Flow drill fastening applications for body shop

Direct joint

A very common flow drill fastening application is a two layer stack with no clearance hole. This allows a direct joint without any pretreatment of the parts. A typical material combination is steel and aluminum. A fastener with undercut accommodates the displaced material. Frequently an adhesive is used to increase the joint strength and prevent contact corrosion between steel and aluminum. The advantages of using applications without clearance holes are cost savings in part production, easy robot teaching and no squeeze out of adhesives. Typical body-in-white application areas are at the front wagon line, the underbody line, the rear wagon line and the framing line.

Thick stack-ups

By using a clamping part with clearance hole, thicker stack-ups with up to four layers can be joined with flow drill fastening. The upper layer can be aluminum or even high-strength steel (HSS). Since no upcoming material needs to be accommodated by the joining element, a cheaper flat head fastener can be used. However, the position of the robot is very important to exactly meet the clearance hole. If used with adhesive, a stitch bead application is recommended to avoid squeeze out.

This joint variant is suitable for body-in-white applications when the design of the car body does not allow direct joining. Also, it is used for applications where serviceability is a priority, for example when joining the cover to the tray in battery assembly for electric vehicles. Due to the clearance hole in the cover sheet, the cover can be easily disassembled and reassembled several times and remain leak tight.

Thick stack-ups with HSS

When thicker stack-ups with three or four layers are needed, two clamping parts with pre-holes can be used. Thus, the upper layer can be a high-strength steel (HSS). Usually an adhesive is applied between steel and aluminum to provide additional joint strength and to avoid contact corrosion. The position of the robot is very important to meet the clearance hole. Considering deviations of the robot, the second pre-hole should have a larger diameter compared to the first. When used with adhesive, a stitch bead application is recommended to avoid squeeze out. Typical body-in-white application areas are at the front wagon line, the underbody line, the rear wagon line and the framing line.

Thinned-out cast

For thicker stack-ups without clearance holes, a thinned-out casting is a cost-effective option. Castings are easy to produce compared to pre-holes in machining. The top sheet can be stamped in the press process to indicate the correct robot position. Due to the thinner layer, less process force is required, and less material is displaced. This avoids gaps between the parts. Adhesives can easily be used with no squeeze-out. This joint is suitable for areas where aluminum castings are used – for example wheelhouses and shock towers at the front wagon.

Thick second layer

For joints with thicker stack-ups, a smaller M4 fastener can be used without a pre-hole. Due to the smaller diameter of the fastener, less material is displaced and less process force is needed. For high volume productions, a smaller fastener can also reduce costs. Adhesives can easily be used with no squeeze-out. However, this application needs a precise flow drill fastening process that can handle the reduced breaking torque of the M4 Fastener. Typical body-in-white application areas are at the front wagon line, the underbody line, the rear wagon line and the framing line.