CHARACTERIZATION OF JOINTS OF FRICTION STIR WELDED 2017A AND 7075, 6013 ALUMINUM ALLOY SHEETS

摘要

The conventional Friction Stir Welding (butt welding) was used to join sheets of 2017A-T6 and 7075-T6, 6013 aluminum alloys by applying different parameters and conditions of welding. Some samples were artificially aged: 2017A/7075 at 150°C and 2017A/6013 at 160°C, both for 8 hours. Samples were investigated using light microscope, scanning electron microscope (SEM), transmission electron microscope (TEM), tensile test and measurement of microhardness was also done. The studied microstructure of the welds shows that the weld nuggets are made out of the alloy form the advancing side. Moreover, in the bottom part of the weld nugget some separated regions of the alloy located at the retreating side were observed. The region above the weld nugget is formed out of recrystallized material. Over this region a band-like arrangement of precipitates indicating the direction of the material flow is visible. The region borders on advancing side of the welds, in the region of stirring of the material exists in a form of a band composed of very fine grains and precipitates. The TEM studies found out in both types of the welds that the weld nuggets are composed of the fine equiaxial grains with a size about 4 urn. In the region of higher density of dislocation, dislocation loops and precipitates with dislocation were visible as well. The distribution of the hardening precipitates indicates that most of them are located inside the grains. These precipitates contain Al, Cu, most likely AI_2Cu phase, in case of the 2017A/7075 weld and Al, Mg in the case of the 2017A/6013 weld. However, the precipitates were also observed at the grain boundaries in 7075 aluminum alloy. Moreover, some larger precipitates with a size of about 2 urn, which contain Fe (most likely AI_3Fe-type phase), were also found within both type of welds. Tensile tests, which were done for the 2017A/7075 welds, indicate the best results of ultimate tensile strength were obtained for the welds welded at parameters of 450 rpm, 280 mm/min. The observation of the structure of the fracture surfaces formed after a tensile test was performed. The fracture had a ductile character at the whole cross-section of the weld with two different areas in the middle of the weld. The brittle precipitates, likely the AI_2Cu phase, are also found there. Microhardness measurements at the cross-section of the welds showed different distributions of microhardness of the investigated welds. The hardness of the welds was changed by artificially ageing process as well. It could indicate metastable state of microstructure of the welds after the FSW process.