Joint Formation And Mechanical Properties Evaluation Of Friction Stir Lap Welding With Drastically Different Thickness

Lightweight design has been an important research direction in the aerospace industry.With the advantages of low density and high specific strength,aluminum alloys have become the alternative materials to relise the target of lightweight design.Friction stir welding(FSW)was a relatively new solid-state joining technology and widely used in the aerospace industry,owing to the advantages of joining aluminum and its alloys that were difficult to fusion welding.There was an urgent demand for achieving high-bearing joints of aircraft skins and panels.In this paper,2.2-mm and 20-mm-thick AA2024-T4 plates were joined via Friction stir lap welding(FSLW)by a circumferential notches shape pin.The joint formation,microstructure and mechanical properties were investaged syetematicly with the variations of welding parameters and the thickness of the alclad layer.The sever plastic deformation of the pin was studied in detial.The diffusion behavor of lap interface and fractography of the FSLW joints with drastically different thickness were clarified.The joint was divided into four zones: stirred zone(SZ),thermo-mechanically affected zone(TMAZ),heat-affected zone(HAZ)and BM.The SZ presented a dumbbell shape and the width of lower part was bigger than that of middle part.The width of the SZ is more than 3 times than the thickness of the upper sheet owing the enlarged pin tip.The pin with an enlarged top improved the material flow behavior and stirred the materials violently,which was beneficial to reducing the interface migration and facilitating the joining of interfaces.The maximium fracture load of 7.97 kN was obtained when the pin length is 0.2 mm longer than the thicnkness of the upper sheet.The fracture load was about 73.2% of the base material(BM),which need to be further improved.The onion skin was the typical features of surface morphology owing to the periodical plastic flow pattern.With the increase of welding speed,the morphologies of onion skin on the top surface were changed and the distance from the wave crest to the trough was directly increased.When the alclad layer was covered on the top surface,the onion skin wasn't clear at low welding speeds and there was a “ridge structure” at high welding speeds.The “ridge structure” was eliminated with higher heat input.With the welding speed of 100 mm/min and the rotational velocity of 800 rpm,the sound joint was achieved,due to the sever plastic deformation from the pin and appropriate heat input from the welding parameters.The interface defects and cavity defects also were eliminated which usually ouucred in the joints by conventional pins.The effective lap width reached 8.28 mm,which was near 4 times as much as the thickness of the upper sheet,and the bearing ability was much higher than that of the HAZ in upper sheet.With the thickness of the alclad layer increasing,the cold lap defect and hook defect occurred at the IZ.When the thickness of alclad reached 0.2 mm,the alclad layer gathered at the IZ,which was detrimental to mechanical properties.When the heat input was mismatching to the joint formation,the cavity defects occurred at the SZ.The interface of FSLW joints was characterized by laminated composites,which consisted of alclad layer and 2A12-T4 aluminum alloys.The thickness of diffusion layer,which consisted of Cu and Mg elements,was no more than 3?m.The laminated composites had relationships with the sever plastic deformation from the pin and the heat input form the welding paramaters.The maximum fracture load of 8.55 kN was obtained with the welding speed of 100 mm/min and the rotational velocity of 800 rpm,which was 92.2% of the BM.During the tensile test,cracks initiated at the top of the cold lap defect or unbonded interface,and then propagated along four different paths.When the heat input was insufficient and the thickness of alclad layer was 0.2 mm,cracks propagated along the lap interface.When the rotational velocity reached 1200 rpm,cracks propagated into the SZ and finially fractured along the SZ.When the heat input was insufficient and the thickness of alclad layer was no more than 0.1 mm,cracks propagated along the interface of the TMAZ and SZ,and than extended in the SZ,finally fractured at the SZ.When the heat input is appropriate,cracks propagated along the interface of the TMAZ and SZ,and than extended in to the TMAZ,finally fractured at the TMAZ.This fracture behavior was ductile,which was beneficial to the mechanical properties.The sound joints were obtained with the circumferential notches shape pin,when the pin length is 0.2 mm longer than the thickness of upper sheet,the welding speed of 100~200 mm/min and the rotational velocity of 800 rpm.