Titanium and titanium alloys made by powder metallurgy. Titanium has high chemical activity and is susceptible to contamination by gases and crucible materials. Therefore, high-quality titanium powder is mainly produced by a centrifugal atomizing powder process under the protection of vacuum or high-purity inert gas. The forming of the product generally does not add a binder, and the blank must be sintered in vacuum [1]. At the end of the 1940s, the first research on the pressing and sintering process using sponge titanium powder as raw material. However, the performance of the products produced by this process can not meet the requirements of the aviation sector, and is mainly used to manufacture corrosion-resistant, filtering and other parts required by the chemical, light industry, metallurgy, marine development and other departments. The first product to be used in industrial production is titanium porous filter material. In the mid-1960s, the development of the pre-alloyed titanium powder by the rotating electrode method and the process of hot isostatic compaction began. The static mechanical properties of the products produced by this process are equivalent to those of smelting and processing products, but the cutting process is significantly reduced, and the utilization rate of materials is increased, which is used in the aviation industry. By the end of the 1970s, the application of titanium powder metallurgy products in corrosion resistance and aerospace had achieved rapid development. China began to conduct research on titanium powder metallurgy processes and products in the early 1970s. Titanium metal valves, bushings, porous tubes and plates, titanium-titanium carbide wear-resistant materials, and titanium-molybdenum corrosion-resistant alloys have all been industrially produced. In the late 1970s, researches were conducted on the production of high-quality titanium alloy powder by centrifugal atomization and hot isostatic pressing.
Industrially produced titanium and titanium alloy powders mainly have the following four methods: ① sodium reduced sponge titanium powder. This kind of powder has large output, low price, good powder plasticity, and is suitable for cold forming. It is the main raw material for the production of general corrosion-resistant products. Because it contains higher sodium and chloride ions, the equipment is easily contaminated during sintering and the welding performance of the material is deteriorated. ② Electrolytic titanium powder. The purity is higher than that of sponge titanium powder reduced by sodium, but the formability of electrolytic titanium powder is inferior to sponge titanium powder. ③ Hydrogenated dehydrogenated titanium powder. The titanium powder and its alloy powder with good quality and fine particle size can be obtained through the hydrodehydrogenation process; however, the batch is small and the price is relatively expensive. ④Centrifugal atomized titanium powder. In the 1960s, American Nuclear Metal Company first made the titanium pre-alloy powder by the arc rotating electrode method. The powder is spherical. This alloy powder has high purity, uniform composition, good fluidity, and packing density of 65% of theoretical value. This type of powder should only be thermoformed, and hot isostatic pressing process can be used to make complex shaped parts. The arc rotating electrode powdering process, because one electrode is a tungsten rod, it is easy to produce tungsten pollution (the tungsten content in the powder reaches 400ppm), which will reduce the fatigue performance of the material. In order to eliminate the pollution of tungsten, a variety of centrifugal atomization powdering processes such as rotating electrodes, arc rotating crucibles, and electron beam rotating discs using electron beams or plasma as heat sources have also been developed. Such powder is the main raw material for manufacturing titanium powder metallurgy aviation parts.
According to the powder raw materials used and the use requirements of the products, different densification processes can be used. The main are: ① sintering of loose or low-pressure forming powder blanks. The sintered body has a porosity of 25 to 70%, and this process is mostly used to produce porous titanium products and materials. ②Pressing and sintering. The pressing can be mechanical or isostatic pressing, the pressing pressure is between 2000-4000kgf/cm2, and the sintering temperature is 1000-1400°C. The relative density of the sintered body can reach 92-95%; this process is used to produce corrosion-resistant parts, general structural parts or ingots for processing. ③ Thermoforming. It is to make the powder or powder ingot to be pressed and formed during heating. The thermoforming process mainly includes hot isostatic pressing and vacuum hot pressing. The titanium alloy powder obtained by centrifugal atomization is formed by hot isostatic pressing. The density of the resulting product can be close to the theoretical density and the performance is uniform.The high-performance aviation structural parts are mainly made by hot isostatic pressing process. . ④Titanium powder and powder powder rolling. Extrusion and forging of blanks. These processes can obtain titanium powder materials or parts with high density and good performance. Among them, powder rolling is an effective and economical processing technology for producing porous titanium plates and dense titanium plates. The typical properties of titanium and Ti-6Al-4V processed by powder metallurgy and smelting are listed in the table above.
Titanium Alloy Active Flux Welding Technology