What are the factors that affect aluminum alloy die-casting ?

People engaged in the die-casting industry know that aluminum alloy die-casting molds are complex equipment that must complete multiple functions simultaneously. Its appearance has a significant impact on the size of aluminum alloy die castings. The fixed or movable core enhances the sensitivity of aluminum alloy die-casting, allowing for the production of complex and precise parts. The geometric shape of the runner mouth affects the filling of the mold. The thermal conditions of aluminum alloy die-casting molds determine the solidification, microstructure, and quality of the parts. In mass production, the thermal conductivity of the mold determines the cycle time. Therefore, it can be said that there are many factors that affect the aluminum alloy die-casting process.

1. Aluminum alloy die-casting is mainly affected by factors such as injection pressure, injection speed, filling time, and mold temperature in production. These elements influence and constrain each other, and adjusting one element will cause corresponding process changes. Therefore, it is important to choose the correct process parameters.

2. The most commonly used in the die-casting industry is aluminum alloy die-casting, which cannot be compared with other alloys during use. Due to the different components in aluminum alloy die-casting, the physical and chemical functions of the alloy exhibited during use are different.

3. The crystallization of aluminum alloy die-casting during manufacturing is also different. It is necessary to choose a reasonable casting method based on the characteristics of its aluminum alloy during application, in order to effectively prevent casting defects.

4. The process of aluminum alloy die-casting is usually understood as filling the mold in application. The most prominent performance in the cooling and crystallization process of equipment is the comprehensive utilization of those functions. These characteristics of aluminum alloys depend on the composition of the alloy, but are also related to casting elements, alloy heating temperature, complexity of the casting mold, gating system, gate shape, etc.

5. The activity of aluminum alloy die casting mainly refers to the filling of the mold in the alloy liquid, and the magnitude of its activity effectively determines whether the alloy can cast complex castings to a certain extent. The activity of eutectic alloys in aluminum alloy die casting is the best.

In fact, there are many factors that affect the aluminum alloy die-casting process, mainly including composition, temperature, and solid particles containing metal oxides, compounds, and other pollutants in the alloy liquid. However, the fundamental internal factors are pouring temperature and pouring pressure. In the theory of aluminum alloy die-casting, when the alloy has been determined, in addition to strengthening the melting process, it is also necessary to improve the casting process, increase the pouring temperature without affecting the quality of the casting, and ensure the activity of the alloy.

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Key equipment for integrated die-casting

Since Tesla launched an innovative trend of integrated die-casting, its application range has extended from the rear bottom plate to the front bottom plate, driving a strong demand for large die-casting machines.

The core of integrated die-casting technology lies in integrating numerous automotive components through one-time die-casting, thus setting strict standards for the raw materials and die-casting technology of vehicle body structural components.

Ultra large die-casting machines are an important development direction of die-casting technology. As the core equipment of integrated die-casting, ultra large die-casting machines have extremely high technical and market value.

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What surface treatments are available for aluminum alloy die casting

 Anodizing: 

The surface corrosion resistance of extruded aluminum alloy profiles is not strong, and surface treatment through anodizing is necessary to increase the corrosion resistance, wear resistance, and aesthetic appearance of the aluminum material.

Electrophoretic painting: 

The surface of electrophoretic painted profiles has a soft luster and can resist the erosion of cement and mortar acid rain.

Powder electrostatic spraying: 

The characteristics of powder electrostatic spraying profiles are excellent corrosion resistance, acid, alkali, and salt spray resistance, which is much better than that of oxidized colored profiles.

Titanium plating and titanium gold plating process: 

The titanium gold plating process on aluminum profiles belongs to the coating technology. It adds pre plating and electroplating process steps on the basis of conventional titanium plating process. The pre plating process is to chemically treat the activated plated parts in an aqueous solution of salt and hydrochloric acid, thereby producing a colorful effect.

Wood grain transfer printing: 

Vacuum wood grain coating technology is used to transfer wood grain onto the surface of a workpiece through thermal transfer printing after electrostatic spraying of metal products. Not easy to fade, realistic wood grain, improving product grade.

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Aluminum alloy die-casting process flow

 1) According to the principles of fluid mechanics, a theoretical analysis and some calculations were conducted on the movement of liquid metal in the pressure chamber of a horizontal cold chamber die casting machine. Based on this analysis, it is believed that the slow injection process of the punch in the horizontal cold chamber die casting machine is a combination of accelerated and uniform motion, and the combined result directly affects the quality of the die casting. The amount of air entrained by liquid metal in the pressure chamber is related to the slow injection acceleration, slow injection velocity, initial filling degree, and pressure chamber diameter, and there exists a critical slow injection velocity and optimal acceleration. At this velocity and acceleration, the amount of air entrained can be minimized, and the porosity of the casting can be minimized.

2) The process of liquid metal filling during die casting production is a process where many conflicting factors are unified. Among the many factors that affect filling, the main ones are pressure, speed, temperature, and time. Time is the result of the coordination and synthesis of process parameters, and various process factors are interdependent and mutually restrictive. When adjusting a certain process factor, it will inevitably cause changes in the corresponding process factor, and may in turn affect the already adjusted process factor, causing it to change. Therefore, only by correctly selecting, controlling, and adjusting these process parameters to meet the needs of die-casting production can qualified die-casting parts be produced under other good conditions.

3) The basic characteristics of aluminum alloy die casting are filling under high pressure and high speed, crystallization under high pressure. During the entire rapid injection stage, the metal liquid enters the mold cavity in the form of a jet at a speed of 30-60m/s, and it is impossible for the metal liquid to not wrap up the gas. In this case, adjusting the process parameters and plan is the key to where and in what form the gas pores are reasonably distributed. Due to the high-pressure jet breaking gas into dispersed small pores and leaving them in the casting, it is not suitable to manufacture important safety components such as shells and outer covers through heat treatment. Improve strength and lower elongation of die castings. Therefore, die casting is generally suitable for producing thin-walled structures that do not require significant impact loads.

4) According to the process characteristics of aluminum alloy die casting, it is difficult to form large-area thin-walled parts. If the wall thickness is too large or severely uneven, defects and cracks are prone to occur. It is hoped that the wall thickness of the die casting will be as uniform as possible. For large aluminum alloy die castings, the wall thickness should generally not exceed 6mm. Under normal process conditions, the wall thickness of die castings should not exceed 4.5mm. To avoid defects such as shrinkage and porosity at the thick wall of the die casting, it is necessary to reduce the thickness of the thin wall and add strengthening ribs.

5) Due to the characteristics of aluminum alloy die-casting process, the alloy used requires a small range of crystallization temperature, low tendency for hot cracking, and low shrinkage coefficient.

6) The ability to cast deeper and smaller holes is a characteristic of die casting technology. For some holes with low precision requirements, they can be directly used without the need for mechanical processing, thereby saving machining time. The diameter and depth of the holes cast on the parts are related, and smaller holes can only be cast at shallower depths. The general aperture is not less than 2mm, and the hole depth is not greater than 4-8 times the aperture. The threaded holes on castings are often made by first die-casting a core hole that meets the requirements, and then machining (mostly by tapping) to create a threaded hole.

7) At the connection between the walls of die castings, whether it is a right angle, acute angle, or obtuse angle, it should be designed as a rounded corner. In order to facilitate the demolding of die castings from the mold cavity and core, prevent surface scratches, and extend the life of the mold, die castings should have a reasonable demolding angle. Its size depends on the wall thickness of the casting and the type of alloy. The thicker the wall thickness of the casting, the greater the clamping force of the alloy on the core, and the greater the demolding angle. The larger the shrinkage rate of the alloy, the higher the melting point, and the greater the demolding angle. In addition, the demolding angle on the inner surface or inner wall of the hole of the casting is larger than that on the outer surface. Within the allowable range, a larger demolding angle should be used to reduce the required ejection force or core pulling force. The general demolding angle is taken as 0.5 °~1.5 °.

8) In terms of die casting technology, threads can be directly cast under certain conditions.

9) Various convex patterns, mesh patterns, characters, logos, and patterns can be cast on the die casting.

10) Metal or non-metallic parts (inserts) can be first embedded in the die casting mold and then cast together with the die casting. This can fully utilize the properties of various materials (such as strength, hardness, corrosion resistance, wear resistance, magnetic conductivity, conductivity, etc.) to meet the requirements for use under different conditions, compensate for the shortcomings caused by poor casting structure and process, and solve the die-casting problem of parts with special technical requirements.

11) Die castings have precise dimensions and good casting surfaces, and generally do not require further mechanical processing. Meanwhile, due to the presence of internal pores in die casting, further mechanical processing should be avoided as much as possible. However, the parts produced by die casting cannot be directly assembled and used in any situation, so in some cases, mechanical processing is also required on some surfaces or parts. The surface of die-casting parts is dense and uniform, with good mechanical and physical properties. The thickness of this surface is approximately 0.5-0.8 mm, so the general machining allowance is recommended to be 0.3-0.5 mm.

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What are the characteristics of aluminum alloy die-casting products

 1) Aluminum alloys can be classified according to their performance characteristics and uses

Four types of rust proof aluminum (LF), hard aluminum (LY), ultra hard aluminum (LC), and forged aluminum (LD)

2) Cast aluminum alloys are divided into different types based on the addition of the main alloying elements

Four types of aluminum silicon system (AL Si), aluminum copper system (Al Cu), aluminum magnesium system (Al Mg), and aluminum zinc system (Al Zn)

Commonly used grades include ADC12 (A383) and ADC10 (A380)

3) Advantages of Aluminum Alloy Die Casting

Good product quality: The casting has high dimensional accuracy, good surface smoothness, high strength and hardness. The strength is generally increased by 25-30% compared to sand casting, but the elongation is reduced by about 70%. The size is stable and the interchangeability is good. Die cast aluminum thin-walled complex castings, for example: the current minimum wall thickness of zinc alloy die casting can reach 0.3mm, and aluminum alloy die casting can reach 0.5mm. High production efficiency: The machine has high productivity. For example, the domestically produced J-III3 horizontal cold air die casting aluminum machine can die cast aluminum 600-700 times per eight hours on average, and the small hot chamber die casting aluminum machine can die cast aluminum 3000-7000 times per eight hours on average. Excellent economic effect: Due to the precise size and smooth surface of die cast aluminum parts. Generally, it is used directly without mechanical processing or with a small amount of processing, which not only improves the metal utilization rate but also reduces a large amount of processing equipment and working hours. The cost of castings is cheap, and composite die cast aluminum can be used with other metal or non-metallic materials, saving both assembly time and metal.

4) Disadvantages of aluminum alloy die-casting

Due to the high filling speed and unstable flow state of liquid metal in the mold cavity during die casting, the general die casting aluminum method is used. However, the casting is prone to porosity and cannot undergo heat treatment. Die casting is more difficult for castings with complex concave surfaces. The lifespan of aluminum alloy die casting molds is relatively low, with a lifespan of approximately 80000 strokes. Small batch production is not suitable, mainly due to the high manufacturing cost of die cast aluminum molds and the uneconomical nature of small batch production. Aluminum alloy die-casting is not easy to achieve anodizing, as it is prone to leaving many bubbles or sand holes after die-casting, resulting in poor repair of the appearance and inability to cover the appearance after oxidation.

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The tips of die casting design

 Under the premise of meeting the product function, the die castings are reasonably designed, the die casting mold structure is simplified, the die casting cost is reduced, the die casting defects are reduced and the quality of die casting parts is improved. Since the injection molding process is derived from the casting process, the die casting design guide is very similar to the plastic parts design guide in some aspects. For detailed die casting part design, refer to the book "Design Guide for Manufacturing and Assembly Products" published by the Machinery Industry Press.

Fillet

(including corners) The casting drawing often indicates the requirements of unfilled corners R2 and so on. When making the mold, we must not ignore the role of these unmarked fillets, and never make them clear corners or too small fillets. Casting fillets can make the metal liquid fill smoothly, so that the gas in the cavity is discharged in sequence, and can reduce stress concentration and extend the service life of the mold. (It is also not easy for the casting to crack at this place or various defects due to improper filling). For example, there are more clear corners on the standard oil pan mold. Relatively speaking, the brother oil pan mold is the best at present, and there are more heavy oil pans.

Demolding slope

It is strictly forbidden to have artificial side concave in the demolding direction (often when the casting is stuck in the mold during the mold trial, and when it is handled by incorrect methods, such as drilling, hard chiseling, etc., it causes local concave).

Roughness

The molding parts and pouring system should be carefully polished as required, and should be polished along the demolding direction. Since the entire process of the molten metal entering the pouring system from the pressure chamber and filling the cavity only takes 0.01-0.2 seconds. In order to reduce the resistance of the molten metal flow and minimize the pressure loss, the surface finish of the flow needs to be high. At the same time, the heating and erosion conditions of the pouring system are relatively severe. The worse the finish, the easier it is to damage the mold.

Hardness of the molding part of the mold Aluminum alloy: about HRC46° Copper: about HRC38° During processing, the mold should leave a margin for repair as much as possible, make the upper limit of the size, and avoid welding.

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The advantages and disadvantages of die casting

 Advantages

Advantages of die casting include excellent dimensional accuracy. This generally depends on the casting material, but typical values ​​are 0.1 mm for the first 2.5 cm of the casting and 0.002 mm for each additional cm. Compared to other casting processes, it produces smooth casting surfaces with corner radii of approximately 1-2.5 μm. It can produce castings with wall thicknesses of approximately 0.75 mm compared to sandbox or permanent mold casting. It can directly cast internal structures such as wire sleeves, heating elements, and high-strength bearing surfaces. Other advantages include the ability to reduce or avoid secondary machining, high production speeds, casting tensile strengths of up to 415 MPa, and the ability to cast highly fluid metals.

Disadvantages

The biggest disadvantage of die casting is its high cost. The casting equipment, as well as the molds and mold-related components, are relatively expensive compared to other casting methods. Therefore, it is economical to produce die castings in large quantities. Other disadvantages include that the process is only suitable for metals with high fluidity and that the casting mass must be between 30 g and 10 kg [5]. In conventional die casting, the final batch of castings will always contain porosity. Therefore, no heat treatment or welding can be performed because the gas in the gap will expand under the action of heat, causing internal micro-defects and surface peeling.

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Die Casting Mold

 The die casting mold consists of two parts, the cover part and the movable part, and the part where they meet is called the parting line. In hot chamber die casting, the cover part has the gate, while in cold chamber die casting, it is the injection port. Molten metal can enter the mold from here, and the shape of this part matches the injection nozzle in hot chamber die casting or the injection chamber in cold chamber die casting. The movable part usually includes push rods and runners, which are the channels between the gate and the mold cavity through which the molten metal enters the mold cavity. The cover part is usually connected to the fixed platen or the front platen, while the movable part is connected to the movable platen. The mold cavity is divided into two mold cavity inserts, which are independent parts and can be removed or installed from the mold relatively easily by bolts.

The mold is specially designed so that the casting will remain in the movable part when the mold is opened. This will push the casting out with the push rods in the movable part. The push rods are usually driven by the platen, which will drive all the push rods at the same time with exactly the same amount of force, so as to ensure that the casting is not damaged. When the casting is ejected, the platen retracts to retract all the push rods and prepare for the next die casting. Since the casting is still hot when it is ejected from the mold, only if there are enough push rods can the average pressure on each push rod be small enough to avoid damaging the casting. However, the push rods still leave marks, so they must be carefully designed so that the position of the push rods does not affect the operation of the casting too much.

Other parts in the mold include core slides. Cores are parts used to make holes or openings in castings. They can also be used to add details to the casting. There are three main types of cores: fixed, movable and loose. Fixed cores are parallel to the direction of the casting out of the mold. They are either fixed or permanently connected to the mold. Movable cores can be arranged in any direction except the ejection direction. Before the mold is opened after the casting solidifies, the movable core must be removed from the mold cavity using a separation device. Sliders and movable cores are very similar. The biggest difference is that slides can be used to create undercut surfaces. Using cores and slides in die casting will greatly increase costs. Loose cores, also called ejector blocks, can be used to create complex surfaces such as threaded holes. Before each cycle, the slide is manually installed and then ejected with the casting. The loose core is then removed. Loose cores are the most expensive cores because they are labor-intensive to make and they increase cycle time.

The ejector is usually thin and long (about 0.13 mm) so that the molten metal cools quickly and reduces waste. Risers are not required in the die casting process because the molten metal is under high pressure, which ensures a continuous flow from the gate into the mold.

Due to the temperature, the most important material properties for the mold are resistance to thermal vibration and flexibility. Other characteristics include hardenability, machinability, resistance to hot cracking, weldability, availability (especially for large molds), and cost. The life of the mold is directly dependent on the temperature of the molten metal and the time of each cycle. The molds used for die casting are usually made of hard tool steel. Because cast iron cannot withstand the huge internal pressure, the molds are expensive, which also leads to high mold opening costs. Metals die cast at higher temperatures require harder alloy steels.

The main defects that can occur during die casting include wear and erosion. Other defects include thermal cracking and thermal fatigue. Thermal cracking occurs when defects appear on the mold surface due to large temperature changes. After too many uses, defects on the mold surface will cause thermal fatigue.

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Die casting equipment classification

 Die casting machines can be divided into two main types: hot chamber and cold chamber. The difference lies in how much force they can withstand, with typical pressures ranging from 400 to 4,000 tons.

Hot chamber die casting

Hot chamber die casting, sometimes called gooseneck die casting, involves a pool of molten liquid or semi-liquid metal that is forced into the die under pressure. At the start of the cycle, the machine's piston is retracted, allowing the molten metal to fill the gooseneck. A pneumatic or hydraulic piston squeezes the metal into the die. Advantages of this system include fast cycle speeds (about 15 cycles per minute), easy automation, and the convenience of melting the metal. Disadvantages include the inability to die cast metals with higher melting points, and also the inability to die cast aluminum, which would pull the iron out of the molten pool. Therefore, hot chamber die casting machines are usually used for zinc, tin, and lead alloys. Also, hot chamber die casting is difficult to use for large castings, and usually the process is used for small castings.

Cold chamber die casting

Cold chamber die casting can be used for metals that cannot be cast using hot chamber die casting processes, including aluminum, magnesium, copper, and zinc alloys with high aluminum content. In this process, the metal is first melted in a separate crucible [2]. A certain amount of molten metal is then transferred to an unheated injection chamber or injection nozzle. The metal is injected into the mold using hydraulic or mechanical pressure. The main disadvantage of this process is the long cycle time due to the need to transfer the molten metal into the cold chamber. Cold chamber die casting machines can be divided into vertical and horizontal types. Vertical die casting machines are usually small machines, while horizontal die casting machines are available in a variety of sizes.

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Die Casting Process

 The traditional die casting process consists of four main steps, or high pressure die casting. These four steps include mold preparation, filling, injection and sanding, which are also the basis of various modified die casting processes. During the preparation process, a lubricant needs to be sprayed into the mold cavity. In addition to helping control the temperature of the mold, the lubricant can also help the casting to be demolded. Then the mold can be closed and the molten metal is injected into the mold with high pressure. The pressure range is about 10 to 175 MPa. When the molten metal is filled, the pressure will be maintained until the casting solidifies. Then the push rod will push out all the castings. Since there may be multiple cavities in a mold, multiple castings may be produced in each casting process. The sanding process requires separating the residue, including the mold gate, runner, gate and flash. This process is usually completed by squeezing the casting through a special trimming die. Other sanding methods include sawing and grinding. If the gate is fragile, the casting can be directly dropped, which can save labor. The excess mold gate can be reused after melting. The typical yield is about 67%.

High pressure injection results in a very fast filling of the mold, so that the molten metal fills the entire mold before any part solidifies. In this way, surface discontinuities can be avoided even in thin-walled sections that are difficult to fill. However, this can also lead to air entrapment, as it is difficult for air to escape when the mold is filled quickly. This problem can be reduced by placing vents on the parting line, but even very precise processes will leave pores in the center of the casting. Most die castings can be completed by secondary operations such as drilling and polishing to complete structures that cannot be completed by casting.

After the sand is dropped, it is time to check for defects. The most common defects include stagnation (under-filling) and cold scars. These defects can be caused by insufficient mold or molten metal temperature, impurities in the metal, too few vents, too much lubricant, etc. Other defects include pores, shrinkage, hot cracks and flow marks. Flow marks are traces left on the surface of the casting due to gate defects, sharp corners or too much lubricant.

Water-based lubricants are called emulsions and are the most commonly used type of lubricant due to health, environmental and safety considerations. Unlike solvent-based lubricants, water does not leave byproducts in the casting if the minerals in the water are removed using the proper process. If the water treatment process is not done properly, minerals in the water can cause surface defects and discontinuities in the casting. There are four main types of water-based lubricants: water-in-oil, oil-in-water, semi-synthetic, and synthetic. Water-in-oil lubricants are the best because the water cools the mold surface by evaporation while depositing the oil during lubrication, which can help with mold release. Typically, this type of lubricant is mixed with 30 parts water to 1 part oil. In extreme cases, the ratio can be as high as 100:1.

Oils that can be used for lubricants include heavy oils, animal fats, vegetable fats, and synthetic greases. Heavy residual oils are viscous at room temperature, but they become thin films at the high temperatures of the die casting process. Other substances are added to the lubricant to control the viscosity and thermal properties of the emulsion. These substances include graphite, aluminum, and mica. Other chemical additives can prevent dust and oxidation. Emulsifiers can be added to water-based lubricants so that oil-based lubricants can be added to water, including soaps, alcohols, and ethylene oxide.

Solvent-based lubricants have long been used, including diesel and gasoline. They facilitate casting ejection, but small explosions occur during each die casting process, which causes carbon to accumulate on the cavity wall. Solvent-based lubricants are more uniform than water-based lubricants.

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The History of Die Casting

 In 1838, die-casting equipment was invented to make molds for movable type printing. The first patent related to die-casting was issued in 1849. It was a small, manual machine used to produce lead type for printing presses. In 1885, Otto Mergenthaler invented the Linotype typesetter, a machine that could die-cast a whole line of text into a single lead type, which brought unprecedented innovation to the printing industry. After the printing industry entered large-scale industrialization, traditional hand-pressed type molds were replaced by die-casting. 

Around 1900, the entry of typesetting into the market further improved the automation technology of the printing industry, so sometimes more than ten die-casting machines could be seen in newspaper offices. With the continuous growth of consumer products, Otto's invention has gained more and more applications. People can use die-casting to manufacture parts and products in large quantities. In 1966, General Dynamics invented the precision die-casting process, which is sometimes also called double-punch die-casting.

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Introduction to Die Casting

 Die casting is a precision casting method that uses high pressure to force molten metal into a metal mold with complex shapes. In 1964, the Japan Die Casting Association defined die casting as "a casting method that presses molten alloy into a precision casting mold at high temperature to mass produce high-precision and excellent casting surface in a short time." The United States calls die casting Die Casting, the United Kingdom calls it Pressure Die Casting, and the most familiar term for domestic general industry insiders is Japan, which is called die casting. Castings made by die casting are called die castings.

The tensile strength of these materials is nearly twice that of ordinary casting alloys, which has a more positive significance for parts such as aluminum alloy automobile wheels and frames that are expected to be produced with higher strength and impact-resistant materials.

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What is Die Casting?

 Die casting is a metal casting process characterized by the use of a die cavity to apply high pressure to the molten metal. The die is usually made of a stronger alloy, and the process is somewhat similar to injection molding. Most die castings are non-ferrous, such as zinc, copper, aluminum, magnesium, lead, tin, lead-tin alloys and their alloys. Depending on the type of die casting, a cold chamber die casting machine or a hot chamber die casting machine is required.

Casting equipment and molds are expensive, so the die casting process is generally only used to batch large quantities of products. It is relatively easy to manufacture die-cast parts, which generally requires only four major steps and the unit cost increment is very low. Die casting is particularly suitable for manufacturing a large number of small and medium-sized castings, so die casting is the most widely used of various casting processes. Compared with other casting technologies, die casting has a smoother surface and higher dimensional consistency.

Several modified processes have been born based on the traditional die casting process, including the non-porous die casting process to reduce casting defects and eliminate pores. Direct injection process, mainly used for processing zinc, can reduce waste and increase yield. There are also new die-casting processes such as precision die-casting technology invented by General Dynamics and semi-solid die-casting.

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