Effect of electropulsing treatment on microstructure and machinability in ultra-precision machining of Mg-9Al-1Zn alloy

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Effect of electropulsing treatment on microstructure and machinability in ultra-precision machining of Mg-9Al-1Zn alloy


Author: Zhang, Duo
Title: Effect of electropulsing treatment on microstructure and machinability in ultra-precision machining of Mg-9Al-1Zn alloy
Degree: M.Phil.
Year: 2011
Subject: Machining.
Metals -- Mechanical properties.
Magnesium alloys.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Industrial and Systems Engineering
Pages: xv, 130 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2507255
URI: http://theses.lib.polyu.edu.hk/handle/200/6487
Abstract: The machinability of metals and alloys is well-known for being affected by the cutting conditions, cutting tools and material properties. A high quality surface finish can be obtained by the use of advanced machine tools based on single point diamond turning (SPDT). However, no matter how accurate the machining system is, the limit of performance is determined by the tool/workpiece interaction during the chip removal process at the micro- and nano-scale levels. In particular, the dimensional accuracy and stability of the machined surface depend on the metallurgical properties of the surface before and after machining, such as the plastic deformation, microstructural changes, phase transformations, etc. Good surface finish and low cutting force are associated with a high shear angle, which is characterized by continuous chip formation and is intimately related to the properties of the workpiece materials. The chip formation process involving the dynamic interaction of the cutting tool and microstructural state of the workpiece is decisive in determining the surface quality and integrity of an ultra-precision machined surface. However, little attention has been paid to studying the effect of material properties on the machined surface. As an alternative to traditional thermal and mechanical processes, electropulsing treatment (EPT) has been widely applied to materials processing to improve the mechanical properties by means of inducing microstructural changes, refining grain size, and improving dislocation distributions, etc. In EPT, critical electric current pulses can be applied to pass through the materials with the high current density. The treatment can be done so efficiently that only one thousandth of the energy is needed and as little as one thousandth of the time is taken compared with a conventional furnace treatment. The electropulsing treated materials are expected to have better ductility and better machinability in terms of the achievable surface finish. An attempt is made in this thesis to design an optimal EPT condition to obtain a work material with better machinability and a fine grain microstructure, which are also conductive to the high-quality surface finish by ultra-precision diamond turning. The thesis is divided into two parts. In the first part, microstructural evolution, phase decomposition and dislocation dynamics are investigated under various types of EPT. In the second part, the micro-cutting experiments on EPT treated work material Mg-9Al-1Zn alloy (AZ91) have been conducted by SPDT processing. The machinability of electropulsing treated materials under different cutting conditions is studied in ultra-precision diamond turning.
In the first part of the study, the effect of EPT on microstructural changes, phase transformations, and dislocation dynamics has been studied under static and dynamic EPTs. In the static EPT with cold-rolled specimens, decomposition and precipitation of the β phase are tremendously accelerated with increasing frequency of electropulsing, and both twins and the dislocation density are reduced in the process. For the specimens under dynamic EPT, it is found that the β phase decomposition is considerably accelerated compared with the conventional thermal processing. The deformation twins disappear, the dislocation density decreases, and a homogenous structure with fine grains is achieved. In the second part of the study, machinability of the EPT treated work materials is investigated under face turning and straight cutting experiments in SPDT. It is found that the cutting forces are significantly reduced and the surface roughness is improved with the increasing frequencies of EPT. The machinability changes are investigated from the point of view of dislocation dynamics. The serrated chip morphology with macroscopic shear bands is identified as one of the characteristics in micro-cutting, and the shear band projection length is extremely high compared with shear band thickness. This research is multi-disciplinary and cuts across disciplines, from materials science to machining engineering. It aims at developing a framework to gain a better understanding of EPT and the diamond turning process, which in turn enables the control of the surface quality of the machined surface. The results of the study lead to the design of high quality materials suitable for ultra-precision diamond turning in a wide range of alloys through EPT. It also gives rise to the provision of practical guidelines and a data basis for creating high quality materials, which possess a fine grain structure and low residual stress for manufacturing precision components that demand ever-higher surface stability.

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