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2022/12/20
What is aluminum die casting? Explanation of its characteristics and manufacturing process
METAL COLUMN

What is aluminum die casting?

This is a casting method that uses a die-casting machine to fill a precision mold with molten metal such as aluminum alloy at high speed and then applies high pressure to it, allowing for the mass production of castings with excellent casting surfaces and high precision in a short period of time.
Die casting is widely used in the automotive industry, as well as in office equipment and home appliances.
It provides superior quality, including high dimensional accuracy, excellent casting surface quality, and high degree of design freedom that cannot be achieved with other casting methods such as sand casting, gravity casting, and metal mold casting.

Die casting requires faster injection of molten metal than regular casting, so the molten aluminum also needs to have high fluidity. For this reason, the composition is slightly different from other aluminum castings. For example, Fe, which is added for aluminum alloy die casting, is an impurity in the base metal for regular aluminum casting, but is intentionally added for aluminum die casting to prevent it from seizing on the mold.
Casting is said to be difficult to achieve precision, but a major feature of aluminum die casting is that it can produce high dimensional precision, making it a manufacturing method suitable for mass production.

About strength

Approximately 90% or more of aluminum die-cast products are manufactured from ADC12, an aluminum alloy (material) for die-casting. Aluminum die-casting is originally strong because it is made from aluminum, but if you want to further increase its strength, it is common to carry out surface treatment.
The structure of die castings differs greatly between the surface layer and the interior. The surface layer has an area called the chill layer, which is made up of fine structures. The chill layer is a dense and very hard area (Vickers hardness approximately Hv110) that is formed when the molten metal is rapidly cooled. The thinner the wall thickness and the lower the mold temperature, the thicker the chill layer will be.

On the other hand, the cooling rate is slow inside, so coarse structures form. There are many defects such as blowholes, and it is inferior to the chill layer in terms of hardness (Vickers hardness Hv85-90). The chill layer is also very important for die casting. The thicker the chill layer on the surface, the higher the tensile strength and breaking elongation. Care must be taken when removing the chill layer indiscriminately through processing, as this will deteriorate the mechanical properties.

The strength of die castings is generally measured using test pieces specified by the ASTM (formerly American Society for Testing and Materials) standard. ASTM tensile test pieces have very few defects such as casting cavities in the parallel section, so they have ideal tensile properties. In comparison, the strength of the actual product (actual strength) shows a low value. Table 1 shows a comparison of the actual strength properties of major aluminum alloy die castings with the strength properties of ASTM test pieces. The tensile strength is about 70% of the ASTM test piece, and the elongation is about 50%.

How to increase the strength of aluminum die casting

Aluminum die casting has a certain degree of strength to begin with, but its strength can be further increased by surface treatment. There are various methods for surface treatment, so please choose the appropriate method according to the application. The following methods can be used to increase strength by surface treatment (painting/plating).

Copper + silicon (painted) → increased strength.
It is used in aircraft.

Manganese (painted) → Rust resistant, stronger, and easier to machine.
It is used in aluminum cans.

Magnesium (painted) → Rust resistant and easier to machine.
It is used in ships and vehicles.

Magnesium + silicon (painted) → Rust resistant and stronger.
It is used in architectural sashes.

Zinc (plating) → Greatly increases strength.
It is used in aircraft and rail cars.

The strength of aluminum die-casting can be increased by increasing the surface hardness through plating or special painting.
Die casting has higher strength than sand casting or metal mold casting. Compared to other casting methods, the cooling rate is faster and the cast structure is finer.

Advantages of aluminum die casting

There are six main advantages to aluminum die casting.
〇Suitable for mass production.
〇Highest dimensional accuracy.
〇The casting surface is the smoothest.
〇Thin-walled products can also be manufactured.
〇You can choose from materials with different characteristics depending on the application.
(Corrosion resistance, impact strength, mechanical properties, castability, wear resistance, etc.)
*Various surface treatments are possible depending on the application, such as plating, chemical conversion coating, anodizing, and painting.

In addition, even complex products that require precision can be manufactured in a single process by pouring metal into a mold, which makes it highly productive and possible to mass-produce products at low cost (however, processing and finishing after casting are required).
Compared to other castings, die casting can achieve high dimensional accuracy. The surface is also finished smoothly, so in some cases no processing is required after casting.

Disadvantages of aluminum die casting

There are six major disadvantages to aluminum die casting:
×Die-cast products are filled with molten metal at high speed and pressure, so the air in the mold product and hot water supply (inside the sleeve) and evaporated mold release agent are mixed into the product during casting, resulting in a few percent of the product's weight being filled with voids (gas inside the product).For this reason, they are not suitable for parts that require strength.
× High mold costs
× The strength and toughness are inferior to those of castings that are less susceptible to blowholes.
× A taper is required for the die※1
×Undercut shapes are not possible※2
×The molds required for die casting are very expensive, and although die casting is suitable for mass production, the cost is not justified for small quantities. Also, when the mold reaches the end of its life, it is necessary to remake the same item at the same cost.

*1 Draft taper: The inclination that allows the molded product to be removed without rubbing against the mold. Without a draft slope, the mold and the molded product will rub against each other when removed from the mold, resulting in a loss of precision in the molded product. Unlike resin, CPFR and carbon in particular contain fibers, resulting in a fuzzy surface.

*2 Undercut: This refers to a convex or concave shape that prevents a molded product from being released from the mold when it is removed.

About the mold

"Aluminum die casting mold material" ... The material of the die casting mold is a metal with a relatively low melting point such as aluminum, zinc, magnesium, or brass (an alloy of copper and zinc). The mold must have the following characteristics:
-Types of casting alloys
・Production quantity
・Used area of the mold
Considering the above, the mold material is as follows:
- High hardness at the mold temperature during casting, and excellent toughness
-Excellent abrasion resistance and heat resistance at the mold temperature during casting
- Resistant to thermal fatigue and has high resistance to softening when heated
- Good hardenability and little hardening distortion
-Good thermal conductivity
- Not easily affected by casting alloys
- Good machinability
Such properties are required.

"Aluminum die-casting mold temperature"...The cavity surface of a die-casting mold can be heated to 450-550° by contact with molten metal at 600-700°. In such high-temperature areas, the aluminum alloy is likely to adhere to the mold and remain when the casting is removed, resulting in "burn-in." In the burn-in area, an Fe/Al reaction layer is formed at the interface between the casting and the mold.

Aluminum die casting manufacturing process

Design

There are many things that must be decided beforehand. First, is the design feasible? From measuring to deciding the thickness, whether to make it in one piece or in separate pieces. These prior discussions have a significant impact on the finished product and delivery date, so they are very important tasks. We perform solidification analysis using CAD to create the optimal product shape and casting method.

Mold making (wooden molds and metal molds)

Once the decision has been made, we begin making the wooden mold. Taking into consideration the size, complexity, and workability of the product, the mold is made by hand by a craftsman or using a numerical controller.

Sand mold

The wooden mold is set into the frame and sand is poured in. As the mold has a front and back, two sand molds are made, one above and one below. Once the sand has hardened, it is removed from the wooden mold and the top and bottom molds are joined together. We use self-hardening sand molds for casting. Sand molds made with a mixture of furan resin and a hardener are also suitable for large, small-lot production items.

METAL MOLD

This method uses a heat-resistant metal mold instead of a sand mold and is widely used industrially. Metal mold casting has a faster cooling rate than sand casting, and has finer crystal grains, resulting in superior mechanical properties.

DISSOLUTION

Alloy ingots (AC4C or AC7A) or returned materials of the same material are placed in a melting furnace and melted. The molten alloy (molten metal) is subjected to removal of impurities such as gas and oxides. The melting temperature is about 660°C, and the casting temperature is 700°C or higher. The casting temperature is determined based on the size of the item, etc.

CASTING

Molten metal is drawn and poured into a mold, then allowed to solidify by natural cooling. The sand mold is destroyed and the solidified casting is removed. The surface sand is removed, and gates and other parts not needed for the product are cut off. The broken sand is recycled and reused. Cut-off gates are also melted down and reused as recycled material.

FINISH

We perform straightening, polishing, and painting to make the product ready for shipment. If necessary, we perform additional machining to achieve dimensional accuracy. We also sometimes undertake surface treatments such as anodizing, painting, and plating.

The difference between aluminum die casting and aluminum casting

Aluminum die casting involves melting aluminum alloys, zinc alloys, etc. and forcing them into a mold to form the shape.
Press fitting is a manufacturing method that makes it possible to mass-produce castings with high precision and beautiful surfaces.
Aluminum casting is a method in which metals such as aluminum alloys and copper are liquefied at a temperature higher than their melting point, poured into a mold, and cooled to solidify. Aluminum die casting uses a metal mold, whereas aluminum casting generally uses a sand mold. Die casting requires the design and production of a mold from a block of metal, which results in high production costs.

Aluminum die casting and rust

Although aluminum is a metal that does not rust easily, it can rust on rare occasions because there are almost no products made from 100% purity aluminum.

There are three reasons why rust occurs:

1. Rust caused by galvanic corrosion
2. Rust caused by pitting of aluminum
3) Corrosion due to grain boundaries in aluminum alloys
Examples include:

Regarding ①, aluminum is a metal with a low electric potential, and if it is used in conjunction with a substance with a high electric potential, corrosion will occur and rust will form.

Regarding ②, pitting corrosion is localized corrosion that can easily occur on aluminum. Chloride ions damage the aluminum oxide, causing rust.

Regarding point 3, aluminum is made up of small grains. There are small gaps between the grains. These gaps are vulnerable to corrosion and are prone to rust.

The first method to prevent rust on aluminum die castings is "anodizing." Anodizing is a major measure to prevent rust and corrosion on aluminum die castings. It prevents rust by creating an oxide film on the surface. It increases the strength of aluminum die castings and keeps them in good condition. It is a widely used processing method, but is not suitable for alloys used for casting.

The second method to prevent aluminum die casting from rusting is "plating." Plating is a process in which another metal is applied to the surface of the aluminum die casting. Coating the surface with a metal that has better corrosion resistance than the aluminum die casting prevents rust.

The third method to prevent rust on aluminum die castings is "ADC12 chemical conversion treatment". ADC12 chemical conversion treatment is a measure to improve corrosion resistance by using ADC12, a type of excellent alloy. ADC12 chemical conversion treatment is mainly divided into two types: trivalent chromium and zirconium.

Aluminum die casting and whether it can be welded

Welding is not considered very suitable for aluminum die casting.
The reason is that aluminum is a material that oxidizes easily, making it difficult to weld properly.
Next, it is prone to distortion and cracking.
Aluminum die casting contains a significant amount of additives other than aluminum, and because it is a casting material with a low melting temperature, it is prone to internal casting voids and these defects can easily expand when heat is applied, presenting many challenges when welding.

Aluminum die casting | ADC materials

・ADC1: The yield strength is slightly lower, so it is not suitable for components that require high strength. However, as mentioned above, it has good castability, so it is also good for complex shapes and thin walls, but it is a standard material that is surprisingly rarely used in practice.

・ADC3...It has fairly good electroplating properties, and is also good in terms of abrasion resistance and machinability. It also has excellent strength properties and airtightness at high temperatures. In addition to pressure resistance, it also has excellent tensile strength and impact resistance, making it useful in applications where such strength is required. Its corrosion resistance is comparable to ADC1, but its castability is good, although not as good as ADC1. Another similar alloy is equivalent to A360.0.

・ADC5: Although it has very good corrosion resistance, it is not suitable for products with complex shapes because it is not easy to cast. In terms of mechanical properties, it has good parameters for elongation and impact value, so it is also an option when strength is required.

・ADC6...Excellent wear resistance and machinability, and also has good polishing properties. An alloy equivalent to 515.0.

・ADC10...In addition to its good machinability and castability, the addition of Cu also improves its strength. It is a well-balanced type that has excellent strength and good castability among aluminum die castings. It is often used because it can be used to make products with excellent mechanical properties and good pressure resistance. However, although it has a similar balance to ADC12, it is not nearly as widely used. As an alloy, it is equivalent to A380.0.

・ADC10Z...As an aluminum die casting, it has similar properties to ADC10, such as castability and mechanical strength, but it contains zinc as an impurity and tends to be slightly inferior in terms of casting crack resistance and corrosion resistance. Materials with a Z at the end are allowed to contain up to 3% zinc (Zn) as an impurity, and are aluminum die castings made to comply with overseas standards.

・ADC12…ADC12 is a well-balanced alloy type for aluminum die casting, with high levels of mechanical properties, machinability, and castability. It is said that over 90% of aluminum die casting production is ADC12, and most of it is used for automotive parts. For this reason, it is well distributed and is an advantageous material in terms of price and availability. If we had to mention its weaknesses, it would be its corrosion resistance, anodizing properties, and chemical coating properties. However, its electroplating properties are very good.

・ADC12Z: It has almost the same properties as ADC12 and a similar balance of parameters, but it tends to be slightly inferior in terms of casting cracking and corrosion resistance. In terms of corrosion resistance, the ADC12 series is not a very strong material.

・ADC14...It has a small coefficient of thermal expansion, and although it has good strength, it has low elongation and is slightly inferior in impact resistance. It also has slightly inferior castability, and is ranked among aluminum die castings as neither good nor bad. However, in terms of castability, its fluidity is the same as ADC1, which has the best fluidity.

・AlSi9…An aluminum alloy die-cast with good corrosion resistance, made from an Al-Si alloy. It also has a fair amount of elongation, so it has a fair amount of impact resistance. However, its yield strength is somewhat low, and it is inferior in terms of casting ability in terms of fluidity. This standard was adopted from the ISO standard into the JIS standard.

・AlSi12(Fe)…An Al-Si aluminum alloy die casting with excellent corrosion resistance and castability. Its strength is slightly weaker. The iron prevents the material from seizing onto the mold. This aluminum alloy die casting is equivalent to A413.0 in the US standard ASTM and ADC1 in Japan.

・AlSi10Mg(Fe)…An alloy of silicon and magnesium, it is characterized by its high impact resistance and strength. It also has the same corrosion resistance as ADC1. It is not as castable as ADC1. It is also said to be similar to Japan's ADC3.

・AlSi8Cu3…This type aims to improve mechanical strength by adding copper as well as silicon, but shows lower parameters in terms of casting crack resistance and corrosion resistance compared to ADC10. It is close to JIS AC4B and is similar to 380.0 in the US ASTM standard.

・AlSi9Cu3(Fe)…An Al-Si-Cu die-casting alloy that intentionally contains Fe (iron) as an ingredient, mainly to prevent seizure on the mold. It is considered to be equivalent to JIS ADC10, but is inferior to ADC10 in terms of casting crack resistance and corrosion resistance.

· AlSi9Cu3(Fe)(Zn)…A silicon-copper aluminum alloy die-cast, which is close to ADC10Z, but does not match ADC10 in terms of casting crack resistance and corrosion resistance.

・AlSi11Cu2(Fe)…A silicon-copper aluminum alloy. Alloys containing silicon and copper in this ratio tend to have excellent mechanical strength, workability, machinability, and castability. Iron is also added.

・AlSi11Cu3(Fe)…A silicon-copper aluminum alloy die-cast with a high copper ratio. This aluminum alloy has excellent mechanical properties and machinability, as well as good castability.

・AlSi12Cu1(Fe)…This type contains a comprehensive range of the main alloying elements for aluminum die casting, including silicon, copper, and magnesium, and has excellent wear resistance. It also has good flowability for casting. It has excellent mechanical properties, but does not stretch much, so it is not an aluminum die casting that is resistant to impacts.

・AlMg9…Among aluminum die castings, this is an Al-Mg alloy that is comparable in composition to ADC5. Compared to ADC5, it has similarly excellent corrosion resistance, but has difficulties with castability and requires attention to stress corrosion cracking and changes over time.

・AlSi17Cu4Mg…This type contains a comprehensive range of silicon, copper, magnesium, and other major alloying elements for aluminum die casting, and has excellent wear resistance. It also has good flowability for casting. In terms of mechanical properties, it has excellent strength and little elongation, so it is not an aluminum die casting that is resistant to impacts. This alloy is equivalent to ADC14 in the Japanese JIS and B390.0 in the US ASTM.

Main products using aluminum die casting

[Automobile parts] (engines, transmissions, etc.)
Engine parts: head cover, cylinder, crankcase, oil pan
Transmission: Transmission case, valve body, cylinder block bracket, transfer case
[Toys] Miniature cars, etc.
[Home appliances] Washing machines, refrigerators, vacuum cleaners, DVD players, HD headphones, sewing machines, etc.
[OA equipment] Computers, copiers, printers, multifunction devices, tablets, etc.
[Other] Fishing equipment (reels, etc.), cameras, zippers, etc.

For aluminum die casting, go to Kiyota!

We have local subsidiaries in Hong Kong and China, allowing us to respond quickly.
We have Japanese staff stationed here to handle any detailed requests you may have for Japan.
Generally speaking, mold costs are expensive, but by localizing them, it is possible to reduce costs.
We have warehouses in Japan, Hong Kong, and China, and by holding inventory, we can avoid risks in the event of an emergency and ensure a stable supply.  

FAQ


Q: What is aluminum die casting?

Answer: Aluminum die casting is a casting method that uses a die casting machine to fill molten metal such as aluminum alloy into a precision mold at high speed and then apply high pressure to mass-produce castings with high precision and excellent casting surfaces in a short period of time.

Q: How can I increase the strength of aluminum die castings?

A: Aluminum die-casting has a certain degree of strength to begin with, but its strength can be further increased by applying surface treatment. There are methods to increase strength by applying surface treatment (painting and plating).
For more details, click herePlease refer to.