Aluminum casting refers to a product in which aluminum alloy is melted at approximately 700-780℃ and poured into a mold. It is a general-purpose material that is manufactured in millions of units per year, from automobile engine parts and electrical equipment cases to residential fences and machine frames. Its characteristics are that it is lightweight, high-strength, and easy to reproduce complex shapes.

In recent years, attention has been rekindled in aluminum casting due to the acceleration of electric vehicles (EVs) and the need for lightweight buildings. In addition, due to its superiority as a recycled material and high energy efficiency, it is also highly regarded as a material that contributes to carbon neutrality and the SDGs.

In this article, we will break down the aluminum casting manufacturing process from the overall picture to each step, and explain it in an easy-to-understand manner, including specific quality control points of view. We aimed to create a structure that is useful not only for procurement personnel and engineers, but also for beginners.

The manufacturing process of aluminum casting is mainly divided into the following six steps:

The precision, strength, and appearance quality of the product are greatly affected by this series of processes. Each process requires unique technology and quality control, and detailed control is required, such as “molten metal flow analysis” and “degassing treatment to prevent gas defects.”

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Design → Wooden mold/mold production → Molding (sand mold/mold) → Melting/degassing → Pouring/solidification → Mold disassembly → Finishing/processing → Heat treatment → Inspection/shipment

A wooden mold or mold is produced based on CAD data, taking into account the product shape, thickness, ease of pouring, etc. Wooden molds are mainly used for sand casting and are suitable for small-lot, multi-variety production. On the other hand, molds are ideal for products that require mass production and precision. In the design stage, “casting methods” such as runners and feeders are also incorporated.

In the process of making a sand mold using a wooden mold, a “self-hardening sand mold” using furan resin and a hardener is common. In the case of a mold, the metal itself is used as the mold, so the cooling speed is fast and the crystal grains are fine. Both processes have a significant impact on the dimensional accuracy and surface finish of the product.

Aluminum alloys such as AC4C and AC7A are heated in a melting furnace (to about 700-780°C), and flux treatment and degassing treatment (removal of impurities and hydrogen) are performed. The molten metal must be poured quietly and quickly, otherwise defects such as casting cavities and oxide films are likely to occur.

After pouring, the molten metal cools and solidifies, and the mold is destroyed or opened to remove the casting. Next, shot blasting, deburring, and machining are performed, and hardness adjustment is performed using T5 or T6 heat treatment if necessary.

Aluminum casting manufacturing is not just a “simple melt and pour” technique. The quality depends on the defects that can occur in each process and how measures to prevent them are taken. In the next chapter, we will delve deeper into each of these processes and explain the key points and quality measures in detail.

The quality of aluminum castings is determined first by the casting plan (how to pour) and the precision of the mold. At the CAD design stage, a “plan” is drawn up that incorporates runners and risers (to prevent shrinkage). Based on this, wooden molds and metal molds are produced by NC machining or by craftsmen. Wooden molds in particular require a correction design that anticipates distortion and uneven thickness that is likely to occur during the subsequent casting process. Sudden changes in thick parts can also cause casting holes due to delayed solidification.

The mold is formed using the mold that has been produced. For sand molds, “self-hardening sand molds” that mix furan resin and hardener are mainstream. Although the cooling speed is slower than that of metal molds, they are suitable for large items and small-scale production. On the other hand, **metal mold casting has excellent mechanical properties because it cools quickly and the structure becomes dense. Each method has its own defect factors, typical examples being gas defects in sand molds and shrinkage cavities due to uneven cooling in metal molds.

Aluminum alloys such as AC4C and AC7A are used for melting, and the melting temperature is about 700 to 780°C. The most important quality control at this stage is “degassing” and “flux treatment”. If hydrogen remains in the molten metal, it will cause gas defects and casting cavities, so stirring with inert gas and adding refining agents are essential. Also, be careful of uneven temperatures in the melting furnace and steam explosions caused by adding wet materials.

In the pouring process of pouring aluminum into the mold, the balance between the “temperature of the molten metal” and the “pouring speed” is the key. If it is poured too quickly, oxide films will be mixed in due to turbulence, and if it is poured too slowly, solidification will be uneven. In recent years, technology that uses casting simulation (molten metal flow analysis) to visualize fluidity and solidification characteristics has become widespread. In addition, by controlling the cooling rate during solidification, a fine crystal structure is obtained and mechanical strength is improved.

After casting, the casting is trimmed by “deburring” and “shot blasting” to improve its appearance, and then heat treated with T5 or T6 treatment depending on the product’s application. Since heat treatment affects strength and wear resistance, optimal management of temperature and holding time is essential. In addition, for parts that require dimensional accuracy, precision machining is performed using CNC. Final tolerance control and the presence or absence of internal defects are confirmed by non-destructive testing such as X-rays and ultrasound.

The manufacturing quality of aluminum castings depends not only on the accuracy of each process, but also on the quality of cooperation and information sharing between processes. In particular, if there is no consistency between the wooden mold design and the pouring conditions (temperature, speed, and runner design), defects such as poor flow, casting cavities, and deformation tend to occur frequently.

Therefore, it is essential to carry out casting simulations using CAE (Computer Aided Engineering) from the early stages of product design to visualize fluidity and solidification behavior. This is an example of an integrated approach between design and manufacturing, which is also recommended in the Smart Manufacturing Support Team Project supported by the Ministry of Economy, Trade and Industry [Source: Ministry of Economy, Trade and Industry “Smart Manufacturing Support Team”].

However, it is also essential not to blindly accept the simulation results, but to compare them with the pouring speed, work sequence, and on-site proficiency in actual work. By understanding the gap with reality and establishing a feedback loop, manufacturing stability will improve dramatically.

In addition, Sharing quality KPIs (Key Performance Indicators) throughout the entire process is also effective. By clarifying and visualizing the following numerical indicators, we can expect to establish a quality culture across departments:

By using these indicators as a common language among the manufacturing, quality assurance, and design teams, the reproducibility and reliability of aluminum castings will be greatly improved.

The quality of aluminum castings cannot be guaranteed by designing according to drawings alone. On-site prototyping and verification, feedback accumulation, and efforts to prevent recurrence are the keys to stabilizing production. Here, we will extract knowledge that will be useful for quality improvement from two cases of success and failure in actual manufacturing sites.

Aluminum casting parts for HV and EV vehicles require both high strength and light weight and the ability to handle complex shapes. In this case, the challenge was the casting design that combined a thick central boss (15 mm) with a thin peripheral wall (3.5 mm).

As a countermeasure, the molten metal flow and solidification behavior were verified in advance using a casting simulation using CAE. The molten metal temperature was stabilized at 740±5°C and the pouring speed was adjusted to 40-60 cm/sec. The feeder arrangement was also optimized to avoid the occurrence of casting cavities and shrinkage defects.

In addition, two small revisions were made to the initial wooden mold based on feedback from experienced workers. Mass production was possible within a tolerance of ±0.1 mm from the first prototype, and a 100% pass rate was achieved for the first product.

In a medium-volume production project, the casting defect rate reached over 15% in the initial mass production lot, leading to increased rework and waste during the inspection process. As a result, a problem occurred that caused a cumulative shipping delay of 5 business days.

After analyzing the cause, the following two points were identified as the main causes:

As a response, the sand hardening time was extended to more than 10 minutes, and a sensor system was introduced to measure the temperature from the melting furnace to the pouring point in real time. As a result, two weeks later, the rate of blowholes was reduced to 1.4% and the process delay was eliminated.

The lesson learned from this incident is nothing less than a reaffirmation of the principle that “adherence to standards in the upstream process and information synchronization with the on-site process fundamentally determine casting quality.”

Q1. What is the difference between aluminum casting and die casting?
A. Casting is poured into a metal mold or sand mold by gravity, while die casting is a method in which metal is poured into a metal mold under high pressure. Die casting has high dimensional accuracy and is suitable for mass production.

Q2. How are internal defects in castings detected?
A. X-ray or ultrasonic flaw detectors are used to non-destructively inspect internal blow holes and cavities.

Q3. Which is more suitable for mass production, a metal mold or a sand mold?
A. A metal mold has a high initial cost, but can be used repeatedly and is suitable for mass production. A sand mold is low cost and suitable for small quantities and complex shapes.

Q4. What are the main defects in aluminum casting?
A. There are blow holes, gas defects, shrinkage cavities, cracks, etc., all of which can be reduced by ingenuity in pouring conditions and mold design.

Q5. Is aluminum casting highly recyclable?
A. Very recyclable. Scrap materials can also be remelted and reused, which helps reduce the environmental impact.

The quality and cost of aluminum castings depend heavily on the understanding and management level of each manufacturing process. From wooden mold design to pouring and finishing, consistency and cooperation between each process is the key to reducing defects and improving reproducibility. In addition, quantifying quality control indicators and sharing information between the field and design is the first step in improving the field.

In future articles, we plan to explain in detail Key points for building a quality assurance system when outsourcing to overseas casting manufacturers. If you are considering global procurement, please stay tuned.