1、 Characteristics of Zinc Alloy
Significant proportion;
Good casting performance, capable of die-casting complex and thin-walled precision parts, with a smooth casting surface;
Surface treatment can be carried out: electroplating, spraying, painting;
During melting and die casting, it does not absorb iron, does not corrode the mold, and does not stick to the mold;
Has excellent mechanical properties and wear resistance at room temperature;
Low melting point, melts at 385 ℃, easy to die cast.
2、 Issues to be noted during the use of zinc alloy
Poor corrosion resistance. When the impurity elements lead, cadmium, and tin in the alloy composition exceed the standard, it causes the casting to age and deform, manifested as volume expansion, significant decrease in mechanical properties, especially plasticity, and even rupture over time. Lead, tin, and cadmium have low solubility in zinc alloys, and therefore concentrate at grain boundaries to become cathodes, while aluminum rich solid solutions become anodes, promoting intergranular electrochemical corrosion in the presence of water vapor (electrolyte). Die castings age due to intergranular corrosion.
Timeliness effect. The microstructure of zinc alloys is mainly composed of zinc rich solid solutions containing Al and Cu and Al rich solid solutions containing Zn, and their solubility decreases with decreasing temperature. However, due to the extremely fast solidification rate of die-casting parts, the solubility of the solid solution is greatly saturated at room temperature. After a certain period of time, this supersaturation phenomenon will gradually dissipate, causing slight changes in the shape and size of the casting.
Zinc alloy die castings should not be used in working environments with high and low temperatures (below 0 ℃). Zinc alloy has good mechanical properties at room temperature. However, both the tensile strength at high temperatures and the impact performance at low temperatures significantly decrease.
3、 Types of Zinc Alloys
Zamak 3: Good fluidity and mechanical properties. Applied to castings with low mechanical strength requirements, such as toys, lamps, decorations, and some electrical components.
Zamak 5: Good fluidity and excellent mechanical properties. Applied to castings with certain requirements for mechanical strength, such as automotive parts, electromechanical parts, mechanical components, and electrical components.
Zamak 2: Used for mechanical parts with special requirements for mechanical properties, high hardness requirements, and general dimensional accuracy requirements.
ZA8: Good fluidity and dimensional stability, but poor fluidity. Applied to workpieces with small die-casting dimensions, high precision and mechanical strength requirements, such as electrical components.
Superloy: With optimal fluidity, it is suitable for die-casting thin-walled, large-sized, high-precision, and complex shaped workpieces, such as electrical components and their boxes.
Different zinc alloys have different physical and mechanical properties, providing a space for selection in die-casting design.
4、 Selection of Zinc Alloy
Which type of zinc alloy to choose is mainly considered from three aspects
1. The purpose of die-casting parts and the performance requirements that need to be met
(1) Mechanical properties, tensile strength, are the maximum resistance of materials when they break; Elongation is a measure of material brittleness and plasticity; Hardness is the resistance of a material surface to plastic deformation caused by the pressing or friction of a hard object.
(2) Working environment status: working temperature, humidity, medium in contact with the workpiece, and air tightness requirements.
(3) Accuracy requirements: achievable precision and dimensional stability.
2. Good process performance
(1) Casting process (2) Mechanical processing processability (3) Surface treatment processability
3. Good economic efficiency
The cost of raw materials and the requirements for production equipment (including melting equipment, die-casting machines, molds, etc.), as well as production costs. Zinc alloy composition controls the role of individual elements in the alloy. Effective alloying elements in the alloy composition include aluminum, copper, and magnesium; Harmful impurity elements: lead, cadmium, tin, iron.
(1) Aluminum effect
A. Improve the casting performance of alloys, increase their fluidity, refine grain size, induce solid solution strengthening, and enhance mechanical properties.
B. Reduce the reactivity of zinc to iron and minimize the erosion of iron materials such as goose necks, molds, and crucibles. The aluminum content is controlled between 3.8% and 4.3%. Mainly considering the required strength and fluidity, good fluidity is a necessary condition for obtaining a complete, accurately sized, and smooth surface casting.
(2) Copper effect
A. Increase the hardness and strength of the alloy;
B. Improve the wear resistance of alloys;
C. Reduce intergranular corrosion.
D. When the copper content exceeds 1.25%, the size and mechanical strength of the die casting will change due to aging; Reduce the extensibility of the alloy.
(3) Magnesium effect
A. Reduce intergranular corrosion
B. Refining the alloy structure to increase its strength
C. Improve the wear resistance of alloys
D. Disadvantage: When the magnesium content is greater than 0.08%, thermal brittleness, decreased toughness, and decreased flowability occur; Easy to oxidize and lose in the molten state of the alloy.
(4) Impurity elements
Lead, cadmium, and tin make the intergranular corrosion of zinc alloys highly sensitive, accelerating their own intergranular corrosion in warm and humid environments, reducing mechanical properties, and causing dimensional changes in castings. When the impurity elements lead and cadmium in zinc alloy are too high, the surface quality of the workpiece is normal when it is just die cast, but after being stored at room temperature for a period of time (eight weeks to several months), bubbles appear on the surface.
(5) Impurity element iron
A. Iron reacts with aluminum to form Al5Fe2 intermetallic compounds, causing loss of aluminum elements and forming floating slag.
B. Forming hard spots in die castings affects post-processing and polishing.
C. Increase the brittleness of the alloy. The solubility of iron in zinc solution increases with temperature, and every temperature change in the zinc solution in the furnace will result in iron supersaturation (when the temperature drops) or unsaturation (when the temperature rises). When the iron element is supersaturated, the supersaturated iron will react with the aluminum in the alloy, resulting in an increase in the amount of floating slag. When the iron element is not saturated, the corrosion of the alloy on the zinc pot and goose neck material will increase, returning to a saturated state. A common result of the two temperature changes is ultimately the consumption of aluminum element, resulting in the formation of more floating slag.
5、 Issues to be noted in the production of zinc alloys
Control the alloy composition starting from the procurement of alloy ingots. The alloy ingots must be based on ultra-high purity zinc, combined with ultra-high purity aluminum, magnesium, and copper, and the supplier has strict composition standards. High quality zinc alloy material is the guarantee for producing high-quality castings.
After purchasing alloy ingots, it is necessary to ensure that there is a clean and dry storage area to avoid long-term exposure to moisture that may cause white rust, or contamination by factory dirt that increases slag production and metal loss. A clean factory environment is very effective in controlling the composition of alloys.
The ratio of new materials to recycled materials such as water inlet should not exceed 50%. Generally, the ratio of new materials to old materials is 70:30. Aluminum and magnesium gradually decrease in continuous remelted alloys.
When remelting the nozzle material, it is necessary to strictly control the remelting temperature not to exceed 430 ℃ to avoid the loss of aluminum and magnesium.
It is best for qualified die-casting factories to use centralized melting furnaces to melt zinc alloys, so that the alloy ingots and recycled materials are evenly proportioned, and the flux can be used more effectively to maintain uniform and stable alloy composition and temperature. Electroplating waste and fine particles should be separately melted in a furnace.
6、 Common defect treatment methods for zinc alloy die-casting parts
Due to the fact that the causes of each defect come from multiple different influencing factors, in actual production, to solve the problem, it is necessary to first adjust the machine in the face of numerous reasons? Should we change the materials first? Or modify the mold first? Suggest processing according to the difficulty level, starting with simplicity and then complexity, in the following order:
Clean the parting surface, clean the cavity, and clean the top rod; Improve coatings and spraying processes; Increase the clamping force and increase the amount of metal poured. These measures can be implemented through simple operations.
Adjust process parameters, injection force, injection speed, filling time, mold opening time, pouring temperature, mold temperature, etc.
Change materials, select high-quality aluminum alloy ingots, change the ratio of new materials to recycled materials, and improve the melting process.
Modify the mold, modify the pouring system, add internal gates, add overflow grooves, exhaust grooves, etc.
For example, the reasons for the occurrence of burrs in die-casting parts are:
1) Die casting machine issue: Incorrect adjustment of locking force.
2) Process issue: The injection speed is too high, resulting in a high pressure shock peak.
3) Mold issues: deformation, debris on the parting surface, uneven wear of inserts and sliders, insufficient strength of the template.
The sequence of measures to solve the flying edge: clean the parting surface → increase the locking force → adjust the process parameters → repair the worn parts of the mold → increase the rigidity of the mold. From easy to difficult, with each step of improvement, first verify its effectiveness, and if not, proceed to the second step.
7. Melting of Zinc Alloy
The 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 molten metal liquid, but more importantly, to obtain a metal liquid with a chemical composition that meets the regulations, which can make the die-casting parts have good crystalline structure and small gas and inclusions. During the melting process, the interaction between metal and gas, as well as the interaction between metal liquid and crucible, causes changes in the composition, 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）. The interaction between metals and gases during the smelting process involves encountering gases such as hydrogen (H2), oxygen (O2), water vapor (H2O), nitrogen (N2), CO2, CO, etc. These gases are either dissolved in the molten metal or undergo chemical reactions with it.
2）. The source of gas can enter the alloy liquid through furnace gas, furnace lining, raw materials, fluxes, tools, and other pathways.
3）. When the melting temperature is too high, the interaction between metal and crucible accelerates the reaction between iron crucible and zinc liquid, and the surface of the crucible undergoes iron oxidation reaction to generate Fe2O3 and other oxides; In addition, iron element will react with zinc solution to form FeZn13 compound (zinc slag), which will dissolve in the zinc solution. The wall thickness of the iron crucible continuously decreases until it is scrapped.
8、 Temperature control of zinc alloy melting
1）Die casting temperature
The melting point of zinc alloy used for die casting is 382-386 ℃, and appropriate temperature control is an important factor in controlling the composition of zinc alloy. To ensure good fluidity of the alloy liquid filling the cavity, the temperature of the metal liquid in the zinc pot of the die-casting machine is 415~430 ℃, and the upper limit of the die-casting temperature can be taken for thin-walled and complex parts; Thick walled and simple components can have a lower limit. The temperature of the molten metal in the central melting furnace is 430~450 ℃. The temperature of the metal liquid entering the gooseneck tube is basically the same as the temperature inside 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.
The harm of high temperature: ① Burning of aluminum and magnesium elements. ② The rate of metal oxidation accelerates, the amount of burned material increases, and the amount of zinc slag increases. ③ The thermal expansion effect can cause the hammer to get stuck. ④ The iron element in the cast iron crucible melts more into the alloy, and the reaction between zinc and iron accelerates at high temperatures. It will form hard particles of iron aluminum intermetallic compounds, causing excessive wear on the hammer and goose neck. ⑤ The corresponding increase in fuel consumption.
Low temperature: The alloy has poor fluidity, which is not conducive to forming and affects the surface quality of die-casting parts.
The current die-casting machine melting pot or furnace is equipped with a temperature measurement and control system. In daily work, the main task is to conduct regular checks to ensure the accuracy of the temperature measuring instrument. Portable thermometers (thermometers) are used to measure the actual temperature of the furnace and calibrate it regularly. Experienced die-casting workers will observe the melt with their naked eyes. If they feel that the melt is not too viscous and clear after scraping the slag, and the slag does not appear quickly, it indicates that the temperature is appropriate; If the melt is too viscous, it indicates that the temperature is too low; After scraping the slag, a layer of white frost quickly appears on the liquid surface. If the slag rises too quickly, it indicates that the temperature is too high and should be adjusted in a timely manner.
2）How to maintain temperature stability
One of the best methods is to use a central melting furnace and a die-casting machine melting furnace as insulation furnaces to avoid significant temperature changes caused by directly adding zinc ingots to the zinc pot for melting. Centralized melting can ensure the stability of alloy composition.
The second best method is to adopt an advanced metal liquid automatic feeding system, which can maintain stable feeding speed, alloy liquid temperature, and zinc pot liquid level height.
If the current production conditions are to directly add materials to the zinc pot, it is recommended to change from adding the entire alloy ingot at once to adding small alloy ingots multiple times, which can reduce the temperature change caused by the addition of materials.
9、 Generation and Control of Zinc Slag
The process of melting alloys from solid to liquid is a complex physical and chemical process. Gas undergoes a chemical reaction with molten metal, with oxygen reacting the most strongly, resulting in the oxidation of the alloy surface and the formation of a certain amount of floating slag. The floating slag contains oxides and intermetallic compounds of iron, zinc, and aluminum. The floating slag scraped off 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 production of the original zinc alloy ingot is less than 1%, within the range of 0.3% to 0.5%; The slag production from remelting nozzles and waste workpieces is usually between 2% and 5%.
1) Control of Zinc Slag Quantity
① Strictly control the melting temperature, the higher the temperature, the more zinc slag there will be.
② Try to avoid stirring the alloy liquid in the zinc pot as much as possible. Any form of stirring will cause more contact between the alloy liquid and oxygen atoms in the air, resulting in more floating slag.
③ Don't scrape the dirt too frequently. When molten alloys are exposed to air, oxidation occurs, forming floating slag. Keeping a thin layer of floating slag on the furnace surface is beneficial for preventing further oxidation of the liquid in the pot.
④ When scraping slag, use a porous (6 mm) disc-shaped scraper to gently scrape under the floating slag, avoiding stirring of the alloy liquid as much as possible. Collect the scraped slag and gently tap the scraper on the edge of the zinc pot to allow the metal liquid to flow back into the zinc pot.
2）. Treatment of Zinc Slag
① Sell back to the raw material supplier or specialized processing plant, as self processing may incur higher costs.
② The die-casting factory handles it on its own. A separate furnace is required, with a zinc slag remelting temperature ranging from 420 to 440 ℃. Simultaneously add fluxing agent. To melt 100 kilograms of slag, 0.5~1.5 kilograms of flux need to be added. First, it should be evenly dispersed on the metal surface. Then, it should be mixed evenly into the molten metal using a stirrer (about 2~4 minutes). After holding for 5 minutes, a layer of more soil like material should be formed on the surface, which should be scraped off.
3. Water nozzle materials, waste materials, garbage bins, 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 undergoes oxidation during the die-casting process, and their zinc oxide content far exceeds that of the original alloy ingot. When these nozzle materials are remelted in the zinc pot, due to the viscous state of zinc oxide under high temperature conditions, a large amount of alloy components will be taken away when they are removed from the zinc pot.
4. Remelting of electroplating waste
Electroplating waste should be melted separately from non electroplating waste, as metals such as copper, nickel, chromium, etc. contained in electroplating waste are insoluble in zinc and will remain in the zinc alloy as hard particles, causing difficulties in polishing and machining.
10、 Precautions during melting operation
1）. Crucible: It must be cleaned before use to remove surface oil stains, rust, slag, and oxides. To prevent iron elements from dissolving in the alloy in cast iron crucibles, the crucible should be preheated to 150-200 ℃, sprayed with a layer of coating on the working surface, and then heated to 200-300 ℃ to completely remove moisture from the coating.
2）. Tools: Smelting tools should be cleaned of surface dirt before use, and the parts in contact with metal must be preheated and coated with paint. Tools should not be contaminated with moisture, otherwise it may cause splashing and explosion of the melt.
3）. Alloy material: Before melting, it should be cleaned and preheated to remove surface adsorbed moisture. To control the alloy composition, it is recommended to use a combination of 2/3 new material and 1/3 recycled material.
4）. The melting temperature must not exceed 450 ℃.
5）. Timely clean the floating slag on the liquid surface of the zinc pot, replenish the zinc material in a timely manner, and maintain the normal height of the molten liquid surface (not lower than 30mm from the crucible surface), because excessive floating slag and low liquid level can easily cause the slag to enter the gooseneck barrel, strain the steel ball, hammer head, and barrel itself, resulting in the jamming of the hammer head, gooseneck, and hammer head, and their scrapping.
6）. The floating slag on top of the melt is stirred calmly with a scraper to gather and facilitate removal.
