Effect Of Ca Add On Microstructure And Performance Of Mg-Al-RE System Heat-resistant Magnesium Alloy

In recent years, the global environment is worsening, the natural resources isexhausting by degrees. The utilization of energy-intensive and pollution-intensivevehicles such as automobiles increases the exhaustion of energy sources. Therefore, it issignificant to seek lightweight materials to cut weight of automobiles, saving energyand reduce consumption. Magnesium, as the lightest element, possesses superiorperformance at room temperature. But it has active chemical property and the strengthreduces radically when the temperature rises. So it cannot be applied widely as alightweight material and the magnesium alloy is restrictively be used as non-structuresalternative materials of automobiles. It is profitable and strategic to develop aninnovative magnesium alloy with excellent heat resistance, excellent processingtechnology and low price. The wide application of this kind of material will be an idealalternative for lightening automobiles.In this thesis, heat-resistant magnesium alloys AE42（Mg-4A1-2RE） and AE52（Mg-5A1-2RE） were utilized as base material; low-priced Ca （0.8¹.5） by differentcontent be added into and a series of samples were tested to reveal the properties ofdifferent samples. The microstructure, mechanical property, foundry technologyperformance and creep mechanism were studied through OM, SEM, EDS, XRD andtensile test. The mechanism and influence of Ca on the mechanical property andstructure of alloy AE42/AE52were studied, and the results are as following:①The as-cast microstructure of AE42has typical dendrites, the second phase arebasically constructed by needle-like Al₁₁RE₃and short rod-like Al₂RE which alongsideor stretch across the grain boundary. AE52has an increased content of A1, so atypicalmassive phase Mg₁₇Al₁₂ increases in amount and enlarges in size, distributing fromdiscrete form gradually to a continuous reticulation.②Ca can refine the grain of alloy; with its increment in content, the short rod-likeand bone-like Al₂Ca, which distributes alongside the grain boundary in the new alloymeet in the middle of phase, will gradually replace the needle-like Al₁₁RE₃of thesecond phase, and the structure changes from dendrites to isometric crystal.③At room temperature, the increment in content of Ca causes increase in yieldstrength of alloy materials with same aluminium content in them, while the tensilestrength and plasticity reduce. The increment of aluminum enhances the strength andreduces plasticity of alloy containing the same Ca content. At the temperature175℃, its strength property reduces compared with that at room temperature, while the elongationprompted. The plasticity reduces as the increment in content of Ca, and the yieldstrength and tensile strength are enhanced.④The tensile fracture analysis at room temperature shows that the addition of Cadoes not change the fracture mode of as-cast alloy AE42/AE52. It is the typicalquasi-cleavage fracture as before, but with the tendency of embrittlement. The result offluidity test shows that Ca reduces the fluidity of the alloy AE42/AE52, the more theincrement of Ca, the more decrement of the fluidity. Al increases the fluidity of alloywith the same Ca content in them.⑤After adding Ca into AE42/AE52, the change tendency of the microstructuresbefore and after creep are similar to AE42/AE52matrix alloy: the shape of secondphase transforms from acicular to short-bar-shaped or granular. A great deal ofshort-bar-shaped and granular precipitated phase appear around grain boundary ofmicrostructure after creep. The high temperature creep testing results shows that theaddition of Ca notably enhances the high temperature creep-resistant of new type alloysunder the condition of175℃/75MPa. More precisely, AE52performs a better creepproperty than AE42. On the other hand, the Al content has no obvious influence on thecreep of the alloys,each reaches10^-9(S^-1) quantity classes.