Recent Advances in Brazing Fillers for Joining of Dissimilar Materials

Brazing fillers for joining applications are essential for manufacturing and designing advanced materials. Several types of brazing fillers have been developed in recent decades to join similar or different engineering materials. Important parts of automotive and aircraft components, including steel, are often joined by brazing. In addition, ceramic components in microwave devices and circuits have been joined with a high level of integration in microelectronic devices. Similarly, in the medical field, metallic implants have been brazed to ceramic dental crowns. These advances have made human life more convenient. However, in brazing, there are certain issues with intermetallic compound (IMC) formation and residual stresses in joints at high temperatures. Nanoparticle-reinforced fillers have been proposed to control IMCs, but there are other dispersion and particle segregation issues at the joints. In this study, various types of brazing fillers, joint fabrication processes, and brazing technologies developed in recent decades are reviewed. Furthermore, new developments in brazing materials and their specific applications are presented. Finally, the emerging areas in brazing, including the recent entropy-modified brazing fillers for various structural and technological fields, are discussed.

Active Brazing of Alumina and Copper with Multicomponent Ag-Cu-Sn-Zr-Ti Filler

The study was designed to investigate the synergic effect of Ti and Sn in the active metal brazing of Al2O3 ceramic to copper brazed, using the multicomponent Ag-Cu-Zr filler alloy. Numerous fine and hexagonal-shaped rod-like ternary intermetallic (Zr, Ti)5Sn3 phase (L/D = 5.1 ± 0.8, measured in microns) were found dispersed in the Ag-Cu matrix of Ag-18Cu-6Sn-3Zr-1Ti alloy, along with the ternary CuZrSn intermetallic phases. An approximate 15° reduction in contact angle and 3.1 °C reduction in melting point are observed upon the incorporation of Ti and Sn in Ag-18Cu-3Zr filler. Interestingly, the interface microstructure of Al2O3/Cu joints brazed by using Ag-18Cu-6Sn-3Zr-1Ti filler shows a double reaction layer: a discontinuous Ti-rich layer consisting of (Cu, Al)3(Ti, Zr)3O, TiO, and in-situ Cu-(Ti, Zr) precipitates on the Al2O3 side and continuous Zr-rich layer consisting of ZrO2 on the filler side. The shear strength achieved in Al2O3/Cu joints brazed with Ag-18Cu-6Sn-3Zr-1Ti filler is 31% higher, compared to the joints brazed with Ag-18Cu-6Sn-3Zr filler. Failure analysis reveals a composite fracture mode indicating a strong interface bonding in Al2O3/Ag-18Cu-6Sn-3Zr-1Ti filler/Cu joints. The findings will be helpful towards the development of high entropy brazing fillers in the future.

Determination of spreading and flow behavior of brazing fillers

The development of new brazing fillers requires knowledge about their properties in comparison to well established fillers. Therefore, the determination of the spreading and flow behavior is necessary. By now, different measuring techniques are combined in order to reach a comprehensive understanding of the mechanisms. Especially in case of flux brazing, conventional measuring techniques cannot be used. Due to the industrial demand of application-related experiments, the intended setup should be simple and easily to arrange. This paper deals with the development of a new procedure to determine the spreading and flow behavior of brazing fillers employing flux. The experimental setup, which consists of a commercially available induction heater, equipped with a temperature controller, is mounted inside a gas and vacuum tight chamber. A CCD camera system is used to record the spreading and flow behavior on surfaces as well as inside narrow gaps. The results of experiments using different fillers and base materials are presented. The measurements of the spreading area and the flow velocity under defined conditions allow a quantitative evaluation of the properties of the fillers. Thus, the comparison of different fillers among each other is possible.

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.

Joining of TiAl Alloy Using Novel Ag–Cu Sputtered Coated Ti Brazing Filler

The aim of this study is to evaluate the potential use of titanium foil coated with sputtered silver and copper films as a novel brazing filler for joining TiAl alloys. For this purpose, a detailed microstructural characterization of the resulting brazing interfaces was carried out. The development of brazing fillers that allow the joining of TiAl alloys without compromising the service temperature is a fruitful prospect. Brazing experiments were performed in a vacuum at 900, 950, and 980°C, with a dwell time of 30 min. Microstructural characterization reveals that brazing joints can be obtained successfully at 950 and 980°C. The interface consists of a large central region of α-Ti with an amount of Al and Ti–Ag compound and thin layers, mainly composed of intermetallic compounds, formed close to the base material. A novel brazing filler consisting of Ti foil coated with sputtered Ag and Cu films inhibits the extensive formation of soft (Ag) zones or coarse brittle Ti–Al–(Cu,Ni) particles. Hence, the need for post-brazing heat treatments for the joining of TiAl alloys was avoided.