Effect of Ag and Er on Microstructures and Properties of Al-Mg-Si-Cu Alloys
|Course||Materials Processing Engineering|
|Keywords||Al-Mg-Si-Cu Ag and Er heat treatment microstructures properties|
Al-Mg-Si alloys have medium strength, excellent corrosion resistance, favorable weldablity and low density, widely used in both cast and wrought forms. Adding Cu can increase the strength of A-Mg-Si alloy, but it introduces the intergranular corrosion which strict the uses of Al-Mg-Si-Cu alloy.The present work investigated the effects of Ag and Er on microstructures, ageing characteristics, tensile properties, electrochemical properties and intergranular corrosion properties of Al-1.1Mg-0.8Si-0.7Cu-0.5Mn-0.02Ti alloy (base alloy) using optical microscopy, scanning electronic microscopy (SEM) and Energy Dispersive Spectroscopy. The results show:Ag has no effect on refining grains of cast alloys. When solution treatment temperature is 520℃, all test alloys are recrystallized, but the recrystallized grains are elongated along rolling direction, the grain boundaries are curved. When solution treatment temperature is 580℃, all test alloys are over-heating. The best solution treatment temperature is 540℃. With the increase of Ag addition, both the strength and elongation of alloys decrease, the strength and elongation significantly decrease 70MPa 4%, respectively, when Ag content is higher than 0.4wt.%. The best aging temperature of alloys is 180℃. When ageing at 140℃, the hardness increases gradually and no peak hardness is observed, the hardness is low. When ageing at 220℃, the time to peak hardness shortened, and the peak hardness is low. Adding Ag into base alloy, the spontaneous corrosion potential firstly decreases and then increases and reaches a maximum value when Ag content is 0.4wt.%, and decreases again when Ag content is 0.7wt.%. The corrosion current density is lower than that of base alloy in the range of 0.1%～0.7%, and is smallest when Ag content is 0.4wt.%. The intergranular corrosion (IGC) test shows that IGC susceptibility of 0.4wt.%Ag alloy is lowest; 0.4Ag alloy has the best intergranular corrosion resistance under aged state, and corrosion resistance deteriorates under peak aging and over-aging states.Er can refine grains of cast alloy. When adding 0.4%Er into base alloy, the grain size is smallest. The best solution treatment temperature is 540℃for test alloys containing Er. Er addition reduces the strength of alloy, particularly adding 0.4wt.%Er. When aging temperature is 140℃, test alloys are under-aged; at 180℃, all test alloys show the maximum age-hardening effect, and at 220℃, alloys are over-aged shortly. The corrosion current density is smallest when adding 0.6%Er into base alloy. The results of intergranular corrosion (IGC) test show that when adding 0.6%Er into the base alloy, intergranular corrosion susceptibility of alloy reduces significantly; the under-aged alloy appears slight intergranular corrosion, while the peak-aged and over-aged alloys display no intergranular corrosion.