Rozprawy doktorskie na temat „AZ80 magnesium alloy”

The multiaxial deformation of magnesium alloys is important for developing reliable, robust models for both the forming of components and also analysis of in service performance of structures, for example, in the case of crash worthiness. This work presents a combination of unique biaxial experimental tests and biaxial crystal plasticity simulations using a visco-plastic self-consistent (VPSC) formulation conducted on AZ80 magnesium alloy in two different conditions - extruded and a more weakly textured as cast condition. The experiments were conducted on tubular samples which are loaded in axial tension or compression along the tube and with internal pressure to generate hoop stresses orthogonal to the axial direction. The results were analyzed in stress and strain space and also in terms of the evolution of crystallographic texture. In general, it was found that the VPSC simulations matched well with the experiments, particularly for the more weakly textured cast material. However, some differences were observed for cases where basal < a > slip and {10¯12} extension twinning were in close competition such as in the biaxial tension quadrant of the plastic potential. The evolution of texture measured experimentally and predicted from the VPSC simulations was qualitatively in good agreement. Finally, experiments and VPSC simulations were conducted in which samples of the extruded AZ80 material were subjected to a small uniaxial strain prior to biaxial loading in order to further explore the competition between basal slip and extension twinning.

In this study the evolution of microstructure and the mechanical properties of an AZ80 magnesium alloy were investigated. It examined prepared samples of AZ80 magnesium alloy before and after processing by severe plastic deformation (SPD) using the High-Pressure Torsion (HPT) technique. An AZ80 magnesium alloy with a chemical composition of Mg-8.7% Al-0.5% Zn was processed using HPT. The processing was conducted at room temperature, 296 K, and at the elevated temperature of 473 K under quasi-constrained conditions, using an imposed pressure of 6.0 GPa at a speed of one revolution per minute (rpm) through different numbers of turns: 1/4, 1, 3, 5 and 10. Processing magnesium alloy by HPT produced excellent grain refinement in the alloy, and it prevented the samples from developing cracks and segmentation at ambient temperature better than the other popular technique of SPD, for instance Equal-Channel Angular Pressing (ECAP). The initial microstructure and the microstructural development after HPT processing were subsequently examined by optical microscopy (OM), scanning electron microscopy (SEM) and Transmission Electron microscopy (TEM). Microstructural investigations for the as-received condition showed an average grain size of ~25 m. Optical microscopy images revealed microstructural evolution at both room and elevated temperature after the HPT process. The small proportion of refined grains at the edges expanded towards the disc centre with consecutive increasing numbers of revolutions. The TEM images demonstrate an evolution toward homogeneity at increasing numbers of revolutions. The final average grain size after 10 turns when the alloy was processed at room temperature was ~200 nm and ~330 nm when the alloy was processed by HPT at 473 K. The selected area electron diffraction (SAED) images of HPT samples after 10 revolutions show a fully developed ring at room temperature, indicating a microstructure with high angles of misorientation grain boundaries, and a less developed ring at 473 K. Microstructural observation through the disc thickness demonstrates more heterogeneity in the vertical than the radial direction. Vickers microhardness (Hv) values were taken along the disc diameter (radial direction) and over the total surface of the discs (colour-coded contour mapping). The results of Vickers microhardness (Hv) measurements along the diameters of the discs verify the heterogeneity of HPT deformation at lower numbers of turns. In the samples, the microhardness values increased rapidly at the edges of the disc, while the centres showed a lower value, and this large difference confirms the heterogeneity of HPT deformation in the early stages. With further straining samples showed a significant increase in microhardness values from the edges towards the disc centre. The microhardness values of samples processed by 5 and 10 turns showed a reasonable homogeneity across the disc diameter, with an average value of ~120 Hv when AZ80 was processed at room temperature and an average value of ~110 Hv when processed at 473 K. Likewise, three selected discs processed by HPT for 1, 3 and 10 turns at both 296 K and 473 K were sectioned vertically across their diameter to be tested by (OM) and Vickers microhardness (Hv) through their thickness (axial direction). The results of (OM) and Vickers microhardness (Hv) confirmed the high heterogeneity in the axial direction than the radial direction. Subsequent to the HPT process at room temperature, tensile specimens were cut from the processed discs and pulled in tension to failure at different tensile test temperatures (473, 523 and 573 K) and strain rates of (1.4×10-4 s-1, 1.4×10-3 s-1, 1.4×10-2 s-1 and 1.4×10-1 s-1). The superplasticity of AZ80 magnesium alloy was confirmed for the first time (to the author’s knowledge) at a maximum elongation of 645% when the alloy was pulled in tension to failure at 573 K using strain rate of 1.4×10-4 s-1. Moreover, the alloy exhibited a lower temperature superplasticity when it attained 423% at 473 K. Despite this superplasticity, AZ80 magnesium alloy does not show the predicted behaviour of increasing ductility with increased imposed strain during HPT process and decreased average grain size. The maximum elongation was reached in a sample processed by HPT for one turn, in which a smaller average grain size and the homogenous microstructure were not achieved.

The continued interest in the use of magnesium alloys for new applications demand the successful production of high quality wrought alloys. Magnesium Elektron seek to reliably produce high quality alloy billets by the DC casting method combined with ultrasonic inspection. The main objectives of this study are to characterize the defects which are currently found in the material and to understand the ability of the ultrasonic inspection technique currently employed to detect the defects. This study began by locating defects using the ultrasonic inspection method which were then characterised using X-ray Computed Tomography (XCT) 3D imaging technique. Attempts were then made to understand and simulate the mechanisms by which the defects form during the casting process. The simulations were used to investigate the flow patterns during casting and the growth kinetics of the intermetallic phase. The initial phase of this research established that the defects found comprised of an entrained oxide film entangled with an abundance of intermetallic phase particles. These defects were found to be present in the size range of 0.5 – 5 mm, and were deleterious to the materials mechanical properties. Greater understanding of the ultrasonic inspection process was achieved and informed improvements to assisting the production of high quality feedstock. Simulation of the formation of the defects indicated that there was a region in which the oxide films could form and be free to enter into the final cast product. Simulation of the growth of the intermetallic particles demonstrated that precipitation from the liquid occurs in the mould during which particles are carried by the melt flow and experiences a complex thermal history. The combination of the two phases was established to be due to entanglement of the oxide and particles which when combined will settle out of the melt as a single defect. Improved filtering and melt handling methods were recommended to eliminate the defects and reliably produce high quality alloys.

碩士<br>國立臺灣科技大學<br>機械工程系<br>95<br>The study mainly investigates into the formability of AZ80 magnesium alloy tube and bar under hot extrusion. The contents of investigation are divided into two parts. The first part is the extrusion of tube. By using Taguchi’s experimental planning method, the study undergoes the construction and analysis of extrusion experiment. The process parameters considered in the study include the heating temperature of materials,initial extrusion speed, heating temperature of ingot container and lubricant. The study also uses Taguchi orthogonal array to make the experimental plan. After the experiment is done, the extruded finished products are brought to receive a test of mechanical properties. The data of mechanical properties acquired in the test are further analyzed by Taguchi method. The study investigates how the extent of influence of different process parameters on extrusion and fabrication is correlated with the mechanical properties of finished products. The second part uses trial and error method to plan the experiment of AZ80 magnesium alloy bar. The extruded finished products have to receive a test of mechanical properties. Then all the data of mechanical properties acquired in the test are analyzed. These results are compared with the results of extruded tube. Finally, it is hoped that the conclusions made by the study can be provided as a reference for the industry of magnesium alloy processing.

碩士<br>國立臺灣大學<br>材料科學與工程學研究所<br>96<br>This study mainly focuses on precipitation hardening behavior and superplasticity of AZ80 magnesium alloy. The precipitation hardening experiment was conducted between 125-300℃. The influence of the precipitate on mechanical properties was measured by microhardness test and tensile test. The precipitate was investigated in detail by XRD, OM, SEM and TEM. The superplastic experiment focuses on three different rolling directions. The temperature ranges from 200-400℃ and the strain rates were 3×10-3、1×10-3 and 3×10-4(s-1). The results were analyzed by OM and SEM to further understand the influence of temperature and strain rate on microstructural evolution. The result of the precipitation hardening experiment indicates that each aging temperature between 150-300℃ shows hardening effect. The highest hardness is occurred at 175℃/256 h and the hardness increment is about 38%. The result shows that the hardness increment is low compared with precipitation-hardenable aluminum alloys. This is influenced by the nature of Mg17Al12 precipitates. The precipitates can be divided into continuous and discontinuous precipitates by their forming mechanism. They can be further divided into lamellar, elliptical, intergranular, Widmanstätten structure and irregular slab by their morphologies. Because the precipitate can not effectively resist dislocation moves, the hardening effect is poor. At different aging temperature the morphology, the size, and the distribution density of Mg17Al12 precipitates are also different. These reasons then lead to different hardness. The result of superplastic experiment shows that the material has best elongation (about 350%) at 300℃/3×10-4s-1. The rapid elongation increase at this temperature has two reasons: one is because 300℃ is close to the recrystallization temperature of the material, dynamic recrystallization then occurs and provides enough grains for grain boundary sliding which produces large deformation; the other reason is because Mg17Al12 precipitates rapidly at 300℃ and most of them are formed on grain boundaries, they can effectively impede grain growth to remain the fine grain structure. These two effects appear at the same time and contribute to high elongation.

碩士<br>國立臺灣大學<br>材料科學與工程學研究所<br>99<br>Magnesium alloys are considered as potential candidates for numerous applications, especially in transportation vehicles or lightweight sectors for 3C products owing to their excellent properties, such as low density, high specific strength, high damping capacity and high recycle ability. The AZ series is mainly based on the Mg-Al alloy system and leads most of the magnesium alloys. If the Al addition in these AZ-series magnesium alloys exceeds 6 mass%, the β-Mg17Al12 inter-metallic compounds will precipitate within the matrix and the mechanical strength increases. Furthermore, adding slight Zn into Mg alloys is to enhance their corrosion resistance. These Zn atoms may also have the effect of solid-solution strengthening. However, it is commonly recognized that magnesium possesses poor formability because of its hexagonal close-packed structure. Adding Lithium with extremely low density 0.534 g/cm3 can improve this shortcoming and further reduce weight. In the present study, by 3mass% addition of Lithium on AZ80 Mg alloy, we study mechanical properties of as-extruded magnesium alloy. Furthermore, to more understand this AZ80+3mass% Mg alloy, the effects of heat treatments (T5 and T6) on its precipitation behavior and mechanical properties will be systematically investigated. Experimental results show that adding 3mass% Li to AZ80 alloy can obviously increase the ductility. Meanwhile, as-extruded AZ80+3%Li specimens will produce the Mg17Al12+AlLi precipitates after aging from 110℃ to 230℃. The 110℃ aged specimen has a maximum tensile strength of 356 MPa with corresponding aging time of 16 hours. The 450℃ solution-treated specimens will produce the AlLi precipitates after aging from 110℃ to 230℃. The 110℃ aged specimen has a maximum tensile strength of 373 MPa with corresponding aging time of 32 hours. However, the addition of 3mass% Lithium on AZ80 Magnesium alloy will reduce the corrosion resistance due to the highly activity of Lithium and hydrolitic reaction of AlLi precipitate.

碩士<br>國立成功大學<br>機械工程學系<br>102<br>In this study, a spit-Hopkinson bar is utilized to investigate the effect of temperature and strain rate on impact response and microstructural characteristics of AZ80 magnesium alloy at different temperatures of -100℃, 25℃and 300℃, under strain rates of 800s-1,1500s-1 and 2200s-1, respectively. The experimental results indicate that the mechanical properties are related to temperature and strain rate. At a constant temperature, flow stress, work hardening rate, strain rate sensitivity and temperature sensitivity all increase with the increasing strain rate, while the thermal activation volume and activation energy decreases. However, at a constant strain rate, flow stress, work hardening rate, strain rate sensitivity, and temperature sensitivity decrease with increasing temperature, while the thermal activation volume and activation energy increases. In addition, the observed impact deformation behavior of AZ80 magnesium alloy under current testing conditions can be described by the Zerilli-Armstrong equation. Optical microstructural observations reveal that the formation of slip band and morphology of deformed grain of Z80 magnesium alloy are strongly dependent on temperature and strain rate. The SEM fracture analysis results indicate that due to the presence of HCP structure, the fracture surfaces of the deformed AZ80 magnesium specimens are dominated by cleavage feature. However, the ductile fracture is found to increase with increasing β phase. TEM observations show that the dislocation density increases with increasing strain rate, but decreases with increasing temperature. A linear relationship between the flow stress and square root dislocation density is observed. Finally, the flow stress, strain rate sensitivity and thermal activation volume are related to the observed dislocation substructure.

碩士<br>國立臺灣大學<br>材料科學與工程學研究所<br>100<br>It is commonly recognized that magnesium possesses poor corrosion resistance because of its high oxidation potential and porous oxide structure. The corrosion resistance is the most important topic for magnesium alloys application. The thesis aims to investigate how cold spray influences the properties of magnesium alloy, including microstructure, mechanical and corrosion property. The sprayed substrate is AZ80 magnesium alloy. IN625, 301 stainless steel and aluminum powder is used to spraying on the surface. In the study, the coating formed by cold spray treatment has some holes, and is a non-dense film. AZ80 substrate occurred grain refinement after spray IN625 and 301 stainless steel process. For the aluminum coating, it was observed that after coating process, substrate surface is obviously harder than after coating IN625 process. As to the corrosion behavior, the result of the potential-dynamic polarization curve tests reveals that, whether in the IN625 or the Al coating, the corrosion resistance are better than magnesium substrate. In the wear and erosion test, the cold spray coating are to improve the wear and erosion resistance of AZ80 magnesium alloy. After annealing, the AZ80 alloy interacts with Al coating to form Mg17Al12 and Al3Mg2 compound at the interface. After anodizing, on the aluminum coating surface formed a high hardness alumina layer that has excellent corrosion resistance than aluminum coating.

碩士<br>國立屏東科技大學<br>車輛工程系所<br>98<br>Lightweighting has become a key issue in automotive industries recently. Magnesium alloy, consequently, has become one of the major materials for structures. In addition, the solid-state bonding and other excellent features of the friction stir spot welding (FSSW) makes it inherently attractive for body assembly and other similar applications. This study aims to investigate the welding characteristics of the AZ80-F magnesium alloy of FSSW. A 3D finite element coupling model is employed to investigate the effects of welding parameters on the thermal-mechanical behavior of the welds. Then, the experimental samples are made by using FSSW process and, the tensile-shear test, direct tensile test, nano-indentation test and metallographic test are performed to understand the corresponding characteristics of the spot welds. The results show that the welding parameters of tool rotation speed and pressure are more sensitive to the temperature distribution of the welds. In addition, the temperature distribution curve can be used to evaluate the properties of the welds. The results obtained from microstructure observation reveal that the geometry of the tool has a strong effect on the plastic flow and grain size in the welds during the welding process. The welds with the dual-conical tool have the best microstructure distribution and welding strength. Its welding strength increases with the increase of tool rotation speed and pressure to a critical value.

碩士<br>國立交通大學<br>機械工程系所<br>97<br>Severe Plastic Deformation (SPD) is widely used to improve the mechanical properties of magnesium. The equal channel angular extrusion (ECAE) is the most simple and convenient technique in SPD. By using ECAE, we obtain a product which has the same cross section with original billet after extrusion .In addition, the force required in extrusion during ECAE process is usually not large, and we can control the microstructure of the billet by changing the die parameters. Our experiments show that low temperature extrusion can achieve in grain refinement, but the lowest extrusion temperature is limited by materials and dies. Imposing a back-pressure is useful for increasing the strain of billet to avoid the cracking on the billet surface by tensile stress. In order words, imposing a back-pressure is beneficial to compromise the extrusion temperature. In this study, the effect on the microstructure of magnesium alloy AZ80 of imposing the back-pressure was investigated. As a result, we found that imposing the back-pressure is helpful in extending the lowest extrusion temperature. At the same temperature, it makes the grain size larger with increasing back-pressure.

碩士<br>國立虎尾科技大學<br>材料科學與綠色能源工程研究所<br>103<br>Lightweight magnesium (Mg) alloys have a low elastic modulus, which value very close to the human bone. Biodegradable Mg alloys can be used for medical hard tissue implants in orthopedic applications without revision surgery. However, Mg alloys are notably characterized by low corrosion resistance in physiological solutions. On the basis of the excellent biocompatibility, bioactive and osteoconductive properties of fluorohydroxyapatite (FHA), a FHA surface coating was hydrothermally synthesized on the AZ80 Mg alloy at 175ºC under a saturated steam pressure atmosphere. The aim of this present study is not only to obtain an optimal FHA coating with the substitution of F─ ions, but improving corrosion resistances and cell responses of the AZ80 Mg alloy. Experimental results show that AZ80 Mg alloy can be protected by the FHA coating even though immersion in the simulated body fluid after 32 days. FHA coating stability is improved after the substitution of F─ ions. In addition, the corrosion resistance of AZ80 Mg alloy can be effectively improved by the deposition of FHA coating. In vitro cell culture results represent that significant proliferation and adhesion of MG63 cells on the FHA-coated AZ80 Mg alloy is demonstrated after testing for 24 hours. It is recognized that FHA-coated AZ80 Mg alloy shows a good biological reponses, and it can be considered as an attractive, excellent hard tissue substitute material for further biomedical applications.

碩士<br>國立臺灣大學<br>材料科學與工程學研究所<br>97<br>Damping capacities (DCs) of AZ80 and LAZ1110 magnesium alloys are investigated by Dynamic Mechanical Analyzer (DMA) at the temperature range of 0-300℃. DC of the high-temperature damping background (HTDB) at 200-300℃ for AZ80 alloy increases significantly after 20% cold-rolling with or without aging. However, the activation energy of HTDB decreases from 1.69eV to 1.37eV, which indicates cold-rolling can accelerate alloy’s diffusion process and promote its creep development simultaneously. Consequently, the contrary cold-rolling effect on DC and creep resistance of AZ80 alloy should be taken into account carefully for its high-temperature applications. Three damping peaks P1, P2 and P3 are observed for LAZ1110 Mg-Li alloy. Severe cold-rolling can effectively improve the DC of LAZ1110 alloy at room temperature. The P3 peak located at about 200℃ shifts to lower temperature if the extent of cold-rolling increases. Experimental results show that, after severe cold-rolling, the formation of P3 peak is well-related to β(200) preferred orientation and caused by recrystallization process. Besides, high damping criterion (tanδ≧0.03) can be met at room temperature for 80% cold-rolled LAZ1110 sheet tested at 0.1Hz low frequency or at 30-35μm operating amplitude.

碩士<br>國立臺灣科技大學<br>機械工程系<br>96<br>Investigation of CO2 laser welding has been done to AZ61 and AZ80 magnesium alloy extrusion sheet. Material without heat treatment has been compared with material which heat treated prior to welding. The aim of this research was to study the effect of different heat input of CO2 laser welding on mechanical properties, microstructure, chemical composition and welding defects of the magnesium alloy extrusion sheet in order to improve the weldability. Base metal and weld metal were observed with the scanning electron microscope. Energy dispersive X-ray spectroscopy analysis reveals that the heat treatment materials have more Al and Zn element segregation in grain boundary. When the alloy with higher aluminum content has wider weld zone in the same heat input relatively. Near the fusion line becomes more Al element segregation and in the weld region the dendritic compounds relatively contain high Al. Experimental results showed that the width of weld cross section was increased with the increment of heat input, and the fusion line has permeated into base metal. The low heat input would increase tensile strength, because the fusion zone has finer crystal microstructure and the heat affected zone width was reduced. It is difficult to produced unstable plasma which leads to weld porosity. But high welding speed brought the big supercooling area ,fusion line solidify in advance prior to partially melted zone (PMZ), cause liquation cracking in the PMZ which lead to loss of tensile strength. Segregation phenomenon of some Al element in liquation cracking has been taken to analyze by EDS.

碩士<br>修平技術學院<br>精密機械與製造科技研究所<br>98<br>This study aimed to investigate the characteristics of AZ80 magnesium alloy by TIG welding for corresponding in the same voltage and current. Tempering heat treatment process by observation of AZ80 magnesium alloy and the mechanical properties change. The experimental results shown in by X-ray diffraction analysis showed that the welded magnesium alloy AZ80 after heat treatment have β-phase (Mg17Al12) precipitation, making AZ80 magnesium alloy in ?Mg matrix phase of the composition, microstructure in the fusion zone. The Microstructure observed after the heat treatment of ?and β phase fraction compared with the tempering temperature and time will change, with the tempering temperature and tempering time meet the increase in ?decreased, the relative increase in β-phase will be . The AZ80 magnesium alloy by tensile test could be that the higher the tempering temperature and tempering under the conditions of a long time, and has the maximum tensile strength/.Hardness the will be slightly decreased when the tempering temperature and time is increased and the magnesium alloy AZ80 after tensile fracture surface is brittle failure types are.

碩士<br>大葉大學<br>車輛工程學系碩士班<br>93<br>ABSTRACT Taiwan is one of the most important bicycle suppliers in the world. As a base of production, there must be a mastery of advantages of manufacturing technology. In the respect of the evolution of bicycle materials from early carbon steel, then aluminum alloy, then carbon fiber, and the current titanium alloy, it is apparent that the traders’ eager desire to find the stronger and lighter materials. Aluminum alloys still dominates the manufacturing of bicycles. The specific gravity of aluminum alloy is about 2.7 g/cm3 and the magnesium alloy is only at 1.8 g/cm3. If product structure size is still remained the same, a use of magnesium alloy can result in 33% reduction of the weight. Not only enjoying low density, magnesium alloy characterized with good specific strength, specific rigidity, machinability, restrict vibration, damping capacity, and recyclability. Aluminum alloy 6061 undergoing T6 treatment has been used frequently in component for bicycles such as frame, handle bar, stem, chain-wheel, seat tube, wheel rim, and brakes. The strength of AZ80A magnesium alloy selected for this experiment is similar to that of T6-treated 6061aluminum alloy that can be extruded, forged and welded and is already used in frame and front-fork. In this experiment, we carried out the tri-temperature aging treatment at 150, 200, 250℃ on the extruding AZ80A magnesium alloy, with the aging time ranging from 0.5 hr to 128 hr. We also worked on tensile test, micro-hardness test, x-ray diffraction analysis and analyzing the fracture surface. Hopefully, we can through the implementation of aging process to find the best precipitating mechanism which can enhance the precipitate phase and resilient characteristics for base metal effectively. Findings of the experimental show that the extruding sheet of AZ80A magnesium alloy produces the discontinuous layer-shaped precipitates that grow form grain boundary into interior with short aging time at 150℃temperature. These layer-shaped precipitates will continue to increase with the lengthening aging time that brings marked improvement to micro-hardness and ultimate tensile strength but, on the contrary, shows poor influence to elongation. When conducting the short-time aging treatment under 200℃, the generation of layer-shaped precipitates is witnessed, and the amount of precipitates also increase gradually with increasing the aging time. After 8 hours aging, mat-shaped precipitates will be precipitated evenly in untransformed grains, there appears no significant change for yield strength and tensile strength except for a slight decrease of micro-hardness. After 32 hours aging, lath-shaped precipitates precipitated inside the grain, with the meager volume cannot determine the influence of mechanical properties. When conducting under 250 ℃, in short aging time specimen, we can observe the discontinuous layer-shaped precipitates and boundary precipitates start to precipitate with the micro-hardness, ultimate tensile strength and elongation to be improved. While at 4hr aging, however, short-rod-shaped precipitates begin to precipitate evenly interior the grain, and boundary precipitates keep growing. Tensile strength and elongation are decreased slowly with increasing the aging time, except the micro hardness is still maintained constant. Our suggestion for thermal treatment of AZ80A magnesium alloy is selecting 200℃aging temperature and 8~16 hr aging time can achieve the better micro-hardness and tensile strength as well as not-so-poor elongation. We expect findings of this research will be helpful to the heat treatment techniques of the bicycle industry and the industry in the hope of making magnesium alloy more extensively applicable in more areas. Key Words: AZ80A Magnesium Alloy, Aging Treatment, Layer-Shaped Precipitates, Mat-shaped Precipitates, Lath-shaped Precipitates, Boundary Precipitates, Short-Rod-Shaped Precipitates.

碩士<br>大葉大學<br>機械與自動化工程學系<br>97<br>The purpose of this study for extrusion AZ80A magnesium alloy welds carry out solid solution and aging treatment, and discussion the precipitates type and distribution to welds micro structure and mechanical property influence situation. The experiment uses the alternating current inert gas tungsten electrode arc welding (TIG) way plate welding. Use the temperature 420℃ solid solution treatment and to keep the temperature 1 hour, and then use the 200℃ temperature, the time set 4, 8, 12, 16, 20 hours to carry out the aging treatment. From the experimental results showed the welds to accompany the aging time increasing, these precipitates can effectively enhance the hardness of materials. Base and welds metal have the same type of precipitates, but the time, pattern and distribution of precipitates will be slightly different. The layer-shaped precipitate is the first emergence in base and welds metal. The precipitates can effectively enhance the micro-hardness, yield strength and maximum tensile strength, but the breakage way easily along precipitates interface separation, thus cause the elongation ratio to drop. When layer-shaped precipitates transform to intermittent layer-shaped precipitates will be led to decline in yield strength, but the short-rod-shaped precipitates begin to precipitate help to enhance the yield strength in welds. The boundary precipitates keep growing, the breakage way easily along grain boundary and precipitates interface separation cause the elongation ratio to drop. In the sum of the data analysis, the AZ80A magnesium alloy base and welds, use the 420 ℃-1hr solution heat treatment, after use the 200 ℃-4hr aging treatment will be get better of mechanical properties.

碩士<br>大葉大學<br>機械工程研究所碩士在職專班<br>93<br>ABSTRACT According to the high development of technology, consumer demands for product are changed from function-oriented to fine quality and light-weight behavior. Also the "Green concept" is highly emphasized in the worldwide. The vehicle industries are selecting light weight materials to fabricate the vehicle to reduce the exhaust gases emission and increase the fuel efficiency. As we know, magnesium alloy exist many excellent characteristics; ex: low specific density, high specific strength and rigidity, good thermal conductivity, high damping capacity, and high electromagnetic interference, etc. Nevertheless, the HCP crystal structure causes the inferior results of formability and ductility. Therefore, not only improve the quality and properties of material but promote the manufacture processing techniques are important. One of the important techniques is the welding technology. In here, we select the most popular welding machine in manufacture industry, gas tungsten arc welding (GTAW), doing this welding research. Hopes that the influence on weld structure and mechanical properties by changing the frequency of alternating current (AC) and pulsing current could be understood, then obtain the welding characteristics and the best welding parameters of AZ80A magnesium alloy. Based on the experiment results shown that, the influence of AC frequency do not show clear affect to the weld property. However, by changing the frequency of pulsing current did obviously reveal the influence to the weld. With increasing the pulsing frequency, the grain refinement effect is very clearly, which will increase the mechanical properties (such as tensile strength, elongation, and toughness) gradually. Especially, at 9 Hz weld can obtain the finest grain size. Hopefully, the result of this study not only can accumulate the knowledge of magnesium welding, but also helps the traditional metal working industries to promote their welding skills and expand the applications of magnesium alloy.