Mechanical properties of friction stir welded dissimilar aluminium alloys (1050 and 5083 aluminium alloy plates)

Abstract

Friction stir welding is a solid-state joining process. This welding technique is energy efficient and environmentally friendly. It is also categorized as the best welding technique when compared to other conventional welding techniques. FSW does not face problems faced by the other conventional welding techniques like the solidification process, the loss of alloying elements, the presence of segregation, porosities, blowhole and cracks formed in the weld joint. It can also be used to join high strength aerospace aluminium alloys and other metallic alloys that are hard to weld by conventional welding. Friction stir welding is considered to be the most important development in the welding of aluminium alloys. Generally, the mechanical properties of conventionally welded aluminium joints are poor and not attractive. This study reports on the analysis of the mechanical properties of the friction stir welded dissimilar aluminium alloys (1050-H14 and 5083-H111). The study was conducted using 6 mm thick 1050-H14 and 5083-H111 aluminium alloy plates which were cut to fit the friction stir welding machine. The profile of the pin was triangular threaded with 20 mm shoulder diameter and 6 mm pin diameter. The triangular threaded pin had 1mm pitch and the height of 5.8 mm. The FSW parameters were chosen using Taguchi method. The rotational speed and transverse that was set during the test was 1000 rpm and 30 mm/min respectively. Tool tilting angle used during all the tests was kept constant at 2 degrees. Computerized numerical control (CNC) wire cutter was used to cut the specimens. This type of cutting was selected because it does not introduce heat during cutting and to ensure that the microstructure integrity is maintained. The microstructure and mechanical properties of the joints were studied using tensile test, bending test, optical microscope, scanning electron microscope (SEM) and microshardness test. The tensile test results showed that the ultimate tensile strength (UTS) of AA5083 parent material is higher than the one of the welded joint. It was also noticed that all tensile test specimens fractured on advancing side (AA1050). The bending results presented good ductility, allowing for very high bend angles and no cracks were observed. The bending load was applied at the center of the specimens but the bending of all the specimens was on the side of AA1050. There was a variation in grain size between AA1050 and AA5083 parent materials where AA1050 had higher grain sizes while AA5083 had smaller grain sizes. The grain size for the base metal AA1050 was ranging between 25 − 33 μm while the grain size for AA5083 base metal ranged between 6.6 − 8 μm. The grain size for the stir zone or welded region ranged between 7.3-11.4 μm. The fracture of the tensile test specimens was also investigated through SEM. All the specimens show a cup like dimpled fracture which is a characterization of ductile failure mode. The microhardness of AA1050 base material is about 40 while the one for AA5083 base material ranges around 80. There is notable decrease in microhardness from the AA5083 side towards the center which then followed by the notable drastic increase towards the center of the weld.