Microstructure and Corrosion Characterization of Squeeze Cast AM50 Magnesium Alloys DEEPIKA SACHDEVA, SHASHANK TIWARI, SURESH SUNDARRAJ, and ALAN A. LUO Squeeze casting of magnesium alloys potentially can be used in lightweight chassis components such as control arms and knuckles. This study documents the microstructural analysis and corrosion behavior of AM50 alloys squeeze cast at different pressures between 40 and 120 MPa and compares them with high-pressure die cast (HPDC) AM50 alloy castings and an AM50 squeeze cast prototype control arm. Although the corrosion rates of the squeeze cast samples are slightly higher than those observed for the HPDC AM50 alloy, the former does produce vir- tually porosity-free castings that are required for structural applications like control arms and wheels. This outcome is extremely encouraging as it provides an opportunity for additional alloy and process development by squeeze casting that has remained relatively unexplored for mag- nesium alloys compared with aluminum. Among the microstructural parameters analyzed, it seems that the b-phase interfacial area, indicating a greater degree of b network, leads to a lower corrosion rate. Weight loss was the better method for determining corrosion behavior in these alloys that contain a large fraction of second phase, which can cause perturbations to an overall uniform surface corrosion behavior. DOI: 10.1007/s11663-010-9433-x Ó The Minerals, Metals & Materials Society and ASM International 2010 I. INTRODUCTION DEMAND is growing in the automotive industry for vehicle weight reduction using lightweight alloys. Mag- nesium alloys, in particular, have shown significant potential for lightweighting in automotive applications because of their lower density compared with aluminum in addition to their excellent machinability and damping capacity. [1] A combination of material design and process development is required to achieve the optimum microstructure for desired strength and corrosion prop- erties for specific automotive applications. Among the currently used magnesium alloys, the Mg-Al systems offer reasonably high strength properties at room temperature and increasingly are finding use in several automotive applications. The following types of Mg-Al alloys are commonly are used for auto applications: Mg- Al-Zn (AZ) and Mg-Al-Mn (AM) series alloys. A review of the microstructure and mechanical properties of AM series alloys and the new Mg-Al-Sn (AT) series alloys has been provided by Luo and Sachdev. [2] A proper combination of strength, toughness, and corrosion resistance has been a major challenge for Mg alloys. Some studies indicate that adding alloying elements such as rare earth elements to Mg alloys produces substantial improvements in strength, toughness, and corrosion resistance. [3–5] In terms of the choice of a manufacturing process, the preferred casting route for producing automotive com- ponents has primarily been the high-pressure die casting (HPDC) process. Although HPDC is a preferred method for producing components with thinner cross sections such as the instrument panel beam, this process cannot be adopted to produce parts with thicker casting cross sections (e.g., control arms and knuckles in the chassis structures) because of porosity issues. For casting thicker sections with Mg, squeeze casting could be a possible choice and currently is being evaluated. Squeeze casting is a liquid-metal-forming process in which the molten metal solidifies under pressure within a reusable die cavity. In this process, an external pressure is applied to a specified quantity of molten metal throughout solidification, which allows the metal to be in close contact with the metallic mold at all times. This intimate contact leads to rapid heat transfer, which yields a high-integrity casting that is substantially free of pores and also has a finer grain size or dendrite arm spacing compared with semipermanent mold casting, the alternative process for making castings with thick sections. Squeeze casting can provide components with mechanical properties similar to wrought products. The most important process parameters in squeeze casting, similar to that in die casting, are melt temperature, melt impurities (i.e., the absence of oxide films and inclu- sions), melt volume, die temperature, applied pressure, and pressure duration. [6,7] Squeeze casting combines the advantages of gravity die casting and closed die forging in a single opera- tion. The squeeze casting process offers the following DEEPIKA SACHDEVA, Researcher, SHASHANK TIWARI, Senior Researcher, and SURESH SUNDARRAJ, Staff Researcher, are with India Science Lab, General Motors Technical Center, Bangalore 560066, Karnataka, India. Contact e-mail: deepika. sachdeva@gm.com ALAN A. LUO, GM Technical Fellow, is with Chemical Sciences and Materials Systems Lab, General Motors Tech Center, Warren, MI 48093. Manuscript submitted August 18, 2010. Article published online September 16, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 41B, DECEMBER 2010—1375