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10 guidelines for reducing casting defects!

During the production process, casting enterprises inevitably encounter casting defects such as shrinkage, bubbles, and segregation, resulting in low casting yield. Re production also faces a significant consumption of manpower and electricity. How to reduce casting defects has always been a concern for casting professionals.

Regarding the issue of reducing casting defects, Professor John Campbell from the University of Birmingham in the UK has been through numerous battles and has unique insights into reducing casting defects. As early as 2001, Li Dianzhong, a researcher of the Institute of Metal Research of the Chinese Academy of Sciences, carried out the organization simulation and process design of the hot working process under the guidance of Professor John Campbell. Today, Intercontinental Media has compiled a list of ten guidelines proposed by international casting master John Campbell to reduce casting defects, hoping to be helpful to colleagues in the casting industry.

1. Good castings start with high-quality melting

Once casting is to begin, the first step is to prepare, inspect, and handle the melting process. If there are requirements, acceptable minimum standards can be adopted. However, a better option is to prepare and adopt a melting scheme that is close to zero defects.

2. Avoid turbulent inclusions on the free liquid surface

This requires avoiding excessive flow velocity of the free liquid surface (meniscus) at the front end. For most metals, the maximum flow velocity is controlled at 0.5m/s. For closed pouring systems or thin-walled components, the maximum flow rate will be appropriately increased. This requirement also means that the falling height of the metal liquid cannot exceed the critical value of the "static drop" height.

3. Avoid laminar inclusions of surface condensation in molten metal

This requires that during the entire filling process, there should be no premature cessation of metal flow at the front end. The metal liquid meniscus in the early stage of filling must remain in a movable state and not be affected by the thickening of surface condensation shells, which will become part of the casting. To achieve this effect, the metal liquid front end can be designed to expand continuously. In practice, only the bottom note 'uphill' can achieve a continuous upward process. (In gravity casting, starting from the bottom of the sprue and flowing upwards). This means that:

Bottom pouring system;

Do not have metal liquid falling or sliding in a "downhill" form;

Do not experience large-scale horizontal flow;

Do not cause the flow of molten metal to stop at the front end due to tipping or waterfall flow.

4. Avoid trapping air (generating bubbles)

Prevent bubbles generated by gas entrapment in the pouring system from entering the mold cavity. This can be achieved through the following methods:

Reasonably design a stepped pouring cup;

Reasonably design the sprue and fill it quickly;

Reasonable use of 'dams';

Avoid using "well" or other open pouring systems;

Using small cross-sectional runners or ceramic filters near the connection between the sprue and the runner;

Use a degassing device;

The pouring process is uninterrupted.

5. Avoid sand core porosity

Prevent bubbles generated by sand cores or molds from entering the molten metal in the mold cavity. The sand core must ensure a very low gas content or use appropriate exhaust to prevent the formation of air holes in the sand core. Clay based sand cores or mold repair adhesives cannot be used unless complete drying can be guaranteed.

6. Avoid shrinkage

Due to the influence of convection and unstable pressure gradient, thick and large cross-section castings cannot achieve upward shrinkage. So it is necessary to follow all the shrinkage rules to ensure a good shrinkage design, using computer simulation technology for verification and actual casting of samples. Control the flying edge level at the connection between sand mold and sand core; Control the thickness of the mold coating (if any); Control the temperature of the alloy and mold.

7. Avoid convection

The convection hazard is related to the solidification time. Both thin-walled and thick walled castings are not affected by convection hazards. For medium wall thickness castings, reducing convective hazards through casting structure or process;

Avoid shrinking upwards;

Flip over after pouring.

8. Reduce segregation

Prevent segregation and control it within the standard range, or in areas where the customer allows the composition to exceed the limit. If possible, try to avoid channel segregation.

9. Reduce residual stress

Do not quench light alloys with water (cold or hot water) medium after solution treatment. If the stress on the casting appears to be low, polymer quenching medium or forced air quenching can be used.

10. Given reference point

All castings must be given positioning reference points for dimensional inspection and machining.