Evaluation of the level of hardening of aluminum alloy chips intended for subsequent pressure treatment

OBRABOTKAMETALLOV Vol. 23 No. 1 2021 TECHNOLOGY The cutting processes that lead to the formation of chips occupy a signi fi cant segment in mechanical engineering [1]. Chips are a second-rate type of scrap metal because they have a developed surface, which creates conditions for more intensive interaction with the surrounding atmosphere. If the chips are transferred to the melt state, the oxide fi lms appear on the surface as a result of heating, or there occurs gas saturation of the metal. These circumstances forced the developers to work out chip recycling avoiding the remelting stage [2,3]. For this purpose, pressure treatment methods are used, such as compaction in a closed container, rolling-pressing, hot pressing with subsequent cold processing of the product, etc. [4]. An additional dif fi culty arises when remelting aluminum chips: unlike heavy non-ferrous metals or even ferrous metals, which can be restored during melting using various reducing materials, aluminum oxides can be converted to a metal state only by electrolysis processes. Hence, there was a limitation to applying only solid-phase recycling processes [5-7]. In the liquid-phase process of aluminum remelting, oxide fi lms are formed on the metal surface; the density of the fi lms is almost the same as the density of the liquid phase, which impedes the separation of these phases due to gravity. The solid phase cannot be separated due to fl oating on the surface of the melt or settling on the surface of the hearth of melting furnaces. The chips have a certain degree of cold work hardening, which is recognized by all researchers. This degree of hardening has little effect on the melting temperature, but it strongly affects the recrystallization temperature; for some metals, it is possible to lower it to room temperature, which allows transferring the metal to a soft state. Conversely, metal hardening should be taken into account in the processes of chip compaction since it causes additional energy consumption. Recent studies have been aiming at determining the properties of chips depending on the cutting conditions [8], including the ones for cutting various aluminum alloys [9], as well as the properties of semi- fi nished products obtained from the chips [10]. The aim of this work is to establish the degree of chip deformation during the aluminum alloys processing by cutting and to predict its impact on the subsequent recycling. The objectives of the study are to formulate the boundary conditions of the cutting process, to obtain a solution, and to adapt the conclusions to the chip recycling process. Methodology of Experimental Research The aluminum alloy chips 2 were briquetted in a mold (Fig. 1, a ), which consists of the upper 1 and lower 4 movable dies, a detachable matrix 3 , and a cage 5 with inclined contact surfaces. a b Fig. 1. Scheme of cold pressing of a billet made of chips ( a ) and a photo of subse- quent hot processing on a combined rolling-pressing process unit ( b )