To evaluate the quality of metallurgy of titanium and titanium alloy ingots

Author Topic: To evaluate the quality of metallurgy of titanium and titanium alloy ingots  (Read 13889 times)

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   To evaluate the quality of metallurgy of and titanium alloy ingots, the main points are as follows: 1 uniform chemical composition, not only meet the standard requirements of various alloy elements, but also stably control at an optimal level; 2 main impurities ( The content of Fe, O, etc. is controlled in an appropriate range, and other impurities meet the standard requirements; 3 there are no metallurgical defects such as impurities, segregation, pores, cracks, shrinkage holes and looseness in the ingot; 4 the surface of the ingot is smooth, no cold separation, wrinkles, etc. Surface defects, small amount of head shrinkage and high ingot yield; 5 reasonable shape and precise size, suitable for pressure processing requirements, otherwise it will increase process waste and reduce the yield. The metallurgical defects associated with the smelting process are mainly points 3 and 4, namely composition segregation and surface quality. The segregation in titanium alloy mainly includes two types of α segregation and β segregation. During the melting process, the ingot continuously solidifies continuously from the bottom to the top in the crystallizer. The cooling conditions, the shape and depth of the molten pool are not constant, and the partition coefficients of the alloying elements during solidification and crystallization are different. The alloying elements or compounds are segregated in dendritic crystals to form segregation. The degree of segregation and the quality of the raw materials, the particle size, the distribution and partition coefficient of the alloying elements in the electrode, the solidification rate, the falling block during smelting, the depth of the molten pool, the natural and forced motion of the liquid phase, the diffusion, the grain size and the crystal formation. The method is related to many factors, such as the specific operating process such as melting speed and magnetic field stirring. 1. Macro segregation
Although the ingot segregation problem of VAR-smelted titanium alloys is very similar to the segregation problem of molten steel and superalloys, titanium alloys still have their own unique features. The near-alloy and pure titanium have a very small solid-liquid phase interval, and their solidification mode is similar to that of pure metal. Only the solidification mode of the beta alloy and the near beta alloy has a dendritic interface. In addition, when the titanium alloy solidifies, it precipitates as solid solution β crystal grains, and generally does not precipitate the precipitate phase once.
The solidification front of α-alloy, near-α alloy and CPTi is planar, and there is only the possibility of macrosegregation during solidification. In large-section ingots, attention is paid to controlling the macrosegregation of Al and trace elements 0, Fe and Cu. Segregation of Al content is mainly caused by an increase in the loss of Al volatilization caused by a decrease in the melting rate in the feeding stage.
The solidification front of the β alloy and the near β alloy is dendritic, and microscopic segregation between dendrites may occur. Such alloys are less prone to macrosegregation, but beta speckle or ring segregation may occur. The beta plaque is a region with a large number of β stable elements. The reason for the formation of cyclic segregation is that there is a trace amount of solute enrichment in the dendritic solidification front. When the melting speed or power changes, the solidification equilibrium is destroyed to cause a change in the solute content, and at the same time, the solute content in the solidification interface changes. This composition change is generally small, below 10% solute content. Therefore, the width of the ring segregation is also small, generally less than 100-300 um.
2, micro segregation
The α segregation can be further divided into type I segregation and type II segregation. A long time ago, when people used titanium materials, they noticed that there are some α-phase enrichment areas in the material. The hardness of these areas is much higher than the hardness of the matrix. The analysis of these areas shows that the N, O, C content is higher. These defects are called type I defects or hard alpha defects. It is caused by localization of a stable element such as N, O, etc. and formation of nitrides and oxides with titanium. Such compounds are characterized by being hard and brittle. Alpha segregation seriously damages the fatigue strength and plasticity of materials, and is a fatal defect in aircraft engines and the like. The main source of N, O, C is titanium sponge and added waste, or it is welded in the process of making consumable electrodes. The precautionary measures are mainly to strictly control the quality of titanium sponge and improve the vacuum and cleanliness of the consumable electrode welding process.
Type II defects are caused by local enrichment of α-stabilizing elements such as Al. It mainly occurs in the upper part of the ingot, showing a local increase in the Al content, which is also known as a soft alpha defect. The hardness of such defects is usually comparable to the hardness of the matrix, with extensibility, no cracks due to processing, and smaller defects do not affect the mechanical properties. Class II defects are not formed by solidification segregation, and class II segregation cannot be satisfactorily explained by conventional solidification theory. D. W. Tripp et al. believe that it is caused by shrinkage cavities and voids in the ingot. Due to the formation of shrinkage cavities in the hot titanium alloy ingot, there is a small amount of air inside the cavity, and the gas pressure in the cavity is very low. In this case, aluminum (or any alloying element that has a relatively high volatility at high temperatures) quickly evaporates from the surface of the hot cavity metal into the void, and when the dew point is reached, the vapor condenses or simultaneously on the colder surface of the void Condensation. Thus, some surfaces of the void may form an enrichment of aluminum, tin or other readily vaporizable elements, and some surfaces may form a depletion of these elements. The precaution is to extend the feeding time, but this will increase the volatilization loss of these elements, especially when the ingot diameter is large. In order to solve the above contradiction, a method of increasing the element content at the feeding position to compensate for the volatilization loss or appropriately reducing the feeding time may be employed.
3, micro segregation
The α+β two-phase titanium alloy, the β alloy and the near β alloy with high content of stable cerium tend to form β segregation, and the main expression form is the so-called β plaque, which is the local enrichment region of β stable element. The reason for the formation of the beta plaque is that during the solidification process, equiaxed products appear at the front of the columnar crystal. These equiaxed crystal clusters are deposited at the bottom of the liquid molten pool. Due to the different equilibrium distribution coefficients of the solute elements, the liquid melting between the equiaxed clusters The solute element content in the pool is segregated and preserved. The method of reducing the beta plaque can be considered from the following three aspects: first, reducing the size of the ingot to rapidly solidify; second, reducing the content of the beta speckle element, and controlling it to a lower limit than the standard. Third, as long as other conditions permit, the melting speed can be reduced and the bath depth can be minimized.
4. Segregation of alloying elements
The depletion segregation of alloying elements is also known as bright segregation, and the main manifestation is the depletion of alloying elements in the matrix. According to the analysis, the reason for this kind of segregation is mainly related to the excessive particle size of the raw materials, the abnormal blocking of the smelting process, and the improper use of the arc material during welding and smelting. The smelting process parameters and electromagnetic stirring only serve as an auxiliary. For such segregation, measures such as increasing the mechanical strength of the electrode, melting a finished ingot in a single ingot, increasing the number of melting times, etc., and strengthening the process supervision and stabilizing the process system are generally adopted.