Investigation On Microstructure Design And Production Of Titanium Matrix Composites Strengthened Non-homogeneously By The Whisker Coated On The Powder

Titanium alloy is widely used in many fields, because of its excellent weldability, high specific strength and other special characteristics. The researchers added high melting point, high hardness Ti B whiskers in titanium alloy to improve the wear resistance. In addition, the thermal expansion coefficient of Ti Bw is similar with matrix, which can improve adhesion strength of the two phase interface, as mean as the wear resistence and high temperature resistant performance effectively. However, the traditional reinforcement measures invariably achieve high strength at the expense of plasticity. In recent years, with the combination of the soft area and the hard area, inhomogeneous reinforcement method has developed into one of the methods to improve strength and toughness effectively. Then the method gained wide attention as well. In this paper, the author utilize preheating craft to gain a kind of mixed powders, where the spherical TC4 powders are coated by the ultra-fine boron powders. Then the several kinds of processes were applied to produce the bulk materials, such as the hot pressing in situ sintering craft, the hot extrusion and the laser cladding process. It was necessary to investigate their affects on the microstructure and mechaniacal properties comparing to those of uniform distributed whiskers.The heating with the furnace and sealed fast heating were used to presinter the powders with different boron contents. On the surface of the coated powders, the Ti B whiskers were emerged with a fine needle shape exist at the surface of TC4 powders. The evolution of the morphology and the distribution of the Ti Bw was investigated through adjusting the preheating parameters, such as heating temperature and heating time, cooling rate. We found that the faster heating method was helpful to obtain the Ti Bw in needle morphology with longer size as well as uniform distribution. It was advisable to fix holding time in 2~4h, promoting holding time longer would reduce the number of Ti Bw. The length of whiskers which emerged on the surface of matrix existence limit size and the average length is about 12μm.In the same condition, the length of whiskers which exist in matrix was always longer than the external ones.Through hot-pressing sintering craft was used to produce sintering the bulk materails with the coated powders with different morphology and the distribution of whiskers. In contrast to the preheating craft, hot-pressing sintering increased the quantity and thelength of whiskers. The whiskers were distributed quasi-continuously in the interface of two adjacent matrix particles when the preheating powders with whiskers in small were used. The defects in the material were quite few. The hardness result showed the improvement was significant. With the powders preheating at 1200℃ and 4 hour, the content of boron weight percent is 3.12, the corresponding hardness of the material is about HV 590.The hot press sintering material was undergone the canning hot-extrusion processing. The large plastic deformation was performed with the extrusion ratio parameter was 16:1. The morphology of whiskers was changed to arranging along with the extrusion direction from the original statistical orientation in a network. With the powders were preheating at 1200 ℃ with 1.56wt% B and fixing the extrusion temperature is 1100℃ the tensile strength of the extruded sample was increased to 1356 MPa, and the elongation was increased to 4.8%. The EBSD results showed the <0001> texture of α phase was remarkably.Utilize the laser cladding process to deal with the preheating powders which holding time is 1 hour and 4 hour respectively. Adjust laser output power to 820 W, the thickness of melting pool is about 740μm. In this area, the distribution of whiskers was regularity. Below the melting pool the matrix powders were not melted, the thickness of this area is about 345μm. It is visible that the distribution of whiskers was uniform on the surface of matrix powders with quasi-continuous shape in space. The avarge hardness number range from HV 480 to 540.