Causes of black bar defects in the rolling process of TC4 titanium rods!

TC4 (Ti-6Al-4V) is a Martens a/3-type two-phase titanium alloy with good comprehensive properties, and its service temperature can reach 450. It is widely used in important structural parts of the aviation industry, such as wing blades, aero engine discs, etc. Because the grade 5 titanium round rod is a two-phase titanium alloy, if the composition of the microscopic area is not uniform, it will inevitably lead to abnormal macrostructure and microstructure, resulting in a significant difference between the hardness of the abnormal area and the normal area, and the overall performance of the material is uneven. Causes fatigue crack sources, brings great hidden dangers to the safety of parts, and reduces the service life of the alloy. Aiming at the black streak defects found in the low-magnification inspection of a TC4 titanium alloy bar product, in order to accurately determine the defect type, a metallographic microscope was used to observe the metallographic structure and determine the abnormal area of ​​the metallographic structure. Then, a scanning electron microscope was used to analyze the chemical composition segregation defects in the black stripe area rich in molybdenum and poor in aluminum. Through the microhardness test, it is determined that the component segregation in the black stripe area is non-brittle segregation.

The experimental results show that the composition segregation and type of TC4 titanium alloy can be effectively determined according to the above method. And it is determined that this type of defect will not affect the use and can be delivered after removal. Such defects can be reduced or eliminated by controlling the selection of raw materials for titanium alloy ingots, material mixing, electrode preparation and voltage and current during smelting. The segregation of titanium alloys can be divided into hard segregation (hardness higher than normal zone, also called brittle segregation) and soft segregation (hardness lower than normal zone, also called non-brittle segregation) according to the difference between the hardness of segregation site and normal zone. If there is only non-brittle segregation in the product, and all properties meet the requirements of the product standard, the product can still be delivered for use after the segregation is eliminated. Fragile segregation is not allowed to be delivered after removal, and should be discarded in batches.

A. For the black streak defect of grade 5 titanium bar found by visual inspection, the structure is observed through a metallographic microscope. The defect area is not much different from the normal area, and the defect type cannot be judged; in addition, the chemical composition of the defect area of ​​the titanium rod is analyzed by scanning electron microscope. It is found that the defect area is the segregation of chemical elements rich in heavy and poor in aluminum. Finally, combined with the microhardness test, it is determined that the segregation type of TC4 titanium rod is the non-brittle segregation of rich aluminum and poor aluminum. Through microstructure observation, micro-area composition analysis and microhardness test, the composition segregation and type of TC4 titanium alloy can be effectively determined.

B. The segregation in TC4 titanium alloy bar is non-brittle segregation of rich and poor aluminum, which does not affect the use and can be continuously transported after cutting; by controlling the selection of raw materials, mixing and electrode preparation parameters, as well as the voltage and current during the smelting process, it can be Reduce or eliminate such defects.