الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 259 |  |  |
| | | from | 259 | | |
Abstract
The extended use of high strength aluminum alloys in structural applications evokes the importance of developing the welding technique of these alloys. As the friction stir welding technique proved to be efficient in welding these high strength alloys, it attracts the researchers to investigate its numerous parameters and their effect on the welded joint efficiency. Each single parameter in friction stir welding (FSW) process has a significant effect on the produced joint, and has different impact on each aluminum alloy and also differs according to its state of heat treatment, which provides a wide range of factors need to be studied to optimize the process for each alloy.
This research investigates FSW of two aluminum alloys, AA7020-O and AA2017-O and presents macro and microstructure evaluation for all welded joints. The variables in this study are two of the most important FSW process parameters which are the welding tool speeds; the rotational speed, rpm and the traverse speed, mm/min, all other process parameters are kept constant to allow comparable results. Two rotational speeds; 1400rpm and 1120 rpm were used with three traverse speeds; 20, 40 & 60mm/min, these speeds provides 6 values of the ratio revolutionary pitch (mm/rev), out of which 0.36mm/min found to be the optimum combination of welding parameters that obtained maximum strength . The effect of post welding heat treatment (PWHT) was not beneficial for the mechanical properties of alloy AA7020-O whereas it caused hardening and strengthening of AA2017-O joints. Welded joints were also examined against elevated temperature up to 300˚C in tension test. The research results were aided by SEM and EDAX examinations.
iii
The experimental program was divided into three stages; first stage was annealing of all specimens and joining each alloy by FSW, second stage was applying PWHT to two groups of specimens); solution treatment (ST-T4) and solution treatment followed by artificially aging (STA-T6), where the third group was left as annealed to compare between three states of heat treatments and the last stage was testing of the welded joints.
Sound weld was produced by the two alloys AA7020-O and AA2017-O by using FSW technique at the selected range of welding parameters, and finer equiaxed grains generated at the nugget zone by dynamic recrystallization. Trapezoidal weld nugget have been obtained with average grain size at the center of the nugget zone of 11.7μm for alloy AA2017-O and increased slightly with the welding speed to 12.7μm, whereas average grain size of AA7020-O has increased with the decrease of welding speed and the increase of rotational speed and varied from 12.8μm to 24μm. Microhardness at the transverse center line of the welded joint has been improved with maximum joint average hardness of 124Hv for AA7020-O and 103Hv for AA2017-O obtained by joints welded by highest heat input welding conditions. PWHT has increased the AA2017-O joint hardness to 123Hv by solution treatment and increased to 144Hv when followed by artificially aging, whereas the heat treatment cycles had no impact on the hardness of AA7020-O. This agrees with the tension test results as the strength of AA2017-O increased from 330MPa to 344MP after (STA-T6) while AA7020-O didn’t show any improve by PWHT. The resistance of welded joints to elevated temperature could have been monitored, and both alloys could have sustained temperature rise up to 300˚C without losing their tensile properties by careful selection of welding parameters and PWHT, also it was observed that AA2017 showed overall superior behavior than AA7020-O under elevated temperature.