Screw Extrusion from various Binary Al-XMg Feed Materials - Effects of Heat Treatment on Microstructure
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Three direct chill (DC) cast aluminium-magnesium (Al-Mg) alloys, with 5, 8, and 10 wt% magnesium, have been used as base materials. These base materials were either rapidly solidified by melt spinning, or granulated by machining DC cast billets. Finally, the rapidly solidified and machined feed stock materials have been screw extruded. This is a novel extrusion method developed at NTNU in cooperation with Hydro Aluminium. In addition, various homogenization treatments have been examined. This was done in order to determine whether it is possible to eliminate the brittle intermetallic beta phase, by subjecting the cast material to a heating schedule in an air circulation furnace and quenching in water. The investigated materials have been characterized by optical microscopy, hardness measurements, scanning electron microscopy and x-ray diffraction. It has been found that screw extrusion produces profiles with good mechanical properties, compared to conventional ram extruded profiles. The grain size was much smaller, by approximately 50 μm. In addition,the ultimate tensile strength of screw extruded Al-8wt%Mg was comparable to conventional extruded Al-10wt%Mg. The homogenized DC cast material shows presence of beta particles in the first stage of the heat treatment schedule, but not in the last stage. It has also been found that time has an effect on the precipitation of the intermetallic beta phase. A DC cast sample homogenized at 430ºC for four hours showed presence of beta, but a DC cast sample homogenized at 430ºC for one week, did not. Homogenization time and temperature was also found to have a rather complex influence on the large amount of porosity already present in the DC cast material.