Currently, aluminum is one of the most important technical metals. Its alloys have many good properties that combine successfully with each other. Their universal applicability is ensured by the strength-to-density ratio, good ductility, corrosion resistance and good thermal and electrical conductivity. Therefore, aluminum alloys are used in many fields, from food, construction, pharmaceutical, electrical, aerospace, automotive and aerospace industries. In the latter three low density to high strength ratio is the key. Therefore, 7XXX alloys, especially EN AW 7075, are the most widely used in these fields. In these alloys, the main alloying element is zinc (up to 6.1 wt. %), and the addition of magnesium (up to 2.9 wt. %) and copper (up to 2 wt. %) is also important. Such alloys are heat-treated, which means that during heat treatment mainly nanometer metastable precipitates η′ (MgZn2) and θ (Al2Cu) are formed. This heat-treatment strengthens αAl matrix the most. Therefore, these alloys achieve strengths up to 500 MPa.
The purpose of the master's thesis was to follow the process of homogenization and heat treatment of alloy EN AW 7075 with differential scanning calorimetry (DSC) and measurements of specific electrical resistivity. First, we calculated a phase diagram, diagram of changing the proportion of phases during equilibrium solidification and the course of nonequilibrium solidification (Scheil diagram). This was followed by casting and preparation of DSC samples and specimens to measure specific electrical resistivity. We made an ETA casting measurement. The maximum homogenization temperature was determined by heating and cooling curves of the DSC cast sample. In order to track homogenization, two-stage homogenization and aging, we performed measurements of specific electrical resistivity in a tubular resistance furnace, the results of which were confirmed by simultaneous measurement on a DSC device. After the measurements, a metallographic examination of the samples was performed.
The results and graphs showed that the maximum homogenization temperature is 460 ° C due to the presence of low-melting phases in the alloy EN AW 7075. Specific resistivity measurements showed that the basic homogenization processes at 460 °C are coming to an end after 10 h, processes in two-stage homogenization (2 h at 460 ° C and 10 h at 520 ° C) are coming to an end after 8,33 h, aging at 120 ° C is coming to an end after 16 h and at 180 ° C after 12 h. DSC measurements performed under the same measurement conditions confirmed all results. The final metallographic investigation confirmed the presence of the phases expected after the Thermo-Calc calculations and the theoretical review.
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