Interface Reactions Responsible for Run-Out in Active Brazing: Part 4

This study examined the interface reaction between Ag-xAl filler metals having x = 0.2, 0.5, or 1.0 wt-% and Kovar™ base materials. The present investigation used the braze joint test sample configuration. The brazing conditions were 965°C (1769°F), 5 min; 995°C (1823°F), 20 min, and a vacuum of 10–7 Torr. Run-out was absent from all test samples. Combining these results with those of the Part 2 study that used high-Al, Ag-xAl filler metals (x = 2.0, 5.0, and 10 wt-%) established these conditions for run-out: Ag-xAl filler metals having x ≥ 2.0 wt-% Al, which result in reaction layer compositions, and (Fe, Ni, Co)y Alz , having z ≥ 26 at.-% Al. The limited occurrences of run-out lobes resulted from the surface tension effect that quickly reduced the driving force for additional run-out events. The interface reactions were controlled by a driving force that was an expressed function of filler metal composition (Ag-xAl) and brazing temperature, as opposed to simply thermally activated rate kinetics. The differences of reaction layer composition and thickness confirmed that the interface reactions differed between the braze joint and sessile drop configurations. Collectively, the findings from the Parts 1–4 investigations concluded that the most-effective means to mitigate run-out is to place a barrier coating on the Kovar base material that will prevent formation of the (Fe, Ni, Co)y Alz reaction layer.

Failures of Brazed Joints

The various methods of furnace, torch, induction, resistance, dip, and laser brazing are used to produce a wide range of highly reliable brazed assemblies. However, imperfections that can lead to braze failure may result if proper attention is not paid to the physical properties of the material, joint design, prebraze cleaning, brazing procedures, postbraze cleaning, and quality control. Factors that must be considered include brazeability of the base metals; joint design and fit-up; filler-metal selection; prebraze cleaning; brazing temperature, time, atmosphere, or flux; conditions of the faying surfaces; postbraze cleaning; and service conditions. This article focuses on the advantages, limitations, sources of failure, and anomalies resulting from the brazing process. It discusses the processes involved in the testing and inspection required of the braze joint or assembly.

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.

Silver braze corrosion in H2S environment: life assessment and methods of preventing

Issues of using silver as the braze material in H2S environment are discussed due to environment related sulfidation which cause corrosion penetration and risk of compromising seals. To explore risk and mitigation techniques, static H2S high temperature silver braze corrosion tests were conducted. The results are presented and interpreted by Arrhenius equation approach. The corrosion rate decreased with time due to the formation of Ag2S corrosion product layer. This corrosion product changed the chemical reaction type from mass transfer controlled to diffusion controlled. The environment temperature vs braze joint thickness vs guarantee period relationships are shown and discussed. Additionally, possible protection options connected with metallic and non-metallic coating were investigated.

Process Development for Vacuum Brazed Niobium–316L Stainless Steel Transition Joints for Superconducting Cavities

The paper describes process development for producing sound, strong, and ductile Nb pipe–316L stainless steel (SS) flange brazed joint suitable for application in superconducting radiofrequency (SRF) cavities. The developed transition joints, made with BVAg-8 braze filler metal (BFM), were free of brittle intermetallic compounds, in contrast to the existing global brazing practice of using oxygen-free electronic copper as BFM which results in the formation of a continuous layer of Fe–Nb brittle intermetallic compound at Nb–braze interface. In view of the large difference in the mean thermal expansion coefficients between niobium and 316L stainless steel, a new design for manufacturing and assembly (DFMA) has been developed to ensure achievement of desired joint thickness with uniformity in circumferential and longitudinal directions. An environment-friendly prebraze cleaning procedure has been qualified and implemented. DFMA has resulted in (i) significant reduction of the out-of-roundness errors (≤10 μm) while machining of the niobium pipe, (ii) simplified clearance fit prebraze assembly at room temperature (RT), and (iii) uniformity of joint thickness. A process flow chart has been developed to ensure repeatability of joint characteristics. The brazed joint, of niobium pipe and 100CF knife edge 316L SS flange made by standardized practice, displayed helium leak tightness better than 5 × 10−10 mbar l/s at RT and at liquid helium temperature (LHT). The braze-joint sustained 873 K/10 h postbraze hydrogen degassing treatment and thermal cycling between RT and LHT without any loss in hermeticity.

Effect of Varying Process Parameters on the Microstructure and Mechanical Properties of Cu-Cr-Zr-Ti Alloy Brazed Using Cu-Mn-Ni-Sn-Fe Foil

Cu-Cr-Zr-Ti alloy is being used widely in aerospace engines, due to its synergetic combination of high strength and thermal conductivity. Brazing is the preferred process being adopted to realize intricate shapes and complex dimensions. In the present work, Cu-Cr-Zr-Ti alloy was brazed using Cu-Mn-Ni-Sn-Fe base brazing foil. This braze foil exhibits liquidus temperature of ~980°C. Brazing experiments were carried out at 1030°C under high vacuum condition. The effect of varying load (0.5-2 kg) has been studied in the current experiment. Microstructural study of the parent materials and joints were carried out using optical microscope (OM). Lap shear testing (1T configuration) of the brazed joints was evaluated to obtain shear strength values. Also, micro-hardness traverse has been carried out across the brazed joint. Applied load plays a significant role in obtaining defect free braze joint.