Foil is a thin metal sheet, such as tin foil, copper foil, aluminum foil, etc. Specifically, any metal sheet with a thickness of <0.2mm is called foil. According to the thickness of the metal foil, it is divided into thick foil, single zero foil and double zero foil. Among them, the thickness of thick foil is <0.2mm-≥0.1mm, the thickness of single-zero foil is <0.1mm-≥0.09mm, the thickness of double-zero foil is ≤0.001mm, and the thickness of the thinnest foil that can be rolled industrially is currently 0.0045mm. At present, the output of aluminum foil is the largest, accounting for almost 85% of the total output of metal foil in the world. China is the world's largest aluminum foil producer and exporter. The output in 2020 will reach 4050kt, accounting for about 65% of the global total. 30% of the output, reaching 1215kt.
Titanium foil is difficult to produce, high in price, and low in output. It is estimated that the world's output in 2020 will not exceed 20 tons. The main producing country is the United States, and it is not widely used. Aircraft skin insulation material is one of its high-end application products. . We all know that when the aircraft is flying in the air, its outer metal skin will heat up due to intense friction with the air and other reasons. In order to prevent heat from entering the interior, a thermal insulation layer must be provided (Figure 1). It consists of four layers of heat shield welded together with a thinner flat sheet sandwiched in between. Figure 2 is a diagram of the parts of the heat shield, which is made of 0.08mm TB2 alloy foil, TB2 is a β-type titanium alloy, Ti-5Mo-5V-8Cr-3Al alloy (mass %): Al2.5-3.5, Mo4.7-5.7, V4.7-5.7, Cr7.5-8.5, Fe0.30, C0.05, N0.04, H0.015, O0.15, other impurities are individually 0.1, total 0.4, the rest Ti.
The United States, France, etc. use the superplastic forming process to process this kind of titanium foil heat insulation board. However, although this process can process high-quality heat insulation boards, the production cycle is long, and the workpiece is in a high temperature state for a long time. Less oxygen is absorbed, titanium no longer has anti-oxidation ability above 649 ℃, it will absorb more oxygen and become brittle, and its mechanical properties will be reduced in an all-round way. In addition, superplastic forming is suitable for preparing workpieces with simple shapes. For complex workpieces shown in Figure 2, superplastic forming is difficult, because machining molds is by no means easy.
Since the heat shield is made of diffusion welding, the oxygen content should be precisely and strictly controlled, so it is best to weld it in a vacuum furnace, but this requires the addition of expensive equipment, increases the investment, and increases the difficulty of operation. To this end, Cheng Xiuquan of Beijing Institute of Aeronautics and Technology proposed to successfully prepare qualified low-cost, high-quality and high-efficiency titanium foil aircraft heat shields with semi-coupling molds and wrapping protection processes, with good results and high praise from users.
During production, the TB2 alloy foil is first placed in the envelope, and the oxygen in the envelope is removed before thermoforming, which can be removed by conventional vacuuming, and the envelope is sealed after suction. The following matters should be paid attention to when adopting the wrapping technology: carefully clean up the oil stains and debris on the surface of the titanium sheet and the wrapping, and be sure to be clean; Materials with low high temperature strength should also be selected as much as possible to reduce deformation resistance, improve mold life, and ensure the dimensional accuracy of the workpiece; a thin and uniform layer of solder stopper should be applied on the sleeve to prevent the sleeve from welding with the titanium foil, but The solder stopper cannot emit any harmful gas at high temperature, and cannot contain substances such as water and glue.
The prepared sleeve is formed at 750 ℃, the pressure should not be too high, preheat for 1min-2min, hold the pressure for 20s-30s, take out the sleeve, and take out the workpiece after it cools down. The surface brightness of the workpiece is almost the same as that of the original foil. . The actual measurement results show that the oxygen content of the surface layer is less than 200ppm, and the thickness of the oxide layer does not change. It can be seen that the wrapping not only protects the workpiece from contamination, but also undertakes most of the uneven deformation, which greatly improves the deformation uniformity of the workpiece. The measured thickness of the deformed workpiece is 0.067mm-0.072mm, which is completely within the thickness deviation range of the original foil. .
