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In this project, the control arms, produced by the four cast- ing processes, were provided by various industrial partners. For the microstructure characterization, twelve samples were cut from one control arm representing each casting process. Characterization analysis included grain size, area percentage of porosity, porosity size distribution, and defect analysis. Four-point bending (FPB) tests are useful for de- termining the flexural properties in simply supported beams because the maximum bending stress is uniformly distrib- uted between the two top loading points, which is far more advantageous than three point bending tests, wherein the maximum stress is located directly under the top load.12


is important for the samples taken from a particular casting location, as they may possess defects anywhere within the center span. In this study, FPB tests were performed on con- trol arm samples from each casting process. Separate sets of samples were cut from two locations on each control arm, representing different thicknesses, with sample location and dimensions being the same for all control arms. The ultimate bending strength (UBS) was plotted with two-parameter Weibull statistics method and the casting process with supe- rior mechanical properties was identified. Monotonic tensile tests were also conducted to characterize the mechanical be- havior. The fracture surfaces were examined with Scanning Electron Microscopy (SEM) to investigate the effects of ox- ide film and porosity defects on the FPB tests.

experimental Procedures Microstructure characterization

One control arm from each casting group was used to exam- ine the microstructure characterization. Twelve samples were extracted from each control arm. The microporosity and grain size distribution were measured for each sample. Significant defects such as shrinkage, crack-like defects and gas pores were also identified. Figure 1 illustrates the locations where the samples were extracted from each control arm.

All samples were mounted and polished to examine the po- rosity distribution and other types of defects. The microstruc- ture and grain size were examined after etching. Two images from each sample were selected for porosity analysis, while other types of defects were analyzed separately. The image analyzer was used to characterize the porosity distribution, including the pore size distribution, average area percentage of porosity, near neighbor distance, and largest pore size. The grain size was measured using the intercept method, in which the number of grain boundary intercepts per unit test line is measured and the grain size is calculated according to the ASTM E112 standard.13

four-point bending (fPB) test

Four-point bending (FPB) test specimens, with dimensions of 70 mm long, 16 mm wide, and 3.5 mm thick, were cut from locations C and D of each control arm (Figure 2) to characterize the casting mechanical properties. The FPB tests were administered on these specimens using an EM Model 5869 Instron load cell at a cross-head speed of 3mm/ min. Figure 3 depicts the sample arrangement employed in the FPB test.

The initial test apparatus used was a smaller bend test setup. However, as the size of this setup was a limiting factor, a larger test setup was used to test samples from processes that did not fracture before “bottoming out” on the small test setup. This included ablation samples from locations C and D and T-Mag samples from location D. Figure 4 illustrates a bend test of an ablation sample from location C, which reached the bottom of the smaller test setup before failure.

Although the larger test setup was used, fracture of the ablation and T-Mag samples previously referenced still did not occur. A major concern regarding the samples during testing in the larger setup was slippage of the ends and contact with the setup in areas other than at the force

location a location B location d location c

Figure 1. Sample regions for the microstructure analysis, totaling 12 samples were extracted from each control arm.


Figure 2. Magnesium control arm (top view of casting) illustrating locations of C and D used for FPB test specimens, and locations of A and B used for tensile test specimens.

International Journal of Metalcasting/Fall 2011

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