Future fastening: mixed-material bonding in body-in-white structures

4 min read

Gestamp's Miguel Angel Ferrandez, material joining manager, Bilbao, explains two particular mixed-material joining technologies in more detail.

This article follows Ignacio Martin’s comments last year about new types of steel and new EV body designs for automotive bodies-in-white, (www.is.gd/cubeye).

Gestamp, a global automotive body panel producer, has developed a process to make body pillars and chassis components from press-hardened, hot-stamped steel. These ultra-high strength steels offer tensile strengths of up to 2,000MPa. These specialised steels provide the ability to meet certain crash scenario requirements (absorbing energy and resisting damage) with relatively thinner gauges, and so less weight, than lower-strength steels.

Where steel abuts steel, spot welding is a familiar, high-strength bonding mechanism. But such advanced steels come at a cost premium, and their high performance is not needed everywhere in the vehicle. To reduce weight, vehicle OEMs are looking to incorporate also aluminium structures in lower-stress areas.

It is not possible to weld steel to aluminium: steel’s melting point is twice that of aluminium’s, and when molten the two materials are insoluble in each other. And when mixed, they form brittle combinations, according to TWI. As a result, Gestamp has been developing a number of production-volume mechanical joining technologies that can be used on dissimilar or similar materials including two types of riveting.

However, neither technology is powerful enough to penetrate the hot-stamped steels that are being developed (from 1,500-2,000MPa) – one such plant, in Valencia, is pictured at top. Over the past decade, Gestamp’s R&D departments have developed several heat-treatment techniques that create softer zones within individual parts, including in-die quenching, laser annealing and TIP. These in-production processes adapt the materials for joining by softening small zones of the part to make the material easier to form with standard industrial fastening processes. The company has already validated the direct-diode laser process for 1,500MPa steel; tests on 2,000MPa steel continue.

One process is self-piercing rivets (SPR), drawing from suppliers Bollhoff and Stanley Engineered Fastening, which Ferrandez describes as requiring a rivet, a gun and a die (process pictured at left on automotive assembly line). Forces involved amount to 50-70kN. He observes: “By adding a soft spot, we can use a standard rivet, a standard die and a standard gun.” In traditional riveting, the hardened material usually goes on top, and softer at the bottom. Then, he adds, “you can deform and open the rivet legs and create this interlock and clamping. However, Gestamp’s local softening process removes that constraint, allowing for greater design freedom in soft spots. The system, pictured top right, has been tested on two- and three-sheet bonding.

A limitation of the SPR process is the requirement for a die. That is something not required by an alternative one-sided mechanical fastening process, FDS, flow-drill screwing, as developed by Ejot. This process involves rotating a pointed, threaded fastener (pictured at left) under pressure into the substrate. As it pushes into the material, it forms a threaded hole, although at much lower axial forces (2.0-3.5kN), because of the additional help of friction caused by the rotating fastener.

Observes Ferrandez: “In one way it [SPR] is cheaper than FDS, because the rivet is not as costly as the screw. But you do need two-sided access to get the proper joining; you can’t work with closed profiles. This is a limitation that you need to take account of in the assemblies.”

Another competitive advantage of the FDS process is that the fastener can be easily removed and replaced. On the other hand, both techniques produce a similar level of shear strength. Other mechanical joining technologies include clinching – deforming one sheet over another – and hemming, bending one sheet over the other.

One essential characteristic of both SPR and FDS technologies is that they avoid the need for a pre-holes, which pose a number of complications for production. Says Ferrandez: “You will need to laser cut the pre-hole after the part is hot-formed. Laser cutting very small holes is very time-consuming. Then you need a very precise robot at a low speed to locate the screw in this hole so it goes through the two or three sheets.”

THE ROLE OF ADHESIVES

In such cases, adhesive is required, and not just to improve bonding, explains Ferrandez. “In case of dissimilar joints, we need to have a certain adhesive between aluminium and steel in order to avoid galvanic corrosion. So you need to avoid that there is adhesive underneath the pre-hole, otherwise the adhesive will come out, spoiling the joint. You need to programme that there is no adhesive there. Then that will be a corrosion point there after the assembly is done.”

Despite the company’s work with mechanical fastenings, it continues to perform research on structural adhesives. “We are now investigating different materials related to the EV body in white architecture. Apart from very hard steels, there are new anti-flame materials used in the battery box. There are new challenge also coming from composites. We are trying to understand what is the best adhesive application.” Trials include aluminium-composite and steel-composite joining using adhesives.

He explains the process conditions required: “First we try to follow the principle and the idea of what the OEM has in mind. You need to have perfect temperature control of the adhesive, whether it’s mono-component or bi-component. It needs to have temperature control for the application. We do numerous tests of that with manual guns or automatic equipment. You need to have everything under control in terms of temperature, and make sure that the surfaces involved in the boding application have a high enough surface energy so they can wet out on the surface and create a good joint.”

This work is in harmony with other development work currently being carried out by industry, according to Ferrandez. “The automotive industry is putting more and more emphasis of using adhesives as a substitution of mechanical joints. That is a clear trend that we are seeing. But we see still limitations with what’s going to be the failure of an adhesive. What’s going to be the behaviour long-term, when only an adhesive is in the joint and you don’t have any mechanical joining? From our perspective, you will still need a mechanical joint to have a secure the right mechanical properties for a crash.”

He concludes that future movements will be determined by adhesives’ performance in the outcome of simulations and the results of real crash testing. Of adhesives, he observes: “They will come more and more; I think we will see a gradual move to reducing spot welds, increasing adhesive, reducing the number of mechanical joints. But [it depends on] what area of the car you’re looking at. Not all the areas of the car are submitted to the same crash requirements. There will be areas where you can use full adhesives. There will be other areas where you have to keep an eye on mechanical or resistance spot welding solutions.”

Miguel Angel Ferrandez gave a presentation at Engineering Solutions Live on 24 March 2022, ‘Joining and BKT Laser Technology on high-strength steels’. The event is set to return on 23 March 2022, at the same venue, the National Motor Museum in Gaydon, Warwickshire.