Study On Strengthening And Toughening Of AZ91 Magnesium Alloy

Magnesium alloys have the advantages of light-weight, high strength/weight ratio, high stiffness/weight ratio, high damping property and electromagnetism shielding, so it will act an important role in automobile, computer and communication as a kind of weight reducing material. AZ91 alloy which is widely used at the present time has many advantages, such as high ambient temperature strength, good casting property and low cost. But it is unsuitable for use at temperatures above 120 degree due to its poor mechanical properties at elevated temperature. Although Mg-Al-Si and Mg-Al-RE alloys were developed for application at elevatedtemperature, their limited castability or higher cost has restricted their casting applications. Therefore, it is important to develop some new magnesium alloys with low cost, high mechanical properties at room and elevated temperature and high toughness on the basis of AZ91 alloy. In this paper, we have studied the influents of adding various amount of A1-3%C and Al-8%Ti-l%C, microalloying elements and conducting different heat treatment processes strengthen AZ91 based alloy.In this thesis, the influences of different A1-3%C and Al-8%Ti-l%C additions, microalloying element and different heat treatment technologies on microstructures and mechanical properties of AZ91 based alloy are systemically studied by using measurements of Optical Microstructure(OM), X-ray Diffraction(XRD), Scan Electric Microscopy(SEM), Energy Dispersive Spectrometer(EDS), Transmission Electron Microscoe(TEM), Hardness and Tensile test. The results of refining addition A1-3%C and Al-8%Ti-l%C master alloy show that: l)The microstructure of AZ91 based alloy can be refined obviously by the addition of 1.0% A1-3%C, 1.0%Al-8%Ti-l%C. Its grain size respectively reduces from 130umwhich is the grain size without refining to 65 urn and 45 urn. When adding content exceeds 1.0%, refining effect is not obvious. Al-8%Ti-l%C refines not only a -Mg matrix ,but also divorced eutectic P-Mg17Al12. 2)The mechanism of the refinement of AZ91 alloy using A1-3%C and Al-8%Ti-l%C master alloy is respectively formation single A14C3 phase , compound A14C3 and TiC phase which can serve as the grain nuclei so as to refine magnesium alloy effectively during the solidification process. The impact value of AZ91 alloy improved 60%, 90% respectively with the addition of 1.0% A1-3%C, 1.0%Al-8%Ti-l%C master alloy. The hardness of AZ91 alloy after refining does not change obviously.The results of refining addition alloy show that:l) With Si added to AZ91 alloy, Chinese Mg2Si compounds are formed at the grain boundaries and within the grains. Al-P, Al-Ti-C-P and Ca addition all promotes the formation of fine polygonal type Mg2Si particles. The mechanism of modification of Mg2Si using Al-P is that single A1P serves as the Mg2Si nucleation and P atoms modify the morphologies of Mg2Si phase, using Al-Ti-C-P is that compound A1P and AIMn phases sreve as the Mg2Si nucleation; additionally,TiC phase improves modifying efficiency of Al-Ti-C-P master alloy; using Ca is CaSi2 phase serves as the Mg2Si nucieation. 2) Tensile strength, yield strength, elongation and impact value improved 1.5%, 13%> 50% and 68% compared to AZ91+0.7%Si alloy after Al-P modification, and improved 12.9%> 21.7%> 66.7% and 60.1% after Al-Ti-C-P modification. 3) Most of Ca dissolves into the ￡ -Mgi7Ali2 phase. With Ca addition the melting point of P -Mg17Ali2 increases 19°C. With RE added to AZ91+0.7%Si+ 0.22%Ca alloy, RE are combined with Al and form AlnRE3. More RE addition caused further decreasing of the 3 -Mgi7Al12 phase and coarsening of AlnRE3 because more Al atoms were consumed by the formation of AlnRE3 phase. 4) Addition 0.9%RE to alloy, hardness of alloy is 69.5, improved 20% and 7.9% compared to that of AZ91 and AZ91+0.7%Si alloy. Addition 03%RE to alloy, impact value of alloy is S.OJcm"2, improved 60% and 52.8% compared to that of AZ91 and AZ91+0.7%Si alloy. 5) Addition Si prolonged solid solution time. The whole {3 -Mg]7Ali2 phase dissolves into the a -Mg after solution 28h. The Si addition to AZ91 retards the aging process, suppresses the discontinuous precipitation and acceleratesthe formation of continuous precipitates. The occurrence of a peak hardness was delayed. The peak value of hardness occurs until aging 36h.