A MicrostructurAl And MechAnicAl ProPerty study of An AM50 hPdc MAgnesiuM Alloy
J. Hines Forsmark, J. Boileau, D. Houston, and R. Cooper Ford Research and Innovation Center, Dearborn, MI, USA
Copyright © 2012 American Foundry Society Abstract
As the need for weight saving and fuel economy has increased, so has the interest in using Aluminum-Manganese (AM) Magnesium alloys. A thorough examination of the existing literature found several competing conclusions on how elongation and microstructure correlate; therefore a study was performed on a high-pressure die cast (HPDC) AM50 alloy. This study focused on understanding the relationship between microstructure and mechanical properties in test bars manufactured from a complex-shaped test casting.
The results from this study found a high degree of variability in the resulting tensile properties, especially in the elongation-to-failure data. To understand the role of
introduction
As the need for weight saving and fuel economy has in- creased, so has the interest in using Mg alloys for a wider variety of automotive applications.1-3
Previously, most auto-
motive applications of Mg alloys focused on powertrain ap- plications such as transfer cases, transmission housings, and small displacement engine blocks. Recently, applications involving automotive body structures such as instrument panels and steering column brackets have greatly increased.
Cast Mg alloys can work well in these applications. The good castability enables more complex designs compared to traditional steel stampings; additionally, the low density gives considerable weight savings. One alloy system that has seen increased usage in these structural applications is the Aluminum-Manganese (AM) series of Mg alloys. These al- loys are used due to their higher ductility (6-10% in AM50) when compared to the Aluminum-Zinc (AZ) series ductility levels (1-2% in AZ91).
In several studies4,5 performed on AM50/60 alloys elonga-
tion-to-failure has been seen to correlate with fracture tough- ness in Mg alloys. In general, higher elongation-to-failure corresponds to better fracture toughness; this is a key con- cern in the prediction of crash properties and optimal design. One of the issues with Mg alloy castings (or any other al- loy casting) is that the casting process yields inherent dif- ferences in mechanical properties. These differences vary
International Journal of Metalcasting/Winter 2012 eutectic in the AM50 alloy.
microstructure on properties, an extensive analysis of the microstructures was performed. No difference in cell size through the sample cross-section was observed. Externally solidified cells (ESCs) were present in large numbers; however, no correlation could be determined between the location of the samples and the size and number of the ESCs. Porosity distribution was random across the sample cross- section. Examination of the fractures surfaces indicated that the fracture did not preferentially occur along the Mg17
Al12
Keywords: magnesium, AM50, tensile properties, microstructure, porosity
both from location-to-location within a complicated part as well as from part to part for a given location. A number of published studies6-10
on AM60 castings have found consider-
able variability in the elongation-to-failure depending on the location where the samples were taken.
Because a thorough examination of the existing literature found several competing conclusions on how elongation and microstructure correlate, a study was performed. This study focused on understanding the relationship between micro- structure and mechanical properties in test bars manufac- tured from a complex-shaped test casting.
Background
While a large body of literature exists on the AM50/60 Mg alloys, only recently have researchers paid particular atten- tion to the elongation-to-failure data. One of the key factors controlling elongation-to-failure in Mg alloys was found to be the aluminum content.11-13
In these studies, Aune et al.
tested high pressure die cast (HPDC) tensile bars in the AM20, AM50, AM60, AM70, and AM80 alloys. Their test results found that elongation-to-failure decreased as the Al content increased. Aune et al.11-13
tectic phase as the Al content increased. It should be noted that the samples used in references 11-13 were as-cast ten- sile bars and had relatively high elongations to failure (an average of 16% for AM50).
to the increased percentage of the more brittle Mg17
suggested that this was due Al12
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