Causes Of Black Streak Defects in The Rolling Process Of TC4 Titanium Bars

TC4 (Ti-6Al-4V) is a martensitic a+/3 two-phase titanium alloy with good comprehensive performance, and its service temperature can reach 450, which is widely used in important structural parts of the aviation industry, such as wing blades and aero-engine disks. Since the TC4 titanium bar is a two-phase titanium alloy, if the micro-zone composition is not uniform, it will inevitably cause abnormalities in the macro-organization and micro-organization, which will lead to significant differences in the hardness of the abnormal region and the normal region, so that the material in the overall performance of the performance of the unevenness, and ultimately sprouting fatigue crack sources, bringing great potential danger to the safety of the parts use, as well as reducing the service life of the alloy. In order to accurately determine the type of defects in the black streak defects found in a TC4 titanium alloy bar product during low magnification inspection, a metallurgical microscope was used to observe the microstructure and determine the abnormal areas of the metallurgical organization; a scanning electron microscope (SEM) was used to analyze the black streak areas as molybdenum-rich and aluminum-poor chemical precipitation defects; and a micro-hardness test was conducted to determine that the compositional precipitation in the black streak areas was non-brittle precipitation. The test results show that, according to the above method can effectively determine the TC4 titanium alloy compositional segregation and its type; and determined that this type of defect does not affect the use of the excision can be delivered. Such defects can be reduced or eliminated by controlling the selection of raw materials for titanium alloy ingots, the mixing and electrode preparation process, and the voltage and current in the melting process. Titanium alloy segregation, according to the segregation site and the normal area of the hardness of the high and low difference classification, can be divided into hard segregation (hardness higher than the normal area, also known as brittle segregation) and soft segregation (hardness is lower than the normal area, also known as non-brittle segregation) two categories. If there is only non-brittle segregation in the product, and the performance meets the requirements of the product standard, the general excision of the segregation of the product can still be delivered for use; brittle segregation is not allowed to be removed from the delivery, the whole batch should be scrapped. The author discusses the analysis and determination method of non-brittle segregation encountered in TC4 titanium alloy bar segregation, aiming to provide reference for product inspection to improve product quality.

a. On the visual discovery of the TC4 titanium bar black streak defects, the use of metallurgical microscopy to observe its microstructure, the defective area is not much different from the normal area, can not determine the type of defects; further scanning electron microscopy on the defective area of the titanium bar chemical composition analysis, the defective area for the aluminum-rich vantage point of the chemical elements of the segregation; and ultimately combined with the micro-hardness test to determine the type of the segregation of the TC4 titanium bar aluminum-rich non-brittle segregation; brittle segregation; and finally combined with the micro-hardness test to determine the type of segregation encountered in the TC4 titanium bar segregation. Brittle segregation; through microstructure observation, micro-area composition analysis and microhardness test combined method, can effectively determine the TC4 titanium alloy composition segregation and its types.
b. The segregation in the TC4 titanium alloy bar is a non-brittle segregation of aluminum-rich and aluminum-poor, which does not affect the use, and can be delivered after excision; such defects can be reduced or eliminated by controlling the selection of raw materials, mixing and electrode preparation parameters, and the voltage and current in the melting process.