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Die casting is a metal casting process characterized by the use of a mold cavity to apply high pressure to the molten metal, similar to plastic injection molding. Die casting is particularly suitable for the manufacture of a large number of small and medium-sized castings, so die casting is the most widely used of various casting processes. Compared to other casting techniques, the die-cast surface is flatter and has a higher dimensional consistency. However, non-standard operations and parameters can also produce a wide variety of defects.
First, the flow marks and patterns
Visual inspection:
There are streaks on the surface of the casting that are consistent with the flow direction of the molten metal. There is a visible non-directional texture that is different from the color of the metal substrate, and there is no development trend.
The causes of the flow marks are as follows:
(1) The mold temperature is too low;
(2) Poor runner design, poor position of the 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.
The reason for the pattern is that the coating in the cavity is too much or the quality of the coating is poor. The solution and prevention methods are as follows:
(1) Adjust the cross-sectional area or position of the gate;
(2) increase the mold temperature;
(3) Adjusting the speed and pressure of the gate;
(4) Appropriate use of paint and adjustment of dosage.
Second, the reticular wing (turtle crack)
Visual inspection:
On the surface of the die casting, there are traces of protrusions or depressions like the reticular hair, which are continuously enlarged and extended as the number of times of die casting increases.
The reasons are as follows:
(1) There is a crack on the surface of the die-casting cavity;
(2) The pre-heating of the die-casting mold is uneven.
The solution to and prevention is:
(1) After the die-casting mold is to be periodically or die-casted for a certain number of times, it shall be annealed to eliminate the stress in the cavity;
(2) If the surface of the cavity has cracked aluminum profile gusset "aluminum profile gusset, the surface should be ground and the crack layer removed;
(3) The mold should be preheated evenly.
Third, cold separation
Visual inspection:
The surface of the die casting has obvious, irregular, and depressed linear patterns (both penetrating and non-penetrating). The shape is small and narrow, and sometimes the joint edge is smooth, and there is a possibility of breaking under the action of external force.
The reasons are as follows:
(1) The two metal flows are butted together but are not completely fused without inclusions, and the bonding force of the two metals is weak;
(2) The pouring temperature or the temperature of the die-casting mold is low;
(3) The runner is not in the correct position or the flow path is too long;
(4) The filling speed is low.
The solution to and prevention is:
(1) Properly increase the pouring temperature;
(2) Increasing the specific pressure of the injection, shortening the filling time and increasing the injection speed;
(3) Improve exhaust and filling conditions.
Fourth, shrinkage (dent)
Visual inspection:
There are smooth indentations (like a dish) on the surface of the majority of the thickness of the die casting.
The reasons are as follows:
(1) caused by shrinkage
Die castings are not designed with too much wall thickness difference;
The sprue is not in the proper position;
The injection pressure is lower than the pressure, and the pressure holding time is short;
The local temperature of the die casting mold is too high.
(2) The cooling system design is unreasonable;
(3) premature opening;
(4) The pouring temperature is too high.
The solution to and prevention is:
(1) The wall thickness should be uniform;
(2) The transition of thickness should be moderated;
(3) Correctly select the introduction position of the alloy liquid and increase the cross-sectional area of ​​the gate;
(4) Increase the injection pressure and extend the pressure holding time;
(5) Properly reduce the pouring temperature and the temperature of the die-casting mold;
(6) local cooling to local high temperature;
(7) Improve the overflow conditions.
Five, imprint
Visual inspection:
Traces left by contact between the surface of the casting and the surface of the die-cast cavity or step marks on the surface of the casting.
The reasons are as follows:
(1) caused by the ejection element
The end face of the jack is worn;
The length of the ejector adjustment is inconsistent;
The die-casting part of the die-casting model cavity is not well matched with other parts.
(2) caused by splicing or moving parts
The mosaic part is loose;
Loose or worn parts;
The side wall surface of the casting is formed by inserts that are interspersed with each other by moving and fixed molds.
The solution to and prevention is:
(1) The length of the ejector should be adjusted to the proper position;
(2) fastening inserts or other moving parts;
(3) Eliminate sharp corners during design, and the aluminum profile gusset is suitable for gap adjustment;
(4) Improve the structure of the casting, so that the die-casting mold eliminates the inserted insert form and improves the structure of the die-casting mold.
Sixth, adhesion marks
Visual inspection:
The small piece and the metal or non-metal are welded to the base portion of the metal, and the small piece is peeled off by the external force, and the surface of the cast after peeling 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 mold cavity;
(2) When pouring, the impurities are first attached to the surface of the cavity.
The solution to and prevention is:
(1) Before the die casting, the cavity pressure chamber and the pouring system should be cleaned to remove metal or non-metallic adhering substances;
(2) The alloy to be poured should also be cleaned;
(3) Choose a suitable coating and the coating should be uniform.
Seven, layering (skin and peeling)
Visual inspection or damage inspection:
There is a distinct layer of metal in the casting.
The reasons are as follows:
(1) The rigidity of the mold is insufficient. During the filling process of the molten metal, the template is shaken;
(2) The crawling phenomenon occurs in the punch during the injection process;
(3) The sprue system is not properly designed.
The solution to and prevention is:
(1) Strengthen the rigidity of the mold, tighten the mold parts to make it stable;
(2) Adjust the cooperation between the injection punch and the pressure chamber to eliminate the creep phenomenon;
(3) Reasonable design of the gate.
Eight, friction ablation
Visual inspection:
The surface of the die casting produces a rough surface at certain locations.
The reasons are as follows:
(1) The position and shape of the gate caused by the die-casting type (mold) is improper;
(2) The cooling of the severe parts of the molten metal at the gate runner caused by the casting conditions is insufficient.
The solution to and prevention is:
(1) improving the position and orientation of the gate and the improper shape;
(2) Improve the cooling conditions, especially to improve the violent part of the molten metal;
(3) adding paint to the ablated portion;
(4) adjusting the flow rate of the alloy liquid so that it does not generate air pockets;
(5) Alloy adhesion on the elimination type (mold).
Nine, erosion
Visual inspection:
The local position of the die casting has pitting or embossing.
The reasons are as follows:
(1) The position of the gate is not set properly;
(2) The cooling conditions are not good.
The solution to and prevention is:
(1) The thickness of the sprue should be appropriate;
(2) Modify the position, direction and setting method of the gate;
(3) The aluminum profile gusset shall be cooled to the eroded portion.
Ten, crack
Visual inspection:
The casting was placed in an alkaline solution with a dark gray at the crack. The destruction and cracking of the metal matrix are linear or wavy, and the grain is narrow and long, which has a tendency to develop under the action of external force.
The reasons are as follows:
(1) The iron content in the alloy is too high or the silicon content is too low; the content of harmful impurities in the alloy is too high, which reduces the plasticity of the alloy; the aluminum-silicon alloy, the aluminum-silicon-copper alloy contains zinc or the copper content is too high; in the aluminum-magnesium alloy Excessive magnesium content;
(2) The mold retention time is too short, the pressure holding time is short; the wall thickness of the casting has a sharp change;
(3) The partial tightening force is too large, and the force is uneven when it is ejected.
The solution to and prevention is:
(1) Correctly control the alloy composition. In some cases, a pure aluminum ingot may be added to the alloy to reduce the magnesium content in the alloy; or an aluminum-silicon intermediate alloy may be added to the alloy to increase the silicon content.
(2) Improve the temperature of the mold (mold), change the structure of the casting, adjust the core pulling mechanism or make the push rod evenly stressed;
(3) Increase the draft angle and use a strong release agent locally;
(4) Increase the mold retention time and increase the pressure holding time.
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