. If you look at some of the reports by Terry Wohlers [of consulting firm Wohlers Associates], there's a huge compound annual growth associated with additive manufacturing. In some cases, we can't actually build machines fast enough. There are a lot of companies out there that are machine vendors where if you order a machine today, you may have to wait a year until that machine arrives at your factory, there's that much demand on the industry.
HIL development and integration of hardware
While the entire frame and body of this vehicle were printed, the powertrain, suspension and components were conventional. Vehicle systems simulations were used for component sizing for the Cobra's electric powertrain. Electrical consumption over multiple drive cycles was used as the baseline to determine the required energy storage system (ESS) capacity. Due to the accelerated nature of the project, components under consideration were limited to commercially available powertrain components using CAN communications.
The capabilities at ORNL's National Transportation Research Center, where vehicle modeling and testing were performed, include powertrain test cells with two 500-kW transient dynamometers suitable for Class 8 truck powertrain testing and a component test cell designed to handle smaller, individual components such as engines or traction motors. They share a 400-kW ESS and each is equipped with a dSPACE HIL real-time platform.
Following the simulation study and component selection, as components became available they were installed in the component test cell. At the same time, yet-to-be-procured vehicle components and the vehicle chassis were modeled on the HIL real-time platform.
The next phase was powertrain-in-the-loop testing, where all electric drive components were physically installed in the dynamometer cell. This included the motor, inverter, battery pack, dc-dc converter, high voltage distribution box, vehicle supervisory controller, driver interface and onboard charger. The real-time platform emulated the rest of the vehicle (transmission, driveline, wheels, chassis, driver and drive cycle) and controlled the dynamometer to subject the motor and electric drive to real-world speed and loads based on the vehicle model. The powertrain-in-the-loop provided a safe and controlled environment to design, construct, debug and validate the system.
The powertrain system was then moved out of the test cell and installed in the actual vehicle, allowing for immediate operation-i.e., the vehicle was fully operational at completion of the wiring.
The front suspension was modified from a commercially available aftermarket suspension kit. The rear suspension was modified from a rear-wheel-drive passenger vehicle. Modifications to the rear suspension included adding shock mounting points and a cradle for the traction motor and gearbox, which were integrated into space originally occupied by the rear differential. All structural modifications to both the front and rear suspension were done through welding of carbon steel. An aftermarket brake system was used.
Mounting was made directly to printed parts. The frame was cross-drilled with conventional drill bits for bolt-on pieces. Care was taken to avoid melting polymer during the cross-drill process. Fasteners were used for all of the polymer-to-metal interfaces.
In addition to mechanical attachments, bonding was used extensively on the printed Cobra. For polymer-to-polymer bonding, Valvoline Pliogrip was used. Some examples include bonding the nose piece of the printed Cobra to the hood, the hood to the fender well section, and the tailpiece to the rear deck lid section. The door skins were bonded to the A-pillar sections, as were a number of smaller support pieces. Bonding was also used for repair of partial delamination and for filling voids.
Additional surface finishing through machining, sanding, filling and polishing allowed for the body of the printed vehicle to be painted with an automotive-grade paint to near Class A finish. Tru-Design helped with this process. Finishing the vehicle consisted of sanding to remove loose fibers followed by a surface prep and coating to fill the ridges created by the printing process. The carbon-fiber-reinforced ABS plastic is a new material, requiring investigation of finishing and painting, and the effects of temperature swings and long-term use are still unknown.
The use of the HIL platform meant that for this case study the powertrain controls were completed in a parallel process to the printing and integration of the vehicle hardware.
Additive Manufacturing Symposium
Event: SAE 2017 Additive Manufacturing Symposium (AMS)
Duration: 2 days including ½ day tour
Location: Knoxville, TN
Dates: March 14-15, 2017
The SAE 2017 Additive Manufacturing Symposium presents:
- Projects and business cases in AM-solutions realized through the implementation of the technology
- Features and benefits and capabilities of 3D industrial-type printers
- Designing for 3D-how and why it is different and the implications
- Development and specifications in AM materials
- Status and activities in AM standards development
- How and why AM will affect your product development, testing, quality assurance and manufacturing
Challenges and opportunities
The ability to go from CAD directly to part with minimal or no additional machining operations holds great promise for BAAM to be used in vehicle prototyping. This case study has demonstrated that as an extension of HIL powertrain development, BAAM can accelerate design to integration for prototyping vehicles. The combination of BAAM plus the HIL development cycle demonstrates the ability to use parallel processes in vehicle prototyping as opposed to more serial processes.
The ability to use BAAM in the HIL development cycle has not been fully explored here, but the potential to integrate design iterations with printing revised components shows promise. A follow-up effort to this project has been completed in which an extended-range hybrid powertrain was developed on the framework described here. Current research is focused on integrating an advanced heat engine with additively manufactured parts into the printed car to study the generation of power for vehicles and buildings.
Industry has been involved in the AM development process, and recently Local Motors indicated an interest in using the BAAM process for low-volume production of neighborhood electric to highway vehicles in local microfactories. However, the ability to directly produce complex parts through AM processes has not been fully exploited in the vehicle space.
Carbon-fiber-reinforced ABS plastic would not be considered an engineering material for direct use in a printed vehicle for the commercial market. The material is not nearly stiff enough to be used alone in the creation of a vehicle frame. It does not take point loads well and cannot be used like steel; hence, the torsional bar system for added support. Researchers at ORNL and elsewhere are investigating other polymer formulations that would be suitable as engineering materials for large-scale printed components, including vehicles.
No documented delamination occurred after final assembly, and no significant distortion in the frame or body has been observed in more than six months of operation. This is notable considering the chassis dynamometer testing and significant on-road driving time.
This study did not focus on meeting specific vehicle performance targets or address crashworthiness, vehicle lightweighting or any other consumer acceptability issues. Studies investigating the crush performance of carbon-fiber-reinforced ABS plastic are ongoing at ORNL.
This article was adapted from: Curran, S., Chambon, P., Lind, R., Love, L. et al., “Big Area Additive Manufacturing and Hardware-in-the-Loop for Rapid Vehicle Powertrain Prototyping: A Case Study on the Development of a 3-D-Printed Shelby Cobra,” SAE Technical Paper 2016-01-0328, 2016, doi:10.4271/2016-01-0328.