In the search for applications for alloys developed under the philosophy of the High Entropy Alloy (HEA)-type materials, the focus may be placed on applications where current alloys also use multiple components, albeit at lower levels than those found in HEAs. One such area, where alloys with complex compositions are already found, is in filler metals used for joining. In soldering (450 °C), filler metal alloys are taken above their liquidus temperature and used to form a metallic bond between two components, which remain both unmelted and largely unchanged throughout the process. These joining methods are widely used in applications from electronics to aerospace and energy, and filler metals are highly diverse, to allow compatibility with a broad range of base materials (including the capability to join ceramics to metals) and a large range of processing temperatures. Here, we review recent developments in filler metals relevant to High Entropy materials, and argue that such alloys merit further exploration to help overcome a number of current challenges that need to be solved for filler metal-based joining methods.

Tailoring the phase constitutions of the interfacial reaction layers under the assistance of ultrasonic vibration is a convenient method to fabricate high-strength Al/Cu brazing joints. In this study, 1060-Al and T2-Cu dissimilar metals were ultrasonically brazed with Zn-3Al (wt. %) filler metals. Effects of ultrasonic brazing time on the microstructure and mechanical properties of joints were investigated. Results showed that the CuZn₅ intermetallic compound (IMC) layer and Cu-based diffusion layer were created on the Cu substrate surface in the joint ultrasonically brazed at 400 ℃ for 2 s. However, the CuZn₅ IMC layer was gradually transformed into a thin Al_4.2Cu_3.2Zn_0.7 IMC layer by increasing the ultrasonic vibration time to 15 s. A well-matched coherent interface was formed between the Al_4.2Cu_3.2Zn_0.7 ternary phase and the Cu-based diffusion layer. The phase transition of the Cu-side interfacial layer correlated closely with the acoustic cavitations induced super-saturation regions near the Cu substrate surface. The measured tensile strength of the Al/Zn-3Al/Cu joint ultrasonically brazed for 15 s was 89.3 MPa, which was approximately 2.5 times higher than that brazed for 2 s, and the tensile failure mainly occurred at the interface between the Al_4.2Cu_3.2Zn_0.7 layer and the Cu-based diffusion layer.

In this investigation, ultrasonic-assisted soldering at 260 °C in air produced high strength and high melting point Cu connections in 60 s using Ni foam reinforced Sn composite solder. Systematically examined were the microstructure, grain morphology, and shear strength of connections made with various porosities of Ni foam composite solders. Results shown that Ni foams as strengthening phases could reinforce Sn solder effectively. The addition of Ni foam accelerated the metallurgical reaction due to great amount of liquid/solid interfaces, and refined the intermetallic compounds (IMCs) grains by ultrasonic cavitation. The joints had different IMCs by using Ni foam with different porosity. Layered (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ phases both existed in Cu/Ni60-Sn/Cu joint while only (Cu,Ni)₆Sn₅ IMCs grew in Cu/Ni98-Sn/Cu joint. As ultrasonic time increasing, Ni skeletons were dissolved and the IMCs were peeled off from substrates and broken into small particles. And then, the IMCs gradually dissociated into refined particles and distributed homogeneously in the whole soldering seam under cavitation effects. Herein, the Cu/Ni60-Sn/Cu joint ultrasonically soldered for 60 s exhibited the highest shear strength of 86.9 MPa, as well as a high melting point about 800 ℃ for the solder seam composed of Ni skeletons and Ni-Cu-Sn IMCs. The characterization indicated that the shearing failure mainly occurred in the interlayer of the soldering seam. The homogeneous distributed granular IMCs and Ni skeletons hindered the crack propagation and improved the strength of Cu alloy joints.