Because of very good properties, aluminium and its alloys are very useful. 7XXX series alloys are wrought alloys. Because of its ability of deformation and precipitation hardening, the alloy EN AW-7075 reaches the highest strengths among all aluminium alloys. The addition of zinc, magnesium and copper increases tensile strength and hardness, but decreases fracture strain, toughness and ductility. The alloy is adequate for forging, because of its low weight and high mechanical properties it is also used in the aircraft industry.
The microstructure is observed at higher magnifications, where crystal grains can be observed. Macrostructure is observed at lower magnifications, where larger defects in the material can be analysed. The surface of the sample must be prepared before the analysis, for the structure to be visible. Microstructure is observed with microscopes, and macrostructure with macroscopes.
Mechanical properties include modulus of elasticity, yield strength, tensile strength, toughness, ductility, hardness etc. These are values, that relate deformation to an applied load or force. They are dependent on the chemical composition, the processing and the state of the material. The values of mechanical properties are acquired with mechanical tests.
The EN AW-7075 alloy was casted with the direct chill casting method into ingots, that were then homogenized. The casted and homogenized microstructure of the ingot was observed on the scanning electron microscope. The homogenized ingot was cut into billets, that were centred, planed and ultrasound examined. The billets were extruded into rods, from which samples were taken. The samples were differently heat treated. The comparison of the macrostructure was done with a macroscope, and the mechanical properties of samples in different states were obtained with the tensile test and hardness measurements. The effectiveness of the individual heat treatment was obtained with the comparison of the values of mechanical properties.
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