Performance of Rolled Threads on 3D Printed Inconel 718 and 17-4PH Stainless-Steel Hex Head Fasteners

The convergence of additive manufacturing (AM), innovation in materials, and advances in software tools has led to significant improvements in airframe and engine manufacture and performance due to superior strength-to-weight characteristics, as well as exceptional mechanical strength for load-bearing applications. Fasteners, due to requirements of high strength and resistance against dynamic loading, are manufactured from traditional wrought alloys rather than printed materials. Fastener threads are typically rolled, rather than cut, for the benefits of cold forming strength and fatigue resistance, material savings, and production efficiency. This study evaluates the potential tensile and fatigue performance benefits of rolled threads on 3D printed Inconel 718 and 17-4 PH stainless steel hex head fastener blanks for aerospace applications using the cylindrical die thread rolling process.

Rolled threads can be up to 30% or more stronger and more fatigue resistant than cut threads in a given material due to the compressive residual stresses, uninterrupted grain flow, and smooth surface finish resulting from the cold work operation. Threads can be rolled before or after heat treatment of the blank, but many critical applications, such as with aerospace, require thread rolling after heat treatment for maximum strength. Experiments were designed to evaluate the tensile strength and fatigue performance of 0.3750-24 UNJF-3A rolled threads and cut threads in Inconel 718 and 17-4PH stainless steel 3D printed hex bolt blanks produced by the powder bed fusion direct metal laser sintering process. The shanks of the 3D printed blanks were turned after printing to the correct pre-roll diameter and were then thread rolled using the infeed rolling process on a two-die cylindrical die thread rolling machine. The tensile strength and fatigue performance are also compared to standard wrought 0.3750-24 UNJF-3A Inconel 718 and 17-4PH rolled thread fasteners for aerospace applications.