Internal Defects of Aluminum Alloy Die Castings: Features, Causes and Solutions | Diecasting-mould

Die casting defects may occur in different stages when producing final metal casting parts, not every defect can be found easily and can be avoided during the die casting process. Casting defects can be divided into internal and superficial defects. In this article, we’ll talk about a variety of internal defects of aluminum alloy die castings from their features, causes, and solutions. 

Internal Defects of Aluminum Alloy Die Castings - Internal Die Casting Defects Causes and Solutions

Die casting internal defects are more difficult to find and this kind of defect is the main reason to weaken the structural resistance of the parts. In addition to common gas porosity, shrinkage cavity, there are also leakage, hard spot, brittleness, and more problems that may appear inside of the aluminum castings. Check out common types of aluminum die casting alloys

1. Brittleness

(1) Features

The crystal grains of the base metal of the die casting is too large or extremely small, which makes the die casting easy to break.

(2) Causes

– The impurities of zinc, iron, lead, and tin in the aluminum alloy exceed the specified range. 

– The alloy liquid is overheated or the holding time is too long, resulting in coarse grains. 

– Intensive overcooling makes the grains too fine.

(3) Solutions

– Strictly control the impurity composition in the metal. 

– Control the smelting process to reduce the pouring temperature. 

– Increase mold temperature.

 

2. Leakage

(1) Features

Air leakage and water seepage appear after the die casting is pressure tested.

(2) Causes

– The pressure is insufficient, and the compactness of the matrix structure is poor. 

– Internal defects such as gas porosity, shrinkage cavities, slag holes, cracks, shrinkage porosity, cold shuts, and patterns occur. 

– The design of the pouring and exhaust system is poor. 

– The die casting punch wears and the injection is unstable.

(3) Solutions

– Increase specific pressure. 

– Take corresponding measures for internal defects. 

– Improve the gating system and exhaust system. 

– Perform infiltration treatment to make up for the defects. 

– Replace the pressure chamber and punch.

 

3. Non-metallic hard spots

(1) Features

It is found that there are tiny particles or lumps that are harder than the metal matrix and make the tool wear seriously on the die castings during mechanical processing, visual inspection or metallographic inspection after processing, which often show different brightness after processing.

(2) Causes

– Oxides on the surface of the alloy liquid are mixed. 

– The aluminum alloy and furnace lining react with each other. 

– Foreign bodies get into metal charge. 

– There are inclusions. 

(3) Solutions

– Do not scoop the oxide on the surface of the alloy liquid into the spoon during casting. 

– After removing the oxide on the surface of the iron crucible, apply paint. Clean the residue on the furnace wall and bottom in time. 

– Remove oxides on tools like spoons. – 

– Use lining materials that do not react with aluminum. 

– Purify metal materials.

 

4. Metal hard spots

(1) Features

It is found that there are tiny particles or lumps that are harder than the metal matrix and make the tool wear seriously on the die castings during mechanical processing, visual inspection or metallographic inspection after processing, which often show different brightness after processing.

(2) Causes

– Metal hard spots are mixed with unmelted silicon elements. 

– Primary crystal silicon: The temperature of the aluminum liquid is low, and the storage time is long. 

– The FE and MN elements segregate to produce intermetallic compounds.

(3) Solutions

– When smelting aluminum-silicon alloys, do not use silicon element powder. 

– Do not directly add silicon element to high-speed alloy composition, and intermediate alloys must be used. 

– Increase melting temperature and pouring temperature. 

– Control alloy composition, especially FE impurities. Avoid segregation of FE, MN, and other elements. 

– Si content in the alloy should not approach or exceed the eutectic composition. 

– Control the amount of primary silicon in the matrix metallographic structure for raw materials.

 

5. Gas porosity

(1)Features

After anatomy, visual inspection or flaw detection, the gas porosity has a smooth surface and a circular shape.

(2) Causes

– The entering direction of the alloy liquid is unreasonable or the flow velocity of the molten metal is too high, resulting in spraying. 

– The exhaust duct is blocked prematurely or the cavity wall is impacted in the front and a vortex is formed to envelop the gas. This kind of gas porosity is mostly caused by poor exhaust or in the deep cavity 

– Because the furnace charge is not clean or the smelting temperature is too high, making more gas in the molten metal not remove. And it precipitates during solidification and cannot be fully discharged. 

– The coating has a large amount of gas or is used too much, and it is cased in the die casting, which is mostly dark gray. The high-speed switching point is not correct.

(3) Solutions

– Use clean furnace charge; control the melting temperature and perform exhaust treatment. 

– Choose reasonable process parameters, injection speeds and high-speed switching points. 

– Guide the metal liquid pressure to balance and fill the cavity in an orderly manner to smoothly discharge the gas. 

– Exhaust and overflow grooves should have sufficient exhaust capacity. 

– Choose paint with a small amount of gas and control the amount of exhaust.

 

6. Shrinkage cavity or shrinkage porosity

(1) Features

– After anatomy or flaw detection, the shape of the hole is irregular and not smooth, and the surface is dark. 

– The large and concentrated one is the shrinkage cavity and the scattered one is the shrinkage porosity.

(2) Causes

– During the solidification process of the die casting, cavities are caused by the metal which shrinks and has no metal compensation. 

– The pouring temperature is too high, and the mold temperature gradient distribution is unreasonable. 

– The injection specific pressure is low, and the boost pressure is too low. 

– The inner gate is relatively thin and has a small area, and premature solidification of the inner gate is not conducive to pressure transmission and liquid metal feeding. 

– There are hot spots or cross-section changes in the die casting structure. 

– The amount of molten metal pouring is too small, and the remaining material is too thin to make up feeding.

(3) Solutions

– Reduce the pouring temperature and reduce the amount of shrinkage. 

– Increase the injection specific pressure and boost pressure to improve the compactness. 

– Modify the inner gate to make the pressure better transmit, which is conducive to the feeding effect of liquid metal. 

– Change the die casting structure to eliminate the metal accumulation, and the wall thickness should be as uniform as possible. 

– Speed up the cooling of thick parts. 

– Thicken the material cake by 15 to 30mm to increase the shrinkage effect.

 

7. Inclusions

(1) Features

Spots or cavities with irregular states, different sizes, colors and heights which are the metallic or non-metallic impurities mixed into the die casting parts can be seen after processing.

(2) Causes

– The furnace charge is not clean, and there are too many returns. 

– The alloy liquid is not refined. 

– The molten slag is brought into the molten metal by a spoon. 

– The graphite in the graphite crucible or the coating falls and mixes with the molten metal. 

– The heat preservation temperature is high and lasts for a long time.

(3) Solutions

– Use clean alloy materials, the dirt on the returns must be removed. 

– The alloy melt must be refined and degassed, and the slag must be removed. 

– When using a spoon to take the liquid, carefully push the liquid surface to avoid mixing slags and oxide skins. 

– Clean the cavity and pressure chamber. 

– Control the heat preservation temperature and reduce heat preservation time.

Common Defects and Influencing Factors of Die Castings - Tips for How to Prevent & Avoid Casting Defects

Since each type of defect is caused by many different influencing factors, it is necessary to solve the problem in actual production. When facing many reasons, should you adjust the machine first or repair the mold first? It is recommended that you should deal with the problem based on the degree of difficulty, from the simple one to the difficult one. The sequences are as the following:

(1) Clean the parting surface, cavity, and ejector rod. Change the paint and improve the spraying process. Increase the clamping force. Increase the amount of pouring metal. These are measures that can be implemented by simple operations.

(2) Adjust process parameters, injection force, injection speed, filling time, mold opening time, pouring temperature, mold temperature, etc.

(3) Change materials; select alloy ingots with high quality; change the ratio of new materials to returned materials, and improve the smelting process.

(4) Modify the mold and pouring system; add internal gates, overflow grooves, and exhaust grooves.


Taking the cause for trimming in die castings for example, how find the problems and solve them?

(1) The die casting machine problem: the clamping force is adjusted incorrectly.

(2) The process problem: The injection speed is too high, resulting in a too high-pressure shock peak.

(3) Mold problems: deformation, debris on the parting surface, uneven wear of inserts and sliders, and insufficient strength of the mold plate.

(4) The sequence of measures to solve trimming: 1. Clean the parting surface. 2. Improve the clamping force. 3. Adjust the process parameters. 4. Repair the worn parts of the mold. 5. Improve the rigidity of the mold.