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Materials of a gradient structure have been shown to possess both high strength and high ductility. To date, materials of a gradient structure can only be produced in small quantities. In this paper, we report a novel 'cyclic skin-pass rolling' (CSPR) technique capable of producing sheets of gradient structure in large quantities. Both experimental and analytical/numerical investigations are reported. In the experiments on aluminium sheets, the outer layer was subjected to 40 passes of CSPR with a reduction ratio of 1% per pass. After CSPR, the sample surface shows an ultrafine-grained microstructure with a mean grain size of 206 nm, while the annealed microstructure is retained in the core of the sample. Compared with cold-rolled aluminium sheets fabricated with the same total reduction ratio, CSPR-processed aluminium sheets have the same yield stress but improved uniform elongation (2.4 times). The scanning electron microscopy was used to study the fracture surface, and The transmission electron microscopy to examine the microstructure near the fracture end, in order to analyse the improvement in ductility. In addition, the finite element method was used to simulate the roll-sample contact pressure and strain distribution as well as residual stress on the sheet surface during CSPR, and to better understand the mechanism leading to improvement of ductility of the sheets by the CSPR technique.

The mechanical properties of the Ti-6Al-4V alloy are significantly determined by the process used for manufacture. In this study, the mechanical properties of the Ti-6Al-4V alloy manufactured using cold rolling, asymmetric rolling and asymmetric cryorolling were characterized by subjecting it to the tensile test and the microhardness test. The evolution in the microstructure was studied using scanning electron microscopy, transmission electron microscopy as well as energy dispersive X-ray spectrometry. Results show that the Ti-6Al-4V alloy sheets subjected to asymmetrical cryorolling with low rolling speed ratio between the upper and lower rolls have the highest tensile strength and microhardness. The tensile stress of the alloy samples after cold rolling was found to be 1008 MPa, 1046 MPa after asymmetric rolling, and 1113 MPa after asymmetric cryorolling. The highest Vickers hardness (HV395) is achieved by asymmetric cryorolling. We discuss the mechanical behavior using the criteria of grain size, size of second phase as well as dislocation density.