Self-piercing riveting (right) is a cold joining process used to hold two or more pieces of material together. During assembly, a controlled force rivet is driven into the stack of material, piercing through the top layer or layers. Under the action of the die, the rivet expands radially into the substrate or sheet, forming a strong mechanical interlock. The rivets do not pierce the final material.
The overlap rivet (left) is similar, but the connection principle is based on plasticity and friction, without causing destruction and deformation of the surface. The process includes two consecutive operations. First machine the dovetail ring holes on the bottom plate. Then insert a half-tubular rivet through the top sheet into the hole in the bottom sheet.
The researchers found that the dovetail holes in the aluminum plate served as mold cavities into which the length of the rivet shaft flowed. The rivets completely filled the 15-degree and 30-degree slant holes, but unfilled pockets of material were visible in the 45-degree slant holes. Even though the 45 degree bevel undercut is larger, unfilled pockets justify choosing a smaller bevel, such as 30 degrees. A smaller angle is also advantageous because less force is required to set the rivet.
Electrical tests have shown that riveted joints have less electrical resistance than similar joints fastened with nuts and bolts.
For the overlap riveting process, researchers have invented a new cutting tool. It consists of two spring-operated inclined blades that gradually open, deepening the dovetail opening as the top table is lowered. The tool can be easily attached to a drilling or milling machine.
Tires are an important part of electric vehicles. This metal strip or rod efficiently distributes electricity from high energy batteries to electric motors and other devices. Typically, an uninsulated busbar must be rigid enough to be supported in the air by insulating posts. This helps cool the conductors and allows engineers to make connections at different points without making new connections.
Busbars are usually made by stamping copper sheets. However, because aluminum is lighter and cheaper than copper, engineers want to use the metal to make tires. The only problem with this idea is that aluminum has a lower current carrying capacity and higher impedance than copper due to its higher resistivity.
To get the most out of these two metals, engineers are developing busbars that use both metals. These hybrid tires combine the excellent electrical conductivity of copper with the low density and cost of aluminum.
Of course, the creation of hybrid tires is a problem of simple and effective connection of two materials without interrupting the power supply. Existing options consist almost entirely of threaded fasteners and welding methods.
Threaded fasteners are the most widely used technology due to their high reliability and ease of assembly and disassembly. However, the fastener creates uneven contact pressure that distorts the current. Mechanical and thermal loads can loosen threaded connections, leading to power outages. In addition, switchboard and busbar housings have limited space for threaded connections.
The main methods for welding copper to aluminum are laser welding and friction stir spot welding. However, welding efficiency is limited by the different chemical, mechanical and thermal properties of the two materials and the formation of hard and brittle intermetallic compounds.
Riveting and self-piercing riveting are also suitable for assembling hybrid tires because they can make joints in one operation at ambient temperature without drilling or punching holes in the material. However, these processes can distort the electrical current due to protrusions of material above and below the surface of the sheet. And, like screw fasteners, self-tapping rivets add a third material to the mix.
In 2019, a team of researchers from the University of Lisbon in Portugal developed a new method for assembling hybrid tires without lugs or additional materials. Their method combines EDM, bending, and volumetric compression of sheets to produce form-fitting joints contained within the thickness of overlapping sheets. The process is effective, but requires a multi-stage sequence of shaping operations.
“This process can be easily implemented in advanced stamping systems,” said Dr. Paulo Martins, professor of mechanical engineering at the University of Lisbon, who helped develop the new process.
Now Martins and his research team have introduced another new method of joining copper and aluminum: the lap rivet. This new technology can be used to build hybrid tires or any other application involving two overlapping metal sheets.
The process proceeds in two stages at ambient temperature. First machine the dovetail ring holes on the bottom plate. Then, half-tubular rivets are inserted through the top sheet into the dovetail holes of the bottom sheet to obtain a lock connection. Its principle of operation is based solely on ductility and friction, unlike the self-piercing rivet, which is based on ductility, friction and fracture causing the rivet to undercut when it penetrates the sheet.
According to Martins, a lap rivet offers many advantages over a self-piercing rivet. These include:
Martins acknowledges that the main disadvantage of cast riveting is that it is a two-stage operation due to the need to drill dovetail holes. This makes the process slower than self-piercing riveting.
The main technological parameters are the tilt angle of the dovetail hole, the depth of the hole, the thickness of the dovetail groove, the inner and outer diameters of the hole, and the length of the rivet shaft.
Although the process is currently limited to the lab, Martins hopes to have it up and running on the assembly line soon. “If our dovetail cutting tool concept can be successfully integrated into existing sheet metal working equipment, there is every chance that this process will become a commercial technology,” said Martins. “We are working hard to fix this.”
Martins added that the technology could be applied to more than just busbars. “The lap rivet is suitable for assembling any metal lapped sheet made from the same or dissimilar materials,” he said.
To test their new process, the researchers made a series of dovetail holes in a 5 mm thick aluminum plate (AA 6082) using special cutting tools. The holes are 3 or 4 mm deep and the angles are set to 15, 30 and 45 degrees. The internal and external diameters of the holes and the thickness of the dovetail grooves remain constant.
The cutting tool consists of two spring-operated inclined blades that gradually open to deepen the dovetail hole as the top table lowers. The tool can be easily attached to a drilling or milling machine. The flexible design of the tool allows you to quickly and easily change inclined knives for sharpening.
Semi-tubular rivets are machined from rods of electrolytic copper with a diameter of 10 mm. Calculate the shank length of the rivet to make sure the hole is completely filled.
Next, the researchers used a press to install rivets into the holes. After assembly, some test joints were cut in half lengthwise for cross-sectional analysis. Some joints were drawn and shear tested, others were amperometrically tested.
The researchers found that the dovetail holes in the aluminum plate served as mold cavities into which the length of the rivet shaft flowed. The rivets completely filled the 15-degree and 30-degree slant holes, but unfilled pockets of material were visible in the 45-degree slant holes. These indentations are caused by hole deformation and are irrelevant if the connection is used for a purely mechanical connection. However, such recesses are detrimental to electrical connections, as in hybrid busbars, since they create additional resistance to the flow of current.
Martins says that while the 45-degree undercut is larger, the unfilled material pockets justify choosing a smaller bevel angle, such as 30 degrees. A smaller angle of inclination is also advantageous because less force is required to set the rivet.
Shear and pull tests showed that 4 mm deep holes performed better than 3 mm deep holes. The maximum force measured in two tests corresponds to failure caused by shearing or separation of the rivet from the sheet.
Electrical tests have shown that riveted joints have less electrical resistance than similar joints fastened with nuts and bolts.
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Post time: Jul-05-2023
