Hailiang Yu

Fabrication of Nanostructured Aluminum Sheets Using Four-Layer Accumulative Roll Bonding

In this paper, an extended accumulative roll bonding (ARB) technique, called the ‘Four-Layer Accumulative Roll Bonding (FL-ARB)’ technique, is presented for the first time. This technique has been employed to produce ultrafine-grained commercial pure aluminum sheets with success. After three FL-ARB passes, the grain size of pure aluminum was seen to reduce to 380 nm. The bonding strength of the sheets after rolling has also been discussed. Theoretical calculations showed that the bonding strength of sheets processed by the FL-ARB technique can be 2–2.2 times greater than that by the traditional ARB technique. The main advantages of the FL-ARB technique are (a) improvement of the interface bonding, with increasing deformation in each pass, (b) applicability of the technique at room temperature to process most metals, and (c) generation of the largest equivalent strain in the workpiece with the same number of passes, compared with other severe plastic deformation techniques.

Fabrication of ultra-thin nanostructured bimetallic foils by Accumulative Roll Bonding and Asymmetric Rolling

This paper reports a new technique that combines the features of Accumulative Roll Bonding (ARB) and Asymmetric Rolling (AR). This technique has been developed to enable production of ultra-thin bimetallic foils. Initially, 1.5 mm thick AA1050 and AA6061 foils were roll-bonded using ARB at 200°C, with 50% reduction. The resulting 1.5 mm bimetallic foil was subsequently thinned to 0.04 mm through four AR passes at room temperature. The speed ratio between the upper and lower AR rolls was 1:1.3. The tensile strength of the bimetallic foil was seen to increase with reduction in thickness. The ductility of the foil was seen to reduce upon decreasing the foil thickness from 1.5 mm to 0.14 mm, but increase upon further reduction in thickness from 0.14 mm to 0.04 mm. The grain size was about 140 nm for the AA6061 layer and 235 nm for the AA1050 layer, after the third AR pass.

Crack Healing in a Low-Carbon Steel Under Hot Plastic Deformation

The behavior of internal crack healing in low-carbon steel samples undergoing hot plastic deformation was investigated using the MMS 200 thermo-mechanical simulator. The characterization of cracks after plastic deformation was analyzed using scanning electron microscopy under different heating temperatures, reduction ratios, numbers of deformation passes, strain rates, and holding time durations. It was found that the degree of crack healing increases with increasing heating temperature, reduction ratio, and holding time duration, and with decreasing number of deformation passes and strain rate.

Thermal―Mechanical Finite Element Analysis of Evolution of Surface Cracks During Slab Rolling

Closure and growth of transverse cracks on slab surface severely affects the quality of rolled products. A finite element (FE)-based modeling on thermal and mechanical behavior of transverse cracks during hot rolling has been carried out using LS-DYNA software. The behavior has been simulated at a range of crack sizes, crack open-angles, friction factor between roll and slab. The maximum compressive and tensile stress at crack tip during rolling, the crack open-angle and the crack height after rolling, and the temperature on crack surface during rolling were analyzed. The mechanism of crack behavior during rolling is discussed.

Tensile fracture of ultrafine grained aluminum 6061 sheets by asymmetric cryorolling for microforming

The size effect on the mechanism of fracture in ultrafine grained sheets is an unsolved problem in microforming. This paper describes a tensile test carried out to study the fracture behavior and the shear fracture angles of both rolled and aged ultrafine grained aluminum 6061 sheets produced by asymmetric cryorolling. A scanning electron microscope was used to observe the fracture surface. The finite element method was used to simulate the tensile test using the uncoupled Cockcroft–Latham and Tresca criteria and the coupled Gurson–Tvergaard–Needleman damage criterion. It was found that the shear fracture angle decreases gradually from 90° to 64° with an increasing number of passes. The results of simulations using the Gurson–Tvergaard–Needleman criterion show trends similar to the experimental ones. The paper also presents a discussion on the fracture mechanism and the size effect during the tensile test.

A new insight into ductile fracture of ultrafine-grained Al-Mg alloys

It is well known that when coarse-grained metals undergo severe plastic deformation to be transformed into nano-grained metals, their ductility is reduced. However, there are no ductile fracture criteria developed based on grain refinement. In this paper, we propose a new relationship between ductile fracture and grain refinement during deformation, considering factors besides void nucleation and growth. Ultrafine-grained Al-Mg alloy sheets were fabricated using different rolling techniques at room and cryogenic temperatures. It is proposed for the first time that features of the microstructure near the fracture surface can be used to explain the ductile fracture post necking directly. We found that as grains are refined to a nano size which approaches the theoretical minimum achievable value, the material becomes brittle at the shear band zone. This may explain the tendency for ductile fracture in metals under plastic deformation.

An Investigation of Interface Bonding of Bimetallic Foils by Combined Accumulative Roll Bonding and Asymmetric Rolling Techniques

The bond strength in bimetallic materials is an important material characteristic. In this study, 0.1-mm thick bimetallic foils (AA1050/AA6061) were produced using one pass of accumulative roll bonding followed by three passes of asymmetric rolling (AR). The AR passes were carried out at roll speed ratios of 1.0, 1.1, 1.2, 1.3, and 1.4 separately. Finite element simulation was used to model the deformation of the bimetallic foils for the various experimental conditions. Particular attention was focused on the bonding of the interface between AA1050 and AA6061 layers in the simulation. The optimization of the roll speed ratio was obtained for improvement of the bond strength of the interface of AA1050/AA6061 bimetallic foils during AR process. In the simulation, the mean equivalent strain at the interface zone between the AA1050 and AA6061 layers was seen to reach a peak value at a roll speed ratio of about 1.2 to 1.3, which also corresponded to a high quality bond at the interface as observed experimentally.

Rolling Technology with Reducing Resources in China

In recent years, steel production has increased rapidly in China, to approximately 489 Mt in 2007. However, any further growth in production presents problems related to the shortage of energy and resources, environmental pollution, and the high cost of raw materials. In view of this, the government of China has changed its steel policy from tax return for export of steel products to adding extra tax on it. The steel companies in China are now forced to devote more attention to reducing the cost of steel production. This articleputs forward idea of rolling steel with declining resources. It includes: 1) Flexible Rolling Technology (FRT) based on the technology of online control of microstructure and properties, which will facilitate using a slab with the same chemical composition to produce the steel products of different properties by means of using different rolling and cooling parameters; 2) new generation cooling system and new generation Thermo-Mechanical Control Processing (TMCP), including High Rate Cooling (HRC) and Cooling Path Control (CPC) for plate mill, hot strip mill, and bar mill; 3) development of high performance steel products with low cost, such as super steel, fine grained steel, and Advanced High Strength Steel (AHSS), such as Dual Phase (DP) and TRIP steels with simple chemical composition. The key points are to save energy and reduce resource consumption (especially the consumption of microalloying elements), aiming at reducing manufacturing cost and helping to protect and conserve the environment, which is considered to be the best way to ensure sustainable development of steel rolling industry.

Experimental and Theoretical Study on Minimum Achievable Foil Thickness during Asymmetric Rolling

Parts produced by microforming are becoming ever smaller. Similarly, the foils required in micro-machines are becoming ever thinner. The asymmetric rolling technique is capable of producing foils that are thinner than those produced by the conventional rolling technique. The difference between asymmetric rolling and conventional rolling is the ‘cross-shear’ zone. However, the influence of the cross-shear zone on the minimum achievable foil thickness during asymmetric rolling is still uncertain. In this paper, we report experiments designed to understand this critical influencing factor on the minimum achievable thickness in asymmetric rolling. Results showed that the minimum achievable thickness of rolled foils produced by asymmetric rolling with a rolling speed ratio of 1.3 can be reduced to about 30% of that possible by conventional rolling technique. Furthermore, the minimum achievable thickness during asymmetric rolling could be correlated to the cross-shear ratio, which, in turn, could be related to the rolling speed ratio. From the experimental results, a formula to calculate the minimum achievable thickness was established, considering the parameters cross-shear ratio, friction coefficient, work roll radius, etc. in asymmetric rolling.

High Strength and Ductility of Ultrathin Laminate Foils Using Accumulative Roll Bonding and Asymmetric Rolling

As product miniaturization is becoming widely popular, many microparts are being produced by microforming of sheets/foils, whose strength needs to be able to maintain structural stability of the micro-components. In addition, their strength and ductility of foils generally reduce with a reduction in the thickness due to the size effect. In this paper, we report the fabrication of an aluminum laminate foil using a combined process of accumulative roll bonding (ARB) and asymmetric rolling (AR). It was found that this improves both strength and ductility. TEM results show that the laminate structures produced by ARB develop an inhomogeneous microstructure with nanoscale grains and abnormal coarsening in some grains during AR processing. Both these effects result in an improved ductility and strength. Using these rolled products, micro-cups of very small wall thickness/cup diameter ratio (1/200) have been successfully fabricated by micro-deep drawing without the need for annealing.