Tong Lin

The problem of stainless steel brazing is still the focus of scientific research. In this work, the Mn-based brazing filler was used to braze 1Cr18Ni9Ti and 1Cr21Ni5Ti stainless steel. The typical microstructure of the 1Cr18Ni9Ti/1Cr21Ni5Ti joint was analyzed in detail, and the interface structure of the joint was determined to be 1Cr18Ni9Ti/Mn(s, s)/1Cr21Ni5Ti. The brazing temperature and holding time were shown to have a great influence on the microstructure of the brazed joint. The tensile strength of brazed joints first increased and then decreased with the rising of the brazing temperature and the holding time. The maximum tensile strength was 566 MPa when the joints were brazed at 1125 °C for 15 min. The diffusion of Mn and Cr was an important factor affecting the quality of the joints. The diffusion distances of Mn and Cr at different brazing temperatures and holding times were measured, and the diffusion activation energy and diffusion coefficient were achieved by the Arrhenius equation.

Reactive air brazing has been widely used in fabricating solid oxide fuel/electrolysis cell (SOFC/SOEC) stacks. However, the conventional Ag–CuO braze can lead to (I) over oxidation at the steel interconnect interface caused by its adverse reactions with the CuO and (II) many voids caused by the hydrogen-induced decomposition of CuO. The present work demonstrates that the Ag particle interlayer can be used to join yttria-stabilized zirconia (YSZ) electrolytes to AISI 441 interconnect in air instead of Ag–CuO braze. Reliable joining between YSZ and AISI 441 can be realized at 920 °C. A dense and thin oxide layer (~2 μm) is formed at the AISI 441 interface. Additionally, an interatomic joining at the YSZ/Ag interface was observed by TEM. Obtained joints displayed a shear strength of ~86.1 MPa, 161% higher than that of the joints brazed by Ag–CuO braze (~33 MPa). After aging in reducing and oxidizing atmospheres (800 °C/300 h), joints remained tight and dense, indicating a better aging performance. This technique eliminates the CuO-induced issues, which may extend lifetimes for SOFC/SOEC stacks and other ceramic/metal joining applications.

To improve repair efficiency and achieve a high‐quality repair layer, wire and arc additive manufacturing process is used in this study to repair missing‐angle Ti‐6Al‐4V blade. The deposition layer with a mixed equiaxed β and columnar grain can be obtained by optimizing the deposition speed and single layer height. According to the characteristics of β grain, the typically repaired thin‐walled part is divided into four representative areas, including heat affected zone (HAZ), bottom layer, middle layer, and top layer. The bottom layer consists of coarsely near‐equiaxed β grains with martensite α′ and basket‐weave structure, and the columnar β grains grow perpendicularly from middle layer to top layer. The heat accumulation in HAZ continues to increase which resulted in a certain degree of cooling rate reduction, resulting in the martensite α′ decomposed to α dots and rods in HAZ near the layer band. The fine near‐equiaxed β grains (78 ± 28 μm), martensite α′ structure, and fine lamellar α structure are presented in HAZ, causing the fracture transition from the HAZ to bottom region. The characteristic of ductile rupture is observed in repaired thin‐wall, the maximum tensile strength is 991 MPa, and the elongation reaches about 10%.