| Summary: | 博士 === 國立臺灣科技大學 === 機械工程系 === 97 === The main purposes of this study are to investigate the forming behaviors of various products and to develop the optimal manufacturing process for obtaining the magnesium alloys products with good mechanical properties under hot extrusion process. Taguchi method, analysis of ANOVA, fuzzy logic theory and artificial neural network are used, not only to investigate the individual quality characteristic, but expand its application to the multiple performance characteristics index (MPCI) of optimal combination of process parameters. In this study, various new products are produced, and a prediction model of searching optimal combination process parameters of product is established. The five topics are involved in this paper:
1.The Taguchi method and analysis of ANOVA are applied to analyze the influence of process parameters on the mechanical properties of magnesium alloy tubes under hot extrusion. Tensile test and flattening test of the extruded tubes are carried out, and the test results are analyzed to find the optimal combination of process parameters under hot extrusion of tubes. The microstructures of extruded tubes are observed to clarify the influence of process parameters on the grain size.
2.The hot extrusion of magnesium alloy sheet under high extrusion ratio is investigated. According to the former reports, the variable speed method can extrude perfect magnesium alloy sheets under high extrusion ratio. In variable speed method, the speed adjustment timing should be determined before the extrusion experiment. In this study, experimental data of different products are collected, then the fuzzy theory is employed to establish the prediction model of magnesium alloy extrusion. This model is applied to predict the speed adjustment timing and the tensile strength of the AZ31 and AZ61 magnesium alloy sheets under hot extrusion with the semi die angles of 20°, 30° and 40°, respectively.
3.Fuzzy theory and Taguchi method are combined to investigate the hot extrusion process of the magnesium alloy sheets. Not only quality characteristic of the smaller-the-better of the extrusion load but also quality characteristic of the larger-the-better of the tensile strength are considered in the method of MPCI to find out the optimal combination of process parameters.
4.The shapes of extruded Mg product are expanded from tube and sheet to more complicated products of the commercial market. The fuzzy-based Taguchi method is used to obtain the optimal process parameters for the hot extrusion of AZ31 and AZ61 magnesium alloy bicycle carriers by the approach of MPCI. The variables considered in MPCI inference model are flattening strength, T-slot fracture strength and extrusion load. In addition, the mechanical properties of the bicycle carriers made from magnesium alloy and aluminum alloy are compared.
5.According to the data of experimental results an artificial neural network (ANN) analytical model is established, the tensile strength of AZ61 structural parts under hot extrusion is analyzed. Through this analytical model, the influence of billet heating temperature and extrusion speed on the tensile strength of the structural parts are investigated. Finally, the mechanical properties of AZ61 magnesium alloy products are compared with those of AZ31 magnesium alloy and A6061 aluminum alloy.
In this study, the fuzzy theory is introduced to establish the prediction and analysis model for obtaining speed adjustment timing and tensile strength of the magnesium alloy sheets under hot extrusion. It can catch the speed adjustment timing under hot extrusion and predict tensile strength of the extruded parts accurately, eventually the effectiveness of the experiments can be improved. Besides, fuzzy theory and Taguchi method are combined to analyze the MPCI of product. Through MPCI inference model, a fabrication method with less extrusion load and better mechanical properties under hot extrusion can be obtained. According to the experimental data collected in this study, an ANN prediction model can be established, and the mechanical properties of extruded products can be predicted accurately. The models established in this study and analytical results of this study can offer insight to the researchers and industries of magnesium alloy forming.
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