1. The stress of gravity casting aluminum can be divided into thermal stress and shrinkage stress according to the different causes.
(1) Thermal stress
The stress caused by the uneven shrinkage of different parts in the process of solidification and cooling is called thermal stress. The greater the wall thickness difference, the greater the linear shrinkage or elastic modulus, and the greater the thermal stress. In the process of directional solidification, due to the different cooling rate of each part of the casting, the thermal stress is large, and it is easy to produce deformation and cracks.
(2) Shrinkage stress
When the casting shrinks in solid state, the stress caused by the obstruction of external forces such as mould, core and riser is called shrinkage stress. Generally speaking, when the casting is cooled to elastic state, the shrinkage stress will be produced when the shrinkage is blocked. Shrinkage stress is usually tensile stress. Once the cause of formation has been eliminated (for example, after the casting has been vibrated or the gate has been removed), the shrinkage stress is also eliminated, so the shrinkage stress is a temporary stress. However, due to the combined action of shrinkage stress and thermal stress before sand falling, when the instantaneous stress is greater than the tensile strength of the casting, the casting will produce cracks.
2. The measures to reduce and eliminate the aluminum stress in gravity casting are as follows: (1) reasonable design of the casting structure, the more complex the shape of the casting, the greater the wall thickness difference of each part, the more uneven the temperature during cooling, and the greater the casting stress. Therefore, the shape of the casting should be simple and symmetrical, and the wall thickness should be uniform.
(2) The process of synchronous solidification is adopted. The so-called synchronous solidification refers to taking certain technological measures to make the temperature difference of each part of the casting very small, almost synchronous solidification. Due to the small temperature difference of each part, it is not easy to produce thermal stress and thermal crack, and the casting deformation is small. Try to improve the core output rate and set riser reasonably. At the same time, the solidification diagram, the process is to add cold iron to the thick wall of the workpiece, and set a riser at the thin wall. Aging treatment is an effective measure to eliminate casting stress.
Aging includes natural aging, thermal aging and resonance aging. The so-called natural aging is to place the casting in the open air for more than half a year to eliminate the internal stress. Thermal aging (artificial aging), also known as stress relief annealing, is to heat the casting to 550-650 ℃, keep it warm for 2-4h, cool it to 150-200t in a furnace, and then put it out of the furnace. The resonance method is to vibrate the casting at its resonance frequency for 10-60 NDN to eliminate the residual stress in the casting.
3. As mentioned before, under the action of thermal stress, the thin part of the casting is subject to compressive stress and the thick part is subject to tensile stress, but the casting always tries to reduce the internal stress by deformation. Therefore, castings often have different degrees of deformation. The deformation of castings often reduces the accuracy of castings. In serious cases, castings can be discarded and should be prevented. Because the casting deformation is caused by the casting stress, reducing and preventing the casting stress is an effective measure to prevent the casting deformation.