Year: 2021
Type: dissertation
Abstract: Friction stir based technologies (FSBT), such as friction stir welding (FSW) and processing (FSP), are assumed as environment-friendly techniques for the joining/mechanical enhancement of several materials. In spite, FSW and FSP have already been explored in important sectors of the transportation industry and in research, an exact knowledge of the thermo-mechanical conditions developed during welding/processing is still missing. There is no systematic information on the contact conditions between the FSW/P tool and the materials being processed and its relationship with process parameters and base materials mechanical, microstructural and surface properties, which difficult the control of the heat generation and plastic deformation during welding/processing. FSW/P processes are widely used for welding/processing heat treatable and non-heat-treatable aluminium alloys. On the other hand, although steels are the most commonly used material for engineering applications, due to its high melting temperature, challenges related to the tool design and wear are the most important limitations in the welding/processing of these materials. In this work, the main friction stir based mechanisms, occurring during the welding/processing of heat treatable (AA2017-T451, AA6082-T651 and AA7075-T651) and non-heat-treatable (AA5083-H111) aluminium alloys and of a mild steel (DC01), a high strength steel (HC420 LA), a dual-phase steel (DP600) and three different galvanised steels (DC01-ZE25/25, DX51D-Z140 and DX51D-Z200) were analysed using experiments and numerical simulation. The welds were produced under a varied range of welding conditions, which enabled to analyse the influence of the tool dimensions, rotational speed, traverse speed, base material mechanical and surface properties on the welding/processing mechanisms. The experimental work included performing a thermal analysis of the welding apparatus and conducting the mechanical, morphological and microstructural characterization of the welds. The thermo-mechanical conditions during welding were assessed by registering the evolution of the welding temperatures and tool torque. The welding/processing mechanisms were also investigated by creating a database, containing experimental results from the literature, and relating the processing parameters with the process outputs, such as the tool torque and temperature. Additionally, the experimental data was coupled with numerical simulation from a 3D thermo-mechanical model for the FSW/P processes. A parametric finite element analysis of the processing parameters, tool dimensions and base material plastic properties on the evolution of the contact condition, strain rate, material flow, temperature and tool torque was conducted. The investigation enabled to conclude that the tool dimensions have a very important influence, not only on the heat generation but also on the volume of material being stirred during welding/processing, while the tool rotational speed governs the heat generation and the strain rates experienced in the stirred volume. The traverse speed and the base material thickness were also found to be important factors governing the heat dissipation during welding/processing. The tool geometry was also found to have a strong influence in the weld formation mechanism. For tools with a pin, it was found that the temperature and the sticking fraction evolve in the same way with the processing parameters. The formation of defects was observed for contact conditions close to full slipping, due to the absence of proper material stirring. Meanwhile, for pinless tools, no material stirring across the base material interface in lap welding was observed, which enabled to produced lap welds with no hooking or cold lap defects, and good mechanical strength, even for very high traverse speeds in linear welding or using very short process cycle times in spot welding. The data from the experiments were used for developing and calibrating analytical models of the process, which were validated by fitting welding/processing data available from previous research works. Equations for predicting the torque and temperature evolution with several process parameters were developed.
Author: D.G. Andrade
Language: English
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W3213050676