Zinc alloy smelting
1. Physical and chemical phenomena of the smelting process
Alloy smelting is an important part of the die-casting process, the smelting process is not only to obtain the molten metal, but more importantly, to obtain the chemical composition that meets the regulations, so that the die-casting parts can get a good crystalline structure and gas, inclusions are very small molten metal.
During the melting process, the interaction of the metal with the gas and the interaction of the molten metal with the crucible causes the composition to change, resulting in inclusions and gas absorption. Therefore, formulating the correct melting process regulations and strictly implementing them is an important guarantee for obtaining high-quality castings.
1. Metal-gas interactions
In the melting process, the gases encountered are hydrogen (H2), oxygen (O2), water vapor (H2O), nitrogen (N2), CO2, CO, etc., which are either dissolved in molten metal or chemically reacted with it.
2. The source of the gas
Gases can enter the molten alloy from furnace gas, furnace lining, raw materials, fluxes, tools, etc.
3. Interaction of metals with crucibles
When the melting temperature is too high, the reaction between the iron crucible and the zinc liquid is accelerated, and the iron oxidation reaction occurs on the surface of the crucible to form oxides such as Fe2O3. In addition, iron will react with the zinc solution to form the FeZn13 compound (zinc slag), which is dissolved in the zinc solution. The wall thickness of the iron crucible is continuously thinned until it is scrapped.
2. Melting temperature control
1. Die casting temperature
The melting point of zinc alloy for die casting is 382 ~ 386 °C, and proper temperature control is an important factor in the control of zinc alloy composition. In order to ensure the good fluidity of the molten alloy filling cavity, the temperature of the molten metal in the zinc pot of the die-casting machine is 415 ~ 430°C, and the upper limit of the die-casting temperature of thin-walled parts and complex parts can be taken; Thick-walled parts and simple parts can take the lower limit. The temperature of molten metal in the central melting furnace is 430 ~ 450°C. The temperature of the molten metal entering the gooseneck is basically the same as the temperature inside the zinc pot. By controlling the molten metal temperature of the zinc pot, the pouring temperature can be accurately controlled. And do the following: (1) the molten metal is a clean liquid without oxides; (2) The pouring temperature does not fluctuate.
Disadvantages of too high temperature:
(1) Aluminum and magnesium element burning.
(2) The oxidation rate of metal is accelerated, the amount of burn-out increases, and the zinc slag increases.
(3) The phenomenon of jamming the hammer will occur due to thermal expansion.
(4) The iron element in the cast iron crucible is fused into the alloy more, and the reaction between zinc and iron is accelerated at high temperature. Hard particles of iron-aluminum intermetallic compounds will be formed, which will cause excessive wear of the hammer head and gooseneck.
(5) Fuel consumption increases accordingly.
The temperature is too low: the alloy has poor fluidity, which 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 daily work is mainly to check regularly to ensure the accuracy of the temperature measuring instrument, and regularly use a portable thermometer (temperature gauge) to measure the actual temperature of the furnace and correct it.
Experienced die-casters will observe the melt with the naked eye, if the melt is not too viscous and clearer after scraping, and the slag is not very fast, it means that the temperature is appropriate; If the melt is too viscous, the temperature is low; After scraping the slag, the liquid level will soon be covered with a layer of hoarfrost, and the slag will rise too quickly, indicating that the temperature is too high and should be adjusted in time.
2. How to keep the temperature stable
(1) One of the best methods: use the central melting furnace (Fig. 4-3) and the die-casting machine furnace as the holding furnace, so as to avoid large temperature changes caused by the direct addition of zinc ingots to the zinc pot for melting. Centralized melting can ensure the stability of the alloy composition.
(2) The second best method is to adopt an advanced automatic feeding system for molten metal (Fig. 4-4) to maintain a stable feeding speed, the temperature of the molten alloy and the liquid level height of the zinc pot.
(3) If the current production condition is to feed directly in the zinc pot, it is recommended to change the addition of the whole alloy ingot at one time to the addition of small alloy ingots multiple times, which can reduce the temperature change caused by feeding.
3. Generation and control of zinc slag
The transformation of alloys from solid to liquid by melting is a complex physical, chemical process. The gas undergoes a chemical reaction with the molten metal, of which the reaction of oxygen is the most intense, and the surface of the alloy is oxidized to produce a certain amount of dross. The scum contains oxides and iron, zinc, and aluminum intermetallic compounds, and the scum scraped off the melt surface usually contains about 90% zinc alloy. The reaction rate of zinc slag formation increases exponentially with the increase of smelting temperature.
Under normal circumstances, the slag yield of the original zinc alloy ingot is less than 1%, which is in the range of 0.3 ~ 0.5%; The slag production of remelt nozzles and waste workpieces is usually between 2 ~ 5%.
1. Control of zinc slag amount
(1) Strictly control the melting temperature, the higher the temperature, the more zinc slag.
(2) Avoid the agitation of the molten alloy in the zinc pot as much as possible, any way of agitation will lead to more contact between the molten alloy and the oxygen atoms in the air, thus forming more dross.
(3) Don't scrape too often. When the molten alloy is exposed to air, it will oxidize, forming dross, and retaining a thin layer of scum on the furnace surface is conducive to preventing further oxidation of the liquid in the pot.
(4) When scraping slag, use a porous (Ф6 mm) disc-shaped slag rake, gently scrape under the scum, avoid agitation of the alloy liquid as much as possible, hold up the scraped slag, and gently knock the slag rake on the edge of the zinc pot to make the molten metal flow back into the zinc pot.
2. Treatment of zinc slag
(1) Sell back to the raw material supplier or specialized treatment plant, as it may be more costly to dispose of it yourself.
(2) The die casting factory handles it by itself. A separate furnace is required, and the zinc slag remelting temperature is in the range of 420 ~ 440°C. At the same time, a flux is added. To smelt 100 kg of slag, 0.5 ~ 1.5 kg of flux needs to be added, which is evenly distributed on the metal surface first, and then evenly mixed into the molten metal with a stirrer (about 2 ~ 4 minutes), after 5 minutes of heat preservation, a layer of more earth-like things will be produced on the surface, and it will be scraped off.
Fourth, nozzle materials, waste parts remelting
Nozzle materials, waste materials, garbage bits, scrapped workpieces, etc., should not be directly put into the zinc pot of the die-casting machine for remelting. The reason is that the surface of these nozzle materials is oxidized during the die-casting molding process, and its zinc oxide content far exceeds that of the original alloy ingots, when these nozzle materials are remelted in the zinc pot, because the zinc oxide is viscous under high temperature conditions, when it is taken out of the zinc pot, a large amount of alloy components will be taken away.
In order to effectively separate zinc oxide from liquid alloys, some solvents must be added to the smelting process and cast into ingots for use.
5. Remelting of electroplating waste
Electroplating waste should be smelted separately from non-electroplating waste, because electroplating waste contains copper, nickel, chromium and other metals that are insoluble in zinc, and will exist as hard particles in zinc alloy, bringing difficulties in polishing and machining.
In the remelting of electroplating waste, pay attention to the separation of the plating substance from the zinc alloy, first put the electroplating waste into the crucible containing the zinc alloy melt, do not stir the melt at this time, and do not add the flux, the use of the high melting point of the plating material, the plating will not melt into the alloy, but will float on the surface of the melt in the initial period of time, when all melted, let the crucible stand for 15 ~ 20 minutes, see if there will be scum on the surface, scrape the scum clean. After this process, see if it is necessary to add refining agent.
6. Precautions in smelting operation
1. Crucible: It must be cleaned before use to remove oil, rust, slag and oxides on the surface. In order to prevent the iron elements in the cast iron crucible from dissolving in the alloy, the crucible should be preheated to 150 ~ 200 °C, spray a layer of paint on the working surface, and then heat it to 200 ~ 300 °C to completely remove the moisture in the paint.
2. Tools: The surface of the melting tool should be cleaned of dirt before use, and the parts that come into contact with the metal must be preheated and brushed with paint. The tool should not be stained with water, otherwise it will cause melt splashing and explosion.
3. Alloy material: clean and preheat before smelting to remove the water adsorbed on the surface. In order to control the alloy composition, it is recommended to use 2/3 virgin material with 1/3 return material.
4. The melting temperature should never exceed 450°C.
5. Clean up the scum on the liquid surface in the zinc pot in time, replenish the zinc material in time, and maintain the normal height of the melt level (not less than 30 mm on the crucible surface), because too much scum and too low liquid level are easy to cause the slag to enter the gooseneck barrel, strain the steel cylinder, the hammer head and the cylinder itself, resulting in the stuck hammer head, gooseneck and hammer head scrapped.
6. The scum on top of the melt is stirred calmly with a slag rake to gather so that it can be removed.