Lihong Su

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.

Superstrength of nanograined steel with nanoscale intermetallic precipitates transformed from shock-compressed martensitic steel.

An increasing number of industrial applications need superstrength steels. It is known that refined grains and nanoscale precipitates can increase strength. The hardest martensitic steel reported to date is C0.8 steel, whose nanohardness can reach 11.9 GPa through incremental interstitial solid solution strengthening. Here we report a nanograined (NG) steel dispersed with nanoscale precipitates which has an extraordinarily high hardness of 19.1 GPa. The NG steel (shock-compressed Armox 500T steel) was obtained under these conditions: high strain rate of 1.2 μs−1, high temperature rise rate of 600 Kμs−1 and high pressure of 17 GPa. The mean grain size achieved was 39 nm and reinforcing precipitates were indexed in the NG steel. The strength of the NG steel is expected to be ~3950 MPa. The discovery of the NG steel offers a general pathway for designing new advanced steel materials with exceptional hardness and excellent strength.

A Fiber-Coupled Self-Mixing Laser Diode for the Measurement of Young’s Modulus

This paper presents the design of a fiber-coupled self-mixing laser diode (SMLD) for non-contact and non-destructive measurement of Young’s modulus. By the presented measuring system, the Young’s modulus of aluminum 6061 and brass are measured as 70.0 GPa and 116.7 GPa, respectively, showing a good agreement within the standards in the literature and yielding a much smaller deviation and a higher repeatability compared with traditional tensile testing. Its fiber-coupled characteristics make the system quite easy to be installed in many application cases.

Transverse and z-direction CVN impact tests of X65 line pipe steels of two centerline segregation ratings

Centerline segregation occurs as a positive concentration of alloying elements in the mid-thickness region of continuously cast slab. Depending upon its severity, it may affect mechanical properties and potentially downstream processing such as weldability, particularly for high-strength line pipe. The segregation fraction in continuously cast slabs and corresponding hot-rolled strips was assessed on API 5L grade X65 line pipe steels with different levels of segregation, rated as Mannesmann 2.0 and 1.4. The results showed that the segregation fraction in hot-rolled strip samples was in accordance with that assessed in the cast slabs, and the segregated regions in hot-rolled strip samples were found to be discontinuous. Transverse and z-direction CVN impact tests were conducted on the two strips and the results showed that centerline segregation does have an influence on the Charpy impact properties of line pipe steel. Specimens located at segregated regions exhibited lower Charpy impact toughness and strips rolled from slabs with higher segregation levels are more likely to exhibit greater variability in Charpy impact toughness. The influence of centerline segregation on z-direction Charpy impact toughness is more severe than on transverse Charpy impact toughness. Lower Charpy impact toughness and brittle fracture surface with cleavage facets along with rod-shaped MnS inclusions were observed for the strip rolled from slab with 2.0 segregation rating if the Charpy specimens were located at segregated regions. The influence on Charpy impact toughness can be associated with the pearlite structure at the centerline and level of MnS inclusions.

Numerical evaluation of a high speed steel work roll during hot strip rolling process

Hot strip rolling process is one of the most promising industrial processes to fabricate finished or semi-finished bulk products. Numerical analysis on the temperature and thermal stress distributions in a high speed steel work roll during hot rolling has been conducted based on a transient thermo-mechanical model. Influence of initial work roll body temperature on temperature and thermal stress has been discussed in detail by assuming different rolling stages. Compared to the work roll surface, stress is much smaller at depth of 2.1 mm and 5.0 mm, respectively. Results showed similar maximum circumferential thermal stress at both depths of 2.1 mm and 5.0 mm when the roll has initial temperature of 25 °C and 100 °C, but they are about 3 times and 8 times larger than at depth of 2.1 mm and 5.0 mm, respectively, when the initial temperature is 200 °C.

Optimization and application of process parameters in an AZ61 alloy twin-roll strip casting

Twin-roll strip casting is a concerned technology for economically producing magnesium alloys sheets. In this paper, numerical simulation of the twin-roll strip casting of an AZ61 magnesium alloy was carried out and the optimal process parameters were obtained. Then, under the conditions obtained through simulation, AZ61 strips of good surface quality were successfully manufactured. The microstructure of the alloy by twin-rolled strip casting is obvious refined compared with that by conventional casting.

A Study on the Aluminum Alloy AA1050 Severely Deformed by Non-Equal Channel Angular Pressing

In order to improve the efficiency of grain refinement, a study on the modified process (called Non-equal channel angular pressing) from the conventional equal channel angular pressing has been conducted. The deformation behavior of aluminum alloy AA1050 deformed by the Non-equal channel angular pressing which has a smaller width in the exit channel than the entry channel was examined based on the finite element simulations. The results revealed that a smaller ratio of dE and dI (dE/dI) leads to a larger equivalent plastic strain. It is not only beneficial to enhance the plastic deformation but also very helpful to get rid of the development of dead zone in the outer corner of die by decreasing the exit channel width by comparing with the conventional process.

Residual Stress Study of Al/Al Laminates Processed by Accumulative Roll Bonding

In this work accumulative roll bonding (ARB) was used to combine AA1050 and AA6061 sheets to produce AA1050/AA1050, AA6061/AA6061 and AA1050/AA6061 laminates with ultrafine grained (UFG) structure. Two sheets of starting materials were roll bonded with 200 °C preheating for 180 s before rolling. The through-thickness residual stress distribution of these laminates processed up to two cycles of ARB was determined by neutron diffraction with spatial resolution of 0.2 mm through 1.5 mm thickness. The measurements also required high accuracy of only few MPa since residual stresses in the laminates peaked at only about 15 MPa. The laminates composed of the same material (AA1050/AA1050 and AA6061/AA6061) showed symmetric residual stress profile with tensile stress at the centre of the sheets and compressive stress at the surfaces. The AA1050/AA6061 laminates showed asymmetric distribution with residual tensile stress in the AA1050 layer and compressive stress in the AA6061 layer. A finite element model (FEM) was used to simulate the residual stress distribution and the results were in agreement with the measured results qualitatively.

An insight into the deformation and orientation development of severely plastic deformed aluminum

Severe plastic deformation (SPD) techniques have attracted significant attention in the last two decades due to its capacity of producing bulk ultra-fine grained (UFG) or nanostructured materials, by imposing very high plastic strain. Equal channel angular pressing (ECAP), or equal channel angular extrusion (ECAE), is one of the most widely used SPD techniques. During ECAP, a sample is pressed through a die which consists of two channels with equal cross-section intersecting at an angle varying from 60° to 150°. Since the cross-section geometry of the sample remains nearly unchanged, the materials can be deformed to a very high strain by repeating the process. The deformation mechanism of ECAP is very complicated and it is dependent on the die geometry and material properties. It has been assumed as simple shear on the intersecting plane of two channels at ideal conditions, such as frictionless, perfectly plastic material and very sharp outer corner. However, from the view of texture evolutions, many expe...

Grain Refinement in the Formability of Aluminium Thin Cup

Refinement of the grain diameter of the micro structure of Aluminium foil with a thickness of 300μm has been done through the ARB process up to fourth cycles with 72 layers that are proven to increase formability in micro forming a cup. Grain size was measured from the full annealed condition, of the ARB process, and the results of ARB process followed by stress relieved. Formability of the formation of a cup which is expressed as the LDR has increased from 1.87 for the material conditions of full annealed to 2.00 for the ARB process followed by stress relieved in a single step process. In addition to improved formability obtained in a cup formation, grain refinement in the microstructure can also reduce cup earing and wrinkle on the cup wall.