Determination of Superplastic Material Properties for Parent Material and Friction Stir Welded Joint of Al-Alloy AA6061-T6
Authors: Velukkudi Santhanam Senthil Kumar , Pasupathy Ganesh , Anantha Padmanabhan Kuppuswamy 
Published: March 2016

Superplastic forming (SPF) takes the advantage of the metallurgical phenomenon of superplasticity (SP) to form complex and highly intricate bulk and sheet metal parts. SP refers to the extraordinary formability of certain metals and alloys, ceramics, composites (both metallic- and ceramic-based), dispersion strengthened materials, nanostructured materials and bulk metallic glasses, which allows them to suffer elongations of several hundred percent under the action of tensile forces. The superplastic forming characteristics of materials like aluminium, titanium and magnesium alloys have been clearly identified in order to produce complicated near-net shapes. These materials are used in the aeronautical manufacturing industry and automotive manufacturing industries due to the significant weight (by ∼ 30%) and cost (by ∼ 50%) saving that is possible. Some research work has proved superplastic forming of friction stir welded (FSW) joints also. The FSW joint efficiencies have been characterized by mechanical and metallurgical examination. Studies are also available on the behavior of FSW joints of similar and dissimilar metals.

Information on the performance of friction stir welded joints during superplastic forming is rather limited, but it is important to achieve excellent properties in the friction stir welded joints also during superplastic forming. FSP (friction stir processing) – SPF (superplastic forming) is presently being promoted as a very viable near-net shape technology for making very large and complicated sheet metal products. To achieve this superplastic material parameters are much required in industry to develop new shapes. One has to understand the flow rule relationship and mechanics involved during sheet metal forming at high temperature to select the material and forming tool with selected process parameters.

This paper deals with the determination of superplastic material properties of non-superplastic aluminum alloy AA6061-T6. The superplastic material properties like strain rate sensitivity index, flow stress and strain rate were determined for both the selected material and friction stir welded sheets at various tool rotation speeds. The superplastic free blow forming experiments were performed for various constant temperatures and pressure for the parent material. Similarly the superplastic free blow forming experiments were performed for the friction stir welded joint for various tool rotation speed at constant temperature. The methods were used to determine the material properties are straight line fit method and polynomial regression method. The superplastic forming height is significantly high in case of the FSW specimens at 2000 rpm, the initial forming rate is faster and the strain rate sensitivity index obtained is also higher when compared to the parent material properties. The strain rate sensitivity index obtained for friction stir welded specimen during superplastic forming was foundto have improved when compared to the parent material.