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Improved strength and ductility of friction stir tailor-welded blanks of base metal AA2024 reinforced with interlayer strip of AA7075
Abstract
Most of welded parts requires secondary forming process to meet the design requirements which makes the
ductility of the joint an important parameter to be improved. To this end, we imped compensation layer of
AA7075 aluminum alloy between two blanks of AA2024 aluminum alloy during friction stir welding. The effect
of compensation layer width on the microstructure, microhardness, tensile strength and failure strain was investigated. XRD results showed that using AA7075 alloy compensation layer helped for precipitation of new
intermetallic phases, Al2Cu and Mg2Zn, at the grain boundaries of Al grains. Tensile test results showed that the
sample with 2 mm width compensation layer possess 18% improved strength and 54.4% improved failure strain
compared with the joint without compensation layer. The failure strain of this joint was 16.8% which is almost
that for AA2024 (17.2%). The improved strength was because of the grain refinement and the precipitation of
Al2Cu and Mg2Zn intermetallics. While the improved failure strain was due to the higher content of Mg and Zn
alloying elements in AA7075 alloy which precipitate on the grain boundaries facilitating the slipping between
grains and provide more deformability of the global structure.
Conclusion
With the aim of improving welded joint strength and ductility, FSW
technique with compensation interlayer was applied to tailor-weld
aluminum blanks. Three different compensation interlayer width is
considered to optimize the interlayer width which achieve barter
combination of strength and ductility. A welded joint without compensation interlayer was prepared to quantify the percentage of improvement using compensation interlayer. SEM, EDX and XRD techniques were used to characterize microstructure changes in welded joints.
Tensile test and microhardness tests were used to characterize the
mechanical strength of the produced joints.
The results showed that using 1 mm wide compensation layer of
AA7075 alloy improves the tensile strength by 14.1% and the failure
strain by 53% compared to welded joint without compensation layer.
The reduced crystallite size and formation of intermetallic phases are
the main reasons for strength improvement, while the existence of Mg
and Zn alloying elements is the main reason for ductility improvement.
The existence of these elements helps for slipping between grains
during deformation which provide frictional deformation between
grains each other which improve ductility of the joint. For samples with
3 mm compensation layer, the strength is increased significantly,
however the ductility is reduced due to the large reduction of crystallite
size. Among the considered compensation layer widths, the optimum
compensation layer width is 2 mm at which the ultimate strength of the
joint is improved by 18% and the failure strain is improved by 54.4%
compared to welded joint without compensation interlayer.
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