Simulation of the flow forming screw process for joining of aluminium sheet metal parts

Vehicle weight reduction has been an important objective in the automotive industry, due to increasingly stringent emission norms and fuel efficiency targets. For achieving this, manufacturers have started the use of lightweight materials such as aluminium and high strength steel. Although, this transition to multi-material design created new challenges for reliable and efficient joining technologies, as many conventional methods (e.g. resistance spot welding, MIG welding) are incompatible or inefficient when used with dissimilar material pairs. The flow forming screw process has emerged as a highly promising solution because of its one-sided access, chipless operation, and strong mechanical performance, especially in aluminium to steel or mixed metal joints. The Flow Drilling Screw process is a multi-phase procedure that combines friction drilling, plastic deformation, and thread forming into a single, continuous operation. It is typically divided into six stages: (1) warming up through friction, (2) penetration of the top workpiece, (3) extrusion forming of a boss on the underside, (4) thread forming within the boss, (5) screwdriving, and (6) final tightening to a specified torque. While this process has proven effective in production environments, there remain gaps in the scientific understanding and numerical modelling of the complex thermo-mechanical interactions that occur during the most critical phases — penetration and thread forming. These stages are highly sensitive to process parameters such as axial force, rotational speed, and temperature rise, and are directly responsible for achieving a defect-free joint with the desired torque and axial clamping force. This thesis, carried out in collaboration with Agrati S.p.A., focuses on improving the simulation accuracy of the penetration and thread forming phases of the FDS process using Hexagon Simufact Forming software. A primary objective is to develop a robust and validated finite element model capable of capturing the dynamic evolution of torque and axial force during screw engagement with a deformable multi-material stack. The numerical model incorporates advanced material definitions (including temperature- and rate-dependent Johnson–Cook constitutive laws), contact formulations, and thermal friction modelling. A literature review of existing academic and industrial simulation methodologies is presented comparing mesh-based and mesh-free approaches, material modelling strategies, and previous validation techniques. During the development of this thesis, a 2D axisymmetric simulation and a 3D simulation were carried out to analyse the behaviour of the screw during the piercing & thread-forming phase respectively. The outcomes of this thesis aim to support the industrial deployment of accurate virtual tools for Flow forming screw process optimization, thereby reducing physical prototyping needs, accelerating development timelines, and contributing to the broader goals of lightweight and sustainable automotive design.