Inclusions in high-pressure die casting are non-metallic particles trapped within the metal, such as oxides, slag, and sand. They weaken the mechanical strength of the casting and reduce the surface quality. This type of defect is more common in aluminum alloy die casting. Still, it can be effectively prevented by purifying the aluminum liquid, optimizing the mold design (proper layout of ventilation and ingates), and maintaining stable process parameters.
This article will explore the types of inclusions, their root causes, their impact on the quality of die casting parts, detection methods, effective prevention strategies, and CEX Casting‘s prevention methods. Read on to learn how to avoid this defect in your next aluminum die casting project.
Types of Inclusions in Die Casting Services
Slag Inclusions
Slag inclusions are solid residues formed by the reaction of flux with oxides and impurities during the smelting process of aluminum alloys.
They are usually in the form of particles or lumps, distributed inside the melt, and have a dark and irregular color.
If the slag is not adequately skimmed, it will enter the mold cavity with the aluminum liquid during the pouring process.
Oxide Inclusions
Oxide inclusions are oxide films formed when molten aluminum momentarily comes into contact with air during flow or splashing.
These films are easily torn, folded, and rolled into the interior during metal movement, forming continuous or layered internal inclusions, which are usually distributed inside the cross-section of the casting.
Sand Inclusions
Sand inclusions are common in die casting processes using sand cores, especially when the sand cores are not compacted enough, damaged, or burned.
When filling the die casting mold, loose sand particles will be flushed into the mold cavity by the aluminum liquid, embedded in the inner wall or transition area of the casting, forming non-metallic impurities that are difficult to remove.
The Root Cause of Inclusions
Improper Melt Treatment
If the aluminum liquid is poured at too fast a speed, at too large an angle, or with too long a free fall, turbulence will be caused, causing oxide film, gas, and scum to be rolled into the melt.
In addition, if the crucible, ladle, and transfer channel are not thoroughly cleaned, slag, dust, or oxide layer may be brought in, which may become the source of inclusions.
Unreasonable Process Parameters
If the temperature is set too high or fluctuates frequently during smelting or pouring, it will accelerate the reaction between the aluminum liquid and air, resulting in the continuous generation of a large amount of oxide film and slag.
At the same time, unstable temperature control will also break the chemical balance of alloy components, causing some elements to precipitate and react, forming new non-metallic impurities and mixing into the melt.
Alloy or Waste Pollution
If the raw materials are of unknown origin and not cleaned properly, they often contain impurities such as sand, iron filings, oil, or an oxide layer.
Especially in recycled materials, such contaminants are difficult to completely melt or separate, and often exist in the melt in the form of solid particles, and enter the cavity with the aluminum liquid to form inclusions.
Mold Design Defects
Irrational design of the ingate, gate, corner, or exhaust system will lead to poor flow of aluminum liquid, forming turbulent areas or dead corners, which are easy to become involved in oxide film and gas.
At the same time, if the mold surface lacks a smooth transition, the aluminum liquid is more likely to bring in suspended impurities during the filling process, increasing the risk of inclusions.
Impact of Inclusions on Die Castings
Decreased Mechanical Strength
Inclusions interrupt the continuous structure of the aluminum matrix and form stress concentration areas.
When subjected to tension, impact, or repeated loads, cracks often originate from inclusions and expand rapidly, reducing the overall strength, ductility, and fatigue performance of aluminium alloy die casting components.
Decreased Surface Quality
Inclusions can cause particle protrusions, pores, spots, or discoloration on the surface of the casting, making the surface rough and uneven with uneven texture.
This not only affects the visual appearance of the die casting but also reduces the adhesion, coverage uniformity, and long-term stability of post-processing processes such as spraying and electroplating.
Decreased Machining Performance
Inclusions form hard spots or non-uniform structures in cast aluminum, which can easily cause accelerated tool wear, chipping, or damage to the machined surface during machining.
Local material instability may also cause dimensional changes, deformation, or cross-sectional defects, affecting machining accuracy and finished product consistency.
Inclusion Detection Method
Visual Inspection
Clean the surface of the casting and observe whether there are abnormal features such as black spots, bubbles, particles, or roughness on the surface with the naked eye or a low-power magnifying glass under strong light or diffuse light.
If necessary, dye penetration or spray development can be used to enhance the visibility of defects.
X-Ray Detection
Fix the casting on the X-ray machine inspection table, set appropriate exposure parameters (voltage, current, time), and generate a perspective image.
Inclusions appear as bright spots or shadows with abnormal density in the image, and can be compared and analyzed through image enhancement software to confirm the location and shape of internal inclusions.
Ultrasonic Detection
Apply a coupling agent to the surface of the casting to make full contact between the probe and the cast aluminum.
The probe emits high-frequency sound waves, and the inclusion position will produce obvious reflections due to different acoustic resistance.
The instrument receives and converts the echo signal and identifies the existence, depth, and size of the inclusion through waveform changes or screen images.
Metallographic Analysis
Cut the sample from the target area, complete cutting, inlaying, polishing, and corrosion according to standard processes, and then observe it under a microscope.
Identify the boundaries, shapes, quantity, and distribution of inclusions at different magnifications, and further image analysis or quantitative evaluation can be performed.
How to Prevent Die Casting Inclusion Defects
Molten Aluminum Treatment
Cast aluminum needs to be degassed immediately after smelting. Nitrogen or argon can be blown into the rotor to remove dissolved gases such as hydrogen.
After adding the flux, stir it thoroughly to make it float up in combination with the oxide inclusions, and thoroughly skim off the surface slag.
Install a ceramic filter or foam filter before pouring to effectively intercept the slag and oxides.
Process Control
The temperature of the aluminum liquid should be stable at 660–690℃, with a fluctuation of no more than ±5℃.
The pouring speed is controlled at 0.5–1.5m/s, the free fall is less than 300mm, and the pouring angle is not more than 45°.
It is recommended to use a servo automatic pouring system with thermocouples and flow meters for real-time monitoring.
Optimization of Die Casting Die Design
The inner runner adopts a streamlined design to avoid sharp turns and sudden changes in cross-sections to ensure laminar filling.
The location of the gate, venting groove, and overflow port should be conducive to the discharge of inclusions.
During the design stage, mold flow simulation software should be used to simulate the filling path to discover and optimize potential inclusion gathering areas in advance.
Use High-Quality Raw Materials
Use high-purity aluminum ingots with inspection reports to eliminate materials of unknown origin.
The recycled materials must be cleaned of oil, moisture, and oxide scale, and stored in batches and tested for composition.
All raw materials should be subjected to spectral analysis or rapid detection in front of the furnace to ensure that there is no inclusion source mixed in before smelting.
Equipment Maintenance
Regularly clean the slag and carbon deposits on the surface of equipment such as furnaces, crucibles, and ladles to prevent impurities from falling into the melt.
The casting tools must be preheated before use to avoid thermal shock and peeling of the oxide layer.
If cracks, peeling, or erosion occur in the refractory materials inside the equipment, they should be repaired or replaced in time to prevent fragments from mixing into the aluminum liquid.
How to Prevent Inclusion Defects in CEX Casting
Centralized Smelting of Raw Materials
CEX Casting implements centralized smelting management, and all aluminum liquids are degassed, fluxed, and manually skimmed in a unified area.
Each furnace of aluminum liquid is purified according to the standard process before pouring, which effectively reduces slag and oxide inclusions and improves the cleanliness of the melt.
Internal Mold Development
CEX Casting has complete internal mold design and manufacturing capabilities, and all molds are independently completed by the internal team.
In the design stage, mold flow simulation is combined to identify areas prone to inclusions, optimize the gate, venting, and overflow structure, and effectively reduce the risk of inclusions entering the cavity.
Raw Material Spectral Analysis
CEX Casting insists on using only standard aluminum ingots that have passed the composition test. All cast aluminum alloys must be confirmed by spectral analysis before entering the furnace.
We have established a strict raw material traceability system to prevent inclusions from entering the raw material end and ensure that each batch of melt is clean and stable.
Conclusion
Inclusion defects will directly weaken the strength and reliability of aluminum die castings.
Controlling such defects requires starting from the entire process of melt quality, process parameters, mold design, and detection.
As an aluminum squeeze casting & die casting manufacturer with ISO 9001 and IATF 16949 certification, CEX Casting provides raw material purification, independent mold design, and strict raw material quality inspection to ensure that the die casting products are free of inclusion defects.
Contact us today to customize high-performance and reliable aluminium die castings for your next project.