Influence Of Y And Ba On Microstructure And Mechanical Properties Of AZ91 Alloy

Magnesium alloys have high specific strength, low intensity, better damping and good machinability, excellent electro-magnetic compatibility and tempting metal polish which can be easily recycled. They have been widely used in automobiles, electronic industries, communication industries and civil or daily manufacturing. However, the commercial die casting magnesium alloys fail to meet the requirement of drive train and other systems in automobiles. This is mainly on account of their imperfect strength and toughness, especially the weak strength in high temperature, and these facts restrict badly the wide use in the automobile field.The casting magnesium alloys, especially the die-casting alloys are applied widely in current industries, and AZ91 magnesium alloys are most used in current industries as its high strength, good castability and low cost properties. However, the AZ91 magnesium alloys would become soft and be out of shape under the stress above 120℃ environment. As usual, the work temperature of the drive train and other systems in automobile is within the scope of 120℃~250℃.So it is restricted of the application of AZ91 magnesium alloys in the automobile field. Based on the facts above, the influences and mechanism of Ba and Y on the structure and mechanical properties were studied in the paper, which is also hoped to offer necessary academic research in developing heat-proof magnesium alloy.In the experiment processes, many kinds of advanced test instruments were adopted to test and analyse the structure and properties of the sample alloys. Microstructure and phase identification of magnesium alloy is analyzed by Scan Electronic Microscope (SEM), X-ray diffraction (XRD), and EDS instrument. In the test of the mechanics properties, the advanced multifunctional electronic experimental machine Instron5569 is employed for measuring the stretchingstrength and extending rate of the alloys in the room-temperature and elevated temperature. The hardness test is carried out on the HB3000 sclerometer. In addition, the fluxion property tests of the alloys are processed in order to study the influence of the added chemical element on the castability.The influences of Y addition to AZ91 magnesium alloy on the structure and mechanical properties were studied in this paper. The results show that just the moderate quantity of added Y can refine the microstructure obviously and increase mechanical properties efficiently. But when Y addition quantity exceeded 1%, the microstructure of the alloys would be coarse and the mechanical function decrease. The hot stability of the original structure can increase effectively due to the segregation of AI2Y phase with high melting point, so the elevated temperature mechanical properties increase validly. The tests also show that the grains tend to be coarse due to the excessive segregation of AI2Y phase, which causes the inhomogeneity of the structure and component of the alloys, even the stress concentration. In this case, the mechanical properties would fall sharply.The influences of Ba addition on the structure, mechanical properties of AZ91 magnesium alloy were studied. The results show that the Ba element can raise the ignition temperature of AZ91 magnesium alloy obviously. A little quantity of Ba addition can refine the structure and increase the mechanical properties of AZ91 magnesium alloy in the room-temperature and elevated temperature efficiently. When Ba addition quantity exceeded 0.3%, the alloy structures are apt to be coarse and the mechanical properties of the alloy decline, especially the tensile properties. The mechanism is studied of the ignition temperature point raise and strengthening mechanism of refining crystals from thermodynamics and dynamics viewpoint. It is Ba addition that result in the improvement of the elevated temperature mechanical strength due to the hot stability of the original Mgi7Ali2 phase enhanceded and the strengthening function of the segregation of AUBa with high melting point.The influences of Y and Ba complex addition on the structure and mechanical properties of AZ91 magnesium alloy were studied. The results show that the refinement tendency of AZ91 alloy increases and the grain quantity increases with the quantity of Y addition in the case of the certain Ba addition. At the same time, the yield strength, tensile strength and the elongation rate of the alloy in the room-temperature and elevated temperature increase obviously in contrast to separated addition of Ba. The complex addition of Y and Ba can bring the raise of the ignition temperature point of AZ91 magnesium alloy obviously, and the alloy developed should be melted and poured directly in air by addition 0.2%Ba and 0.5%~0.7%Y to the AZ91 alloy, possessing high strength, ignition-proof, heatproof and other good property. It is supposed that the new-type alloy would be developed with bright prospect due to its high elevated temperature strength and the best synthesized mechanical properties.The influences of Y and Si complex addition on the structure and mechanical properties of AZ91 magnesium alloy were studied in the paper. The results show that the original structure is refined obviously, and the mechanical property is increased efficiently with Y and 1.0%Si addition. It is observed that Y addition can play an important part in the transformation of the original massive Chinese shape of Mg2Si phase, followed by the improvement of the elevated temperature strength efficiently.In these designed alloys , the alloy with 1.0%Si and 0.5~0.7%Y addition possessed the best synthesized mechanical properties.A series of contrast tests are carried out to measure the tensile strength, yield strength and toughness among AZ91, AS41,AZ91+0.9%Y,AZ91+0.2%Ba and AZ91+1.0%Si+0.5%Y these alloys processed by thermal rate treatment. The results indicate that AZ91+0.9%Y and AZ91+1.0%Si+0.5%Y these two alloys have the best synthesized mechanical properties, especially elevated temperature tensile strength and yield strength, and the AZ91+1.0%Si+0.5%Y alloy is advantaged in cost in the two alloys. It is found that the complex addition of Si and Y is superiorto separate addition of Ba and Y in refining AZ91 alloy. It is viewed that the AZ91+1.0%Si+0.5%Y alloy, processed by thermal rate treatment would have bright prospect as a heat-proof magnesium alloy applied in automobile field.