Abstract
This work aims to investigate parameter dependency of an innovative welding technique which uses the focused ion beam (FIB) to perform microscale cold welding of aluminum (Al) and copper (Cu), with focus on welds to be used in electrical components. The welding is done by inserting a custom-made Cu-tip micro manipulator needle into a hole in an Al slab. Welding is made possible due to vacancy enhanced diffusion, resulting from plastic deformation, and the welds are characterized by FIB-scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show prominent grain refinement and deformation in both the Al and Cu, which enhance the solute diffusivity in the system. The limiting factor of successful bonding is breaking of the surface oxide layers, which seems to be heavily dependent on the geometry of the system.
As manufacturers of electrical devices push for improved performance, enhanced functionality, cost-efficiency and sustainability, the development of electronics increasingly depends on multi-material metal components [1, 2], and more specifically often Al and Cu components, with trends towards product miniaturization [3]. This evolution calls out for development of new joining methods.
A welding setup in the focused ion beam has been developed. The welding setup consists of a custom-made Cu-tip micromanipulator needle, which is inserted into a pre-made hole in an Al slab [4]. The Cu tip is made of an oxide free, high purity, strain hardened Cu, and the slab is a softer 1050-aluminum alloy. During insertion, the metals are heavily deformed, breaking the oxide layers at the surface and bringing virgin material into direct contact. A bonded interface is created by the plastic deformation and formation of excess vacancies at a low temperature. The goal is to induce vacancy enhanced diffusion and get a strong bond between the metals [4, 5], with limited formation of inter-metallic layers [6]. Several welds are made with different process parameters, like strain rate. The welds are analysed by systematic cross-section images made by FIB. TEM is used to perform a detailed characterization of the joints, with focus on both weld structure and chemical composition.
The FIB-SEM cross-sections show prominent deformation zones with grain refinement around the copper needle and no prominent pores at the interface. Formation of intermetallic phases are seen at local areas at the Al-Cu interface. Deformations in both the Al and Cu are seen by TEM imaging, with varying dislocation densities along the interface. Chemical mapping by energy dispersive x-ray spectroscopy and electron energy loss spectroscopy reveals a variation in residue of oxide layers along the interface, and accumulation of Fe and Ga within areas of high dislocation density in the Al. The residue of oxides acts as a diffusion barrier at the interface and hinder the formation of strong bonding. The accumulation of Fe and Ga indicate an enhanced diffusion within the Al matrix because of the deformation and introduction of vacancies. Fe is the main solute in the Al alloy, while Ga is from sample preparation.
The results show that the deformation of the system was sufficient to enhance the solute diffusivity, causing the solute Fe and implanted Ga to accumulate in the Al matrix. However, the cracking of the oxides at the surface seems to be heavily dependent on the geometry of the setup, with only some local areas along the needle experiencing forces strong enough to break the oxides and allow for diffusion and bonding.
[1] Jorg Kaspar, Martina Zimmermann, A. Ostwaldt, G. Goebel, J. Standfuß, and Berndt Brenner. Challenges in joining aluminium with copper for applications in electro mobility. Materials Science Forum, 783-786:1747–1752, 05 2014.
[2] Jamin Ling, Tao Xu, Raymond Chen, Orlando Valentin, and Christoph Luechinger. Cu and al-cu compositematerial interconnects for power devices. pages 1905–1911, 05 2012.
[3] Y. N. Zhou. Microjoining and Nanojoining, pages xix–xxii. Woodhead Publishing, 2008.
[4] Ambra Celotto, Randi Holmestad, Filippo Berto, and Per Erik Vullum. In-situ microscale cold welding using
a focused ion beam-scanning electron microscope. Micron, 200:103922, 09 2025.
[5] J. Robson. Deformation enhanced diffusion in aluminium alloys. Metallurgical and Materials Transactions
A, 51, 08 2020.
[6] Per Erik Vullum, Ambra Celotto, Øystein Grong, and Randi Holmestad. Cold bonding of aluminium to
copper by deformation-enhanced diffusion. Scientific Reports, 15:41656, 11 202