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Top Ten Causes of Defects in Aluminum Alloy Die Casting

Die casting is a metal casting process that utilizes a mold cavity to apply high pressure to melted metal, similar to injection molding of plastics. Die casting is particularly suitable for manufacturing large quantities of small and medium-sized castings, making it the most widely used casting process among various casting techniques. Compared with other casting techniques, the surface of die-casting is smoother and has higher dimensional consistency. However, non-standard operations and parameters can also result in a wide variety of defects. 

1. Flow marks and patterns

Appearance inspection

There are stripes on the surface of the casting that are consistent with the flow direction of the molten metal, and obvious non directional patterns that are different from the color of the metal substrate, with no development trend.

The reasons for the formation of flow marks are as follows:

1) The mold temperature is too low; 2) Poor design of the sprue and poor positioning of the sprue; 3) The material temperature is too low; 4) Low filling speed and short filling time; 5) Unreasonable pouring system; 6) Poor exhaust; 7) The spray is unreasonable.

2. Mesh like hairy wings (turtle cracks)

Appearance inspection

There are network like protrusions or indentations on the surface of the die-casting parts, which continuously expand and extend with the increase of die-casting times.

The reasons for this are as follows:

1) There are cracks on the surface of the die-casting mold cavity; 2) Uneven preheating of die-casting molds.

3. Cold insulation

Appearance inspection

The surface of die-casting parts has obvious, irregular, sunken linear patterns (with both penetrating and non penetrating types) with small and narrow shapes. Sometimes the junction edges are smooth, and there is a possibility of breakage under external force.

The reasons for this are as follows:

1) Two metal flows are connected to each other, but they are not completely fused and there are no inclusions between them, and the bonding force between the two metal flows is very weak; 2) The pouring temperature or die-casting mold temperature is too low; 3) The pouring position is incorrect or the flow path is too long; 4) Low filling speed.

4.Shrinkage (dent)

Appearance inspection

There are smooth indentations (resembling disks) on the surface of the thick part of the die cast.

The reasons for this are as follows:

1) Improper design of die-casting parts caused by shrinkage resulting in significant wall thickness differences; Inappropriate pouring position; Low injection ratio and short holding time; The local temperature of the die-casting mold is too high. 2) Unreasonable design of cooling system; 3) Premature mold opening; 4) The pouring temperature is too high.

5. Imprint

Appearance inspection

The traces left by the contact between the surface of the casting and the surface of the die-casting model cavity, or the appearance of step marks on the surface of the casting.

The reasons for this are as follows:

1) Wear on the end face of the top rod caused by the ejection component; The length of the top pole adjustment is inconsistent; The joint part of the die-casting model cavity does not match well with other parts. 2) Loose inlay caused by splicing or moving parts; Loose or worn activity parts; The side wall surface of the casting is formed by inserts interspersed between the moving and fixed molds. 

6. Adhesive traces

Appearance inspection

Small pieces of metal or non-metal are welded to the metal substrate, and under the action of external force, the small pieces are peeled off. After peeling, the surface of the casting is either shiny or dark gray.

The reasons for this are as follows:

1) There are metal or non-metal residues on the surface of the die-casting model cavity; 2) During pouring, impurities are first introduced and attached to the surface of the mold cavity.

7. Layering (skin inclusion and peeling)

Appearance inspection or damage inspection:

There are obvious layers of metal in the local area of the casting.

The reasons for this are as follows:

1) The rigidity of the mold is insufficient, causing the template to shake during the filling process of the metal liquid; 2) During the injection process, the punch experiences crawling phenomenon; 3) Improper design of the pouring system.

8. Friction erosion

Appearance inspection

The surface of the die-casting part produces rough surfaces at certain positions.

The reasons for this are as follows:

1) Inappropriate position, direction, and shape of the sprue caused by the pressure casting mold (die); 2) Insufficient cooling of areas with severe metal liquid erosion at the sprue caused by casting conditions.

9. Erosion

Appearance inspection

There are pockmarks or raised marks in some parts of the die-casting parts.

The reasons for this are as follows:

1) Improper positioning of the internal pouring channel; 2) The cooling conditions are not good.

10. Crack

Appearance inspection

Place the casting in an alkaline solution, and the crack appears dark gray. The damage and cracking of metal substrates are linear or wavy, with narrow and long patterns that tend to develop under external forces.

The reasons for this are as follows:

1) The iron content in the alloy is too high or the silicon content is too low; The excessive content of harmful impurities in the alloy reduces its plasticity; Aluminum silicon alloy and aluminum silicon copper alloy contain excessive zinc or copper content; Excessive magnesium content in aluminum magnesium alloy; 2) The mold retention time is too short and the holding time is short; There are drastic changes in the wall thickness of castings; 3) Excessive local clamping force results in uneven force during ejection

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