Formation and evolution of a subsurface layer in a metalworking process

Abstract

The effect of metalworking on the structure and composition of the near-surface workpiece region was studied using, as an example, the hot rolling of an aluminum alloy. In this work, it was found that hot rolling resulted in the formation of a subsurface layer 1.5–8 μm thick with structure and properties different from those of the underlying bulk metal. Electron microprobe analysis, transmission electron microscopy, and electron diffraction studies showed this layer to be composed of magnesium and aluminum oxides mixed with the metal, and to be non-uniform in thickness and composition. The thickness of the subsurface layer gradually decreased in the sequential stages of hot rolling. The metal grain size ranged from 0.04 to 0.2 μm, which was at least 25 times smaller than the grain size of the underlying bulk material. The oxides in the subsurface layer were found to contain, in addition to amorphous Al 2O 3, a mixture of crystalline oxides. The particle size of the crystalline oxides in the subsurface layer was found to be in the range from 28 to 300 Å, with almost half of the particles in the range 28–33 Å. After the first passes the oxide mixture was composed of MgO, γ-Al 2O 3, and MgAl 2O 4 spinel, later of MgO and γ-Al 2O 3, and after the last pass only of MgO. The subsurface mixed layer was covered with a continuous surface oxide layer 250–1600 Å thick containing MgO and Al 2O 3. A hypothesis that the subsurface grain growth was retarded by Zener pinning by the small oxide particles was suggested and tested. Image analysis of transmission electron micrographs was used to obtain the size distribution and volume fraction of small oxide particles used to test this hypothesis. The experimental metal grain size was found to be in good agreement with the results of three-dimensional computer simulation studies of grain growth inhibition reported by Anderson et al., in literature which supported the Zener pinning hypothesis.