Molten Braze Research Articles

In-situ observation of molten brazing filler metal permeating gap

Generally, brazing is completed when the brazing filler metal permeates the gap by uniform wetting. It is believed that the joining process is completed with the formation of joint defects ‘voids’ in the process of uniform wetting. However, previous studies have shown that what occurs when brazing is performed is non-uniform wetting. It is suggested that this non-uniform wetting is the cause of void generation. Wetting and spreading of brazing filler metal is tested on a metal plate, as described in JIS Z 3191. This is used to investigate the wetting of the brazing filler metal. However, this method does not cover the wetting of the molten brazing filler metal as it moves into the gap. Therefore, it is suggested that the JIS Z 3191 experiment is insufficient to evaluate the wetting of the molten brazing filler metal that penetrates into the gap. In this experiment, we created new specimen. It is called us ‘V-groove specimen’. Specimens were created with two base metals. They are pure copper and lead-free brass in which bismuth was used as an alternative element. In order to do in situ experiments, it was not possible to observe the inside of a conventional electric furnace. Therefore, we produced a furnace with a window for in-situ observation and experimented with it. As a result of experiments with V-groove specimens, it was found that there are two types of wetting of brazing filler metal. They are called ‘primary wetting’ and ‘secondary wetting’. The shape of the specimen was measured and evaluated for two types of wetting. The results showed that the primary wetting was measurable in shape and spread evenly. However, the shape of the secondary wetting could not be measured, and it was found to be non-uniformly spread.

Enhancing interfacial strength of T-joint for vacuum-brazed thin-walled structures comprising TiAl and GH3536 alloys via in-situ synthesis of Al2TiO5 ceramic coatings

The joining of dissimilar materials to fabricate lightweight components of thin-walled structures for aerospace applications has garnered significant interest. A pioneering technique has been proposed to achieve an optimal interfacial microstructure and fillet of brazed joints. This technique involves a layer of Al deposited on the surface of TiAl base metal, which aids in the brazing process of the thin-walled structure comprising TiAl and GH3536 alloys. Notably, the addition of the Al layer has a significant impact on the spreading area of molten brazing filler metal, the size of the fillet, and the interfacial microstructure evolution. Consequently, these factors contribute to a substantial increase in the load-bearing capacity of the structure. A comprehensive analysis and discussion are conducted to explore the impact of the Al layer on both the interfacial microstructure and mechanical performance. Under the brazing parameters of 1040 °C/5 min, the peak tensile load reaches a peak value of 362 N, which is 100% higher compared to the joints without Al layer. It is worth noting that fracture occurred in the GH3536 foil after undergoing tensile testing. The dimension of fillet can be controlled through the in-situ synthesis Al2TiO5 ceramic phase on the surface of TiAl alloy. This research offers valuable insights into the production of engineered biomimetic materials and devices, specifically focusing on honeycomb-inspired structures.

A Review on the Friction Stir Brazing for Joining Dissimilar Materials

Friction stir brazing (FSB) is a new technology developed for its ability to join similar and dissimilar metals and alloys resulting in a joint with considerable characteristics through the use of interlayer (braze) material under the action of a pinless rotating tool and other FSB parameters. The frictional heat during FSB is responsible for the melting of the braze material between the two workpieces, while the shoulder action must be satisfactory for the extrusion of the excess braze liquid phase depending on the FSB parameters used. The parameters of FSB also have a considerable impact on the microstructure and mechanical characteristics of friction stir brazed (FSBed) joints. Sound interfacial bonding can be observed in the central zone of FSBed joints, where intermetallic compounds (IMCs) can be formed by direct diffusion among the dissimilar workpieces after extrusion of liquid phase rather than by mechanical mixing or solidification of braze material. Increasing the transverse speed at a constant rotational speed has an influence on the peak temperature, but it remarkably reduces the holding time owing to the increased cooling rate. The use of vibration in the FSB increments the fluidity of the molten braze material among the joining workpieces resulting in a more homogeneous distribution of IMCs particles. In this review article, FSB parameters, bonding mechanisms, as well as the microstructure, and mechanical properties of FSBed joints are reviewed.

Effects of gravity on the capillary flow of a molten metal

Repair and construction in space by means of brazing requires understanding of the effect of gravity on the capillary flow of a molten metal. We perform two sets of experiments: (1) spreading of a sessile drop of liquid aluminum-silicon-flux (Al-Si-KxAlyFz) composite braze on a horizontal alumina (Al2O3) substrate (a non-wetting surface), and (2) capillary flow of the same braze alloy on an inclined aluminum-manganese (AA3003) substrate (a wetting surface). We vary the mass of the brazing liquid and the inclination of the substrate (i.e., the relative direction of gravity).In the composite metal/flux sessile drop experiments, we observe a secondary liquid flux meniscus forming at the contact line between the liquid Al-Si and the alumina substrate. We demonstrate that the equilibrium contact angle appears to be close to 180°, while the apparent contact angle depends on the mass of the braze. In the second set of experiments, we study the molten braze alloy on different inclinations of AA3003 in a wedge-T wetting/non-wetting assembly. As the inclination angle decreases, the wetting distance increases and the surface profile changes from a non-symmetric bag-like shape to a more symmetric pancake shape. With the mass decreasing, the surface profile on the vertical substrate approaches a symmetric shape when the wetting distance is equal or smaller than the capillary length. While the non-homogeneous melting and hence, the non-homogeneous microstructure of the melt, may prevent a full symmetry. Computational predictions of equilibrium shapes are in good agreement with experimental results.

A systematic study on the effect of coating type and surface preparation on the wettability of Si-Bronze brazing filler material on GI and GA-coated DP600

Zn-coated advanced high strength steels are popular in the automotive industry due to their excellent combination of mechanical strength and ductility as well as superior corrosion resistance provided by the Zn-coating – with hot-dip galvanized and galvannealed coatings being the most popular. Gas metal arc brazing technology is a non-fusion joining method, proposed as an alternative to the gas metal arc welding process due to several advantages: The arc-brazing process delivers significantly lower heat input to the substrate, which minimizes the Zn-coating burn-off leading to higher corrosion protection for the joined parts, and reduces the HAZ softening phenomenon which affects the mechanical integrity of the substrate, while at the same time reduces welding defects such as porosity and blowholes. The strength of an arc-brazed joint depends on the spreading of the molten filler material over the joint to create a bond between the parts as the molten braze solidifies. Existing literature on the subject suggests that heat input is the main driving factor controlling the wettability of the molten braze material with the type of shielding gas used also having an effect. However, the influence of different types of Zn-coatings and their respective surface condition (i.e., clean, or unclean) on the wettability of Cu-based molten braze materials during arc-brazing has not been investigated. The present work investigates the effect of surface morphology of galvanized and galvannealed DP600 steel in the as-received and plasma cleaned conditions on the wettability of molten Si-Bronze (CuSi3Mn1) brazing filler material in the bead-on-plate configuration. The findings of this work clearly demonstrate that the type of coating and its surface condition has a significant effect on the spreading of the molten filler material and on the growth of the intermetallic compound layer at the joint interface and therefore, should be taken into consideration as a factor that can impact the efficacy of an arc-brazed joint. • Evaluating surface morphology of different Zn-coatings to calculate their surface free energy. • Establishing a clear link between the surface roughness of Zn-coated steels on the wettability of molten Si-Bronze filler material. • The study shows that the wettability of the molten filler material is also affected by the presence of organic contaminants on the coating surface. • This work shows the relevance of surface morphology in controlling the wetting length of weld brazed joints. • The thickness of the intermetallic compound layer at the braze interface can be controlled by using oxygen plasma cleaning.

Dissimilar Brazing of Ti-15Mo-5Zr-3Al and Commercially Pure Titanium Using Ti-Cu-Ni Foil.

Dissimilar brazing of Ti–15Mo–5Zr–3Al (Ti-1553) to commercially pure titanium (CP-Ti) using Ti–15Cu–15Ni foil was performed in this work. The microstructures in different sites of the brazed joint showed distinct morphologies, which resulted from the distributions of Mo, Cu, and Ni. In the brazed zone adhered to the Ti-1553 substrate, the partitioning of Mo from the Ti-1553 into the molten braze caused the formation of stabilized β-Ti without Ti2Cu/Ti2Ni precipitates. In the CP-Ti side, the brazed joint displayed a predominantly lamellar structure, composed of the elongated primary α-Ti and β-transformed eutectoid. The decrease in the Mo concentration in the brazed zone caused the eutectoid transformation of β-Ti to Ti2Cu + α-Ti in that zone. The diffusion of Cu and Ni from the molten braze into the CP-Ti accounted for the precipitation of Ti2Cu/Ti2Ni in the transformed zone therein. The variation in the shear strength of the joints was related to the amount and distribution of brittle Ti2Ni compounds. Prolonging the brazing time, the wider transformed zone, consisting of coarse elongated CP-Ti interspersed with sparse Ti2Ni precipitates, was responsible for the improved shear strength of the joint.

Investigation into microstructure and mechanical behaviors of joints made by friction stir vibration brazing between low carbon steels

In the current investigation, a new version of brazing entitled FSVB (friction stir vibration brazing) is developed. In this process, the adjoining samples are vibrated normally to the brazing line while FSB is performed. Low carbon steel sheets are lap joined together using FSB and FSVB methods while SiO2 nanoparticles are incorporated in the joint. The microstructure and mechanical behaviors of the joints developed under different conditions are analyzed. %67wt Sn-%33wt Pb alloy is used as braze metal. The results show that the strength of the FSV brazed specimen is higher than that of the FS brazed sample (about 30%); additionally, the presence of SiO2 nanoparticles increases the strength of joints made by FSVB. Second-phase particles reduce the mobility of dislocations and strengthen the braze material. The vibration of adjoining specimens, during FSVB, increases the stirring of the molten braze metal between the adjoining specimens and the molten fills the space thoroughly. Workpiece vibration during FSVB also leads to the production of higher heat and temperature compared to those when using FSB. The higher temperature results in a higher fluidity of molten and it enhances the homogeneity of second-phase particle distribution and as a result, the joint strength increases. The results show that the strength and hardness of FSV brazed samples increase by 16% and 18%, respectively, and the thickness of the intermetallic component decreases as vibration frequency increases from 20 Hz to 50 Hz.

New attempt to improve friction stir brazing

An improved method of friction stir brazing (FSB) by application of mechanical vibration was introduced in this investigation. This modified version is entitled friction stir vibration brazing (FSVB) as joining specimens are vibrated normally to the brazing line. During this investigation, low carbon steel sheets are joined together using FSB and FSVB, and %67 wt Sn-%33 wt Pb alloy is used as brazing metal. The influence of FSB and FSVB processes on the microstructure and mechanical characteristics of brazed samples is examined. The results indicate that the application of vibration during FSB causes the grain size to reduce in the joint region and it improves the homogeneity of intermetallic components (IMCs) distribution. The vibration of joining specimens, during FSVB, enhances the eutectic reaction of the molten brazing metal between the joining specimens and the molten fills the space between them thoroughly. The results also imply that the strength of FSVB-ed specimens is higher than that of FSB-ed specimens.

Microstructure and heat transfer characteristics of active brazed Ceramic–Metal joints

This work reports on the bonding of Al2O3–Cu, AlN–Cu, and AlN–Ni by active brazing for the thermoelectric module support plate. Ag63Cu35.25Ti1.75 braze alloy in the form of paste and sheet was used for this purpose. Investigation of the microstructure using scanning electron microscope (SEM) attached with energy dispersive spectroscopy (EDS) showed that Al2O3–Cu brazed joint alone had an interface with a continuous reaction layer. On the other hand, AlN when brazed to Cu and Ni showed a relatively weak bonding due to the poorly developed reaction layer at the interface. The growth of reaction layer while brazing of AlN–Cu and AlN–Ni was impeded by the dissolution of the respective metals into the molten braze alloy. In AlN–Ni, the decrease in Ti activity due to such dissolution is so low that the interface reaction layer was absent. The temperature difference (ΔT) measured across the ceramic–metal joints showed little resistance to heat transfer in Al2O3–Cu. On the other hand, other joints have shown a significant ΔT, mainly due to the poorly developed and less thermally conductive (TiAl)1+xN interface reaction layer. Thus, this study demonstrates that a stable ceramic–metal joint with high thermal conductivity could be made with Al2O3 using Cu by active brazing.

<i>In Situ</i> Observation during Copper Alloy Brazing Using an X-Ray System

In general, a flux is used to braze a copper alloy. In many cases, when the molten brazing filler metal spreads in the set joint gap, vaporised flux and its residue are produced, and defects (mainly voids) are formed. Voids, which are formed on the brazed layer, cause deterioration in the strength and other properties. However, with conventional evaluation methods (e.g. ultrasonic or X-ray radiography tests), the behaviour of the molten brazing filler metal during the brazing process cannot be visually observed from the outside of the joint. Therefore, the void formation process cannot be clarified. To improve the quality of the brazed layer, it is necessary to elucidate the mechanism of void formation. The purpose of this study is to observe the behaviour of the molten brazing filler metal by performing an X-ray radiography test at the same time as brazing and to study how to reduce voids. In this study, a brass specimen was brazed with a Cu–P-based brazing filler metal. The specimen was brazed by heating in an electric furnace, and the specimen was irradiated with X-rays. The state where the molten brazing filler metal spread into the gap was photographed as the transmission image. Thereafter, the behaviour of the molten brazing filler metal was analysed.

The influence of the ratio between the wire and process velocities on the wetting process during laser brazing

Laser brazing is used in automotive production for the joining of car body parts with customer-visible seams, whereby the highest optical seam quality is demanded. During the wetting process, a constriction formation can occur within the molten brazing material, which subsequently collapses; this induces a stepwise progression of the wetting front, thereby decreasing the optical seam quality. The aim of this study is to investigate the influence of the wire and process velocities on this constriction formation. For this purpose, high-speed recordings of the process zone are performed during laser brazing and then evaluated concerning the wetting process. The results show that the constriction formation depends on the relationship between the process velocity and the process direction component of the wire velocity. The constriction formation only occurs when the process velocity exceeds the parallel-oriented component of the wire velocity. This constriction formation is unstable and collapses repeatedly. In this study, increasing velocity differences stabilizes the constriction existence. This stabilization correlates with a uniform wetting process and a reduced frequency of the occurrence of blowout events, probably due to improved outgassing of zinc bubbles from the melt pool.

Strong and reliable dissimilar brazing titanium to copper using niobium diffusion barrier

Dissimilar brazing of titanium (Ti) and copper (Cu) using Ag-based braze and niobium (Nb) diffusion barrier was investigated. It was found that the addition of a Nb interlayer can effectively block the interaction between Ti substrate and molten braze, thus completely eradicate the formation of brittle intermetallic compounds (IMCs). This leads to a brazed joint consisting of Ti-Nb solid solution, remnant Nb interlayer and Ag-Cu braze. The resultant IMCs free joint exhibits superior structural integrity with bonding strength exceeding that of the Cu substrate. Meanwhile, negligible dissolution of Nb into molten braze and sluggish solid-state diffusion at the Ti/Nb interface keep the Nb interlayer intact, enabling excellent reliability of the joint.

Interfacial microstructures formation mechanism between SiO2 substrate and AgCuTi braze alloys

The wetting characteristics of braze alloys (AgCu eutectic and AgCu eutectic −2 wt% Ti) on SiO2 substrate are observed and the results show that small percentage additions of Ti into the AgCu eutectic alloy leads to a dramatic decrease in the steady contact angle. The interfacial microstructures are responsible for the steady contact angle decreasing of different braze alloys on the SiO2 substrate. The aim of this work is to investigate the interfacial microstructures formation mechanism between SiO2 substrate and AgCuTi braze alloys. Scanning electron microscopy (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM) were employed to analyze the interfacial microstructures. The results show that the molten AgCuTi alloy infiltrates into the SiO2 side, resulting in the formation of SiO2/Ti2O/(Ti4Cu2O+AgCu)/AgCu/(AgCu+Ti5Si3) structures. The Ti atoms in the molten braze alloy firstly accumulated along the surface of SiO2 and then reacted with SiO2 to form the Ti2O layer, at the same time, Si liberated from thermal degradation of SiO2 dissolved into the molten AgCuTi solution and reacted with Ti to form Ti5Si3 compounds. Ti-Cu rich in the molten AgCuTi would react with Ti2O at the interface to form the Ti4Cu2O M6X compounds. Both the experiment results and the theoretical thermodynamics analysis support the proposed viewpoint of interfacial microstructures formation mechanism between the SiO2 substrate and AgCuTi braze alloys.

Bonding Characterization Between 316L and Porous Stainless Steel Pipes by Vacuum Brazing

Vacuum brazing between porous stainless steel (PSS) tube and 316L stainless steel has been performed using BNi-7 brazing paste as a filler metal. And the interface microstructure of the joint was analyzed by SEM and EDS. The results show that Ni(Cr, Fe)-P intermetallic compounds and chromium-rich nickel-iron-based solid solutions phases are the main phases in the brazing seam. The wettability of the molten braze alloy to PSS is better than that to 316L. The molten braze filler is pulled into the pore of PSS by a capillary action during brazing. The highest tensile strength of 245 MPa is acquired from the joint brazed at 980°C for 15 min. Longer processing time or higher processing temperature does not improve the joint strength further.

Microstructure and Joining Properties of High Nb-containing TiAl Alloy Brazed Joints

High Nb-containing TiAl alloy (Ti-45Al-8.5Nb-(W, B, Y) (at%), abbreviated as TAN alloy) was brazed at 1100 °C using Ti-28Ni (wt%) eutectic brazing alloy. The typical interfacial microstructure was TAN/τ3-Al3Ti2Ni+B2/α2-Ti3Al layer/α2-Ti3Al + δ-Ti2Ni/α2-Ti3Al layer/τ3-Al3Ti2Ni + B2/TAN. The effects of holding time on interfacial microstructure and joining properties of TAN brazed joints were investigated. The results demonstrate that the diffusion of Ni from molten brazing alloy to TAN substrate plays an important role in the interfacial microstructure evolution, which results in an increase of diffusion zone and a reduction of brazed seam with the prolongation of holding time. Shear test indicates that the maximum shear strength at room temperature and high temperature (600 °C) reaches 248.6 MPa and 166.4 MPa, respectively when holding time is 15 min. Fracture analyses reveal that brittle fracture prefers to initiate and propagate in the continuous intermetallic layers during shear test.

Formation of the reaction zone between tin‐copper brazing fillers and aluminum‐silicon‐magnesium alloys: Experiments and thermodynamic analysis

AbstractA primary challenge in brazing is the controlled formation of phases resulting from interactions of elements of the liquid filler metal with those of the base material. The morphology of the brazed joint, which is decisive for the mechanical properties of the joint, is influenced by present elements and process parameters such as brazing temperature and time. Furthermore, the wetting of the base material is a crucial factor in joining of aluminum because of the low wettability of the alumina layer by molten brazing filler metals. In order to remove the alumina and prevent reoxidation of the substrate surface, the brazing process can be conducted in vacuum or inert gas atmosphere. Again, selection of process parameters is crucial for the quality of the brazed seam. In this work, we focus on the influence of the process parameters on the wetting behavior and the formation of aluminum‐copper phases theoretically by means of thermodynamic calculations using a CALPHAD database as well as by means of in‐situ observations in the large‐chamber scanning electron microscope (LC‐SEM) and by brazing experiments. Both the critical temperatures with respect to the wetting and the reaction kinetics as well as the crucial stages of the brazing process and the resulting phases were determined.

Effect of Copper-based Fillers Composition On Spreading and Wetting Behaviour

Wetting and spreading of molten brazing filler material are important factors that influence the brazing ability of a joint to be brazed. Several investigations into the wetting ability of a brazing filler alloy and its spreading area in the molten state, in addition to effects of brazing temperature on the contact angle, have been carried out. Generally, the composition of copper-based filler and temperature affect spreading of molten brazing filler material during brazing. Wetting by and interfacial reactions of the molten brazing filler material with the metallic substrate, especially, affect strongly the spreading of the filler material. In this study, the effects of filler composition and brazing temperature on the spreading of molten brazing filler metallic alloys were investigated. MBF 2005 (Cu, 5.7wt.%Ni, 9.7wt.%Sn, 7.0wt.%P), MBF 2002 (Cu, 9.9wt.%Ni, 4.0wt.%Sn, 7.8wt.%P) and VZ 2250 (Cu, 7.0wt.%Ni, 9.3wt.%Sn, 6.3wt.%P) alloys were used as brazing filler materials. Pure copper block and a rectangular plate were employed as the base metal. Brazing filler material and metallic base plate were first washed with acetone. Brazing was performed at 750°C and 800°C under Ar gas for 30 minutes using an electrically heated furnace, after which, the original spreading area, defined as the sessile drop area, and the apparent spreading area were both evaluated. It was observed that the spreading area and wetting angle influenced by the composition of copper-based filler.

Vacuum brazing of TZM alloy to ZrC particle reinforced W composite using Ti-28Ni eutectic brazing alloy

Reliable brazing of TZM alloy and ZrC particle reinforced (ZrCp) W composite was achieved in this study by using Ti-28Ni eutectic brazing alloy. The typical interfacial microstructure of TZM/Ti-28Ni/ZrCp-W brazed joint consisted of a Ti solid solution (Ti(s, s)) layer, a continuous Ti2Ni layer and a diffusion layer mainly composed of W particles and (Ti, Zr)C particles. With an increase of brazing temperature, more ZrC particles and W particles entered the molten brazing alloy, which broadened the brazing seam and diminished the Ti2Ni layer, resulting in the disappearance of the Ti2Ni layer eventually. Meanwhile, more Ti(s, s) stripes were observed on the TZM side. The presence of continuous Ti2Ni intermetallic phase and Ti(s, s) stripes structure in joints deteriorated the joining properties, which resulted in the formation of brittle fracture under shear test. In addition, the fracture path was related to the brazing temperature, and cracks initiate and propagate in the continuous Ti2Ni layer at lower temperatures. However, the fracture path tended to be located at the TZM substrate close to the interface between TZM and the brazing seam when the brazing temperature exceeded 1040°C. The optimal room temperature shear strength reached 120.5MPa when brazed at 1040°C for 10 min and the fracture surface exhibited cleavage fracture characteristics, and the shear strength at high temperature of 800°C for the specimens with highest shear strength at room temperature reached 77.5MPa.

The base material: A key factor in sinter-brazing

Brazing can be considered as an attractive method for joining sintered steel components since it can be integrated into the sintering process. The behavior and quality of the joint rely on the braze filler design. Among others, the interaction between the base material and the filler depends on the brazing temperature, wettability, infiltration ability, holding time, and sintering atmosphere. The use of this joining technique in components based on FeCuC is very common, and in this case the presence of a second liquid phase during sintering can limit the response of braze fillers. In this study, with the aim of analyzing the effect of the base steel composition, a NiCu-based brazing will be used together with different Fe-base substrates for sinter-brazing tests: iron is considered as the reference, FeC to assess the carbon addition effect, and finally FeCCu substrate tries to explain the interaction between the two liquid phases (the molten brazing alloy and the Cu transient liquid phase). Characterization of the final joint is supported with SEM and LOM microscopy, and EDS is used for determining the distribution of the alloying elements. Moreover, liquid phase features such as infiltration capacity and diffusion are evaluated by using image analysis. On the other hand, the study is completed with wetting evaluation of the brazing alloy.

Effect of dilution on reaction properties and bonds formed using mechanically processed dilute thermite foils

Thermites have been used for over a century for joining applications and in this paper we present fully dense diluted thermite foils that react in a self-propagating manner and produce sufficient heat and molten braze to join aluminum-, magnesium-, and iron-based alloys. Al:NiO, Al:CuO, and Al:Cu2O thermite systems were systematically diluted with Ni or Cu to decrease the maximum reaction temperature and hence the amount of gas generated during the self-propagating reactions. Velocities and mass ejection were measured for reactions within free-standing foils as a function of dilution. The dilution that leads to quenching during propagation within a bond is identified, and finally, Al:NiO:10wt%Ni and Al:CuO:40wt%Cu foils were used to demonstrate the ability to join aluminum-, magnesium-, and iron-based alloys.