الفهرس 
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Abstract
In the present investigation, lap joints of the AA7020-T6 aluminum sheets were joined using friction stir welding (FSW). The AA7020-T6 sheets has 3 mm thickness. The FSW was carried out at three different tools rotational speeds of 1200 rpm, 1400 rpm and 1600 rpm; and three different welding speeds of 20 mm/min, 40 mm/min, and 60 mm/min. During FSW, the tool tilt angle and plunging depth were kept constant at 3o and 0.5 mm, respectively. The FSW was performed using a tool with a tapered pin profile and a flat shoulder. The tool is made of K110 tool steel.
The surface quality of the FSW specimens was evaluated by the arithmetic average roughness value (Ra). Both macro- and microstructural characteristics of the friction stir (FS) welded AA7020-T6 Al alloy lap joints were also investigated.
Tensile-shear and microhardness tests were carried out to evaluate the mechanical characteristics of the AA7020-T6 lap joints. Moreover, the corrosion rate at the center of the stirred zone (SZ) were measured. A statistical analysis of variance
(ANOVA) was performed to find which FSW process parameters (i.e. the tool
rotational and welding speeds) are statistically significant. With the signal to noise (S/N) ratio and ANOVA analyses, the optimal levels of the process parameters can be determined. Also, an artificial neural network (ANN) model was developed to
predict the microstructural, mechanical and corrosion haracteristics of the AA7020-T6 Al alloy lap joints. The results revealed that increasing of the tool rotational speed and/or the welding speed increases the surface roughness of AA7020-T6 lap joints. Increasing
the tool rotational and/or reducing the welding speeds increase(s) the average grain size and corrosion rate, but reducing the microhardness at the center of SZ of the FS welded regions of AA7020-T6 lap joints. The reduction of the tool rotational speed and/or increasing the welding speeds increase(s) the tensile-shear load of the AA7020-T6 aluminum friction stir welded lap joints. The minimum corrosion rate and maximum microhardness at the center of SZ, as well as the maximum tensile
load of AA7020-T6 Al FS welded lap-joints were obtained for those joints FS welded using tool rotational and welding speeds of 1200 rpm and 60 mm/min, respectively.
The welding speed showed the highest statistical significance on the
microhardness as well as the tensile-shear load of the AA7020-T6 aluminum lap FS welded joints when compared with the tool rotational speed. In contrast, the tool rotational speed has more statistical significance on the average grain size and
corrosion rates at the center of the stirred zone of the welded regions as well as the surface roughness of the AA7020-T6 welded joints when compared with the welding speed.
The developed ANN model showed a good agreement between the predicted
and experimental results. The absolute mean errors (AME) for the average grain size, surface roughness, microhardness, tensile-shear load, and corrosion rate of 4.768%, 2.513%, 1.039%, 2.261% and 10.595%, respectively. These results
indicate the reliability of ANN approach as one of the most accurate prediction approaches. However, the influence of the process parameters on the aforementioned characteristics for other joint configurations such as lap-joints, Tjoints …etc. was seldom reported.