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Causes of black bar defect in TC4 titanium rod rolling process

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The TC4 (Ti-6Al-4V) is a two-phase titanium alloy at temperatures up to 450. widely used in the aviation industry important structural parts, such as wing blade, aviation engine wheel. Because TC4 titanium rod for two-phase titanium alloy, if the micro region composition is uneven, will inevitably lead to macroscopic organization and microstructure, abnormal area and normal area hardness has significant difference, make the overall material shows uneven performance, eventually lead to fatigue crack source, bring great hidden trouble to the safety of the parts, reduce the service life of the alloy.

According to the black stripe defects found in the low-fold test of a TC4 titanium alloy rod product, in order to accurately determine the type of defects, a metallographic microscope was used to observe the metallographic tissue and determine the abnormal area of the metallographic tissue. Then the chemical segregation defects of molybdenum-rich and poor aluminum in the black stripe zone were analyzed by SEM. Compositional segregation of the black-stripe region was determined by microhardness testing.

The experimental results show that the composition analysis of TC4 titanium alloy can be effectively determined according to the above method. It is also determined that this type of defect does not affect use and can be delivered after removal. Such defects can be reduced or eliminated by controlling the voltage and current of titanium alloy ingot raw materials, material mixing, and electrode voltage and current during preparation and smelting. The segregation of titanium alloy can be divided into hard segregation (hardness is also called brittle segregation), and the hardness is higher than normal area) and soft segregation (hardness is lower than normal area). If only non-brittle segregation is in the product and all performance meets the product standards, the product remains deliverable after segregation elimination. Frittlement segregation is not delivered after removal and shall be discarded in batches.

A. For the black stripe defect of TC4 titanium rod, the tissue was observed by metallophase microscope, and the defect area is not different from the normal area, and the defect type cannot be judged; moreover, the chemical composition of the defect area of titanium rod is analyzed by scanning electron microscope, and the defect area is rich in heavy and aluminum-poor chemical element segregation. Finally, combined with the microhardness test, the segregation type of the TC4 titanium rod was determined. The gregation and its type of TC4 titanium alloy can be effectively determined by microtissue observation, microregion composition analysis, and microhardness testing.

B. The segregation in T C 4 titanium alloy rod is a non-brittle segregation of heavy-rich and poor aluminum, which does not affect the use and can be continuously transported after cutting; this defect can be reduced or eliminated by controlling the raw material selection, mixing and electrode preparation parameters as well as the voltage and current in the smelting process.