Development of a novel low-pressure lapping method for superfinishing aluminium alloys

Abstract

Aluminium alloy (Al6061) is a common material used for reflective mirrors. It can be machined with a good surface finish by Single Point Diamond Turning (SPDT). As a high optical reflectance is needed for mirrors. Due to the material is relatively soft, it is difficult to apply post-processing such as ultra-precision lapping and ultra-precision polishing, as they may scratch the diamond turned surface. In addition, a recent study showed that the reflectance on a diamond turned aluminium alloy (Al6061) does not have a simple correlation with good surface finish. The highest reflectance is found on a surface with small tool marks, but how the tool marks affect the optical properties is not fully known. In this study, a novel low-pressure lapping method has been developed to reduce the surface roughness and preserve the reflectance at the same time. In this thesis, the research work consisted of four parts. The first part focuses on the study of the influence of feed rate on the surface roughness and reflectance of Aluminium alloy (Al6061) by single-point diamond turning (SPDT). The results show that even through the surface roughness is decreased when the feed rate is reduced. When the feed rate is lower than a critical value, the reflectance is also decreased due to the Mg2Si particles being embedded on the machined surface rather than being removed by the cutting. As a result, the low-pressure lapping technology in this research is focused on reducing the surface roughness and preserving the reflectance of the aluminium alloy (Al6061) surface is machined under the critical feed rate. The second part of the research focuses on the investigation of the relationship between the lapping parameters and surface roughness of the aluminium alloy (Al6061) surface by using a novel lapping technology. This novel lapping method uses a brush to drive the abrasive particles over the workpiece surface. During the lapping process, the fiber on the rotating brush provides a small normal force and sufficient lateral force for the abrasive particles to roll at high speed on the surface of the workpiece. When the moving abrasive particles reach adequate speed, they impinge and damage the asperities of the surface and eventually wear away the material. In this section, Taguchi method was used to determine the optimal lapping parameters. The lapped surface under the optimal lapping parameters was further analyzed by the spectrum analysis method so as to evaluate the performance of the lapping technology. The results showed that the new low-pressure lapping technology can successfully remove the tool marks, reduce the surface roughness and preserve the reflectance of aluminium alloy (Al6061) surfaces. In the third part of the research, the mechanism of the lapping technology was investigated. To gain a deeper understanding of the surface generation mechanism, the hydrodynamic effect, different lapping tools and slurry in the lapping technology were investigated. The influence function was further studied in this chapter. Finally, the surface generation mechanism was discussed. A Finite Element model has been built to simulate the mechanism of the lapping technology in the fourth part of the research. The simulation results were compared with the experimental results so as to gain a better understanding of the lapping mechanism. The present study does not only contribute to determine the relationship between the optical reflectance and surface finish on machining aluminium alloy (Al6061), but also contributes significantly in the development of a novel low-pressure lapping technology which can be used to lap soft aluminium alloy (Al6061) workpiece with nanometric surface finish. The results provide an important means for superfinishing of soft materials.