| The hybrid component of titanium alloy/stainless steel has the advantages of excellent corrosion resistance,weight reduction and low cost.Implementing the reliable joining between titanium alloy and stainless steel could utilize their comprehensive advantages,which is significantly valuable for the practical application.There are huge differences between titanium and steel on the physical and chemical properties,which would easily result in the formation of brittle intermetallic compounds and large residual stress in titanium/steel welded joints.Nevertheless,the titanium/steel joining problems can be effectively solved by brazing method.And the titanium/steel vacuum brazing problems lie in deficient designing theory of filler metals,low joint strength,unclear relationship between the alloying constituents and joint performance and indistinct reaction mechanism of joint interfaces.In present study,TC4 titanium alloy/316L stainless steel dissimilar materials were employed as the object of study,and we mainly investigated the effect of alloying constituents of Ti-Cu-based filler metals on the performance of brazed joints.The alloying element composition and brazing parameters were optimized and brazing mechanism was revealed,aiming to improve the mechanical properties of the brazed joint.The investigation in this study was expected to provide the theoretical foundation and scientific evidence for the engineering application of the titanium/steel brazed joints.The novel Ti-Cu-based amorphous filler metals were designed based on the "cluster method".Focusing on how the alloying elements affect the performance the brazed joint,the following investigation was conducted.With the characterization methods of electron probe micro-analyzer(EPMA),scanning electron microscope(SEM),electron back-scattered diffraction(EBSD),X-ray diffraction(XRD)and scanning transmission electron microscope(STEM),the interfacial microstructure of the TC4 titanium alloy/316L stainless steel brazed joint was studied in detail.And the mechanical properties of the brazed joints were studied with the methods of compress-shear test and nano-indentation test to optimize the filler metal composition and brazing parameters.Meanwhile,the formation mechanism and growth behavior of the reaction zones in the joint were studied.Combining the fracture path of the brazed joint,lattice misfit of the interfaces and nano-indentation test,the fracture mechanism of the brazed joint was revealed.The main results and conclusions are listed as follows:(1)Based on the cluster formular[Ti-Cu6Ti8]Cu3 of filler metal,three kinds of Ti-Cu-based amorphous filler metals(Ti33.3Zr16.7Cu50-xNi.,Ti50-xZrxCu39Ni11 and Ti38.8-xZr11.2VxCu39Ni11)were designed with the substitution of Cu by Ni,and Ti by Zr and V,which implement the large-scale regulation of alloying constituents.The effect of Ni,Zr and V content on the characteristic temperatures of filler metals and wettability of base metals was investigated.The liquidus temperature of the filler metal was improved,attributed to the substitution of Ni,Zr and V.At 11 at.%Ni and 16.7 at.%Zr,Ti33.3Zr16 7Cu50-xNix and Ti50-xZrxCu39Ni11 filler metals respectively showed inferior wettability to the base metals.At 2.8 at.%V,Ti38.8-xZr11.2VxCu39Ni11 filelr metal had superior wettability to the titanium alloy,and the addition of V to the filler metal had little effect on the wettability to the stainless steel.The filler drops on both titanium alloy and stainless steel spreading samples consisted of matrix phase and primary phase,and increasing the content of Ni,Zr and V promoted the dissolution separation of titanium alloy base metal.(2)The interfacial microstructure of vacuum-brazed joint of TC4 titanium alloy/Ti-Cu-based filler metal/316L stainless steel was characterized through elemental distribution,quantitative analysis,TEM slected-area electron diffraction and high-resolution TEM test.The brazed joint featured the layered interfacial microstructure:TC4 alloy substrate/diffusion zone/brazed seam/interfacial zone/316L steel substrate.The diffusion zone consisted of Widmanstatten microstructure and ?-Ti,which resulted from the diffusion of Cu from the filler metal to titanium alloy substrate.Residual elements zone and FeTi reaction layer formed in the brazed seam,and most alloying components in the filler metal retained in residual elements zone.Three reaction layers,including Fe2Ti,FeCr and ?-Fe,formed in order in the interfacial zone,which resulted from the solid diffusion of Ti from filler metal into stainless steel.The formation of FeCr and ?-Fe layers resulted from the up-hill diffusion of Cr casued by the aforementioned Ti diffusion.A submicron ?-Ti phase precipitated around the FeTi/Fe2Ti interface.The submicron ?-Ti phase had the orientation relationship(OR)with FeTi and Fe2Ti phases,which is favorable to improve the joint strength.(3)The evolution of reaction layers in the brazed joint was studied under varied brazing parameters and different content of Ni,Zr and V in filler metals.With increasing the brazing temperature and prolonging the brazing time,more Ti atoms diffused from filler metal to stainless steel and long-distance diffusion of Ti occurred,which promoted the up-hill diffusion and segregation of Cr.Thus the thickness of interfacial zone improved,particularly the ?-Fe layer in the interfacial zone.The brazing temperature is the key factor that affects the thickness of FeTi layer.Increasing the brazing temperature intensified the metallurgical reaction between the filler metal and stainless steel,resulting in the thickening of FeTi layer.The increase of Ni content weakened the mutul diffusion and reaction between between the filler metal and stainless steel,which reduced the thickness of FeTi layer.The Ti content of the filler metal decreased because of the substitution of Zr and V,thus the metallurgical reaction between the filler metal and substrates weakened and further decreased the thickness of FeTi reaction layer.Besides,less Ti atoms diffused from the filler metal to stainless steel,which thinned the interfacial zone.(4)The fracture behavior of the brazed joint was revealed based on the crystal structure at interfaces,and the relationship between interfacial microstructure and mechanical properties was established.During the brazing process,the interface of FeTi/Fe2Ti was a liquid/solid interface with large residual stress,which was the crack initiation of the brazed joint because it was difficult to form the orientation relationship between Fe2Ti and FeTi phases.Both Fe2Ti and FeCr layer has close nano-indentation hardness(14.8 GPa/14.9 GPa)and elastic modulus(215.8 GPa/222.2 GPa),and they have the smaller lattice misfit with the crystal plane misfit of 13.92%for and crystal direction misfit of 10.21%.Therefore,the spreading behavior of the initiated cracks was regardred similar in Fe2Ti and FeCr reaction layers.The thickness of FeTi layer was the controlling factor for the shear strength of the brazed joint,since increasing the thickness of FeTi reaction layer was beneficial for the improvement of the joint shear strength.The optimized content of Ni,Zr and V was 11 at.%,16.7 at.%and 5.8 at.%,respectively.At the brazing parameter of 990?/10 min,the shear strength of the joint brazed with Ti33.3Zr16.7Cu39Ni11 amorphous filler metal reached the maximum value of 318 MPa,which was higher than the public reports of titanium/steel joint brazed with Ti-Cu-based filler metals. |
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