Scaling in aluminum die casting is an oxidation defect caused by high temperatures, violent metal flow, and exposure to air. It usually appears as dull, flaky patches. Scaling reduces the corrosion resistance of the casting and affects the adhesion of the coating. Effective preventive measures include precise temperature control, smooth metal flow, improved exhaust system, and purification of the metal melt.
This article will explore the causes of scaling, its impact on casting quality, prevention methods, post-occurrence treatment methods, and CEX Casting‘s advanced preventive measures. Read on to help you fully understand and control this aluminium die casting surface defect.
Causes of Scaling
Thermal Oxidation at High Temperatures
When the aluminum melt temperature is too high or it is at a high temperature for a long time, it tends to react with oxygen to form a dense oxide film.
Once this type of oxide layer adheres to the surface of the casting, it forms scaling after cooling, which is common in the ladle outlet, pouring channel, and high-temperature exposure area of the mold.
Violent Metal Flow
Unstable flow behaviors such as splashing, tumbling, or rewinding will continuously refresh the contact interface between the aluminum liquid and the air, significantly accelerating the oxidation reaction.
At the inlet, thin-walled area, or the end of filling, it is particularly easy to form unevenly distributed, loose flake oxides.
Irrational Design of the Pouring System
If the pouring system has a sudden change in cross-section, a sharp corner, or uneven pouring, it will cause metal flow disorder, forming turbulence and air inclusion.
As a result, the aluminum liquid is frequently in contact with the air, and the oxide film accumulates rapidly, which makes it easy to form multi-point scaling on the surface of the casting, affecting the appearance and post-processing.
Insufficient Mold Exhaust
Improper mold exhaust design will cause air stagnation. When the high-temperature aluminum liquid is filled, the residual gas contacts it to produce a local oxidation reaction, forming scaling.
This is common at the parting line, the end of the exhaust groove, or the dead corner of the cavity, with concentrated distribution and obvious visual characteristics.
Impurity of Aluminum Liquid
If the aluminum liquid contains impurities such as hydrogen, oxide inclusions, or slag, it is very easy to become the trigger point of the oxidation reaction.
Hydrogen combines with the oxide film to form loose, powdery scaling, which often occurs in areas where impurities are concentrated, the high temperature stays time is long time, or the degassing process is weak.
Scaling
Effect of Scaling on Casting Quality
Damaged Surface Appearance
Scale often adheres to the surface of castings in the form of flakes, powder, or rough patches, causing it to lose its proper metallic texture and finish.
The surface is grayish-white or uneven in color, which significantly reduces the appearance grade and makes it difficult to be directly used for products with appearance requirements, such as automotive parts, home appliance housings, etc.
Increased Risk of Coating Failure
The oxide layer isolates the bonding surface between the metal substrate and the coating material, reducing adhesion.
The coating is prone to blistering, peeling, or cracking under thermal cycles or mechanical stress, seriously affecting corrosion resistance, electrical insulation, and appearance stability.
Such problems are particularly prominent in powder spraying, electroplating, and anodizing processes.
Increased Processing Difficulty and Cost
Scaling can cause uneven processing surfaces, unstable tool-cutting loads, increased wear rates, and reduced processing accuracy.
Additional roughing, cleaning, repositioning, and other processes are required to correct, resulting in longer overall processing time, increased labor costs, and scrap rates.
Structural Integrity Is Threatened
During the solidification process of the casting, gas or impurities may be trapped under the scaling layer, forming tiny pores or interlayers.
Such hidden defects are difficult to detect by conventional detection methods, but they will reduce the local strength at the stress-bearing part and easily become the starting point of fatigue cracks or leaks.
Degradation of Mechanical Properties
The oxide film blocks the effective connection between metal grains, destroys the local stress conduction path, and leads to reduced fatigue strength, impact resistance, and load-bearing performance.
Under dynamic load, high temperature, or alternating stress environment, the scaling area is more likely to undergo plastic deformation or early failure.
Effective Measures to Prevent Scaling
Precise Temperature Control
Control the temperature of the aluminum liquid within a reasonable range (no more than 740°C) to avoid overheating and long-term high-temperature insulation.
Use an automatic temperature control system to monitor the entire process of melting, transferring, and pouring.
The die casting mold should be equipped with a temperature control device or cooling water circuit to keep the cavity temperature stable and prevent local overheating.
Optimize the Pouring and Injection Process
Optimize the runner and gate layout through mold flow analysis to ensure metal laminar filling and reduce splashing and rewinding.
Adjust the injection speed and switching point to prevent turbulence. Use a closed-loop controlled injection system to improve the stability of the filling process.
Use a Protective Atmosphere or Cover Flux
During the smelting and insulation process, use inert gases such as nitrogen or argon to protect the surface of the aluminum liquid, or add a special flux cover layer to isolate oxygen contact.
This operation should be carried out throughout the entire molten aluminum processing process, especially during the high-temperature exposure stage.
Improve Mold Exhaust Design
Set exhaust grooves in thin-walled areas, ends, and cavity-closed parts, and the depth and width are designed according to standards (such as more than 0.3mm) to ensure that air is discharged in time.
Combined with mold flow analysis to determine the exhaust position, and regularly clean the exhaust channel before using the mold.
Ensure the Purity of Molten Aluminum
Remove hydrogen from the melt by rotor or nitrogen degassing. Set a ceramic foam filter or glass fiber filter at the furnace or ladle outlet to remove oxide inclusions.
All tools that contact the melt must be preheated and kept clean to prevent secondary contamination during the high-pressure die casting process.
How to Deal with Scaling After It Occurs
Mechanical Method
A sandblasting machine can be used to perform high-pressure cleaning on large-area aluminum parts to quickly remove oxide scale.
Use an angle grinder or wire brush to grind the local area, which is suitable for corners, holes, and other parts.
The force and angle should be controlled during the operation to avoid damaging the size or surface quality of the die casting parts.
Chemical Method
Sodium hydroxide solution is commonly used for alkaline washing, which is suitable for removing the surface aluminum oxide layer.
Acid washing uses a mixture of phosphoric acid or nitric acid, which can deeply dissolve the scale in complex structures.
The treatment time and concentration should be strictly controlled and carried out under ventilation and protective conditions.
Surface Cleaning
Ultrasonic cleaning can remove residual particles and impurities and is suitable for parts with blind holes and precision structures.
Solvent cleaning can use acetone, isopropyl alcohol, etc., to remove oil and scum by soaking or wiping, providing a clean surface for subsequent coating, electroplating, and other processes.
Quality Control and Testing Methods
Visual Inspection
Inspect the surface of the casting at a 45° angle under 500-1000 lumens of strong light, focusing on gray and white spots, flaky adhesion, or discoloration.
A magnifying glass or high-definition camera system can be used to quickly screen out obvious scaling parts, which is suitable for initial inspection of the production line.
Electron Microscope Inspection (SEM)
Use a scanning electron microscope with a magnification of 500x–5000x to inspect the scaling area and identify the thickness and distribution of the oxide film.
Combined with energy spectrum analysis (EDS) to confirm the oxide composition, it is used for process verification and problem tracing, suitable for key parts and abnormal analysis.
Surface Roughness Inspection
Surface Roughness InspectionUse a contact roughness meter to measure the Ra value, focusing on the functional surface of the casting. High Ra is often related to scaling or impurities.
The inspection should be sampled at multiple points and controlled within the process allowable range (such as ≤3.2μm) to ensure the stability of subsequent processing and surface treatment.
Coating Adhesion Test
According to ISO 2409 or ASTM D3359 standards, perform cross-cutting, peeling, or thermal cycle tests to determine whether the coating is easy to peel or peel.
If the adhesion is insufficient, it means that there is scaling or oxidation residue on the surface, and additional descaling, cleaning, or roughening treatment is required to ensure reliable subsequent coating.
CEX Casting’s Ability to Control Scaling
Centralized Smelting and Purification System
CEX Casting is equipped with a centralized raw material smelting system, integrated with automatic temperature control and purification devices, and closed control of the temperature and purity of molten aluminum throughout the process.
Ensure that the aluminum liquid is always in a stable and clean state, effectively prevent high-temperature oxidation and impurity contamination, and reduce the generation of scaling defects from the source.
Internal Mold Development
All projects are completed by our internal team to complete mold design and mold flow simulation, optimize gate layout and exhaust path, and ensure smooth metal filling without turbulent rewinding.
For complex structures or scaling-prone areas, we customize cooling and exhaust solutions for customers to reduce scaling risks from the design level.
X-Ray Detection
We are equipped with X-ray machines to identify internal defects and scaling-related areas of castings.
Combine visual inspection with roughness detection to establish a standardized surface quality screening process.
Timely discovery and isolation of workpieces with scaling risks to ensure that there is no oxidation residue on the surface of the shipped die casting products.
Conclusion
Scale is a surface defect caused by oxidation, but it can be effectively prevented by strictly controlling the melting temperature, metal flow, mold exhaust, and melt cleanliness.
When a single prevention method is insufficient, targeted post-casting treatment can remove residual scale and restore surface quality.
As a professional aluminium alloy die casting manufacturer, CEX Casting provides full process control and advanced detection methods to prevent scale defects from the source.
Contact us now to obtain aluminium die casting components with excellent surface quality and reliable performance.