Porosity control is key to quality.

"With the increasing trend in the use of aluminum foundry alloys for the production of critically stressed components in the automotive and aerospace industries, the aluminum industry has had to focus sharply on the quality and reliability of such component castings. To consistently produce high-quality castings, the alloy or metal quality must be optimized prior to casting."

With this introductory statement, A.M. Samuel and F.H. Samuel, University of Quebec-Chicoutimi, set the tone for this year's aluminum program, which stressed the need for control of the melt prior to pouring--including fluxing, modification and grain refinement--to achieve mechanically and physically sound aluminum castings.

Porosity is one of the major factors confronting aluminum metalcasters in improving overall quality levels. In their discussion on the basic issues concerning the effects of porosity on aluminum castings, the Montreal researchers pointed out that "The presence of porosity, inevitable to a certain extent in any casting, can be very detrimental, in terms of surface quality and a deleterious effect on the mechanical properties and corrosion resistance.

"Hydrogen, the only gas capable of dissolving to a significant extent in molten aluminum, is basically responsible for the gas porosity in a casting, as opposed to shrinkage porosity that results from the volume shrinkage associated with solidification and from inadequate feeding.

"Porosity is controlled by factors like casting geometry, grain refining, modification and inclusion content. Feedability, dissolved hydrogen levels and melt quality are, therefore, critical parameters in the quality control of castings."

Several other authors also stressed the need to understand and control porosity levels in aluminum castings in this year's aluminum sessions. "Although numerous theoretical and empirical studies have appeared, no clear consensus has emerged as to what the underlying mechanisms are for porosity formation," explained G. Sigworth, Metalworking Technology, Inc., Johnstown, Pennsylvania, while reviewing the effects of porosity on casting quality.

"The properties of a casting determine, to a large extent, the quality of the final product. In particular, porosity and shrinkage voids are usually extremely undesirable. From our commercial experience in the casting area," Sigworth said, "it would appear that a half to three-fourths of all scrap castings are lost to factors related, directly or indirectly, to porosity and shrinkage. And while this problem has been studied in detail for nearly 50 years, there appears to be no clear agreement as to the important mechanisms which control the formation of porosity and shrinkage."

With that in mind, Sigworth described a two-part research project on the "Evolution of Porosity During Solidification." Part one consisted of a literature review of the subject, and part two studied a number of theoretical models that were derived to determine how porosity forms during solidification. According to Sigworth, "The predictions of the theoretical models proposed have been compared to results in actual castings, and the correlation appears to be excellent for steel and for aluminum alloys having a modest freezing range."

But Sigworth added that the models may not apply to sand or investment castings of aluminum alloys having a wide freezing range. He proposed that a dimensionless group be developed to characterize the freezing range of the alloy, as compared to thermal conditions in the mold, in order to establish the validity of the models.

While many researchers look for the reasons that lead to porosity in aluminum castings, others have turned their efforts toward determining the amount of porosity in the casting. A team comprised of researchers from Image Analysis Research Center, Medical College of Ohio, Edison Industrial Systems Center, and Doehler-Jarvis, all of Toledo, Ohio, described a high-tech nondestructive method of locating and measuring porosity in die castings.

Called volumetric computed tomography (CT), this technique produces internal images of objects with enough detail to characterize important features such as porosity in die castings. As the authors explained, "During production of aluminum die castings, regions of porosity frequently coalesce in the part. These regions may or may not have an effect on the overall quality of the part, depending on the location of the porosity. If significant porosity intersects a machined surface or if the porosity provides a pathway for passage of air or liquid from one chamber to another, the result is a defective part. The object |of this research~ was to locate porosity and measure it in a nondestructive manner."

In this case, CT was used to produce 3-D images of internal porosity in an aluminum water pump. The CT scanner was capable of producing hundreds of inspection planes in a single scan yielding 3-D image data of the internal structure.

The image data was then evaluated using 3-D gray-scale mathematical morphology algorithms custom developed to extract regions of porosity while maintaining the geometric integrity of the data. Once the regions of porosity were extracted from the CT images, validation and visualization of the data was done. The validity of the porosity image was verified by destructive metallographic test methods.

According to the authors, "The significance of this development for the foundry engineer is that the results of a volumetric CT scan, one that quantifies porosity characteristics in 3-D, can lend a quantitative understanding of the product for problem solving and process optimization and reduce process development time."