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Microstructure and mechanical properties of friction stir welded aluminum 6101-T6 extrusions

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This investigation correlates the mechanical performance of friction stir welded 6101-T6 panels with the microstructural characteristics of the weld nugget. Tin plated 6101-T6 aluminum extrusions were friction stir welded in a 90° butt-weld configuration. A banded microstructure of interleaved layers of particle-rich and particle-poor material comprised the weld nugget. Scanning and transmiss[...]

A coupled thermal/material flow model of friction stir surfacing applied to AlMg9Si

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Friction stir surfacing (FSS) (also known as friction stir processing - FSP) utilizes the same process principles as friction stir welding (FSW). However, instead of joining samples together, FSS modifies the microstructure of surface layers in monolithic specimens to achieve specific and desired properties. As in FSW, the tool induces plastic flow during FSS, but depending on the process parameters, i.e. applied force, tool velocity and rotation speed, the material flow can yield a modified microstructure that is beneficial to the performance of the material. FSS, therefore, is an exciting technique for microstructural development and property enhancement [1, 2]. The mechanical properties of cast aluminum alloys are significantly limited by porosity, coarse acicular silicon phases and coarse aluminum dendrites. These three factors can significantly degrade the fracture toughness and fatigue resistance of the alloy. Various foundry and heat treatment schedules are traditionally employed to modify the aluminum microstructure in order to minimize the impact of these factors. Friction stir surfacing, however, offers the ability to locally modify the microstructure and reduce, in particular, the porosity, thus potentially improving ductility, fracture toughness and fatigue [3, 4]. In the present study, friction stir surfacing was applied to samples of cast aluminum alloy AlSi9Mg. A coupled thermal/material flow model of the process is presented, and the effect of tool velocity and tool rotation speed on the material flow and temperature characteristics of the process is discussed. Experimental procedures Friction stir surfacing was performed utilizing a typical milling machine specifically adapted for the processing trials. The FSS tool was made of HS6-5-2 high speed steel, having a 20 mm diameter shoulder without a pin. The tool tilt angle during processing was held constant at 1.5°. The rotational speed r and tool velocity v vari[...]

Microstructure and mechanical properties of friction stir welded Sc-modified Al-Zn-Mg-Cu alloy

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Conventional Al-Zn-Mg-Cu (7000 series) alloys are widely used in aerospace applications due to their favorable mechanical properties. Their strength, the highest among all aluminum alloys, arises from a high density of minute precipitates produced upon aging. However, the mechanical properties degrade above 150°C due to the coarsening and/or dissolution of the strengthening phases within the microstructure. There is currently strong interest in adding zirconium and scandium to Al alloys in order to improve the overall performance of these alloys [1, 2]. Additions of scandium and zirconium to these alloys can stabilize the microstructure at elevated temperatures and can augment the mechanical properties through the formation of fine, secondary strengthening phases such as Al3(Sc, Zr) [3, 4]. Addition of Zr together with Sc improves the effectiveness of Sc as an inhibitor of recrystallization and increases the stability of the alloy during prolonged exposure to elevated temperatures [5]. What is more, the addition of Zr reduces the susceptibility of the Al3Sc precipitates to coarsening. Because the nanometer-sized Al3(Sc, Zr) particles stabilize the microstructure in elevated temperatures, the potential for enhanced properties following joining operations, such as friction stir welding (FSW), arises. Friction stir welded joints display excellent mechanical properties when compared to conventional fusion welds, and as such, the aerospace industry is embracing the FSW technology and implementing its capabilities into their manufacturing sectors [6]. Over the last fifteen years, numerous investigations have sought to characterize the principles of FSW and to model the microstructural evolution [7÷12]. The current status of FSW research pertaining to aluminum alloys has been recently well summarized by Threadgill et al. [13]. The following paper contains introductory results on the characterization of microstructure and mechanical pr[...]

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