Surface Micromechanics of Polymer Coated Aluminium Sheets during Plastic Deformation By Dierk Raabe,* Michael Sachtleber , Luis F. Vega, and Hasso Weiland Plastic straining of polymer-coated polycrystalline metals is accompanied by the development of microstructural de- fects at the sample surface as well as in the interface between the metal and the polymer. These defects are due to a variety of dynamical mechanisms which are essentially induced by bulk plasticity of the metal substrate. They micromechani- cally interact with the polymer coating and transfer some of the metallic roughness through that layer to the surface. It is pertinent in this context to differentiate between intrin- sic and extrinsic defects at the surface and in metal-polymer interface. Intrinsic defects induced by bulk plasticity occur in the form of net residual surface displacement fields as a result of microstructure dynamics during plastic straining. Exam- ples are glide steps, shear bands, or grain cluster deformation patterns. As extrinsic defects we denote all surface changes that occur through the influence of the environment, i.e. by mechanical contact (tools, friction, non-homogeneity of forces and material flow) or by corrosion. Intrinsic surface and metal-polymer interface defects occur at different spatial scales and can have different microstruc- tural origin, similar to the corresponding hierarchy of bulk defects associated with plasticity. Figure 1 and Table 1 show some examples concentrating on those defects which origi- nate from plastic deformation of the metal substrate. They include interface defects generated by elastic distortions, point defects, atomic slip steps, crystallographic slip steps created by sets of dislocations on parallel or identical glide planes in the metal (Fig. 1a), athermal transformations, mechanical twinning (Fig. 1b), non-crystallographic glide traces caused by dislocation bands which contain slip activity on parallel and non-parallel glide systems, fracture and delamination, orange peel phenomena where metallic crys- tals abutting on the metal-polymer interface undergo individ- ual out-of-plane displacements (Fig. 1c), individual surface deformations by hard and soft phases, as well as ridging and roping phenomena, which are characterized by the collective deformation of larger sets of grains typically resulting in a banded topology (Fig. 1d). Grain-scale micromechanical effects at the sheet surface or respectively metal-polymer interface are essentially deter- mined by orange peel and ridging phenomena. They decide about surface roughness, friction conditions during forming, strain localization, optical appearance, and failure of con- structional materials at an engineering scale. For this reason we concentrate in this article on grain-scale effects. We pre- sent experimental results on the plastic deformation of coated aluminium sheets. The work aims at understanding the microstructural and micromechanical fundamentals of the co-deformation of polycrystalline aluminium sheets (1 mm thickness) and siloxane layers (1.5 lm). Important aspects in this investigation are the heterogeneity of microstructure and microstrains in the aluminium sheets and the resulting sur- face roughness of the coated and uncoated materials after plastic straining. We used a set of Al-Mg-Si (AA6022, T4 con- dition) aluminium samples with identical composition but different processing history and microstructures. One of the materials (sample A) represents production conditions while the other (sample D) was fabricated using outdated experi- mental conditions. The experiments include the measurement of the accumulated plastic surface microstrains (photograme- try), surface topography (whitelight confocal microscopy), particle distribution, microtexture (orientation mapping by electron back scatter diffraction), and grain size distribution in the same sample regions. Spin-off from such studies may be expected for products where microstructures, mechanics, and engineering of struc- tural lightweight materials with functional polymer layers play a key role such as for instance in the automotive, illumi- nation, packaging, food, construction, and design industries. Particular attention must be placed in this context on the growing interest in the finish first ± fabricate later philosophy. This new approach means that sheet metal is first coated and subsequently formed to a high quality surface finish. This new method of manufacturing semi-finished parts requires that both, the sheet substrate metal and the coating(s) preserve good surface appearance during forming. Furthermore, the surface roughness evolving during plastic loading has strong influence on friction, thereby affecting phenomena such as springback, strain localization, and failure during medium and large strain sheet metal forming operations. Figure 2a shows the surface distribution of the accumu- lated plastic equivalent strain for two of the afformentioned samples (referred to as A and D) in the uncoated condition after 7 % plastic straining in transverse direction (flat tensile test). The in-plane strain distribution was calculated from the displacement field which was determined by use of photo- grametry. [1] The production representative material (Sample A) reveals a rather homogenous distribution of the von Mises strain with only modest equiaxed localizations in the sheet COMMUNICATIONS ADVANCED ENGINEERING MATERIALS 2002, 4, No. 11 859 ± [*] Prof. D. Raabe, M. Sachtleber Max-Planck-Institut für Eisenforschung Max-Planck-Str. 1, 40237 Düsseldorf (Germany) Dr. L. F. Vega, Dr. H. Weiland Alcoa Technical Center 100 Technical Drive Alcoa Center, PA 1506 (USA) Ó 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1438-1656/02/1111-0859 $ 17.50+.50/0