**5. Conclusions**

Production trials of aluminum alloy extruded semi-products were performed both with conventional air cooling of the extrusion mold and with newly installed liquid nitrogen cooling of the mold. The semi-products analysis led to the identification of two main categories of defects found in the joint areas: pick-up and dye pick-up.

The metallographic analysis pointed out the absence of any hot shortness, grain boundary segregation or coarse intermetallic compound formation phenomena occurring within aluminum semi-finished products as concurrent causes for pick-up generation. The liquid nitrogen dye-cooling did not have any influence on the extruded aluminum grain size. However, the use of liquid nitrogen cooling reduced the adhesion phenomena between the flowing aluminum and the mold, hindering the generation of the detected defects. Moreover, liquid nitrogen dye cooling provided inertizing, detaching and cooling effects. Consequently, the best results, in terms of absence of defects, were achieved at the lowest working temperatures through the use of liquid nitrogen cooling. Furthermore, low working speeds also hindered the generation of defects.

Finally, liquid nitrogen mold cooling, compared to classic air cooling, granted smoother surfaces featured by a high density of valleys along the extrusion direction and the almost complete absence of peak-like defects emerging from the surface which would lead to flaws in the subsequent finishing processes.

**Author Contributions:** A.F.C., S.B., C.D.C., A.G., L.M. took part to the experimental campaign relative to the metallographic investigation and the defects detection and analysis. C.M. is the research group leader and supervised the work, G.M. and M.B. performed the extrusion an collect the process data using the software produced by A.t.i.e. uno.

**Funding:** This research received no external funding.

**Acknowledgments:** This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

**Conflicts of Interest:** The authors declare no conflicts of interest.
