Internal stress and deformation of liquid forming

1. Castings in the solidification after the cooling process, its solid state contraction is hindered, casting internal stress will be produced. According to different causes, divided into:

• Mechanical stress (shrinkage stress)

Line contraction of the alloy by the casting mold, core, pouring and bubbling system of mechanical obstruction and the formation of internal stresses. Mechanical stresses are temporary stresses.

• Phase Strain

During cooling, some alloys undergo a solid-state phase change and the specific volume changes. May be temporary or permanent (residual stresses).

• Thermal Stress

Thermal stresses are stresses caused by gradual wall thickness non-uniformity and different cooling rates in various parts, so that the casting shrinks inconsistently in various parts during the same period. Thermal stress causes the thick wall or heart of the casting to be stretched and the thin wall or surface to be compressed.

2. casting deformation and prevention

Solidification in three stages:

• No stress at high temperature
• When solidification begins

Thin part of the tensile, thick part of the compression, this time the stress can be offset by plastic deformation

• At the end of solidification

At the end of solidification, the thick part of the casting is subjected to tensile stresses and the thin part to compressive stresses under the action of thermal stresses.

The result of deformation is that the part subject to tensile stress tends to shorten deformation, and the part subject to compressive stress tends to elongate deformation, so as to reduce or eliminate the residual stress in the casting.

Uneven wall thickness of the casting or the cooling rate of each part of the casting of the thick wall or the heart of the casting by tensile stress, thin wall or surface by compressive stress, and with the casting of the wall thickness difference increases, the cooling rate of each part of the difference between the casting alloy line shrinkage increases, as well as its elastic modulus increases, casting of the thermal stress increases.

Methods to prevent deformation:

• Selection of alloys with small elastic modulus; l Make the casting wall thickness as uniform as possible.
• Make the casting wall thickness as uniform as possible.
• Adopt the principle of simultaneous solidification.

Methods:The inner gate is opened at the thin wall of the casting to slow down its cooling rate; and the cold iron is placed at the thick wall of the casting to speed up its cooling rate.

• Adopt anti-deformation method: make the anti-deformation amount equivalent to the deformation amount of the casting in advance on the mold, in order to offset the gradual deformation.