| Author: | Guo, Sai |
| Title: | Microstructural evolution, surface integrity and material removal mechanisms in high-speed precision grinding of Al/SiCp metal matrix composites |
| Advisors: | Cheung, C. F. Benny (ISE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Machining Grinding and polishing Composite materials Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Industrial and Systems Engineering |
| Pages: | xxv, 155 pages : color illustrations |
| Language: | English |
| Abstract: | Silicon carbide (SiC) particle reinforced aluminum (Al) metal matrix composites (Al/SiCp MMCs) represent a new generation of materials with significant technological importance owing to their desirable properties. Al/SiCp MMCs are therefore being used to replace conventional materials in various engineering applications due to the increasing performance requirements. However, the heterogeneous constituents of hard reinforced SiC particles and ductile Al alloy matrix in Al/SiCp MMCs leads to serious surface defects and subsurface damage (SSD) in conventional machining techniques, thus substantially deteriorating the mechanical and fatigue properties of the machined materials. To address the problems in conventional machining of Al/SiCp MMCs, high-speed grinding (HSG) is used due to the promising benefits of an increased grinding speed in improving surface integrity of ductile and brittle materials according to extensive studies. However, research on the variation of microstructural evolution, surface integrity and material removal mechanisms with grinding speed in Al/SiCp MMCs has not been sufficiently investigated considering the synergetic influences of size, thermal, and strain-rate effects. An approach has not been identified for efficiently revealing the microstructural evolution and SSD without introducing extraneous damage during SSD characterization. Moreover, SSD features such as dislocations, stacking faults and cracks at a subgrain scale for different constituents of Al/SiCp MMCs have not been adequately analyzed. In this regard, this study aims to carry out HSG on Al/SiCp MMCs to investigate the effect of grinding speed on microstructural evolution, surface integrity and material removal mechanisms. Extensive characterization methods are employed to comprehensively reveal the microstructural evolution and features of surface defects and SSD at multiple scales. Factors affecting surface and subsurface formation are revealed and separately analyzed to explain the evolution of microstructure and surface integrity of Al/SiCp MMCs with increasing grinding speed. In this thesis, three parts are included to elaborate two different grinding methods carried out on two types of workpieces. In the first part, Al6061T6 alloy with a single ductile phase is used to perform preliminary investigation on the influence of speed in HSG of a ductile material. Since Al6061T6 alloy is commonly known as a difficult-to-grind material due to its high ductility, this study carries out HSG on the Al6061T6 to explore microstructural evolution, surface integrity and material removal mechanisms under high strain-rate conditions. It reveals that surface morphology varies from surface smearing to grinding streaks, reflecting the increased surface quality. The refined equiaxed nanograins in the ground subsurface is ascribed to the mechanism of continuous dynamic recrystallization (cDRX). In addition, the decreased depth of discontinuous dynamic recrystallization (dDRX) with an increase in grinding speed is attributed to the high strain-rate field and the reduced depth of the heat-affected layer below the ground surface, indicating a result of damage skin effect. The results imply that better surface integrity with a reduced SSD depth can be attained through HSG, which lays down a solid foundation for further investigation in HSG of Al/SiCp MMCs. In the second part, HSG is performed at speeds up to 307.0 m/s to achieve high-efficiency, high-accuracy, and low-damage machining of Al/SiCp MMCs. The corresponding microstructural evolution, surface integrity and material removal mechanisms are investigated considering surface morphology, surface roughness, and SSD at different scales. The alteration of surface defects and SSD with increasing grinding speed is clarified. For the first time, the material removal mechanisms of Al/SiCp MMCs are revealed and clarified based on comprehensive and multiscale surface defects and SSD characterization methods, considering thermal, size, and strain-rate effects. Surface morphology analysis indicates that a higher grinding speed improves surface quality, which is demonstrated by the reduced grinding scratches, surface pits, and surface roughness. Based on the dislocation features of the Al grains at a subgrain scale, both cDRX and dDRX mechanisms are revealed to govern Al grain refinement in grinding Al/SiCp MMCs. dDRX is more likely to occur in the upper part of a ground surface due to higher temperature induced by severe plastic deformation. The distribution of the O-rich zone is closely associated with subsurface cracks. A workpiece ground at a higher grinding speed sustains less damage than when a lower grinding speed is used due to the reduced coverage area of the O-rich zone. In the subsurface, three different layers of microstructural evolution are identified based on various features. The topmost hybrid layer features refined Al grains and fragmentary SiC particles at a scale of nanometers. Below the hybrid layer, lateral cracks are found in the Al alloy matrix, forming a plastic flow layer. Underneath the plastic flow layer is the plastic deformation layer, in which refined and elongated Al grains are observed around the migrated SiC particles. The depths of the hybrid layer and plastic flow layer in HSG are relatively narrower in comparison to those in low-speed grinding. The range of plastic deformation of the Al alloy matrix is suppressed in HSG. Distinctly denser dislocation kinks formed at the boundary of SiC particles in HSG indicate the increased ductility of SiC particles. The responses of these two constituents in HSG facilitate reducing the property discrepancies between them. As a result, improved surface integrity of Al/SiCp MMCs is realized through HSG. In the third part, to compare the features of the ground surface and subsurface and the corresponding material removal mechanisms with HSG, ultra-precision grinding (UPG) of Al/SiCp MMCs are conducted at a linear grinding speed from 20.9 m/s to 62.8 m/s on an UPG machine. This work reveals two layers of SSD in the subsurface of Al/SiCp MMCs in UPG based on the features of the two constituents. The topmost hybrid layer features the refined Al grains and the fragmentary SiC particles at a nanometer scale, which becomes shallower at an increased grinding speed. Along the depth direction from the ground surface, microstructural evolution of Al grain refinement and SiC particle fragmentation are further weakened in the underlying plastic deformation layer. cDRX is the dominant mechanism governing Al grains refinement with dislocation arrays in evidence. Strain-rate effect prevails in Al alloy matrix deformation as shown by the larger Al grains in UPG. As a result, an improved surface integrity of Al/SiCp MMCs is realized by reducing SSD depth in UPG. With an increase in grinding speed, the Al/SiCp MMCs workpieces ground by HSG and UPG are characterized by a decreased plastic deformation of the matrix and a significantly reduced depth of SSD similar to those revealed in HSG of Al6061T6 alloy. It is worth mentioning that the damage skin effect occurs in both HSG and UPG of Al/SiCp MMCs, although the maximum linear grinding speed in UPG is approximately only one-fifth of that in HSG. This indicates that the damage skin effect is widely applicable to grinding of Al/SiCp MMCs at a higher speed through mitigating the plastic deformation of the matrix. This thesis presents an original study of microstructural evolution, surface integrity and material removal mechanisms in HSG and UPG of Al/SiCp MMCs based on comprehensive surface defects and SSD characterization at multiple scales. The originality and significance of this thesis can be identified as: (1) establishing an HSG platform that enables a linear grinding speed of 307.0 m/s; (2) developing a strategy to perform comprehensive surface defects and SSD characterization at multiple scales; (3) revealing the microstructural evolution of surface integrity with an increase in grinding speed based on the features of different constituents; (4) discovering material removal mechanisms by respectively analyzing the major influencing factors of thermal, size, and strain-rate effects in grinding Al/SiCp MMCs. The results of the study contribute significantly to a better scientific understanding of the machinability of Al/SiCp MMCs and shed some light on the optimization of the surface quality in grinding Al/SiCp MMCs. |
| Rights: | All rights reserved |
| Access: | open access |
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