Ford has cooperated with Magna International to explore a technical route to mass-produce the front frame of light vehicles in large quantities using carbon fiber composite materials as much as possible to meet the needs of a high-volume (200,000 vehicles / year) car. And revealed the limitations, challenges and technical solutions adopted.
By co-molding the chopped fiber SMC and the non-crimped fabric SMC, Ford and Magna explored the production methods for mass-producing the front sub-frame of automobile chassis.
Mass-produced CFRP front subframe: Ford and Magna International cooperated to explore a way to meet the needs of a high-volume (200,000 vehicles / year) car and use as much carbon fiber composite materials as possible for mass production. The technical route of the front subframe of the quality car, and revealed the limitations, challenges and technical solutions adopted (picture from Magna International)
The upper and lower parts of the formed box section: the topologically optimized hollow box section design ensures that the least material is used to provide the rigidity required by the subframe. The upper and lower parts are formed separately, and then they are connected by adhesive and rivets (picture from Magna International)
The stainless steel sleeve is overmolded into the SMC subframe, realizing 4 body mounting connections and two steering support connections (picture from Magna International)
In order to resist the high load in the high load area (such as where the upper and lower parts are riveted and bonded together), it is also important for the co-molding of the SMC patch without curling fabric (picture from Magna International)
For decades, carbon fiber reinforced plastic (CFRP) body frame / chassis components have been used in high-end sports cars and racing cars. In recent years, CFRP's performance and lightweight features have expanded its application to higher-volume models, including BMW's i3, i8 and 7 series, and Audi's R8 and A8 luxury cars. However, most of these components, including roof rails and beams, lower side sills, B-pillars and rear walls, use continuous fibers.
In a joint research project, Ford Motor Company and Magna International developed a carbon fiber composite subframe made of SMC through the combined application of continuous fiber SMC and chopped fiber SMC. This application is very innovative, because the subframe is located at the front of the car to support the engine and chassis components, including the steering gear and the lower control arm used to fix the wheels, so it needs to withstand high loads.
"There are dozens of engineers participating in the project." Brian Krull, Global Innovation Director of Magna Exteriors, said, "We have structural engineers, manufacturing engineers, test engineers, product engineers and computer-aided engineering (CAE) experts, and have Support from various departments of our customer, Ford, including vehicle modeling and simulation. "In addition, the Magna Composites Center of Excellence in Toronto also supported this project. "Just like what we did at Cosma's body and chassis team." Gabriel Cordoba, Global Director of Cosma International R & D, added.
"We want to explore the challenges of using CFRP components in high-volume vehicles." Ford Motor Company technical director David Wagner recalled, "Our goal is to use as much CFRP structure as possible, while adopting the ability to adapt to large quantities The manufacturing method required for production (200,000 vehicles / year), see how much weight can be reduced, and what are the limitations and challenges. "
From preliminary discussion to completion of the design, the project took more than a year. At the end of 2017, the prototype subframe was delivered to Ford and entered the testing phase in 2018.
Redefine the design framework
Based on the stamped steel subframe of Ford Fusion models. "Ford provided design space for the project-vehicle-level design input, and held a design meeting once a week." Wagner said.
Cordoba explained: "Then, Cosma adopted that containment space and began to explore the weight loss that can be achieved. How can we change the design to fit that space while still meeting the rigidity, strength and durability requirements?"
Rigidity and corresponding containment space are early challenges. "When moving from steel to composite materials, the modulus is usually reduced." Wagner said, "Compared with the benchmark size, we defined how much the new design can change, and shared the initial topology optimization with Magna . "
Topology optimization (TO) is a CAE analysis that optimizes the placement of materials in a given design space, including loads, boundary conditions, and constraints, and aims to maximize performance and minimize weight. "We started with our own topology optimization to understand the critical load path." Cordoba said that the loads involved at the connection points of the subframe include: control arm and engine load, road load, engine torque distortion and collision requirements.
"We have to see how our stiffness differs from steel, and the performance we want," Krull recalled. Since the composite material provides a variety of choices of resin, fiber, and fiber orientation, its performance can be customized. "But this is much more complicated than just inserting properties into steel properties." He pointed out.
"Plugging in properties" refers to the input of material data into the software tool. "We used all standard software, including Nastran (MSC Software, Newport Beach, California) for static analysis, Abaqus (Dassault Systèmes, Waltham, Mass.) For nonlinear static analysis, and overlay coverage. Simulated Fibersim (Siemens PLC, Waltham, Massachusetts, USA) and HyperWorks (Altair Engineering, Troy, Michigan) for topology optimization to generate finite element models for analyzing different load conditions.
The multiple frame analysis for different loads, boundary conditions and materials has improved the subframe model. Krull said they evaluated many composite materials. "We started to think of compression molding as a process that can achieve mass production." He added, "SMC is very suitable for this part, and we have developed carbon fiber SMC in-house."
"Topology optimization produced a box-shaped cross-section as the best solution." Cordoba recalled, "When we studied the SMC compression molding process, the design that emerged was to use two parts to do this." Therefore, the The frame consists of separately formed upper and lower halves, which are joined together by polyurethane structural adhesive and rivets.
Co-molding two types of SMC
Magna used its glass fiber SMC development experience and a trial production line it acquired to develop carbon fiber SMC. "We developed proprietary technology to handle this carbon fiber for SMC, and found that we can also use this line to produce non-crimp fabric (NCF)." Krull explained. This material is similar to prepreg and is infiltrated before molding, but the unique feature is that it is produced using the same resin on this SMC production line, so it is called NCF SMC. "When we were improving the design analysis of the subframe, we used the performance test results of these materials."
Using Zoltek's 50K tow chopped carbon fiber and Ashland internally modified vinyl ester resin, Magna formulated EpicBlend SMC. This vinyl ester resin has good adhesion to carbon fiber and can wet it well. This will be used for local reinforcement and co-molding with 6 layers of NCF SMC laid at 0 ° / 90 °. The NCF SMC is also made by Magna, which uses the same vinyl ester resin and NCF fabric provided by Zoltek, and is cut into small patches before molding.
The combination of short fiber SMC and long fiber SMC is the key to the design, but it is also a real challenge. Short fiber SMC can achieve complex shapes and overmolding of steel inserts for engines and steering brackets, while NCF patches can withstand high loads at the engine and lower control arm fixing. Compared with stamped steel subframes, the combined use of these two SMC materials has reduced weight by 9.3 kg. "It took a lot of effort to develop this 0 ° / 90 ° SMC patch for co-molding." Krull recalled, noting that the flow of this chopped fiber SMC was achieved during the molding process to ensure no dry spots or Issues that need attention when integrating NCF patches without other quality issues.
Bolted
Bolting is also a problem. "In the place where the control arm and the steering gear are fixed to the subframe with bolts, the point load of the composite material is very high, reaching 80 ~ 100kN." Wagner said. Where four vehicle body mounting connections and two steering brackets are connected, stainless steel sleeves need to be overmolded into composite parts. "For fixation, each body mounting bush is pressed into the sleeve to achieve an interference fit." Krull explained, "Stress is carried into the molded part through the circumference of the sleeve. We observed the force when inserting the bush And entered these into the computer design model. We were also looking for cracks in the composite material during the mechanical tests, but we saw nothing. "
Wagner said that the bolts used are large, usually M12 and larger, and must have tight position tolerance, diameter tolerance and angle tolerance. Once the subframe is formed and assembled, the necessary post-processing is required.
Test and teamwork
In 2018, the prototype subframe was tested. Ford has completed a series of corrosion tests at the component level and the vehicle level to explore various corrosion inhibition measures. Durability tests at the component and vehicle levels include stone impact tests, bolt load retention tests, and high temperature cycle tests. The tests on components include high cycle fatigue test, joint overload test, vibration and safety test.
"We tested the prototype subframe ourselves." Krull said, "This is a very large and complex part made of SMC, made by a co-molding process. In order to understand where we might use these materials, We are studying how to change the SMC flow and fiber arrangement in design, molding. "
"We want to understand what are the cost drivers of such CFRP-intensive parts," Wagner said. "Secondary processing is one of the most important costs. We need more creative thinking to eliminate the processing of parts after molding." "He also said that one of the biggest challenges is to develop the absolute material properties of molded parts for design. "We spend a lot of time characterizing materials for design analysis." He explained.
Cordoba said that the biggest achievement is teamwork. "Not only the cooperation with our customer Ford, but also the cooperation of the global team."
Wagner agrees: "This is the best example of how we push our suppliers and ourselves to use advanced lightweight materials."