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Microstructure formation during friction stir welding of high strength aluminum alloys

Name
Anton
Surname
Naumov
Scientific organization
Peter the Great St. Petersburg Polytechnic University
Academic degree
PhD
Position
Associate Professor
Scientific discipline
New materials, Manufacturing technologies & Processes
Topic
Microstructure formation during friction stir welding of high strength aluminum alloys
Abstract
In the present research the results of Friction Stir Welding of two Al-based alloys with Mg and Cu are presented. The temperature distribution during FSW with the help of thermocouples was measured and was compared with the results of numerical FE-simulation with program ANSYS. Fourteen points of the welds were taken for the detailed analisys of thermocycles and microstructure formation. The thermocycles in these points were simulated on the Gleeble-3800 System, the received microstructures were compared to the microstructures of the points of the welds.
Keywords
Friction Stir Welding, Numerical and Physical Simulation, Microstructure, Gleeble tests
Summary

The Friction Stir Welding (FSW) is the solid state process widely used to provide similar and dissimilar joints. The main advantages of this process are the relatively low temperatures of welding (0,5-0,8 of the melting point) and the possibility to provide similar and dissimilar joints of materials combinations which are unweldable by conventional methods of welding. The relatively low temperatures of FSW allow to prevent hot cracking and porosity which are typical for conventional welding methods of high-strength aluminium alloys, for example, and to decrease the distortion of welded sheets.

The parameters of FSW for 2 mm butt joints were set for two different materials Al-Mg and Al-Cu alloys to get solid welds. The tool rotation was varied in order to change the temperature of process that has direct influence on the microstructure formation. The number of thermocouples was used during FSW to measure the thermocycles in certain points of the welds. Using this data the numerical FE-model of temperature calculation made in ANSYS was calibrated.

The phanoramic pictures of the welds after FSW were received. The net of 14 points was set for the phanoramic pictures of the microstructure in order to calculate the thermocycles in these points with the help of the calibrated FE-model. The net has 2 rows, the first is 0.5 mm below the upper surface and the second is 0.5 mm over the bottom serface. The analisys of themosycles in upper and lower raws shows the uniform distribution of temperature through the thickness of the 2 mm metal sheets.

The calculated thermocycles were physically simulated using Gleeble-3800 System with Pocket Jaw MCU. The microsructure of received samples was analyzed and compared with microstructure of the points of the welds. The main influence on the microstructure formation has the deformation at the elevated temperatures during FSW.