Ti-4Al-2V alloy is a ternary titanium alloy, which contains a small amount of α-stable element Al to improve the thermal strength of the alloy, and a small amount of β-stable element V can refine the grain and improve the plasticity of the process. Its cold workability is better than that of GR5 alloy. Ti-4Al-2V alloy has medium strength, excellent welding performance and water corrosion resistance, high static and cyclic strength, heat treatment and subsequent sandblasting have no effect on the strength, plasticity and low cycle fatigue performance of the alloy. Ti-4Al-2V alloy was developed by Prometheus Central Institute of Structural Materials Science of the Soviet Union. Its international designation is ЛТ-3B alloy. It is mainly used in shipbuilding, atomic energy, chemical industry, aviation and other fields, especially as a ship shell material. , its output once accounted for 1/3 to 1/2 of the Soviet titanium alloy finished products. The main product forms of this alloy in my country are plates, forgings, bars and wires.
Compared with traditional common metal materials, titanium alloys are more difficult to process, so various defects will occur in the production process. Even so, there are few research reports on surface crack defects of Ti-4Al-2V alloy cold-rolled sheets. Researchers analyzed the strip crack defects produced in the cold rolling process of Ti-4Al-2V alloy sheets, and found out the reasons for their formation, so as to provide technical support for the production of Ti-4Al-2V alloy sheets with qualified surface quality.
The basic process of Ti-4Al-2V alloy cold-rolled plate production is: alloy smelting→slab forging→hot rolling→cold rolling. Sampling was carried out on the cold-rolled sheet with surface strip cracks for chemical composition analysis, and the surface and cross-section samples of cold-rolled sheet with strip cracks were sampled for morphology and micro-area composition analysis. Stereoscopic observation was carried out on a DM6000 stereoscope, and a JSM-7001 scanning electron microscope and an attached INCA-7557 energy spectrometer were used to analyze the crack surface, cross-sectional morphology and micro-area composition.
The analysis found that the strip cracks contain a large amount of oxygen, and it can be seen that the appearance of strip cracks on the surface of the cold-rolled sheet has a great relationship with the oxidation of the sheet. The Ti-4Al-2V alloy sheet will not be oxidized during cold rolling, and the sheet oxidation can only occur during the annealing process. Therefore, it is inferred that these oxidation defects were not completely removed during the alkali pickling process after annealing and remained on the plate surface, resulting in strip cracks along the rolling direction in the subsequent further cold rolling process.
First of all, during the annealing process before cold rolling, the surface of the plate is oxidized, and where there are quality defects on the surface of the plate, such as grain boundary outcropping, inclusions and other defects provide channels for oxidation, and the oxidation of these parts is deeper. In the subsequent surface cleaning process such as alkali pickling, the parts with surface defects are deeply oxidized and the alkali pickling time is insufficient, so the oxide layer cannot be completely removed and remains on the board surface. These remaining oxidation defects are elongated along the rolling direction and broken into a series of small holes in the subsequent cold rolling process because of their poor plasticity and cannot cooperate with the deformation of the matrix. The small holes in the steel are further elongated and enlarged, separated from each other, and the brittle oxides partly fall off, finally forming linear jagged strip crack defects. The exposed part of the plate falls off due to a certain amount of solid solution of oxygen, which makes the plasticity worse than that of the matrix. During the rolling process, it cannot produce consistent plastic deformation with the matrix and a large number of microcracks appear, because the oxygen content of this part of the oxygen-enriched layer is low. At the same time, the part with high oxygen content has fallen off, so that the oxygen in the remaining part cannot be detected by energy spectrum, so no obvious oxygen element is found in the energy spectrum analysis part.
The above analysis shows that in order to eliminate the strip cracks of Ti-4Al-2V slab, it is first necessary to ensure that the alloy has no metallurgical inclusion defects, and vacuum consumable arc melting has a limited effect on alloy removal and purification, so the quality of raw materials must be controlled, namely To ensure the quality of sponge titanium and V-Al alloy. The second is to ensure sufficient alkali pickling time after annealing to remove surface oxidation defects. Third, quality defects such as surface holes caused by metallurgical defects should be cleaned up before cold rolling.
