Abstract
The aim of this study is to improve the properties of aluminum and magnesium alloys and expand their applications. The unique method, named “reciprocating extrusion”, was applied on conventional aluminum alloys, such as 5056, 5083 and 7075, and new dual-phase magnesium alloys, such as Mg-15Al-Zn and Mg-20Al-Zn. The evolution of microstructures and mechanical properties at room temperature and superplasticity at high temperatures were examined. The relationship between refined microstructures and enhanced properties were built up, and theoretical calculations and models were proposed to prove some outstanding phenomena. The grain size in 5056 aluminum alloy was reduced to 4.6 microm and the coarse inclusions refined to 2 microm after ten passes of reciprocating extrusion. A subgrain structure was formed in the interior of the fine grains, indicating that dynamic recrystallization occurred during extrusion. Dynamic recrystallization in the billet is proposed to be repeatedly induced with the number of extrusion passes until a limiting grain size was obtained. Thereafter, dynamic recrystallization was no longer activated because grain boundary sliding, instead of dislocation gliding, accommodated the deformation strain required for extrusion. The fine-grained structure and its substructure were stable up to 500℃. The maximum elongation of the alloys after ten passes extrusion is 460% at 5x10^-3 s^-1 and 500℃ with a m value of 0.45. After calculation, the dominant of superplasticity in the alloys is grain boundary sliding. The alloys extruded with ten extrusion passes exhibited a superior combination of strength and ductility over commercial 5456-H34. The enhancement of mechanical properties is related to the fine grain structure and refined inclusions. The 5083 aluminum alloys modified with grain refiner, 0.25% Zr and 0.46% Mn, were processed by reciprocating extrusion to yield high-strain-rate superplasticity above 400 ℃ and superior room-temperature mechanical properties. Without any prior homogenization treatment, ten extrusion passes could give the cast billets an equiaxed grain structure with a grain size of about 4.5 microm and a subgrain size about 0.2 microm, and a uniform distribution of fine inclusions and dispersoids in the matrix. The fine-grained structure was stable up to 525 ℃, giving the alloy a high-strain-rate and low-stress superplasticity over a wide operating temperature of 400-500 ℃. In the tensile test at 500 ℃, a maximum elongation of 1013% and a low flow stress of 7.7 MPa at 5x10^-2 s^-1 were achieved. The apparent and true activation energies for low temperatures (300-400 ℃) without high-strain-rate superplasticity were 220.6 and 208 kJ/mol, respectively, whereas those at high temperatures (400-500 ℃) were 88.4 and 98.7 kJ/mol, respectively. Further analysis confirms that grain boundary sliding is the dominant mechanism over the high-strain-rate region from 1x10^-2 s^-1 to 5x10^-1 s^-1 at 500 ℃, and power-law breakdown mechanism occurs over the strain rate from 5x10^−4 s^−1 to 1x10^−2 s^−1 at 300 ℃. The high-strain-rate superplasticity was more strongly enhanced by Zr addition than addition of Cr and Mn. Two enhancing mechanisms for the maximum superplastic elongation and the optimum strain rate are proposed. This study concludes that the effectiveness of Zr is caused by the fineness and the coherency of Zr-rich dispersoids in the matrix. A superior combination of specific strength and low-temperature high-strain-rate superplasticity of two-phase Mg-15Al-1Zn and Mg-20Al-1Zn alloys could be achieved with reciprocating extrusion directly from as-cast billets. The volume fraction of b phase increases from 10% to 45% with increasing Al content from 9 wt% to 20 wt%. The average grain size of Mg-15Al-1Zn and Mg-20Al-1Zn alloys could be refined to 2.8 and 2.5 microm, respectively, after ten passes at 325℃. The yield strength increases but elongation reduces with increasing aluminum content in Mg-Al alloy. In the as-extruded state, the Mg-20Al-1Zn alloy has less ductility. However, the elongation, yield strength and ultimate tensile strength of Mg-15Al-1Zn were 3.6%, 306 MPa and 376 MPa, respectively. After T6 heat treatment, Mg-15Al-1Zn alloy has an excellent improvement on yield and ultimate tensile strength, which are 363 MPa and 432 MPa, respectively. It is noticed that the specific strength of Mg-15Al-1Zn is larger than 7075-T6. The pronounced strengthening mechanism was attributed to the high volume fraction of fine-grained hard Mg17Al12 phase in the refined a-Mg matrix. At 275, 300 and 325℃, both the Mg-15Al-1Zn and Mg-20Al-1Zn have better high-strain-rate superplasticity than AZ91D. The optimum elongation of Mg-15Al-1Zn at least 1610% in company with a high m value of 0.7 was obtained at the strain rate of 1x10^-2 s^-1 when tested at 325 ℃. The excellent superplasticity at high strain rates was also mainly contributed by the high volume fraction of Mg17Al12 phase, which displays easier grain boundary sliding than alpha-Mg phase. The apparent activation energy for superplastic flow is thus smaller than that of the boundary diffusion in Mg. In addition, the mechanisms for the filament formation and cooperative grain boundary sliding are discussed. The increase on volume fraction of beta phase could improve the performance of high-strain-rate superplasticity in Mg-Al alloys. A cooperative grain boundary sliding mechanism is proposed to explain how beta phase could enhance the superplasticity in two-phase Mg-Al alloys. The reciprocating extrusion method was also applied on 7075 Al alloy to refine the grains and inclusions with the aim of improving mechanical properties. Various extrusion passes, that is 1, 5, 10, 20, were chosen to investigate the microstructure evolution and property variation. Experimental results indicate that grain refining largely ceases after five extrusion passes whereas significant inclusion refining continues up to the 20th pass. Comparing the properties of the 20-pass extrudates with those of the starting material with zero pass, yield strength and tensile strength decrease by 9% and 11%, respectively, while elongation, reduction of area, fracture strain and KIC increase by 78%, 270%, 390%, and 210%, respectively. However, compared to the typical properties of 7075 alloys, yield strength, tensile strength, elongation and KIC are all improved and increase by 11%, 3 %, 68%, and 200%, respectively. The ductility and toughness improvement is attributable to the refinement of both grains and inclusions, while the strength loss results from the increased volume fraction of soft PFZ associated with grain refinement. Investigation of tensile fracture surfaces demonstrates the transition of the fracture mode from predominantly intergranular to completely transgranular. This phenomenon in company with the extensive necking behavior is consistent with the significant increasing of ductility and toughness. The reciprocatingly extruded 7075-T6 alloys exhibit superior strength-ductility and strength-toughness combinations to conventional ones. This characteristic could largely increase the designed stress standard for the airframe structure based on the fail-safe design concept.