Affect of cast thickness on microstructure and mechanical properties of produced sheets with twin roll casting

Countinious slab casting has been done by commercially for 50 years by using twin roll casting discovered 1940.Compared to other convensional teqniuques, twin roll casting has lower investment cost and needs low energy for production.However, with this technique aluminium alloys which have lower solidification ratio can be produced continiously. Twin roll casting technology can be used for production of aluminium sheet from 0.5 to 10 mm thickness from melted aluminium.With the variation of casting thickness there can be seen defects in microstucture and this defets can affect mechanical properties of final material, therefore the thickness of casted material must be optimised. In aluminium sheet production including two types of casting, continuous and semi continuous. Semi continuous casting method is direct chill and continuous casting methods are known as strip casting processes and twin roll continuous casting method. DC casting technique was invented by VAW(Germany) and Alcoa(USA) at the beginning of 1930's. Although it has lower amount DC casting technique also used in casting of copper, zinc and magnesium metals. Semi finished products are obtained from primary or secondary recycled aluminium by melting and shaping in DC casting technique. DC casting technique provide more fine grained structure beside prevent variations and segregations in microstructure by applied through direct cooling. DC technique casted slabs are used for sheet and foil in rolling mills. The slabs 1.1 meters in diameter, although generally sent to the extrusion process also can be used in forging process. Although the DC method allows to casting of alloys with wide solidification range, because it provides the advantages of microstructure, makes it possible to produce final products have better quality. Investment costs are higher when compared with TRC method. Melted metal contacts with water-cooled molds and solidified, in DC technique. The water circulating in molds provide the heat reduces necessary for melted metals solidification. Casting speed is dependent on the type of alloy and it's between 1-3 mm/s. Continuous casting methods have proven itself in production of foil semi-finished products and sheet products. There are several methods of continuous casting and casting thickness is between 3mm and 20 mm. Melted metal solidifes through out cooled rollers and molds. Poured sheet can be fed directly to the roll or may be allow to cool. In the production of Conventional which named as DC technique, slab casted product heated to 500 0C and hot rolled to 4mm or 6mm thickness. The most beneficial advantage of the continuous casting techniques is that allows to removal of some production processes. In Hazelett casting line, melted metal solidified between two rotating and cooled strips which is around 1.5 mm thickness. Slabs are produced to 15 mm-25 mm thickness and 2000 mm width by this technology. These machines are used for long plates and 1750 mm width sheets. The working principle is cast in the same thickness of the sheet can be poured of melted metal, through the casting nozzle in the space between bands. The metal solidifies in the space between bands and pulls out of the machine by the pinch roll. Casting range is between 5m/min or 9m/min. The Kaiser casting technique's surface quality is not smooth enough for foilstok.The concept based on continuous canstok production sub processes are as follows: -Melted metal hold in furnace, -degassing and filtering, "Kaiser Caster", - Hot rolling mill, - heat treatment (annealing) and quenching, -cold rolling mill and spinning. For example; three sub products of the can; body,lid and label should be produced in this line. Although twin roll continuous casting technique's origin based on Hazelett and block casting techniques which is a little different from other techniques. This technique allows the melt solidifies the metal through water-cooled rollers rotating in opposite directions out. Rolling occurs in an amount in the solidified sheet and wound in rolls with tension . The temperature of solidifying sheet is approximately 300 0C. Physical property with respect to hot rolled product %15-%20 deformation sustain has the same properties. Although sheets produced by TRC have relatively smaller-grained microstructure, in the literature the grain might be observed for casted sheets in next process. In addition to this growth can be managed to using more grain refiners or application of thermomechanical processes. Aluminium is known as a metal has low density ,2.7 g/cm3,high light reflection, high thermal and electricial coductivity. In athmospheric conditions aluminium has high corrosion resistance due to oxide film on the surface. Aluminium has face centered cubic (FCC) microstructure. Aluminium has corrosion resistance in severeal conditions, for example athmosphere conditions. It provides to forming oxide film layer on the surface. Formation of the oxide film layer occurs in 10-4 seconds at the room tempreture and normal conditions. The layer has been formed prevents to decomposition of metals in future. The thickness of this layer in normal conditions is between 5 or 20 nanometers and the thickness increasing in aqua environment. Aluminium has high affinity to oxygene. Ellingham diagram shows the aluminium can be reduced with carbon at 2200 0C. Due to carbide maker metal and has high reduction temperature, aluminium can not be reduced with carbon. Aluminium alloys are divided into two groups including processing and casting alloys. Processing alloys are converted products by applying afterward rolling, extrusion, forging, deep drawing. The casting alloys, often called the alloy will be redirected to the product close to the semi-finished or final shape after casting. In condition identification initial capitalization defines the class of therapy. F letter meant as produced, defines the products casting form. For thermal condition has not been done any additional processing and there is not defined any mechanical properties range for processing alloys. O letter meant, which means annealed.Which describes the condition after performed to improve ductility and workability in process alloy cast products. H letter meant condition after the product has been hardened with deformation. In condition defining there is two or more number after H letter. W letter meant condition of products have applied solution heat treatment. T letter shows stable conditions.It defines the alloys have been conditioned with a heat treatment and used one or two number for special defining. In this study, effects of ranging casted coil thickness on microstructure and mechanical properties of final material were investigated by using 8006 alloy with different thicknesses, produced by twin roll casting technology. Samples having a different thickness from each other are subjected to homogenization annealing. After homogenization annealing process, samples were deformed uniformly and they were rolled to the same thickness. The samples which have same thickness were subjected to final heat treatment process. Samples casted state, homogeniously annealed stete, uniformly deformed state, and rolled to same thickness state were etched to exmine grain structure. Optical microscopy and scanning electron microscopy investigations have been completed for the characterization of microstructure. Precipitated intermellallic have seen in the center plane of the samples which has same distance from both surface of sample (center line) during optical microscopy and scanning electron microscopy investigations. These intermetallics observed with different magnifications in both optical microscopy and scanning electron microscopy. In addition to that, from various part of center line were analized during scanning electron microscopy investigation to determine alloying and the other element content of center line. As a result of scanning electron microscopy analysis iron and manganese elements was found to be rich in the center line intermetallis. The etched samples was observed under a polarized light. During the investigation for grain structure in optical microscopy under polarized light of the samples which are casted state, grains which close to the surface of sample are larger and versetile, grains which close to the center of the sample has are coaxially oriented. During the investigation for grain structure in optical microscopy under polarized light of the samples which are homogeniously annealed, after annealing process the grain size of the samples close to each other due to re-cristallization in annealing process. As a result of annealing process showed that, after homogenization annealing grains re-crystallized and size of grains are close to each other. Samples which is cast thickness were rolled to same thickness and also were deformed uniformly for indicating mechanical properties. Traces of stifness were constructed on different parts of sample for hardness testing. Samples which are deformed same thickness annealed finally and used for the erichsen test.