Application of Niyama criterion for predicting of shrinkage porosity of shaped castings

Method for determining of the critical value of the Niyama criterion based on computational experiment is developed. It is found that the critical value of the Niyama criterion depends on the alloy cooling conditions and therefore is applicable only to the given technological process. The critical values of the Niyama criterion for casting s made of ML10 and 14Kh17N2L alloys obtained in permanent metal moulds and in moulds of coldhardening mixtures are determined. The obtained results are confirmed on shaped casting.

Discussion on Usability of the Niyama Criterion for Porosity Predicting in Cast Iron Castings

The paper refers to previous publications of the author, focused on criteria of casting feeding, including the thermal criterion proposed by Niyama. On the basis of this criterion, present in the post-processing of practically all the simulation codes, danger of casting compactness (in the sense of soundness) in form of a microporosity, caused by the shrinkage phenomena, is predicted. The vast majority of publications in this field concerns shrinkage and feeding phenomena in the cast steel castings – these are the alloys, in which parallel expansion phenomenon does not occur as in the cast irons (graphite crystallization). The paper, basing on the simulation-experimental studies, presents problems of usability of a classic, definition-based approach to the Niyama criterion for the cast iron castings, especially of greater massiveness, for prediction of presence of zones of dispersed porosity, with relation to predictions of the shrinkage type defects. The graphite expansion and its influence on shrinkage compensation during solidification of eutectic is also discussed.

Numerical Prediction of Shrinkage Defect in Aluminum Alloy Electron Beam Weld Based on Niyama Criterion Program

The root shrinkage cavity (SCRT) is one special kind of high-energy welding defect which can remarkably decreases mechanical strength and induces fatigue crack, so it is of great significance to numerically study its formation mechanism. In this paper, a set of Niyama criterion program based on temperature field was developed and adopted to numerically predict it in non-penetration 2219 aluminum alloy electron beam weld. The results indicate that the formation possibility of SCRT defect decreases with the increased welding speed. In addition, the error of simulated SCRT in the weld penetration direction is 8.76%, while the error in the weld width direction is 23.46%, which shows good agreement with experimental comparison samples. All the simulation and experimental results validate the correctness and feasibility of the developed Niyama criterion program.