Vacuum melting process research and analysis

Aluminum-lithium alloys, as aerospace materials, are highly valued at home and abroad. Due to the high chemical activity of lithium and the difference in the specific gravity and atomic radius of aluminum and lithium, the lithium content of aluminum-lithium alloys prepared by common smelting processes does not exceed 3%. Otherwise, severe segregation of the alloy occurs. We first proposed the use of micro-gravity electromagnetic simulation device to prepare aluminum-lithium alloy ingots, and smelt an aluminum lithium alloy experimental sample with a lithium content of 5% to 10%. By testing and analyzing the experimental samples, it is proved that the aluminum-lithium alloy ingot prepared by this method overcomes the serious segregation problem that occurs with the increase of the lithium content in the common smelting process. The principle of electromagnetic simulation of microgravity is to place metal melts in electromagnetic fields that cross each other vertically. The force of the electromagnetic force excited in the liquid acts on each unit volume of the substance. Its physical properties are similar to those of gravity, and to a certain extent Plays a role in compensating gravity. In the experiment, given the magnetic induction intensity, the effective gravity of the dispersoid and the matrix is ​​equalized by continuously adjusting the current, so that the entire melt is always in a quasi-weightless state.

High-purity graphite crucibles and high-purity graphite crucibles coated internally with three layers of boron nitride were tested. High-purity graphite crucible experiment results: From the appearance of the crucible, it can be seen that the milky white particles seep out from the large oval shape around the crucible. The graphite crucible oozes out of the loose structure. The milky white particles reacted with lithium and carbon to form lithium carbide during a long period of heating and holding. At the same time, the semicircular bottom of the sample was black, indicating that carbon was involved in the reaction. The use of high-purity graphite crucibles coated with three layers of boron nitride coatings does not have this phenomenon. The semi-circular bottom of the sample is slightly brown and the appearance of the crucible is free of precipitates, and at the same time, the mold release is easier. It is proved that the polymer compound boron nitride has the characteristics of high temperature resistance, corrosion resistance, good lubricating performance, and stable chemical properties. From XPS energy spectrum analysis, it can also be seen that the sample smelted from high-purity graphite crucibles has carbon in both the combined state and the elemental state, and the content is high. In the case of a high-purity graphite crucible coated with three layers of boron nitride coating, carbon was only present in the form of a simple substance and the content was low. Determination of heating temperature As the lithium content increases, there is an immiscible zone in the melt of the aluminum-lithium alloy. The heating temperature must be higher than a certain temperature in order to demix the two elements to a single-phase liquid state, and the temperature can be lowered. Limit the evaporation of alloying elements and the reaction of materials such as lithium and cesium. This experiment was based on theoretical calculations and previous experience. The experimental temperature was determined to be 760°C. The heating method is based on the conditions provided by the equipment with full power heating.

The choice of atmosphere protection was tested without atmosphere protection. The aluminum in the alloy sample was present in both the chemical and atomic states, indicating that some of the aluminum was oxidized and not involved in the combination. However, under the condition of argon gas for protection, the aluminum in the alloy sample exists only in one form of compound state. Experimental sample testing According to the above smelting process, a total of three binary aluminum-lithium alloy samples containing 5.0%, 7.5% and 10.0% lithium were tested for the metallographic and mechanical properties. Metallographic test using JSM-5600LV scanning electron microscope on the surface morphology of the sample after corrosion, we can see that its organization is more uniform and dense.

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