(1) The failure of the sequential solidification system to realize the sequential solidification is usually the cause of the coarse grains. For castings with sharp section changes, attention must be paid to the number and location of internal gates. In order to carry out feeding, keeping the hot molten metal in the action area of riser will reduce the cooling rate of thick section to the extent of producing coarse grains. Improper riser design, such as too long riser neck, improper riser pad design, or too large riser size, will cause excessive heat collection at the thicker section.
(2) In order to make up for the thick section, it often causes excessive heat collection in the local area. For example, because the side riser will overheat the thick section and slow down the cooling speed, it is sometimes not easy to use in actual operation. In actual production, reasonable riser design is needed to reduce the size of riser as much as possible.
(3) The short neck of the inner gate or riser at the joint of the inner gate or riser and the casting is beneficial to the feeding, but it will make the transverse gate or riser too close to the casting and slow down the cooling speed of the part. Increasing the riser neck will bring problems to the feeding. Therefore, the best measure is to adopt effective riser design, reduce the size of riser as much as possible, prevent the cross sprue and riser from being too close to the key section which is easy to form coarse particles, and properly set the cross sprue and riser to realize feeding.
(4) If the number of inner gates is insufficient, it will not only cause sand flushing, but also cause local hot spot and coarse grain structure. This phenomenon is common in all cast metals, even in low pouring temperature aluminum alloy. In some cases, shrinkage defects may occur due to the small number of gates. This shrinkage defect may cover up the defects with coarse grains caused by the same reason. In fact, when the coarse defects of grains get worse seriously, they become a kind of shrinkage defect. Therefore, the prevention and control measures for these two defects are often the same.