Hello.

My assumption.

It was not only extrusion I guess. It was maybe hot extrusion or cold extrusion with subsequent annealing. After internal stress annealing during cooling such grain boundary pattern can form inside the tube because of heating and cooling cycle. At the outer surface of the tube or at some flat sample you cannot probably find the same effect.

Dear Dr. Amol Suresh Jadhav,

I did not understand if you produced the tubes. If you hadn't done it, could it be possible that during the extrusion process lubricants were used which were then removed from the inside of the tube with an aggressive solution making the microstructure present in the photo visible?

Best regards, Pierluigi Traverso.

Abdul Malik

Anton Pshenichnikov The process is hot extrusion and the (MPE) tubes are subsequently quenched in water immediately after exiting the dies. No annealing was done on the tubes. Definitely the same pattern cannot be seen on the outer surface of the tube.

Pierluigi Traverso Yes, we do extrude the tubes. And like I mentioned no surface preparation was done on the sample. To be more specific this pattern was visible randomly on only few ports. I also anlayzed it under the SEM but could not visualize the pattern.

Initially I assumed it could be a case of Intergranular corrosion when viewed on a LM but no evidence of corrosion was found when I checked it on a SEM.

Could it be possible that the patterns (grain boundaries?) are visible as the surface roughness is very low? In some other ports of the same tube the patterns are not visible due to higher surface rougness as a result of extrusion die lines.

Dear Dr. Amol Suresh Jadhav ,

here are the first two ideas after your welcome reply:

1) the fact that the microstructure occurs inside the tube randomly on only a few ports could indicate a phenomenon of poor oxidation of the material or abrasion of the oxidized layer;

2) I would try to verify, through 3D no-Contact optical profilometry, the difference in surface roughness at the different points.

In general, it is not optimal, from the corrosive point of view, to have a surface system that presents chemical-physical inhomogeneities.

Best regards, Pierluigi Traverso.

Dear Pierluigi Traverso ,

By poor oxidation of the material do you imply that the aluminium oxide layer on the surface is not of optimal thickenss?

And if abrasion of oxidised layer takes place wont aluminium generate another layer of oxide on its own in ambient conditions?

Best regards,

Amol Suresh Jadhav

Dear Dr. Amol Suresh Jadhav ,

I am very sorry but not knowing the exact characteristics of your plant and the production parameters I can only make assumptions that can be more or less realistic. To avoid unpleasant surprises (... and unfortunately I deal with Corrosion of Materials and I have had many anomalous behaviors of materials not foreseen in the design phase) it is better to investigate and prevent, rather than having problems over time.

Best regards, Pierluigi Traverso.

After all I am still for compression stresses on the inner surface of the tube due to rapid cooling. This was the main reason. Plus also assume that the extrusion process was not immediate and it required some time to finish the extrusion of the required tube length. During this time at the temperature a recrystallization was surely possible. During the rapid cooling the inner tube surface had definitely compression stresses, that slightly strained the grains so that you could see the boundaries at the inner surface. Outer surface had tensile stresses during cooling and you could not see anything there.