Engineering topics

Asthana, Rajiv

Publications and source records attributed to Asthana, Rajiv.

14 records

NASA NTRS2011

Fabrication and Characterization of Brazed Joints for SiC-Metallic Systems Utilizing Refractory Metals

Metal to ceramic joining plays a key role for the integration of ceramics into many nuclear, ground and aero based technologies. In order to facilitate these technologies, the active metal brazing of silicon carbide (CVD beta-SiC, 1.1 mm thick, and hot-pressed alpha-SiC, 3 mm thick) to the refractory metals molybdenum and tungsten using active braze alloys was studied. The joint microstructure, composition, and microhardness were evaluated by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Knoop hardness testing. The braze alloys, Cusil-ABA, Ticusil and Copper-ABA, all formed sound joints with excellent wetting and chemical bonding with the SiC substrate. Despite the close thermal expansion match between the metal substrates and SiC, hairline cracks formed in alpha-SiC while beta-SiC showed no signs of residual stress cracking. The use of ductile interlayers to reduce the effect from residual stresses was investigated and joints formed with copper as an interlayer produced crack free systems utilizing both CVD and hot-pressed SiC.

Coddington, Bryan↗

NASA NTRS2010

Joining and Integration of Silicon Carbide for Turbine Engine Applications

The critical need for ceramic joining and integration technologies is becoming better appreciated as the maturity level increases for turbine engine components fabricated from ceramic and ceramic matrix composite materials. Ceramic components offer higher operating temperatures and reduced cooling requirements. This translates into higher efficiencies and lower emissions. For fabricating complex shapes, diffusion bonding of silicon carbide (SiC) to SiC is being developed. For the integration of ceramic parts to the surrounding metallic engine system, brazing of SiC to metals is being developed. Overcoming the chemical, thermal, and mechanical incompatibilities between dissimilar materials is very challenging. This presentation will discuss the types of ceramic components being developed by researchers and industry and the benefits of using ceramic components. Also, the development of strong, crack-free, stable bonds will be discussed. The challenges and progress in developing joining and integration approaches for a specific application, i.e. a SiC injector, will be presented.

Halbig, Michael C.↗

NASA NTRS2010

Microstructural and Mechanical Evaluation of a Cu-Based Active Braze Alloy to Join Silicon Nitride Ceramics

Self-joining of St. Gobain Si3N4 (NT-154) using a ductile Cu-Al-Si-Ti active braze (Cu-ABA) was demonstrated. A reaction zone approx.2.5-3.5 microns thick) developed at the interface after 30 min brazing at 1317 K. The interface was enriched in Ti and Si. The room temperature compressive shear strengths of Si3N4/Si3N4 and Inconel/Inconel joints (the latter created to access baseline data for use with the proposed Si3N4/Inconel joints) were 140+/-49MPa and 207+/-12MPa, respectively. High-temperature shear tests were performed at 1023K and 1073 K, and the strength of the Si3N4/Si3N4 and Inconel/Inconel joints were determined. The joints were metallurgically well-bonded for temperatures above 2/3 of the braze solidus. Scanning and transmission electron microscopy studies revealed a fine grain microstructure in the reaction layer, and large grains in the inner part of the joint with interfaces being crack-free. The observed formation of Ti5Si3 and AlN at the joint interface during brazing is discussed.

Singh, M.↗

NASA NTRS2007

Effects of Different Braze Materials and Composite Substrates on Composite/Ti Joints

An ever increasing number of applications require robust joining technologies of dissimilar materials. In this study, three types of ceramic composites (C-C, C-SiC, and SiC-SiC) were vacuum brazed to commercially pure Ti using the Cusil-ABA (63 Ag - 35.5 Cu - 1.75 Ti) active metal braze alloy. The study also compared composite specimens as-fabricated and after surface grinding/polishing. A butt-strap tensile shear strength test was used to evaluate the joined structures at room temperature, 270 and 500 C. The elevated temperatures represent possible use temperatures for some heat rejection type applications. Joint strength will be discussed in light of braze wetting and spreading properties, composite properties, and test temperature.

Morscher, Gregory N.↗

NASA NTRS2006

Effect of Composite Substrates on the Mechanical Behavior of Brazed Joints in Metal-Composite System

Advanced composite components are being considered for a wide variety of demanding applications in aerospace, space exploration, and ground based systems. A number of these applications require robust integration technologies to join dissimilar materials (metalcomposites) into complex structural components. In this study, three types of composites (C-C, C-SiC, and SiC-SiC) were vacuum brazed to commercially pure Ti using the active metal braze alloy Cusil-ABA (63Ag-35.3Cu-1.75Ti). Composite substrates with as fabricated and polished surfaces were used for brazing. The microstructure and composition of the joint, examined using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), showed sound metallurgical bonding in all systems. The butt strap tensile (BST) test was performed on bonded specimens at room and elevated temperatures. Effect of substrate composition, interlaminar properties, and surface roughness on the mechanical properties and failure behavior of joints will be discussed.

Singh, M.↗

NASA NTRS2006

Robust Joining and Integration Technologies for Advanced Metallic, Ceramic, and Composite Systems

Robust integration and assembly technologies are critical for the successful implementation of advanced metallic, ceramic, carbon-carbon, and ceramic matrix composite components in a wide variety of aerospace, space exploration, and ground based systems. Typically, the operating temperature of these components varies from few hundred to few thousand Kelvin with different working times (few minutes to years). The wide ranging system performance requirements necessitate the use of different integration technologies which includes adhesive bonding, low temperature soldering, active metal brazing, diffusion bonding, ARCJoinT, and ultra high temperature joining technologies. In this presentation, a number of joining examples and test results will be provided related to the adhesive bonding and active metal brazing of titanium to C/C composites, diffusion bonding of silicon carbide to silicon carbide using titanium interlayer, titanium and hastelloy brazing to silicon carbide matrix composites, and ARCJoinT joining of SiC ceramics and SiC matrix composites. Various issues in the joining of metal-ceramic systems including thermal expansion mismatch and resulting residual stresses generated during joining will be discussed. In addition, joint design and testing issues for a wide variety of joints will be presented.

Singh, M.↗

NASA NTRS2006

A Simple Test to Determine the Effectiveness of Different Braze Compositions for Joining Ti-Tubes to C/C Composite Plates

A simple tube-plate joint tensile test was implemented to compare the effectiveness of commercial brazes, namely, TiCuNi, TiCuSil, and Cu-ABA, used for bonding Ti-tubes joined to C-C composite plates. The different braze systems yielded different; yet, repeatable results. The Cu-ABA system proved to have about twice the load-carrying ability of the other two systems due to the fact that the bonded area between the braze material and the C-C plate was largest for this system. The orientation of the surface fiber tows also had a significant effect on load-carrying ability with tows oriented perpendicular to the tube axis displaying the highest failure loads. Increasing the process load and modifying the surface of the C-C plate by grooving out channels for the Ti-Tube to nest in resulted in increased load-carrying ability for the TiCuSil and Cu-ABA systems due to increased bonded area and better penetration of the braze material into the C-C composite.

Morscher, Gregory N.↗

NASA NTRS2005

Joining and Testing Composite Plates to Ti Tubes

The joining of metal tubes to composite plates is required for heat-rejection components in several space applications. Currently a number of different braze compositions are being evaluated as to their effectiveness. Such tube-plate configurations cannot be represented by traditional methods of testing, e.g., lap joints. The joined region is not between two flat surfaces, but rather between a flat surface and a curved surface. Therefore, several tests have been employed to ascertain the effectiveness of the different braze approaches in tension that are both simple and representative of the actual system and relatively straightforward in analysis. The results of these "tube-tests" will be discussed for the three different braze compositions, Cu-ABA, Ti-Cu-Sil, and Ti-Cu-Ni. In addition, fracture analysis of the failed joints was performed and offers insights into the cause of joint failure and the distinctions which need to be made between the "strength" of a joint versus the "load carrying ability" of a joint.

Morscher, Gregory N.↗

NASA NTRS2005

Mechanical Characterization of Reactively Brazed Metal-Composite Joints for Heat Rejection Systems

The joining of metal tubes to composite plates is required for heat-rejection components in several space applications. Currently a number of different braze compositions are being evaluated as to their effectiveness. Such tube-plate configurations cannot be represented by traditional methods of testing, e.g., lap joints. The joined region is not between two flat surfaces, but rather between a flat surface and a curved surface. Therefore, several tests have been employed to ascertain the effectiveness of the different braze approaches in tension and in shear that are both simple and representative of the actual system and relatively straightforward in analysis. The results of these "tube tests" will be discussed for the three different braze compositions, Cu-ABA, Ti-Cu-Sil, and Ti-Cu-Ni. In addition, fracture analysis of the failed joints was performed and offers insights into the cause of joint failure and the distinctions which need to be made between the "strength" of a joint versus the "load carrying ability" of a joint.

Morscher, Gregory N.↗

NASA NTRS2005

Brazing of Stainless Steels to Yttria Stabilized Zirconia (YSZ) Using Silver -Base Brazes

Three silver-base brazes containing either noble metal palladium (Palcusil-10 and Palcusil-15) or active metal titanium (Ticusil) were evaluated for high-temperature oxidation resistance, and their effectiveness in joining yttria stabilized zirconia (YSZ) to a corrosion-resistant ferritic stainless steel. Thermogravimetric analysis (TGA), and optical- and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS) were used to evaluate the braze oxidation behavior and the structure and chemistry of the YSZ/braze/steel joints. The effect of the braze type and processing conditions on the interfacial microstructure and composition of the joint regions is discussed with reference to the chemical changes that occur at the interface. It was found that chemical interdiffusion of the constituents of YSZ, steel and the brazes led to compositional changes and/or interface reconstruction, and metallurgically sound joints.

Singh, Mrityunjay↗

NASA NTRS2004

Reactive Brazing of Carbon-Carbon Composites to Titanium

The Ti-metal/C-C composite joints were formed by reactive brazing with three commercial brazes, namely, Cu-ABA, TiCuNi, and TiCuSil. The joint microstructures were examined using optical microscopy, and scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS). The results of the microstructure analysis indicate solute redistribution across the joint and possible metallurgical bond formation via interdiffusion, which led to good wetting and spreading.

Shpargel, Tarah↗

NASA NTRS1996

Zone Directional Solidification Of NiAl Composites

Interfacial shear strengths increased. Zone directional solidification investigated for use in toughening nickel aluminide matrices in NiAl-matrix/Al(2)O(3)(sapphire)-fiber composite materials.

Bowman, Randy R.↗

NASA NTRS1993

Melt infiltration of silicon carbide compacts. II - Evaluation of solidification microstructures

Microstructural aspects of alloy solidification within the interstices of porous compacts of platelet-shaped single crystals of alpha-SiC, when the latter are infiltrated with a hot metal under pressure, have been described. Microstructural evidence is presented of selective reorientation of platelets and nonhomogeneous solute distribution under shear of pressurized melt, of constrained growth of primary solid within finite width zones, and of the modulation of coring due to microsegregation as a result of variations in the pore size of compacts.

Asthana, Rajiv↗

NASA NTRS1992

Melt infiltration of silicon carbide compacts. I - Study of infiltration dynamics

Countergravity, pressure-assisted infiltration with a 2014 Al alloy of suitably tamped porous compacts of platelet shaped single crystals of alpha (hexagonal) silicon carbide was used to measure particulate wettability and infiltration kinetics under dynamic conditions relevant to pressure casting of composites. A threshold pressure P(th) for ingression of the infiltrant was identified based on the experimental penetration length versus pressure profiles for a range of experimental variables which included infiltration pressure, infiltration time, SiC size and SiC surface chemistry. The results showed that P(th) decreased whereas the penetration length increased with increasing SiC size and infiltration time. Cu-coated SiC led to lower P(th) and larger penetration lengths compared to uncoated SiC under identical conditions. These observations have been discussed in the light of theoretical models of infiltration and the kinetics of wetting.

Asthana, Rajiv↗