Technical Overview of Joining Solutions for Aluminum Body Assemblies
Fasteners commonly used for aluminum alloy vehicle bodies can be categorized by material type into steel and aluminum standard parts. They can also be classified based on how they connect to the vehicle body, which includes mechanical fasteners and welded fasteners. In this summary, we will briefly outline these categories.
Mechanical Fasteners
1. Self-Piercing
Self-piercing bolts or nuts, also referred to as self-piercing rivet bolts or nuts, are cold-formed components designed to penetrate sheet metal using a flanged end structure, creating a secure interlocking connection. Their design is illustrated in Figures 1 and 2.
The production process for self-piercing standard parts involves four key stages: clamping, piercing, forming, and releasing. These self-piercing parts have several advantages, including the ability to withstand standard torque levels, the elimination of the need for pre-drilled holes in sheet metal, and excellent bolt sealing properties. However, they tend to be relatively expensive and are only suitable for thin-walled aluminum sheet metal with a thickness of less than 2.0 mm.
Figure 3 illustrates the use of self-piercing bolts in the front wall and floor areas of the Aion LX. Self-piercing standard parts are widely used in aluminum alloy body structures and aluminum stamping components.
2. Riveting
Riveting, also referred to as press riveting or self-fastening, is a method of creating connections between standard parts that are interlocked with a plate using either a normal structure or a radial structure. There are two main types of interlocking press riveting: tooth-shaped interlocking press riveting and hem-shaped interlocking press riveting. These techniques are commonly used in various components of aluminum alloy car bodies produced through stamping, extrusion, and casting. The following sections will introduce the two common types of riveting standard parts mentioned above.
(1) Tooth-shaped interlocking press riveting standard parts: This method involves forming an interlocking structure by compressing the tooth structure at the base of the standard part against the cylindrical surface of a perforated plate through an interference fit. This process completes the connection between the standard part and the plate. The structural design is illustrated in Figures 4 and 5.
The process consists of three stages: matching the upper part, applying force, and closing the mold. Tooth-shaped interlocking press riveting features a simple structure and is cost-effective, as the lower die only requires a flat surface. However, the push-out force is minimal. Therefore, it is recommended for installation points with open sections and low precision requirements. Caution is advised when using this method in critical load-bearing areas.
(2) Flanged interlocking riveted standard parts
The flange structure at the bottom of the standard part is nested with the surface of the perforated plate under the influence of the pressure head. This causes the flange to be extruded and bent within a specific mold, resulting in an interlocking structure that securely connects the standard part to the plate. The structural design is illustrated in Figures 6 and 7.
The process involves three main stages: aligning the upper part, applying force, and closing the mold. This type of riveted standard part provides a strong push-out force and a reliable connection; however, its structure is complex, and the costs are high. Additionally, a specific lower mold is required for proper alignment and formation. This method is typically used for critical load points or important installation sites.
Figure 8 shows the application of self-riveted nuts and bolts on the door sill and front bumper beam connecting plate of the Aion V. Self-riveted standard parts can be widely used on various aluminum alloy body parts.
3. Pull Rivets
Pull rivets utilize the tension and compression created by the head of a standard component to deform and expand. This action effectively clamps the plate openings with the flanges positioned above and below, resulting in a stable and reliable connection. These rivets are commonly used to fasten various metal sheets and pipes in the manufacturing industry and are particularly prevalent in aluminum vehicle bodies. Their structure can be seen in Figures 9 and 10.
The process of pulling rivets involves four stages: automatic loading, insertion into the installation hole, pulling the rivet, and reverse unscrewing, as illustrated in Figure 11.
Pull rivets have several advantages, including a simple structure, reliable connections, easy assembly and disassembly, and one-way operation. However, a disadvantage is that the end flanges of most standard pull rivet parts protrude from the sheet metal, which hinders direct contact between the mounting element and the body panel.
Pull rivet nuts and bolts are mainly used on aluminum alloy body profiles, castings, and thin-walled components. They provide mounting structures for critical points such as piping, wiring harnesses, and trim accessories.
Figure 12 illustrates the use of pull rivet nuts and bolts in vehicles like the Model X. Standard pull rivet parts are commonly found on closed-section aluminum profiles and open-section cast aluminum components.
Welding Technology
Aluminum Stud Welding
Aluminum stud welding is a process that involves bringing the end of an aluminum alloy stud into contact with the surface of an aluminum sheet. Electricity is then applied to create an arc, which melts the contact surfaces. After melting, pressure is applied to the stud to complete the weld. This process is illustrated in Figure 13.
The aluminum stud welding process consists of four main stages: loading, pre-tightening, fusion, and removal. This method offers several advantages, including low cost, speed, and flexibility. However, a notable disadvantage is that the joint strength is weaker compared to steel. Specifically, aluminum stud welds of the same type and specification do not have the same strength as steel stud welds. Aluminum stud welding is commonly used to secure wiring harnesses, piping, and other fixtures on stamped aluminum plates.
Figure 14 shows aluminum stud welding in use on models such as the Jaguar XE. Aluminum studs are widely used on aluminum stamping plate components.
Other Types
1. Threaded Bushings
A threaded bushing is a standard component designed to engage with aluminum alloy castings through external threads. It provides a mating thread for external connectors via its internal threads. Its structure is illustrated in Figure 15.
Threaded bushings enable strong connections even when using low-strength metal materials, a concept that has been validated over more than 40 years of practical application. These bushings are made of high-quality stainless steel and are embedded in the aluminum castings of the vehicle body, creating a durable internal thread. The mounting shank can be disconnected at a designated cutout known as the pre-set disconnect point.
Threaded sleeves are available in two types: standard and self-locking. Standard threaded sleeves are constructed by winding a coil that has a precisely formed diamond-shaped cross-section. This design creates a standard internal thread that can be utilized on both ends. On the other hand, self-locking sleeves provide the same technical benefits as standard threaded sleeves, but they also include an internal thread-locking feature. This feature consists of one or more turns of the polygonal coil, which secures the threads of the mounting screw, effectively locking it in place.
Threaded sleeves are commonly used in aluminum die casting parts, serving as mounting points for critical chassis components such as subframes, power batteries, and vehicle body parts. For instance, Figure 16 illustrates the application of a threaded sleeve at the front longitudinal beam joint of the NIO ES6, where it serves as a mounting point for the reinforcement plate that connects the longitudinal beam joint to the door sill.
2. Aluminum Alloy Sleeve Nuts
Aluminum alloy sleeve nuts are typically made from 7-series special aluminum and feature internal threads, providing high connection strength. With a thread length that is double the usual size, these nuts can reliably meet PC10 load requirements. The sleeve utilizes a cold connection method to join the two ends of the profile, allowing both ends to effectively share the load. This structure is illustrated in Figure 17.
Aluminum alloy turnbuckles provide several advantages, including reliable connections, the ability to handle high torque requirements, and ease of assembly. However, they also have some drawbacks, such as their high cost and the necessity to customize them to match the cross-sectional dimensions of the profile beam being used.
These turnbuckles are primarily utilized at critical mounting points on body custom aluminum parts, including the subframe, suspension, motor assembly, power battery, and other essential hardpoints, as well as seatbelt mounting locations. For instance, Figure 18 illustrates an aluminum alloy turnbuckle used on the front longitudinal beam of the CT6, which serves as a mounting point for the subframe.
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