Quantitative analysis of porocity in die casting

Aluminum alloy high-pressure die casting is widely used, particularly in the automotive components industry, for manufacturing near-net shape products with a high production rate. In the die-casting process, the formation of pores in components is unavoidable. The porosity has a harmful effect on the strength and pressure tightness of die castings. Porosity is primarily caused by shrinkage due to solidification and by entrapped gases.

For instance, when the porosity is identified to be predominantly caused by shrinkage, die casters should check the metal temperature or the components should be designed without thicker sections. On the other hand, when the predominant cause of the porosity is gas, degassing the metal or increasing the metal pressure should be considered. In practice, however, it is frequently difficult to tell the difference between porosity due to shrinkage and that due to gas from observing die castings because these effects often interact with each other when pores form. The accurate identification of pores requires a huge amount of knowledge about the diecasting process, the lack of which may lead to misjudgments instance, when the porosity is identified to be predominantly caused by shrinkage, die casters should check the metal temperature or the components should be designed without thicker sections. On the other hand, when the predominant cause of the porosity is gas, degassing the metal or increasing the metal pressure should be considered. In practice, however, it is frequently difficult to tell the difference between porosity due to shrinkage and that due to gas from observing die castings because these effects often interact with each other when pores form. The accurate identification of pores requires a huge amount of knowledge about the diecasting. To identify the porosity accurately and to take corrective action in the die-casting process, the quantitative estimation of the morphology of pores such as their shape or spatial distribution can be a source of useful information. If such estimation can be applied easily at the foundry floor to identify the cause of porosity, it is expected that the burden of die casters will be substantially reduced.

Here the use of fractal analysis applied to obtain a quantitative indicator of porosity. Fractal analysis is widely used to quantify the self-similarity and complexity of natural structures such as the perimeters of clouds, coastlines, and various other geometries. In this study, two types of fractal analyses are proposed to characterize the porosity in terms of the shape of individual pores and the spatial distribution of multiple pores. In die castings, it is known that the predominant cause of porosity in the surface region is gas. On the other hand, pores formed by shrinkage and gas are scattered in the inner region.

Experimental procedure.

First, a component was sectioned, and the surface was polished using SiC emery sheets of increasing fineness from #800 up to #2400 before being buff-polished using alumina with an abrasive finenesses of 1 mm. Then, optical microscopy images were imported into a personal computer using a CCD camera. The pores of inner region & surface region has been identified for separate analysis. In images we will find black area which are are pores & grey areas which indicates aluminum oxide. Then the perimeters of the pores are emphasized. Finally, fractal analyses of the perimeters of individual pores and their spatial distribution were conducted.

The cross sections of typical pores predominantly caused by shrinkage are irregularly shaped, and those predominantly caused by gas are approximately circular. From this perspective, the fractal dimension of the porosity perimeter is expected to have a high value for pores predominantly caused by shrinkage and low value for pores predominantly caused by gas.