Weldability And Joining Mechanism Of Titanium-stainless Steel Dissimilar Metals By MIG/TIG Double-sided Arc Welding

To solve the problem of titanium-stainless steel(Ti-SS)joint embrittlement caused by Ti-Fe hard-brittle phases,this dissertation proposed to apply MIG/TIG double-sided arc welding(DSAW)to join Ti and SS and conducted studies on the weldability and joining mechanism of the Ti-SS dissimilar metals.The joining modes of Ti-SS joints welded by MIG/TIG DSAW using silicon-bronze wire were studied,and the microstructures of typical joints with different joining modes were analyzed emphatically;the effect of welding parameters on the mechanical properties of the joins was studied,and the relationship among welding parameters,microstructures(joining modes)and mechanical properties was revealed;the prediction models for Ti side and SS side interface joining mechanisms and the joint tensile strength were established by using the back-propagation neural network optimized by genetic algorithm(GA-BPNN),and the process windows of different joining modes of the joints were determined according to the predicted results of interface joining mechanisms,further,the process parameters were optimized by the use of genetic algorithm(GA).The main research results are as follows:The titanium-stainless steel joints with excellent performance were obtained by MIG-TIG double-sided arc welding using silicon bronze wire without grooving and backing.All three types of Ti-SS joining modes,such as brazing,welding-brazing and welding modes,were obtained by welding process control.In the typical brazing joint,the Ti side brazing interface was mainly composed of Ti-Cu(Ti2Cu,TiCu)intermetallic compound(IMC)layer with dispersed Ti5Si3 phase,with a thickness of about 60 ?m;the SS side brazing interface was mainly composed of a flat diffusion layer,containing the brittle TiFeSi and ?(Ti(FexSi1-x)2)phases,with a thickness of about 0.5 ?m.According to the joining mechanism of Ti side and SS side interfaces,the welding-brazing mode can be distinguished into two forms:welding(Ti)-brazing(SS)and welding(SS)-brazing(Ti)joining modes.In the typical welding(Ti)-brazing(SS)joint,the Ti side welding interface was mainly composed of Ti-Cu(Ti2Cu,TiCu and TiCu2)IMC layer with Ti(CuxSi1-x)2 and Ti5Si3 phases dispersed on it,showing a thickness of about 165 ?m;the SS side brazing interface was mainly composed of a 3-mm-thick flat diffusion-reaction layer,which was composed of brittle TiFeSi phase.In the typical welding(SS)-brazing(Ti)joint,the Ti side brazing interface showed an about 100 ?m-thick reaction layer,which was mainly composed of Ti-Cu(Ti2Cu,TiCu,Ti3Cu4,Ti2Cu3 and TiCu4)IMCs and dispersed Ti5Si3 phase,the X phase would be formed in the layer with the large melting of the SS base metal;the SS side welding interface showed the peninsula structure,which was mainly composed of brittle TiFe2Si,Fe(s,s)and Cu(s,s)phases.In the typical welding mode joint,the Ti side welding interface was mainly composed of Ti-Cu(Ti2Cu,TiCu,TiCu2,Ti2Cu3 and TiCu4)IMC layer with dispersed Ti5Si3 and ? phases,showing a thickness of about 180 ?m;the SS side welding interface exhibited the island structure mainly composed of ? brittle phase and intergranular Fe(s,s)and Cu(s,s)phases under the action of complex molten pool flow.Based on the optimal parameters determined by the orthogonal test,the influence of main control factors on the joining mode and performance was studied.With other parameters kept constant,the joining modes of the joint were not changed with the variation of the MIG voltage,remaining welding(SS)-brazing(Ti)mode;the joints transformed from welding(SS)-brazing(Ti)mode to welding mode with the increase of TIG current or decrease of welding speed and from welding(SS)-brazing(Ti)mode to brazing mode with the decrease of the TIG current or increase of the welding speed;the joints transformed from welding(SS)-brazing(Ti)mode to brazing mode when the TIG torch offset to the Ti side or further to the SS side.The tensile strength of Ti-SS joints was determined by IMC characteristics at the Ti side and SS side interfaces.The brazing and welding(Ti)-brazing(SS)joints were compromised by the brittle and flat IMC layer on the SS side,showing low tensile strength;the tensile strength of the welding joints was limited by the thicker IMC layer on the Ti side interface;the welding(Ti)-brazing(SS)joints showed high tensile strength because the thin IMC layer at the Ti side brazing interface and the peninsular/island structure at the SS side welding interface were beneficial to reduce the interface brittleness.The prediction models for joint interface joining mechanism and tensile strength were established using the Genetic Algorithm optimized Back-propagation Neural Network(GA-BPNN),the models showed excellent fitting and generalization ability.Based on accurate prediction of interface joining mechanism,the prediction of joining mode process window was realized:with the MIG torch placed on the weld center,when the TIG torch was offset on the SS side or moderately on the Ti side,the joints tend to be joined by brazing mode under low heat input;when the TIG torch was considerably offset on the Ti side,the joints tend to be joined by welding(Ti)-brazing(SS)mode under large heat input;when the TIG torch was offset on the SS side or moderately on the Ti side,the joints tend to be joined by welding(SS)-brazing(Ti)mode under moderate heat input;when the TIG torch was moderately offset on the Ti side or SS side,the joints tend to be joined by welding mode under high heat input.The GA optimization algorithm was used to optimize the process parameters.The optimized process parameters were:MIG voltage 12.605 V,TIG current 78.543 A,MIG-TIG longitudinal relative position 0.663 mm,and the transverse position of TIG-1.742 mm.Under this parameter,the joint was predicted to be welding(SS)-brazing(Ti)mode,and the nominal strength of the joint reached 359.9 MPa.According to the optimized parameters,the obtained actual nominal strength of the welding(SS)-brazing(Ti)joint was 348.4 MPa.