Numerical/experimental investigation of plunge stage and effect of donor material in friction stir welding.

摘要

Friction Stir Welding (FSW) was developed in 1991 as a robust solid-state joining technique that uses a specially shaped rotating tool to generate heat and plasticize material around the tool. The tool then mixes plasticized material along the joint line to produce the weld. Over the last decade, FSW has become increasingly popular for welding aluminum. The combination of attractive properties of the weld and cost-efficiency has led researchers to investigate the feasibility of using FSW for steel. One of the major impediments for using friction stir welding for harder materials such as steel is tool wear. It is well-documented that a large part of this wear occurs during the initial plunge phase. This dissertation focuses on developing a better understanding of the plunge stage of FSW and also proposes a novel concept to mitigate tool wear during plunge. The commercial FEA code, ABAQUS is used to simulate the plunge phase and the results are compared with experimental results obtained from literature. Plunge experiments on AA 2024 were also conducted during the course of this research and the axial load and temperature were measured. The 'donor material' concept was proposed for the reduction of tool wear at the plunge by providing localized pre-heating at the plunge area using a softer material as a 'donor.' This process creates heat in a relatively soft 'donor' material, which is transferred to the much harder workpiece material by conduction. The expected advantage of this localized process is that it reduces the chances of altering the microstructure of the base material due to excessive heat, which is a possibility in conventional pre-heating methods. This research includes several numerical simulations of the donor material concept with different donor materials and plain carbon steel as the workpiece. It was observed in the case of using a donor material that the axial load during the plunge decreased by approximately 80%. Additionally, the contact stresses at the tool workpiece interface also decreased by approximately 75% when a donor material was used in the plunge area. Decrease in both the axial force and contact stress should contribute to the decrease of tool wear. Proof of concept experiments are also demonstrated with copper as the donor and AA 2024 as the base material.