Thermo-Mechanical Characterization of Friction Stir Spot Welded AA7050 Sheets by Means of Experimental and FEM Analyses.

Gianluca D’Urso,Claudio Giardini

doi:10.3390/ma9080689

Abstract

The present study was carried out to evaluate how the friction stir spot welding (FSSW) process parameters affect the temperature distribution in the welding region, the welding forces and the mechanical properties of the joints. The experimental study was performed by means of a CNC machine tool obtaining FSSW lap joints on AA7050 aluminum alloy plates. Three thermocouples were inserted into the samples to measure the temperatures at different distance from the joint axis during the whole FSSW process. Experiments was repeated varying the process parameters, namely rotational speed, axial feed rate and plunging depth. Axial welding forces were measured during the tests using a piezoelectric load cell, while the mechanical properties of the joints were evaluated by executing shear tests on the specimens. The correlation found between process parameters and joints properties, allowed to identify the best technological window. The data collected during the experiments were used to validate a simulation model of the FSSW process, too. The model was set up using a 2D approach for the simulation of a 3D problem, in order to guarantee a very simple and practical solution for achieving results in a very short time. A specific external routine for the calculation of the thermal energy due to friction acting between pin and sheet was developed. An index for the prediction of the joint mechanical properties using the FEM simulations was finally presented and validated.

Highlights

When joining difficult-to-be-welded materials, it is normal practice, nowadays, to refer to friction stir welding (FSW)technology
The process parameters defining the joint mechanical characteristics [26] are the rotational speed of the pin, the plunge velocity, the dwell time, the maximum pin depth, and the pin and shoulder geometries
friction stir spot welding (FSSW) cannot be simulated at steady-state conditions because of its short cycle time

Summary

Introduction

When joining difficult-to-be-welded materials (typically Al, Ti, and Mg alloys, and advanced high-strength steel [1]), it is normal practice, nowadays, to refer to friction stir welding (FSW). The process parameters defining the joint mechanical characteristics [26] are the rotational speed of the pin, the plunge velocity, the dwell time, the maximum pin depth, and the pin and shoulder geometries (pin diameter, shoulder diameter, pin length) All of these parameters influence the heat generated, the process duration (that influences the temperature level and its distribution within the material), and the stirring effect. FSSW cannot be simulated at steady-state conditions (as normally happen for FSW) because of its short cycle time (a few seconds) This means that the model validation must be based on experimental measurements of temperature and welding forces during the whole process. An experimental study aimed to evaluate how the FSSW process parameters affect the temperature distribution in the welding region, the welding forces and the mechanical properties of the joints when considering AA7050 alloy, is reported. An index for the prediction of the joint mechanical properties using the FEM simulations was presented and validated

Experimental Procedure

Welding

Shear Test

Contour

FEM Model

Predictive Index of the Joint Resistance