The Development and Validation of a Novel Thread Forming Fastener for High Strength Steel Applications

Abstract

Automotive OEMs continue to lightweight their fleets to increase fuel economy and electric drive range to meet government requirements and consumer expectations. This push towards lightweighting has led to increasing use of advanced high strength steels in both car body and safety critical systems, such as seats. These steels present different challenges for traditional joining technologies, especially thread forming fasteners. Thread forming fasteners are externally threaded fasteners like machine screws that are driven into a pilot hole where they form their own threads and then tighten down to secure the target joint. Traditional thread forming fastener designs struggle to form threads into steels with tensile strengths greater than 600MPa. This paper utilizes simulation coupled with experimental validation to evaluate one of the most popular traditional thread forming fasteners that is currently used in low strength steel to determine why it fails to form threads in high strength steels. A new thread forming fastener design targeted for high strength steels is developed through simulation. The new fastener design is manufactured and evaluated across three different screw diameters, and six different high strength steels (> 600MPa tensile strength) using the same heat treatments, materials, and platings as traditional thread forming fasteners. Validation tests for the new fastener include drive-to-failure to identify a target tightening torque and clamp load at the target tightening torque. Both tests are conducted across a hole size tolerance window. It is shown that the new thread forming fastener design performs well in steels up to 1200MPa without any special heat treatments but can benefit from specialized heat treatments in steels of tensile strength 1200MPa or higher.