CN1310066A - Vacuum travelling-wave electromagnetic fining high temperature alloy precision casting method - Google Patents

Abstract

The precision casting process features that when the alloy melt begins to solidify, one pair of symmetrical traveling wave magnetic fields in opposite direction and of 50-200 mT magnetic field strength are applied. The method is easy to control and the high temperature alloy has fine crystallite, compact structure and less holes.

Description

Vacuum Traveling Wave Electromagnetic Refinement Superalloy Precision Casting Method

The invention relates to a casting method of metal materials, and in particular provides a precision casting method of vacuum traveling wave electromagnetically refined superalloy.

Superalloys are materials used in the manufacture of high-temperature resistant components such as aviation, aerospace vehicles, rocket engines, ships, nuclear reactors, industrial gas turbines, and petrochemical equipment. The main methods of obtaining these superalloy parts are casting, forging and powder metallurgy. Since casting superalloys can make parts precisely formed at one time, it has great advantages in terms of manufacturability, economy and performance, so it is widely used. However, the usual investment casting superalloy precision castings often have low mechanical properties due to their coarse grain size. Therefore, since the early 1980s, relevant scholars and experts at home and abroad have been researching and developing superalloy fine-grain casting technology. The purpose is to control the grain size and shape of superalloy investment castings so that their structure and properties are similar to those of forgings, thereby replacing expensive forgings or even powder metallurgy. In order to refine the superalloy grain structure, three refinement methods, thermal control method, chemical method and mechanical method, have been developed at home and abroad. The thermal control method is characterized by reducing the refining temperature and time, retaining carbides, reducing the pouring temperature, accelerating cooling, and limiting grain growth; equiaxed crystals and dendritic equiaxed crystals with a size of 0.3-1.0mm can be obtained. The chemical method is to add an effective solid nucleating agent to the melt to form a large number of heterogeneous crystal nuclei; equiaxed crystals with a size of 0.5-0.8mm can be obtained; the mechanical method is to stir the melt by rotating the mold and mechanical vibration , refine grains; equiaxed grains with a size of 0.1-1.6mm can be obtained. There are limitations and defects in the above-mentioned process methods. For example, the disadvantage of thermal control method is that it is not easy to remove air bubbles and inclusions, which reduces the purity of castings and makes it difficult to cast large parts; its application is limited due to extremely low superheating temperature and strict temperature control. Because the mechanical method uses mechanical force such as centrifugal force, it is difficult to make the melt move uniformly and is limited by the shape of the part. The disadvantage of the chemical method is that it is difficult to control the chemical composition, and the nucleating agent is easy to form oxides and cause fatigue. Therefore, grain refinement has always been an unresolved problem in fine-grain casting of superalloys.

The purpose of the present invention is to provide a vacuum traveling wave electromagnetic refining superalloy precision casting method, which is easy to control, and the obtained superalloy grains are finer and have less porosity and voids.

The invention provides a precision casting method for vacuum traveling wave electromagnetic refinement superalloy, which is characterized in that: when the alloy starts to solidify, a pair of symmetrical but opposite traveling wave magnetic fields are applied to the alloy melt, although the magnetic field intensity is 50-200mT within range.

The so-called traveling wave magnetic field is a magnetic field distributed in a sinusoidal waveform along a straight line. Because it is a translational sine wave, it is also called a plane parallel wave magnetic field. In the invention, during the pouring and solidification process of the vacuum casting workpiece, a pair of symmetrical but opposite traveling wave magnetic fields are applied to the alloy melt, and the magnetic field and the melt interact to form a pair of force couples in the melt, causing the melt to rotate. Electromagnetic stirring is formed. Stirring force can break dendrites during the solidification process of the melt, increase the solid phase nucleation rate, promote the transformation of columnar crystals to equiaxed crystals, and make the melt solidify and crystallize into uniform and fine equiaxed crystal structures. Specifically, the main reason for the stirring of the traveling wave electromagnetic field to refine the grains is that under the action of the electromagnetic field, an electromagnetic force is formed in the alloy melt, and the electromagnetic force makes the melt flow, and the tip of the columnar crystal during solidification is sheared or cut by the flow force. Fusion becomes the core of new solid phase grains, and these micro grains later serve as nuclei for equiaxed crystal growth, so the nucleation rate increases during solidification, and the solidification mode changes from columnar crystals to equiaxed crystals; and due to the solidification front and The convective heat transfer between a large number of newly formed free grains makes the residual superheat in the liquid quickly eliminated, the temperature gradient at the crystallization front is reduced, and the temperature field is more uniform. These will increase the equiaxed grain structure. Generally, the traveling wave magnetic field is generated by a linear electromagnetic stirrer, and the intensity of electromagnetic stirring is mainly determined by the strength of the magnetic field, which can be adjusted by adjusting the magnitude of the winding current, so that the degree of grain refinement can be easily controlled. In the method of the present invention, the magnetic field strength can be adjusted steplessly within the range of 50-200mT. The magnetic field action time depends on the size of the casting, and is generally controlled at the end of solidification. In addition to refining grains, traveling wave electromagnetic stirring has other advantages, which can reduce metallurgical defects such as composition segregation and porosity in superalloy castings. The method overcomes the disadvantages of other existing thinning methods of vacuum precision superalloy castings, and can achieve effects that cannot be achieved by the existing thinning methods.

In a word, the present invention can solve the problems that the existing methods cannot completely solve, and obtain a more uniform and finer grain structure than the above three methods, and its grain size is in the range of 0.1-0.5 mm. The invention reduces defects such as porosity and voids, which cannot be solved by the thermal control method. In terms of structure, the fine-grain cast superalloy of the present invention can be similar to forgings, and is expected to replace forgings. Because the strength of the traveling wave electromagnetic field is easy to adjust, it is easier to control the grain refinement than the existing three fine-grain casting methods. The present invention is described in detail below by way of examples.

Accompanying drawing 1 is the schematic diagram of the principle of grain refinement by traveling wave electromagnetic stirring.

Figure 2 shows the influence of traveling wave electromagnetic stirring and stirring intensity on the macrostructure of GH4169 superalloy a no electromagnetic stirring b weak electromagnetic stirring c strong electromagnetic stirring.

Figure 3 shows the effect of traveling wave electromagnetic stirring and stirring intensity on the loose and dendrite structure of GH4169 superalloy a without electromagnetic stirring b electromagnetic stirring.

Example

Using the vacuum traveling wave electromagnetic thinning superalloy precision casting method invented, GH4169, K17 and other superalloys were smelted and casted, and the expected effect was obtained. Now take the GH4169 deformed superalloy as an example for a specific description.

GH4169 alloy, its chemical composition is (wt.%) C<=0.01, Mn<=0.5, Si<=O.75, Cr17～21, Ni, Mo2.8～3.3, Nb4.5～5.75, Al0. 2～0.8, Ti0.3～1.3, B0.006 (according to the calculated amount), Fe17～20, S＜=0.015, P＜=0.015.

Place the GH4169 master alloy in the crucible of a vacuum induction furnace, and heat it until it melts. Before pouring, adjust the winding current that generates the traveling wave magnetic field to achieve the required electromagnetic stirring intensity. The adjustable range of the winding current of the traveling wave electromagnetic field is 10-50A, and the corresponding magnetic field strength is adjusted to 50 and 200mT respectively, and then the melt is poured into the casting mold applied with the traveling wave electromagnetic field. During the solidification process of the melt, it is subjected to electromagnetic stirring. When the melt is completely solidified, adjust the current of the traveling wave magnetic field winding to zero, and stop the electromagnetic stirring at this time.

After the GH4169 alloy casting was taken out, it was cut open to determine the macroscopic grain structure. The result showed that the grain size reached 0.1-0.3 mm, and the distribution was uniform. Metallographic observation shows that the loose voids are significantly reduced, as shown in Figure 2 and Figure 3.

The chemical composition (wt.%) of K17 is as follows: B C Zr Mo Cr V Ti co al Si mn Fe Ni 0.0163 0.163 0.071 3.01 8.94 0.73 4.37 10.21 5.55 <0.2 <0.2 <1.0 balan Ce

Through the method, a fine-grain cast structure of 0.1-0.5 mm is obtained.

comparative example

Still take GH4169 deformed superalloy as an example. The macroscopic crystal grains of the alloy ingot applied with traveling wave electromagnetic stirring are equiaxed crystals and shape crystals, while the alloy ingots without applied traveling wave electromagnetic stirring are mostly penetrating columnar crystals. With the increase of the applied traveling wave magnetic field stirring intensity, the crystal state From columnar crystals to equiaxed crystals, and more and more fine and uniform. (See Figure 2)

Metallographic observation results show that the alloy without traveling wave electromagnetic stirring has a lot of porosity and voids at the neck, while the alloy with traveling wave electromagnetic stirring has significantly reduced porosity and voids (see Figure 3).

Claims (1)

1. A vacuum traveling wave electromagnetic refinement high temperature alloy precision casting method, characterized in that: when the alloy starts to solidify, a pair of symmetrical but opposite traveling wave magnetic fields are applied to the alloy melt, and the magnetic field strength is in the range of 50-200mT.

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