In aluminum alloy die casting production, surface defects directly determine product yield and post-processing costs. Blackening, flow marks, oxide scale are the three most common and easily confused problems! Many companies, unable to accurately identify causes of these defects, blindly adjust their processes, which not only fails to eradicate them but may also lead to increased costs and decreased efficiency.
This article, drawing on experience from integrated die casting and precision die casting production sites, breaks down core causes, differentiation methods, and directly implementable comprehensive solutions for these three types of defects, helping companies quickly identify and efficiently rectify them.

 

In production, it's crucial to clearly identify defect type to avoid misdiagnosis and incorrect treatment. Following three key characteristics can quickly distinguish them without need for specialized testing equipment; frontline operators can master them:
1. Localized Blackening (accounting for over 70% of surface defects)
Appearance characteristics: Localized black spots or dark marks appear, without obvious protrusions/indentations, and gloss is significantly lower than surrounding area; some black spots cannot be removed after shot blasting or sandblasting, blackening becomes more pronounced after machining.
Core judgment point: Location of black spots is often fixed (corresponding to a specific area of mold), or they appear randomly throughout batch of products, without obvious patterns. They have no grainy feel to touch and cannot be removed by wiping.
2. Flow Marks (forming defects, easily misjudged as oxidation)
Appearance characteristics: Irregular stripe or band-like patterns appear on the surface of casting, with direction of patterns consistent with flow direction of molten metal. Color is slightly darker than base material, without obvious blackening, and some flow marks have slight protrusions or indentations.
Key identification points: Continuous streaks distributed along flow path of molten metal, often appearing near gate, at abrupt changes in wall thickness, and in deep cavities. Polishing can reduce or eliminate these defects.
3. Oxide Scale (Material/Smelting Defects)
Appearance characteristics: A gray or black oxide layer forms on the surface. It is brittle and easily peels off, leaving black powder residue when rubbed. In severe cases, oxide scale can embed into casting surface, resulting in a rough and uneven surface.
Key identification points: Often distributed over a large area or concentrated at the front of molten metal flow path or near overflow channel. It is not directly related to mold location and is often accompanied by internal defects such as inclusions and porosity.
On-site mnemonic: Fixed black spots indicate mold; streaks are flow marks; powdery black scale is oxide. Quickly distinguish between them to avoid pitfalls.

These three types of defects may seem similar, but their causes differ greatly. Blackening is often related to molds and release agents, flow marks are often related to molding processes, oxide scale is often related to smelting and aluminum melt quality. Specific causes are broken down below, with precise correspondences to production scenarios:
(I) Localized Blackening: 5 Core Causes
Blackening is the most recurring defect, primarily caused by three main issues: "residue, oxidation, and mold abnormalities." Specifically, it can be divided into:
Residual Release Agent Carbonization (70%): Excessive or uneven application of release agent, or low mold temperature, leads to incomplete volatilization of release agent, resulting in carbonization and black spots at high temperatures. Residue is more severe in poorly vented areas such as deep cavities, dead corners, and rib areas. Excessively high concentrations of water-based release agents or incomplete drying can also exacerbate carbonization and blackening.
Poor mold venting (15%): Blockage or insufficient venting in localized venting channels leads to air entrapment during molten metal filling. Gas reacts with molten metal at high temperatures, forming a black oxide scale. Abrupt changes in wall thickness and areas with dense ribs cause turbulent molten metal flow, exacerbating air entrapment, resulting in more severe blackening.
Mold surface abnormalities (10%): Lack of timely mold maintenance leads to carbon buildup, rust spots, and roughening on the surface. Castings sticking to mold can cause black spots. Damage to mold's nitriding layer allows aluminum to adhere and accumulate, resulting in blackening of corresponding casting area. Leaks in cooling channels can also cause localized rusting in mold, which can transfer to casting surface.
Poor molten aluminum quality (3%): Incomplete degassing of molten aluminum allows hydrogen and oxide to rise to the surface, forming black spots on casting. Excessive proportion of recycled materials, excessive impurities and iron content, and inadequate refining allow oxide slag to be entrapped into casting, causing localized blackening.
Uneven cooling (2%): Excessive localized water flow in mold causes a sudden temperature drop, or uneven wall thickness leads to significant differences in cooling rates. The fastest-cooling areas are prone to dark spots and color differences, mistakenly identified as blackening.
(II) Flow marks: 4 core causes
Flow marks are essentially "textures formed when surface cools too quickly during molten metal filling, preventing subsequent molten metal from completely covering it." Core causes are concentrated in molding process and mold design:
Inappropriate molten metal filling speed: Excessive speed in low-speed stage leads to uneven cooling of molten metal surface; excessively slow speed in high-speed stage prevents subsequent molten metal from merging after initial molten metal cools, forming flow marks; improper high-speed switching point settings can also cause flow turbulence and flow marks.
Uneven mold temperature: Low mold temperature near gate and cavity edges causes rapid cooling of molten metal upon contact with mold, forming a cold film that cannot be eliminated when subsequent molten metal covers it, forming flow marks; an overall low mold temperature also exacerbates flow marks.
Unreasonable mold design: Insufficiently small or improperly positioned gates obstruct molten metal flow, resulting in uneven flow rates; poor cavity venting hinders molten metal flow, forming irregular flow marks; abrupt changes in wall thickness cause sudden changes in molten metal filling speed, easily leading to flow marks.
Abnormal aluminum melt temperature: Too low an aluminum melt temperature results in poor fluidity and rapid cooling during filling, forming flow marks; too high a temperature leads to severe oxidation of molten metal, also causing flow marks accompanied by oxide spots.
(III) Oxide Scale: 3 Core Causes
Oxide scale is an oxide layer formed when molten aluminum oxidizes upon contact with air during melting, transfer, and filling. It is mostly related to melting process and aluminum melt treatment, and is difficult to completely eliminate through post-treatment:
Incomplete aluminum melt refining: Insufficient degassing and slag removal during melting results in excessive oxide inclusions in molten aluminum. During filling, these oxide inclusions float to the surface, forming oxide scale; insufficient argon, nitrogen degassing time and excessive hydrogen content also exacerbate oxidation.
Improper aluminum molten material transfer and filling: During transfer of aluminum molten material from furnace to injection chamber, excessive contact with air leads to secondary oxidation; excessively high injection speed causes severe turbulence in molten metal, entraining air and forming oxide scale.
Improper control of raw materials and recycled materials: An excessively high proportion of recycled materials (over 30%) leads to accumulation of impurities and oxide layers, which cannot be completely removed during refining; insufficient purity of raw materials, containing too many impurities, exacerbates oxidation reaction and forms oxide scale.

Based on above causes, a targeted three-step solution of "investigation-rectification-prevention" is formulated. No new equipment is required; significant improvements and a reduction in defect rate can be achieved through process adjustments and mold maintenance. Specific solutions are as follows:
(I) Localized Blackening: Scenario-Specific Response

(II) Flow Marks: Primarily process adjustment, supplemented by mold optimization
Core corrective measures:
Adjust injection process: Control speed at 0.3-0.5 m/s during low-speed stage to prevent excessively rapid cooling of molten metal surface; adjust speed at 1.5-3.0 m/s according to product structure during high-speed stage, optimize high-speed switching point, and ensure smooth molten metal filling.
Uniform mold temperature: Control the overall mold temperature at 180-220℃. Mold temperature near gate and at cavity edge can be appropriately increased by 5-10℃ to reduce cold film formation; avoid direct cold air blowing onto cavity surface.
Optimize mold: Enlarge gate size and adjust gate position to ensure smooth molten metal flow; clean venting channels to prevent gas from obstructing flow; perform transition treatment at abrupt changes in wall thickness to reduce sudden changes in flow rate.
Adjusting Aluminum Liquid Temperature: Control aluminum liquid temperature between 680-720℃. If temperature is too low, appropriately increase temperature to improve fluidity; if temperature is too high, extend settling time to reduce oxidation.
Preventive Measures: Establish a logbook of injection process parameters and dynamically adjust parameters according to product structure and batch aluminum liquid quality; regularly clean mold gate and cavity to prevent slag and carbon buildup from affecting flow of molten metal.
(III) Oxide Scale: Eradicate problem at smelting stage to prevent oxidation at source.
Core Rectification Measures:
Strengthen Aluminum Liquid Refining: Use argon gas + refining agent for combined refining, with a degassing time ≥15 minutes, thoroughly remove slag, and prevent oxide slag from being drawn into aluminum liquid; regularly test hydrogen content of aluminum liquid to ensure that hydrogen content is ≤0.12ml/100gAl.
Standardize Aluminum Liquid Transfer: Use sealed transfer bags to reduce contact between aluminum liquid and air; avoid violent shaking during transfer to prevent turbulent flow and air entrapment in aluminum liquid.
Strictly control raw materials and recycled materials: Proportion of recycled materials should be controlled below 30%. Recycled materials must be cleaned before use to remove surface oxide layer. Use raw materials of qualified purity to avoid introducing impurities.
Optimize filling process: Reduce injection speed to decrease turbulence in molten metal and prevent air entrapment. Increase venting channels to ensure smooth gas discharge during filling and reduce oxidation reactions.
Preventive measures: Establish smelting process specifications, clearly defining refining time, temperature, and degassing standards. Regularly inspect smelting equipment to ensure good sealing and prevent air ingress. Strengthen raw material inspection; unqualified raw materials are strictly prohibited from use.

Many companies fall into trap of "blindly rectifying" problems during production. This not only fails to solve problem but also increases costs. Following three common mistakes, if corrected promptly, can quickly improve rectification efficiency:
Mistake 1: Blindly increasing shot blasting intensity for blackening → Correction: Shot blasting can only remove surface dust and slight oxidation. It cannot solve blackening caused by mold release agent carbonization or internal oxide inclusions. First, check mold and molten aluminum for problems before considering further treatment.
Mistake 2: Increasing molten aluminum temperature for flow marks → Correction: Excessively high molten aluminum temperature will exacerbate oxidation and may even cause flow marks accompanied by oxide spots. Prioritize adjusting injection speed and mold temperature, then fine-tune molten aluminum temperature.
Mistake 3: Removing oxide scale with acid pickling → Correction: Acid pickling will damage casting surface, leading to secondary oxidation, and cannot remove oxide slag embedded inside casting. Root cause should be addressed in smelting and filling processes to reduce oxide scale formation.

Rectification of blackening, flow marks, and oxide scale on the surface of die-cast parts follows core logic of "first locate, then cure, prevent," with priority order as follows: first address high-frequency blackening problem (optimize release agent, mold venting), then rectify flow marks (adjust molding process), and finally cure oxide scale (standardize smelting process).
For scenarios with high surface quality requirements, such as integrated die casting and precision die casting, it is recommended to establish a "defect log" to record defect locations, frequency of occurrence, rectification measures, and effects, forming a standardized process. This can significantly reduce defect recurrence rates, improve product qualification rates, reduce post-processing costs, and increase production efficiency.