铝合金的熔炼与铸造（Melting and casting of aluminum alloy）Melting and casting
Melting and pouring of aluminum alloy is the main link in casting production. The whole process of melting and casting is strictly controlled, which plays an important role in preventing casting defects such as pinholes, inclusions, castings, cracks, porosity and shrinkage. Because the aluminum melt absorbs the hydrogen tendency, the oxidation ability is strong, dissolves the iron easily, in smelting and the casting process, must take the simple and careful preventive measure, obtains the high-quality casting.
1 、 preparation and quality control of aluminum alloy burden
In order to smelt high quality aluminum melt, the qualified raw material should be selected first. To carry out scientific management and proper processing of raw materials, otherwise it will seriously affect the quality of the alloy, the production practice has proved that the raw materials (including metal materials and auxiliary materials) lax control will make batch scrap castings.
(1) raw materials must have qualified chemical composition and organization, and the specific requirements are as follows:
In addition to the analysis of the main components and impurities in the alloy ingots, the microstructure a
nd fracture of the alloy were examined. Practice has proved that the use of serious shrinkage cavity, pinhole, and bubbles of aluminum liquid, it is difficult to obtain dense castings, and even cause
the whole furnace, batch castings scrapped.
It was found in the study of Al Si alloy ingots of Aluminum Alloy pinhole, does not appear in the molten pure sand casting pinhole test block, when the aluminum silicon alloy ingot with low and unqualified specimens, pinhole serious, and the grain size large. The reason is the heredity of the material. The heredity of Al Si alloy and heredity increased with the increase of content and the amount of silicon reached 7%. Continue to increase silicon content to eutectic component, heredity decreases slightly again. In order to solve the casting defects caused by the heredity of the burden, aluminum ingots, intermediate alloys and other charging materials with high metallurgical quality must be selected. Specific standards are as follows:
(1) there should be no pinholes or holes in the fracture surface
Pinhole shall be within grade three, and local (not exceeding 25% of the inspected area) shall not exceed three grade. Over three grade shall be taken by means of heavy smelting to reduce the degree of puncture. Remelting refining method and the general Aluminum Alloy smelting, casting temperature
should not exceed 660 degrees, for the original grain large aluminum ingot, alloy ingot, should be the first to use the lower mold temperature, making them rapid solidification, grain refinement.
2 、 burden treatment
Before using the burden, it should be treated by blowing sand
to remove the surface rust, grease and other dirt. The time is not long, Aluminum Alloy ingot and metal scrap surface is clean without blowing sand, but should be in charge of the elimination of mixed iron filters and inserts, all shall be in charge of preheating furnace, to remove the surface of the water, shorten the melting time in 3 hours above.
3 、 management and storage of burden
Reasonable storage and management of burden is important to ensure the quality of alloy. The burden shall be stored in a dry warehouse with little change in temperature.
2 、 preparation of crucible and melting tools
(1) crucible casting aluminum alloy commonly used iron crucible, also can use cast steel and steel plate welding crucible.
New and old is not for a long time in the crucible crucible, before use should be blowing sand, and heated to 700--800 degrees, to keep 2--4 hours, to burn water and attached to the inner wall of the crucible of combustible material, to be cooled to 300 degrees below, carefully clean the inside of the crucible, at a temperature not lower than 200 degrees when spraying paint.
The crucible should be preheated to dark red (500--600 degrees) before use and kept warm for more than 2 hours. Before the new outer crucible melting, melting scrap with the best grades of a furnace.
(two) preparation of smelting tools
Zhong Zhao, press ladle, mixing spoon, ladle
At the other before use shall be preheated, and at 150 degrees ---200 degrees temperature, coated with a protective coating, and thorough drying, the drying temperature is 200--400 degrees, holding time of 2 hours, after use should be thoroughly removed attached on the surface of oxide and fluoride (preferably blowing sand).
3, smelting temperature controlpouring
The melting temperature is too low, is not conducive to the dissolution of alloying elements and gas inc
lusions, discharge, tendency segregation, cold shut, undercasting increase formation, but also because of insufficient heat riser, the casting without reasonable feeding, has information that,
The melting temperature of all aluminum alloys should be up to 705 degrees and should be stirred. The melting temperature is too high, not only a waste of energy, more serious is because the higher the temperature, the hydrogen absorption of the grain becomes thick, aluminum oxide is more serious, some of the burning loss of alloy elements is more serious, which leads to a decrease in the mechanical properties of the alloy, casting the deterioration of mechanical properties and modification, weaken the effect of air the castings reduce.
The production practice shows that the molten alloy rapid heating to high temperature, reasonable stirring to dissolve all alloy elements (especially refractory metals), scraping
scum down after the pouring temperature, so that the minimum degree of segregation, melting of hydrogen is less favorable, to obtain the uniform and compact alloy mechanical properties high. Because the aluminum melt temperature is difficult to determine with the naked eye, so no matter what type of the melting furnace, should use temperature control instrument. The instrument should be regularly checked and the maintenance cycle should thermowell with metal brush clean, coated with a
protective coating, in order to ensure the accuracy and prolong the service life of the measurement result.
4 、 control of smelting time
In order to reduce the oxidation, gettering and dissolution of molten aluminum, the residence time and rapid melting of molten aluminum should be shortened. From the beginning of the melt to the end of the casting, the sand casting shall not exceed 4 hours, the die casting shall not exceed 6 hours, and the die casting shall not exceed 8 hours.
In order to speed up the smelting process, should first join the scrap aluminum silicon intermediate alloy medium size, low melting point, in order to accompany the formation of molten pool as soon as possible in the crucible bottom, then add the returns for larger pieces and pure aluminum ingot, so that they can gradually expand slowly immersed in molten pool, fast melting. When the main part of the furnace is melted, the intermediate alloy with higher melting point and small quantity is melted and stirred to accelerate the melting. Finally, cool down and press the oxidizable alloy elements to reduce the loss.

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