Influence Of Bi Addition On The Microstructure And Properties Of Hypoeutectic Al-Mg₂Si Alloy

In recent years,Al-Mg₂Si alloys have attracted much attention from scholars at home and abroad.Mg₂Si as reinforcing phase improves the specific stiffness,specific strength,wear resistance and heat resistance of the Al-Mg2Si alloys.However,the Mg2Si phase in as-cast alloys is in the shape of thick flakes or Chinese characters,which reduces the mechanical properties of the alloys.In this study,hypoeutectic Al-10%Mg2Si alloy was modified by adding different amounts of Bi.The microstructures and modification mechanism of Bi were analyzed by optical microscopy（OM）,scanning electron microscopy（SEM）and X-ray diffraction（XRD）.The mechanical properties of the alloy were measured by electronic universal testing machine.The fracture morphology was analyzed by SEM,and the electrochemical workstation and optical microscope were used to study the effect of Bi on the corrosion resistance of hypoeutectic Al-10%Mg2Si.The main results of the research are as follows:Adding an appropriate amount of Bi to the hypoeutectic Al-10%Mg₂Si alloy can achieve a good modification on eutectic Mg₂Si.The addition of Bi significantly reduces the mean size of the eutectic Mg2Si from 13.92μm to 4.23μm.And the shape of eutectic Mg2Si was transformed from plate-like structure into a fine fiberous one.Bi element distributes near eutectic Mg2Si.During solidification of liquid metals,Bi element enriches at the front of the solid-liquid interface of Mg₂Si,which results in the undercooling of composition and hinders the growth of Mg₂Si phase.After the addition of Bi,the ultimate tensile strength and elongation of hypoeutectic Al-10%Mg2Si alloy showed a trend of increasing first and then decreasing slightly with increasing the amount of Bi.When the amount of Bi was increased from 0 wt%to 0.8 wt%,the ultimate tensile strength increased from 213MPa to 270MPa,and the elongation increased from 2.7%to 5.9%,which was increased by about 26.8%and 118.5%,respectively.The effect of Bi on grain refinement of eutectic Mg2Si increases the total area of grain boundary in the alloy.Because the grain orientation of the grains on both sides of the grain boundary is different,when the dislocation passes through the grain boundary,the boundary of crystal will hinder the movement of dislocations,thus effectively inhibiting the growth and proliferation of cracks at the interface of eutectic Mg₂Si or matrix,and transforming the fracture mode of the alloy from brittle fracture to ductile fracture to strengthen the mechanical properties of alloy.The results of electrochemical corrosion tests conducted in a 3.5%NaCl solution simulating seawater corrosion shows that the corrosion tendency of the alloys ranges from small to large,Bi content is 0%,0.5%,0.08%,0.2%,0.8%and 1.2%,respectively.According to the low-limit potential E_p and the high-limit potential E_t,the passivation potential widths of the alloys of the respective components were 0.16V,0.38V,0.4V,0.23V,0.43V and 0.33V,respectively.The passive film of the modified alloy was more stable than that of the unmodified alloy,and the alloy was less susceptible to be damaged and more resistant to corrosion.Bi as a modifier is not dissolved in the alloy,it existed in the form of a Bi-rich phase between the Mg₂Si phases,which made the phase in the alloy increased compared to the unmodified Al-10%Mg₂Si alloy.This leaded to an increase of the tendency to corrosion.In addition,the addition of Bi changed the morphology of the eutectic Mg₂Si phase,which also affects the corrosion performance of the alloy.The passivation zone of the alloy increases significantly,indicating that the corrosion rate of the alloy became smaller,which also made the corrosion pit smaller.In general,the corrosion resistance of the modified alloy has a certain improvement.