Aluminum Die Casting Defects Causes and Solutions

2024-08-07 By Cherry  

1 Surface casting defects

1.1 Strain

(1) Characteristics:

a. Linear scratches on the casting surface along the mold opening

direction, with a certain depth, and in severe cases, the entire

the surface is scratched;

b.The metal liquid adheres to the mold surface, resulting in a

material shortage on the casting surface.

(2) Causes:

a.The mold cavity surface is damaged; 

b. There is no slope or too small slope in the demolding

direction; 

c. Unbalanced ejection; 

d. The die casting tool is loose; 

e.The casting temperature is too high or too low, and the mold

temperature is too high, resulting in alloy liquid adhesion;

f.The release agent is not effective;

g.The iron content of the aluminum alloy is less than 0.8%;

h.The cooling time is too long or too short.

(3) Solutions:

a. Repair the surface damage of the mold;

b.Correct the slope and improve the surface finish of the mold;

c.Adjust the ejector pin to balance the ejection force;

d. Tighten the die casting tool;

e. Control the reasonable casting temperature and mold temperature

1 80-250. ;

f. Replace the release agent:

g. Adjust the iron content of the aluminum alloy;

h. Adjust the cooling time;

i.Modify the gate and change the direction of the aluminum liquid.

1.2 Bubbles

(1) Characteristics: There are rice-sized bulges on the surface

of the casting and cavities it is formed under the surface.

(2) Causes

a. The alloy liquid is too low in the pressure chamber, which

makes it very easy to produce air entrainment and the injection

speed is too high as well;

b. The mold is poorly vented;

c. The molten liquid is not degassed and the melting temperature

is too high;

d. The mold temperature is too high, the metal solidification

time is not enough, and the strength

is not enough, and the mold is opened too early to eject the casting,

and the pressurized gas expands;

e. Too much release agent;

f. The gate is not opened properly and the filling direction

is crossed.

(3)  Solutions:

a. Reduce the diameter of the pressure chamber to increase the

filling degree of the molten metal;

b. Extend the injection time, reduce the injection speed of the

the first stage, and change the switching point between low-speed

and high-speed injection;

c.Reduce the die casting tool temperature to maintain thermal

balance;

d. Add exhaust grooves and overflow grooves to fully exhaust

the air, and promptly remove the oil and waste from the exhaust

grooves;

e. Adjust the smelting process and perform degassing treatment;

f. Appropriately extend the mold retention time:

g. Reduce the amount of mold release agent;

1.3 Cracks

(1) Features:

a. There are straight or wavy lines on the surface of the

casting, which is narrow and long and tends to develop under

the action of external force;

b.The metal is not oxidized at the cracking point of cold

cracks;

c. The metal has been oxidized at the cracking point of

hot cracks;

(2) Causes:

a. The iron content or silicon content in the alloy is too high;

b.The content of harmful impurities in the kettle is too high,

c. which reduces the plasticity of the alloy;

d. The zinc content of aluminum-silicon-copper alloy is too high

or the copper content is too low;

e. The overall temperature of the mold, especially the mold cavity,

is too low;

f. The shrinkage of the casting wall thickness and thickness

changes drastically and stress is formed at the corners;

g. The mold retention time is too long, and the stress is large;

h.The force is uneven during ejection;

(3) Solutions:

a. Correctly control the alloy composition. In some cases,

pure aluminum ingots can be added to the alloy to reduce

the magnesium content in the alloy or aluminum-silicon

An intermediate alloy can be added to the aluminum alloy to

increase the silicon content;

b. Change the casting structure, add angles, change the mold

taper, and reduce the wall thickness difference meantime;

c. Change or increase the ejection position to make the ejection

force uniform;

d. Shorten the mold opening and core pulling time to increase the

mold temperature and maintain the thermal balance of the

die casting tool;

1.4 Deformation

(1) Characteristics:

a. Overall deformation or local deformation;

b. The geometric shape of the die casting does not conform

to the drawing;

(2) Causes:

a. Poor casting structure; 

b. Too early mold opening, insufficient rigidity of the

casting;

c. Improper ejector setting, uneven force during ejection;

d. Incorrect gate position or too thick gate thickness, easy

to deform when cutting the gate;

e. The rough surface of the mold causes large resistance, and 

the product is deformed during ejection;

The local temperature of the mold is too high, and the product is

not completely solidified, and the ejection

force is large, causing product deformation;

(3) Solutions:

a.Improve the structure of the casting;

b. Reasonably adjust the pressure holding and mold opening

date;

c. Reasonably set the ejection position and the number of

ejector pins, and open places;

d. Change the gate position so that the gate has a point

and reduce the gate thickness to ensure the casting

quality of the product.

In this way, the product is not easy to deform when the

the gate is cut; 

e. Strengthen the mold surface treatment to reduce the

demolding resistance;

f. Control the local mold temperature to maintain the

thermal balance of the mold;

1.5 Flow marks and patterns

(1) Characteristics:

There are stripes on the casting surface that are consistent

with the direction of the flow of the molten metal.

There are obvious non-directional lines that are different

in color from the metal matrix and have no development trend.

(2) Causes:

a.The molten metal that first enters the cavity forms an

extremely thin and incomplete metal layer, which is then

filled by the subsequent molten metal and leaves traces;

b. The die casting tool temperature is too low and the mold

temperature is uneven;

c. The cross-sectional area of the inner runner is too small

and the position is improper, resulting in splashing;

d. The pressure acting on the molten metal is insufficient;

e. Pattern: Too much paint is used.

(3) Solutions:

a. Increase the temperature of the molten metal by 620% to 650℃;

b. Increase the mold temperature to maintain a thermal balance

of 200~C~250"(2;

c. Thicken the cross-sectional area of the inner runner and

change the inlet position;

d. Adjust the filling speed and the length of the injection

time stroke;

e. Select a suitable coating and adjust the concentration.

1.6 Cold shut

(1) Characteristics

The surface of the die casting has obvious, irregular,

sunken linear lines (with two types: penetrating

and non-penetrating).

The shape is small and narrow, and some of the intersection

edges are very smooth. There is a possibility

of development under the action of external force.

(2) Causes:

a. The two metal flows are connected,but they are not

completely fused and there is no inclusion in between.

b. The bonding force between the two metals is very weak;

c. The pouring temperature or the temperature of the

die casting mold is low;

d. The alloy is improperly selected and the fluidity is poor;

e. The runner position is not correct or the flow path is too long;

f. The filling speed is low and the injection pressure is low.   

(3) Solutions:

a. Appropriately increase the pouring temperature and mold temperature;

b. Increase the injection pressure ratio and shorten the filling time;

increase the injection speed and increase the cross-sectional area of

the inner gate at the same time; 

c. Improve the exhaust and filling conditions;

d. Correctly select the alloy and improve the fluidity of the alloy.

1.7 Discoloration and spots

(1) Characteristics: Different colors and spots appear on

the surface of the casting.

(2) Causes:

a. Inappropriate mold release agent;

b. Excessive use of mold release agent, local accumulation;

c. Graphite in the lubricant containing graphite falls onto

the surface of the casting;

d. The mold temperature is too low, and the temperature of

the molten metal is too low, resulting in irregular

solidification.

(3) Solutions:

a. Replace high-quality mold release agent; 

b. Strictly control the spraying amount and spraying operation;

c. Control the die casting tool temperature to maintain thermal balance;

d. Control the temperature of the molten metal.

1.8 Net hair wings

(1) Characteristics:

There are net-like raised or depressed marks on the surface

of the die-casting, which continue

to expand and extend with the increase in the number of

die-casting times.

(2) Causes:

a. Cracks on the surface of the die-casting model cavity;

b. Improper die-casting mold material or incorrect heat

treatment process;

c. Large temperature difference between hot and cold die-casting mold;

d. Too high pouring temperature;

e. Insufficient preheating of the die-casting mold;

f. The rough surface of the cavity.

(3) Solutions:

a. Correctly select die-casting mold materials and heat

treatment processes;

b. The pouring temperature should not be too high, especially

for high-melting-point alloys;

c. The mold should be fully preheated;

d. After the die casting tool is manufactured, perform low-temperature

long-term aging treatment or chemical oxidation treatment on the surface;

e. Grind the surface of the molding part to reduce the surface

roughness Ra value, Ra0.8~Ra0.4;

f. Reasonably select the mold cooling method;

g. Avoid strong cooling of the mold surface.

1.9 IEI sink

(1) Characteristics:

Depressions appear on the smooth surface of the casting.

(2) Causes:

a. The wall thickness of the casting is too different, and

the depression is mostly generated in the thick wall;

b. The mold is partially overheated, and the overheated part

solidifies slowly;

c. The injection pressure is low;

d. The high temperature of the aluminum die casting tool causes

the cavity gas cannot be discharged and is compressed between

the cavity surface and the metal liquid interface.

(3) Solutions:

• The wall thickness of the casting should be designed to be as uniform as possible;

• Adjust the local cooling of the mold;

• Increase the injection pressure;

• Improve the cavity exhaust conditions.

1.1 Undercasting

(1) Characteristics:

There are insufficient areas on the surface of the casting;

the contour is unclear.

(2) Causes:

a. Poor fluidity;

b. The alloy liquid absorbs air, oxidizes inclusions, and

c. has a high iron content, which makes its quality poor and

reduces fluidity;

d. Low pouring temperature or low mold temperature;

e. Poor filling conditions;

f. Too low pressure;

g. Too much gas is involved, the back pressure of the cavity becomes

high, and filling is blocked;

h. Poor operation, excessive spraying of paint, paint accumulation,

and gas cannot evaporate.

(3) Solutions:

a. Improve the quality of alloy liquid;

b. Increase the pouring temperature or mold temperature;

c. Increase the specific pressure and filling speed;

d. Improve the flow diversion method of the metal liquid

in the pouring system, and add overflow grooves and exhaust

grooves in the undercast parts;

e. Check whether the die-casting machine capacity is sufficient.

1.11 Burr and flash

(1) Characteristics:

Metal flakes appear on the edge of the parting surface of the die-casting.

(2) Causes:

a. Insufficient clamping;

b. Excessive injection speed, resulting in a high-pressure shock peak;

c. The debris on the parting surface is not cleaned up;

d. Insufficient mold strength causes deformation;

e. The insert and slider are worn and the parting is not flush.

3) Solutions:

a. Check the clamping force and pressurization, and adjust

the die casting process parameters;

b. Clean the cavity and parting surface;

c. Trim the mold;

d.It is the best way to use a closed injection end-time

control system to achieve flash-free die casting.

2. Internal defects of die castings

2.1  Porosity

(1) Characteristics and inspection methods:

After dissection, the appearance inspection or flaw detection

inspection shows that the pores

have a smooth surface and are circular.

(2) Causes:

a. The alloy liquid is introduced in an unreasonable direction or

the metal liquid flow rate is too high, resulting in jetting;

b. The exhaust duct is blocked too early or the front impact

on the mold wall forms a vortex that envelops the air.

c. This type of pore is mostly caused by poor exhaust or deep cavities;

d. Due to unclean furnace charge or high melting temperature, more

gas in the metal liquid is not completely removed, and it precipitates

during solidification and cannot be fully discharged;

e. The coating has a large amount of gas or is used too much, and

it is not poured out before pouring so that the gas is drawn into

the casting;

f. This gas is mostly dark gray on the surface; 

g. the high-speed switching point is incorrect.

(3) Solutions:

a. Use clean charge, control melting temperature, and perform

exhaust treatment;

b. Select reasonable process parameters, injection speed, and

high-speed switching point;

c. Guide the pressure balance of molten metal, fill the cavity

in an orderly manner, and facilitate gas discharge;

d. The exhaust groove and overflow groove must have sufficient

exhaust capacity;

e. Select coatings with low gas emissions and control the exhaust volume.

2.2 Shrinkage cavities and shrinkage

(1) Characteristics and inspection methods:

a. Dissection or flaw detection inspection, the hole shape is

irregular, not smooth, and the surface is dark;

b. Large and concentrated holes are shrinkage cavities and small

and dispersed holes are shrinkage cavities.

(2) Causes:

a. During the solidification process of the casting, the

shrinkage is not compensated by the metal, resulting in holes;

b. The pouring temperature is too high, and the mold temperature

gradient distribution is unreasonable;

c. The injection pressure is low and the boost pressure is too low;

d. The inner gate is thin and the area is too small, which causes

premature solidification, which is not conducive to pressure

transmission and metal liquid shrinkage compensation;

e. There are hot spots or drastic changes in the cross-section

of the casting structure;

f. The amount of molten metal poured is too small, and the residual

material is too thin, which cannot play a shrinkage compensation role.

(3) Solutions:

a. Lower the pouring temperature to reduce the shrinkage;

b. Increase the injection pressure and boost pressure to

improve the density;

c. Modify the inner gate to make the pressure better transmitted,

which is conducive to the liquid metal shrinkage compensation effect;

d. Change the structure of the casting, eliminate the metal accumulation

area, and make the wall thickness as uniform as possible;

e. Accelerate the cooling of thick and large parts;

f. Thicken the material handle by 15 to 30 mm to increase

the shrinkage compensation effect.

2.3 Inclusions

(1) Characteristics and inspection methods:

Metallic or non-metallic impurities mixed into the die castings,

irregular in state, with points or holes of different sizes,

colors and heights can be seen after processing.

(2) Causes:

a. The charge is not clean, and there is too much return charge;

b. The alloy liquid is not refined;

c. Slag is brought in when pouring with a spoon;

d. Graphite contained in the graphite crucible or coating

falls off and mixes with the molten metal;

e. The temperature is high and the duration is long during insulation.

(3) Solutions:

a. Use clean alloy materials, especially the dirt on the return

a charge must be cleaned;

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

must be cleaned;

c. When pouring with a spoon, carefully open the liquid surface

to avoid mixing with slag and oxide scale;

d. Clean the cavity and pressure chamber;

e. Control the insulation temperature and reduce the insulation time.

2.4 Brittleness

(1) Characteristics and inspection methods:

The grains of the base metal of the casting are too coarse

or very small, making the casting easy to break or smash.

(2) Causes:

a.The impurities of zinc, iron, lead, and tin in the aluminum

alloy exceeds the specified range;

b. The alloy liquid is overheated or the insulation time

is too long, resulting in coarse grains;

c. Intense overcooling makes the grains too fine.

(3) Solutions:

a. Strictly control the impurity composition in the metal;

b. Control the smelting process and reduce the pouring temperature; 

c. Increase the mold temperature.

2.5. Leakage

(1) Characteristics and inspection methods:

The die casting has air leakage and water seepage after

the pressure test.

(2) Causes:

a. Insufficient pressure and poor density of the base structure;

b. Caused by internal defects, such as pores, shrinkage

cavities, slag holes, cracks, shrinkage, cold shut, and patterns;

c. Poor design of the pouring and exhaust system;

d. Wear the die casting punch and unstable injection.

(3) Solutions:

a. Increase the specific pressure;

b. Take corresponding measures for internal defects;

c. Improve the pouring system and exhaust system;

d. Carry out infiltration treatment to make up for defects;

e. Replace the pressure chamber and punch.

2.6 Non-metallic hard spots

(1) Characteristics and inspection methods:

During the machining process or after machining, the

appearance inspection or metallographic inspection found

that there were fine particles or lumps with a hardness

higher than the metal matrix on the casting, which caused

serious wear of the tool and often showed different brightness

after machining.

(2) Causes:

a. Non-metallic hard spots;

b. Oxides mixed on the surface of the alloy liquid;

c. Reactants between aluminum alloy and furnace lining;

4. Foreign matter infiltrated by metal materials;

5. Inclusions.

(3) Solutions:

a. Do not scoop the oxides on the surface of the alloy

liquid into the spoon during casting;

b. Remove the oxides on the surface of the iron crucible

before applying the paint. Clean the residues on the furnace

wall and bottom in time;

c. Remove the oxides on tools such as spoons;

d. Use furnace lining materials that do not react with aluminum;

e. Pure metal materials.

2.7  Metal hard spots

(1) Characteristics and inspection methods:

During the machining process or after machining, the appearance

inspection or metallographic inspection found that there

were fine particles or lumps on the casting with a hardness

higher than that of the metal matrix, which caused serious

tool wear and often showed different brightness after machining.

(2) Causes:

a. Unmelted silicon was mixed into the metal hard spots;

b. Primary silicon: the temperature of the aluminum liquid

was low and the parking time was long; 

c. FE and MN elements are segregated to produce

intermetallic compounds.

(3) Solutions:

a. When melting aluminum-silicon alloy, do not use silicon

powder;

b. When high-speed alloy composition, do not directly add

silicon elements and intermediate alloys must be used;

c. Increase the melting temperature and pouring temperature;

d. Control the alloy composition, especially the amount of

FE impurities; avoid the segregation of elements, such as

FE and MN;

e. The Si content in the alloy should not be close to or

exceed the eutectic composition;

f. Control the amount of primary silicon in the matrix

metallographic structure of the raw materials.

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