Ensuring the melting quality of die-casting aluminum alloy is one of the most important issues in die-casting production. Based on years of production experience, this paper proposes ways to solve the chemical composition, porosity, and hard points from the perspective of raw materials and the melting process for the melting quality of die-casting aluminum alloys in small and medium-sized enterprises.
Table of Contents
Raw material problem
Most of the die casting industry is now purchased from the aluminum alloy ingot production factory. This formulated alloy ingot is mostly produced by recycling recycled aluminum products as the main material after composition adjustment (adding pure aluminum ingot and part of the intermediate alloy). Therefore, this alloy ingot’s cost and selling price are lower than that of a pure aluminum ingot as the main material, but the impurity content is higher.
In response to this situation, it is necessary to do the chemical composition inspection of raw materials for the purchased alloy aluminum ingots. Appropriate adjustments should be made according to GB/T8733 when signing technical requirements with alloy aluminum ingot manufacturers, and then make further adjustments according to the requirements of die-casting aluminum alloys, for example, the following adjustments are recommended for commonly used YL113 and YL112 die-casting aluminum alloy ingots (see Table 1).
Due to the requirements of the aluminum alloy in the gas content and hard mass point, require the aluminum ingot production plant must do a good job refining, gas removal, and slagging to prevent high gas content in the ingot, impurities, hereditary to the die-casting aluminum liquid, require the ingot surface smooth (after picking up the slag) fracture fine no crystalline silicon bright grains (aluminum bloom surface bubble hole is the ingot mold coating water is not dried, the surface is not bright slag is not scraped clean, the ingot fracture has Bright grains is the casting temperature is too high, there is silicon crystallization.
Die-casting production of 30%-50% back to the furnace material, for back to the furnace material has oil to be burned and then pressed into the aluminum liquid, the crushed aluminum slag material must be sieved to remove dust, remove sand and gravel before back to the furnace, where the use of back to the furnace material of the aluminum liquid, refining agent, slag remover dosage must be increased by an appropriate amount, generally according to the upper limit of proportional control (should control the addition of back to the furnace material at less than 40%.
Melting of aluminum liquid
Die-casting aluminum alloy melting, the use of rapid submersion, that is, in the melting furnace to retain a certain amount of aluminum liquid, do not use up (will be pressed into the aluminum liquid melting to prevent the direct burning of aluminum scrap oxidation formation of aluminum oxide at high temperatures for a long time, when the scrap material is finished, the liquid level increases before adding aluminum ingots (aluminum alloy melting do not stir to prevent oxidation increase burn loss.
Aluminum alloy liquid refining and slagging
Refining and slagging are among the most important processes in the melting process of die-casting aluminum alloy, which is an effective way to avoid many pores and hard spots.
Refining and slagging in the furnace and outside the furnace
The crucible and small and medium-sized pool-type melting furnace can take in-furnace refining and slag because it can quickly use the processed aluminum liquid. For a large pool-type melting furnace, it is better not to use the furnace refining and slagging treatment but should be processed in the special insulation package or insulation furnace, reducing the amount of melt.
If the aluminum liquid after refining is placed for too long, it will oxidize and absorb gas in contact with the atmosphere (if placed in the cast iron crucible for too long, it will lead to increase iron and produce precipitates (practice proves that when the aluminum liquid in the cast iron crucible with poor coating protection or failure is placed for more than 40min, the iron content will increase, sometimes up to 0.2% increase, the longer it is placed, the more iron increase, so in the stoppage of work The aluminum solution in the insulated cast iron crucible should be transferred to other crucibles for use as soon as possible. If necessary, it can use again after refining with a small amount of refining agent. Some enterprises use a large furnace to refine Aluminum and then use the transfer package to allocate to each station holding furnace. For this case, if the aluminum solution after melting in the special holding furnace refining and slagging, it must be put once the aluminum solution refining once. Do not use it while replenishing aluminum solution, so the refining effect is not good.
Suppose a large furnace melting is directly transferred to each station after using the holding furnace. In that case, you can take the method of refining and slagging in the transfer package and then pour the aluminum liquid into the holding furnace after treatment. You can also refine and slag in the holding furnace.
Refining and slagging method and the choice of flux
The purpose of aluminum alloy refining and slagging is to remove the gas and impurities in the aluminum liquid, mostly using a combination of physical adsorption and chemical reaction methods.
The first method is blowing powder refining, and it is the use of cheap nitrogen (or argon) after dehumidification blowing into the aluminum liquid, in the blowing gas at the same time add powder refining agent, so that the aluminum liquid in the furnace fully contact with the gas and powder generated by the bubble adsorption of gas and impurities in the aluminum liquid up to the liquid surface. Therefore, the size of the refining bubble how much, bubble time, and refining quality is closely related to the length. Refining reaction time can not be too short, generally required in 6-10min, sometimes up to 15min. Bubble size is also very important. Some refining melt reaction is intense, and bubbles out of the liquid are very high. It is not good but increases oxidation and re-aspiration. I hope the melt reaction after the bubbles produced evenly, long time, bubbles above the liquid surface of 10cm or less.
The second is the use of refining blocks, pressed into the aluminum liquid with a bell, constantly stirring along the lower part of the aluminum liquid to achieve the purpose of full contact between the aluminum liquid and bubbles. At present, there are many refining block manufacturers on the market. The quality varies. When choosing the block refining agent, one should pay attention to the material it is adapted to, the minimum refining temperature, bubble size, and time length, whether there is harmful gas release, and whether there is harmful impurities residue in the refining agent. A good block refining agent should be mixed with a molten agent after melting, crushed, and used. It can be in 650-680 ℃ can produce decomposition reaction bubble release, in the reaction at the same time there is a certain warming effect, in the refining bubble time is long, bubble evenly bubbles out, rather than very intense bubble after bubble becomes a very small bubble, can completely remove refining residue, the bubble without odor harmless to humans. Refining block, such as bubbling time below 5min, should be carried out again and must guarantee refining time in 6-10min.
Slagging and de-slagging
Aluminum alloy slag making and slag removal is the process after refining. Its purpose is to make the aluminum liquid impurities and oxides into the slag to remove. Often in slag removal, the slag contains aluminum liquid. Therefore, it is hoped that the slag contains as little liquid aluminum as possible, and the slag should be scrambled again. The purpose of frying ash is to make the slag in the aluminum liquid extrusion sink in the bottom of the frying pan so that the slag is soft and scattered in the upper so that the slag ash and aluminum liquid separation. To achieve this purpose is to choose a good slagging agent. Slag remover is scattered on the liquid surface. To make it react well, use the pressure spoon, press into the aluminum liquid, repeatedly gentle pressure as far as possible, to make full contact with the aluminum liquid surface layer, up to the role of slag aluminum separation. Experience shows that the slag remover made of slag is loose, not black. Grayish white is a better slag remover.
To sum up, the refining and slagging die-casting of aluminum alloy is a time process. It is impossible to complete quickly; shortening the refining time is the wrong operation. The adsorption of gas and impurities in the aluminum liquid needs a certain amount of time, only to ensure that there is sufficient adsorption time and impurities floating time, to achieve the purpose of refining, refining to ensure that the aluminum liquid is fully in contact with the bubbles, constant stirring is necessary, the gas in the aluminum liquid to remove the net, impurities removed, to ensure that the purpose of refining. To ensure that the porosity of the product is reduced, the hardness is reduced, and the quality is guaranteed, the gas in the aluminum solution is removed, and the impurities are removed.