Defect Analysis of Medical TC4ELI Titanium Alloy Plate
Abstract: A certain medical TC4ELI titanium alloy plate was found to have penetrating bright band defects during low-magnification inspection. The types and causes of defects were analyzed through metallographic examination, scanning electron microscope analysis, energy spectrum analysis and hardness testing. Results It shows that this defect is a titanium-rich + interstitial element segregation defect, which is caused by the uneven particle size of titanium sponge and the uneven distribution of the intermediate alloy mixture during the production process of titanium alloy ingots. It is recommended to reduce or eliminate this defect by controlling the raw materials and smelting process. defect.
TC４ELI titanium alloy has become a medical surgical implant due to its good biocompatibility, low elastic modulus, low density, good anti-corrosion properties, non-toxicity, high yield strength, long fatigue life, large plasticity at room temperature, and easy forming. An ideal material for medical devices [1-2]. Medical TC4ELI titanium alloy plates are mainly used in skull repair, bone grafting, etc., which have higher requirements on strength, fatigue life, plasticity, etc. According to GB/T 13810-2017«Surgical Implantation Titanium and titanium alloy processing materials for medical use», if segregation, metallic or non-metallic inclusions and other visually visible metallurgical defects are found in the low-magnification structure of titanium alloy materials used in implant products, the batch of products will be judged to be unqualified .Segregation is the manifestation of uneven micro-region composition of titanium alloy materials in the structure. Medical TC4ELI plate is an α+β type two-phase titanium alloy. If its micro-region composition is uneven, it will cause abnormalities in the macro and microstructure, leading to abnormalities. There is a significant difference in hardness between the normal area and the normal area, which will lead to uneven overall performance of the titanium alloy material, thereby reducing the material strength, fatigue life and plasticity, and ultimately leading to early material failure [3-10].
During low-magnification observation of a certain medical TC4ELI titanium alloy plate, an abnormal band-shaped area with a width of about 5mm was found. When a part of it was intercepted and observed at low-magnification, it was found that the band-shaped area was a bright band. In order to accurately determine the type of defect, it is necessary to identify the defect. The author has examined and analyzed the causes.
1 Physical and chemical testing
1.1 Metallographic examination
Use Observer. The AIM-type ZEISS metallographic microscope was used to conduct metallographic examination of the bright band area and normal area of TC4ELI titanium alloy plate. As can be seen from Figure 2, the bright band area is a single-phase equiaxed α structure, showing segregation-like structural characteristics, while the normal area is In the structure of a typical TC4ELI titanium alloy processed in the α+β two-phase region, all the original β grain boundaries are fully broken, so it can be determined that the bright band area is a segregation defect.
1.2 Scanning electron microscopy analysis
The JSMG6700 cold field emission scanning electron microscope (SEM) was used to analyze the morphology of the bright band area and normal area of the TC4ELI titanium alloy plate. As can be seen from Figure 3, the single-phase equiaxed structure in the bright band area is clearer, and the normal area shows α+β The characteristics of the processed structure of the two-phase area are consistent with the results of metallographic examination, and it is further determined that the bright band area is a segregation defect.
1.3 Energy spectrum analysis
The energy spectrometer (EDS) attached to the scanning electron microscope was used to perform micro-component analysis on the bright band area and normal area of the TC4ELI titanium alloy plate. The analysis results are shown in Table 1. It can be seen that the vanadium content in the normal area is slightly higher than the standard value. In addition, the contents of other elements are in compliance with the requirements of GB/T 3620.1-2016 "Titanium and titanium alloy grades and chemical compositions"; the titanium, aluminum and vanadium contents in the bright band area are not within the standard range, and there is obvious titanium-rich content. , poor in aluminum and poor in vanadium, and the content of oxygen element is the upper limit of the standard range, it is judged that the TC4ELI titanium alloy plate has titanium-rich segregation defects.
1.4 Hardness test
The segregation of titanium alloys can be divided into hard segregation (the hardness of the segregation part is higher than the hardness of the normal zone, also known as brittle segregation) and soft segregation (the hardness of the segregation part is lower than the hardness of the normal zone) according to the difference between the hardness of the segregation part and the normal zone. Also known as non-brittle segregation). Micro-Vickers hardness tests were conducted on the bright band area and normal area of the TC4ELI titanium alloy plate. The measured results were 383HV and 327HV respectively. It can be seen that the hardness of the bright band area is significantly higher than that of the normal area. The segregation type in the bright band area is brittle segregation[11].
2 Analysis and Discussion
The bright band area of TC4ELI titanium alloy sheet is a segregation defect. This defect is caused by the incomplete alloying of the intermediate alloy particles. It is a titanium-rich segregation, but it is not a common titanium-rich segregation because the hardness of the titanium-rich segregation area should be It is lower than the normal area [12], and the hardness of the segregation defect area (bright band area) of TC4ELI titanium alloy sheet is higher than that of the normal area, which is consistent with the characteristics of segregation of interstitial elements. Interstitial elements specifically refer to oxygen, carbon, and nitrogen. elements. The high content of oxygen in the segregation defect area verifies this result. The enrichment of interstitial elements will increase the beta phase transformation temperature of titanium alloys, increase the alpha phase hardness, and make the material brittle. In summary, TC4ELI titanium The type of segregation defects in alloy sheets is titanium-rich + interstitial element segregation.
The cause of this segregation defect is mainly related to the smelting process of titanium alloy. The segregation defect has already been formed in the production of ingots. At present, China's titanium alloy production enterprises generally adopt the three-pass vacuum melting consumable electric arc furnace smelting method, which is operated during the electrode preparation process. Improper use can easily lead to metal contamination or the formation of refractory oxides and nitrides. Improper selection of current and voltage will cause the melting zone to fail to achieve thermal balance during the melting process, and will also cause changes in the depth of the molten pool, resulting in uneven particle size of titanium sponge. The uneven distribution of the master alloy mixture will cause the enrichment and depletion of alloy elements in local areas of the material, causing the phase transformation point in this area to deviate. During the subsequent hot processing process, it will gradually evolve into an abnormal structure and form segregation defects. [12G18].
3 Conclusions and suggestions
TC4ELI titanium alloy sheets have titanium-rich + interstitial element segregation defects. This defect is caused by the uneven particle size of titanium sponge and the uneven distribution of the intermediate alloy mixture during the production process of titanium alloy ingots.
It is recommended to reduce or eliminate such defects by strengthening the control of raw materials and mixtures, as well as the selection of voltage and current during electrode preparation and smelting processes.
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