Microstructure modification for improved mechanical properties in A356 aluminum using friction stir processing

Friction Stir Welding (FSW) is a revolutionary solid-state joining technology patented by The Welding Institute (UK) in 1991. This technique uses a rapidly rotating, non-consumable high strength tool steel-in that extends from a cylindrical shoulder to generate frictional heating at the point of welding and to induce intense plastic deformation of work-piece material. The rotating tool assembly is traversed along the joint line, forcing the plasticized material to forge behind to form a solid phase joint. The rotation of the tool produces intense plastic deformation due to stirring action leading to significant microstructural refinement in a single step.; The microstructural refinement obtained after FSW has given rise to the concept of friction stir processing (FSP), wherein, instead of joining, the technique is used to locally modify the microstructure. In this case instead of joining two plates, FSP is used on a monolithic sheet and advantage is taken of the severe plastic deformation caused during the process. In recent years, cast alloy microstructure, in relation to both crack initiation and propagation, has gained much attention considering that advanced casting techniques such as squeeze casting, thixocasting, rheocasting, etc. give rise to significantly fewer defects than conventional cast structures. Moreover, new developments in conventional casting technologies continuously lower defect levels, especially in critically loaded areas. This research work was initiated to develop FSP as a tool for locally modifying the microstructures in a various aluminum alloy and studying the change in mechanical properties in the modified regions. Al-7wt. %Si-Mg alloys with Mg contents in the range of 0.25 to 0.65 wt.% (A356, A357 and C355 alloys) are widely used to cast high-strength components in the aerospace and automobile industries because they offer a combination of high strength to weight ratio with good casting characteristics. Al-Si alloys were selected for the current study because of the above facts.; During FSP, microstructural refinement is induced in the stir zone due severe plastic deformation around the rotating pin, leading to a reduction in the number of small porosities along with reduced aspect ratios of second phase particles. The weld integrity, microstructural refinement and hence the mechanical properties are significantly influenced by the processing parameters and tool geometry. During this study, critical processing parameters were determined and a process window was established to obtain sound welds. Multiple tool designs and processing parameters were studied to study the influence of these parameters on mechanical properties in the stir zone. A correct combination of tool design and processing parameters was determined to achieve optimum tensile properties.; An in depth understanding of the FSP microstructure was achieved through the use of optical, scanning and transmission electron microscopy. It was seen that after FSP, there is a homogenous distribution of the secondary particles in the aluminum matrix; a significant reduction is the porosity volume fraction and a development of finer grain structure in place of a cast microstructure. The influence of heat treatment on the FSP microstructure was also studied and it was found that homogenization treatment during the casting process leads to abnormal grain growth in a FSP microstructure. However, this abnormal grain growth does not adversely affect the mechanical properties in case of FSP A356. (Abstract shortened by UMI.)...