High-Strength Lightweight Joints Using ‘Flow Drill Screw’ Technology

The FDS® flow drill screw is an innovative single-sided fastening solution for assembly into extruded/hydroformed profiles and difficult-to-reach locations.

By Terry Tripp, Semblex technical marketing manager and Eric Breidenbaugh, Semblex FDS® product line manager

Today’s designers continue to search for ways to maximize performance and efficiency in their product designs. One approach that has gained huge momentum in the automotive industry and elsewhere is lightweighting through material selection. The integration of thinner, lighter and mixed materials has led to significant weight reductions. Using these materials has created new challenges in joining and brought upon an evolution in fastening technologies.

The majority of the fastening for lightweight sheet joints requires two-sided access for installation (riveting, clinching, etc.). The FDS® flow drill screw manufactured by the Semblex Corporation was developed as an innovative single-sided fastening solution perfect for assembly into extruded/hydroformed profiles and difficult-to-reach locations. The design originated with EJOT GmbH & Co in Germany and has been used for many years by multiple European auto manufacturers. Its popularity has risen alongside the lightweighting initiatives of recent years.

FDS fasteners bring together the technology behind flow drilling and thread-forming screws into a single multiple-stage process (see Figure 1) detailed below.

  1. Warming up the sheet metal by axial end load and high speed
  2. Penetration into the material
    1. Extrusion formation
    2. Chip-less forming of female thread
    3. Installation
    4. Tightening with the pre-set torque

     


    The combination of speed and end load create heat that allows the fastener to not only penetrate unprepared layers, but to maximize thread engagement by creating deep extrusions. This results in very strong joints.

    The FDS can be used in combination with many material types and thicknesses. Aluminum (typically 6000 series) is the most common with joint thickness upward of 5.0 mm. This can be mixture of sheet or cast material. Engagement into steel is also possible, but is very dependent on the thickness and material strength.

    2.0 mm—up to 350 MPa
    1.5 mm—up to 600 MPa
    1.0 mm—up to 800 MPa

    Mixed-material joints are very common and achievable with the FDS joining process.

    Typical installation speed requirements for engagement into aluminum only are 5,000-8,000 rpm depending on the capability of the equipment. Steel or mixed (steel and aluminum) engagement requires 2,000 rpm.

    When selecting, or designing joint stacks, it is important to follow a thin-on-thick or soft-on-hard strategy. Since we are dealing with a single-sided installation process, we do not have back support during joining. Therefore, if the weaker material is on the bottom side of the joint, there is a risk of the material pulling away or deflecting. If you are unable to overcome this limitation by changing the layers or driving direction, a pilot hole can be added to the clamped layer.

    The FDS design features are detailed in Figure 2. The head configuration and drive system can be selected based on the customer’s preference. However, the joint configuration plays an important role.

     

     

    Depending on the materials and number of layers within a joint, it may be possible to have no pilot hole in the clamped (top) layer. This scenario creates flexibility in the joint positioning. When driving into the clamped layer, the material will naturally flow upward and could prevent the FDS fastener from fully seating. To prevent this, undercut designs (under-head cavities) are integrated into the part to effectively capture this material. These designs are normally paired with external drive systems to maintain adequate head strength.

    When fastening clamped layers with pilot holes, there is no up flowing material from the clamped layer to capture. Here, there is an opportunity to use simpler FDS configurations with internal drive systems and little-to-no undercut.

    The FDS thread was designed to create a metric female thread. Therefore, an FDS can be removed and replaced by an equivalently sized machine screw. The most common diameters are M4, M5 and M6.

    The thread-forming zone allows the fastener to slowly form the female thread (versus cutting). The slow rise of the thread crest minimizes the thread-forming torque during assembly.

    The point and flow-drilling section enables the fastener to engage the joint materials, generate the required heat for penetration and to form the long extrusion for maximized thread engagement.

    FDS fasteners are produced from neutral (aluminum-only applications) or case-hardened carbon steel. Zinc-aluminum flake coatings are most commonly applied for corrosion protection and reduced assembly torque. A plating such as zinc-nickel may also be used in higher-heat steel applications where a robust coating is preferred.

    FDS assembly requires high-speed automated drive systems (see Figure 3) that control and adjust speed, torque, axial load and depth throughout the multi-stage installation process.

Programmed assembly parameters are dependent on the following joint characteristics:

  • Layer thicknesses
  • Number of layers
  • Material properties
  • Surface treatment
  • Overall joint requirements

FDS-installation equipment is most commonly paired with robotics to allow for pre-programmed precise locating and assembling of joints in a variety of spaces and positions, thereby improving cycle times over traditional methods.

Read the rest of this article in the March/April edition of Lightweighting World Magazine being distributed this week.

 

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