1.Principle introduction
During metal solidification, the volume shrinkage and solidification shrinkage between liquid and solid phase lines are the main reasons for the formation of shrinkage cavity and shrinkage porosity defects. When the feeding channel is smooth and the dendritic crystal does not form a skeleton, the volume shrinkage shows a concentrated shrinkage cavity defect, which is located on the upper part of the flowable unit of sand casting; When the dendritic crystal forms a skeleton, the macro feeding channel is blocked. At this time, the volume shrinkage of the liquid part surrounded by segmentation shows shrinkage porosity defects. In the whole process, liquid shrinkage and solid shrinkage have little effect on the formation of shrinkage cavity and porosity, which are not considered in the calculation.
In the actual solidification process of sand casting, when the sand casting shrinks, the remaining liquid will automatically feed the shrinking part. At this time, under the action of gravity, the flowing liquid will automatically feed from top to bottom, that is, the macroscopic liquid level will be reduced, and shrinkage defects will occur at the reduced liquid level.
2.Prediction of shrinkage defects
Taking a single isolated area sand casting as an example, this paper expounds the calculation process of shrinkage defects. The formation of shrinkage defects is continuous. During the calculation, judge whether the total shrinkage of sand casting is greater than the unit volume at any time. If it is greater than the unit volume, subtract the shrinkage from the riser or the unit with flow feeding at the highest position of sand casting in time. Since each unit is discrete, only one unit can be subtracted at a time, and the reduction is repeated until the total shrinkage is less than the volume of one unit of a network, and then the shrinkage less than one unit volume is accumulated in the next calculation for the next calculation. When the shrinkage amount is subtracted from the top of the sand mold casting, first judge whether the subtracted grid unit is liquid phase. If it is liquid phase, subtract it from the unit with the highest temperature, and then expand the search for the unit with the solid rate less than the street solid rate, that is, the unit with solid composition, but can also flow feeding. When the shrinkage defect is formed, the shrinkage defect unit and the sand mold casting unit will be composed of different substances, At this time, new boundary layers will continue to form, and the heat transfer characteristics and temperature field will change. The air unit needs to be marked again, and the boundary conditions will be reset automatically.
For this program, the isolated region of hot spot is calculated, so the above ideas and methods are applied to each isolated region respectively, that is, the overall program for shrinkage defect prediction is formed. When calculating the shrinkage, calculate the shrinkage of each isolated domain; When judging the shrinkage defect, judge the feeding of the corresponding isolated region.