Aluminum alloy gravity casting common defects
First, shrinkage hole


This type of defect often occurs in the thick part of the casting, or in the thick and thin junction. Sometimes the surface of the casting is white, which is actually shrinking.



Insufficient filling of the casting during crystallization;

The position where the alloy liquid is introduced is incorrect;

The temperature of each part of the metal type is not appropriate and does not conform to the principle of sequential solidification;

Improper paint or paint peeling off;

Pouring temperature is too high;

Pouring too fast;

Casting cooling is too slow;

The edging of the casting is too large.


Prevention method:

The riser is arranged in the thick part of the casting, and the size and height of the riser should be suitable to achieve the final solidification and increase the feeding effect of the riser;

Evenly distribute the inner runner along the circumference of the casting, or add a supplementary runner from the root of the riser for supplementary casting;

Adjust the temperature specification of each part of the metal type to facilitate the sequential solidification of the casting;

According to different requirements of casting working part and pouring riser part, different coating ingredients and coating thickness should be selected, and the material should be evenly replenished;

Properly reduce the pouring temperature;

Slow down the pouring speed;

In places where shrinkage is likely to occur, copper cold iron or a vent plug is embedded to accelerate cooling.



Second, cold separation



Such defects typically result in thin wall castings on larger horizontal surfaces, as well as in the final confluence of the alloy. After the casting is out of shape, it can be found by visual inspection.



The mold temperature is too low;

The temperature of the aluminum liquid is too low;

Poor mold exhaust;

Poor design of the gating system, the number of ingates is small, and the cross section is too small;

Pouring speed is too slow or pouring is interrupted;

The cast design is too thin or lacks proper fillet.


Prevention method:

Properly increase the mold temperature;

Properly increase the pouring temperature of the aluminum liquid;

a venting groove or a venting plug is provided on a portion where the gas is not easily discharged, and the exhaust gas is kept good;

Appropriately increase the number of gates and the cross section of the gate;

Properly increase the pouring speed to avoid interruption of pouring of aluminum liquid;

Design reasonable minimum wall thickness and casting fillet according to the casting design process requirements.



Third, the stomata



Pores are often produced in the upper part of the casting and often occur on the surface of the projecting portion of the casting. The hidden pores inside the casting must be visible through X-ray and as the casting is processed.



Pouring too fast, getting into the air;

Die exhaust gas is poor;

The aluminum liquid flows too fast;

The melting temperature is too high;

Poor degassing of the alloy;

Pouring temperature is too high;

The core is not dry, the exhaust is poor, or the amount of gas is too large.


Prevention method:

Smoothly pouring molten metal;

Adding a venting groove or a venting plug to a portion where the metal gas is difficult to remove, and cleaning it frequently;

Pouring the ladle as close as possible to the sprue cup;

Strictly control the temperature of the aluminum liquid to prevent overheating;

The aluminum liquid is properly degassed;

The core should be dried, the vent hole should be unblocked, the mud core should be filled after the damp, and the extra large core should be hollowed out;

After the metal coating, the coating should be dried before it can be poured.



Fourth, the crack



Most of the cracks appear at the inner corner of the casting, and the transition of the thick section; the part where the alloy liquid is introduced into the casting and the part where the casting stress is the largest can be found by coloring inspection, air tightness test, and X-ray inspection. Cold cracks on aluminum castings can be found by visual inspection after cleaning the sand core.



There are sharp corners on the castings, and the thickness is very different;

Local overheating of the mold or excessive casting temperature;

Cold iron is not placed correctly;

Poor filling of castings;


Prevention method:

Improve the design, remove the sharp corners of the castings, try to make the casting wall thickness evenly transition and round the corners;

Correctly select the gate, the position of the runner, control the pouring temperature, the thickness of the coating, properly place the cold iron, and increase the feeding capacity of the feeder;

Apply asbestos insulation coating to the mold riser.



V. Segregation



Segregation is generally distributed in the center and upper part of most of the thickness of the casting, which can be found in macroscopic analysis.



The aluminum liquid component is not stirred evenly before pouring;

Pouring temperature is too high;

The metal type temperature is too high, the coating is uneven and too thick.


Prevention method:

Stir the alloy solution as much as possible before pouring;

Properly reduce the pouring temperature and the preheating temperature of the metal type;

Design cold iron, vent plug or air-cooled and water-cooled in areas with slow cooling;

Adding an element that hinders the segregation of the alloy;

The wall thickness of the casting is appropriately thinned to accelerate solidification.

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