Foils are thin sheets of metal such as tin foil, copper foil, aluminium foil etc. Specifically, any sheet of metal <0.2mm thick is called a foil. The thickness of the metal foil varies dividing it into thick foil, single-zero foil and double-zero foil. Among them, thick foil is <0.2mm - ≥0.1mm thick, single zero foil is <0.1mm - ≥0.09mm thick, double zero foil is ≤0.001mm thick and the thinnest foil that can be industrially rolled is currently 0.0045mm thick.Nowadays, aluminium foil is the largest producer, accounting for almost 85% of the world's total metal foil production, and China is the world's largest producer and exporter of aluminium foil. Production in 2020 amounts to 4050kt, about 65% of the global total, and exports account for 30% of production, amounting to 1215kt.
The United States, France, etc. use superplastic forming technology to process this kind of titanium foil heat insulation trigger, however, although this process can be processed out of high quality heat insulation board, but the production cycle is long, and the workpiece for a long time at high temperature, will be more or less absorb oxygen, titanium in 649 ℃ above no longer has the ability to resist oxidation, will absorb more oxygen and become brittle, so that the mechanical properties of the overall decline. In addition, superplastic forming is suitable for the preparation of simple workpiece shapes, for the complex workpiece shown in Figure 2 is difficult to superplastic forming, because the processing of the mould is not an easy task.
As the heat shield is diffusion welded, the oxygen content should be precisely and strictly controlled, so it is best to weld in a vacuum furnace, but this requires additional expensive equipment, which increases the investment and also makes the operation more difficult. For this reason, Cheng Xiuquan and others from the Beijing Institute of Aeronautical Technology have successfully prepared qualified low-cost, high-quality and efficient titanium foil aircraft heat insulation plates using a semi-coupled mould and a protective cladding process, which has been highly praised by users.
During production, the TB2 alloy foil is first placed inside the envelope, and the oxygen inside the envelope is removed prior to thermoforming, which can be removed by conventional evacuation methods, and the envelope is sealed after evacuation. The following matters should be noted when using the cladding technology: carefully clean the titanium sheet and cladding surface of the oil and debris, must be dry and clean; strict selection of cladding materials, in high temperature does not leak, itself does not emit any gas, should also be as far as possible to choose high temperature strength of lower materials, in order to reduce deformation resistance, improve mould life, to ensure that the workpiece size accuracy requirements; cladding should be coated with a thin and uniform layer of stop welding flux, so as to avoid cladding A thin and uniform layer of solder stop should be applied to the cladding to prevent the cladding from being welded to the titanium foil, but the solder stop should not emit any harmful gases at high temperatures and should not contain water, glue or other substances.
The prepared package is shaped at 750°C, the pressure must not be too high, preheated for 1min-2min, kept under pressure for 20s-30s, the package is removed and the workpiece can be taken out after it cools down. The measured results show that the oxygen content of the surface layer is <200ppm and there is no change in the thickness of the oxide layer. It can be seen that the wrapper not only protects the workpiece from contamination, but also takes up most of the uneven deformation, greatly improving the uniformity of the deformation of the workpiece. The measured thickness of the deformed workpiece is 0.067mm-0.072mm, which is well within the deviation of the original foil thickness.
