Complex elements of aerospace equipment from reaction silicon carbide ceramics

The results of studying the process of obtaining complex-profile elements of the substrate of mirrors of optical telescopes from reaction-sintered silicon carbide ceramics are presented. It is shown that the strength of silicon carbide ceramics depends on the dispersion of the silicon carbide powder and on the temperature of reaction sintering. An increase in the sintering temperature from 1500 to 1650 °C leads to an increase in strength by 60 MPa, and to 1800 °C – to a decrease in strength by 40 MPa. An increase in strength is explained by a decrease in free silicon and an increase in the content of secondary silicon carbide, a decrease in strength is explained by an increase in the size of carbide grains. The study of the influence of the modes of soldering of hexagonal elements to obtain a complex-profile element of the substrate of the mirror of an optical telescope on the strength of the soldered seam showed that the introduction of silicon carbide powder 7 μm in size and amorphous boron in an amount of 6 % into the solder composition based on silicon carbide has a positive effect on the strength of the soldered seam. Tests of the brazed specimens at three-point bending showed that fracture occurs along the body of the specimens being brazed, and not the brazed seam. The structure of the brazed joint depends on the composition of the braze alloy and the gap between the samples to be brazed.

Microstructure and Oxidation Behavior of Narrow Gap Brazing and Wide Gap Brazing Joints With Boron/Silicon-Free Nickel Base Braze Alloys

In this study, the microstructure and solidus and liquidus of several Ni-Co-Hf-Zr-Ti-Al braze alloys were first examined with the objective to develop a B- and Si-free low-melting braze alloy for narrow gap (NGB) and wide gap brazing (WGB) and turbine component repair applications. Among various alloys examined, differential scanning calorimetry (DSC) was used to measure the solidus and liquidus during heating and cooling cycles. Following the measurements of liquidus and solidus, the microstructure was evaluated using SEM. Equations for calculating solidus and liquidus based on alloy's compositions were established, and the functions of each elements on these two characteristic temperatures were discussed. One selected alloy with a liquidus of 1201 °C was further employed for NGB and WGB experiments. The results showed that it was able join Cannon Muskegon single crystal (CMSX)-4 at 1240 °C without interfacial voids, and with the use of externally applied pressure and extended homogenization treatment, the interfacial intermetallic compounds were substantially removed. Furthermore, the same braze alloy was used to fill a large artificial cavity in a WGB scheme at a reduced temperature of 1200 °C. The braze alloy was able to fully bond the filler powder alloy in addition to join the two alloys to an IN 738 substrate. Finally, oxidation test was conducted at 1050 °C (isothermal in static air) for 100 h after NGB of CMSX-4 and WGB of IN 738. The results showed that the oxide formed on the standalone braze alloy is very dense and there is no sign of spallation. It contained primarily NiO (+CoO) with no other elements measured. For the NGB joints, large amount of scale spallation was observed on base alloy CMSX-4 while the NGB joint remained spallation free. The oxide formed on the NGB was NiO with partitions of Co, Al, Ti, Cr, and W. The WGB joint region in IN 738 showed oxide scale spallation on the IN 738 substrate side, leaving behind steps and depression on the sample surface. In the WGB joint itself, there were three notable phases after oxidation test; however, no scale spallation could be found. For the majority part of the surface, a Ni-rich oxide covered the surface. There were areas of smaller oxide particles with higher Cr content. Overall, the new boron/silicon-free braze alloy was found to be able to join several superalloys in both WGB and NGB schemes without occurrence of defects and the oxidation resistance was superior to both substrate alloys examined in this study.

Microstructure and Oxidation Behaviour of NGB and WGB Joints With Boron/Silicon Free Nickel Base Braze Alloys

In this study, the microstructure and solidus and liquidus of several Ni-Co-Hf-Zr-Ti-Al braze alloys were first examined with the objective to develop a B and Si free low melting braze alloy for narrow gap (NGB) and wide gap brazing (WGB) and turbine component repair applications. Among various alloys examined, DSC was used to measure the solidus and liquidus during heating and cooling cycles. Following the measurements of liquidus and solidus, the microstructure was evaluated using SEM. Equations for calculating solidus and liquidus based on alloy’s compositions were established and the functions of each elements on these two characteristic temperatures were discussed. One selected alloy with a liquidus of 1201 °C was further employed for NGB and WGB experiments. The results showed that it was able join CMSX-4 at 1240°C without interfacial voids; and with the use of externally applied pressure and extended homogenization treatment the interfacial intermetallic compounds were substantially removed. Furthermore, the same braze alloy was used to fill a large artificial cavity in a WGB scheme at a reduced temperature of 1200°C. The braze alloy was able to fully bond the filler powder alloy in addition to join the two alloys to a IN 738 substrate. Finally, oxidation test was conducted at 1050°C (isothermal in static air) for 100 hours after NGB of CMSX-4 and WGB of IN 738. The results showed that the oxide formed on the standalone braze alloy is very dense and there is no sign of spallation. It contained primarily NiO (+CoO) with no other elements measured. For the NGB joints, large amount of scale spallation was observed on base alloy CMSX-4 while the NGB joint remained spallation free. The oxide formed on the NGB was NiO with partitions of Co, Al, Ti, Cr, and W. The WGB joint region in IN 738 showed oxide scale spallation on the IN 738 substrate side, leaving behind steps and depression on the sample surface. In the WGB joint itself, there were three notable phases after oxidation test, however, no scale spallation could be found. For the majority part of the surface, a Ni-rich oxide covered the surface. There were areas of smaller oxide particles with higher Cr content. Overall, the new boron/silicon free braze alloy was found to be able to join several superalloys in both WGB and NGB schemes without occurrence of defects and the oxidation resistance was superior to both substrate alloys examined in this study.

Vacuum Brazing Ti–15–3 with a TiNiNb Braze Alloy

Among all types of brazing fillers, Ti-based fillers show satisfactory joint strengths in brazing titanium alloys. However, the major concern in using such fillers is the formation of Cu/Ni/Ti intermetallic compound(s) in the joint. In this study, a Ti–15–3 alloy was vacuum brazed with a clad Ti–35Ni–25Nb foil. The brazed zone consisted of a Ti2Ni intermetallic compound in a (β-Ti,Nb)-rich matrix for specimen brazing at 1000 °C/600 s. Raising brazing temperature and time resulted in the Ti2Ni dissolving into the (β-Ti,Nb)-rich matrix. For the specimen brazing at 1100 °C/600s, Ti2Ni could only be observed at the grain boundaries of the (β-Ti,Nb)-rich matrix. After further raising it to 1200 °C/600 s, the Ti2Ni intermetallic compound was all dissolved into the (β-Ti,Nb)-rich phase. The average shear strength was significantly raised from 140 (1000 °C/600 s) to 620 MPa (1100 °C/3600 s). Crack initiation/propagation in the brittle Ti2Ni compound with the cleavage fractograph were changed into the Ti–15–3 base metal with a ductile dimple fractograph. The advantage of using Nb in the TiNiNb filler foil was its ability to stabilize β-Ti, and most of the Ni in the braze alloy was dissolved into the β-Ti matrix. The brazed joint could be free of any intermetallic phases with a proper brazing cycle applied, and the joint was suitable for a few harsh applications, e.g., repeated stresses and impact loadings.

Effect of Gold Nano Dots in Microwave Brazing: A Novel Approach to Join Ti6Al4V to MACOR®

Nano structured surface generation is useful in inducing specific functionalities to the surface. This work attempts on generation of such surface through thermal dewetting. Enhanced adhesion behavior of such surface is utilized for joining MACOR® ceramic to Ti6Al4V alloy. Ti6Al4V alloy is brazed with MACOR® by microwave energy using TiCuSil as a braze alloy. MACOR® ceramic is subjected to pre-treatment called gold dewetting. For comparison plain ceramic is also used for joining. The reaction zone formed on joining Ti6Al4V to gold dewetted MACOR® is more uniform than the untreated MACOR® ceramic interface. Energy Dispersive Spectroscopy (EDS) analysis of the reaction zone suggests the formation of Ti2Cu and Ti3Au intermetallic compounds. The shear strength of the pre-treated samples is observed to be higher than that of plain joints.

Microstructure and Interface Bonding Strength of WC-10Ni/NiCrBSi Composite Coating by Vacuum Brazing

WC flexible cloth was fabricated by rolling WC powder that was pre-mixed with a nano-scale binder. BNi-2 braze alloy was used to braze the WC flexible cloth to a workpiece surface in a vacuum to form a compact composite coating. The composite coatings with different percentages of WC (wt.%, 30 %, 50 % and 80 %) have been successfully brazed onto Q345 steel substrate. Microscopic morphology and interfacial structure were characterized in terms of porosity, segregation and the distribution of WC particles. The bonding strength between the coating and the substrate was evaluated by shear test. The results showed that the segregation of WC particles appeared in all three coatings. With the increase of WC content in the coating, the segregation phenomenon was gradually reduced, and the WC particles were more closed. Furthermore, the thickness of the coating and matrix reaction layer was narrower, and more holes were found in the coating. Fracture of the 30 % and 50 % WC coatings was mainly plastic fracture, and the fracture of the 80 % WC coating was mainly brittle fracture accompanied by local plastic fracture. The maximum bonding strength of the coating to the substrate was determined to be 302 MPa.

Effect of Temperature and Hold Time of Induction Brazing on Microstructure and Shear Strength of Martensitic Stainless Steel Joints

1Cr12Mo martensitic stainless steel is widely used for intermediate and low-pressure steam turbine blades in fossil-fuel power plants. A nickel-based filler metal (SFA-5.8 BNi-2) was used to braze 1Cr12Mo in an Ar atmosphere. The influence of brazing temperature and hold time on the joints was studied. Microstructure of the joints brazed, element distribution and shear stress were evaluated at different brazing temperatures, ranging from 1050 °C to 1120 °C, with holding times of 10 s, 30 s, 50 s and 90 s. The results show that brazing joints mainly consist of the matrix of the braze alloy, the precipitation, and the diffusion affected zone. The filler metal elements diffusion is more active with increased brazing temperature and prolonged hold time. The shear strength of the brazed joints is greater than 250 MPa when the brazing temperature is 1080 °C and the hold time is 30 s.