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A Study of Hot Tearing in Typical Mg-Al Magnesium Alloys

Author: LiuWenJun
Tutor: JiangBin
School: Chongqing University
Course: Materials Engineering
Keywords: Magnesium alloy Hot tearing Alloying Cooling rate HCS
CLC: TG146.22
Type: Master's thesis
Year: 2012
Downloads: 67
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Abstract


With the development of thin-walled and complex shape casting of Mg-Almagnesium alloys, the hot tearing has become one of the most common and seriousdefects that impact on the production capacity and mechanical properties of alloys.Based on the numerical simulation results of casting process, the hot crackingsusceptibility (HCS) curves, hot tearing fracture microstructure, fracture morphologyand secondary phase were observed and analyzed in this work, and the effectmechanism of alloying and cooling rate on hot tearing were discussed also.Numerical simulation of casting process of AZ91D magnesium alloy underdifferent cooling ways were investigated,and the results showed that both modes of theair-cooled and two-way insulation had similar filling behavior. The unstablephenomenon of surface forefront in the filling process became more serious with thisorder of air-cooled mode, two-way insulation mode and overall insulation mode. Thenumber of isolated melting zones was increased with this order of overall insulationmode, two-way insulation mode and air-cooled mode during the solidification.The action mechanism of alloying on hot tearing was discussed by the observationand analysis of HCS curve, hot tearing fracture microstructure, fracture morphology andsecondary phase. Under the influence of Al, the HCS of Mg-xAl-1Zn was proportionalto grain size, and not significantly related to the discontinuous distribution of Mg17Al12.As to the Ce, the HCS of Mg-9Al-1Zn-xCe (0≤x≤1.0) was not only proportional tothe grain size, but closely related the discontinuous precipitation of reunion in pairs ofAl11Ce3The relationship between HCS and cooling rate was studied,and the resultsindicated that both the grain size and the mount of secondary phase were increased withthe decrease of the cooling rate, while the value of HCS was increased also. Meanwhile,there is a large amount of shrinkage and micro-porosity appeared beside the freedendrite, solute-riched secondary phase and cracks in the fracture surface of alloy withlow cooling rate.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Non - ferrous metals and their alloys > Light non-ferrous metals and their alloys > Magnesium
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