Study On Microstructures And Properties Of Friction Stir Welding Joints Of6005A-T6Aluminum Alloy

With the rapid development of high-speed railway technology, aluminum alloys havegained wide application in manufacturing of high-speed trains. As the key technology for thehigh-speed train manufacturing, the welding of aluminum alloys strongly influences on thequality and cost of trains and traffic safety. However, the conventional fusion welding foraluminum alloys often brings some defects such as porosities, hot cracks, distortion andsoftening. It is very difficult to solve these weldability problems because they are relatedwith physical properties of aluminum alloys. These problems limit more widespreadapplications of aluminium alloys in manufacturing of high-speed trains. As a solid-statejoining technique, friction stir welding (FSW) was considered to be the more desired methodfor joining aluminium alloys. Therefore, it has great theoretical significance and practicalvalues to carry out the study on microstructures and properties of friction stir welding jointsof6005A-T6aluminum alloy used in high-speed trains.Firstly, the characteristics of microstructures and properties of6005A-T6aluminumalloy FSW joints were studied systematacially. The results showed the FSW joint of6005A-T6aluminum alloy was composed of nugget zone (NZ), thermo-mechanicallyaffected zone (TMAZ), heat-affected zone (HAZ) and base metal (BM). The BM exhibitedfibrous microstructure, and a large number of fine needle-shaped β’’ precipitates, which arebasically coherent with-Al matrix, were observed within BM grains. Elemental Si and Qprecipitates continuously distributed at grain boundaries of the BM. Additionally, wideprecipitate-free zones (PFZ) were also found along grain boundaries of the BM. NZunderwent dynamic recrystallization and precipitates dissolution during FSW. TMAZ ischaracterized by elongated grains with a high density of dislocations. A few elemental Si wasfound at grain boundaries of NZ and TMAZ. The microstructure of HAZ was non-uniform.HAZ close to the TMAZ (named HAZ1) exhibited equiaxed grains, while HAZ close to the BM (named HAZ2) had fibrous grain morphology similar to BM. The predominantprecipitates in HAZ were rod/lath-shaped β’, lath-shaped Q’ and needle-shaped β’’ phases.With the distance decreasing away the weld center, the density of β’’ phases within HAZdecreased while β’ and Q’ phases within HAZ increased and coarsed. It should be noted thatβ’ and Q’ phases are semi-coherent with-Al matrix. Elemental Si and Q precipitatesdiscontinuously distributed at grain boundaries of HAZ. In addition, the PFZ width of HAZwas smaller than that of BM. The hardness distribution of FSW joint approximates to theW-type. BM has the highest hardness value, and NZ has higher hardness than TMAZ. Theminimum hardness is located at the HAZ1. The hardness of HAZ increased with increasingthe distance away the weld center. The hardness changes are associated with theintragranular precipitate evolution, i.e. the kind, size and density of the intragranularprecipitates. The results of tensile tests showed that the tensile strength and elongation ofFSW joints are higher than70%and75%of those of BM, respectively. Moreover, FSW jointexhibited stronger intergranular corrosion (IGC) resistance than BM. The presence ofcontinuous cathodic precipitates (Si and Q phases) at grain boundaries and the PFZ alonggrain boundaries of the BM resulted in the severe IGC susceptibility. The significantelimination of IGC in NZ is related to the low volume fraction of intergranular precipitates.Secondly, effects of welding parameters (welding speed, rotational speed and plungedepth) and postweld treatments on microstructures and mechanical properties of FSW jointsof6005A-T6aluminum alloy were studied. The results indicated that the increasing ofwelding speed, decreasing of rotational speed or decreasing of plunge depth resulted in thedecreasing of heat input, grain refining for NZ and HAZ1, precipitates (β’ and Q’) refining forHAZ1and slight increasing in hardness of NZ. Inadequate welding parameters should causedefect formation in the joint such as holes and flash, which deteriorate the tensile properties.Postweld natural aging (PWNA) treatment affected microstructures and hardness distributionof FSW joints. The microstructure evolution in NZ and TMAZ during PWNA wassupersaturated solid solution (SSS)â†'clustersâ†'GP zones. As a result, the hardness increasedwith increasing PWNA time. However, the microstructure and hardness of HAZ wereunaffected by PWNA. Postweld artificial aging (PWAA) led the solution atoms to form β’’ precipitates which are benefit for increasing the hardness level and tensile strength of joints.Furthermore, the welding parameters were optimized by means of quadratic regressionorthogonal combination test, and the optimal parameters for6005A-T6aluminum alloy FSWare the welding speed of370mm/min, rotational speed of1381r/min, and plunge depth of0.11mm. The calculated tensile strength of FSW joint for the optimal parameters was222.4MPa and the experimental one was212.7MPa, the error between the calculated value andexperimental result being4.36%.Furthermore, an analytical model for the heat generation in FSW was established, basedon the hypothesis of sticking condition between tool and workpiece. Results showed that thecontributions for heat generation of shoulder concave, pin side, shoulder side and pin tip are83.9%,11.0%,2.7%and2.4%, respectively. This implies that it is neccesary to consider theeffect of shoulder side on heat generation in condition of numerical modelling for FSWthermal process. Hence, the temperature fields of joint during FSW were analyzed by meansof numerical simulation with the ANSYS software. Results showed excellent concordance inthermal cycles of different locations between simulated ones and experimentalmeasurements. The effects of welding parameters on the temperature distribution could beclarified by the thermal model.Finally, based on the microstructural investigation for6005A aluminum alloy joints,considering cylinder-shaped morphology for precipitates is closer to the real situation. Then,mirostructural evolution model and hardening model of6005A aluminium alloy wereestablished, based on the precipitation thermodynamics, growth kinetics and hardeningtheory. The quantitative description of the microstructural evolution during FSW and theprediction of mechanical properties for the FSW joints were realized by means of themirostructural evolution model and hardening model combined with the thermal model. Thecalculated results of hardness distribution of FSW joints showed concordance withexperimental measurements.