Research On Microstructure And Properties Of Mg-4.5Al-1.5Sn-0.5Ca Soluble Alloys

Magnesium and its alloys are easy to degrade because of their high chemical and electrochemical activity.In addition,their degradation products are harmless,making them have broad application prospects in unconventional energy exploitation such as shale oil and gas.Unconventional oil and gas are usually exploited by staged fracturing technology in horizontal wells,where plugging tools are required.Conventional plugging tools in horizontal wells require secondary construction to remove them for oil/gas exploitation.Soluble plugging tools prepared by magnesium alloy can effectively save construction time and reduce costs.Therefore,the research of soluble magnesium alloy in unconventional oil and gas energy exploitation has attracted great attention.In practical application,both mechanical properties and degradation properties of soluble magnesium alloys should meet the construction standards.The mechanical properties of most traditional commercial magnesium alloys have already met the mechanical requirements,but the dissolution rate is too low to be directly used as soluble materials.Therefore,it is necessary to design new alloys based on commercial magnesium alloys.In this paper,the mechanical properties and degradation rate of Mg-4.5Al-1.5Sn-0.5Ca alloy（ATX410,wt.%）were systematically studied.The as-cast alloy was obtained by conventional solidification and sub-rapid solidification.Thereafter,homogenization and aging treatment were employed to regulate the microstructure and improve the properties of the alloy.XRD,SEM,TEM and three-dimensional morphology analysis were used to characterize the microstructure and corrosion morphology of magnesium alloy,and the corrosion rate was measured by hydrogen evolution,weight loss and electrochemical test.The main conclusions are as follows:（1）There are two phases in conventional solidification（CS）as cast alloy,namely rod-like Ca Mg Sn phase and island-like Mg₁₇Al₁₂ phase.However,only rod-like Ca Mg Sn phase exists in sub-rapid solidification（SRS）as cast alloy.Ca Mg Sn phases gather together in CS alloy,while the rod-like Ca Mg Sn phases in SRS alloys are shorter and evenly distributed.After homogenization,Mg₁₇Al₁₂ phase in CS alloy disappeared and Ca Mg Sn phase was left.Furthermore,the rod-like Ca Mg Sn phase was spherical and refined to～1μm in SRS alloy after homogenization.（2）After homogenization,the ultimate tensile strength（UTS）and fracture elongation（EL）of SRS-T4 alloy increase simultaneously,which are 236 MPa and 24%,respectively.The mechanical properties of SRS-T4 alloy are better than those of CS alloy（UTS and EL of CS-T4 are～220 MPa and～21%,respectively）.The refined eutectic Ca Mg Sn in SRS-T4 alloy exists in the form of near-spherical particles,the large contact area and short distance between near-spherical Ca Mg Sn particles and a-Mg matrix increase the number of electrochemical active sites,therefore results in fast corrosion rates(～145.25 mm y^-1 at 25℃and～2739.45 mm y^-1 at 93℃).In addition,SRS-T4 alloy exhibits a macroscopic uniform corrosion,while CS-T4 alloy shows a macroscopic local corrosion due to the aggregation of rod-like Ca Mg Sn phase.The corrosion rate of CS-T4 is 20.63 mm y^-1.（3）The UTS of CS and SRS alloys increases with aging time and reaches the maximum at 8 h.The sample aged for 8 h has the best mechanical properties.The maximum tensile strength of CS-T6-8h alloy is 230 MPa and the fracture elongation is13.7%.The maximum tensile strength of SRS-T6-8h alloy is 246 MPa and elongation at break is 18.5%.In the aging alloy,the corrosion rate of CS alloy firstly increases with aging time,then decreases,and reaches the maximum value at 4h,with P_H of 24.83 mm y^-1.The corrosion rate of SRS alloy is positively correlated with the aging time and reaches the maximum at 8 h,with a P_H of 109.26 mm y^-1.In this paper,the relationship between microstructure and solubility of alloy is systematically studied.The corrosion mechanism and related factors of alloy are explored.The corresponding results can provide support for the corrosion control of magnesium alloy,and provide experimental and theoretical basis for the development and application of soluble magnesium alloy.