Filler Alloy Research Articles

Interfacial reaction and wetting behavior of active Ag-Cu filler alloys on surface of cBN grits and its influence on failure patterns of brazed joint

Cubic boron nitride (cBN) abrasive grits are often brazed on steel to produce monolayer grinding wheels. Establishing a strong cBN brazement is a challenge. In the current work, performance of three different active Ag-Cu alloys have been investigated in-depth considering microstructure of alloys, interfacial reaction phase formation and wetting capability of the reaction product towards the alloy. In the current study, all three alloys contain Ti as active element but with different wt%. One of alloys was so chosen that Indium (In) was present as an additional alloying element, which is responsible for its low liquidus temperature, high Young's modulus and good ductility. Segregation of Ti towards the interface was critically examined. To evaluate joint strength, a miniature twin-ring grinding wheel was produced with these alloys. Interestingly, the alloy containing higher Ti, which required higher brazing temp, resulted in intense reaction but with inferior joint strength. On the other hand, the very presence of indium in Ag-Cu alloy, containing low wt% of Ti not only lowered the brazing temperature but also led to effective wetting and higher joint strength. Failure pattern indicates the prevalence of high residual stress at the bond level for the alloy containing the highest percentage of titanium.

Microstructural and mechanical assessment of transient liquid phase bonded commercially pure titanium

ABSTRACTDiffusion joining of commercially pure titanium was successfully prepared via transient liquid phase bonding in vacuum environment. The process was carried out using AMS 4772 silver-based filler alloy at 900–1000°C for various holding time under the vacuum of 6 × 10−7 Torr. Optical and scanning electron microscopy equipped with an EDS analyzer was conducted for microstructural evaluations. Mechanical properties were also investigated by shear test, fractographic assessment and X-ray diffraction analyses. The tendency to achieve isothermally solidified joint increased by increasing bonding time. No sign of athermal solidification was detected of sample bonded at 1000°C for 90 min. Consequently, the bonding condition of a high quality joint was obtained. Elemental analyses revealed that filler alloy’s elements (Ag, Cu) distributed more uniformly in fully isothermal solidified bond, whereas the aggregation of these elements is considerable in athermally solidified bond. Shear test results represented that the highest shear strength attributed to the sample bonded in isothermal solidified condition (bonded at 1000°C for 90 min).

Effect of Bonding Temperature on Microstructure Development during Transient Liquid Phase Bonding of Ti<sub>2</sub>AlNb Alloy Using Ti-Zr-Cu Based Filler Alloy

Transient liquid phase (TLP) bonding of Ti2AlNb, used for vacuum brazing furnace, was carried out using Ti-Cu-Zr based foil as filler alloy at 950, 1000 and 1050°C. The effect of bonding temperature on joint interface, phase constitutions and their distributions were investigated by taking advantages of OM, SEM, EDS and XRD analyses. The result revealed that the TLP joint consisted of isothermally solidified zone and diffusion affected zone. A non-isothermally solidified zone existed only when the bonding temperature was not high enough. The interface morphologies of the joints were found to be very sensitive to the bonding temperature. With the bonding temperature increased from 950°C to 1000°C, the width of non-isothermally solidified zone decreased from 69 μm to 23 μm. When the bonding temperature was 1050°C, the non-isothermally solidified zone disappeared. Meanwhile, more alloying elements of Cu and Zr diffused most adequately into the base material. Phase analysis showed that along with the increasing of bonding temperature, the secondary phase constitution of joint changed from Ti (Cu,Al)2 + AlNb2 + Ti solid solution to Ti solid solution + Nb (CuAl) + Al4Cu9 + Al2Zr3, and the proportion of secondary phase was 35.7%, 20.2%, 6.7%, respectively. The morphology of base metal changed because of the relatively high bonding temperature was higher than 980°C, the α→β transition temperature.

Reactive brazing Cf/SiC to itself and to Mo using the NiPdPtAu-Cr filler alloy

The NiPdPtAu-Cr filler alloy was proposed for joining Cf/SiC composites. The wettability on Cf/SiC composite was studied by the sessile drop method at 1200°C for 30min. Under the brazing condition of 1200°C for 10min, the Cf/SiC-Cf/SiC joint strength was only 51.7MPa at room temperature. However, when used a Mo layer, the Cf/SiC-Mo-Cf/SiC joint strength was remarkably increased to 133.2MPa at room temperature and 149.5MPa at 900°C, respectively. At the interface between Cf/SiC and Mo, Mo participated in interfacial reactions, with the formation of Cr3C2/Mo2C reaction layers at the Cf/SiC surface. The improvement in the joint strength should be mainly attributed to the formation of MoNiSi. The Cf/SiC-Mo joint strength was 86.9MPa at room temperature and 73.7MPa at 900°C, respectively. After 10 cycles of thermal shock test at 900°C the Cf/SiC-Mo joint strength of 71.6MPa was still maintained.

Interfacial microstructure and mechanical properties of porous-Si3N4 ceramic and TiAl alloy joints vacuum brazed with AgCu filler

Abstract Porous Si 3 N 4 ceramic was firstly joined to TiAl alloy using an AgCu filler alloy. The effects of brazing temperature and holding time on the interfacial microstructure and mechanical properties of porous-Si 3 N 4 /AgCu/TiAl joints were studied. The typical interfacial microstructure of joints brazed at 880 °C for 15 min was TiAl/AlCu 2 Ti/Ag-Cu eutectic/penetration layer (Ti 5 Si 3 +TiN, Si 3 N 4 , Ag (s, s), Cu (s, s))/porous-Si 3 N 4 . The penetration layer was formed firstly in the brazing process. With increasing brazing temperature and time, the thickness of the penetration layer increased. A large amount of element Ti was consumed in the penetration layer which suppressed the formation and growth of other intermetallic compounds. The penetration layer led the fracture to propagate in the porous Si 3 N 4 ceramic substrate. The maximum shear strength was ~13.56 MPa.

Microstructure and mechanical behavior of a titanium-to-stainless steel dissimilar joint brazed with Ag-Cu alloy filler and an Ag interlayer

The microstructure and mechanical behavior of a titanium-to-stainless steel dissimilar joint with a characteristic Ti(base)/TiAg/Ag-rich solid solution/stainless steel(base) layered structure were investigated under tensile loading. Elastic deformation in the layered joint structure occurred under the isostress condition observed in multi-phase materials, and the elastic interaction stresses were readily generated at the interphase boundaries to maintain strain continuity. This elastic interaction retarded mechanical yielding in the Ag-rich solid solution layer, thereby leading to the high joint strength. After yielding, the plastic deformation was mostly concentrated on the Ag-rich solid solution layer, and the final fracture proceeded in a ductile mode.

Brazing SiC matrix composites using Co-Ni-Nb-V alloy

Co-13.5Ni-16.2Nb-10 V (wt.%) filler alloy was designed for joining silicon carbide (SiC) matrix composites, including Cf/SiC and SiCf/SiC composites. The wettability of the Co-Ni-Nb-V alloy on two kinds of composites was studied by the sessile drop method. After heating at 1573 K for 20 min, the new filler alloy exhibited a low contact angle of 18° and 2° on the Cf/SiC and SiCf/SiC composite, respectively. The interfacial microstructure was investigated by SEM equipped with EDS. At the interface between the Cf/SiC composite and the brazing alloy, sandwiched reaction band VC/NbC/VC was formed. In the case of SiCf/SiC, only VC band was formed at the interface, signifying that the activity of Nb was lower than V. The elements of Co and Ni mainly existed in the form of Co-Si or (Co,Ni)-Si compound. The Co-Ni-Nb-V alloy was fabricated into the filler foils with thickness of 100 μm by rapid solidification technique. The Cf/SiC-Cf/SiC joints and SiCf/SiC-SiCf/SiC joints were produced using the filler foil at 1553 K for 10 min. The bend strength of the Cf/SiC-Cf/SiC joints brazed with double-layer filler foil is 21.3 MPa tested at room temperature, whereas the maximum strength of the SiCf/SiC-SiCf/SiC joints brazed with single-layer filler foil reaches 30.8 MPa.

Evaluation of mechanically alloyed Cu-based powders as filler alloy for brazing tungsten to a reduced activation ferritic-martensitic steel

80Cu-20Ti powders were evaluated for their use as filler alloy for high temperature brazing of tungsten to a reduced activation ferritic/martensitic steel (Eurofer), and its application for the first wall of the DEMO fusion reactor. The use of alloyed powders has not been widely considered for brazing purposes and could improve the operational brazeability of the studied system due to its narrower melting range, determined by DTA analysis, which enhances the spreading capabilities of the filler. Ti contained in the filler composition acts as an activator element, reacting and forming several interfacial layers at the Eurofer-braze, which enhances the wettability properties and chemical interaction at the brazing interface. Brazing thermal cycle also activated the diffusion phenomena, which mainly affected to the Eurofer alloying elements causing in it a softening band of approximately 400 μm of thickness. However, this softening effect did not degrade the shear strength of the brazed joints (94 ± 23 MPa), because failure during testing was always located at the tungsten-braze interface.

A novel method of brazing Cu/Cu‐7.0Ni‐9.3Sn‐6.3P/Cu using microwave hybrid heating

AbstractBulk copper and Cu‐7.0Ni‐9.3Sn‐6.3P (in wt %) filler alloys were brazed by microwave hybrid heating (MHH) and the results were compared with brazing using conventional furnace heating. Characterizations of the brazed joint were investigated through field emission scanning electron microscope, X‐ray diffraction, shear strength test and Vicker's microhardness test. It was observed that microwave heating decreased to 94 % of total processing time compared to conventional heating. Metallurgical study showed the formation of the alpha‐copper, copper phosphide and nickel phosphide intermetallics at the joint interface for both heating methods. The reaction layer thickness of microwave brazed joint was three times thinner compared to the conventionally brazed joint. The optimum shear strength of the microwave brazed joints was found to be 108.8 MPa while it was 16 % higher for the samples brazed using the conventional method. The failures of the microwave brazed joints showed a brittle cleavage structure while a mixed cleavage structure with tearing regions was observed for the conventionally brazed joints.

Compressive Strength and Interface Microstructure of PCBN Grains Brazed with High-Frequency Induction Heating Method

In order to prepare monolayer brazed superabrasive wheels, the polycrystalline cubic boron nitride (PCBN) grains were brazed to AISI 1045 steel matrix with Ag–Cu–Ti filler alloy using the high-frequency induction heating technique. The compressive strengths of brazed grains were measured. Morphology, chemical composition and phase component of the brazing resultant around PCBN grain were also characterized. The results show that the maximum compressive strength of brazed grains is obtained in the case of brazing temperature of 965 °C, which does not decrease the original grain strength. Strong joining between Ag–Cu–Ti alloy and PCBN grains is dependent on the brazing resultants, such as TiB2, TiN and AlTi3, the formation mechanism of which is also discussed. Under the given experimental conditions, the optimum heating parameters were determined to be current magnitude of 24 A and scanning speed of 0.5 mm/s. Finally, the brazing-induced residual tensile stress, which has a great influence on the grain fracture behavior in grinding, was determined through finite element analysis.

Interfacial microstructure and mechanical property of brazed copper/SiO2 ceramic joint

Vacuum brazing of copper to SiO2 electroceramics was carried out using commercially obtained Ag-Cu-Ti filler alloy. The effects of brazing temperature and bonding time on the interfacial microstructure and mechanical properties were investigated. The microstructure observation revealed that the typical interfacial structure could be expressed as SiO2 ceramic/Ti5Si3 + Ti4O7/Ti2Cu + Ti3Cu3O/Ag (s,s) + Cu (s,s)/Copper. The migration of Ti in brazing seam, which promoted the reaction of Ti with the SiO2 ceramic, was a positive factor on the interfacial microstructure and mechanical properties. With increased the brazing temperature or holding time, the thickness of Ti5Si3 + Ti4O7 reaction layer thickened, meanwhile, the amount of AgCu eutectic structure decreased. The shear strength firstly increased and then decreased with brazing temperature increasing or bonding time prolonging. The maximum shear strength of 22 MPa was obtained at 850 °C for 5 min. Its fracture occurred both along the ceramic substrate adjacent to the bonding interface and along the reaction layer, showing a mixture fracture mode.

Corrosion behavior in high-temperature pressurized water of Zircaloy-4 joints brazed with Zr-Cu-based amorphous filler alloys

The compositional effects of ternary Zr-Cu-X (X: Al, Fe) amorphous filler alloys on galvanic corrosion susceptibility in high-temperature pressurized water were investigated for Zircaloy-4 brazed joints. Through an Al-induced microgalvanic reaction that deteriorated the overall nobility of the joint, application of the Zr-Cu-Al filler alloy caused galvanic coupling to develop readily between the Al-bearing joint and the Al-free base metal, finally leading to massive localized corrosion of the joint. Contrastingly, joints prepared with a Zr-Cu-Fe filler alloy showed excellent corrosion resistance comparable to that of the Zircaloy-4 base metal, since the Cu and Fe elements forming fine intermetallic particles with Zr did not influence the electrochemical stability of the resultant joints. The present results demonstrate that Fe is a more suitable alloying element than Al for brazing filler alloys subjected to high-temperature corrosive environments.

Continuous nanoscale Al2Fe transition layer strengthened magnesium-steel spot joints

Cold metal transfer welding was employed to produce Mg to steel spot joints. A continuous and compact plate-like intermetallic Al2Fe layer, with a thickness of 100–300nm, was formed at the interface between solidified AZ61 Mg filler alloy and Zn-coated DP600 steel sheet, resulting in a very high tensile-shear strength of up to 180MPa. On the contrary, when 100nm thick, island-type Al2Fe was observed at the interface between Mg and uncoated DP600 steel sheet, a much lower joint strength was observed regardless of energy input.

Feasibility study of fluxless brazing cemented carbides to steel

One of the most important brazing processes is the joints between cemented carbides and steel for the tool industry such as in rotary drill hammers or saw blades. Even though this technique has already been used for several decades, defects in the joint can still occur and lead to quality loss. Mostly, the joining process is facilitated by induction heating and the use of a flux to enhance the wetting of the filler alloy on the surface of the steel and cemented carbide in an ambient atmosphere. However, although the use of flux enables successful joining, it also generates voids within the joint, which reduces the strength of the connection while the chemicals within the flux are toxic and polluting. In this feasibility study, a fluxless brazing process is used to examine the joint between cemented carbides and steel for the first time. For this, ultrasound is applied during induction heating to enable the wetting between the liquid filler metal and the surfaces of the cemented carbide and steel. The ultrasound generates cavitations within the liquid filler metal, which remove the oxides from the surface. Several filler metals such as a silver based alloy Ag449, pure Zn, and an AlSi-alloy were used to reduce the brazing temperature and to lower the thermal residual stresses within the joint. As a result, every filler metal successfully wetted both materials and led to a dense connection. The ultrasound has to be applied carefully to prevent a damage of the cemented carbide. In this regard, it was observed that single grains of the cemented carbide broke out and remained in the joint. This positive result of brazing cemented carbides to steel without a flux but using ultrasound, allows future studies to focus on the shear strength of these joints as well as the behavior of the thermally induced residual stresses.

Studies on Hot Cracking Susceptibility and Establishment of Welding Procedure of Austenitic Stainless Steel Grade UNS S31035 for Power Plant Application

Austenitic stainless steel Sandvik Sanicro 25 (UNS S31035) was developed in European Advanced ultra super critical boiler program called Thermie-AD700 project. Compared to all other commercial heat resistant austenitic stainless steels, Sanicro 25 has the highest creep strength. Hence, this material has been identified as one of the candidate materials in reheater and super heater lines of advanced ultra super critical boiler for up to 700 °C temperature usage. In this austenitic stainless steel, along with Fe, Ni and Cr, solid solution hardening element, like W, Co & N and precipitation hardening elements like Nb and Cu are added to enhance the creep and high temperature oxidation resistance. Addition of more alloying elements and full austenitic microstructure may lead to the possibility of solidification cracking susceptibility during welding. Hence, investigation on the weldability of this material becomes inevitable to establish the welding procedure. In this paper, solidification cracking behaviour of UNS S31035 has been studied with varestraint test. From the test results, it is concluded that solidification cracking susceptibility is higher in this steel and similar composition filler cannot be used for welding. Based on this conclusion, Ni based super alloy filler ER NiCrCoMo-1 is used for establishing welding procedure in gas tungsten arc welding process. Test results are presented and discussed in this paper.

Microstructure Evolution During Stainless Steel-Copper Vacuum Brazing with a Ag/Cu/Pd Filler Alloy: Effect of Nickel Plating

Vacuum brazing of stainless steel and copper plates was done using a silver-based filler alloy. In one set of experiments, around 30-µm-thick nickel coatings were electrochemically applied on stainless steel plates before carrying out the brazing runs and its effect in making changes in the braze-zone microstructure was studied. For brazing temperature of 830 °C, scanning electron microscopy examination of the braze-zone revealed that relatively sound joints were obtained when brazing was done with nickel-coated stainless steel than with uncoated one. However, when brazing of nickel-coated stainless steel and copper plates was done at 860 °C, a wide crack appeared in the braze-zone adjacent to copper side. Energy-dispersive x-ray analysis and electron microprobe analysis confirmed that at higher temperature, the diffusion of Cu atoms from copper plate towards the braze-zone was faster than that of Ni atoms from nickel coating. Helium leak rate of the order 10−11 Pa m3/s was obtained for the crack-free joint, whereas this value was higher than 10−4 Pa m3/s for the joint having crack. The shear strength of the joint was found to decrease considerably due to the presence of crack.

Reactive air brazing of YSZ-electrolyte and Al2O3-substrate for gas sensor sealing: Interfacial microstructure and mechanical properties

A YSZ/Al2O3 layered structure was successfully reactive air brazed using AgCuO fillers. The interaction mechanism between the filler and ceramics was systemically analyzed. The effects of filler composition on the interfacial microstructure and joint properties were investigated. AgCuO fillers possessed good wettability on the YSZ and Al2O3 ceramics. Ag had diffused into the ceramic substrates, but the width of the diffusion layer at the YSZ interface (∼10 nm) was narrower than that at the Al2O3 substrate (∼20 nm). Further HRTEM observations confirmed interatomic bonding between the CuO and the ceramics. The lattice mismatches at the Al2O3/CuO and YSZ/CuO interfaces were 0.98% and 2.97%, respectively. The filler alloy composition had a major influence on the microstructure and shear strength. The CuO phase preferentially precipitated at the Al2O3 interface. Consequently, the continuous CuO and CuAl2O4 layers gradually formed at the Al2O3 interface with increasing CuO content in the filler. The maximum shear strength (∼45 MPa) was obtained when the composition was Ag-8mol%CuO.

Disk Laser Weld Brazing of AW5083 Aluminum Alloy with Titanium Grade 2

Disk laser weld brazing of dissimilar metals was carried out. Aluminum alloy 5083 and commercially pure titanium Grade 2 with the thickness of 2.0 mm were used as experimental materials. Butt weld brazed joints were produced under different welding parameters. The 5087 aluminum alloy filler wire with a diameter of 1.2 mm was used for joining dissimilar metals. The elimination of weld metal cracking was attained by offsetting the laser beam. When the offset was 0 mm, the intermixing of both metals was too high, thus producing higher amount of intermetallic compounds (IMCs). Higher amount of IMCs resulted in poorer mechanical properties of produced joints. Grain refinement in the fusion zone occurred especially due to the high cooling rates during laser beam joining. Reactions at the interface varied in the dependence of its location. Continuous thin IMC layer was observed directly at the titanium–weld metal interface. Microhardness of an IMC island in the weld metal reached up to 452.2 HV0.1. The XRD analysis confirmed the presence of tetragonal Al3Ti intermetallic compound. The highest tensile strength was recorded in the case when the laser beam offset of 300 μm from the joint centerline toward aluminum alloy was utilized.

Brazing of Pure Aluminum by Aluminum Silicon Filler Metal Alloys

The aim of this work is to study the effect of clearance, on the metallurgical aspects of diffusion rate and mechanical properties of the brazing joint using two types of Silicon-Aluminum filler metal alloys on the commercially pure aluminum base metal. The brazing process experiments done by joining Al-alloys (1100) type by a brazing process using (4043, 4047) filler metals at 600-650oC. Two types of joint accomplished, inclined and curvature design. To indicate the brazing joint performance the specimens tested for single shear tensile test and metallurgical testing using optical and scanning electron microscope assisted with energy dispersive detector. Diffusion rate results according to joint clearance and brazing time accomplished using optical microscope images for joints cross sections and data gained with assisting of ImageJ® software. The joint sections analyzed using EDS detector and X-Ray analysis to observe the produced phases. The major phases of brazed joints using 4047 filler alloy gives (Al, Fe3Si, Al 0.3Fe3Si0.7) and (Al 0.3Fe3Si0.7) for the 4043 filler alloy. The two filler alloys (5 and 12%Si) had equivalent tensile strength of 176 and 145MPa respectively, therefore the maximum joint efficiencies are 172% for AL5%Si filler alloy and 142% for AL12%Si filler alloy which mean that the tensile strength of the brazed joint had values greater than 100%.

Electron beam welding of thick-walled copper components

Electron beam welding facilitates a very high energy density, which is a significant advantage when welding materials with a high thermal conductivity like copper. Nevertheless, in EBW of copper, unique root defects are observed in full penetration welding of thick components. Therefore, it is of scientific interest and relevant for applications to determine the reason and to develop countermeasures. A novel attempt to address this problem is using filler material. First, with a copper–tin alloy filler metal in form of sheets, a sufficient solidification range can be established. Second, a pure tin plasma-sprayed layer is applied to achieve an alloyed weld zone. With these approaches and in combination with proper welding parameters, defect free joints are established.