Mechanical Properties And Fracture Behavior Of Al Alloys Subjected To Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) technique is one of the important techniques for producing ultrafine-grained (UFG) materials. In this paper, cast alloys Al-0.63%Cu, Al-3.9%Cu and Al-2.77%Mg were subjected to ECAP for the experimental materials. After ECAP, the microstructures of the alloys were observed, and tension, fatigue and impact tests were performed, to investigate the corresponding mechanical properties as well as deformation and fracture mechanisms. The following conclusions can be drawn:The grains are refined to submicron-meter scale after four ECAP passes. In addition, the precipitation phaseθalong grain boundaries in cast alloy Al-3.9%Cu can be broken into disperse particles. As a result, the tensile strength is improved, while elongation decreases. It is noted that the static toughness of the alloys increases after multi-passes. For Al-2.77%Mg alloy subjected to four ECAP passes and subsequent annealing at 523K, comprehensive mechanical properties are enhanced due to the bimodal distribution of grain size.The tensile failure modes of the alloys display different features. For Al-0.63% Cu alloy, necking degree decreases gradually, and the shear feature becomes more obvious with increasing the number of ECAP passes. For Al-3.9%Cu alloy, the cast sample displays fracture normal to the tensile axis, while the ECAPed samples fail in shear mode. For Al-2.77%Mg alloy, the cast sample exhibits obvious necking before failure, while the samples subjected to different number of ECAP passes display shear fracture with different shear fracture angles. Based on the results above, the tensile failure mechanisms of the ECAPed alloy are discussed.The ECAPed Al-0.63%Cu alloy displays obvious cyclic softening during strain-controlled fatigue tests. Meanwhile, It is found that multipass ECAP can decrease the shape parameter of hysteresis loops, indicative of a large Bauschinger effect. The shear bands orient at about 45°to the cyclic loading axis on XZ-plane, while make an angle of around 90°with respect to the cyclic loading axis on XY-plane. The current research proves that there is no one-to-one relationship between the shear bands induced by fatigue and the shear plane in the last ECAP pass. In addition, it is noted that fatigue cracks can also occur in the area away from shear bands. Stress-controlled fatigue life of Al-2.77%Mg alloy increases with increasing the number of ECAP passes. Meanwhile, It is found that fatigue cracks can propagate along shear bands, or across the shear bands on the surface of the alloy ECAPed for one pass. The fatigue fractograph consists of several different zones, such as fatigue crack initiation, propagation and final fracture.Impact properties of Al-0.63%Cu and Al-2.77%Mg alloys are enhanced after ECAP. While for Al-3.9%Cu alloy, its impact toughness is not improved as a result of more second-phase in the alloy. It is shown that the impact properties have a close relation with the static toughness of the alloys.