Influence of Surface Preparation on the Diffusion Welding of High Strength Aluminium Alloys

Abstract

Diffusion Bonded joints have been produced between sheets of the high strength aircraft and aerospace Aluminium alloys AA 7475 and Al-Li 8090/2091. For these materials the combination of Diffusion Bonding and Superplastic Forming would lead to considerable design flexibility and cost reductions. So an enormous increase of their application potential for aircraft structural components would result from the development of DB/SPF-technologies (1).

This paper reports some effects of different surface preparation methods on the bond quality which were investigated in connection with a modified bonding process. The preparation methods included grinding, brushing and “RF ion etching” (sputter cleaning) with a subsequent sputter deposition of Cu, Ag or Au. Grinding was accomplished in air and also in argon atmosphere in order to minimize the immediate formation of the natural oxide layer. From the argon chamber the ground specimens were then directly shifted into the bonding chamber without beeing exposed to air.

Surface and depth profile analyses were carried out by means of Glow-Discharge-Source (GDS) Plasma-Emission-Spectroscopy in order to verify the surface characteristics of the sheets to be bonded.

The DB-joints were produced in a Diffusion Bonding Hot-Press which admits a defined application of the weld pressure during the heating phase and also rapid cooling after the bonding process. This process was especially developed for the Diffusion Bonding of high strength Aluminium sheets.

Shear tests at room temperature exhibited that excellent strengths up to 410 MPa are possible. Besides the shear strength also the peel strength was measured at room temperature and at SPF temperature (516°C). The results were correlated with joint characteristics found by metallographic methods and observations by Scanning-Electron-Microscope (SEM).