Ambient And Cryogenic Mechanical Properties Of Friction Stir Welded AA2195 Alloy Joints

AA2195 Al-Li alloy is an important structural material widely used in aerospace industry,especially applied in rocket liquid fuel tank.For large structural parts,joining process is inevitable.Friction stir welding?FSW?is a novel and efficient solid-state joining process,which is an ideal technology for the joining of aluminum alloys.2 mm thick AA2195-T8 Al-Li alloy plates were friction stir welded at tool rotation rate of 1000 rpm and traverse speeds?TS?of 100⁴00 mm/min under three types of faying surface conditions,i.e.1)without faying surface treatment,2)faying surface milled,and 3)bead-on-plate welding.The hardness test and tensile test were carried out to obtain the properties of joints.The macrostructure of as-weld specimens and the fracture location of as-tested specimens were studied by optical microscope.The microstructure of as-weld specimens and the fracture surface of as-tested specimens were studied by scanning electron microscope.Whether the joint line remnant?JLR?will affect the mechanical properties were studied by comparing the different faying surface conditions.The influence of the change of the traverse speed on the microstructural evolution?especially JLR?,tensile property and fracture behavior was studied.In addition,through comparing the mechanical properties of the base metal and the joints tested at room temperature and cryogenic temperature respectively,influence mechanism of testing temperature on joint property was studied.The results achieved are as follows:In the stir zone of the butt welding joint,there exists a joint line composed of alumina-particle arrays and microcracks generated from initial faying surface.The joint line remnant?JLR?evolves from smooth to serrate as TS increases.It still exists after milling the faying surface prior to welding but becomes relatively obscure.For high-strength heat-treatable 2195-T8,JLR deteriorates the tensile strength of the joints at high TS?TS?200mm/min?,with the joint prematurely fracturing along JLR.The increase of TS not only leads to less reduction in the strength of lowest hardness zone(S_LHZ)due to lowered heat input,but also decreases the bonding strength of JLR(S_JLR)due to more serrate JLR with higher density of alumina particles and microcracks inside.Faying surface treatment prior to welding can improve S_JLR.The lower one between S_JLR and S_LHZ determines the fracture location of 2195-T8 FSW joints.From room temperature to cryogenic temperature,the ultimate strength improvement of the joints is higher than that of base metal from room temperature to cryogenic temperature because strength improvement of the weld zone is higher than that of base metal.Thus,the welding efficiency of joints at cryogenic temperature is higher than that at room temperature on most welding conditions.