Comprehensive Evaluation of Common Die-Cast Aluminum Defects
Die casting is a metal casting process that involves applying high pressure to molten metal within a mold cavity, similar to the process used in plastic injection molding. This method is particularly effective for producing a large number of small to medium-sized castings, making die casting the most commonly used among various casting techniques. Compared to other casting methods, die casting offers a smoother surface finish and greater dimensional consistency. However, deviations from standard operations and parameters can lead to a variety of defects in the final products.
1. Flow marks and patterns
Appearance Inspection:
The surface of the casting exhibits stripes that align with the flow direction of the molten metal. Additionally, there are clearly visible non-directional lines that differ in color from the metal matrix, and no significant trends in development are observed.
The reasons for flow marks are as follows:
- Mold temperature is too low;
- Poor runner design and poor internal gate position;
- Material temperature is too low;
- Low filling speed and short filling time;
- Unreasonable pouring system;
- Poor exhaust;
- Unreasonable spraying.
2. Net hair wing (turtle crack)
Appearance inspection:
There are net-like hair or raised and depressed marks on the surface of the die-casting, which continue to expand and extend with an increasing number of die-casts.
The causes are as follows:
1) There are cracks on the surface of the die-casting cavity; 2) The die-casting mold is preheated unevenly.
3. Cold shut
Appearance inspection:
The surface of the die casting features noticeable irregular sunken lines that can be categorized into two types: penetrative and non-penetrative. These lines are small and narrow, and the edges where they intersect can sometimes be smooth. This creates a risk of disconnection when subjected to external forces.
The causes are as follows:
- The two metal flows are butted together but not completely fused and without inclusions, and the bonding between the two metals is weak;
- The pouring temperature or die-casting mold temperature is low;
- The runner position is incorrect, or the flow path is too long;
- The filling speed is low.
4. Shrinkage (dents)
Appearance inspection: There are smooth dents (shaped like a plate) on the surface of the thick part of the die casting.
The causes are as follows:
- Die castings can be improperly designed due to factors such as shrinkage, significant differences in wall thickness, incorrect runner positioning, low shot pressure ratios, short holding times, and excessively high local temperatures in the die casting mold.
- The cooling system may also be poorly designed.
- Opening the mold too early can lead to issues.
- Additionally, pouring at excessively high temperatures can cause problems.
5. Imprint
Appearance inspection involves checking for marks left by contact between the casting surface and the die casting mold cavity, as well as for any step marks on the casting surface.
The causes are as follows:
1) Causes Related to Ejector Components
- The end face of the ejector pin is worn.
- The ejector pin has been adjusted to different lengths.
- The alignment between the die-casting model cavity splicing part and other components is not optimal.
2) Issues with the Inlay Part
- The inlay part is loose due to improper splicing or movement.
- The movable part is either loose or worn.
- The side wall surface of the casting is formed by the inserts from both the movable and fixed molds.
6. Adhesion traces
Appearance inspection reveals small flakes and metal or non-metal base parts that are partially welded; small flakes may peel off due to external forces. The surface of the casting after peeling appears shiny or dark gray.
The causes are as follows:
1) The surface of the die-casting model cavity may have metal or non-metal residues;
2) Impurities are introduced during pouring and adhere to the cavity surface.
7. Delamination (skinning and peeling)
Appearance inspection or destructive inspection: There are obvious layers of metal in the local part of the casting.
The causes are as follows:
The mold lacks rigidity, causing it to shake during the metal filling process. Additionally, the punch moves inconsistently during injection. The design of the runner system is inadequate.
8. Friction ablation
Appearance inspection: Rough surfaces appear on the surface of the die casting in certain locations.
The causes are as follows:
1) The improper position, direction, and shape of the gate are caused by the die casting mold; 2) Insufficient cooling of the gate leads to violent erosion of the liquid metal due to casting conditions.
9. Erosion
Appearance inspection: There are pitting or convex patterns in the local position of the aluminum die casting.
The causes are as follows:
1) Improper setting of the ingates; 2) Poor cooling conditions.
10. Cracks
Appearance Inspection: Immerse the casting in an alkaline solution, and the cracks will appear dark gray. The damage and cracking of the metal matrix can be straight or wavy, characterized by narrow and elongated lines. These cracks tend to develop under the influence of external forces.
The causes are as follows:
1) The iron content in the alloy may be too high, or the silicon content may be too low. Additionally, the presence of harmful impurities in the alloy can reduce its plasticity. The aluminum-silicon alloy and aluminum-silicon-copper alloy may also contain excessive amounts of zinc or copper, while the aluminum-magnesium alloy may have too much magnesium.
2) The mold retention time and pressure holding time may be too short. Furthermore, there may be areas in the casting where the wall thickness changes dramatically.
3) The local clamping force may be too high, leading to uneven force distribution during the ejection process.
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