Microstructure and PerforMance of four casting Processes for MagnesiuM alloy aZ91
L. Wang, R. Lett, S. Felicelli and J. Berry
Center for Advanced Vehicular Systems and Mechanical Engineering Department, Mississippi State University, Mississippi State, MS, USA
J. Jordon
Department of Mechanical Engineering, The University of Alabama, Tuscaloosa, AL, USA D. Penrod
Manufacturing Services and Development, Inc., Cape Haze, FL, USA Copyright © 2011 American Foundry Society abstract
The performance of four different casting processes for magnesium alloy AZ91 were evaluated through microstructure characterization, mechanical testing, and SEM analysis of fracture surfaces. Passenger car control arms were cast by indirect squeeze cast, low pressure permanent mold (LPPM), T-Mag, and ablation processes. Samples were cut from twelve locations of the control arms for microstructure characterization. The microstructure, grain size, porosity distribution, and defect analyses were performed using optical microscopy and an image analyzer. Five different defects including microporosity, oxide film, sponge shrinkage, gas pore, and a crack-like
introduction
The magnesium alloy AZ91 has been used in many automo- tive applications because it is lightweight and has excellent castability and high fluidity.1
Most magnesium automotive
parts are produced by a high pressure die casting (HPDC) process. However, due to the high filling speeds in HPDC, the melt flow is turbulent and air can be entrapped, causing porosity when the melt solidifies.2
In turn, porosity is one of
the primary detrimental factors controlling fatigue life and total elongation in cast light alloy components.3
Many efforts have been devoted to investigate the mecha- nisms of porosity formation in the last 20 years. More recently, new mechanisms of pore formation based on the entrainment of oxide films during the filling of aluminum alloy castings have been identified and documented.4-9
Oxide film defects
may be entrained in most reactive liquid metals such as Al and Mg due to surface turbulence during the melting, pour- ing and transfer processes in casting. These defects have been observed on the fracture surfaces of tensile test specimens and the oxides have been identified by SEM-EDX analysis.8-10 In contrast to the efforts devoted to Al-based casting alloys, few studies have been done for Mg alloy castings. Griffiths
International Journal of Metalcasting/Fall 2011
The objective of this research is to investigate common and emerging casting technologies for magnesium alloys in order to evaluate their capabilities in meeting current and future en- gineering challenges facing the automotive industry. Through the production of magnesium passenger car control arms, we studied indirect squeeze cast, low pressure permanent mold (LPPM), and two new casting processes: T-Mag and Abla- tion. The T-Mag process was invented by the Commonwealth Scientific and Industrial Research Organization (CSIRO), which is the national government body for scientific research in Australia. T-Mag is a permanent mold casting process for magnesium, which can fill the die smoothly from the bottom up. This process can minimize air entrapment and oxide gen- eration, which leads to superior mechanical properties for Mg castings. The ablation process removes the aggregate mold with water, which is sprayed so as to ablate away the mold, allowing the water to impinge directly on the casting. This technique can easily remove the mold and achieve high cool- ing rates within the casting.11
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defect were identified. The mechanical behavior was characterized using four-point bending (FPB) and tensile tests. The four casting processes were evaluated in terms of reliability using two-parameter Weibull statistics of the ultimate bending strength (UBS) determined from the FPB test samples. Metallographic analyses were performed on these samples, revealing noticeable microstructural differences between them, with some showing possible evidence of oxide films.
Keywords: squeeze cast, LPPM, T-Mag, ablation, AZ91, oxide films
and Lai10 investigated the nature of the oxide film defects in
unfluxed commercial purity Mg alloy castings. They found double oxide film defects comprised of folded MgO films on the fracture surface of tensile test bars taken from the castings.
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