Fig. 2. Porosity analysis revealed the porosity area percentage (left) and grain size for each casting process.
to the process’ high cooling rates. While none of the processes exhibited significant amounts of microporosity, other defects were evident. In general, all four cast- ing groups presented some sponge shrinkage, as well as gas porosity. Long crack-like defects were found in some squeeze casting samples, and separated irregular pores were found in the ablation castings. Four relative defect ratings were
defi ned to compare the presence of defects for the different processes: • Level 0—sample without defect. • Level 1—sample with either a crack- like defect less than 2,500 µm or sponge shrinkage with average diameter less than 3,000 µm.
• Level 2—samples with a larger crack- like defect or sponge shrinkage, or a
gas pore less than 600 µm in diameter.
• Level 3—sample with gas pores larger than 600 µm and/or sponge shrinkage larger than 3,000µm. More than half the T-Mag and
ablation samples did not have appar- ent defects. According to the defect rating summary, 92% of the T-Mag and ablation samples achieved a Level 1 or Level 0 rating. Fifty-eight percent of the squeeze castings and 33% of the low pressure permanent mold castings achieved similar rat- ings (Table 2).
Factoring Fracture Surfaces Fracture surfaces from the four-
point bend test were investigated for casting defects, as well. In the squeeze and low pressure permanent mold samples, the main casting de-
Research Lab vs. Real World Results from the Mississippi State Univ., Mississippi State, Miss., magnesium
casting evaluation tests showed the ablation and T-Mag casting methods produced the cleanest microstructure and superior mechanical properties. But does that mean the other methods aren’t viable? The main researcher, Liang Wang, and Mike Marlatt, a lead investigator in the
high integrity magnesium automotive castings project that authorized the test, both pointed out low pressure permanent mold should not be left out, despite results that detected signifi cant shrinkage porosity and oxide fi lm defects. “The results don’t mean the low pressure and squeeze casting processes
are not good, just that ablation and T-Mag castings are cleaner,” Wang said. Various factors could improve casting results, he said, including a different heat treatment choice or better casting methods and the metalcasting facility. “It’s kind of a skewed result,” Marlatt said. “The castings came from dif-
ferent cast dates with different process settings.” Marlatt believes low pressure permanent mold and T-Mag (which is a
Fig. 3. Casting defects on the fracture sur- faces of four-point bend test specimens are shown for (from top) squeeze casting, low pressure permanent mold, ablation and T-Mag.
32 METAL CASTING DESIGN AND PURCHASING
proprietary method that must be licensed), are the two processes the industry will most likely fi nd as viable commercial casting methods. Ablation earned the highest performance marks in the testing and will
likely be included in further magnesium casting research, but like T-Mag, the process is patent-protected, which could limit its growth.—Shannon Wetzel, Senior Editor
JULY/AUGUST 2011
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