Effects Of Minor Additions Of RE,Silicon And Calcium On The Microstructure And Mechanical Properties Of AZ91 Magnesium Alloy

The high demand for weight reduction in engineering structural metals specifically in the automotive manufacturing sector has motivated researches in the improvements in the mechanical properties of light-weight magnesium alloys.AZ91 alloy is frequently used for diecasting purposes in the automobile industry,on the other hand,its coarse as-cast grain size is detrimental to the mechanical properties.Grain refinement is one of the key microstructural parameters that can improve the mechanical properties of Mg alloys.Extensive work in the literature has revealed several methods in achieving fine grain size in AZ91 alloys including mechanical,thermal and melt additions techniques.This thesis focused on the grain refinement of RE(Ce or Sm)with and without Ca or Si and their influences on the mechanical properties of AZ91 alloy.This thesis focused on the grain refinement of AZ91 alloy achieved by additions of single Ce or Sm additions and in combination with Si or Ca additions and the influences on mechanical properties were explored.The experimental work comprises of alloys of AZ91,AZ91-1.0Ce,AZ91-1.5Sm,AZ91-1.0Ce-0.5Si,AZ91-1.0Ce-0.5Ca,AZ91-1.5Sm-0.8Si and AZ91-1.5Sm-0.8Ca.It was established that Mg–9Al–0.8Zn alloy was mainly composed of α-Mg and β-Mg? ? Al? ? phases.The results indicated a significant modification and refinement of the β-phase by intermetallic compounds of Al-Ce,Al-Sm,Al-Ca,and Mg-Si,as well as a decrease in the volume fraction of β-Mg? ? Al? ? phase.The grain refinement attained in combined additions alloys were significant with recorded average grain size reduction from 239.7 ± 16.9 μm in AZ91 to 57.8 ± 1.4 μm and 66.34 ± 5.10 μm in AZ91-1.0Ce-0.5Ca and AZ91-1.5Sm-0.8Ca respectively.All the alloys studied were subjected to aging treatment.It was observed that Ca in combination with Ce or Sm notably suppressed the discontinuous precipitates.Furthermore,the enhancement of room temperature as-cast yield strength and total elongation achieved with AZ91-Ce/Sm-Ca were optimum as compared to other alloys.The yield strength improved from 91 MPa in the AZ91 to 119 MPa and 135 MPa in AZ91-Ce-Ca and AZ91-Sm-Ca indicating that about 31 % and 48 % increase in yield strength respectively.While the elongation improved from 6.7 % to 10.7 % for the former and a decrease,4.32 % for the later indicating about 60 % increment and 36 % decrement respectively.The improvements were as results of grain refinement strengthening,fine and modified β-Mg? ? Al? ? phase distribution at the grain boundary and intermetallics strengthening.The coarse Al? Ca particles formed in the AZ91-Sm-Ca alloy are brittle,which could subsequently worsen the elongation of the alloys at room temperature.Ca additions to AZ91-Ce/Sm alloys exhibited optimum yield strength of 164 MPa and 154 MPa with elongation of 11.8 % and 7.1 % respectively at T6 treatment due to the grain refinement attained in the solutionized alloys.Hence the addition of Ca to AZ91-Ce/Sm has superior effects on the overall grain refinement,microstructure and mechanical properties of both as-cast and as-aged.