Aluminum Insert with Mysterious Stripes

Aluminum 6061-T6 inserts sourced from China exhibited stripes on the inside and outside surfaces after applying a trivalent conversion coating. Furthermore, the inserts appeared to be welded, although seamless tubing was specified. Four good and four bad (striped) inserts were submitted for metallurgical examination to determine the cause of the stripes and the effect of the stripes and/or welds on the insert strength. Examination of the inserts included: (1) chemical analysis of the conversion coating using a scanning electron microscope equipped with energy dispersive spectroscopy (SEM/EDS) to determine if chemistry differences cause the stripes; (2) examination of the tubing microstructure to determine if banding, welds, or other microstructural features cause the stripes; and (3) ring tensile tests of good and bad (striped) inserts to determine the effect of the stripes and/or welds on the insert strength.

The striped appearance of the bad inserts was found to be due to significant microstructural heterogeneity in precipitate size and grain size. Although root cause determination was beyond the scope of this effort, the striped condition is most likely due to improper thermal (and possibly mechanical) processing. Perhaps the inserts were contacting each other or fixture/furnace surfaces during heat treatment or hot forming processes. No seam welds were observed on the good or bad inserts. The strength of the good inserts is approximately twice as high as that of the bad inserts.