You don't even collect this: Analysis of the Technical Reasons for the Top Ten Quality Defects in Aluminum Alloy Die Casting

Die casting is a metal casting process that utilizes a mold cavity to apply high pressure to molten metal, similar to plastic injection molding. Die casting is particularly suitable for manufacturing a large number of small and medium-sized castings, therefore it is the most widely used among various casting processes. 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 generate numerous types 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 there are clearly visible non directional patterns that are different from the color of the metal substrate, without any development trend.

The causes of flow marks are as follows:

1) The mold temperature is too low;

2) Poor sprue design and poor position of inner gate;

3) The material temperature is too low;

4) Low filling speed and short filling time;

5) The pouring system is unreasonable;

6) Poor exhaust;

7) The spray is unreasonable.

2、 Reticular hairy wing (turtle crack)

Appearance inspection: There are traces of network like protrusions or depressions on the surface of die-casting parts, which continuously expand and extend with the increase of die-casting times. picture

The reasons for this are as follows:

1) Cracks on the surface of the die-casting mold cavity;

2) Uneven preheating of die-casting molds.

3、 Cold insulation

Appearance inspection: There are obvious, irregular, and concave linear patterns on the surface of the die-casting parts (there are two types of penetrating and non penetrating), with small and narrow shapes. Sometimes the intersecting edges are smooth, and there is a possibility of disconnection under external forces.

The reasons for this are as follows:

1) The two metal streams are interconnected, but they are not fully fused and there are no inclusions between them, and the bonding force between the two metals is very weak;

2) The pouring temperature or die casting mold temperature is too low;

3) The position of the sprue is incorrect or the flow path is too long;

4) Low filling speed.

4、 Indentation

Appearance inspection: There are smooth dents (like discs) on the surface of the thick majority of the die-casting parts.

The reasons for this are as follows:

1) Caused by contraction

Improper design of die castings with large wall thickness differences;

Improper pouring position;

Lower injection pressure and shorter 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 marks 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) Caused by ejecting elements

The end face of the top rod is worn;

Inconsistent adjustment of the top rod length;

The splicing part of the die-casting model cavity does not fit well with other parts.

2) Caused by splicing or moving parts

Loose splicing parts;

Loose or worn moving parts;

The side wall surface of a casting is formed by the insertion of moving and fixed molds.

6、 Adhesion marks

Appearance inspection: Small flakes and metal or non-metal to metal matrix parts are fused, and small flakes are peeled off under external force. The surface of the peeled castings may be shiny or dark gray.

The reasons for this are as follows:

1) There are metal or non-metallic 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 pinching and peeling)

Visual inspection or destructive inspection: There are obvious layers of metal in local parts of the casting.

The reasons for this are as follows:

1) Insufficient rigidity of the mold causes shaking of the template during the metal liquid filling process;

2) During the injection process, the punch exhibits crawling phenomenon;

3) Improper design of the sprue system.

8、 Frictional erosion

Appearance inspection:

Rough surfaces appear on the surface of die castings at certain locations. picture

The reasons for this are as follows:

1) Improper position, direction, and shape of the runner caused by die-casting molds (molds);

2) Insufficient cooling of the parts with severe metal liquid erosion at the inner runner caused by casting conditions.

9、 Erosion

Appearance inspection:

There are pits or protrusions in local positions of the die-casting parts. picture

The reasons for this are as follows:

1) Improper placement of the internal pouring channel;

2) Poor cooling conditions.

10、 Cracks

Appearance inspection:

Place the casting in an alkaline solution, and the cracks appear dark gray. The failure and cracking of the metal matrix 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 have high zinc or copper content; Excessive magnesium content in aluminum magnesium alloys;

2) The mold retention time is too short, and the pressure holding time is short; There are significant changes in the wall thickness of the casting;

3) Excessive local wrapping force, uneven force during ejection.

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Prepared by: Zhao Xiao 

Reviewed by: Wang Kunpeng