Fabrication of Ultrathin Metallic Sheets by Accumulative Pack Rolling

Ultrathin metal foils with thicknesses down to a few microns are used for a wide range of applications, including batteries, cable and component shielding and packaging. These foils are difficult to be produced using conventional rolling method because the minimum achievable thickness of conventional rolling is generally larger than the required thickness. More complex rolling techniques such as use of Sendzimir mills or use double rolling method. However, the Sendzimir mills are expensive to set up, the double rolling method still has a thickness limit. Severe plastic deformation (SPD) methods such as accumulative roll bonding (ARB) has also been investigated to produce ultrathin metal foils. The method could reduce the metal layers to ultrathin level but it often requires additional separation process to extract useful metal layers. It makes the process complex and limiting the manufacturability of some reactive metal materials.

In our study, we proposed a novel accumulative pack rolling (APR) process, which has successfully been applied to fabricate ultrathin metal foils. By repetitively rolling sandwich sheet and replacing the pack sheet, extreme high reductions could be achieved. This work investigated the minimum achievable thickness, surface morphology, microstructure, texture, and defects that develops in the APR process.

The APR method demonstrated remarkable capability in reducing the minimum achievable thickness of metal foils. For pure Al, Cu, Zn and Ni, the minimum achievable thickness could reach 1-3 μm. Study of the surface profiles showed that during the APR process, material surfaces underwent a noticeable roughness increase as a result of plastic deformation. This phenomenon was typically more pronounced in the early stages of deformation. The surfaces could exhibit peak-valley type patterns or particle like forms. The former could evolve to defects such as pinholes and cracks, leading to early failure and affecting the minimum achievable thickness. The peak-valley type of surface patterns could be supressed by replacing pack sheets after certain reduction, usually between 60% to 80%. Then the pinhole formation could be hindered and the thickness reduction could continue.

To explain the surface roughening observed in the APR process, we drew insights from research on surface roughening during deformation of single layer metal sheets, as well as interface roughening or thickness irregularity in multilayer sheet deformation. It has been found that plastic instability is the primary reason for interface roughening or irregularity in the stacked metal sheets. At the macro scale, this is mainly attributed to differences in material flow properties between layers. At the micro scale, through Electron Backscatter Diffraction (EBSD) analysis of cross-sections of APR copper foils, we found that differences in grain flow properties resulting from variations in grain orientations can also lead to interface roughening. Numerical simulation techniques were also employed to validate this conclusion. We constructed polycrystalline models to simulate the deformation process of multilayer sheets and the results verified that differences in mechanical properties between grains or differences in the soft and hard orientations significantly influenced the surface roughening behaviour at the interface.

Using the developed APR method and the theory of surface/interface roughening, we successfully produced AA1050/Ni and AA7075/Ni multilayered composite sheets with good layer continuity. Multilayered metal composites are usually fabricated by ARB. The method has some drawbacks such as the harder layer usually experiences necking and fracturing during the process so the layer continuity is poor. This could severely influence the application of materials such as Al/Ni composites as they discontinued layers could reduce the amount of heat released during exothermal reaction. Two strategies were adopted in our work: one is to use AA7075, whose mechanical properties are close to Ni, and the other is to use AA1050, but the AA1050 and Ni sheets were first reduced to thicknesses of 5-10 μm and then subjected to roll bonding to prepare AA1050/Ni multilayers. The Al/Ni multilayers maintained excellent layer continuity in both material combinations, with AA7075/Ni outperformed the AA1050/Ni sheets. The heat release of AA7075/Ni sheets in the differential scanning calorimetry (DSC) experiments also showed values close to the calculated theoretical values. This combined process of APR and roll bonding helps to control thickness irregularity during multilayer sheet deformation processes.