Zinc alloy melting
1. Physical and chemical phenomena
of the melting process Alloy melting is an important part of the die casting process, the melting process is not only to obtain the molten metal liquid, but more importantly, to get the chemical composition in accordance with the regulations, which can make the die casting parts have a good crystalline structure and gas and inclusions are very small metal liquid.
During the melting process, the interaction of metals with gases and the interaction of metal liquids with the crucible causes changes in the components, resulting in inclusions and suction. Therefore, formulating correct melting process procedures and strictly implementing them is an important guarantee for obtaining high-quality castings.
1. Interaction
between metal and gas During the melting process, the gases encountered include hydrogen (H2), oxygen (O2), water vapor (H2O), nitrogen (N2), CO2, CO, etc., which are either dissolved in the metal liquid or chemically react with them.
2. The source
gas of the gas can enter the alloy liquid from furnace gas, furnace lining, raw materials, flux, tools and other ways.
3. Interaction
between metal and crucible When the melting temperature is too high, the reaction between the ferrous crucible and the zinc liquid accelerates, and the oxidation reaction of iron occurs on the surface of the crucible to form oxides such as Fe2O3. In addition, iron will react with zinc liquid to form FeZn13 compound (zinc slag), which will be dissolved in zinc solution. The thickness of the iron crucible wall continues to thin until it is scrapped.
2. Melting temperature control
1. Die casting temperature
The melting point of zinc alloy used in die casting is 382 ~ 386°C, and appropriate temperature control is an important factor in the control of zinc alloy composition. In order to ensure the good fluidity of the alloy liquid to fill the cavity, the temperature of the metal liquid 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 be removed at the lower limit. The temperature of the metal liquid in the central melting furnace is 430 ~ 450 °C. The temperature of the metal liquid entering the gooseneck is basically the same as that of the zinc pot. By controlling the temperature of the zinc pot metal liquid, the pouring temperature can be accurately controlled. And achieve: (1) the metal liquid is a clean liquid without oxides; (2) The pouring temperature does not fluctuate.
Harms of excessive temperature:
(1) Aluminum and magnesium are burned.
(2) The metal oxidation rate is accelerated, the burning loss increases, and the zinc slag increases.
(3) The phenomenon of jamming the hammer head will occur due to thermal expansion.
(4) More iron elements are melted into the alloy in cast iron crucibles, and the reaction between zinc and iron is accelerated at high temperatures. It will form hard particles of iron-aluminum intermetallic compounds, which will cause excessive wear on the hammer head and gooseneck.
(5) Fuel consumption increases accordingly.
Too low temperature: The fluidity of the alloy is poor, which is not conducive to forming and affects the surface quality of the die casting.
The current die-casting machine melting pot or furnace is equipped with a temperature measurement and control system, which is mainly checked regularly to ensure the accuracy of the temperature measurement instrument, and the actual temperature of the furnace is regularly measured and corrected with a portable thermometer (temperature gauge).
Experienced die-casting union will observe the melt with the naked eye, if the slag is not too viscous and clear, and the slag is not very fast, it means that the temperature is suitable; if the melt is too viscous, it means that the temperature is low; After scraping the slag, the liquid surface quickly appears with a layer of white frost, and the slag rises too quickly, indicating that the temperature is high and should be adjusted in time.
2. How to maintain the stability
of the temperature (1) One of the methods: use the central melting furnace (Figure 4-3) and the die-casting machine furnace as the insulation furnace, so as to avoid large temperature changes caused by direct addition of zinc ingots to the zinc pot and melting. Concentrated melting can ensure the stability of the alloy composition.
(2) Method 2: The advanced automatic feeding system of metal liquid is used (Fig. 4-4), which can maintain a stable feeding speed, the temperature of the alloy liquid and the liquid level height of the zinc pot.
(3) If the current production condition is to add the whole alloy ingots directly in the zinc pot, it is recommended to add the whole alloy ingots at one time to add small pieces of alloy ingots multiple times, which can reduce the temperature change caused by feeding.
3. Generation and control
of zinc slag The alloy changes from solid to liquid through melting, which is a complex physical and chemical process. The gas reacts chemically with the molten metal, in which the reaction of oxygen is strong, and the surface of the alloy is oxidized to produce a certain amount of scum. The scum contains oxides and intermetallic compounds of iron, zinc and aluminum, and the scum scraped from the surface of the melt usually contains about 90% zinc alloy. The reaction rate of zinc slag formation increases exponentially with the increase of melting temperature.
Under normal circumstances, the slag yield of the original zinc alloy ingots is less than 1%, in the range of 0.3 ~ 0.5%; The residue yield of remelting water outlets 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 alloy liquid in the zinc pot as much as possible, as any kind of agitation will cause more alloy liquid to come into contact with oxygen atoms in the air, resulting in the formation of more scum.
(3) Don't pick up the slag too often. When the molten alloy is exposed to air, it will oxidize and form scum, which is conducive to the liquid in the pot from further oxidation.
(4) When removing slag, use a porous (Ф6 mm) disc-shaped slag rake, gently scrape from under the scum, avoid stirring the alloy liquid as much as possible, hold the scraped slag, and gently knock the slag rake on the edge of the zinc pot to make the metal liquid flow back into the zinc pot.
2. Treatment
of zinc slag (1) Sell back to raw material suppliers or specialized treatment plants, as self-processing may be more costly.
(2) The die casting plant handles it by itself. A separate furnace is required, and the remelting temperature of zinc slag is in the range of 420 ~ 440°C. At the same time, add a flux. To melt 100 kg of slag, you need to add 0.5 ~ 1.5 kg of flux, first evenly dissipate it on the surface of the metal liquid, and then use a stirrer to evenly mix it into the molten metal (about 2 ~ 4 minutes), and after 5 minutes of insulation, the surface will produce a layer more like soil, scrape it off.
4. Water spout material, waste parts remelt
water spout material, waste material, garbage position, scrap workpiece, etc., should not be directly put into the die-casting machine zinc pot for remelting. The reason is that the surface of these nozzles is oxidized during the die-casting process, and the content of zinc oxide far exceeds that of the original alloy ingots.
The remelting of the water outlet material is to effectively separate the zinc oxide and liquid alloy, and some solvent must be added to the melting, and it must be cast into ingots for use.
5. Electroplating waste Remelting
electroplating waste should be melted separately from non-electroplating waste, because the metals containing copper, nickel, chromium and other metals in the electroplating waste are insoluble in zinc, and will exist as hard particles in the zinc alloy, bringing difficulties in polishing and machining.
In the remelting of electroplating waste, pay attention to separating 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, do not add flux, use the high melting point of the plating substance, the plating layer will not melt into the alloy, but will float on the surface of the melt for a period of time at the beginning, when it is 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 agents.
6. Precautions
in melting 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 element 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 to 200 ~ 300 °C to completely remove the water in the paint.
2. Tools: Melting tools should be cleaned of surface dirt before use, and the parts in contact with metal must be preheated and painted. The tool should not be stained with moisture, otherwise it will cause melt splashing and explosion.
3. Alloy materials: Before melting, clean and preheat to remove the moisture adsorbed on the surface. In order to control the alloy composition, it is recommended to use 2/3 of the new material and 1/3 of the reflow material.
4. The melting temperature should not exceed 450°C.
5. Clean 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 surface (not less than 30 mm of the crucible surface), because too much scum and too low liquid level can easily cause the slag to enter the gooseneck cylinder, strain the steel barrel, hammer head and the cylinder itself, resulting in the jamming of the hammer head, gooseneck and hammer head scrapping.
6. The scum on the melt is gently stirred with a slag rake to make it gather and be taken out.