Mechanical properties of 6000 series aluminum substrates joined together using a flux-free brazing technology in a graphite vacuum furnace

Hadian Amir - University of Applied Science Northwestern Switzerland (Switzerland)

technology, especially for the 6000 and 7000 series alloys used in the automotive and aviation industries. The persistent oxide layer that forms on the surface is a ceramic material with a high melting point and a high affinity for chemicals. Contrary to welding, this layer can be easily broken down in brazing using flux materials primarily composed of chloride and fluoride salts, however it is not an environmentally friendly process [1].

The concept of flux-free aluminum brazing was developed in the 1990s to reduce the efforts related to flux application and post-brazing corrosion risk caused by flux residues. To advance flux-free brazing technology into a more efficient and environmentally friendly process, this study focused on reducing the magnesium (Mg) content in the braze filler material. Traditionally, the evaporation of Mg during brazing was used to facilitate the reduction of the surface oxide layer and provide interaction between the fresh aluminum surface and the molten braze alloy. However, evaporated Mg can deposit on critical components within the furnace, leading to contamination between brazing cycles and potential long-term short circuits in the furnace’s electronic connections [2]. Our approach involves adding nickel (Ni) to the brazing alloy, following the same concept but avoiding furnace contamination. The energy from the exothermic reaction between Ni, Al, and Si partially disrupts the surface oxide layer, facilitating interaction with the filler metal.

Thermodynamic simulations and differential thermal analysis were primarily used to develop our braze alloy. To study the gap-filling behavior of the developed filler metal, the samples were analyzed using ultrasonic imaging. Additionally, cross-sections of the samples were prepared and investigated using light microscopy and scanning electron microscopy. In this work, a new shear testing fixture was designed, evaluated, and constructed to measure the shear strength of the joints, ensuring more reproducibility by applying pure shear forces on the joints. Using the in-house made shear test fixture, the shear strength of the samples was measured. The average shear strength of the brazed samples was 85 MPa after brazing, increasing to 100 MPa after precipitation hardening. Compared to the shear strength of the full-material substrate, the joints exhibited 30% lower shear strength. Additionally, micro-hardness measurements were conducted to evaluate the hardness of the phases formed in the braze and diffusion zone and to find the best brazing practice to achieve a fine distribution of brittle intermetallic phases.

Keywords: Aluminum, vacuum brazing, flux-free, shear strength, hardness

1. Humpston, G., S. P. S. Sangha, and D. M. Jacobson. “New filler metals and process for fluxless brazing of aluminium engineering alloys.” Materials science and technology 11, no. 11 (1995): 1161-1168.
2. Hawksworth, D. K. “Fluxless brazing of aluminium.” In Advances in brazing, pp. 566-585. Woodhead publishing, 2013.