Brazed Joints Research Articles

Study of Corrosion Mechanism in Brazed Cu Joints

The mechanism behind corrosion of Cu and Cu alloys has been the subject of extensive studies within their applications in various engineering components. These studies explore the electrochemical mechanisms along with the contributing factors impacting the corrosion behaviors such as pH, temperature, and solution chemistry. Thanks to these studies, the corrosion behavior of Cu and Cu alloys is reasonably well understood to a degree to predict their reliability under conditions that have been a concern of related industries. However, it is noted that studies are very limited to the impact of a microscopic structure that forms a galvanic pair with Cu and Cu alloys. One of the most obvious but least studied examples of such a case is the corrosion located in the joint that is created by brazing material. The Cu or Cu alloy components with a brazed joint is commonly found in water handling devices and facilities (such as a heat exchanger). Since the filler material used for the brazing of Cu and Cu alloys contains a substantial number of elements other than Cu the joint changes chemically and microstructurally different. It then becomes a source of a galvanic couple. In this case, the most corrosion prone part is the area near or at the joint as the corrosion is accelerated by the galvanic potential. While such joints are abundantly placed in various engineering components (and therefore the concern for corrosion should be focused more on those parts), studies on the corrosion mechanism with the brazing joint are extremely limited. This is probably due to sufficient material redundancy in the joint and thus causing immunity to failure by corrosion. However, a recent development of an advanced cooling system, where heat from electronic devices such as a microcontroller is removed by the use of recirculating coolant, the redundancy is no longer valid and the corrosion at the brazed joint becomes a matter of critical and practical concern.Motivated by the growing need to better understand the corrosion mechanism active in advanced cooling system, we have investigated the nature of corrosion in Cu plate with a variation in filler materials (Cu-Ag eutectic alloy, Cu-Ag-P alloys), liquid type (water, water mixed with PG), and temperatures. One of the most challenging part of our investigation is the fact that there are multiple sources of galvanic potential due to the existence of multiple phases in the sample. For example, when a Cu-Ag-P alloy was chosen as a filler material, the joint contained three phases, Cu-rich, Ag-rich and Cu3P intermetallic phases. An example of such a microstructure is shown in Fig.1, where an SEM micrograph of the joint is displayed. In this case, the galvanic potential exists not only between the filler materials as a whole and the surrounding Cu, but also among these three phases. In order to overcome the difficulty of studying corrosion behaviors in such a structure, we adapted two different experimental approaches. The first is to measure the corrosion current under the configuration of ZRA (zero-resistance-analysis). The ZRA uses the Cu and the joint as the counter/working electrode as shown in Fig.2 and measures the galvanic potential and current across the joint. The second is to make a mini-galvanic cell consisting of a pure Cu plate and a Cu plate with the brazed part. Spaced by an O-ring, these two plates are forced to make a galvanic cell with a liquid filled the gap and an external short circuit. We were able to measure the resulting corrosion current, direction and the amount of which can be related to the galvanic corrosion created by the braze.This study presents some of our highlighting findings suggesting that the galvanic corrosion can be better characterized by the use of techniques employed in our study. An example of such evidence can be found in Fig.3 where the ZRA scan signal (voltage and current) in 1M KCl solution is shown with the filler area as the working and surrounding Cu as the counter electrode. Note the instability in the galvanic potential with time, which is attributable to the lack of sustainable passivation layer on top of Cu3P and is responsible for the complex corrosion response of the brazed part to the solution chemistry. A detailed description of such findings along with theoretical explanation will be provide in this paper. Figure 1

Investigation on microstructure and mechanical properties of a vacuum brazed joint of γ′-strengthened Co-based single crystal superalloy before and after the post-bond heat treatment

Transient liquid phase bonding of γ′-strengthened Co-based superalloy was carried out using Ni-based filler alloy. The effects of post-bond heat treatment (PBHT) on the microstructure and mechanical properties of the γ′-strengthened Co-based superalloy joint were systematically investigated. Two distinct regions were observed in the isothermally solidified zone: primary γ′ phase zone (PZ) and secondary γ′ phase zone (SZ). The diffusion of Al from the substrate to the liquid interlayer proves to be the main reason for the precipitation of γ′ phases, showing a significant growth trend after the PBHT. The joint after the PBHT shows higher microhardness in most areas of the bonding zone. However, some parts of the diffusion affected zone show the contrary result. This could attribute to the competition consequence of the solid solution strengthening and second phase strengthening. Furthermore, the post-bond heat treated joint exhibited higher yield strength and longer stress rupture life. Our experimental results help to better understand the microstructural characteristic of the new type Co-based superalloy and provide a direction for further optimizing the brazing joint.

Evaluation of Biomedical Ti/ZrO2 Joint Brazed with Pure Au Filler: Microstructure and Mechanical Properties

Titanium and zirconia (ZrO2) ceramics are widely used in biomedical fields. This study aims to achieve reliable brazed joints of titanium/ZrO2 using biocompatible Au filler for implantable medical products. The effects of brazing temperature and holding time on the interfacial microstructures and mechanical properties of titanium/Au/ZrO2 joints were fully investigated by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and X-ray diffraction (XRD). The results indicated that the typical interfacial microstructure of the titanium/Au/ZrO2 joint was titanium/Ti3Au layer/TiAu layer/TiAu2 layer/TiAu4 layer/TiO layer/ZrO2 ceramic. With an increasing brazing temperature or holding time, the thickness of the Ti3Au + TiAu + TiAu2 layer increased gradually. The growth of the TiO layer was observed, which promoted metallurgical bonding between the filler metal and ZrO2 ceramic. The optimal shear strength of ~35.0 MPa was obtained at 1150 °C for 10 min. SEM characterization revealed that cracks initiated and propagated along the interface of TiAu2 and TiAu4 reaction layers.

Development of a boron-free filler metal for brazing nickel-based superalloys utilizing the Cu–Mn–Ni system

A new material for boron-free brazing of nickel-based superalloys is described that utilizes a transient liquid-phase bonding (TLPB) mechanism. Commercially available AMS 4764 powder (52.8Cu–37.5Mn–9.7Ni) was mixed with pure Ni powder to increase Ni content in the filler metal. This alloy is a good candidate for TLPB in Ni superalloy applications because of its high strength, low liquidus temperature, low cost and the absence of brittle intermetallic phases. Analysis of in situ braze joint formation with IN625 base metal using differential scanning calorimetry measurements allowed for optimization of Ni content, braze temperature and hold time conditions of the brazing process. Calculated ternary phase diagrams coupled with EDS analysis of braze joints identified a low-temperature copper-rich segregated region in braze joints, which, through interdiffusion with the base metal, increases melting temperature with increasing hold time. This work concludes that a 50-µm half braze joint can be formed through full isothermal solidification with an 80-min hold at 1050 °C. Solidification occurs primarily through interdiffusion of base metal and filler metal.

Optimization of process parameters for titanium alloy to itself and stainless steel brazed joints using BAg22 filler metal

In the presents study, the optimization done for brazing process parameters for joining Ti-6Al-4V to itself and stainless steel using low temperature silver based alloy in the form of a foil type brazing filler metal in the vacuum furnace. The brazing joint clearance, brazing temperature and the brazing time are chosen to be the design parameters to have massive impact on the bonding potential of a brazed joint. Taguchi method is employed as a statistical method for designing of experiments with an effort for optimizing the brazing conditions in terms of shear strength. L9 (33) orthogonal array is design for conducting the experiments. The relative impact of the design parameters and also their interaction over the response are also discussed. The results obtained from the experiments showed that the brazing conditions found in this work by Taguchi technique could generate the brazed joints between Ti-6Al-4V to itself and stainless steel with maximal shear strength.

Assessing the feasibility of using a carbon material in the design of a radiative cooler for spacecraft

The paper discusses the results of feasibility studies for using a carbon web as the radiating surface of a spacecraft radiative cooler. It proposes a design and manufacturing process solution that provides for a link between heat-transfer devices and the carbon web that has the necessary strength and minimizes heat loss. It presents results of experimental studies of temperature distribution across the radiative surface of the carbon web with mockups of the heat transfer devices. An analysis of the obtained results showed that the use of a carbon web in the design of a spacecraft radiative cooler is both feasible and promising. Key words: spacecraft, heat-conductive carbon web, radiative cooler, heat pipe, braze joint, heater.

Microstructure and strength of ultrasonic-assisted brazed joints of Si3N4/6061Al composites

The ultrasonic-assisted brazing of 30 vol.% Si3N4/6061Al composites in air using a filler metal comprising a Zn-Al alloy was investigated. Effects of the ultrasonic action duration and brazing temperature on the microstructure and mechanical properties of the brazed joints were focused on. The surface oxide layer between the composites and filler metals appeared to be broken and gradually removed by the ultrasonic action, and a composite bond with uniformly distribution of the reinforcements was formed eventually. As the ultrasonic action duration and brazing temperature increased, the proportion of Si3N4 particles and the atomic percentage of Al in the bond layer increased. Increasing the brazing temperature led to grain coarsening of the α-Al phase. The maximum shear bond strength of the joints was 183 MPa, and fracture occurred on the Al metal matrix composites side. The higher strength was attributed to the α-Al phase and Si3N4 particles being uniformly distributed in the bond layers.

Fatigue of brazed joints made of X5CrNi18-10 and Cu110 and derivation of reliable assessment approaches

Brazing in a continuous belt furnace is a quite cost-effective way of joining components consisting of many individual parts. It is extensively used in many industrial sectors like the automotive or in the energy industry. Even though many components are cyclically loaded during service up to now, no approaches to assess the lifetime under in-service loads are available. In order to assess the fatigue strength of brazed joints, three different specimen types have been investigated: peel and shear as well as a component-like specimens. The specimens were characterized and tested under load control using constant and variable amplitudes. Subsequently, the fatigue life of these joints was assessed by linear-elastic notch stresses. A reference S-N curve and characteristic damage sums were derived that enable to perform a reliable fatigue assessment. Further parameters which seem to have a strong influence on the fatigue life, the surface topology, and the overall quality of the braze were identified. Their influence is discussed.

Design TiZrCuNi filler materials for vacuum brazing TA15 alloy

TA15 alloy with near α microstructure offers excellent high temperature mechanical properties for future gas turbine blades. To developing for the thin-wall and complex structure of this material, brazing is preferred. However, a new brazing material and process are required. In this study, a new theoretical modeling approach for designing Ti-based filler material for TA15 is presented by simulation on the bond strength in the crystal unit for the brazing filler material (BFM). The BFM chemical composition was determined as Ti-(17∼19)Zr-15Cu-15Ni wt.% to balance its strength, the toughness and relative lower melting temperature for TA15. All the fracture happened in the matrix rather than the brazing joints. It means that the strength of the brazed TA15 joint at room temperature was at least as strong as that of the matrix. Furthermore, under its service temperature (600℃), the strength of the brazing joint maintained almost 70 % of strength. The high joint strength and the ductile connection were attributed to the formed Widmanstätten structure and the sufficient diffusion of Cu and Ni solutes from the BFM across the interface. No intermetallics compound was formed in the joint. This model is established to optimizing a Ti-based filler material for achieving the excellent properties of brazed joints.

Mechanical properties and microstructure evolution of CrMnFeCoNi HEA/304 SS dissimilar brazing joints

CrMnFeCoNi, a typical face-centered-cubic high-entropy alloy (HEA), was selected to fabricate a dissimilar joint with 304 stainless steel by using Ag72Cu filler metal (FM) in a vacuum furnace for the first time in this study. Neither microcracks nor pores were found in the joints, and remarkable penetration of the liquid FM was observed at the HEA/FM interface and adjusted area. Such penetration is attributed to the low solid–liquid interface energy (σSL) between HEA and the liquid FM. The penetration coefficient (D∗) of the liquid FM decreased with decreasing brazing temperature and increasing brazing time. During brazing, HEA atoms diffused into the liquid FM through dissolution and penetration. Compared with the other elements of HEA, Mn exhibited strong diffusivity and considerable dissolubility in the liquid FM, resulting in a low σSL between HEA and the liquid FM. The tensile strength of the joint could reach 459.7 MPa under the solution strengthening of Mn and the increased contact areas at the HEA/FM interface. Penetration also rendered the HEA/FM interface the weak part of the joints due to the formation of a stress concentration. This finding indicates that the strength of the joints can be enhanced through the controlled penetration of FM.

Исследование влияния содержания ZnCl2 во флюсе для пайки Al-Mg сплавов на его технологические свойства

Aluminum-magnesium alloys are ductile alloys with good weldability, corrosion resistance, and a high fatigue strength level. Aluminum-magnesium alloys contain up to 6 % magnesium. The higher its content, the stronger is the alloy. However, these alloys with high magnesium content are difficult to solder. Therefore, the task is to create flux compositions for high-temperature brazing of these alloys providing high-quality brazed joints. The work investigated the influence of the ZnCl2 activator on the technological properties of the flux. The authors tested the flux compositions with ZnCl2 content of 0 to 12 %. The tests were carried out on the AMg2 alloy using the AK12 solder. The authors evaluated the influence of ZnCl2 content on the spreading area of solder, the spreading uniformity, and the surface condition. The study revealed a significant increase in the spreading area of solder when introducing the ZnCl2 activator into the flux and the introduction of 4 % zinc chloride made it possible to increase the spreading zone of a solder drop by 50–55 %. The authors considered the samples brazed with a flux that did not contain ZnCl2 and using flux with the addition of 4 % ZnCl2. Activator content in the flux increased by 4 % before reaching 12 %. The study identified the strong interaction between the flux and the base metal with the release of gaseous products leading to the pore formation. As a result of the work, it was revealed that ZnCl2 significantly affects the flux properties, allowing increasing the solder spreading area; however, in the result of reactions with the formation of gaseous products, it can lead to the porosity of the brazed joint.

Investigation of the Effect of Manufacturing Methods and Sr Addition on the Structure and Properties of Solder 34А (Al – 27% Cu – 6% Si)

The advantages and disadvantages of Russian solder 34А (Al–27% Cu–6% Si) used for brazing aluminium alloys are described. It is demonstrated that modification of solder by Sr significantly reduces the size and morphology of phases (Si and CuAl2 ), but does not have a marked affected on mechanical and production properties. The structure and properties of solder bars prepared by vacuum suction casting, hot extrusion, and continuous casting are compared. The main regimes are determined for continuous casting and hot isothermal extrusion of solder metal rods 3-4 mm diameter. It is shown that the best mechanical properties are exhibited by rods prepared by hot extrusion with a degree of reduction of 300. It is shown that the manufacturing method of filler metal rods does not have a marked effect on spreading kinetics during brazing and on the sizes of structural components within brazed joints.

Pulsed current-assisted joining of copper to graphite using Ti-Cu brazing layers

Copper-graphite joints are functional elements of many electronic and electromechanical devices. Unique properties of graphite, such as the ability to withstand high temperatures, a low friction coefficient and high thermal and electrical conductivities make it an attractive material for various applications. For enabling efficient heat sink or making sliding electrical contacts, graphite requires joining to metallic copper. Poor wettability between copper and graphite is a problem that needs to be overcome to form a reliable joint. In the present work, we used a Ti25Cu75 alloy to braze graphite to copper in a pulsed current-assisted process. Brazing was carried out in a Spark Plasma Sintering (SPS) apparatus under a uniaxial pressure of 13 MPa. The SPS conditions – a possibility of localized heating under an applied pressure in dynamic vacuum – are beneficial for the joining processes of conductive materials. For joining graphite and copper plates, a die-free configuration was used. The microstructure and the phase composition of the copper-graphite brazed joints were investigated. Strong joints were formed when the upper electrode temperature measured by a thermocouple reached 700 °C. At the graphite-alloy interface, a TiC layer 0.5–1 μm thick was formed; in addition, TiC particles were found in the metallic matrix in a layer 5–10 μm thick adjacent to the graphite plate. The tensile strength of the brazed joint was higher than the tensile strength of graphite (13 MPa).

Quality inspection scheme for automotive laser braze joints

Laser triangulation and image processing are two major methods for surface quality inspection. The former has the advantage of three-dimensional information, while the latter can provide more detailed characteristics. However, there are few researches on effective integration of the two methods. In this paper, a lab scale laser braze quality inspection system utilizing information from laser triangulation scanning and optical image acquisition was able to inspect an automotive laser braze at a scanning speed of 1.5 m/min with 100% accuracy. The process started with defect zoning to sort out braze areas with and without defect based on the joint surface profile reconstructed from laser triangulation. Feature extraction was then carried out on the images of defect-containing braze areas, in which the location, size, geometry, and texture features of the defects were extracted from the gray-scale images, and since these features were extracted from only defect-containing areas, a lot of computational cost was saved, which reduced time consumed greatly. For defect classification, in the training stage the extracted features were processed by the principal component analysis and the first three principal components were calculated for 5 typical defects: surface cavity, pore, hump, burn-through, and lack-of-braze. In the test stage, the first three principal components of the images of interest were calculated and compared with the established principal component values of the 5 defect types for defect classification.

Microstructure and mechanical properties of Ti6Al4V alloy and sapphire joint brazed with graphene-AgCuTi

The sapphire (α­Al2O3) and Ti6Al4V (TC4) alloy was successfully brazed with GNSs (Graphene)-AgCuTi composite filler. The effects of different GNSs content and different brazing temperatures on the interface and mechanical properties of brazed joints were studied. The typical interfacial microstructure of the brazed joint was: Ti6Al4V/Ti2Cu/TiCu/Ti3Cu4/Ag(s.s)/TiC + GNSs /TiCu/Ag(s.s)/Ti3Cu3O/sapphire. The result shows the diffusion layer (I) and Ti-Cu layer (II) got thickened gradually as the brazing temperature rose. At the same time, the thickness of Ti3Cu3O reaction layer increased from 1.9 m to 4.7 m when the brazing temperature increased from 810 °C to 870 °C. The thermal expansion coefficient of the composite filler and the mechanical properties of the joint were calculated and analyzed. The addition of GNS can reduce the mismatch of thermal expansion coefficient between the brazing joint and Ti3Cu3O reaction layer. When the 0.1 wt% GNSs-AgCuTi composite filler was put into use, the shear strength of the joint reached 29.1 MPa. However, the agglomeration of excess graphene hindered the diffusion of the elements, making the Ti3Cu3O layer discontinuous and causing a mismatch in the coefficient of thermal expansion among the materials, thus reducing the shear strength of the joint.

Possibilities of using scanning acoustic microscopy to analyze incompatibilities in braze welded joints

This article explores the possibilities of using non-destructive ultrasonic techniques to analyze the quality of lapped braze-welded joints. The tests were performed for 4 material groups (DC03+ZE steel and X5CrNi18-19 steel, aluminum alloys AW-5754 and AW-6061, titanium Grade 2 and copper Cu-ETP). As part of the work, additional materials and joint processes and its parameters were selected (TIG, MIG, laser). The quality of joints was monitored using scanning acoustic microscopy. Based on the A-scan andC-scan images, potential joints imperfections were determined. The possibilities of using advanced ultrasonic techniques to analyze the quality of braze joints was assessed.

Interfacial microstructure and mechanical properties of ultrasonic-assisted brazing joints between Ti–6Al–4V and ZrO2

Ultrasonic-assisted brazing (28.8 kHz, 180 W) was introduced to achieve high-quality joints between Ti–6Al–4V alloy and ZrO2 ceramic with Al-5wt.%Si brazing filler in air. The interfacial microstructure of intermetallic compounds (IMCs) and the phase constitution of joints ultrasonic-assisted brazed at 700 °C for different ultrasonic time were investigated in detail using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer and X-ray diffractometer. When the ultrasonic time was 20s, the average shear strength of the joint reached a maximum value of 90.68MPa and two important types of IMCs Ti(Al,Si)3 and Ti7Al5Si12 formed at the interface between the Ti–6Al–4V and the filler. Ultrasonic played a vital role in the formation of Si segregation regions at the interface near the Ti–6Al–4V and in the centre of the joint. A change in the Ti molar fraction, resulting from the cavitation effect of ultrasonic on the surface of Ti–6Al–4V, increased the chemical potential gradient of Si across the joint. Meanwhile, driven by the ultrasonic standing wave field in the liquid Al-5wt.%Si filler, Si atoms moved to the ultrasonic antinode-plane spontaneously. Ultrasonic-assisted brazing proved to be an effective method of joining Ti–6Al–4V and ZrO2 with Al-5wt.%Si filler.

The microstructural evolution and formation mechanism in Si3N4/AgCuTi/Kovar braze joints

The braze joints of Si3N4-Kovar using AgCuTi braze alloys are investigated systematically with the brazing temperatures between 860 °C and 950 °C for 2–30min. The correlation between the joint strength and the microstructures of the braze joints is discussed. These results show that the reaction layer plays an important role in interface bonding. Growth of the reaction layer is a reaction-diffusion process, which mainly depends on the diffusion of Ti. The diffusion activation energy (Q) is estimated as 170.9–248.7 kJ/mol in this system, which may provide data support for Si3N4/metal braze joints. As the holding time is further prolonged (in the range of 10–30min), intermetallic compounds (IMCs) forms in the braze seam, which will prevent Ag solid solution from alleviating interfacial thermal stress. With the increasing of the brazing temperature, the residual thermal stresses increases. As a results, larger residual thermal stress is produced in the braze joints at higher braze temperature, which result in a weaker bonding.

Challenges in achieving hermetic brazing joints in single-insulator multi-stage depressed collector of travelling wave tubes

Travelling wave tube (TWT) being a vacuum electronic device, demands achieving hermetic sealing in ~350 braze and weld joints to attain ultra-high vacuum. One such critical brazing joint exists in its multi-stage depressed collector (MDC) which is generally employed to enhance the TWT efficiency. As an alternative to conventional MDC, single-insulator MDC technology has the capability to offer: (i) increased high-voltage breakdown margins, (ii) reduced diameter/weight and (iii) simpler fabrication. Here, all the electrodes and their corresponding radial feedthrough connections are brazed with a single insulator. Although this technology appears attractive, the major challenge has been in attaining hermetic brazing joints, particularly between the insulator and the radial feedthrough pins. Authors have experimented with different configurations of the feedthrough pins to achieve hermetic braze joints and the details of which have been presented in this paper.

Microwave-assisted alumina-zirconia brazed joint for microwave tubes

Alumina-zirconia joint was fabricated by microwave-assisted brazing technique at 980 °C for 15 min using TICUSIL (68.8Ag26.7Cu4.5Ti in wt%) as the brazing alloy. The brazed joints were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, Vickers microhardness evaluation, brazing strength measurement, and helium leak test. X-ray diffraction analysis confirmed the formation of Ti-based compounds at the substrate-filler alloy interfaces of the brazed joints. SEM images showed the substrate-filler alloy interfaces of the brazed joints. The EDX analysis determined the elemental compositions along the joint cross-section. Vickers microhardness measurement indicated reliable joint performance for the microwave-assisted brazed joints during actual application. Brazing strength measurement and helium leak test proved the formation of sound alumina-zirconia brazed joint by both microwave and conventional heating techniques. Scratch tests were performed on the cross-sectional surfaces of the alumina-zirconia joints. Scratch test results established the strong interfacial strength of the microwave brazed joints.