Study On Microstructures And Mechanical Properties Of Laser Welded Joints Of Ti₃Al Alloy And Stainless Steel

With a variety of advantages,such as high specific strength,low density,stability at high temperatures etc.,Ti₃Al alloy is considered to be potential structural material in the aerospace,automotive,and other fields.Therefore,the joining of Ti₃Al and stainless steel can realize the complementarity of the economical and performance advantages of the two materials,which has strong practical significance and important value.However,few related studies were reported so far and they are focused on non-melting welding methods such as brazing and diffusion welding.This has limitations on the form of the joint and the welding efficiency is also low.In contrast,laser welding technology has its unique advantages.It is of great practical significance to study Ti₃Al/stainless steel heterogeneous laser welding.In this paper,the microstructures and mechanical properties of Ti₃Al/stainless steel laser welded joints were studied.The weldability of the two base metals was studied.The effects of Cu and V interlayers and welding parameters on microstructures and mechanical properties of joints were discussed.The results showed that Ti₃Al/stainless steel cannot be effectively joined without the interlayer,and the joint immediately cracked after welding.A huge crack can be found throughout the middle of the weld.A large amount of Fe and Ti elements were detected in the weld.Under the optical microscope,there were light and dark areas in the weld.The main microstructures were Fe₂Ti+FeTi and??Fe,Cr?+Fe₂Ti,and the microhardness values were 853HV and 560HV,respectively.Cu interlayer can improve the microstructure of weld metal.The Cu interlayers of 250?m and 500?m can realize the joining of Ti₃Al/301L and the strength of the joints were 30MPa and 78MPa,respectively.The joint with 250?m copper interlayer was poorly formed,the weld microstructure was uneven.Defects such as cracks and collapses were likely to occur.The joint with 500?m copper interlayer was well formed,and the weld can be divided into three regions.The weld near the Ti₃Al side was mainly composed of CuTi,CuTi₂ intermetallics,Cu solid solution and Ti solid solution.The center of the weld was mainly Cu solid solution and a small amount of CuTi and CuTi₂ intermetallics.The 301L side weld was mainly composed of a Cu isle,?-Fe solid solution and a small amount of CuTi intermetallics.Ti₃Al/weld interface mainly contained Ti and Cu elements,CuTi₂ phase and a small amount of CuTi phase,the microhardness was the highest,and tensile fracture also occured at this interface.The 301L/weld interface area was well formed and consisted mainly of?-Fe and Cu solid solution.The 250?m V intermediate layer cannot realize the joining of Ti₃Al/301L,and cracks occurred at the center of the weld after welding.Because it cannot improve the weld composition and the content of Fe and Ti in the weld was high.There were inevitably more brittle FeTi and Fe₂Ti intermetallics in the weld,which lead to joint fracture.The 500?m V interlayer can realize the joining of Ti₃Al and 301L.The joint shape was well formed.There were a large number of??Ti,V?solid solutions in the weld.There were??Fe,V?and??FeV?near the 301L side.The microhardness value reached the largest in this area,because the??FeV?phase had a large brittleness,which will adversely affect the joint.The fracture also occurred here in the tensile test,and the tensile strength was 74 MPa.Optimized welding parameters and welding process can improve the tensile properties of the joint with Cu interlayer.In this test,the joint strength was 78MPa when the pulsed current and welding speed were optimized.The interface region obtained by appropriate welding parameters not only realized the interface joining,but also avoided the interface area being too thick and brittle.When the laser beam was shifted to the interlayer/301L interface,the tensile strength was further increased to 131 MPa.The offset of the laser beam changed the melting ratio of the base metal on both sides,the Ti content in the weld decreased,and the weld composition was improved.According to the test results,the influence of Cu/V multi-interlayer on the microstructures and properties of the joint was studied.The results showed that the Cu/V composite interlayer can realize the effective joining of Ti₃Al/301L.The welded specimen was well formed without defects and cracks.The average content of the main elements in the weld zone was 51 at.%Cu,18 at.%V,9 at.%Ti and 13 at.%Fe.The weld zone was mainly composed of??Ti,V?,Cu and V solid solution.The elemental composition of the Ti₃Al/weld interface region was improved,and the Cu content in the interface region was also significantly reduced.The interface was mainly composed of Cu-Ti intermetallics and V solid solution.There were?-Fe phase and a small amount of??FeV?and??Fe,V?in the 301L/weld interface region.The Cu/V multi-interlayer significantly improved the microstructures of the Ti₃Al/301L joint,and the tensile strength was significantly increased to 219 MPa.A large amount of Cu solid solution in the weld reduced the welding stress and slowed the crack propagation.The fracture location of the specimen was at the Ti₃Al/weld interface.Compared with the laser welded joints with Cu and V interlayers,the joint strength with Cu/V multi-interlayer was increased by 180.77%and 195.95%,respectively.