Study On The Joining Mechanism Of TA0/Q235 Dissimilar Metals With Cu Intermediate Layer

Ti/steel composite plate was fabricated by explosive welding.Ti/steel composite plate was widely used in mechanical industry,chemical industry,sea water desalination,power plant desulfurization,oil and gas storage and transportation because of its advantages such as low cost,high corrosion resistance as Ti alloy and higher mechanical performance as steel.A large amount of TiC and Ti-Fe phase formed in the Ti/steel dissimilar alloy joint during fusion welding.Further,a great difference between Ti and steel could cause a large internal stress resulting in the formation of cold crack.In this case,Ti and steel dissimilar alloy could not be directly welded by fusion welding in the engineering application.In the engineering application,Ti/steel composite plate was welded step by step in which Ti was firstly welded,steel was welded subsequently.In this process,Ti melt and steel melt would not contact together resulting in that no metallurgical bonding was created with a low bonding strength,poor gas tightness.This kind of joint can only be used in fabricating chimney such as the chimney used in the thermal power plant.In this paper,Ti/steel dissimilar alloy plasma arc welding technique was developed,aiming for creating metallurgical bonding between Ti and steel reinforcing the bonding strength and gas tightness promoting the application of Ti/steel dissimilar alloy welded pressure vessel in seawater desalination,oil transportation.In this paper,pure Ti powders widely used in the industry was plasma arc deposited on Q235 steel plate by using intermediate layer Cu or not,in which DC plasma arc welding machine was applied and coaxial power feeding system was well-designed.The thickness of the weld was accurately controlled and gas protection system was designed to avoid the oxidation of Ti alloy.The weldability of Ti and Q235 steel welded by plasma arc welding with using Cu intermediate layer or without using intermediate layer were investigated.Effects of welding current?85,90,95,100,105 A?,welding speeds?18,19,20 cm/min?on the microstructure and mechanical performance of the joint were investigated.It was shown that cold crack was formed and Titanium cladding was even fallen when TA0 pure Titanium powders was directly deposited on Q235 steel plate.When pure copper as an intermediate material was applied at higher welding than 90 A,the brittle intermediate phase and cold crack could be avoided efficiently by using plasma arc cladding method.A lower heat input was produced when a welding current lower than 90 A was used resulting in that Cu powder and steel plate could not be adequately melted.Continuous Cu intermediate cladding layer could not be formed caused Titanium contacting steel molten pool in the local area.As a result,a cold crack was formed.A sound joint with better surface appearance was obtained at a welding current of 100 A.And good metallurgical bonding interfaces between Ti and Cu and between Cu and Q235 steel plate were formed and no defect was formed in the joint.Fine needle-like equiaxed dendrites,coarse dendrites were respectively formed in Ti cladding layer and Cu cladding layer.A similar hardness distribution on the cross section of the joints was observed,in which the higher hardness appeared in the region near Cu/Ti interface.The highest hardness with the value of 448 HV_0.2.2 was obtained at a welding current of 100A.A good joint with better surface appearance was produced and no cold crack,lack fusion,slag inclusion defect or other defects were observed in the inside of the joint.The highest hardness with a value of 441 HV_0.2.2 was observed at the welding speed of 18 cm/min.Copper based solid solution and a less brittle intermetallic compounds such as CuTi2,CuTi was formed and on a brittle intermetallic compound like Ti-Fe and TiC under XRD analysis.As a result,the formation of Ti-Fe phase and TiC intermetallic compound could be avoided efficiently reducing cold crack tendency,by using Cu intermediate layer under the low heat input of plasma arc welding.Temperature is key factors of the dynamics of intermetallic compound formation for plasma arc deposition and plasma arc welding.To understand the mechanism of the formation of the plasma arc deposition and the plasma arc welding joint,numerical simulation of temperature and stress fields were studied through finite element analysis.For plasma arc heat resource,high temperature residence time was short and heat input was quite low.It was also found temperature even reached 2753?and a large temperature gradient was formed for directly Ti/Q235 plasma arc deposition.The highest temperature dramatically decreased to2276?and temperature gradient also reduced obviously.Similar results were found in the plasmas arc welding comparative study between Ti/Q235 dissimilar metal directly welding and Ti/Q235 dissimilar metal welding by using Cu intermediate layer.A larger volume of materials were melted during Ti/Q235 plasma arc deposition than that of plasma arc welding resulting causing higher temperature and residual stress.For two situations,high temperature can be reduced dramatically and cooling rate can be increased dramatically due to good heat dissipation capacity of copper.As a result,the formation tendency of brittle phases can be avoided showing good controlling effect of the microstructure of the Ti/Q235 dissimilar metal.Further,residual stress and the stress gradient are reduced dramatically after using Cu intermediate layer resulting in decreasing the tendency of the cold crack formation of Ti/Q235 dissimilar metal joint.