What You Want to Know About Die-Casting Melting Alloys

A melting process of physical and chemical phenomena

Alloy melting is an important part of the die-casting process; the melting process is not only to get molten metal liquid but, more importantly, to get the chemical composition in line with the regulations; it can make die castings get good crystalline organization and gas, inclusions are very small metal liquid.

In the melting process, the interaction between metal and gas and between metal liquid and crucible makes the components change and produce inclusions and gas absorption. Therefore, developing correct melting process procedures and their strict implementation is important to obtain high-quality castings.

Interaction between metal and gas

In the melting process, the gases encountered are hydrogen (H2), oxygen (O2), water vapour (H2O), nitrogen (N2), CO2, CO, etc. These gases either dissolve in the metal liquid or interact chemically with it.

Source of gas

Gases can enter the alloy liquid from furnace gas, furnace lining, raw materials, melts, tools, etc.

Interaction of the metal with the crucible

When the melting temperature is too high, the reaction between the iron crucible and the zinc liquid is accelerated, and oxidation of iron occurs on the crucible surface to produce oxides such as Fe2O3; in addition, the iron element reacts with the zinc liquid to produce FeZn13 compound (zinc slag), which is dissolved in the zinc liquid. The wall thickness of the iron crucible is constantly thinned until it is scrapped.

Melting temperature control

Die-casting temperature

The melting point of zinc alloy for die-casting is 382 ~ 386°C. Proper temperature control is an important factor in controlling zinc alloy composition. To ensure good liquidity of the alloy to fill the cavity, the temperature of the metal liquid in the zinc pot of the die-casting machine is 415 ~ 430 ℃; thin-walled parts, complex parts die-casting temperature can be taken as the upper limit; thick-walled parts, simple parts can be taken as the lower limit. The temperature of the metal liquid in the central melting furnace is 430 ~ 450℃. The temperature of the liquid metal entering the gooseneck tube is the same as the temperature inside the zinc pot. By controlling the temperature of the metal liquid in the zinc pot, the pouring temperature can be accurately controlled. And to achieve: (i) the metal liquid is a clear liquid without oxides; (ii) the pouring temperature does not fluctuate.

The harmful effects of high temperatures are

① Aluminum and magnesium elements are burnt.

② The oxidation rate of the metal is accelerated, the number of burnout increases, and the zinc slag increases.

③ Thermal expansion will occur, jamming hammer head phenomenon.

④ More iron is fused into the alloy in the cast iron crucible, and the reaction between zinc and iron is accelerated at high temperatures. Hard particles of iron-aluminium intermetallic compounds will be formed, making the hammerhead and gooseneck excessively worn.

⑤ Fuel consumption increases accordingly.

Temperature is too low: poor fluidity of the alloy is not conducive to forming and affects the surface quality of die castings.

Now the die-casting machine melting pot or furnace is equipped with a temperature measurement and control system; the main daily work is to regularly check to ensure the accuracy of the temperature measuring instrument and regularly use the portable thermometer (temperature table) to measure the actual temperature of the furnace, to be corrected.

Experienced die-casting union with the naked eye to observe the melt; if scraping slag after feeling the melt is not too dense, but also clearer, slag is not very fast, indicating that the temperature is appropriate; melt is too dense, it means that the temperature is low; scraping slag after the liquid surface quickly flooded with a layer of white frost, slag too fast, indicating that the temperature is high, should be adjusted in time.

When the melting temperature is too high, the reaction between the iron crucible and the zinc liquid is accelerated, and oxidation of iron occurs on the crucible surface to produce oxides such as Fe2O3; in addition, the iron element reacts with the zinc liquid to produce FeZn13 compound (zinc slag), which is dissolved in the zinc liquid. The wall thickness of the iron crucible is constantly thinned until it is scrapped.

How to keep the temperature stable

① One of the best methods: use a centralized melting furnace, or die-casting machine furnace, for holding the furnace, thus avoiding large temperature changes caused by adding zinc ingots directly in the zinc pot when melting. Centralized melting can ensure stable alloy composition.

② The second best method: Adopt an advanced automatic feeding system of metal liquid, which can maintain stable feeding speed, the temperature of alloy liquid and the liquid level of the zinc pot.

③ If the current production condition is to add material directly in the zinc pot, it is recommended to change from adding the whole alloy ingot at one time to adding small pieces of alloy ingot several times, which can reduce the magnitude of temperature change caused by adding material.

Generation and control of zinc slag

It is a complex physical and chemical process of melting the alloy from a solid to a liquid state. The gas reacts chemically with the molten metal, among which oxygen reacts most strongly, and the surface of the alloy is oxidized, and a certain amount of slag is produced. The slag contains oxides and intermetallic compounds of iron, zinc and aluminium, and the slag scraped from the surface of the melt usually contains about 90% of the zinc alloy. The reaction rate of zinc slag formation increases exponentially with the melting temperature.

Under normal circumstances, the dross production of the original zinc alloy ingot is less than 1%, in the range of 0.3 ~ 0.5%. In contrast, the dross production of remelted water spouts, waste workpieces, etc., is usually between 2 ~ and 5%.

Control of zinc slag quantity

① Strictly control the melting temperature; the higher the temperature, the more zinc dross.

② Avoid stirring the alloy liquid in the zinc pot as much as possible; any way of stirring will lead to more contact between the alloy liquid and oxygen atoms in the air, thus forming more floating dross.

③ Do not skim the slag too often. When the molten alloy is exposed to air oxidation, the formation of slag, retaining a thin layer of slag on the surface of the furnace, is conducive to the liquid in the pot not oxidizing further.

④ slag raking, the use of a porous (Ф6 mm) disk-shaped slag rake, gently scraped from under the floating slag, as far as possible to avoid stirring the alloy liquid, will be scraped out of the slag Sheng up, raking slag rake in the side of the zinc pot gently knocked, so that the metal liquid flow back into the zinc pot.

Handling of zinc slag

① Sell it back to the raw material supplier or special treatment plant because it may cost more to treat it yourself.

② Dispose of it by the die-casting plant itself. A separate furnace is required, and the remelting temperature of zinc dross is in the range of 420 ~ 440°C. Also add-flux. Melting 100 kg of slag, you need to add 0.5 ~ 1.5 kg of flux, first evenly distributed in the metal liquid surface, followed by a stirrer to mix it evenly into the molten metal (about 2 ~ 4 minutes), insulation for 5 minutes, the surface produces a layer of more like mud-type things, scrape it off.

③ Water mouth material, waste parts remelt water mouth material, waste, garbage bits, scrap workpieces, etc., should not be put directly into the zinc pot of the die-casting machine to remelt. The reason is that the oxidation of the surface of these water mouth materials occurs during the die-casting and forming process, and the content of zinc oxide far exceeds that of the original alloy ingot. When these water mouth materials are remelted in the zinc pot, the zinc oxide is sticky under high-temperature conditions. When it is removed from the zinc pot, it will take away a large number of alloy components. The purpose of remelting the scrap is to separate the zinc oxide from the liquid alloy, and some solvent must be added to the melt, which is used after the ingot is cast.

Plating scrap remelting

Electroplating scrap should be melted separately from non-electroplating scrap because electroplating scrap containing copper, nickel, chromium and other metals are insoluble in zinc, which will be left in zinc alloy as hard particles and bring polishing and machining difficulties.

In the remelting of electroplating scrap, it is important to separate the plating material from the zinc alloy, first put the plating scrap into the crucible with the zinc alloy melt, do not stir the melt at this time, and do not add the melt, use the high melting point of the plating material, the plating will not melt into the alloy. Still, it will float on the surface of the melt in the first period; when all melted, let the crucible stand for 15 ~ 20 minutes to see if there is still slag on the surface and scrape the scum clean. After this process, see if adding a refining agent is necessary.

Precautions in melting operation

  1. Crucible: It must be cleaned before use to remove oil, rust, slag and oxides from the surface. To prevent the iron elements in the cast iron crucible from dissolving in the alloy, the crucible should be preheated to 150 ~ 200℃, sprayed with a layer of paint on the working surface, and then heated to 200 ~ 300℃ to remove the water from the paint thoroughly.
  2. Tools: The melting tools should be cleared of surface dirt before use, and the parts in contact with the metal must be preheated and painted. The tool should not be covered with water. Otherwise, it will cause melt splash and explosion.
  3. Alloy material: Clean and preheat before melting to remove any moisture adsorbed on the surface. To control the alloy composition, it is recommended to use 2/3 of the new material with 1/3 of the retorted material.
  4. Melting temperature should never exceed 450°C.
  5. Clean up the slag on the liquid surface in the zinc pot in time, replenish the zinc material in time and keep the melt surface at a normal height (not lower than 30 mm above the crucible surface) because too much slag and the too low liquid surface can easily cause the slag to enter the gooseneck sizer and strain the steel purline, the hammerhead and the sizer itself, leading to jamming of the hammerhead, scrapping of the gooseneck and the hammerhead.
  6. molten liquid above the floating slag with slag rake calmly stirring so that the collection to remove.

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