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Core considerations for reducing casting defects!

Update:13 Dec 2019
Summary:

During the production process of the casting company, i […]

During the production process of the casting company, it is inevitable that it will encounter casting defects such as shrinkage, bubbles and segregation, which will cause the low yield of castings, and it will face a lot of manpower and electricity consumption when it is returned to the production. How to reduce casting defects is a problem that casting people have always been concerned about.

1.Good casting starts with high quality smelting

Once the casting needs to be started, the smelting process needs to be prepared, inspected and handled first. If required, an acceptable low standard can be used. However, a better choice is to prepare and adopt a melting scheme that is close to zero defects.

2.Avoid turbulent inclusions on the free surface

This requires that the front free liquid (meniscus) flow rate be avoided to be too high. For most metals, the maximum flow velocity is controlled at 0.5m / s. For closed gating systems or thin-walled parts, the large flow velocity will increase appropriately. This requirement also means that the drop height of the metal liquid cannot exceed the critical value of the "static drop" height.

3. Avoid laminar flow inclusions of surface condensation shells in metal liquids

This requires that the entire front end of the filling process does not need to stop the flow in advance. The liquid metal meniscus in the early stage of filling must be kept in a movable state, regardless of the thickening of the surface condensation shells, which will form part of the casting. To achieve this effect, the front end of the molten metal can be designed to continuously expand. In practice, only the bottom note "uphill" can achieve a continuous ascent. (For example, in gravity casting, the flow starts from the bottom of the runner). This means:

Bottom casting system

There is no need for a "downhill" form of metal liquid to fall or slide;

Does not require large-scale horizontal flow;

There is no need for a liquid metal front-end flow stop due to dumping or waterfall flow.

4. Avoid entrainment (produce bubbles)

Avoid air bubbles from entrainment of the pouring system into the cavity. This can be achieved by:

Reasonably design the stepped gate cup;

Reasonably design the sprue and fill it up quickly;

Rational operation of "dams";

Avoid "well" or other open casting systems;

Use ceramic runners with small cross-section runners or near sprues at the junction of runners;

Operating degassing devices;

No interruption in the pouring process.

5. Avoid sand core pores

Avoid air bubbles from the sand core or mold entering the cavity metal liquid. The sand core must ensure a particularly low air content, or appropriate venting can be used to prevent the generation of core air holes. Unless you can guarantee complete dryness, you cannot use clay-based sand cores or mold repair glue.

Avoid shrinkage

Due to convective effects and unstable pressure gradients, thick and large cross-section castings cannot achieve upward shrinkage. Therefore, it is necessary to follow all the filling rules to ensure a good filling design, use computer simulation technology to verify, and actually cast the sample. Control the level of flash at the joint between the sand mold and the sand core; control the thickness of the mold coating (if any); control the alloy and mold temperature.

Avoid convection

Convection hazards are related to freezing time. Both thin-walled and thick-walled castings are immune to convection hazards. For medium-thickness castings: reduce convective hazards through casting structure or process;

Avoid upsizing

Flip after filling.

8.Reduce segregation

Prevent segregation and control within the standard range, or the customer's allowable composition overrun area. If possible, try to avoid channel segregation.

9.Reduce residual stress

After solution treatment of light alloys, water (cold water or hot water) medium quenching is not required. If the casting stress does not seem to be too great, a polymer quenching medium or forced air quenching can be used.

10. Given a reference point

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