純鈦與鈦合金銲接技術應用於抗海水腐蝕環境之特性研究

Taking the advantage of its low density, high specific strength and strong corrosion resistance, titanium and its alloys have high marine adaptability and can be directly used in various marine engineering equipment that will expose to seawater or directly contact with seawater. Seawater desalination is an important technology helping to reach the goal of water resources stability in droughting regions, where people struggle for survival that depending on sufficient water supply. Heat exchangers, condensers and pipeline systems are main units for the seawater desalination facility. The seawater heater has to withstand 90~120 °C corrosion in seawater. This unique working condition makes titanium and its alloys superior materials which can significantly lower the heater construction cost. The brazed plate heat exchanger currently available on the market are made of stainless steels by copper brazing. A gasket is traditionally used to avoid fluid leakage. However, the brazing process and its gaskets have low resistance to high temperatures and corrosion, which limits the function and life of the heat exchanger. In this study, friction stir welding was used to join the dissimilar materials of aluminum alloy 6061 and commercially pure titanium by means of butt joint and lap joint. To discuss the influence of different rotating speeds and travel speed, the specimens were analyzed for their microstructure, mechanical properties, and elemental analysis was carried out, finally the residual stress measurement and corrosion resistance analysis were performed followed by the friction stir welding. Experimental results showed that the best welding properties of the butt joint can be obtained by setting the rotational speed of 1000 rpm and the travel speed of 100 mm/min; and the lap joint can be obtained by setting the rotational speed to 1200 rpm and the travel speed of 120 mm/min. Comparing the two joining methods, the butt joint has better tensile strength than the lap joint. In addition, the two welding methods can observe the effect of grain refinement in the stirring zone, but the grain in the heat-affected zone is larger, and the heat-affected zone has a tendency to decrease in hardness, causing failure in the heat-affected zone when using butt joint. As for the lap joint, the interface between the two materials will form intermetallic compounds, the hardness of which is nearly 300HV, higher than that of the pure titanium base material, and fractured at the interface, which was attributed to the presence of intermetallic compounds. The residual stress portion is opposite to the typical butt residual stress, showing compressive stress at the weld bead, which is good for the weld bead. In addition, the stir zone is superior to other regions in the corrosion resist test due to grain refinement.