Is the overall quality of castings deteriorating? The response plan for the three major 'fatal defects' is fully disclosed! ​

In the production process of the foundry industry, casting defects have always been a major challenge for practitioners. These defects not only affect product quality, but may also cause economic losses. Today, we will delve into the causes of five common casting defects and provide effective solutions.

01 Casting shrinkage cavity

The core cause of shrinkage is the volume contraction during the solidification process of the alloy, as well as the formation of pores due to the dissolution of gases in the molten metal and the release of gases during solidification. The combination of the two easily leads to the formation of shrinkage cavities in the final solidification zone (such as the hot spot).

Precise response strategy

1. Scientific supplementation and contraction

Reasonably set up risers (gold balls) in the hot section or final solidification area of castings to provide channels for liquid metal shrinkage.

2. Optimize the pouring system

Increase the cross-sectional area of the pouring system (increase the diameter of the casting channel) or shorten the pouring distance (shorten the length of the casting channel), accelerate the filling process, and shorten the solidification time.

3. Ensure sufficient metal liquid

Appropriately increase the amount of metal feeding to ensure sufficient metal liquid to compensate for shrinkage.

4. Fine control of temperature field

Set up a cooling point (cooling channel) at the root of the casting channel to accelerate solidification in that area and prevent the structure from sinking towards the direction of the casting channel.

5. Optimize the charging method

When designing the sprue, avoid vertical impact of molten metal on the cavity wall and use curved or inclined sprues to achieve smooth filling and reduce turbulent air entrainment.

02 Surface roughness

The roughness of the casting surface is mainly due to the microscopic state of the cavity surface and the physical or chemical reactions that may occur between the metal and the cavity material (embedding material) at high temperatures.

Specific incentives include:

1. The particles of the embedding material are too coarse; 2. The roasting process heats up too quickly, causing severe evaporation of water; 3. Insufficient roasting temperature or short roasting time, low cavity strength or high residual volatile matter; 4. Overheating of molten metal or high temperature of casting mold (ring); 5. The local temperature of the mold is uneven (too high).

Precise response strategy

1. Strictly control raw materials

Select high-quality embedding materials with appropriate particle size.

2. Standardize the roasting process

Strictly control the roasting curve, especially during the heating stage, which should be slow and uniform, and fully eliminate moisture and gas. For phosphate embedded materials, it is recommended to maintain a stable roasting temperature within the range of 800-900 ℃ and ensure sufficient insulation time.

3. Optimize the melting and casting temperature

Accurately control the melting temperature of metals to avoid overheating; Reasonably control the temperature of the mold before entering the furnace.

4. Improve the surface of the cavity

Apply a layer of wetting and high-temperature resistant separating agent (anti burning and adhesive liquid) evenly on the surface of the wax (investment mold) to reduce the adhesion reaction between the metal and the mold cavity.

5. Prevent hot spot depression

Control the shrinkage cavity to avoid uneven surface caused by shrinkage depression on the tissue surface.

03 Casting cracking

When it comes to casting cracking, its mechanism is quite complex, but there are two common causes of casting cracking: cold/hot cracking caused by rapid solidification, and casting cracking caused by high temperature thermal stress.

1. Rapid solidification leads to cold/hot cracking: The metal liquid fails to fully fuse in the mold cavity, forming weak joints. This is closely related to factors such as wax structure (such as uneven thickness), pouring system design (position and size of the casting channel), insufficient filling pressure, and poor permeability of the embedded material that hinders gas discharge.

2. High temperature thermal stress cracking: Factors such as excessive casting temperature, high temperature strength of the embedded material (which greatly hinders the shrinkage of the casting), poor high-temperature plasticity of the alloy itself (low ductility), etc., lead to excessive thermal stress and cracking of the casting during the cooling process.

Precise response strategy

1. Optimize process parameters

Reduce the casting temperature of the molten metal appropriately.

2 Selected embedding materials

On the premise of meeting the strength requirements, priority should be given to using embedding materials with moderate high-temperature strength, good matching thermal expansion coefficient, and excellent breathability. Sometimes, low-strength embedding materials can actually reduce stress through moderate "yielding".

3. Carefully choose alloys

Try to avoid using alloys with high temperature brittleness and poor ductility (such as certain nickel chromium alloys and cobalt chromium alloys), especially on castings with complex structures and large differences in wall thickness. Choose alloys with better plasticity.

4 Optimization Design

Improve wax structure (such as adding rounded transitions), optimize pouring system design (ensuring smooth filling, sufficient pressure, and smooth exhaust).

When you have dealt with these five major obstacles, do you feel that the road to casting becomes much brighter in an instant?