Due to the continuously growing market demand for higher fuel economy and the electric vehicle wave in the automotive industry, every auto maker is striving to reduce the gross weight without sacrificing the safety and performance of the vehicle. One common approach is to use lighter materials such as aluminum, high-strength steel, magnesium, carbon fiber, highly-engineered plastic, rubber, and etc. This has forced auto makers to rethink and adopt new joining technologies for a multi-material body assembly in recent years.
Fusion joining, solid-state joining, and mechanical joining are three main categories of joining technologies defined by the temperature required on the workpiece materials being joined. Fusion joining involves melting and coalescing two materials. Common examples in automotive body assembly include resistance spot welding, laser welding, and arc welding. Solid-state joining doesn’t require the materials to be melted for joining, however, the workpieces are commonly heated. Common solid-state joining in automotive body assembly includes friction stir welding, laser brazing, magnetic pulse welding, and ultrasonic spot welding. Mechanical joining, as the name implies, is performed through mechanical methods, such as self-piercing riveting, blind riveting, clinching, flow-drill screwing, adhesive taping, and adhesive gluing.
The selection of different joining technologies can be confusing and frustrating due to the complexities involved. Given the nature of mass production of the automotive industry, the following are the common criteria when it comes to choosing appropriate joining technologies.
- Process productivity and flexibility
- Process safety
Feasibility is a fundamental requirement, dictated by the kind of materials and the geometry of the workpieces to be joined. For example, if there are non-metals such as polymer, plastic, or carbon fiber involved in the joining, fusion welding technology is not a good fit because these non-metal materials tend to degrade before the metal melts. Another example is when using resistance spot welding to join aluminum with steel, special consideration must be put into the process design because 1) aluminum has low resistance and high thermal conductivity, and 2) the high resistivity of oxide layer on the aluminum workpiece can cause strong heat development — which has significant influence on electrode life and weld quality consistency. Geometry of the workpieces can be another important consideration of joining feasibility. For example, joining sometimes can only be performed from one side. That will bring down the list of joining technologies to just a few, such as flow-drill screwing and blind riveting.
Durability of the joint refers to the strength and other performance requirements (for example, corrosion-resistance through the designed life cycle). Joint strength is important to the stiffness and crashworthiness of the body assembly. Considerations such as fusion welding tends to create a larger intermetallic compound, and if not controlled well compared to solid-state or mechanical joining technologies, becomes the weak point of the joint. Corrosion can be introduced by water leakage into the body cavity. It can also be introduced by galvanic corrosion through dissimilar metal materials touching each other for extended duration. Adhesive can be great in building a water seal and preventing dissimilar metal touching each other for galvanic corrosion. As an example, German auto makers using adhesives offer 12 years, unlimited miles of corrosion warranty (see the link https://www.motor1.com/features/253277/comparing-new-vehicle-warranties/).
High productivity and high quality consistency are crucial to the automotive industry, but trade-offs often have to be made to strike a balance between the two. One joining technology, such as friction stir welding, is limited by how fast it can travel (typically up to 100 mm/s for a 5 mm thick aluminum alloy). However, friction stir welding excels in minimizing the intermetallic compound and joining non-metal materials with a good surface appearance. This is why friction stir welding is mainly used for joining small joints of automotive assembly such as bumper beams, rear spoilers, intake manifolds, etc.
Process flexibility can be another important factor since new models continue to be introduced and huge savings in re-tooling can be realized if the process is flexible enough to accommodate new part variants with minor changes. Adhesive bonding has a great flexibility advantage in this regard since it doesn’t require access to both sides of the workpieces for joining and it can easily adapt to new part geometries.
Process safety is also important since manufacturers have to minimize manufacturing hazards to operators on the floor. For this particular reason, shielding, special access doors, light curtains, and fume extract systems are commonly seen in the fusion joining cells to prevent arc, splash, fume, and robot movement injuries. These safety measures add cost and floor space to the cell.
Cost can be also a big deciding factor for joining technology selection. Let’s take adhesive taping and adhesive gluing as an example. At one OEM facility, traditionally they have been using adhesive tape for joining the wheel house. The average material cost of adhesive tape used is $0.38 per wheel house. If they switch to adhesive gluing, the material cost will be less than $0.05 per wheel house with equivalent sealing performance. By switching to the robotic adhesive gluing process alone, it will provide $162,000 worth of annual material saving, not to mention the reduction of labor cost of 2 full-time operators to manually apply the adhesive tape by switching to the robotic adhesive gluing process.
Finally, the appearance of the joint surface can also be important if it is on Class A surfaces such as closure panels because this has a great impact on customers’ quality perception. For example, the self-piercing rivet head being flush with the surrounding material makes itself a great fit for joining exterior body parts.
In conclusion, there is a huge variety of different joining technologies out in the market. For the automotive industry, which is very sensitive to productivity, quality, and cost, it is common to see a combination of multiple joining technologies such as resistance spot welding, adhesive gluing, self-piercing riveting, etc. applied on the same joint for performance enhancement. Adhesive gluing and self-piercing riveting do seem to be gaining momentum in recent years.