Effects of lubrication and strain path on constrained groove pressing of commercially pure aluminum sheets

Constrained groove pressing (CGP) is a promising method for fabricating ultra-fine grained sheet metals based on severe plastic deformation (SPD). However, its efficiency is lower than other SPD methods. Thus, in this work, the effects of lubrication and strain path on CGP of commercially pure aluminum sheets were investigated via experimental and numerical methods, and it was expected to explore a suitable way of improving its efficiency of grain refinement and property enhancement. By either using Teflon layers as lubricant or importing cross-CGP procedure, an effective pass number of six was successfully imposed to the sheets without distinct cracks appearing at the surface. The elongation to failure of the sheets was improved by both modifications while the strength level was much lower when using Teflon layers even than that obtained by an originally four-pass parallel-CGP with MoS2 as lubricant. This revealed that an appropriate contact condition between the dies and sheet sample should be determined to ensure a sufficient and uniform shear deformation during CGP. The optimum tensile properties with yield strength of about 120 MPa and elongation of about 10% were obtained during the last passes of cross-CGP. TEM and electron backscatter diffraction results also demonstrated that the best grain refinement was achieved by cross-CGP indicated by a homogeneous distribution of regular and equiaxed subgrain sizes and a high proportion of high-angle grain boundaries. It was concluded that the constant rotation of pressing orientation during cross-CGP contributed a lot to the higher process efficiency through introducing a more complicated deformation field and a more homogeneous distribution of equivalent plastic strain.