Degree Name

Doctor of Philosophy


School of Mechanical, Materials and Mechatronics Engineering


Surface roughness affects surface quality and wear and fatigue of metal forming products. The objective of this study is to improve our understanding of the evolution of surface asperity (surface roughness) during cold uniaxial planar compression of aluminium samples. Experimental research and simulation analysis were applied to study the evolution of surface asperity during compression.

Uniaxial planar compression experiments of FCC (Face Centred Cubic) aluminium alloy have been carried out using the designed compressing equipment on an INSTRON 8033 Material Testing Machine to examine the characteristics of surface asperity during compression. These characteristics have been investigated by instruments such as an Atomic Force Microscope (AFM), a 3D profile meter, a Vickers hardness tester, and surface profile meter. The Field Emission Guns- Scanning Electron Microscope (FEG-SEM) with Electron Backscattered Diffraction (EBSD) technique was also used to analyse the surface microstructures of cold, uniaxial planar compressed samples.

The results obtained from these experiments show that compressing parameters such as the reduction (strain), strain rate (deformation velocity), friction (lubrication), and wavelength (a technical term of surface roughness, refers to the length of surface roughness wave because the surface roughness curve is the filtered wave, which has the features of wave) have significant effects on the evolution of surface asperity (surface roughness). In uniaxial planar compression, surface asperity evolves in three stages: when reduction is less than 10 %, plastic deformation takes place in an aggregate of elastic deformation. Elastic deformation plays an important role in the evolution of surface asperity. When reduction exceeds 10 % the influence of plastic deformation on surface asperity becomes obvious, while the influence of elastic deformation is insignificant. When reduction exceeds 40 %, and continues to increase, there is no significant decrease in surface roughness (Ra). The influence of lubrication on the evolution of surface asperity is also very complicated. For example, when reduction is small (10 %), the layer of lubrication has no obvious influence on surface roughness of the sample. However, when reduction is in a certain range (in this study, it is from 10 to 40 %), lubrication can significantly limit the flattening of surface asperity. When reduction exceeds 40 %, the lubrication is squeezed out and has no obvious effect on the evolution of surface asperity. In cold uniaxial planar compression (CUPC), the relationship between surface roughness Ra and strain is non-linear, which is different from the tensile experiment. The influence of strain rate on surface asperity flattening can be divided into the elastic and plastic deformation stages. During elastic deformation, strain rate has no significant influence on the evolution of surface asperity, but during plastic deformation, an increasing strain rate can lead to a larger surface roughness. At a lower strain, an increasing strain rate can result in a higher hardness, while at a larger strain (60 %) this tendency is reversed, an increasing strain rate leads to a lower hardness. Under the same reduction, an increasing strain rate can lead to a higher flow stress.

Surface roughness shows an obvious sensitivity to the orientation of grains near the surface. In this study the oriented {111} grains are the main source of localised strain. With an increase in reduction, the grain size tends to decrease and the cubic texture {001}is weak, while the brass orientation {110}becomes stronger. At the same time the Schmid factor (also called “orientation hardness”) of the surface area will shift from “hard” (about 0.3) to “soft” (about 0.5). When reduction exceeds 40 %, the in-grain shear band appears in some grains which are 4- 5 grains away from the edge and localised strain starts in this area. When reduction exceeds 60 %, most grains have plastic slips, and a few transgranular shear bands that resulted from the deformation twins of brass orientation were observed in the surface area. Under the same reduction, the surface roughness of the sample with a large grain size tended to decrease very slowly, but after compression, the sample with the largest grain size had the maximum surface roughness. The surface roughness Ra is a power exponent function of grain size. Influences of recovery and recrystallisation are not significant in the CUPC process of aluminium.

The initial data of the sample surface roughness, friction, and microstructures (textures and grain orientations) have been applied in the 2D and 3D crystal finite element models. The simulated results obtained from the experimental results confirmed the influence of deformation parameters (reduction, lubrication, strain rate, and wavelength), and microstructures (textures and grain orientations) on the evolution of surface asperity in uniaxial planar compression.

It is recommended that BCC (Body Centered Cubic) metal and Hexagonal close-packed (hcp) metals such as magnesium (Mg) are used to analyse the evolution of surface asperity in uniaxial compression with different deformation parameters, friction state, and microstructures. The initial surface roughness of compression tool should also be discussed, although there are some difficulties with simulation, and various lubricants should be used to study how they affect the evolution of surface asperity. 3D EBSD mapping by the FIB (Focus Ion Beam) technique is also recommended as a good method to show the practical development of surface microstructure in three dimensions. High temperature deformation processes are also needed to analyse the influence of temperature and oxidation on the evolution of surface asperity, with the revised crystal plasticity constitutive model. The effect of the workpiece size is discussed.