Research On The Cause Analysis And Detection Method Of Defects In The White Bright Block In The Center Of TC11 Titanium Alloy Bar

Research On The Cause Analysis And Detection Method Of Defects In The White Bright Block In The Center Of TC11 Titanium Alloy Bar

 

TC11 is a kind of α+β thermally strong titanium alloy with good overall performance, with a maximum long-term operating temperature of up to 500℃. It is widely used in rotating parts such as aero-engine compressor discs, blades, drums, etc., and aircraft structural parts in our country. The alloy has good thermal processing technology and can produce a variety of products such as bars and forgings.

 

The internal quality of titanium alloy products directly affects the reliability of aviation engines and aircraft. At present, the conventional inspection method for the internal quality of TC11 products by titanium factories in Baoji area is ultrasonic flaw detection. However, ultrasonic flaw detection has limitations in detecting certain types of defects. Recently, when a certain batch of TC11 titanium alloy bars were all qualified for ultrasonic flaw detection, it was found that there was a continuous white bright block defect in the center of the bar during the low-magnification inspection. Through metallographic testing, energy spectrum analysis and mechanical properties testing, this paper systematically analyzes the cause of the defect and provides technical reference for subsequent production process control.

 

1. Test Materials And Methods

The test material is TC11 titanium alloy bar, and the nominal composition is Ti-6.5 Al-3.5 Mo-1.5Zr-0.3Si. The preparation process route is as follows: Φ750mm ingot is obtained by three vacuum self-consumption smelting→ two upsetting and two drawing blanks in the β phase area→ one upsetting in the α+ β phase area + multiple fire times drawing length→ Φ230mm bar is finally obtained→ double annealing and heat treatment according to the standard → turning light→ ultrasonic flaw detection. The ultrasonic flaw detection results showed that all the bars in this batch were qualified and there were no abnormal reflected waves.

 

The low-magnification test sample was intercepted on the bar, and it was found that there were silver-white bright spots (that is, “white bright spots”) in the central area. Continuous sampling and re-inspection at the same location, the white bright blocks reappeared repeatedly, and it was determined to be a continuity defect. For the white bright block area, a metallographic microscope was used to observe the tissue differences, a scanning electron microscope was used to observe the microscopic morphology, and an energy spectrometer was used to quantitatively analyze the composition of the micro-region. At the same time, samples were taken from the white bright block and away from the white bright block for room temperature tensile tests to test tensile strength, yield strength and elongation.

 

2. Test Results And Analysis

1) Low Magnification And Metallographic Inspection

The low magnification tissue showed that there were obvious silver-white bright spots in the central area of the bar, and there was no obvious boundary between the edge and the substrate, showing a continuous distribution. High-magnification metallographic observations show that the α-phase content of the white bright block area is significantly lower than that of the β-phase, and the tissue morphology is significantly different from the surrounding matrix, and there is no obvious transition zone or boundary between the white bright block and the matrix, excluding the possibility of inclusions, cracks and other discontinuous defects, it was initially determined to be component segregation.

2) Energy Spectrum Analysis

The multi-point energy spectrum composition analysis of the white bright block area and the matrix area were carried out separately. The results showed that the Al content at the white bright block was about 4.82%, while the matrix was 6.45%, a decrease of about 25.3%; the Mo content was 2.15%, and the matrix was 3.52%, a decrease of about 38.9%; the Zr content was 0.91%, and the matrix was 1.48%, a decrease of about 38.5%; the Si content was 0.18%, and the matrix was 0.31%, a decrease of about 41.9%.At the same time, the Ti content at the white bright block increased significantly relative to the matrix. The above data clearly show that the four main alloying elements (Al, Mo, Zr, and Si) at the white bright block are all significantly depleted, showing typical titanium-rich segregation characteristics, which belong to the negative segregation of alloying elements.

3) Tensile Properties At Room Temperature

The results of the room temperature tensile test show that the tensile strength of the sample taken from the white bright block is about 1012MPa, the yield strength is about 935MPa, and the elongation is about 11.8%; while the tensile strength of the sample taken from the normal area is about 1035MPa, the yield strength is about 960MPa, and the elongation is about 14.5%.Comparison shows that the elongation at the white bright block decreased by about 18.6%, and the tensile strength and yield strength were also slightly reduced, indicating that the segregation area has a significant adverse impact on the plastic reserve of the material, which will weaken the material's processing and forming ability and service reliability.

 

3. Defect Cause Analysis And Process Control Recommendations

1) Determination Of The Nature Of The Defect

Based on the differences in metallographic structure (decrease in α phase, relative increase in β phase) and energy spectrum composition data (Al, Mo, Zr, Si are all low, Ti is high), it can be clearly determined that the white bright block is a component segregation defect, the specific type is titanium-rich segregation (alloying element depleted type).In TC11 alloy, Al is an α-phase stable element, and Mo, Zr, and Si are β-phase stable or enhanced elements. The depletion of these elements directly leads to a decrease in the α-phase content and a relative increase in the β-phase content in the white bright block area. Tissue imbalance, and cause a decrease in mechanical properties, especially plasticity.

 

2) Analysis Of The Source Of Segregation

The segregation has a certain depth and is continuously distributed, excluding the possibility of local accidental factors. The analysis believes that the root cause lies in the uneven mixing of raw materials. Vacuum self-consuming smelting is a regional smelting. Melting and solidification are carried out at the same time. The bath is shallow and the solidification is fast. The macroscopic convection and diffusion effects are limited, and the homogenization effect of smelting is relatively poor. If the sponge titanium and the intermediate alloy are not fully mixed evenly in the mixing and electrode pressing stages, the problem of uneven composition of the ingot after smelting will continue, and eventually a strip-like or flocculent titanium-rich segregation area will be formed in the bar.

 

3) Process Improvement And Testing Recommendations

Based on the above analysis, the following suggestions for process control and inspection improvement are put forward:

① Strengthen the quality control of the mixing process. The particle size matching of sponge titanium and intermediate alloys should be optimized, the mixing time should be extended and the uniformity should be verified regularly, and automatic mixing equipment and online uniformity monitoring methods should be introduced if necessary to ensure that the mixture is fully uniform before the electrode is pressed.

② After the ingot is smelted, the uniformity of the composition is sampled. Conduct multi-point sampling and analysis of the head, middle, tail and edges of each batch of ingots to identify the tendency to segregation in advance and avoid problems flowing into subsequent processing processes.

③ For aviation-grade TC11 bars, low-magnification inspection should be listed as a mandatory inspection item for each batch on the basis of ultrasonic flaw detection, and the number of samples should be increased, paying special attention to the low-magnification organization in the central area. For batches where segregation is found, energy spectrum analysis should be further used to confirm the type and degree of segregation, and determine whether the product is available or needs to be scrapped based on the degree of influence of segregation on mechanical properties.

④ For the existing segregation bars, it can be explored to remedy them through subsequent high-temperature and long-term homogenization annealing, but its effect and cost feasibility need to be verified.

 

4. The Conclusion

In this case, the white bright block defect of TC11 titanium alloy bar belongs to a typical titanium-rich segregation, which is due to uneven mixing. It was completely missed in ultrasonic flaw detection, and finally found by low magnification inspection. This defect significantly reduces the elongation of the material and poses a potential threat to the safe use of aviation rotating parts. Through the combination of metallographic inspection, energy spectrum analysis and mechanical properties testing, such segregation defects can be effectively identified and determined, and provide a scientific basis for product quality determination and process improvement. It is recommended that relevant production enterprises attach great importance to the uniformity of mixing and low-magnification inspection links, and improve the quality control system to ensure that the internal quality of TC11 titanium alloy products is stable and reliable, and meets the high standards of aviation applications.