Major defects of aluminum die castings and prevention

2020-08-27 16:13:25

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 obviously non-directional lines that are different from the metal matrix in color, and there is no development trend.

 

1. The causes of flow marks are as follows:

(1) The mold temperature is too low.

(2) Poor runner design and poor position of the 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. The cause of the pattern is too much paint in the cavity or poor paint quality. The solutions and prevention methods are as follows:

(1) Adjust the cross-sectional area or position of the inner runner.

(2) Increase the mold temperature.

(3) Adjust the speed and pressure of the inner runner.

(4) Appropriate selection of paint and adjustment of dosage


Reticulated wing (crack of tortoise)

Appearance inspection: The surface of the die-casting parts has the same raised or depressed traces as net-like hair, which will expand and extend as the number of die-casting increases.

The reasons are as follows:

(1) There are cracks on the surface of the die-casting cavity.

(2) Uneven preheating of die casting mold.

The solutions and prevention methods are:

(1) The die-casting mold should be annealed regularly or after a certain number of die-casting to eliminate the stress in the cavity.

(2) If a tortoise crack has appeared on the surface of the cavity, the molding surface should be polished to remove the crack layer.

(3) Preheat the mold evenly.

 

Cold barrier

Appearance inspection: The surface of the die casting has obvious, irregular, and sinking linear lines (both penetrating and non-penetrating). The shape is small and narrow. Sometimes the junction edge is smooth, and it may be broken under the action of external force.

The reasons are as follows:

(1) The two metal streams are connected to each other, but they are not completely fused and there are no inclusions in between, and the bonding force of the two metal streams is very weak;

(2) Pouring temperature or die-casting temperature is too low;

(3) The runner position is wrong or the flow path is too long;

(4) The filling speed is low.

The solutions and prevention methods are:

(1) Increase the pouring temperature appropriately;

(2) Improve the injection ratio, compress the filling time and increase the injection speed.

(3) Improve exhaust and filling conditions.

 

Shrinkage (dent)

Visual inspection: There are smooth dents (like discs) on the most thick surface of the die casting.

The reasons are as follows:

(1) Caused by contraction

1.1 Improper design of die castings and the wall thickness difference is too large;

1.2 Improper runner position;

1.3 Low injection pressure and short holding time;

1.4 The local temperature of the die casting mold is too high.

(2) The cooling system design is unreasonable;

(3) Mold opening is too early;

(4) Pouring temperature is too high.

The solutions and prevention methods are:

(1) The wall thickness should be uniform;

(2) The thickness transition should be eased;

(3) Correctly choose the alloy liquid introduction position and increase the cross-sectional area of the inner runner;

(4) Increase the injection pressure and extend the pressure holding time;

(5) Appropriately reduce the pouring temperature and die temperature;

(6) Partially cool the local high temperature;

(7) Improve overflow conditions;

   

 

Imprint

Visual inspection: the traces left by the contact between the surface of the casting and the surface of the die-casting cavity or the step marks appear on the surface of the casting.

The reasons are as follows:

1. Caused by ejector components

(1) The end face of the ejector rod is worn;

(2) The adjustment length of the ejector rod is inconsistent;

(3) The splicing part of the die-casting cavity does not fit well with other parts;

2. Caused by splicing or moving parts

(1) The mosaic part is loose;

(2) The movable part is loose or worn;

(3) The surface of the side wall of the casting is formed by the inserts intersected by the movable and fixed molds.

The solutions and prevention methods are:

(1) The length of the ejector rod should be adjusted to an appropriate position;

(2) Fasten inserts or other movable parts

(3) Eliminate sharp corners when designing, and adjust the gap to fit;

(4) Improve the structure of the castings to eliminate the interspersed inlay form of the die-casting mold and improve the structure of the die-casting mold;

 

Adhesive traces

Appearance inspection: small flakes and metal or non-metallic and metal base parts are welded, and the small flakes are peeled off under the action of external force. After the peeling, the surface of the casting is bright and some are dark gray.

The reasons are as follows:

(1) There are metallic or non-metallic residues on the surface of the die-casting cavity; (2) Impurities are first brought in and attached to the cavity surface during pouring.

The solutions and prevention methods are:

(1) Before die-casting, the cavity pressure chamber and pouring system should be cleaned up to remove metal or non-metal adherents;

(2) Clean the cast alloy;

(3) Choose suitable paint, and the coating should be even.


Layering (pinch and peeling)

Visual inspection or damage inspection: There are obvious layers of metal in the casting part.

The reasons are as follows:

(1) The mold rigidity is not enough. During the molten metal filling process, the template will shake;

(2) The punch appears to crawl during the injection process;

(3) Improper design of runner system.

The solutions and prevention methods are:

(1) Strengthen the rigidity of the mold and tighten the mold parts to make them stable;

(2) Adjust the cooperation of the injection punch and the pressure chamber to eliminate the crawling phenomenon;

(3) Reasonably design the inner runner.


Friction ablation

Visual inspection: The surface of the die-casting part has rough surfaces in some positions.

The reasons are as follows:

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

(2) Insufficient cooling of molten metal scouring parts at the inner runner caused by casting conditions.

The solutions and prevention methods are:

(1) Improving the location and direction of the inner runner for improper injecting;

(2) Improve cooling conditions, especially the areas where the molten metal is violently washed;

(3) Add paint to the ablated part;

(4) Adjust the flow rate of the alloy liquid to prevent cavitation

(5) Eliminate the alloy adhesion on the mold (mold)


Erosion

Visual inspection: There are pitting or embossing in the local position of the die casting.

The reasons are as follows:

(1) Improper setting of the inner runner;

(2) Poor cooling conditions.

The solutions and prevention methods are:

(1) The thickness of the inner runner should be appropriate;

(2) Modify the position, direction and setting method of the inner runner;

(3) Strengthen the cooling of the eroded parts.

   

Crack

Visual inspection: Put the casting in alkaline solution, the cracks are dark gray. The destruction and cracking of the metal matrix are straight or wavy, with narrow and long lines, which tend to develop under the action of external forces.Cracks in aluminum alloy castings.


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