Research On The Weldingand Mechanism Of In Situ Reinforced Titanium Matrix Composites

Discontinuously reinforced titanium matrix composites (TMCs), espec-ially in situ synthesized TiB/Ti composites are more and more widely used in engineering applications, because they offer a combination of high specific strength, specific stiffness, and high temperature strength, better corrosion resistances, and so on. Beises high price, the processing capacity of TMCs made into the structural component, especially their welding techniques have hidered the further development and engineering applications of this kind of materials, which is one of the key problems to be urgently resolved. Thence, it is significant to investigate the weldability and of TMCs.In view of the characteristics and practical applications of in situ TMCs,(TiB+La2O3)/Ti composites were employed to study the weldability of TMCs by means of three fusion welding methos, gas tungsten arc welding, pulsed current GTAW and laser beam welding. The formation mechanism of welded joints, together with the existing state and distribution of reinforcements in the weld bead were studied using optical microscope, X-ray analysis, SEM and TEM techniques, tensile tests and microhardness measurement, and the microstructure and mechanical behavior of TMCs welded joints of were studied. The research results are summarized as follows:During the fusion welding process of (TiB+La2O3)/Ti composites thin sheets, the weld heat input has played a significant effect on the weld quality and appearance of weld. With increasing heat input, the depth of penetration, weld with and the back width to weld width ratio (Rw) increase, under the reasonable welding parameters, the welded joint process the steady aspect ratio of weld with uniform and good appearance. Howerver, excess heat input will lead to welding defects such as burning-through and surface depression without the addition of weld metal. Contrarily, too low heat input will also result in the formation of incomplete penetration and gas porosity. When the pulsing current is applied to reduce the heat input, the aspect ratio of weld is enhanced, and at the same time, the weld microsturucture were refined, and the perfect scaly weld bead was obtained when the rational pulsed frequency was employed, because it can strengthen the effect of stirring the molten bath and enhance the fluidith of liquid metal. Because the LBW shows the lowest heat input with very fast cooling rate, the smooth, uniform continuous and narrow “X”weld were produced with appropriate welding parameters, and the joint show little welding deformations. The welding experiment results exhibit that on the basis of meeting the formation of weld, the welding parameters with low heat input are usually preferable.Due to the effect of welding thermal cycle, the welded joint of TMCs shows three different characteristic zones, namely fusion zone, heat-affected zone and base metal. The microstructure of fusion zone exhibits typical columnar grains, and the weld crystallization of fusion zone joint is formed by epitaxial growth. The grain size of heat-affected zone near the fusion line becomes coarsened because it is overheated, and the heat-affected zone far away from the fusion line almostly remains unchanged. Further examinations show that the microstructure of the fusion zone and the heat-affected zone near the fusion line are wholly composed of acicular martensite, and the heat-affected zone far away from the fusion line consists of the mixture of martensite and primary phases. However, the heat-affected zone near the base metal shows any unchanged transformations. Besides, with increasing heat input, the martensites display poor orientation and exhibit more dense and dispersed distribution, which is beneficial to the mechanical properties of welded joint.Based on the results, we have found that the weld heat input has played an important effect on the formation and distribution of TiB reinforcements in the welded joint. In the weld zone and bon area, TiB whiskers were visibly refined and redistributed at grain boundaries forming the unique network structure, which is good for the room and high temperature of welded joint. Comparatively, only a small portion of TiB whiskers in the heat-affected zone far away from the fusion line changed their sizes and shape because of much lower heat input, and TiB whiskers near the base metal showed similar morphological features with that in the base metal.The microhardness of fusion zone and heat-affected zone in the TMCs welded joints produced by three different welding methods are higher than that of the base metal, and the microhardness in the fusion zone reach the peak value. Moreover, the hardness decrease continuously in steps along the direction from the fusion zone to base metal, and this can be attributed to the nonuniform microstructure of welded joints. In addition, the welded joints of TMCs presents excellent tensile strengths, and but show poor ductility, which is ascribed to numerous martensites and the network structure of TiB in the weld. The TMCs joints demonstrate good room temperature strengths, which are more than85%of the base metal strength, and can almost reach the full-strength when the laser welding process is employed with optimum welding parameters. At high temperature, TMCs can obtain the full-strength joint, even higher than that of the base metal. At600℃,the LBW joint strength is823MPa, increased by11%compared with that of the base metal (740MPa). Besides, with temperature increasing the welded joints exhibit a slower degradation trend in sftrength than that of the base metal due to the network strtucture of TiB in the weld zone, and this is more favorable and significant for the practical engineering application of TMCs.