The investigation of production parameters, mechanical properties and microstructures of 5005 aluminum alloys produced by steel/copper and steel/steel shell pairs in twin roll casting method

This study was carried out on the rollers where solidification occurs in the twin-roll casting method, which is one of the flat product production methods. Flat product production can be carried out by 2 different casting methods. One of these is the twin roll casting method and the other is the direct cooled casting method. In the direct cooled casting method, the products cast as slabs must be reduced to a thickness of 8-10 mm before cold rolling. Therefore, the hot rolling process and many equipment in this process are needed. In the twin-roll casting method, it is frequently preferred industrially, as material with a thickness that can be directly cold rolled is cast. In the twin-roll casting method, the rollers where solidification occurs consist of two components: core and shell. Pure water circulating in closed circulation in the copper pipes between the core and the shell is kept within the specified temperature range in the cooling tower, ensuring continuous solidification on the roller. The shell and the core material are connected to each other using a tight fitting method. Since cast rolls are exposed to high loads and thermal cycles during solidification, alloy forged steel is frequently used as a shell material in the industry. It is known that the copper element has a much higher thermal conductivity coefficient than steel. For this reason, copper shell material is used industrially instead of steel shell due to important factors affecting material and production such as rapid solidification and increased productivity. In this thesis study, steel/copper shell pair was used instead of alloy steel/steel shell. In the twin-roll casting method, beryllium alloy forged copper shell is used here, as it is the lower roll where the first solidification begins. Alloy standard steel shell is used in the upper roller. The effect of the steel/copper shell pair on the parameters, microstructures and mechanical properties in the production of 5005 aluminum alloy, which is rich in magnesium and is frequently preferred in almost every sector, compared to the steel/steel shell pair, was investigated. Within the scope of this study, optical microscope and SEM devices were used in microstructural analysis. In the analysis of mechanical properties, tensile tester, Erichsen collapse tester and microhardness measurement device were used. In addition, electrical conductivity measurements were also made. According to the results, when the productivity is compared in t/h/m, which is an output of the production parameters, an approximately 40 - 60% productivity increase was achieved on the St/Cu shell pair side. One of the parameters used in calculating productivity is line speed. In production with copper shell, line speed increased by 1.5 times compared to steel shell. The amount of graphite spray running parallel to the line speed increased by 1.5 times at the same rate. Similarly, the cycle time of one full rotation of the rollers around their own axis was completed approximately 50% faster in production with copper shell. Since band marks are not visible on the surface and its strength is lower than steel, roller pressures are kept approximately 8-10% lower. The service life of the steel shell, which is 1200 tons, was measured as 400 tons on the copper shell side. This shows an approximately 66% reduction in lifespan. In the microstructure examination of the casting thickness, it was determined that there was a significant difference in the grain structures on the surfaces in contact with the shell. It has been observed that the surface in contact with the copper shell has a coarser grained structure compared to the surface in contact with the steel shell. The event known as the Zener Drag phenomenon occurred and a supersaturated matrix was obtained with rapid solidification and partial dynamic recrystallization took place with the effect of some rolling. In the BF images, it was seen that the inclusions decreased significantly on the copper shell side and the MHS was shifted to the steel shell side by 0.503 mm. Since the thermal conductivity of copper is higher than steel, a supersaturated matrix structure is obtained by rapid solidification. Grain growth occurred on the partially dynamic recrystallized copper shell side due to the effect of the load and temperature applied during casting. SEM EDS analyzes also proved that intermetallics were reduced, dispersed in the matrix, and the matrix was oversaturated. In 8 different processes, 1,2,3 and 4 represent samples produced with St/St shell, and 5,6,7 and 8 represent samples produced with St/Cu shell. Processes 1 and 5 were annealed at 330ºC, 2 and 6 at 350ºC, 3 and 7 at 370ºC and finally 4 and 8 at 390ºC for 4 hours. In the intermediate annealing processes applied for 4 hours at different temperatures, it seems that recrystallization can be achieved at a lower intermediate annealing temperature due to the partial dynamic recrystallization that occurs during casting. When the microstructures taken with an optical microscope were examined, it was seen that the structure produced with St/St shell and annealed at 370ºC was very similar to the structure produced with St/Cu shell and annealed at 350ºC. When the mechanical properties were examined, there was an increase of approximately 30% in the microhardness measurements taken in Vickers in the as-cast samples. Tensile and Erichsen test results also confirm the superiority of the effect of the steel/copper shell couple on mechanical properties. In addition, it has been confirmed that there is a gain in the intermediate annealing temperature on the copper shell side, since the values of the previously mentioned processes 3 and 5 are close to the intermediate annealing application. Electrical conductivity measurements also gave results proving that the matrix was oversaturated. In addition, when the results of intermediate annealed samples in conductivity tests were examined, it was seen that samples produced with copper shell gave higher conductivity results compared to those produced with steel shell. This led to the conclusion that the event known as the Zener Drag phenomenon experienced during the casting process triggered the precipitation potential after intermediate annealing. In addition, it can be shown as evidence that dispersion occurs. As a result, copper shell has the disadvantage of short lifespan; high productivity, high line speed, low annealing temperature etc. It provides many advantages such as and can be used successfully in industrial TRC casting method. It supports process and energy efficiency by slightly reducing the intermediate annealing temperature. In addition, it is anticipated that it will provide many industrial advantages in terms of the strength and electrical conductivity properties it brings to the structure through partial dynamic recrystallization and dipersion. It has become a method that can turn the cost incurred due to its short lifespan into an advantage with its more efficient and high tonnage production capacity in a short time.