**1. Introduction**

Extrusion is one of the most widely used forming processes for aluminum and its alloys [1,2]. In this process, the internal liquid nitrogen cooling of the extrusion mold represents a significant innovation of recent years [3,4]. Indeed, during the extrusion process, different thermal exchanges occur, for example, heat generation due to material deformation, heat generation due to the friction between the billet and the liner and between the material and the dye, heat generation due to the internal friction between the flowing material and the dead-metal zone, and heat conduction through the components of the system (billet, ram, chamber, dye, etc.). Furthermore, due to the high output speed of this forming technology, the output speed of the is semi-product also high. Thus, the contact time between the mold and the extruded material is very short and the temperature increments are concentrated within a thin surface layer. This will extensively increase the generation of surface defects [5–10].

The new liquid nitrogen cooling technology, used directly in the matrix, allows the exploitation not only of the nitrogen inertizing effect, but also of its use as coolant, providing a much broader

range of benefits. The liquid nitrogen enters into the channels inside the matrix at a temperature of −196 ◦C and, during evaporation, it cools the mold. After the change in state, the gas creates an inert atmosphere, which inhibits the formation of oxides. The most significant advantage of this process, which justifies the higher installation and operating costs, is the increase in productivity due to the decrease in heat generation during extrusion. Indeed, the modern liquid nitrogen extrusion mold cooling systems optimize the consumption of nitrogen and guarantee sufficiently precise control over the whole process [3,4,11–13]. A schematic sketch of this technology is reported in Figure 1.

**Figure 1.** Schematic drawing of the liquid nitrogen extrusion mold cooling.

Furthermore, both the corrosion resistance and the aesthetic aspect of the final products may be relevant in some application fields of aluminum alloy extruded semi-products. Thus, surface quality may prove to be crucial [14–17].

Since the temperature increments of the extrusion process are concentrated in a thin surface layer, they drastically affect the surface finishing [2,4,18,19]. It is thus clear that the use of liquid nitrogen cooling of the mold and good temperature control of the overall process will beneficially affect these properties as well.

In the present work, the AA6060 alloy (0.30–0.60 wt % Si, 0.35–0.60 wt % Mg and 0.1–0.3 wt % Fe) is tested. This aluminum alloy is extremely ductile and therefore, is one of the most commonly used aluminum alloys. Further, its mechanical properties can be adjusted by subsequent heat treatment through precipitation hardening. It is used in many technical applications, such as automotive, aerospace and structural frame structures [20–22].

This work aims to assess the occurrence of a relationship between the surface quality and the use of internal liquid nitrogen cooling of the extrusion mold. Indeed, for structural frame applications, such as in this case, the extruded surface can be threated via the following aesthetic surface processes that might highlight surface defects. In the present work, in detail, the extruded semi-products undergo a 20 μm painting process and a 1 μm thick chromium plating process (Figure 2).

**Figure 2.** Flowchart of the production process of extruded AA6060 structural frames.

Thus, all the defects (such as pick-up, dye-lines and blisters), which are not completely hidden within these two overlaying layers, are highlighted in the final products by these aesthetic processes. The surface finishing can accordingly prove a major instance in this application field and even small progresses could emerge as significant.
