Development of new FSP method for enhancing the mechanical properties of FSW and TIG welded dissimilar aerospace aluminium alloy joints

Abstract

The main aim of this study was to develop a new friction stir processing method to enhance the mechanical properties of the FSW and TIG-welded AA6082-T651/AA8011-H14 dissimilar aluminium alloy joints. The processing conditions used included friction stir processing under normal (air at room temperature) and underwater (submerged) at room temperature. The dissimilar plates were first welded using the tungsten inert gas welding (TIG) and the friction stir welding (FSW) techniques, taking into consideration the material positioning. The first batch was welded with the AA6082-T651 alloy on the advancing side, while the AA8011-H14 was positioned on the retreating side. The second batch was welded with the AA8011-H14 on the advancing side and AA6082-T651 on the retreating side. A similar pattern was followed during the friction stir processing of the aforementioned welded joints. This was done to evaluate the impact of material positioning on the microstructure and mechanical properties of the said joints. The welded and friction stir processed (FSP) plates were cut and prepared for different tests. The tests conducted include the x-ray diffraction analysis, chemical composition, macrostructure and microstructure analysis, tensile tests, fractographic analysis, flexural tests and hardness tests. The test results of the said joints produced distinctive FSP conditions were studied comparatively. The FSW and TIG-welded average mean grain sizes preceding FSP were 23.247 μm and 31.765 μm respectively. The post-FSP results revealed greatly refined nugget zone microstructural grain sizes under both FSP conditions applied. The submerged FSP TIG-welded (SFSP-TIG) average mean grain size was 4.24 μm, while the normal FSP TIG-welded (NFSP-TIG) average mean grain size was 10.373 μm. The TIG-welded joints had a maximum ultimate tensile strength (UTS) of 85.42 MPa with a maximum yield strength of 68.336 MPa and tensile strain of 23.889 %. The NFSP-TIG showed a maximum UTS of 89.777 MPa with a maximum yield strength of 71.822 MPa, while the SFSP-TIG had a maximum UTS of 90.25 MPa with a yield strength of 72.2 MPa. The maximum tensile strain rates of the same were 27.994 % and 28.829 % respectively. The tensile results correlated with the grain sizes obtained. The nugget zone maximum average Vickers hardness of the TIG-welded joints was found to be 58.5 HV while the NFSP-TIG one was 61.5 HV and the 69 HV of the SFSP-TIG. The fracture surface morphology correlated with the strain rate results with the TIG-welded joints showing a brittle nature of fracture and the FSP specimens under both conditions showing a ductile behaviour. The normal friction stir processed FSW (NFSP-FSW) joints average nugget zone mean grain size was significantly refined to 12.475 μm and the submerged friction stir processed FSW (SFSP-FSW) joints to 5.611 μm. The FSW joints had a maximum UTS of 84.444 MPa with a yield strength of 67.555 MPa and a strain rate of 23.035%. The NFSP-FSW maximum UTS was 89.611 MPa with a yield strength of 71.289 and strain rate of 24.609 %, while for the SFSP-FSW joints was 92.511 MPa with a yield strength of 74.004 MPa and a strain rate of 25.975 %. The nugget zone maximum average Vickers hardness was found to be 48 HV for the FSW joints, 53 HV for the NFSP-FSW joints and 61 HV for the SFSP-FSW joints. The fracture surface morphology for the FSW and FSP joints all showed a ductile behaviour with the SFSP-FSW joints showing more ductility. With regards to specimen positioning the positioning of the harder alloy AA6082-T651 on the advancing side and the weaker strength alloy AA8011-H14 on the retreating side resulted in improved results than when the AA8011-H14 alloy was positioned on the advancing side.