Microstructural analysis of ultrasonic welded AA6061 by electron backscattered diffraction

Zhu, Z. and Ghassemieh, Elaheh and Wynne, B. and Amir, Muhammad (2009) Microstructural analysis of ultrasonic welded AA6061 by electron backscattered diffraction. Rare Metal Materials and Engineering, 38. pp. 147-151.

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Abstract

Ultrasonic welding can be used to join plastic and metal through high-frequency (more than 20 kHz) acoustic vibrations. Aluminium alloy is widely used in electronics, automotive and aerospace. The mechanical vibrations used during ultrasonic metal welding are introduced horizontally. During ultrasonic metal welding, a complex process is triggered involving static forces, oscillating shearing forces and a moderate temperature increase in the welding area. As the energy is introduced to the weld zone, there are three important, phenomena occurring which are surface effect, volume effect and thermal effect. As it is known, microstructure is directly linked to the mechanical properties including fatigue resistance, fatigue life and fracture strength. So the microstructure analysis is important to evaluate the ultrasonically welded aluminium alloy's properties. Also, mechanical properties of metal sheets, such as plastic anisotropy and formability, can be improved by a proper crystallographic texture control. That means the texture has great influence on the plastic anisotropy of the final recrystallized sheets. Optical microscopy (OM) can be used for showing the grain size and grain shape. However, to examine the relationship between mechanical behaviour and microstructure of the weldment, optical microscopy(OM) is not sufficient. OM cannot delineate a grain structure in the weld zone. The etched surface exhibited granularity at the micron level, but it was not clear if that reflected the grain structure. Electron backscatter diffraction (EBSD) in an SEM has become the most widely used technique for determination of microtexture. In this research, high-resolution electron backscatter diffraction is used to study the effects of the vibration on the evolution of microstructure in AA6061. The orientation image maps (OIM), polar figures (PF), and orientation distribution functions (ODF) of the samples are obtained. The characteristics of the crystallographic orientation, the grain structure and the grain boundary are analysed to find the effect of ultrasonic vibration on the microstructure and texture of the bond. The ultrasonic vibration will lead to exceptional refinement of grains to a micron level along the bond area and affect the crystallographic orientation. The grain size of upper foil is much smaller than that of the lower foil. And ultrasonic vibration results in a very weak texture with some characteristic intensity of a rotated cube-orientation and the Brass-orientation and S-orientation. In the lower foil, there is no much influence of the ultrasonic vibration. So there is very strong cube orientation in the lower foil.

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