Benign joining of ultrafine grained aluminum alloys

Researchers at Empa in Zurich Switzerland have successfully joined nanostructured aerospace grade aluminum alloys with a minimal loss in mechanical properties using nanotechnology. The authors outline in a communication to Advanced Materials how it is possible to braze and solder benignly ultrafine grain aluminum. Nanostructured reactive foils were used as local heat sources. These foils rapidly release thermal energy directly at the interface between two materials leading to a metallurgical joint. The heat affected zone and the duration of heating are substantially limited, which in turn minimizes damage to the bulk. This is a first in the literature and is extremely relevant to several industries that have been struggling with the problem of joining temperature-sensitive materials while avoiding grain growth. This oven-less joining demonstration increases the attractiveness of nanostructured aluminum alloys as lightweight replacements material for conventional alloys.

Metal alloys are strengthened by refining their grain size. This is true down to the nanoscale, yet nanostructured alloys have a limited thermal stability. Recovery, recrystallization and grain growth even at mild temperatures lead to weakening. As a consequence, conventional and widespread joining technologies such as welding, brazing and high temperature soldering are inapplicable. This fact severely hinders alloy commercialization and restricts the type and number of applications in which they can be implemented. The obvious solution of using rivets or adhesives is not always possible and has its respective drawbacks. If the preferred joining options are fusion-based, then heat input must be carefully controlled and/or localized.

For the past decade, the focus in this field has been on characterizing the nanostructured materials and improving their production methods, namely the severe plastic deformation processes. Despite the impressive progress, the joining issue has remained open, even though several companies are now making commercial offerings of nanocrystalline metal articles. This is problematic as advanced materials have limited applicability unless they can be joined properly into useful products and assemblies in a way that preserves their unique properties. i.e. their nanoscale characteristics. Luckily, the joining problem has been solved in part here. Only the application performance remains to be assessed.

Longtin, R., Hack, E., Neuenschwander, J. and Janczak-Rusch, J. (2011), Benign Joining of Ultrafine Grained Aerospace Aluminum Alloys Using Nanotechnology. Advanced Materials.
DOI: 10.1002/adma.201103275 (abstract)

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