Brazed Samples Research Articles

Comparative Analysis of Dip-Brazing and TIG Welding on the Properties of Al-64430 Joints

This study compares dip brazing and TIG welding operations carried out under optimal conditions on the parameters of strength, deformation, and ability to resist shocks for Al-64430 joints, which find extensive use in automobile and telecommunication industries. The fabricated samples were tested for bump test, surface deformation, microhardness and tensile strength, and the joint properties were studied through SEM, EDAX mapping and EDX. Compared to the dip-brazed samples, the TIG welded samples achieved 161% greater strength and 1000% greater elongation but subsequently 1330% higher surface deformation. Both joints were able to pass the bump test; however, some degree of cracks were observed post testing in both samples. Both samples achieved similar joint microhardness values, but notably, compared with the TIG welded sample, the brazed sample showed no HAZ zones in terms of microhardness variation, where a substantial decrease in microhardness was recorded. This absence of the HAZ region was also confirmed through the microstructure. Both primary and secondary silicon were observed in brazed samples, which had finer grain sizes than did the welded samples. Dendrite formation was observed for the brazed samples, whereas a uniform distribution of aluminium was observed for the welded samples. Fractographic analysis revealed brittle intergranular fracture with small dimples present at the fracture point for the brazed joints, while out-of-plane fracture with sharp edges indicated a more ductile nature for welded joints.

Metallurgical Processing for Impregnated Thermionic Cathodes Manufacturing

A metallurgical process using a very high temperature Mo-Ru brazing filler alloy to join a controlled porous tungsten cathode button and a single-crystalline molybdenum cathode body for microwave tubes manufacture was developed†. The Mo-Ru brazing alloy was obtained by mixing and milling powders in the eutectic composition with a binder, and a braze paste was applied on the surface cathode parts. Brazing was performed in two temperatures by using a resistive dry hydrogen cold wall furnace for 10 minutes: at 1890 °C and 1967 °C. It was observed a fillability by the Mo-Ru system only in the tests performed in temperatures above 1967°C. The brazed samples were analyzed by Scanning Electron Microscopy coupled to Energy Dispersive Spectroscopy. It was observed absence of microstructural defects in the interface between the tungsten porous and dense molybdenum joint. Stress-strain tests, followed by SEM analysis were performed to determine the mechanical behavior of the brazing joining. The results indicate the origin region of the cracking and show an intergranular propagation; some evidence as grain cleavage indicates a brittle failure behavior.

Investigation into novel multipass friction stir vibration brazing of carbon steels

ABSTRACT In this examination, an advanced version of friction stir brazing (FSB) entitled friction stir vibration brazing (FSVB) was implemented to produce a joint between low carbon steels. %67 wt Sn-%33 wt Pb alloy was used as brazing metal. This article aims to study the effect of mechanical vibration and brazing pass number on the structure and mechanical behaviors of the brazed samples. In addition, the thermal analysis, the characteristics of intermetallic compounds (IMCs) layers, and void volume percentage at the bond interface were investigated. It was found that the coherency of the joint interface increased as the brazing pass number increased. Additionally, the mechanical performance, namely, shear strength and hardness of brazed samples enhanced as the vibration was applied during FSB, and brazing was done in two passes. The results showed that the metallurgical reaction between the parent metal and the filler enhanced as FSB was replaced by FSVB. Two passes of FSVB decreased the thickness of the IMC layer, void volume percentage, and the formation of micro-cracks in the brazed sample.

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.

Internal Corrosion of Warm Formed Aluminum Alloy Automotive Heat Exchangers

The objective of this study is to evaluate the effects of sheet temper, forming conditions, and the resultant post-braze microstructure on the corrosion behavior of Al alloy heat exchangers. Accelerated corrosion tests were performed by flowing a corrosive water solution through brazed heat exchanger samples fabricated from O and H24 temper sheet formed between room temperature and 250 °C. Sheet perforations within the brazed samples were found to be concentrated at the coolant channel inlets, due to crevice corrosion, or around other formed features within the coolant channel, which was attributed to erosion corrosion. A clear relationship between corrosion initiation sites and local microstructure was not observed in the accelerated test. Potentiodynamic polarization measurements were performed using the corrosive water solution and sheet samples pre-strained to 4 and 10% and subjected to a simulated brazing heat treatment. The average corrosion current density was the same for the two conditions, but liquid film migration occurred during brazing in the sample strained to 4%, resulting in localized corrosion and greater variability in the polarization measurements. It is concluded that warm forming does not impair the corrosion resistance of H24 temper sheet, but the corrosion resistance of brazed samples fabricated from O temper sheet is altered by warm forming.

Brazing of Be with CuCrZr-bronze using copper-based filler metal STEMET

At the D.V. Efremov Scientific Research Institute of Electrophysical Apparatus (SRIEA), elements of the ITER first wall (beryllium tiles and heat removal CuCrZr-bronze) are currently joined by ``fast'' brazing (heating with a rate of 150K/min) using the method of passing a current through brazed samples. For this purpose, rapidly quenched brazing filler metal STEMET 1101 is used. The thermal cycle of brazing consists of a fast heating of the assembly to a temperature of 700–720°C and a subsequent cooling (rate of 60K/min). However at these temperatures, processes resulting to a loss of the tensile and shear strength of CuCrZr can occur. Therefore, reducing the brazing temperature is a very important task.Optimization of the composition of the Cu–Ni–Sn–P system filler metals and comparative tests of filler metals of various compositions have been carried out in this paper to reduce the brazing temperature of beryllium with CuCrZr. Alloys of the following compositions Cu–6.4Ni–9.2Sn–6.3P (STEMET 1105) and Cu–9.1Ni–3.6Sn–8.0P (STEMET 1101) were made in the form of rapidly quenched ribbons with a thickness of 50µm and a width of 50mm. They were used to perform furnace brazing by Joule heating (with a rate of 15K/min) of beryllium with CuCrZr (Be/CuCrZr) at temperatures of 650, 700 and 750°C for 15min. Metallographic investigations of the zone of brazing and mechanical shear tests of joints before and after the heat treatment at 350°C for 30h have been conducted. It was found that the joints of Be/CuCrZr brazed at 650°C using STEMET 1105 (τs=230MPa) and at 750°C using STEMET 1101 (τs=260MPa) had the best shear strength properties. However, there is a significant decrease of the microhardness of CuCrZr from 1570 to 1140MPa at 750°C, which indicates a significant loss of its strength. The results obtained suggest that the brazing of beryllium with CuCrZr using STEMET 1105 at 650–700°C will not adversely affect the CuCrZr.

Basic principles of creating a new generation of high- temperature brazing filler alloys

The development of new materials is based on the formation of a structural-phase state providing the desired properties by selecting the base and the complex of alloying elements. The development of amorphous filler alloys for a high-temperature brazing has its own features that are due to the limited life cycle and the production method of brazing filler alloys. The work presents a cycle of analytical and experimental materials science investigations including justification of the composition of a new amorphous filler alloy for brazing the products from zirconium alloys at the temperature of no more than 800 °C and at the unbrazing temperature of permanent joints of more than 1200 °C. The experimental alloys have been used for manufacture of amorphous ribbons by rapid quenching, of which the certification has been made by X-ray investigations and a differential-thermal analysis. These ribbons were used to obtain permanent joints from the spacer grid cells (made from the alloy Zr-1% Nb) of fuel assemblies of the thermal nuclear reactor VVER-440. The brazed samples in the form of a pair of cells have been exposed to corrosion tests in autoclaves in superheated water at a temperature of 350 °C, a pressure of 160 MPa and duration of up to 6,000 h. They have been also exposed to destructive tests using a tensile machine. The experimental results obtained have made it possible to propose and patent a brazing filler alloy of the following composition: Zr-5.5Fe-(2.5-3.5)Be-1Nb-(5-8)Cu-2Sn-0.4Cr-(0.5-1.0)Ge. Its melting point is 780 °C and the recommended brazing temperature is 800°C.

Evaluation of the Thickness and Bond Quality of Three-Layered Media using Zero-Group-Velocity Lamb Waves

Bonding conditions such as jointing agent thickness and bond strength (quality) at the interface between two components is a critical concern in an assembled product because the jointing area is often the weakest component in products. Using a laser ultrasonic technique, zero-group-velocity (ZGV) Lamb waves were utilized to characterize the thickness of an adhesive layer for an epoxy-bonded sample and the bond quality for brazed samples. Using two modes of the ZGV Lamb waves, the thickness of the adhesive layer ranging from 0.2 to 0.8 mm can be estimated with high accuracy. In bond quality measurements, poor bond quality lowered the frequency of ZGV Lamb waves. The decrease in the frequency depended on the amount of shear stress along the interface caused by ZGV Lamb waves.

Thermal cycling behavior of alumina-graphite brazed joints in electron tube applications

Alumina was joined with graphite by active metal brazing technique at 895, 900, 905, and 910 °C for 10 min in vacuum of 0.67 mPa using Ti-Cu-Ag (68.8Ag-26.7Cu-4.5Ti; mass fraction, %) as filler material. The brazed samples were thermal cycled between 30 and 600 °C and characterized. X-ray diffraction results show strong reaction between titanium and carbon as well as titanium and alumina. Scanning electron microscopy and helium leak tests show that the initial and thermal cycled brazed samples are devoid of cracks or any other defects and hermeticity in nature. Brazing strength of the joints is found to be satisfactory.

The Effect of Braze Interlayer Thickness on the Mechanical Strength of Alumina Brazed with Ag–CuO Braze Alloys

The effect of braze interlayer thickness on the strength of alumina brazed with silver‐copper oxide reactive air braze (RAB) alloys was evaluated using a four point bend test configuration. The brazed samples had an average fracture strength of 180 MPa or approximately 60% of the average monolithic alumina strength. The joint strength values obtained exceeded the yield strength and ultimate tensile strength of the silver interlayer indicating strong ceramic to metal adhesion and the development of a triaxial stress state in the braze interlayer. The average fracture strength was relatively constant (190 ± 60 MPa) in the thickness range of 0.030–0.230 mm for all test conditions. The braze fracture strength then decreased to 100 ± 30 MPa as the braze thickness increased from 0.230 to 0.430 mm indicating a loss of triaxial constraint with increasing interlayer thickness. In addition, four different fracture modes were observed.

Experimental Study of Free Convection in Open-cell Aluminum Foam

Free convection in air-saturated open-cell aluminum foam was studied experimentally. The influence of pore density, foam height and bonding technologies are examined. For this research, the base plate temperature is varied between 55°C and 95°C. The ratio of the sample length-to-width is kept to a fixed value of 10. The investigated foams have a pore density of 10 and 20 PPI (Pores Per linear Inch) with a porosity of 93%. The foam height is varied between 6 and 40mm. The bonding of the foam is done by either brazing, or by applying a single epoxy.An effect of the bonding methods is observed. The brazed samples showed a higher heat transfer rate in all cases. The influence of the pore density is more pronounced. However, the strongest influence can be attributed to an increase in sample height.

Characterization and brazing of sintered Ni-Al-Co powder mixtures containing intermetallics

AbstractPowder metallurgy is a progressive branch of engineering that enables engineers to fabricate difficult-to-make parts and materials that are used in many industrial areas. Joining this class of materials is a difficult task due to their intrinsic limitations, such as porosity and thermal properties. In this study, varying ratios of Co powder additions to Nix+Aly powder mixture were made prior to sintering at 600°C. The sintered samples were brazed in both microwave and traditional tube furnaces by placing brazing filler alloy between the sintered specimens without added weight at 950°C for 15 min. Scanning electron microscopy and X-ray diffraction techniques were employed to characterize the brazed samples and the joints. Shear strength and hardness of brazed joints were also determined.

Experimental study of buoyancy-driven flow in open-cell aluminium foam heat sinks

Air-saturated buoyancy-driven flow in a heat sink with open-cell aluminium foam as extended surface is studied. The experiments were performed in a carefully validated test rig. Influences of the pore density, the foam height and the bonding method are examined in this paper for Rayleigh numbers varying between 4000 and 6500, while maintaining the length-to-width ratio of the substrate on 10. The studied foams have a pore density of 10 and 20 PPI, both with a porosity of 93%. The height of the 10 PPI foam is varied between 6 and 40 mm and for 20 PPI between 6 and 18 mm. The bonding of the foam is done via brazing or by applying a single epoxy.A difference in heat transfer rate is observed between both bonding methods: the brazed samples are superior in all cases. The pore density, has a more pronounced influence. The strongest influence can be attributed to a variation in sample height. A correlation for the Nusselt number is derived, for the studied 10 and 20 PPI foam.

Synthesis and characterization of Ni-Mo filler brazing alloy for Mo-W joining for microwave tube technology

A brazing process based on Ni-Mo alloy was developed to join porous tungsten cathode bottom and dense molybdenum cathode body for microwave tubes manufacture. The Ni-Mo alloy was obtained by mixing and milling powders in the eutectic composition, and applied on the surface of the components. The brazing was made at 1400 °C by using induction heating in hydrogen for 5 minutes. Alumina surfaces were coated with the binder and analyzed by Energy Dispersive X-rays Fluorescence. The brazed samples were analyzed by Scanning Electron Microscopy coupled to Energy Dispersive Spectroscopy. Stress-strain tests were performed to determine the mechanical behavior of the joining. The quality of the brazing was evaluated by assuring the presence of a "meniscus" formed by the Ni-Mo alloy on the border of the tungsten and molybdenum joint, the absence of microstructural defects in the interface between the tungsten and molybdenum alloys, and the adhesion of the brazed components.

An experimental study of natural convection in open-cell aluminum foam

Natural convecton n air-saturated alumnum foam has been measured. A carefully designed experimental setup is built for his ask. The calibraton is done by comparing he results of a flat plate wh literature data, revealing excellent agreement. The nvestigated foams have a pore densiy of 10 and 20 PPI. The bondng of the foam is performed via brazing, or by applying a single epoxy which is enriched wh highly conductve alumna particles. The Rayleigh number is varied between 2500 and 6000, wh he rato of he surface area o he perimeter of he substrate as characteristc length. The foam height is varied between 12 and 25.4 mm.A major difference between both he bondng methods is observed. The brazed samples showed a beter heat ransfer n all cases. Furthermore, when ncreasing he foam height, a clear augmentaton of he heat ransfer is observed. Based on hese results, a correlaton is presented.

Fatigue Properties of Narrow and Wide Gap Braze Repaired Joints

With the increasing utilization of braze repair in the gas turbine industry, the properties of braze joints under simulated service conditions become vital in selecting braze repair over other processes. While braze repair has often been claimed to deliver mechanical properties equivalent to that of the parent material, this is largely based on the results of tensile or accelerated creep tests for most gas turbine hot section components failure occurs as a result of thermal fatigue or thermomechanical fatigue. The damage that occurs under such conditions cannot be assessed from tensile or creep testing. This study was undertaken to characterize the fatigue properties of narrow and wide gap brazed X-40 cobalt-based superalloy and compare these properties to that of the X-40 parent material. Butt joint narrow gap and wide gap specimens were vacuum brazed using BNi-9 braze alloy. X-40 and IN-738 were used as additive materials in wide gap braze joints. To characterize the fatigue properties of the braze joints and parent material, isothermal fatigue tests were conducted at 950°C and under load control using a fully reversed sinusoidal wave form having stress amplitude of 75% of the yield strength of the parent material. The braze specimens were fatigue tested in the as-brazed condition. The fatigue test results showed that the fatigue lives of the brazed specimens were lower than that of the parent material, particularly for the narrow gap samples and wide gap samples containing IN-738 additive alloy. All fatigue failures in the brazed samples occurred in the braze joints. An analysis of the fracture surfaces using a scanning electron microscope revealed that porosity was the major contributing factor to fatigue failures in the wide gap braze joints. The testing life debit observed in the narrow gap braze samples can be attributed to the presence of brittle boride phases in the braze joint. This study also included examination of techniques for reducing the aforementioned porosity and presence of brittle intermetallic phases.

Flux-free brazing of Mg-containing aluminium alloys by means of cold spraying

In the present study, AlSi12 and AlSi10Cu4 were deposited onto Mg-containing aluminium alloys 6063 and 5754 by cold spraying. The influences of the two brazing alloys and spray parameters on coating formation were investigated. The microstructure of the coatings was characterized. Some coated samples were heat-treated at 590°C and 560°C in air to investigate the effect of the rupture of oxide scales on the diffusion of elements during heat-treatment. Some coated samples were brazed under argon atmosphere without any fluxes. The results show that AlSi12 had much better deposition behaviour than AlSi10Cu4. Due to the rupture of oxide scales, Cu and Si diffused into the substrate and a metallurgical bond formed between the brazing alloys and the substrates during heat-treatment. The coated samples could be brazed without any fluxes. Because the oxide scales prevented the formation of a metallurgical bond locally, the brazed samples had relatively low shear strengths of up to 43 MPa.

Effect of Process Variables on Microstructure and Mechanical Properties of Wide-Gap Brazed IN738 Superalloy

This study investigated the microstructure and mechanical properties of a wide-gap region brazed with various process variables. The IN738 and DF 4B alloy powders were used as additive and filler metal powder for the brazing process. The wide-gap brazing process was carried out in a vacuum of 2×10-5 torr. The wide-gap region brazed with 60wt.% IN738 additive powder had a microstructure consisting of IN738 additive and (Cr, W)2B. The fracture strength of the wide-gap region (60 wt.% additive and 40 wt.% filler metal powder) brazed at 1230°C for 30hr was as high as 862MPa (93% of base material strength). It was observed that the brazing temperature was the main process variable affecting the mechanical properties of the wide-gap brazed region. The creep rupture life of the region brazed with 60wt.% additive and 40 wt.% was longer than that of other brazed samples. The Cracks in the wide-gap brazed region initiated at the (Cr, W)2B and propagated through them. It was found that the (Cr, W)2B and the pore in the brazed region are important microstructural factors affecting the mechanical properties of the wide-gap brazed region.

Analysis of Residual Stresses and Distortions in Brazed Joints of Cemented Carbide and Steel

Brazing of cemented carbides to steel bodies gives rise to the development of complex stress states and distortions which influence the fatigue behaviour of the parts. It is quite important to estimate the residual stresses with numerical methods whose agreement is to be guaranteed with experimental characterization of the brazed parts. In this work FEM simulations and X-ray as well as neutron stress analysis were used to examine the residual stresses of brazed samples. Joints with different geometries and dimensions made of cemented carbide and different steels showing different phase transformation behaviours were investigated.

Effects of thermal cycling and thermal aging on the hermeticity and strength of silver–copper oxide air-brazed seals

Thermal cycle and exposure tests were conducted on ceramic-to-metal joints prepared by a new sealing technique. Known as reactive air brazing, this joining method is currently being considered for use in sealing various high-temperature solid-state electrochemical devices, including planar solid oxide fuel cells (pSOFC). In order to simulate a typical pSOFC application, test specimens were prepared by joining ceramic anode/electrolyte bilayers to metal washers, of the same composition as the common frame materials employed in pSOFC stacks, using a filler metal composed of 4 mol% CuO in silver. The brazed samples were exposure tested at 750 °C for 200, 400, and 800 h in both simulated fuel and air environments and thermally cycled at rapid rate (75 °C min −1) between room temperature and 750 °C for as many as 50 cycles. Subsequent joint strength testing and microstructural analysis indicated that the samples exposure tested in air displayed little degradation with respect to strength, hermeticity, or microstructure out to 800 h of exposure. Those tested in fuel showed no change in rupture strength or loss in hermeticity after 800 h of high-temperature exposure, but did undergo microstructural change due to the dissolution of hydrogen into the silver-based braze material. Air-brazed specimens subjected to rapid thermal cycling exhibited no loss in joint strength or hermeticity, but displayed initial signs of seal delamination along the braze–electrolyte interface after 50 cycles.