Study On Microstructures And Properties Of The Joint Of High Entropic Welding Titanium/steel Bimetallic Sheets

Titanium/steel bimetallic sheet is a type of layered metal matrix composite material prepared by explosive welding or hot rolling,with titanium as the flyer plate and structural steel as the base plate.It has the advantages of excellent corrosion resistance of titanium and low cost,high strength of steel,which is an ideal corrosion resistant protective material for marine engineering equipments.However,it is harmful to the weld forming and mechanical properties of the joint,due to the formation of Fe-Ti intermetallic compounds（IMCs）in directly welding of titanium/steel bimetallic sheets.Therefore,the welding process of titanium/steel bimetallic sheets is complicated and many difficult problems are existed.High-entropy alloys（HEAs）have high mixing entropy effect,which could reduce the formation of intermetallic compounds.Therefore,this work aimed to fusion welding of titanium/steel bimetallic sheets and promote in-situ forming of a high-entropy alloy in the weld.Then the weld formation,microstructures,phases,and mechanical properties of the joint were studied.Try to optimize the welding process of titanium steel bimetallic sheet and improve the service performance of joints.Relevant research has important theoretical research and practical significance.Firstly,the feasibility of welding TA1/Q345 bimetallic sheets by using the HEA of Fe Co Cr Ni Mn as filler material was explored.The difference of weld forming ability of two joints by using two kinds of V-shape groove is compared.It is found that the welding of TA1/Q345 bimetallic sheet can be realized by using Fe Co Cr Ni Mn as a single filler material,and the weld forming ability when the groove root was opened in the Q345 base plate is better than that was opened in the TA1 flyer plate.However,due to the weld dilution and low powder filling efficiency,high mixing entropy thermodynamic environment was not formed in the weld.There are a lot of Fe and Ti elements and form hypoeutectic of Fe₂Ti+（α-Fe+Fe₂Ti）_E in the weld zone（WZ）.Three kinds of IMCs of Fe₂Ti+Fe Ti+Ti C were observed near the TA1/Q345/WZ interface,which is harmful to the formation of the weld and leads to cracks.WZ has high hardness with the maximum hardness of 841 HV_0.2 and low strength of 117 MPa.Subsequently,to solve the low efficiency of filler materials,a Ni-Co-Al multi-principal welding wire for arc welding was developed by taking high-strength HEA of Fe Co Ni Al Ti as the target component of the weld.The high entropy environment of WZ was invesgated and the relationship between chemical composition,microstructures and mechanical properties of WZ were established.It is found that the welded joint of TA1/Q345 bimetallic sheet without cracks and pores could be obtained by using Ni-Co-Al multi-principal wire.The WZ had five main elements and formed high mixing entropy.Multiphases of FCC+BCC+Fe-Ti IMCs were formed in the WZ.However,there are a transition zone（TZ）with the wide of about 370μm at the TA1/WZ interface,and a narrow region（NR）with the wide of about 50μm at Q345/WZ interface,only Fe-Ti IMCs were formed due to the low mixing entropy?S_mix and high mixing Gibbs free energy?G_mix.The study also found that Ti element has a significant impact on the mechanical properties of the high entropy weld.Excessive Ti elements in weld metal lead to extremely high hardness(853 HV_0.2)and low strength（123 MPa）of the WZ.By expanding the groove angle,the content of Ti element in WZ was effectively reduced,the hardness of WZ was reduced,the joint plasticity was increased,and the joint strength was improved.In addition,the surface of the weld has better corrosion resistance than the base metal of titanium/steel bimetallic sheet in 3.5 wt.%Na Cl solution.In order to in-situ form the eutectic high entropy alloy with good crack resistance and mechanical properties in the weld,four kinds of multi-principal filling materials of Fe_25-xCo₂₅Ni₂₅Cu₂₅Ti_x（x=5,10,15,20）were designed and prepared to weld TA2/Q235bimetallic sheet.By changing the composition of the filler powder,the composition window of the weld forming a large amount of eutectic structure was explored,and the microstructures and mechanical properties of the weld were analyzed.It is found that the formation of eutectic structures with high entropy characteristics in WZ could reduce the generation of large size cracks.A large amount of eutectic structure is formed in the weld metal that welded by filling Fe₁₀Co₂₅Ni₂₅Cu₂₅Ti₁₅.No cracks were found in the WZ,and only a 286μm long crack was found at the interface of TA2/WZ/Q235.In addition,with the increase of Ti element in the filler powders,the phase in weld was changed from FCC+Co Fe＿B2 phase to FCC+Co Fe＿B2+（Co,Ni,Fe）Ti＿B2 phase,then converted to FCC+Fe₂Ti＿Laves phase.The hardness of weld metal was significantly increased.The tensile strength and plasticity of the joint were reduced with the increase of Ti element in the filler materials.