Reducing the weight of vehicles, known as "lightweighting," has become increasingly popular in the automotive industry, supported by major manufacturers such as Ford and Tesla. Driven by the need to improve fuel economy and reduce carbon emissions, automakers are discovering that lightweighting vehicles through the use of advanced materials triggers new applications of aluminum extrusions.
Aluminum's characteristics make it an ideal candidate to replace heavier materials in vehicles. These qualities include its high strength stiffness-to-weight ratio; ability to be machined, formed and shaped; non-sparking capability so it can be used near flammable substances; and resistance to corrosion. In addition, the thermal management of vehicles is serving to expand aluminum extrusions. This reflects the metal's facility to conduct heat well, which is a crucial characteristic for use in electric vehicles that present a thermal management challenge.
Electric vehicle's benefits have become well known as major manufacturers introduce more electric models. And stringent new fuel-economy standards will prompt automakers to produce increasing numbers of electric cars ahead. Manufacturers are concerned especially by the weight of batteries in plug-in hybrid vehicles as they find ways to pare the weight of the other parts of the vehicle.
Electric-drive vehicles (EDVs) rely heavily on power electronics to distribute the proper amount and type of power into and out of the appropriate subsystem at the right time. Power electronic components include inverters, converters and chargers. Power electronics also are responsible for determining the exact nature and timing of the current and voltage waveforms to the motor.
The National Renewable Energy Laboratory (NREL) defines electric motor thermal management as the multifaceted interaction of motor operation conditions, heat load distribution, material temperature limitations, passive thermal heat spreading and active convective cooling. Industry partners seek to better understand heat transfer in electric motors in efforts to develop motors that reliably operate with lower size and cost.
Just as the battery in a mobile device or laptop becomes hot to the touch after extensive use, so does the battery in an electric vehicle when storing energy. Engineers face the challenge of cooling down the battery and converted energy quickly via the battery enclosure to avoid an internal fire.
Thermal management strives to remove the heat from the source and dissipate it into the surrounding atmosphere as far from the electronics and as fast as possible. There are two types of thermal management: "passive," which uses an aluminum extrusion attached to the source of heat, and "active," which requires the addition of a device – often a fan – to assist in moving the air over the heat sink. A liquid cooler or cold plate offers another form of active thermal management, when the two other types don't dissipate enough heat to meet the requirements of the electronics.
Most materials have the ability to conduct heat, some better than others. This ability is referred to as thermal conductivity. Copper, for one, has a high level of thermal conductivity (390 W/mK), but also possesses two significant drawbacks compared to aluminum – it weighs roughly three times more and typically costs up to five times as much. Aluminum extrusions have a thermal conductivity of 200 -215 W/mK, while aluminium castings are typically 120-140 W/mK.
To help determine the proper thermal management solution, engineers usually work with specialized software that models the products and their thermal characteristics. As the nation's largest thermal management manufacturer as well as aluminum extrusion company, Sapa possesses the capacity and capabilities to determine the proper thermal management solution. Sapa's North American Technical Center (NATC), staffed by a team of scientists, engineers, metallurgists and other product specialists, offers solutions based on the latest technological expertise.
Computational Fluid Dynamics (CFD) is used to simulate the thermal conductivity of the product along with Finite Element Analysis (FEA), which examines the structural integrity of the component. A CFD example is shown in Figure 1. Each product entails numerous variables dependent on product size, shape and application. By combining FEA and CFD along with the variables, it is possible to design the most cost-effective product that meets the needs of both design and thermal engineers.
Extrusions have been the primary thermal management (heat sink) solution for over 30 years. As stated earlier, passive solutions are typically the least expensive and most reliable solution. Extrusions provide a cost-effective way to provide the surface area required to remove the heat from the source.
A fin ratio – the height of the fins divided by the width between them – serves as an important factor in creating surface area. In the example below, the fin ratio is 7.5, a typical ratio for good heat sink extruders. However there are tradeoffs. For example, a 21-inch-wide extrusion is more expensive to manufacture than a 10-inch-wide extrusion, the same way a 12:1 ratio is more expensive than a 3:1. It is possible to extrude ratios of 19:1, the process is slower. It is important to maximize the heat sink to provide the needed ratio.
Aluminum extrusions and aluminum castings are two of the methods most often used to develop thermal management solutions. Extrusion growth is being driven by better thermal efficiency, design flexibility and the cost advantages of aluminum extrusions versus castings – providing proof that many manufacturers are discovering that aluminum castings aren't the solution for most of their applications.
The automotive industry uses several types of castings. For lower-volume applications, sand castings are the product of choice. Other types of castings include permanent mold castings, used for mid-volume applications, and die castings, used for high-volume applications.
When considering the associated costs, sand castings have the lowest tooling costs, typically within the $5,000-to-$10,000 range, while offering the highest price per piece. Pricing for permanent mold castings typically range from $15,000 to $30,000. Piece part pricing for both sand and permanent mold products depend highly on the amount of secondary machining required. Die castings have lower piece prices versus sand and permanent mold castings and usually require the least amount of secondary operations. Still, tooling can range from $50,000 to $100,000. Additionally, all forms of aluminum cast tooling have a specified life expectancy where tooling needs to be replaced. By comparison, tooling for a typical large extrusion costs $5,000 to $7,500, and aluminum extruders typically cover all replacement tooling costs.
In terms of thermal conductivity, a clear advantage exists in using extrusions over castings. Aluminum extrusions prove to be 53 percent more efficient than castings because they are less porous and deliver a higher level of thermal conductivity. The collective conductivity of the types of castings cited above is typically within the 120-140 W/mK range, while the conductivity of aluminum extrusions usually ranges from 200-215 W/mK.
The nature of the casting process creates problems with gas porosity. If the porosity is near the area generating the heat, it acts as an oven, holding the heat in that area. This reduces the life of the electronics, which is a problem especially with foreign casters that may have lower-quality procedures and standards. Porosity isn't an issue with the aluminum extrusion process.
CFD analysis has been performed on aluminum die castings and compared to similar designs of aluminum extrusions. Figure 2 shows such a review, illustrating that the extrusion process allows the fins of the extrusion to be designed without draft, which is required for die castings. This allows for the longer fins that provide additional surface area. In general, the greater the surface area, the greater the natural convection of the heat into the surrounding atmosphere. The combination of the increased surface area and higher thermal conductivity of the extrusion over the die casting reduces the maximum temperature by 23 percent.
Temperature: 23% reduction of max temperature utilizing the extrusion design
The increase in thermal conductivity of extrusions versus castings enables an automotive manufacturer to use less material to obtain the same thermal efficiency. Less material plus a smaller footprint usually equates to lower total costs (and lower weight). Additionally, high-volume CNC machining lets extruders machine in features in a cost-effective manner.
At times, the final component is larger than typical extrusion capabilities. A typical aluminum extrusion needs to fit into a "circle size" of 12 inches. Although extruders, including Sapa, can extrude parts up to a 21-inch circle size, the tooling and minimum amount of material required to extrude shapes of that size are often larger than the budget allows. Sapa has seen an increased need for the development of housings up to 12 inches by 24 inches to hold various types of batteries or capacitors. These rechargeable devices generate a significant amount of heat, therefore requiring the housing to be thermally efficient and structurally sound. Sapa solves this by extruding a low-cost, light-weight corner extrusion capable of mating with itself to form a rectangular box. To join the extrusions together, Sapa has used FSW. This process allows Sapa to extrude and fabricate the housings in long lengths, then cut to the final length required.
FSW is a method of joining two pieces of metal with no filler material by subjecting the components to heavy plastic deformation at high temperatures, but lower than the melting point.
A rotating tool is plunged into the components and friction heat is generated. The tool produces severe plastic deformation under high pressure, at which time the weld interfaces are stirred together, forming a homogenous structure.
Compared to fusion welding, Friction Stir Welding provides:Increased strengthImproved sealing with void-free leak proof jointsReduced heat distortionImproved repeatabilityThe ability to join two different alloys
Extrusions possess other design advantages, as well. There is more flexibility in terms of size. Extruders can create products upwards of 21 inches wide and offer fin ratios of 19:1. Two methods provide wider products, and they include a snap-fit design, often used for enclosures or boxes, and a technology called Friction Stir Welding (FSW), which allows extruders to join two or more pieces of aluminum together with no filler material. Sapa has used this technology to hermetically seal an extrusion by welding a cover on the top.
If the source of the heat is larger than an air-cooled heat sink can handle, the engineer often will use a liquid cooler or cold plate. A cold plate is a closed-loop system that allows fluid to absorb the heat faster than air would. The heat is then dissipated over the large cold plate.
Typically, the cold plate is manufactured by inserting copper tubing into aluminum castings, rolled aluminum plates or aluminum extrusions. Sapa has found that aluminum extrusions are the best options thermally and economically. Numerous vehicle manufacturers that used castings in the past have been forced to coat the casting with a water-proof layer of protection due to leakage problems from porosity, it has been determined.
Although plates are equal to extrusions in thermal conductivity, they must be machined to create a flow for the liquid. Sapa has lowered the cost of manufacturing by extruding the holes into the cooler. This process also increases the thermal efficiency of the cold plate by creating more surface area for the liquid. Sapa then uses FSW end caps to close the loop of the flow path. The required pressure in the liquid coolers is typically less than 50 psi. Sapa has burst-tested an FSW cold plate and it withstood pressure of up to 2,700 psi.
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