Equal channel angular pressing (ECAP) is a widely known processing procedure for the fabrication of ultra-fine grained (UFG) metal and alloys, in which a sample is pressed through a die comprising two channel portions having an L-shaped configuration. [1] Typically, samples are pressed for several consecutive passes through the die to impose very high strains. This technique is especially attractive for the production of semi-finished products from aluminum alloys with the UFG structure for several reasons. First, conventional tool steels, used as a structural material for the ECAP die, allow ECAP processing of aluminum alloys in a wide temperature range. Second, ECAP can be applied to fairly large billets due to the fact that this processing requires a reasonable low load capacity. Third, ECAP is a relatively simple procedure where high strains are introduced into a billet by a simple shear providing sufficient homogeneity of microstructure evolved in this billet under the processing. It is worth noting that the simple shear nature of the plastic deformation during ECAP is very effective for extensive grain refinement in aluminum alloys in comparison with the other deformation methods.At the same time the potential viability of ECAP processing to be implemented into industrial environment is currently limited by three factors.(i) In order to achieve a high impose strain a repetitive ECAP processing is used. The billet must be removed from the ECAP die and reinserted, with or without an interpass rotation. This operation is time-consuming one and requires high labor cost. (ii) Extensive surface cracking and an irregular shape of the outer corner appeared after each pass lead to a necessity of scalping and cutting operations between ECAP passes.A two-step process consisting of modified equal channel angular pressing (ECAP) and subsequent isothermal rolling (IR) was developed to produce thin sheets of aluminum alloys with ultra-fine grained (UFG) structure. Significant increase in the efficiency of ECAP was attained by using flat billets and a back pressure system. The incorporation of final IR into technologic route provides a reduced strain which is necessary to impose for the fabrication of thin sheets with UFG structure. In addition, it allows producing relatively ''long billets.'' In order to demonstrate the feasibility of this technique an Al-5.1Mg-2.1Li-0.17Sc-0.08Zr (wt %) alloy was subjected to ECAP at 325 8C to a total strain of $8 using processing route CX. The operation time of this processing did not exceed 15 min. Subsequent IR at the same temperature with a total reduction of 88% was applied to produce thin sheets with a 1.8 mm thickness and an average size of recrystallized grains of $1.6 mm. These sheets exhibit extraordinary high superplastic ductilities. In addition, this material demonstrated almost isotropic mechanical behavior at room temperature. The maximum elongation-to-failure of $2700% was attained at a temperature of 450 8C and an initial strain rate of 1.4 Â 10 À2 s À1 . Thus it was dem...
Abstract. An AA2139 alloy with a chemical composition of and an initial grain size of about 155 μm was subjected to annealing at 430°C for 3 h followed by furnace cooling. This treatment resulted in the formation of a dispersion of coarse particles having essentially plate-like shape. The over-aged alloy exhibits lower flow stress and high ductility in comparison with initial material in the temperature interval 20-450°C. Examination of microstructural evolution during high-temperature deformation showed localization of plastic flow in vicinity of coarse particles. Over-aging leads to transition from ductile-brittle fracture to ductile and very homogeneous ductile fracture at room temperature.
The effect of liquid hot isostatic pressing (LHIP) on microstructure and mechanical properties of a high-strength cast Al-6Zn-2Mg-0.5Fe-0.7Ni alloy was examined. LHIP eliminates shrinkage porosity that highly improves strength and fatigue limit. Yield stress (YS) and ultimate tensile strength (UTS) in T6 condition increased from 135 to 470 MPa and from 410 to 510 MPa, respectively. Endurance limit on the base of 107 cycles increased from 95 to 140 MPa. However, a small number of gas pores with an average size less than 2 μm retains. LHIP suppresses the crack initiation on coarse cavities. However, brittle intergranular fracture occurs in the hipped alloy through the breaking of eutectic phase Al9FeNi. As a result, elongation-to-failure was of 1.2% and the fatigue strength is equal to one of AA356.02 alloy subjected to LHIP.
Effect of liquid hot isostatic pressing (LHIP) on structure and mechanical properties of an A356.0 alloy was examined. Samples from this alloy were produced by gravity die castings. Part of these samples was subjected to homogenization annealing, and the other part was subjected to LHIP following homogenization annealing. All samples were water quenched from the temperature of prior homogenization annealing or LHIP and finally aged. It was shown that the LHIP processing leads to increase in yield stress, ultimate stress and total elongation. A significant increase in fatigue strength and decreased the scattering of fatigue data takes place too. This is caused by the fact that the fatigue crack initiation mostly occurs on lateral surfaces of the samples subjected to LHIP, whereas shrinkage voids in the non-hipped condition play a major role in crack initiation. In addition, crack propagation under fatigue occurs in samples subjected to LHIP in essentially ductile manner. Thus, LHIP eliminating shrinkage porosity enhances significantly mechanical properties and reliability of aluminum casting.
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