The T-Mag casting schematic in the starting position of the cycle is shown here.
on the uncovered casting. The rapid solidification of an ablated casting leads to high mechanical properties and a fine-grained microstructure due to the rapid solidification rate. The technology associated with ablation is patent-protected, so the current avail- ability of North American suppliers of ablation castings is limited.
Determining Defects For each of the test castings, four-
point bending tests were performed to determine flexural properties. be- cause the maximum bending stress is uniformly distributed between the two top loading points. 70- x 16- x 3.5-mm specimens were taken from two loca- tions on each control arm and placed in a load cell to be bent until fractur- ing. The ultimate bending strength was plotted with the two-parameter Weibull
statistics method, which provides a single value to show the spread of a property. A higher Weibull value, or modulus, indicates a narrower spread of properties associated with low num- bers of defects and greater reproduc- ibility of properties. Fracture surfaces resulting from the
four-point bending tests were exam- ined with an SEM to investigate the ef- fects of oxide film and porosity defects. During the tests, every ablation
sample and a few of the T-Mag samples required a larger test setup because they did not fracture under the stresses of the original, smaller setup. Even in the larger setup, T- Mag samples from one location and all of the ablation samples could not be broken, indicating the ablation castings show significantly higher ductility than the other processes.
Table 1. Porosity Analysis Results for Each Casting Group Casting Groups No. per mm2
)
Squeeze LPPM T-Mag
Ablation
34.95 76.75 69.16
194.52
132.94 84.10 76.18 41.65
Avg. porosity Avg. porosity min. Avg. max. size (µm2
15.84 15.31 12.43 9.64
As-cast magnesium wheel rims were pro- duced on a T-Mag machine during initial development of the process.
Several ablation samples were bro- ken manually in order to investigate the fracture surfaces. The test specimens exhibited mi-
croporosity ranging from 5 to 20 µm, with the nearest neighboring poros- ity ranging from 30 to 70 µm away, depending on the casting groups. Average porosity analysis showed that microporosity was not a major defect for any of the castings tested (Table 1 and Fig. 2). The T-Mag and squeeze cast processes showed fewer pores than low pressure permanent mold and ablation. The average area per- centage of porosity for all processes was in the range of 0.1 to 0.2% by weight. Larger grains were found in the T-Mag castings compared with low pressure permanent mold and squeeze castings. Ablation castings exhibited the smallest grain size due
Aspect ratio
0.53 0.47 0.57 0.58
Avg. near
71.53 65.69 50.33 38.34
Avg. area
diameter (µm) diameter (µm) (min/max) neighbor dist. (um) percentage (%) 8.37 7.06 7.07 5.52
0.44 0.84 0.49 0.83
Table 2. Summary of the Weibull Analysis of Ultimate Bending Strength for Four-Point Bend Test Results Casting Process Weibull Distribution Equation Weibull Modulus Scale Parameter (MPa) Mean Value of UBS (MPa) Squeeze Cast LPPM T-Mag
y = 7.93x - 45.59 y = 11.54x – 67.60 y = 17.29x – 101.94
July/August 2011 7.93
11.54 17.29
307 348 358
295.80 334.93 352.84
MetAl CAsting Design AnD PurChAsing 31
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