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

OBRABOTKAMETALLOV Vol. 23 No. 1 2021 TECHNOLOGY 20...60 times. Agreater effect is achieved in the metal layers on the convex surface of the chip, and a smaller effect is achieved on the concave surface, which corresponds to the resulting deformation distribution. A speci fi c factor that has to be taken into account when assessing the chip deformed state is the degree of deformation achieved at the previous stages of processing since the parts are often cut after preliminary cold deformation. The level of this deformation is set by the engineering speci fi cations for using high- strength metal in the part. Additionally, the cold-worked metal is characterized by a higher level of brittleness, which yields loose chips instead of drain chips. The latter is easier removed from the cutting zone as fragmented material. In the mechanics of plastic deformation, there is a hypothesis that plastic deformations achieved in various processes have the property of additivity, i.e. they can be summed up. Then the resulting degree of shear deformation   obtained by the chip is the sum of the degrees of shear deformation accumulated by the workpiece at the preliminary stages of its processing  1 ,  2 , ...,  i-1 , and the degree of shear deformation obtained by cutting  d :    =  1 +  2 +…+  i– 1 +  d . (3) Pure aluminum rarely acts as a structural material. It is usually considered a functional material and used as a conductor of electric current or a heat transfer element. Accordingly, chips rarely occur in the manufacture of wire or conductive tires since these products are obtained by drawing. Thermally non-hardenable aluminum alloys are hardened by cold plastic deformation. Thermally hardenable alloys undergo the stages of quenching and subsequent natural or arti fi cial age-hardening, with the possibility of additional hardening by cold or warm plastic deformation. The metal that has undergone these stages of hardening can be processed by cutting with the formation of chips. The degree of deformation to failure depends on the type of alloy and can exceed 90 % [20]. An additional degree of shear deformation  i -1 , which the workpiece metal underwent during the cutting operation, causes a decrease in the temperature of the recrystallization beginning. This circumstance will have to be taken into account for the heat treatment or hot deformation treatment operations [21]. In particular, the recrystallization temperature decrease may be inhomogeneous within the workpiece due to the inhomogeneities of the chips themselves. As a result, the formation of a multi-grained structure is not excluded, which, in turn, will cause heterogeneity in the distribution of the physical and mechanical properties of the fi nished product. Another factor that has to be taken into account when assessing the chip condition concerns the thermal processes that occur during cutting. The process itself can be carried out in various conditions. The cutting work is converted into heat; as a result, the chips are heated. This process coincides with the process of cooling it by the lubricoolant supplied to the cutting center. At the same time, at high cutting speeds, heat may not have time to be removed, so the heating temperature may exceed the recrystallization temperature, which will lead to the annealing of the metal. However, a large number of aluminum alloys have a very high recrystallization temperature, which allows obtaining cold-worked metal in such operations as pressing. Thus, although pressing is usually referred to as hot processing, for aluminum alloys it is often a cold or, in extreme cases, warm processing. As a result, a semi- fi nished product is obtained, for example, a rod that enters the cutting operation with a residual level of accumulated deformation. Here, the same situation develops as in the case of processing a cold-formed semi- fi nished product: to assess the hardening of the resulting chips, the degrees of shear deformation must be summed up. The paradox is that an increased degree of deformation, in this case, leads to a decrease in the recrystallization temperature of the chip material, i.e. the process of the chips’ hot deformation can be carried out at lower temperatures. Importantly, the recrystallization temperature for aluminum chips also depends on the deformation rate: when it increases, the recrystallization temperature decreases. Therefore, if a less durable state of aluminum chips is required, the recommended process of hot briquetting is better performed at high loading speeds, which contradicts the concept of high-speed hardening characteristic of other metals. Further research can be aimed at the study of the structural consequences for the chips to be used as a raw material for the manufacture of semi- fi nished products.