Analysis of rolling fracture of the conticasted and tandem rolled blanks of low alloyed aluminum

Optical microscopy, electron microscopy and energy spectrum were used to test the morphology of grains, as-cast microstructure and secondary phases in confiscated and tandem rolled planks of 8011 low alloying aluminum alloy. It can be concluded that the existence of inhomogeneous secondary FeSiAl phases lead to the fracture of planks during rolling.


Introduction
In some plants, the conticasted and tandem rolled blanks with 6mm thickness can be directly rolled into foils with different size. rolling fracture often happened during the rolling process, however, the chemical ingredient of alloys was in agreement with the deviation of standard. In order to investigate the reason of fracture, the representative product 8011 aluminium alloy was analyzed by optical microscopy (OM) and scanning electron microscopy (SEM).

Samples and Experiments
Two kinds of optical microscopy samples were used, one was as-cast rolled plank of 8011 aluminum alloy with 6mm thickness (1# sample), the other were two bulks cutted from as-cast ingot of 8011 aluminum alloy (2# and 3# sample). The cooling conditions of continuous cast was simulated during casting small ingot.
The chemical constitution of samples was in agreement to the standard of 8011 aluminum alloy, namely (wt%): Si=0.5~0.9, Ti=0.03, Fe=0.6~1.0, Cu<0.1, the content of other elements were lower than 0.05, the balance was Al.
After electrolytic polishing and anode tectorial membrane these samples were observed by POLYVAR-MET optical microscope, and the microstructure images were taken by CCD.

Non-metal impurities
Non-metal impurities in sample 2# were observed in different fields of view by EDS, the results were almost same with Fig10. It could be seen that in 8011 alloy there were little content and fine non-metal impurities.

Analysis and Discussion
From the experimental results it could be known that: (1) the microstructure of confiscated and tandem rolled planks was hot-rolling microstructure, however, the existance of fibrous microstructure which was composed of bruised and elongated grains and broken grains showed that the microstructure of alloys was hot-rolling microstructure far from complete recrystallization. Although cogging temperature (about 600 ℃ ) was higher than recrystallization temperature, tandem rolling velocity reached above 1000M/min. During this high velocity rolling process, the velocity of work-hardening in metals was far higher than that of recrystallization softening, so it was impossible to complete the recrystallization. When this kind of planks were rolled into a certain thickness (especially rolling temperature gradually decreased), strain hardening accumulated significantly, so in some brittle micro-regions (for example, local area inwhere large brittle AlFeSi phases existed) fracture happened. The existance of broken crystal in microstructure (See Fig2 and Fig3) also proved this.
(2) the investigation on as-cast microstructure, the second phases and non-metal impurities showed that: this kind of material included trace content of non-metal impurities with very fine size, namely that the alloy was clear, however, AlFeSi phases with different size and component distributed in homogeneously in microstructure. The case of tandem rolled planks was different to that of small ingot, but the inhomogeneous distribution of second phases was inevitable. During the tandem rolling process, in high deformation degree these brittle AlFeSi (especially those AlFeSi phases at the place of triangle grain boundaries) probably became the first origin of fracture and leaded to ultimate fracture. The broken grains in conticasted and tandem rolled planks probably resulted in the fracture during the subsequent processing (as-cast rolled planks were directly rolled into foils), especially when planks were rolled into smaller thickness.

Conclusion and Suggestion
(1) The inhomogeneous distribution of brittle AlFeSi phases was the internal reason of fracture during subsequent cold-rolling of confiscated and tandem rolled planks in 8011 alloy. The microstructure without recrystallization in confiscated and tandem rolled planks was the condition of fracture during subsequent cold-rolling process.
(2) If suitable technology could be performed to change the size, morphology and distribution of AlFeSi phases and acquire complete recrystallization microstructure, it was possible to avoid the fracture during subsequent rolling process.