Infrared Brazing of CoCrFeMnNi Equiatomic High Entropy Alloy Using Nickel-Based Braze Alloys.

Infrared vacuum brazing of CoCrFeMnNi high entropy alloy (HEA) using BNi-2 and MBF601 fillers has been investigated. Both brazes show poor wettability at temperatures only 20 °C above their liquidus temperatures. However, the wettability of BNi-2 and MBF601 fillers on CoCrFeMnNi HEA is greatly improved with increasing the test temperatures, 50 °C above their liquidus temperatures. The BNi-2 brazed joints are dominated by Ni-rich matrix with huge CrB and a few tiny boride precipitates. Average shear strengths of joints increase with increasing brazing temperature and/or time, and fracture location changes from blocky CrB in the brazed zone to grain boundary boride in the substrate. The MBF601 brazed joints are composed of CoCrFeMnNi-based matrix, particles of B/Co/Cr/Fe/Mn/Ni/P compounds, and some phosphides form along the grain boundaries of the substrate. The specimen brazed with MBF601 filler foil at 1050 °C for 600 s has the highest average shear strength of 321 MPa, while that brazed at 1080 °C for 600 s has a lower average shear strength of 271 MPa due to the presence of solidification shrinkage voids.

Zirconia and Crofer Joint Made by Reactive Air Brazing Using the Silver Base Paste and Cu-Ti Coating Layer.

This study proposes a method to enhance the airtightness of the joint between the ZrO2 and Crofer alloy using coating technology. With the aid of vacuum sputtering technology, a titanium-copper alloy layer with a thickness between 1.5 µm and 6 µm was first deposited on the surface of ZrO2 and Crofer, respectively. The chemical composition of the deposited reaction layer was 70.2 Cu and 29.8 Ti in at%. Then, using silver as the base material in the reactive air brazing (RAB) process, we explore the use of this material design to improve the microstructure and reaction mechanism of the joint surface between ceramics and metal, compare the effects of different pretreatment thicknesses on the microstructure, and evaluate its effectiveness through air tightness tests. The results show that a coating of Cu-Ti alloy on the ZrO2 substrate can significantly improve bonding between the Ag filler and ZrO2. The Cu-Ti metallization layer on the ZrO2 substrate is beneficial to the RAB. After the brazing process, the coated Cu-Ti layers form suitable reaction interfaces between the filler, the metal, the filler, and the ceramic. In terms of coating layer thickness, the optimized 3 µm coated Cu-Ti alloy layer is achieved from the experiment. Melting and dissolving the Cu-Ti coated layer into the ZrO2 substrate results in a defect-free interface between the Ag-rich braze and the ZrO2. The air tightness test result shows no leakage under 2 psig at room temperature for 28 h. The pressure condition can still be maintained even under high-temperature conditions of 600 °C for 24 h.

The Effect of Homogenization Heat Treatment on 316L Stainless Steel Cast Billet.

This investigation aims to analyze the effect of homogenization heat treatment at 1240 °C for 2 and 6 h on the hardness, distribution, morphology, and chemical composition of the δ-ferrite and sigma phases in 316L stainless steel cast billet. A field emission scanning electron microscope, combined with electron back-scattered diffraction, a field emission electron probe microanalyzer with a wavelength dispersive spectrometer, and a Vickers microhardness tester are applied to identify various phase evolutions in the cast billet. The morphology of the δ-ferrite and sigma phases in the austenite matrix of the 316L cast billet are strongly related to the subsequent hot and cold wire drawings. The homogenization heat treatment is expected to provide a driving force to form spheroid interdendritic δ-ferrite and to minimize the amount of the brittle sigma intermetallic compound in the austenite matrix. The homogenization heat treatment at 1240 °C effectively spheroidized all δ-ferrites into blunt ones in the cast billet. The transformation of δ-ferrite into sigma is dominated by temperature and cooling rate. The fast air cooling after homogenization between 1240 and 850 °C retards the precipitation of the sigma in the δ-ferrite. There are two δ-ferrite transformation mechanisms in this experiment. The direct transformation of the δ-ferrite into sigma is observed in the as-cast 316L stainless steel billet. In contrast, the eutectoid transformation of the δ-ferrite into the sigma and austenite dominates the 316L cast billet homogenized at 1240 °C, with a slow furnace cooling rate.

Failure Analysis of a C-276 Alloy Pipe in a Controlled Decomposition Reactor.

Failure analysis was carried out on a ruptured C-276 pipe heated externally at 1050 °C, which had been used for a few months in a controlled decomposition reactor (CDR) system. To catch the decomposed perfluorinated compounds (PFCs, e.g., CF4, SF6, NF3, C3F8 and C4F8) present in the exhaust gas, the C-276 reactor was periodically purged with water mist, which caused a temperature gradient from the external to the inner surface of the pipe. The precipitation of large amounts of intermetallic compounds along the grain boundaries were found to be corroded preferentially. The internal surface of the used pipe was covered with many fine cracks. The corrosion and cracking of grain boundary precipitates accounted for the short service life of the C-276 pipe. Compositional measurements by electron probe micro-analyzer (EPMA) and phase identification by electron backscatter diffraction (EBSD) confirmed the presence of δ and µ phases in the ruptured pipe. The coarse intergranular precipitates were the δ phase (Mo7Ni7), which were enriched in Mo and Cr. Moreover, the fine precipitates dispersed intergranularly and intragranularly were the µ phase (Mo6Ni7), which were abundant in Mo and W. The numerous precipitates present in the matrix and along the grain boundaries were responsible for an obvious loss in the strength and ductility of the used C-276 pipe.

Vacuum Brazing of Metallized YSZ and Crofer Alloy Using 72Ag-28Cu Filler Foil.

The study focused on dissimilar brazing of metallized YSZ (Yttria-Stabilized Zirconia) and Crofer alloy using BAg-8 (72Ag-28Cu, wt%) filler foil. The YSZ substrate was metallized by sequentially sputtering Ti (0.5/1 µm), Cu (1/3 µm), and Ag (1.5/5 µm) layers, and the Crofer substrate was coated with Ag layers with a thickness of 1.5 and 5 µm, respectively. The BAg-8 filler demonstrated excellent wettability on both metallized YSZ and Crofer substrates. The brazed joint primarily consisted of Ag-Cu eutectic. The metallized Ti layer dissolved into the braze melt, and the Ti preferentially reacted with YSZ and Fe from the Crofer substrate. The globular Fe2Ti intermetallic compound was observed on the YSZ side of the joint. The interfacial reaction of Ti was increased when the thickness of the metallized Ti layer was increased from 0.5 to 1 µm. Both brazed joints were crack free, and no pressure drop was detected after testing at room temperature for 24 h. In the YSZ/Ti(0.5µ)/Cu(1µ)/Ag(1.5µ)/BAg-8(50µ)/Ag(1.5µ)/Crofer joint tested at 600 °C, the pressure of helium decreased from 2.01 to 1.91 psig. In contrast, the helium pressure of the YSZ/Ti(1µ)/Cu(3µ)/Ag(5µ)/BAg-8(50µ)/Ag(5µ)/Crofer joint slightly decreased from 2.02 to 1.98 psig during the cooling cycle of the test. The greater interfacial reaction between the metallized YSZ and BAg-8 filler due to the thicker metallized Ti layer on the YSZ substrate was responsible for the improved gas-tight performance of the joint.

Dissimilar Brazing of Ti-15Mo-5Zr-3Al and Commercially Pure Titanium Using Ti-Cu-Ni Foil.

Dissimilar brazing of Ti-15Mo-5Zr-3Al (Ti-1553) to commercially pure titanium (CP-Ti) using Ti-15Cu-15Ni foil was performed in this work. The microstructures in different sites of the brazed joint showed distinct morphologies, which resulted from the distributions of Mo, Cu, and Ni. In the brazed zone adhered to the Ti-1553 substrate, the partitioning of Mo from the Ti-1553 into the molten braze caused the formation of stabilized ß-Ti without Ti2Cu/Ti2Ni precipitates. In the CP-Ti side, the brazed joint displayed a predominantly lamellar structure, composed of the elongated primary α-Ti and ß-transformed eutectoid. The decrease in the Mo concentration in the brazed zone caused the eutectoid transformation of ß-Ti to Ti2Cu + α-Ti in that zone. The diffusion of Cu and Ni from the molten braze into the CP-Ti accounted for the precipitation of Ti2Cu/Ti2Ni in the transformed zone therein. The variation in the shear strength of the joints was related to the amount and distribution of brittle Ti2Ni compounds. Prolonging the brazing time, the wider transformed zone, consisting of coarse elongated CP-Ti interspersed with sparse Ti2Ni precipitates, was responsible for the improved shear strength of the joint.

Infrared Brazed Joints of Ti50Ni50 Shape Memory Alloy and Ti-15-3 Alloy Using Two Ag-Based Fillers.

The wettability, microstructures, and bonding strength of infrared brazing Ti-15-3 and Ti50Ni50 shape memory alloy using 72Ag-28Cu (wt.%) and 68.8Ag-26.7Cu-4.5Ti (wt.%) filler metals have been investigated. Only Ticusil® active braze readily wets both Ti50Ni50 and Ti-15-3 substrates. Wetting of eutectic 72Ag-28Cu melt on Ti50Ni50 base metal is greatly ameliorated by adding 4.5 wt.% Ti into the alloy. The brazed Ti-15-3/BAg-8/Ti50Ni50 joint consists of Cu-Ti intermetallics in the Ag-rich matrix. The formation of interfacial Cu(Ti,V) and (CuxNi1-x)2Ti intermetallics next to Ti-15-3 and Ti50Ni50 substrates, respectively, is attributed to the wetting of both substrates. The brazed Ti-15-3/Ticusil®/Ti50Ni50 joint shows a vigorous reaction, which results in the formation of a large amount of Ti2Ni intermetallics in the joint. The maximum joint strengths using BAg-8 and Ticusil® filler metals are 197 MPa and 230 MPa, respectively.