PP - 25.9

PP - 55

NWAS

NWAS

(Spectrographic surface analysis of the slug near the cast surface)

The Brinell hardness values were identical for these 2 samples but the tensile values define the differences in the properties more clearly. The NWAS samples had higher strength and elongation. This was interesting to note even though the PP sample had significantly higher Si content. This higher Si content is also reflected in the lower conductivity in the PP sample.

Regarding the chemistry, we noted that the structure of the PP sample changed significantly from the surface to the center of the slug parallel to the casting direction. We analyzed the surface, center and a re-melt section to compare differences. Fe % varied significantly from the surface to the center in the PP slug. This type of variation in NWAS "Air Slip" cast ingot has no statistical differences in chemistry throughout the cross section (see chemistry above).

Looking at the structure even at low magnifications indicates a much coarser structure in the PP samples. This is primarily due to a slower solidification rate during casting with the largest inter-metallic particles in the center as it is the last area to freeze. Another reason why these particles (AlFeSi - inter-metallic phase) are so coarse is that the material appears to have never been homogenized. This is noted in the 400X micrograph of the PP sample showing a plate like structure (sometime referred to as "Chinese Script"), which is typical of an as-cast structure in aluminum containing Si and Fe. Homogenization rounds and coalesce these particles and can be seen with optical magnification. More importantly, homogenization redistributes the Si, Fe and Cu more uniformly throughout each grain so that optimum properties can be attained. This process occurs mostly on a submicroscopic scale and can only be seen with an electron microscope. We homogenized a section of the PP so show the differences in the structure after this heat treatment (compare the PP micrographs at 400X). It is generally accepted in the aluminum industry that all wrought products are homogenized prior to forming of the final product. Inter-metallic AlFeSi particles can act at stress concentration points and cause premature structural failure under certain load conditions.

Discussion - 6061

The Brinell hardness values indicate that all of the samples had been annealed but that the NWAS sample had the lowest hardness, which is preferable for cold impact extrusion.

With regard to conductivity and the variations to the microstructure seen in the PP samples, we measured the conductivity at the surface and at the center of the slugs. In both PP cases the conductivity was lower in the center because more of the alloy constituents had precipitated from solution primarily due to a slow solidification rate. This is also confirmed by the higher hardness in the center. NWAS slugs have uniform conductivity and hardness regardless of sampling location.

We then compared properties of the PP and NWAS material after they were heat treated to T6 using standard ASTM methods. In all but 1 value, the NWAS material was superior in strength and the ductility was more than 50% better than either of the PP samples . Because PP sample #2 had considerable porosity in the center of the slug, tensile properties were tested at both the surface and the center. The properties in the center of this material were the lowest in strength with an elongation value that would not meet minimums in the finished product.

All of the samples were tested for chemical composition. Again, NWAS chemistry did not change significantly as determined by spectrographic analysis from the center to the outside of the slug while the 2 PP samples had significantly varying Si, Fe, Cu and Mg values when tested from the surface of the slug compared to the center. This is due mostly to the slow solidification rate but also due to homogenization, which does not appear to be complete. All of the samples were cut through the center, machined and etched to show the differences in macrostructure. Not only are these differences quite obvious but sample PP2 had porosity that could be seen without magnification. Clearly a defect that is unacceptable in a wrought aluminum product and cause for total rejection and scraping if it occurred in NWAS material. The changes that can occur in the microstructure (or lack thereof in the case of the PP samples) during homogenization are similar to those noted above in the 1100 discussion. Large coarse particles can be seen throughout both PP samples with the largest in PP2. These inferior structures can reduce strength levels as seen in the tensile values but are particularly detrimental to fatigue properties where large coarse precipitates can act as stress concentration points leading to premature failure.

Conclusion

In all areas of material quality and industry standards, it is obvious that NWAS material is superior to punched plate in this case. In fact, the quality of PP2 does not meet industry standards for wrought aluminum impact extrusion starting stock. While punched plate might be adequate for some applications where properties are not critical, this material should be thoroughly evaluated if used in a pressure container and avoided altogether if used in a low or high frequency fatigue application. NWAS is committed to a higher level of quality than the standard acceptable and sometime questionable quality standards for aluminum impact and hot forge material on the market today and will continue with this commitment in the future.