At first glance, the Ford F-150 and the Chevrolet Corvette would appear to have nothing in common, beyond the fact that they’re both gas-powered vehicles. After all, you wouldn’t tow a bass boat with a Corvette any more than you would zip a 5,000-pound pickup around a test track.

However, the same small component plays a key role in the design and manufacture of both vehicles: the friction-drilling screw.

That’s because the bodies of both vehicles are largely made from aluminum, which helped reduce their weight significantly. For example, the aluminum frame of the 2014 Corvette is 99 pounds lighter and 57 percent stiffer than the previous-generation steel frame. The 2015 F-150 is 700 pounds lighter than its predecessor.

Faced with the need to join aluminum to aluminum and aluminum to steel, GM and Ford have been forced to find alternatives to the tried-and-true spot welding technology they had been using for decades to join all-steel assemblies. Friction-drilling screws are one such alternative.

Although friction-drilling screws have been around since the late 1990s, the fasteners are increasing in popularity due to the continued emphasis on “lightweighting” in the auto industry; a greater familiarity with the technology; and advances in the equipment for installing them.

“The early adopters in this country are now on their second or third programs [with friction-drilling screws],” says Jim Graham, president of Weber Screwdriving Systems Inc. “In Europe, the technology has been on solid footing with automotive OEMs since 2002.”

Besides the Corvette and the F-150, friction-drilling screws are being used in many other vehicles, including the Cadillac CT6; Acura NSX; Mercedes-Benz SLS; Audi TT, A4, A6 and A8; Porsche 911 and Boxter; Lotus Evora; Jaguar XK; Ferrari California; and Lamborghini Gallardo. Auto parts suppliers are also taking advantage of the technology. For example, Hella is using friction-drilling screws to assemble its Bi Xenon headlamp modules.

The fasteners are also seeing interest from manufacturers of white goods, aircraft, fabricated metal products, and buses and heavy trucks.

Going With the Flow
A friction-drilling, or flow-drilling, screw is a self-piercing and extruding fastener for joining layers of sheet metal. Combining the properties of friction drilling and thread forming, the screw acts as both a fastener and a drilling-and-tapping tool. It penetrates the layers, extrudes a short boss, forms its own threads, and applies clamping force between the sheets.

The fastener has a wide, flat head; a relatively thick shank; and a pointed tip. The head can be designed for external or internal drive systems (including hex head, TORX, TORX plus and cross recesses), and the bottom surface of the head can be undercut. The shank is divided into three zones: a pointed, unthreaded tip (for drilling); a short partially threaded midsection (for thread-forming); and a fully threaded upper section (for applying clamp load).

The installation process has six distinct steps: heating, penetration, extrusion forming, thread forming, screwdriving and tightening.

“It takes 2 to 3 seconds to drive one of these fasteners,” says Boris Baeumler, applications engineer at DEPRAG Inc. “In that time, we change the driving parameters four times.”

In the heating step, the tip of the fastener is pushed against the material with high force and rotated at high speed. Friction between the screw and the material heats the surface to 150 to 250 C, depending on the materials and how thick they are.

“At the beginning of the cycle, you want a lot of down-force to help generate friction,” says Baeumler. “With aluminum, you need less force, say, 1,500 to 2,000 newtons. With steel, it might be 1,800 to 2,500 newtons.

“Driver speed is also important. Aluminum tends to dissipate heat quickly, so we run at a higher speed, say, 6,000 rpm. With steel, we tend to run at slightly lower speed, perhaps 4,000 rpm.”
As the materials heat up and get soft, the fastener starts to penetrate the stack and create a hole. The material extrudes up and down along the points of the screw, forming a boss.

“As soon as the fastener penetrates the material, we reduce the down-force,” says Baeumler.

The fastener continues to penetrate the material stack until the tip penetrates the bottom of the stack. The conical geometry of the fastener helps to extrude a short boss on the bottom side of the stack.

Next, female threads are created in the extrusion by the thread-forming zone of the fastener. This step is performed at a lower speed—approximately 2,000 rpm. After the threads are created, the screw is threaded into the newly created nut member until its head seats against the top sheet. This is performed at 200 rpm, to avoid damaging the newly created threads.

Finally, the fastener is tightened to a preset value.

“We monitor all the variables—torque, speed, thrust and fastener depth—all the way through the process, and we feed that data back to the motion controller,” says Graham.