| With the rapid development of science, technology and modern industry, all industries have higher requirements on the function of the product materials, especially surface materials which should not only withstand high speed, high-pressure conditions, but also have good wear resistance, corrosion resistance and thermal resistance.At present, Ti-Si eutectic alloy system, as an important metallic silicide, has gained wide attention. It also has been successfully used in aerospace and military fields because of its excellent performance with high melting point, low density, high strength, high hardness, good corrosion resistance and excellent wear resistance. In Ti-Si eutectic alloy system, Ti5Si3 has the highest melting point and is the most stable in thermodynamics among the five TixSiy compounds. It has excellent high temperature tensile strength, high temperature stability, excellent creep strength and strong oxidation resistance, which becomes the new universal structure material at high temperatures.As the argon arc cladding technique has the advantages of simple operation, inexpensive equipment, a large heat and high efficiency, a simple TIG welding machine was used in this paper to make use of the high temperature from arc under the protection of Ar gas to process the pure titanium (TA2) substrate materials pre-deposited with Si powders (particle size of 100μm) by surface cladding. The heat-resistant surface layers reinforced by Ti5Si3 phase were prepared. During surface cladding the pure titanium substrate samples were fixed by a home-made clamping fixture and cooled by water. The traveling speeds were controlled by an automatic travelling device. The Ti5Si3 surface layers with different microstructures were prepared by setting arc currents supplied from the TIG welding machine. After Ar arc cladding, optical microscope, scanning electron microscopy and X-ray diffractometer were used to observe, measure and analyze the macro-morphologies, microstructures, phase compositions and the defects formed in cladding process of Ti5Si3 surface layers. The difference of the heat-resistant property among the Ti5Si3 surface layers and also the substrate was determined by the cyclic oxidation test which was carried out with a box heat treatment furnace at a set temperature in which the typical samples of Ti5Si3 surface layers were kept for a certain time together with a cleaned and dried crucible. Before the cyclic oxidation test the samples were ground with 800 # water mill sandpaper and then weighed at room temperature after being washed by acetone and dried. The samples were taken out of the furnace on schedule, cooled to room temperature and weighed again to determine the heat-resistant effect of the Ti5Si3 surface layers. Finally, the typical metallographic specimens of the Ti5Si3 surface layers were chosen to do dynamic surface oxidation experiment.By observing the metallographic specimens of the Ti5Si3 surface layers, it was found that with the increase of arc currents, the width of the layers was increased. But when the current was below 130A, the surface morphologies of the surface layers were not satisfied. Only when the current was numbered to or over 130A the surface morphologies could be satisfactory. Arc currents affected the microstructures of the Ti5Si3 surface layers obviously. When the currents were 100A and 130A the hypereutectic microstructures were gained because the Si content was high in surface layers. The primary Ti5Si3 phases with the shapes of bars and blocks precipitated from the liquid molten pool and the eutectic components were composed of lamellar Ti5Si3 andβ-Ti solid solution phases. As the current was increased to 150A, the surface layer was characteristic of totally eutectic microstructures. If the current was 180A, the dendriticβ-Ti solid solution precipitated from the liquid first because of low Si content. And then the left liquid transformed to two solid phases -- theβ-Ti solid solution and Ti5Si3, i.e. eutectic transformation. Finally, hypoeutectic microstructures composed of theα-Ti solid solution and Ti5Si3 were gained. Some effective measures should be taken to prevent the defects such as holes, cracks and pores which may take place in the Ti5Si3 surface layers.The microhardness of the pure Ti substrate was 132-154HV. It was increased to about 200HV at the interface area between the substrate and the layer. Microhardness was increased obviously inside the Ti5Si3 surface layers. For example, the highest microhardness of the hypereutectic surface layer cladded at the current of 130A was 752HV.Under the conditions of 800℃, 900℃and 1000℃experimental temperatures, the comparison test of the heat resistant performance between the industrial pure titanium substrate graded TA2 and Ti5Si3 surface layers showed that the property of the Ti5Si3 surface layers with different microstructures was significantly improved compared with the Ti substrate. With the cyclic oxidation at 800℃, oxidation resistance of the surface layer cladded at 130A was a little better than that cladded at 150A and 180A. But at 900℃and 1000℃, the heat resistant performance of the surface layer cladded at 180A was the best. The surface layer cladded at 150A was the second and the layer cladded at 130A was the worst. The results were related to the inside crack surface oxidation phenomenon in the samples during oxidation process at high temperatures. The titanium dioxide (TiO2) layers grew on the outside surfaces of the pure Ti substrate and the Ti5Si3 surface layers by observing and analyzing the sectional morphologies of the final oxidation experiment samples. The oxidizing process of the pure Ti substrate belonged to the laminate oxidation mechanism. The oxygen diffusion from outside to inside resulted in a TiO2 laminate on the sample surface. Then the oxygen concentration reduced on the interface between the TiO2 laminate and the substrate. Oxygen continued diffusing to the interface through the laminate. When the composition requirement to compose the TiO2 was met by oxygen accumulating, a new TiO2 laminate formed. Thus the thick oxidized layer constituted of many TiO2 laminates. But the oxidized layers on the outside surfaces of Ti5Si3 surface layers without that characterization were only thickened by prolonging time or increasing temperature.The dynamic surface oxidation experiment was carried out at 900℃by the typical metallographic specimens of the Ti5Si3 surface layers. After 1 min oxidation, a oxide film was formed on the surface with purple color on substrate area but copper color on the layer area. After 5 min oxidation, oxide powders were formed on the substrate surface, especially at the grain boundary ofα-Ti solid solution. That changed the surface color from purple to grey. At the same time the surface of the layer area exposed the faded color without apparent oxide powders. After 25 min oxidation, a large number of granular oxides were accumulated on the substrate surface and the oxidation appearance was obvious on the surface of the layer area. But slight oxidation took place on the surface of the hypereutectic Ti5Si3 layer and more oxidation on the surface of the eutectic one. Moreover, heavier oxidation occurred on the surface of the hypoeutectic Ti5Si3 layer, especially at the regions ofα-Ti solid solution.Ti5Si3 surface layers prepared by arc cladding were metallurgically bonded to the substrate. Their microstructures were hypereutectic, eutectic, or hypoeutectic. And their heat-resistant properties were superior to that of the pure Ti substrate. It is easily realized that arc cladding the pure titanium (TA2) substrate pre-deposited with Si powders is a simple and practical technique to prepare Ti5Si3 layers and has potential application value and development prospects.
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