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Rao T.S.,ACE Engineering College Ghatkesar | Reddy G.M.,Defense Metallurgical Research Laboratory | Rao S.R.K.,SSN College of Engineering
Metallurgia Italiana | Year: 2016

High strength aluminum alloy 7075 is widely used in aerospace applications. In this investigation, Al-Zn-Mg-Cu alloy- AA7075-T651 was friction stir welded, in two thicknesses-10 mm and 16 mm, in order to investigate variations in microstructure and hardness profiles across the joint in through thickness direction. Process parameters resulting in defect free welds were identified using trial and error method. The variations in microstructures and hardness profiles were studied by means of optical microscopy, transmission electron microscopy (TEM) and hardness tests. 10 mm thick welds were found to show better performance in terms of hardness compared to 16 mm thick welds. However, both welds were found to exhibit significant loss in the hardness in the heat-affected zone (HAZ). Furthermore, the loss of hardness due to the welding is more in 16 mm thick welds. Hardness across the joint increases as the survey moves away from the surface of the specimen to the root of the weld. The weld nuggets in both the welds reveal very fine recrystallized grains and the grain size was found to decrease from surface to root of the joint along the thickness of the weld. TEM studies reveal that the weld nugget suffers from dissolution of precipitates and the HAZ consists of wide precipitate free zones, which are considered responsible for lower hardness in the HAZ. It is concluded that sound welds can be produced on thick plates ofAA7075- T651 and welds made on thicker plates exhibit inferior hardness values.

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