Surface defect analysis of aluminum die casting

by Anderson Uncategorized

1.Pulling mold

Features and inspection method:

There are strip-like strain marks on the surface of the casting along the opening direction, with a certain depth, and facial scars in severe cases. The other is that the metal liquid is bonded with the mold, adhered to and pulled, so that the surface of the casting has more material or lack of material.
Causes:

1. Cavity surface damage (collapse or knock).

3. The ejection is unbalanced or the top is decline.

4. The casting temperature is too high, the mold temperature is too high, resulting in the alloy liquid adhesion. 5. The release agent effect is not good.

6. Aluminum and gold composition iron content is less than 0.6%.

  • The cavity is rough and not smooth, and the mold hardness is low.
    Preventive measures:
  • Repair the damaged part of the mold surface, correct the release slope, improve the hardness of the mold (HRC45°~48°), and improve the finish of the mold.

2. Adjust the ejector rod, so that ejector balance.

3. Replace the release agent with good release effect.

4. Adjust the iron content of the alloy.

5. Reduce the pouring temperature, control the mold temperature stable and balanced.

6. Adjust the direction of the inner gate to avoid the metal liquid directly punching the core and the wall.

2. Blister

Features and inspection methods:

The surface of the casting has a bulge of different sizes, or a hole formed under the skin.
Causes:

1. The metal liquid in the injection chamber is too low (controlled at 45%~70%), easy to produce enrolling gas, the initial injection speed is too high.

2. Mold pouring system is unreasonable, poor exhaust.

3. The melting temperature is too high, the gas content is high, and the molten liquid is not degassed.

4.The mold temperature is too high, the mold time is not cooling enough, the metal solidification time is not enough, the strength is not early enough to open the mold, the pressure gas expands.

5.Excessive dosage of release agent and injection head oil.

6. The blowing time after spraying is too short, and the water on the surface of the mold is not dried.
Preventive measures:

  1. Adjust the switching point of die casting process parameters, injection speed and high pressure injection speed.

2. Modify the mold runner, add overflow groove, exhaust groove.

3. Reduce the mold temperature of the defect area, thereby reducing the pressure effect of the gas.

4. Adjust the melting process,

5. Extend the mold time, adjust the blowing time after spraying.

6. Adjust the release agent, injection oil dosage.

3. Cracks

Features and inspection methods:

The surface of the casting has a linear or irregular narrow and different lines, and has a development trend under the action of external forces. Cold crack – The metal is not oxidized at the crack. Thermal cracking – where the metal is oxidized.
Causes:

1. The iron content in the alloy is too high or the silicon content is too low.

2. The content of harmful impurities in the alloy is too high, reducing the plasticity of the alloy.

3. Aluminum silicon alloy: aluminum silicon copper alloy zinc or copper content is too high; There is too much magnesium in aluminum-magnesium alloy.

4. Mold temperature is too low.

5.There is a drastic change in the wall thickness of the casting, and the contraction is blocked.

6.The mold time is too long, the stress is large.

7. Uneven force when ejecting.
Preventive measures:

1. The correct control of alloy composition, in some cases can be added to the alloy pure aluminum ingots to reduce the amount of magnesium in the alloy; Or add an aluminum-silicon intermediate alloy to the alloy to increase the silicon content.

2. Change the casting structure, increase the rounded Angle, increase the release slope, reduce the wall thickness difference,

3. Change or increase the ejection position, so that the ejection force is uniform.

4. Shorten mold opening or core pulling time.

5. Improve the mold temperature (mold operating temperature 180° -280 °).

　

4.Deformation

Features and inspection method:

The geometric shape of the die casting does not conform to the drawing.

Overall deformation or local deformation.
Causes:

1. Casting structure design is poor, resulting in uneven shrinkage.

2. Opening too early, casting rigidity is not enough.

3. Deformation of drawing die.

4. The setting of the ejector rod is unreasonable, and the force is uneven when ejecting.

5. Improper gate removal method.
Preventive measures:

1. Improve the casting structure.

2. Adjust the mold opening time.

3. Set the position and number of ejector rods reasonably.

4. Choose a reasonable way to remove the gate.

5. Eliminate the drawing factor.

　　

5.Marks and patterns
Features and inspection method:

Appearance inspection, the casting surface has stripes consistent with the direction of liquid metal flow, there are obviously visible non-directional lines that are different from the color of the metal matrix, and there is no development trend.
Causes:

1. The metal liquid that first enters the cavity forms an extremely thin and incomplete metal layer, which is made up by the later metal liquid.

2. Mold temperature is too low.

3. The cross-sectional area of the inner gate is too small and the improper position causes splashing.

4. The pressure on the metal is insufficient.

5. Pattern: Excessive amount of paint and injection oil.
Preventive measures:

1. Increase mold temperature.

2. Adjust the cross-sectional area or position of the inner gate.

3. Adjust the speed and pressure of the metal liquid in the inner runner.

4. Choose the appropriate paint, injection oil and adjust the amount of paint injection oil.

　　

6.Cold shut

Features and inspection methods:

Appearance inspection, die-casting surface has obvious, irregular sag linear lines (penetrating and not penetrating two) small and narrow shape, sometimes smooth edge, under the action of external force development is possible.
Causes:

1. The two metal flows butt to each other, but not fully integrated and there is no inclusion between them, the two metal binding force is very weak.

2. Casting temperature or mold temperature is low.

3. Improper selection of alloy, poor fluidity.

4. The location of the runner is wrong or the flow line is too long.

5. Low filling speed.

6. Low injection ratio.

7. The metal liquid flow in the cavity is not smooth.
Preventive measures:

1. Appropriately increase the pouring temperature (controlled at 630-73 °C, can be adjusted according to the product and aluminum) and mold temperature.

2. Improve the injection pressure, shorten the filling time.

3. Improve the injection speed, while increasing the cross section of the inner gate.

4. Improve the exhaust filling conditions.

5. Choose the appropriate alloy to improve the fluidity of liquid metal.

6. Improve the smooth flow of metal liquid in the cavity.

　

7. Reticular edge

Features and inspection methods:

Appearance inspection, the surface of the die casting has a network of hair-like convex or concave traces, with the increase of the number of die casting and shall not expand and extend.
Causes:

1. Die casting mold cavity surface cracks.

2. The use of die casting mold material or heat treatment process is not correct.

3. The temperature difference between hot and cold molds changes too much in a very short time.

4. The pouring temperature is too high.

5. Uneven and insufficient preheating before mold production.

6. The surface of the mold cavity is rough.
Preventive measures:

1. The correct selection of mold materials and heat treatment process.

2. The pouring temperature should not be too high, especially the high melting point of the alloy. Under the condition of meeting the production demand, the lowest pouring temperature should be selected as far as possible.

3. The mold preheating should be full and uniform.

4. The mold production to a certain mold after annealing, eliminate internal stress.

5. The surface of the runner and the cavity is not regularly polished to ensure the surface finish.

6. Reasonable choice of mold cooling method to ensure the mold heat balance.

8.Depression

Features and inspection method:

The concave part appears on the smooth surface of the casting.
Causes:

1. the wall thickness of the casting is uneven, the difference is too large, and the depression is mostly generated in the wall thickness part.

2. mold local overheating, overheating parts slow solidification.

3, low injection ratio.

4. caused by suffrage cavity gas can not be discharged, compressed between the surface of the cavity and the metal liquid interface.

5. not open pressurization, insufficient feeding.
Preventive measures:

1. casting wall thickness design as uniform as possible.

2. mold overheating parts cooling adjustment.

3. improve the injection pressure.

4. improve the cavity exhaust conditions.

5. improve the pressurization specific pressure.

9.Short filling

Features and inspection methods:

The surface of the casting is not filled enough or the outline is unclear.
Causes:

1. Poor liquidity reasons: ① liquid metal suction, oxidizing inclusions, high iron content, so that its quality is poor and reduce mobility. ② Low casting temperature or mold temperature.

2. The filling conditions are poor: ① the injection ratio is too low. ② Too much involved gas, the back pressure of the cavity becomes high, and the filling is blocked.

3. Poor operation, spray paint, injection oil too much, paint, injection oil accumulation, gas volatilization can not go out.
Preventive measures:

1. Improve the quality of liquid metal.

2. Improve the pouring temperature or mold temperature.

3. Improve the specific pressure of die casting and filling speed.

4. Improve the flow diversion mode of the metal liquid in the pouring system, and add an overflow tank and an exhaust tank in the under-casting part.

5. The correct die casting operation.

　　

10.Burrs and flash edges

Features and inspection methods:

The die casting has a metal burrs on the edge of the parting surface.
Cause:

1. Clamping force is not enough.

2. The injection speed is too high, forming the pressure impact peak is too high.

3. The debris on the parting surface is not cleaned up.

4. Mold strength is not enough to cause deformation.

5. Insert, slider wear and parting surface is not flush.

6. Die casting machine hinge wear deformation.

7. The pouring temperature is too high.
Preventive measures:

1. Check the clamping force and pressurization, adjust the die casting process parameters.

2. Clean the cavity and parting surface.

3. Repair mold, repair die casting machine.

4. Using closed injection end time control system, to achieve no flash die casting.

　

11.Color change, spot

Features and inspection methods:

The casting surface has different color spots from the base metal.
Causes:

1. Release agent selection is not appropriate.

2. Too much release agent.

3. The graphite in the lubricant containing graphite falls into the surface of the casting.
Preventive measures:

1. Replace high-quality release agent.

2. Strict spraying amount and spraying operation.