경량합금의 마찰교반접합(FSW)에 의한 접합부의 역학적 거동 및 금속학적 특성 에 관한 연구

Abstract

Magnesium and aluminium alloy are becoming the materials of choice for many lightweight transports component applications. In the alloyed form, both magnesium and aluminium are structural metal, thereby providing considerable opportunity to improve fuel economy and reduce harmful emissions produced in powering transport when substituted for steel design. As the density of Mg alloy is one-fifth of iron alloy, it is the lowest among the developed alloy till now. The Mg alloys also have a modulus of non-intensity or non-elasticity enough to stand comparison with other lightweight materials. It is not only excellent in absorptiveness for wave such as vibration, shock and electronic, but also distinguished in quality such as electricity conduction, thermal conduction and processing, or exhaustion and shock on high temperature. It is used for material miniaturization in the industries such as automobile, aerospace and defense machinery. The welding of magnesium alloy turned out to be difficult due to certain factors such as the low melting point, the low evaporation point, the high chemical attraction with oxygen and the generation of metal compound. Even though Aluminium alloys are light in weight some of them have strength exceeding the mild steel. Al alloys have good ductility at subzero temperatures and have high resistance to corrosion, non-toxic and have an approximate melting range from 482 to 660℃. Due to its high thermal conductivity high rate of heat input in need for the fusion welding of Aluminium alloys. It requires new welding method to be developed for productivity and high quality of products because of the difficulty in applying the conventional welding processes (resistance and arc welding, etc) to aluminium alloy. In short the similar and dissimilar welding of these alloys using the fusion welding is not feasible. According to the trend, the uses of Magnesium and aluminum alloy are increasing in auto mobile and aerospace industries. As the use of these alloys increase, the research concerns about the similar and dissimilar welding of these alloys are also increase. Friction stir welding (FSW) offer great promise as a means of joining similar and dissimilar alloys and producing welds with superior metallurgical properties and strengths. Significant cost reductions and novel combinations of materials and designs may be achieved by FSW. Although FSW is performed at temperatures well below the melting point of the alloys, complex thermal and plastic strain gradients are developed during the stirring process. So the mechanical and metallurgical characteristics of FSW welds are to be analyzing in order to supply the fundamental information for the criteria of welding design and construction. This study is based on dissimilar joining of Mg AZ31B and Al6061-T6 alloy by FSW based on the manufacturing process and examine the systematic or mechanical characteristic that can be generated while welding of these dissimilar alloys. The sheets of Al alloy and Mg alloy were friction stir welded under various combinations of rotating speed and traveling speed and the temperature distributions at the resulting fsw joint was measured using the thermocouples. Also the resulting microstructures were analyzed, using optical microscope (OM) and scanning electron microscope (SEM). The dissimilar FS welded specimen we subjected to tensile testing and after the tensile test SEM test along the tensile fracture surface showed that the crack originated in the intermetallic compound layer with β-Mg17Al12 phase and then grown to the base metals with α-Mg and Al phase. In the x-ray diffraction test, diffraction profiles of the fractured area of the dissimilar welded joint Mg17Al12 intermetallic compound were identified. Shortly it is indicated that the welded joint was fractured at the Mg17Al12 intermetallic compound layer. The identified results of the intermetallic compound were agreed to the results determined by EPMA. Due the presence of this intermetallic compound layer the tensile strength of the dissimilar welded Mg AZ31B and Al6061-T6 alloy is less compared to the similar welded Al-Al and Mg-Mg alloy specimens. The choice of process parameter especially the rotational speed and traveling speed have a significant effect on the control and optimization of the process. So various FS Welding was carried out in various combinations of travel speed and rotation speed for similar and dissimilar welding of this alloys. The results indicate, Al6061-T6 alloy, rotating speed 1000rpm, travel speed 200mm/min. AZ31B-H24 rotating speed 2000rpm, travel speed 100mm/min. and Al6061/AZ31B dissimilar FS welding rotating speed 450rpm, travel speed 15mm/min shows good bead and optimization conditions. In order to numerically calculate temperature and residual stress distribution in welds, finite element heat source model is developed on the basis of experiment results and characteristics of temperature and residual stress distribution in dissimilar welds are understood from the result of simulation. For the finite element stimulation of the heat transfer and residual stress analysis in the FSW, three models with different tool pin shape such as Cylindrical shaped pin, Frustum shaped pin and Threaded pin were used. The thermal and residual stress distributions for the three types of pin configuration were simulated and results were compared. Al6061 FSW Calculated Max. Temperature distribution follows; Frustum pin (448.47) > Cylindrical pin (442.76) > Threaded pin (441.02). And, AZ31B-H24 FSW Calculated Max. Temperature distribution follows; Frustum pin (474.27) > Cylindrical pin (469.21) > Threaded pin (463.56) (Unit :℃). The maximum temperature measured using the thermo couple at Stir zone (AS) of Al6061 alloy is about 470℃, in Stir zone (RS) is about 380℃, which is lower than Mg alloy. And, Al6061/AZ31B dissimilar FSW temperature distribution for Cylindrical pin was about 509℃, Frustum pin 467℃, Threaded pin 464℃. Al6061-T6 FSW Calculated Max. Residual stress distribution follows; Frustum pin (50.15) > Cylindrical pin (49.48) > Threaded pin (49.25). And, AZ31B-H24 FSW Calculated Max. Residual stress distribution follows; Frustum pin (49.48) > Cylindrical pin (45.09) > Threaded pin (34.51). (Unit:Mpa). So from the simulation and experimental results, the Threaded pin case is advantages from the point of view of weld efficiency and process stability. Also maximum temperature and residual values are less compare to other pins. The aluminum has large thermal expansion coefficient and the mechanical melting point is lower than mg alloy. Numerical (finite element) thermal models are used to predict the thermal histories in trial welds. The main contribution of this thesis is as follows: (a) two-dimensional optimization of thermo-elasto-plastic process, (b) evaluation of material property sensitivity to welding residual stress, (c) FE analysis for elastic rate-independent plastic material with equilibrium equation.