Influence of welding parameters on the microstructure, thermal fields and defect formation in AA7075-T6 friction stir welds

Abstract

This study focuses on the influence of the welding parameters on defectiveness, thermal field and grain size in friction stir welding of aluminium alloy 7075-T6. Different welded specimens were produced employing various rotational speeds and welding speeds. In the first part of the analysis, the quality of the joints was evaluated by microscopic observation of the jointed cross-sections to investigate the modification of the shape of the stirred zones and the occurrence of internal voids. Afterwards, temperature and grain size of welds obtained at welding speed of 60 mm/min have been measured to study the influence of the rotational speed on the heat input. Temperature measurements have been carried out with thermocouples and infrared camera. Grain size evolution of the material was analysed by optical observations of the jointed cross-sections and the average grain size in the nugget zone (NZ) has been estimated through electron backscatter diffraction. The results globally show that the first internal defects arose at the same time as an unexpected zone on the advancing side between the shoulder affected zone (SAZ) and NZ. In sound welds, from 800 to 1400 rpm, the peak temperature showed an increase between 5% and 15%, depending on the considered zone, while the grain size in the stirred zone increased from 2 to 3.6 µm. Overall, it has been shown that the approach to the defective condition is due to the low heat input which is responsible for improper material flow during the process.Keywords friction stir welding • nugget zone • temperature measurement • grain size • process parameters • AA7075-T6

Relationships between welding parameters, aging conditions, and weld properties in AA7075-T6 friction stir welds

“…The tested welding configurations and the different chosen process parameters are listed in Table 1. The process parameters were chosen based on previous work [19] and to assess the influence of the rotational speed (WC 1 ) and welding speed (WC 3 ) against a reference configuration (WC 2 ). In other words, the choice was first of all driven by the willingness to analyze the influence of the maximum temperatures reached in the nugget zone (rotational speed) and the rate of thermal cycles (welding speed) on the mechanical properties and secondly by the attempt to investigate their influence on the aging process of friction stir welded joints.…”

A semi-empirical model for peak temperature estimation in friction stir welding of aluminium alloys

“…Similarly, it is valid for rotations from 500 rpm onwards because this was the lowest rotation speed used in the experiments to develop the model. In the literature, while it is possible to find many studies with thermocouples fixed away from the welding line [24], few are those in which the peak temperature within the NZ has been measured experimentally. Fehrenbacher et al [10] represent, to the authors' knowledge, the only work in which the tool and plate geometries used are similar to ours and in which the temperature in the NZ was experimentally measured.…”

“…In the literature, while it is possible to find many studies with thermocouples fixed away from the welding line [24], few are those in which the peak temperature within the NZ has been measured experimentally. Fehrenbacher et al.…”

Plastic behavior-dependent weldability of heat-treatable aluminum alloys in friction stir welding

“…By sound weld is meant a joint that has neither internal nor external defects. From the analysis of the work provided on AA6xxx-T6 [4,5,6,7,8] and AA7075-T6 [9,10,11,12,13,14,15,16], it was evident the greater weldability of the former, as shown in Fig. 1.…”