Search

To meet the requirements of automatic production, a new type of green BAl88Si cored solder was developed. The lap brazing experiments were carried out with copper and aluminum as brazing substrates. The microstructure, phase composition, and corrosion behavior of solder joint interface were studied by field emission scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, electron backscattering diffraction, tensile testing machine, and electrochemical workstation. The results show that the brazing joint of Cu/BAl88Si/Al is metallurgical bonding, and the brazing joint of Cu/BAl88Si/Al is composed of Cu9Al4, CuAl2, a-Al, (CuAl2 + a-Al + Si) ternary eutectic. In addition, there is no obvious preference for each grain in the brazing joint, and there are S texture {123}, Copper texture {112}, and Brass texture {110}. The interface of Cu9Al4/CuAl2 is a non-coherent crystal plane and does not have good lattice matching. The average particle size of CuAl2 is 11.95 µm and that of Al is 28.3 µm. However, the kernel average misorientation (KAM) value at the brazed joint interface is obviously higher than that at the brazed joint interface copper, so the defect density at the brazed joint interface aluminum is higher than that at the brazed joint interface copper. At the same time, due to poor corrosion resistance at the interface on the aluminum side of the brazed joint, serious corrosion spots and corrosion cracks occur at the same time, which leads to the shear performance of the brazed joint decreasing by about 75% after salt spray test for 240 h.

Refilled friction stir spot welding of aluminum alloy to galvanized and uncoated steel was conducted, and the microstructure and the corrosion behavior of joints were investigated. Al5FeSi phases distributed in TMAZ and broke during tool stirring in NZ, which induced the pitting corrosion in 3.5 wt% NaCl solution. The maximum corrosion depth of ST-Al/ST06 Z joint was up to 21.3 μm at the mixed layer, which was 2.8 μm deeper than that of ST-Al/ST16 joint at the interface between aluminum alloy and steel. Zn and Mg segregated at the grain boundaries of the mixed layer, resulting in serious intergranular corrosion. The corrosion current density of ST-Al/ST06 Z joint ((8.1 ± 0.2) × 10–6 A/cm−2) is higher than that of ST-Al/ST16 joint ((7.9 ± 0.4) × 10–6 A/cm−2) after immersion for 1 h in 3.5 wt% NaCl solution. The corrosion current densities of samples were minimum for 6 h immersion time, which was related to the passivation films and corrosion products formed on the joint surface.

The newly designed cryogenic high-Mn steel plates (Fe-23Mn-1.5Al-0.45C) were joined by metal inert gas (MIG) multi-pass welding process. To evaluate the inhomogeneity of microstructure and cryogenic toughness in heat-affected zone (HAZ) of welded joint, the impact samples were prepared by zonal sampling method. The microstructure features and embrittlement mechanism in HAZ of welded joint were systematically investigated. The results reveal that grains obviously grew in the coarse-grain HAZ (CGHAZ). C element segregated and nano-sized lamellar and granular (Cr, Mn)7C3 carbides precipitated at the grain boundary in HAZ, and the size of precipitate carbides in CGHAZ and fine-grain HAZ (FGHAZ) was about 100 nm and 40 nm, respectively. The interface between (Cr, Mn)7C3 and γ-austenite matrix in CGHAZ was incoherent with the lattice misfit of 45.13%. Higher fraction of low angle grain boundaries (LAGBs) appeared in CGHAZ (25.89%) and FGHAZ (11.99%), compared with the base metal (4.33%). Such microstructural differences can degrade the impact toughness in CGHAZ. From cryogenic Charpy impact tests, the impact toughness in different zones of the joint at each test temperature follows this sequence: base metal > fine-grain HAZ > coarse-grain HAZ > fusion line. These results can provide a fundamental understanding and guidance on the brittle failure of cryogenic high-Mn steel welded joint during service.

An innovative weld with high tensile strength was fabricated for gas tungsten arc welding of TC4 titanium alloy and 304 stainless steel with Cu foil as interlayer and Ni-based alloy as filler metal simultaneously. Due to the high longitudinal residual stress, transverse cracks easily generated in the weld seam with pure Cu wire as filler metal. With the newly designed weld, the longitudinal residual stress in the weld seam was much lower than the fracture strength of Ni-based deposited metal, hence the transverse cracking of the joint was remarkably inhibited. The Cu foil enfolded at the end face of titanium alloy sheet effectively reduced the dissolution of titanium alloy, suppressing the formation of Ti-based brittle compounds in the interfacial zone. The Ti-side interface consisted of β-Ti, Ti2Ni, TiNi, TiNi3, (Cr,Mo), Ti(Fe,Cr,Ni)2, (Cr,Fe,Ni) solid solution, and Ni–Fe–Cr–Ti phases, resulting in high microhardness of Ti-side interface up to 855 HV0.1. At the weld toe, TixCuy phases with lower microhardness and brittleness are formed between the unmelted Cu foil and titanium alloy substrate, reducing the cracking sensitivity of Ti-side interface. The tensile strength of the corresponded TC4/304SS joint reached 485 MPa, and all joints fractured at the Ti-side interface that consisted of β-Ti and Ti2Ni phases.

TiAl alloy and 316L stainless steel were vacuum-brazed with Zr−50.0Cu−7.1Ni−7.1Al (at.%) amorphous filler metal. The influence of brazing time and temperature on the interfacial microstructure and shear strength of the resultant joints was investigated. The brazed seam consisted of three layers, including two diffusion layers and one residual filler metal layer. The typical microstructure of brazed TiAl alloy/316L stainless steel joint was TiAl alloy substrate/α2-(Ti3Al)/AlCuTi/residual filler metal/Cu9Zr11+Fe23Zr6/Laves-Fe2Zr/α-(Fe,Cr)/316L stainless steel substrate. Discontinuous brittle Fe2Zr layer formed near the interface between the residual filler metal layer and α-(Fe,Cr) layer. The maximum shear strength of brazed joints reached 129 MPa when brazed at 1020 °C for 10 min. The diffusion activation energies of α2-(Ti3Al) and α-(Fe,Cr) phases were −195.769 and −112.420 kJ/mol, respectively, the diffusion constants for these two phases were 3.639×10−6 and 7.502×10−10 μm2/s, respectively. Cracks initiated at Fe2Zr layer and propagated into the residual filler metal layer during the shear test. The Laves-Fe2Zr phase existing on the fracture surface suggested the brittle fracture mode of the brazed joints.

Ratcheting is an important mechanical behavior of metals and alloys, which is caused by the repeated accumulations of tensile and compressive strain in circle load. However, the current characterization methods of ratcheting effect are mostly based on standardized testing and uniform data, and more comprehensive field measurement data cannot be used. This paper focuses on how to make full use of field measurement data to characterize ratcheting effect and identify the corresponding kinematic constitutive. A nonlinear virtual field method that can invert the parameters of Chaboche constitutive from strain field data is proposed. And a return mapping strategy driven by iteration of internal variables is used to reconstruct the stress field, which ensures the convergence speed and global convergence in the black box search of the nonlinear virtual field method. By using the finite-element model to generate the strain field data, the numerical experiment shows that the ratchet path identified by the nonlinear virtual field algorithm is basically consistent with the prior ratchet path generated by the finite-element simulation. The adaptability of the algorithm to data density and noise amplitude was also verified: under lower data noise interference, more strain field training data makes the inversion results more accurate; but in the case of high sound amplitude, it is necessary to reduce the data size to obtain accurate fitting results of the ratchet path. By training the measured field data from 3D-digital image correlation, it is shown that the algorithm can also run effectively under the complex working conditions of non-uniform deformation.

The effect of repair welding on the microstructure and mechanical properties of the welded joint was investigated. The results show that the difference of microstructure in different locations of the original welded joint (OWJ) is mainly related to the production process. The AT5 side is an extruded profile, exhibiting equiaxed grain characteristics, while the AT4 side is a rolled sheet, displaying flat long grain. The grain boundary orientation difference in different positions of repair welded joint (RWJ) is various from that of OWJ, especially the change of proportion of small angle grain boundary in AT4-HAZ II is large. The tensile strength of the OWJ and RWJ are 283 MPa and 280 MPa, respectively, and the corresponding welded joint coefficients are 78.6 % and 77.8 %, respectively, which can fulfill the demands of practical engineering applications. Due to the existence of pores and inclusions, the crack of OWJ originates in the junction of HAZ I near the 7N01-T4 side, then expands along the welded zone, while the RWJ fails in the junction of the original weld and the weld passes.

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 AlCoCrFeNi2.1 eutectic 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-Al3NiTi2 + TiAl, τ3-Al3NiTi2 + TiAl + 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 TiAl phase into Al(Co, Ni)2Ti and τ3-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 of τ3-Al3NiTi2 + 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 °C. And the brittle fracture generally initiated at the interface between Al(Co, Ni)2Ti and τ3-Al3NiTi2 + TiAl layers.

The inhomogeneous properties of 5052 aluminum alloy/304 stainless steel rotary friction welded joint and the growth behavior of intermetallic compound (IMC) layer were investigated. The ultimate tensile strength (UTS) of as-welded joint in 0R location with discontinuous IMC layer reached the maximum value of 203 MPa, and that in R/2 and 3R/4 locations decreased with the increase of IMC layer thickness, but that in R/4 location was the lowest due to the initiation of crack. After post-weld heat treatment at 250 °C/20 min, the thickness of IMC layer remained invariant, but the UTS of entire joint increased from 153 MPa to 161.4 MPa. In addition, the difference of UTS along the radius direction decreased, which benefited the engineering application. During post-weld heat treatment at 450 °C, the growth of IMC layer in R/4, R/2 and 3R/4 location were faster than that in 0R location. However, the growth of IMC layer deviated from parabolic law, which was attributed to (i) a large number of dislocations, grain boundaries and storage energy in the interface, (ii) the linear growth of Fe4Al13 phase, and (iii) the effect of Cr element.

A novel Ni-Cr-Si-B filler metal with the cluster formula of [Cr-Ni12]B2Cr + [B-Ni8Cr]BSiCr 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 of γ-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 at 1240 °C consisted of borides, Laves phase and δ phase. 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 °C/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.

Aluminum alloy and steel hybrid structures offer clear advantages in weight reduction and energy conservation for the automotive industry. This paper exhibits sound refilled friction stir spot welded joints between the aluminum alloy and ST16 steel sheets with the maximum tensile/shear and cross-tension fracture loads of 3745 N and 1073 N, respectively. All samples failed through the interface of aluminum alloy/steel joints during tensile/shear test and cross-tension test. The microstructure and fracture mode of the joints were studied with the scanning electron microscope, electro-probe microanalyzer and X-ray diffraction. For the joints where the sleeve had not plunged into the ST16 steel, the effect of the sleeve plunging depth and rotation speed on the tensile/shear fracture load cannot be identified, as an intermetallic compounds layer formed between Al and Fe, which was almost uniform across the interface. The mechanical performance of joints increased dramatically when the sleeve plunged into the ST16 steel because of metallurgical bonding and mechanical interlocking in the sleeve-affected zone. However, this process caused considerable wear to the tool and shortened its service life. The bonding mechanism was investigated by studying joints between the aluminum alloy and galvanized steel for the same welding parameters.

The reaction layer of rotary friction welded 5052 aluminum alloy/304 stainless steel dissimilar joints consisted of Fe2Al5 and Fe4Al13 phases, and the distribution of intermetallic compounds (IMCs) and mechanical properties were inhomogeneous due to the inhomogeneity of heat generation along the radius direction. Almost no brittle IMCs formed in center part, while continuous and thicker IMCs formed in R/4, R/2, 3R/4 and edge locations, and the thickness of IMCs reached the maximum value in 3R/4 rather than in edge location. The joint efficiency was 99.51% in center location, higher than that in R/4, R/2, 3R/4 and edge locations. Meanwhile, the width of heat-affected zone on 304 stainless steel side decreased, but that of softened zone on 5052 aluminum alloy side increased from center to edge location. The entire joint fractured through the interface and predominantly presented brittle fracture feature. Cracks mainly propagated along the IMCs/AA5052 substrate interface for samples from center and R/4 locations, between the IMCs and the IMCs/AA5052 substrate interface alternately from R/2 and partial 3R/4 locations, and along the IMCs/304SS substrate interface from partial 3R/4 and edge locations.

The feasibility of (CoCrFeNi)100-xCux high-entropy alloy filler metal for laser welding between TC4 titanium alloy and 304 stainless steel is investigated. Reliable metallurgical bonding was achieved between weld zone and 304 stainless steel. The weld zone consisted of ductile FCC solid solution phase and Cu-rich phase which segregated at the grain boundary of FCC phases. The Ti/Cu transition zone is composed of Ti-rich layer near TC4 titanium alloy substrate and Ti-depleted layer adjacent to the weld zone. Ti-rich IMCs and β-Ti phases formed in Ti-rich layer, and Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases formed in Ti-depleted layer. With increase of Cu content in filler metal, the segregated Cu-rich phase in weld zone increased, and Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases decreased along with the increase of (Fe,Cr)-rich phase and segregated Cu-rich phase. The tensile strength of the joint with CoCrFeNi filler metal was 158 MPa, and it decreased with increase of Cu in filler metal until the atomic percent of Cu reached 27.27 at. %, but raised to 161 MPa while using (CoCrFeNi)66.67Cu33.33 filler metal. All joints fractured through the transition zone, and the maximum microhardness of the transition zone reached 803 HV, higher than those of substrates and weld zone. The formation of Ti(Fe,Co,Cr)2 and Ti(Fe,Ni)2 phases resulted in the limited tensile strength and high microhardness in the transition zone.

TC4 titanium alloy was vacuum-brazed to 316L stainless steel (SS) with Ti-Zr-Cu amorphous filler metal. The effect of brazing time and temperature on the interfacial microstructure and mechanical properties of joints was investigated. Electron probe micro-analyzer (EPMA) and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) were used to study the joint microstructure; meanwhile, the reaction phases on fracture surfaces were identified by X-ray diffraction (XRD). The results show that all joints had similar interfacial microstructure of TC4 titanium substrate/Widmanstatten/β-Ti + Ti2Cu/(α-Ti + λ-Cu2TiZr) + Ti2Cu/Ti-Fe-Cu/TiFe/(Fe, Cr)2Ti/σ-phase + Fess/316L stainless steel substrate. Three reaction layers TiFe/(Fe, Cr)2Ti/σ-phase + Fess formed close to 316L stainless steel substrate and could benefit the mechanical properties of joints. The maximum shear strength of 65 MPa was obtained at 950 °C for 10 min. During shear test, cracks initiated at the interface of Ti-Cu-Fe layer/TiFe layer and propagated along the brazed seam/316L interface with a large amount of cleavage facets existing on the fracture surface.

The feasibility of direct fusion welding of TC4 alloy to ST16 steel by GTAW using CuSi3 filler wire was investigated. Defect-free fusion welded joints were obtained. Various intermetallic compounds, such as Ti2Cu, TiCu, AlCu2Ti, TiCu2 and Ti5Si3 phases, orderly formed in Ti/weld transition zone. Ti and Si diffused into ST16 steel, and an interface formed between copper weld and ST16 steel substrate. The resultant joints fractured at the ST16 steel substrate, and the tensile strength reached 282 MPa. The formation of various TixCuy IMCs and the solution strengthening induced by diffusion of Ti and Si led to the increase of microhardness in Ti/weld transition zone and copper/steel interface, respectively.

A series of KxH1.1-xCu0.2Cs1(NH4)1.5PVMo11O40 (KxCuCsNH4PVA) catalysts with different x values were synthesized to catalyze the selective oxidation of methacrolein (MAL) to methacrylic acid (MAA). The effects of potassium (K) ions on both the structure and catalytic activity were studied in detail. The optimum K0.6CuCsNH4PVA exhibited a large surface area, more acid sites, and abundant active species (V4+/VO2+) in the secondary structure of the Keggin structure, consequently offering good catalytic performance. Furthermore, K ions increased the MAA selectivity at the expense of carbon monoxide and carbon dioxide (together defined as COX). Additionally, several process parameters for MAL oxidation were evaluated in the processing experiments. The effects of aldehyde impurities (formaldehyde and propanal) on the catalytic performance were investigated. Possible detrimental effects (catalyst poisoning and structural damage) of aldehyde impurities were excluded. A light decrease in MAL conversion could be attributed to the competitive adsorption of aldehyde impurities and MAL on the catalyst. Hopefully, this work contributes to the design of stable and feasible catalysts for the industrial production of MAA.

The present work investigated the composition evolution of the TMS series of Ni-base single crystal (SC) superalloys in light of the cluster formula approach systematically. The cluster formula of SC superalloys could be expressed with $${[}\overline{{{\text{Al}}}} {-} \overline{{{\text{Ni}}}} 12{](}\overline{{{\text{Al}}}} {, }\overline{{{\text{Cr}}}} {)}m$$ [ Al ¯ - Ni ¯ 12 ] ( Al ¯ , Cr ¯ ) m , in which all the alloying elements were classified into three groups, Al series ($$\overline{{{\text{Al}}}}$$ Al ¯ ), Cr series ($$\overline{{{\text{Cr}}}}$$ Cr ¯ ), and Ni series ($$\overline{{{\text{Ni}}}}$$ Ni ¯ ). It was found that the total atom number (Z) of the cluster formula units for TMS series of superalloys varies from Z ~ 17 to Z ~ 15.5, and then to Z ~ 16 with the alloy development from the 1st to the 6th generation, in which the superalloys with prominent creep resistance possess an ideal cluster formula of $${[}\overline{{{\text{Al}}}} {-} \overline{{{\text{Ni}}}} 12{](}\overline{{{\text{Al}}}} 1.5\overline{{{\text{Cr}}}} 1.5{)}$$ [ Al ¯ - Ni ¯ 12 ] ( Al ¯ 1.5 Cr ¯ 1.5 ) with Z = 16. Similar tendency of composition evolution also appears in the PWA and CMSX series of SC superalloys. Typical TMS series of superalloys with prominent creep properties generally exhibit a moderate lattice misfit of γ/γ′ which could render alloys with appropriate particle size of cuboidal γ′ precipitates to acquire a maximum strength increment by precipitation strengthening mechanism. More importantly, the relationship between the lattice misfit (δ) of γ/γ′ and the creep rupture lifetime (tr) of superalloys was then established, showing a linear correlation in the form of lgtr–lg|δ|3/2 at both conditions of 900 °C/392 MPa and 1100 °C/137 MPa. Combined with the lattice misfit, the cluster formula approach would provide a new way to modify or optimize the compositions of Ni-base superalloys for further improvement of creep property.

The financial sustainability of large-scale sport stadiums has become a challenging issue for sport organizations in China due to increasing market competition, lack of professional sport franchises to tenant the facilities, and gradual slow-down in Chinese GDP growth. Previous findings about operational performance of sport organizations identify organizational structure and public-private partnership (PPP) as important predictors. The aim of the current study was (a) to propose a predictive model for operational performance of large-scale stadiums in China and (b) to examine the relationships among organizational structure, PPP, and operational performance. We conducted a literature review to establish a theoretical framework for the proposed model, selected Yangzhou Sports Park and Xuzhou Olympic Sports Center to examine the relationships, and conducted expert interviews to examine the research questions. We found that Xuzhou&rsquo, s operational performance was more effective due to several mechanisms related to both organizational structure and PPP: incentive, supervision, and assessment. Notably, using built-in benchmark monitoring procedures, Xuzhou managers identified a variety of constraints early on to address onsite problems while maintaining efficient communication among key PPP stakeholders.

Butt joining of Al-Mg-Si alloys with 6.0 mm in thickness was made with conventional rotational shoulder friction stir welding (RSFSW) and non-rotational shoulder friction stir welding (NRS-FSW) process, respectively, and the resultant mechanism of different shoulders affecting the microstructure and mechanical properties of joint was discussed. A new zone named “triangle zone” (TZ) in the RSFSWed joint was first clearly defined, while the shoulder and pin affected zone (SPAZ) did not exist in the NRS-FSWed joint. The distribution of average grain sizes in each zone of the joints (“M” shape) and the changes in the proportion of high angle grain boundaries (HAGBs) (“W” shape) are unified with the typical “W” shape distribution of microhardness in the joints. Compared to rotational shoulder, the non-rotational shoulder significantly reduced the width of softened area. In particular, the microstructure of the joints by NRS-FSW with triple facets on the pin surface is finer, denser and more homogeneous. The tensile strength coefficient of the joint welded by NRS-FSW and NRS-FSW with the pin of triple facets increased by 2.3% and 11.6%, respectively, compared to the joint welded by RSFSW. Accordingly, the fatigue performances increased by 1.72 times and 3.19 times, respectively.

Replacing steel with aluminum alloy is appealing to realize the lightweight of metal structures. Novelis AC 170 PX aluminum alloy and ST06 Z galvanized steel sheets were lap welded by gas tungsten arc welding (GTAW) with ER4047 filler wire and the corrosion behavior of the joint was characterized. The results revealed that the zinc-rich zone had the highest corrosion susceptibility. The difference of localized corrosion between the adjacent area near the rod-shaped and the layered Fe4Al13 phases was explained by galvanic effect. When Zn in the zinc-rich zone was exhausted, the corrosion process of the joint was controlled by the second phases and corrosion products.

TC4 titanium alloy was successfully welded to 304 stainless steel using gas tungsten arc welding with a CuSi3 filler wire. The Ti/weld transition zone in the joint consisted of β-Ti, Ti2Cu, AlCu2Ti, TiCu, and Ti5Si3 intermetallic compounds (IMCs) when welded with low current. As the welding current increased, more Fe dissolved into the weld pool, and massive ternary Ti-Cu-Fe and AlCu2Ti IMCs formed in the Ti/weld transition zone. The segregated Ti5Si3 phase disappeared, and complex Fe-Ti-Si-Cr IMCs formed outside the Ti/weld transition zone. The microhardness was much higher in the Ti/weld transition zone than that in the TC4 substrate and copper weld, reaching a maximum value of 619 HV. The highest tensile strength was 328 MPa when the welding current was 120 A and the traveling speed was 4 mm/s. All joints failed through the Ti/weld transition zone, and brittle cleavage features were present on the fractured surfaces. Due to the large difference in thermophysical properties between the two base metals, cracks were easily generated, which restricted the improvement in the tensile strength of the TC4/304 SS joint.

Vacuum brazing of TC4 titanium alloy and 316 L stainless steel (SS) was conducted using Ti-Zr-Cu-Ni-V amorphous filler metal. The effect of brazing temperature and brazing time on the interfacial microstructure and mechanical property was investigated. The cross section of brazed joint could be divided into four distinctive reaction zones. The typical interfacial microstructure of the joint brazed at 930 °C/15 min was TC4 substrate/Widmanstatten/β-Ti + Ti2Cu/(Ti, Zr)2(Cu, Ni)/Ti(Cu, Ni)2 + TiFe/TiFe2 (high Cr)/α-(Fe, Cr) + τ/γ-(Fe, Ni) + σ/316 L SS matrix. Residual filler metal disappeared at 960 °C due to complete mutual diffusion between molten filler metal and base metal. The diffusion of Cu atoms from residual filler metal region into β-Ti region dominated the precipitation of Ti2Cu phase. A transition zone formed adjacent to 316 L SS matrix, which consisted of three reaction layers, i.e., TiFe2/α-(Fe, Cr) + τ/γ-(Fe, Ni) + σ. With the increase of brazing temperature and brazing time, the shear strength of brazed joints first increased and then decreased. The maximum shear strength of 70 MPa was obtained at 960 °C/25 min. Fracture analysis indicated that cracks initiated at the interface of (Ti(Cu, Ni)2 + TiFe)/TiFe2, and then propagated along the zone of TiFe2 and α-(Fe, Cr) + τ with cleavage and intergranular fracture pattern. The sectionalized morphology of fracture surface was resulted from the joint distortion and uneven load distribution within the joint clearance.

Dissimilar T-joints between 4-mm-thick wrought 6061-T6 and cast A356-T6 aluminum alloy plates are made with pulse metal inert gas welding (P-MIG) process, and the weld geometry, microstructure and mechanical properties of the joints are investigated. The average tensile load of the joints is 9488 N at travel speed 12 mm/s and welding current 165 A with 6061 aluminum alloy as web plate and A356 aluminum alloy as flange plate. The weld zone (WZ) is composed of dendrite zone and columnar zone, which is wide at bottom of weld making it become a weak zone. Besides, grain boundary (GB) liquation, consisting of GB eutectic and solute-depleted α-Al, occurs in the partially melted zone on 6061 aluminum alloy side. The T-joint with highest tensile load fails at the root of WZ and the flange plate near the weld with different fracture modes.