Multiscale microstructural consideration of enhanced shear strength in TiAl intermetallic/K4169 alloy composite joints prepared by vacuum brazing with(Ti,Zr)-Ni-based amorphous filler metal

TiAl intermetallic could be used to replace Ni-based alloy in assemblies to generate excellent specific strength.A(Ti,Zr)-Ni-based amorphous filler metal Ti_(21.25)Zr_(25)Ni_(25)Cu_(18.75)(at.%)was designed using a cluster-plus-glue-atom model to successfully vacuum braze K4169 and TiAl bimetallic assemblies.At various brazing temperatures and holding time,the quantitative relationships between lattice distortion,grain boundary,dislocation density,and hardness,elastic modulus,shear strength of the joints were investigated.Meanwhile,the fracture mechanism of the joints was revealed.The brazed seam mainly consisted of solid diffusion reaction layers(ZonesⅠandⅢ)and filler metal residue zone(ZoneⅡ).When the brazing temperature increased to 1030℃,grain refinement occurred in the brazed seam.ZoneⅠwas primarily composed of(Ni)ss[0-11]+TiNi[011]/(Cr,Fe,Ni)ss[0-11]/(Ti,Zr)Ni[0-1-1]+(Cr,Fe,Ni)ss[0-11].The(Ti,Zr)(Ni,Cu)[001]and(Ti,Zr)(Ni,Cu)[101]intermetallic compound-based solid solutions were formed in ZoneⅡ.And the lattice distortion of(Ti,Zr)(Ni,Cu)[101]and(Ti,Zr)(Ni,Cu)[001]was 32.05%and 14.82%,respectively.As a result,the proportion of low angle grain boundaries(LAGBs)and deformed grains in ZoneⅡrose to 38.6%and 38.7%.In ZonesⅠandⅢ,the proportion of LAGBs reduced to 8%and 3.4%,respectively.As the holding time increased,the long-range diffusion of Al in ZoneⅡcaused the(Ti,Zr)(Ni,Cu)[001]with cubic structure to transform into(Ti,Zr)(Ni,Cu,Al)[00-1]with hexagonal crystal system structure,where the lattice distortion was 4.42%and 10.49%for a and c.At 1030℃/10 min,the average geometrically nec-essary dislocation densities(GNDs)in ZonesⅠ,ⅡandⅢwere 9.87×10^(14)m^(-2),8.55×10^(14)m^(-2)and 11.4×10^(14)m^(-2),respectively.Therefore,the shear strength of joints reached 322 MPa due to the lattice distortion,dislocation strengthening and fine grain strengthening.Meanwhile,the plastic and brittle hard phases were generated in ZoneⅡand displayed a mechanical interlocking structure that contributed to the performance of the joint.Both(Ti,Zr)(Ni,Cu)[001]and(Ti,Zr)(Ni,Cu)[101]in ZoneⅡformed along differ-ent low-index cleavage planes during transgranular fracture.The cracks initiated in this region extended to the interface between Zones I andⅡand exhibited bimodal grain characteristics.

Vacuum brazing TiAl intermetallic to K4169 alloy using amorphous filler metals Ti_(56.25–x)Zr_(x)Ni_(25)Cu1_(8.75)

A series of Ti_(56.25-x)Zr_(x)Ni_(25)Cu1_(8.75)(x=0–25,at.%) filler metals were designed based on a cluster-plus-glue-atom model to vacuum braze TiAl intermetallic to K4169 alloy. The impact of Zr content on the interfacial microstructure and shear strength of joints was examined. And the relationship between the interfacial lattice structure and the fracture behavior of the joint was investigated. The findings reveal a sectionalized characteristic with three reaction zones (Zone I, Zone II and Zone III) in the microstructure of the TiAl intermetallic to K4169 alloy joint. As the Zr content in filler metals increased, the diffusion of Ti transitioned from long-distance to short-distance in Zone I, changing the initial composition from TiNi_(3) /TiNi/NiNb/(Cr, Fe, Ni)SS to NiCrFe/(Cr, Fe, Ni)SS /TiNi. In Zone II, the initial composition altered from TiNi_(3) /TiNi to TiNi/Ti_(2) Ni/TiNi_(3) /TiCu/TiNi. The interface between Zones II and III altered from a non-coherent and semi-coherent interface of TiNi/TiAl/Ti_(3) Al with significant residual stress to a semi-coherent interface of TiNi/TiNi_(3) /TiAl_(2) /Ti_(3) Al with a gradient distribution. The shear strength of the joint initially decreased and then increased. When the Zr content of filler metal was 25 at.%, the shear strength of the joint reached 288 MPa. The crack initiation position changed from non-coherent TiNi/TiAl interface with high angle grain boundaries (HAGBs) and lattice mismatch of 65.86 at.% to a semi-coherent Ti3 Al/TiAl2 interface with a lattice mismatch of 20.07 at.% when the Zr content increased. The brittle fracture was present on the fracture surfaces of all brazed joints.

Diffusion bonding of AlCoCrFeNi2.1 eutectic high entropy alloy to TiAl alloy

High entropy alloys have special microstructure and remarkable properties.To explore their potential engineering application in high temperature structures,the microstructure evolution of bonding interface,the elemental diffusion behavior and mechanical property of the diffusion bonded joint between AlCoCrFeNi2.1eutectic high entropy alloy(EHEA)and TiAl alloy were investigated.Four reaction layers(rodlike B2 phase,Al(Co,Ni)2Ti,τ3-Al3NiTi2+TiAl,τ3-Al3NiTi2+TiAl+Ti3Al)formed in the diffusion zone near FCC phase of EHEA,but three layers(Al(Co,Ni)2Ti,τ3-Al3Ni Ti2+Ti Al,τ3-Al3Ni Ti2+Ti Al+Ti3Al)formed near B2 phase.Al and Ni controlled the reaction diffusion of EHEA and TiAl alloy,coarsened the acicular precipitated B2 phase and turned Ti Al phase into Al(Co,Ni)2Ti andτ3-Al3NiTi2 phases.All these reaction layers grew in a parabolic manner as a function of bonding temperature.Rodlike B2 phase has the lowest growth activation energy of 125.2 kJ/mol,and the growth activation energy ofτ3-Al3Ni Ti2+TiAl layer near B2 phase is much lower than that near FCC phase.The penetration phenomenon and convex structure formed in the diffusion zone,which resulted in interlocking effect and enhanced the strength of resultant joints.The highest shear strength of 449 MPa was achieved at 950℃.And the brittle fracture generally initiated at the interface between Al(Co,Ni)2Ti andτ3-Al3NiTi2+TiAl layers.

CuTiNiZrV Amorphous Alloy Foils for Vacuum Brazing of TiAl Alloy to 40Cr Steel

Vacuum brazing of TiAl alloy to 40Cr steel sheets was conducted with newly developed CuTiNiZrV amorphous foils. It was found that a diffusion layer,filler metal and reaction layer existed in the brazed seam. The diffusion layer in the joint brazed with Cu43.75Ti37.5Ni6.25Zr6.25V6.25(at.%) foil was flat and thin,containing Ti19Al6 and Ti2Cu intermetallic compounds; however,the diffusion layer brazed with Cu37.5Ti25Ni12.5Zr12.5V12.5 foil was uneven with bulges,consisting of essentially Ti-based solute solution. The foil with 12.5 at.% V showed inferior spreadability compared to that with 6.25 at.% V at brazing temperature. However,fracture happened along the diffusion layer with 6.25 at.% V foil due to the formation of brittle intermetallic phases,but the joints brazed with 12.5 at.% V foil failed through the TiAl substrate. These results show that designing amorphous alloy with less Ti and more V for brazing TiAl alloy to steel is appropriate.

Microstructure and Mechanical Properties of Pulse MIG Welded 6061/A356 Aluminum Alloy Dissimilar Butt Joints

The microstructure and mechanical properties of pulse metal inert-gas （MIG） welded dissimilar joints between 4 mm thick wrought 6061-T6 and cast A356-T6 aluminum alloy plates were investigated. The tensile strength of the joints reached 235 MPa, which is 83% of that of 6061 aluminum alloy, and then decreased with the increase of travel speed while keeping other welding parameters constant. The microstructure, composition and fractography of joints were examined by the optical microscopy （OM）, scanning electron microscopy （SEM） and electron probe microanalysis （EPMA）. Grain boundary liquation and segregation occurred in the partially melted zone （PMZ） on 6061 aluminum alloy side, and brittle Fe-rich phases were observed in partially melted zone on A356 aluminum alloy side. The minimum microhardness appeared in heat-affected zone （HAZ） near A356 aluminum alloy substrate. The samples during tensile test failed mainly in PMZ and HAZ on A356 aluminum alloy side through mixed fracture mode with quasi cleavage and dimples on fracture surface.

Effect of Post-weld Heat Treatment on Properties of Friction Welded Joint Between TC4 Titanium Alloy and 40Cr Steel Rods

Dissimilar metal joining of Ti-6AI-4V （TC4） titanium alloy to as-rolled 40Cr steel rods was conducted with friction welding, and the effect of post-weld heat treatment （PWHT） on the microstructure and mechanical properties of the resultant joints was investigated. The average tensile strength of the as-welded joints reached 766 MPa and failure occurred in 40Cr steel base metal. However, after PWHT at 600℃ for 0.5, 1, 2 and 3 h, the tensile strength of the joints decreased and fracture happened through the interface with quasi-cleavage features. The bending angle of specimens was improved from 9.6° in as-welded state to 32.5° after PWHT for 2 h. The tensile strength of the joint was enhanced by martensitic transformation near the interface in as-welded state. Sorbite formed near the interface in PWHT state and improved the bending ductility of the joint. TiC brittle phase formed at the interface after PWHT for 0.5 h and deteriorated the tensile strength and bending ductility of the joint. After PWHT for 2 h, no TiC phase was detected at the interface. The microhardness on the interface in as-welded state was higher than that after PWHT, indicating that the decrease of microhardness around the interface could be accompanied by degradation of tensile strength but improvement of bending ductility of the joints.

Performance of GH4169 brazed joint using a new designed nickel-based filler metal via cluster-plus-glue-atom model

A novel Ni-Cr-Si-B filler metal with the cluster formula of[Cr-Ni12]B2Cr+[B-Ni8Cr]BSi Cr based on the cluster-plus-glue-atom model was designed for vacuum brazing GH4169 alloy.The effect of brazing temperature and brazing time on microstructure and shear strength of GH4169 alloy joints was investigated.The brazed seam was mainly composed ofγ-Ni solid solution.(Nb,Ti)-rich phase and(Cr,Nb,Mo)-rich borides distributed in diffusion zones.The diffusion and aggregation of B,Cr,Nb,and Mo resulted in the variation of phase contrast and morphology of borides.Coarse precipitations in the joint brazed at1240℃consisted of borides,Laves phase andδphase.The shear strength of joints was principally dominated by the brittle precipitations in diffusion zone,and the homogenization of microstructure improved the room-temperature shear strength to 820 MPa with the high-temperature shear strength of 627 MPa for the joint brazed at 1240℃/20 min.The joint fractured in diffusion zone and brazed seam,and the existence of borides and Laves phase in diffusion zone provide the potential origin for crack growth.

Effects of Gap Width and Groove on the Mechanical Properties of Butt Joint Between Aluminum Alloy and Stainless Steel

Butt joining of 5A02 aluminum alloy to 304 stainless steel sheets was conducted using gas tungsten arc welding process with Al-12%Si （wt.%, the same below） and Zn-15%Al flux-cored filler wires. The effects of gap width and groove in steel side on the microstructure and tensile strength of the resultant joints were investigated. For the joint made with 0 mm-wide gap and without groove in steel side, severe incomplete brazing zone occurred along the steel side and bottom surfaces, and consequently seriously deteriorated the joint strength. However, presetting 1.5 mm-wide gap or with groove in steel side could promote the wetting of molten filler metal on the laying surfaces, and then significantly enhance the resultant joint strength. Moreover, post-weld heat treatment could further improve the tensile strength of the joints. During tensile testing, the specimens from the joints made with AI-12%Si flux-cored filler wire fractured through the weld or interracial layer, but those from the heat-treated joints made with Zn-15%AI flux-cored filler wire fractured in the aluminum base metal.

Carbide precipitates and mechanical properties of medium Mn steel joint with metal inert gas welding

Medium Mn steel was metal inert gas(MIG)welded with NiCrMo-3 and 307 Si filler wires.The effect of filler wires on the microstructure and mechanical properties of joint was investigated,and the carbide precipitates were contrastively discussed.The results revealed that the microstructure of weld metal,heat-affected zone and base metal are austenite.Obvious grain coarsening occurred in the heat-affected zone(HAZ),and the maximum grain size grew up to 160μm.In HAZ,C and Cr segregated at grain boundaries,the carbides was identified as Cr7 C3.The dispersive(Nb,Mo)C phase was also found in weld metal with NiCrMo-3 filler wire.All the welded joints failed in HAZ during tensile tests.The tensile strength of welded joint with NiCrMo-3 filler wire was 675 MPa,which is much higher than that with307 Si filler wire.In comparison to base metal,higher microhardness and lower impact toughness were obtained in HAZ for these two welded joints,which was attributed to the precipitation of Cr7 C3 phase and grain coarsening.The impact toughness around the fusion line is the worst for these two welded joints.