FRICTION STIR ASSISTED JOINING AND FORMING OF DISSIMILAR METALS

Abstract

Enhancement of energy efficiency and to reduce harmful emissions simultaneously, most industries intend to incorporate lightweight multiple materials structures. To construct such structures joining and forming are the basic needs. Further, localized enhancement and use of bi or multi-metallic components are also recommended due to lack of stiffness, corrosion property, and poor conductivity for some lightweight non-ferrous metals. The conventional fusion welding techniques induce solidification defects, base material softening, and excessive formation of detrimental intermetallic compounds. Moreover, the preexisting hot forming techniques involve the requirement of external energy, causing prolonged manufacturing time and higher energy consumption. Friction stir processing (FSP) is a solid-state manufacturing process generating frictional heat, which can evade defects related to solidification and does not require any external energy source. The energy input for this process for dissimilar material combinations especially can be perplexing, and its optimization requires several types of operating parameter variation. Thus, an in-depth study involving microstructural and mechanical property correlation by varying energy input conditions for different material combinations used for forming or joining is required for establishing this process as an alternative to the existing process. To validate the claims, several different combinations of materials are butt and lap spot welded. In addition, the standard FSW processes were also modified to invent a noble process of simultaneous joining and forging of the Al-Mg and Al-Cu bimetallic blanks. Low nickel austenitic stainless steel (LN1) and 409M ferritic stainless steel is friction stir welded to study the effect of material flow on weld quality and also dynamic recrystallization phenomena by carrying out the electron microscopy. Later it was correlated with the measured mechanical properties. The ferritic stainless steels show more severe dynamic recrystallization, resulting in a very fine microstructure, probably due to the higher stacking fault energy. Elemental mapping of the dissimilar joints clearly indicates that the material flow pattern during FSW depends on the process parameter combination. Dissimilar joining of mild steel and aluminum 5052-O aluminum is carried out successfully by offsetting the tool pin towards the harder steel material by 0.5 mm with a convex scrolled tool made of tungsten carbide. 1000 rpm and 75 mm/min weld parameters resulted in better material mixing, which further enhances the mechanical properties of the joint. 1060 pure aluminum was strengthened using a novel technique by combining additive manufacturing and friction stir processing. Additively manufactured 7 series aluminum inside a grooved 1series Al were friction stirred to incorporate more refined grain structure and precipitations of 7 series aluminum resulting dual strengthening. The mechanical properties of the AM+FSP ed sample were enhanced significantly. The friction stir spot welding process was modified to invent a new simultaneous joining and forming process and was successfully carried out on both Al-Mg and Al-Cu bimetallic blanks to form a bimetallic ring component. The frictional heat generated by the rotating tool forges and the bimetallic blanks generates favorable conditions for diffusion, resulting in joining by intermetallic formation. Since the material combination was altered the composition of the IMC is altered significantly. Due to the poor formability of Mg, the energy required to accomplish the process was much higher. In contrast, due to good formability and high reactivity of Al and Cu, much lower energy input was adequate to carry out the process.