tive gas to produce two tensile bar (12 mm in diameter and 50 mm gage length) at each pouring. Commercial alloy AZ91 was also cast as a baseline for comparison.
/CO2
Three alloys, AM60, AM80 and AZ91, were selected for high-pressure die casting trials. Super-vacuum die casting (SVDC) process using a very high vacuum level of < 60 mbar, recently developed for magnesium alloys,12
was used to make
a tophat casting as shown in Fig. 1. The tophat castings were produced using a 1600 ton conventional die cast machine that was retrofitted with the SVDC technology. Process simula- tion tools (both EKK and MagmaSoft) were used to validate the gating system and determine initial process parameters. The die casting process parameters were optimized to provide minimal shrink and gas porosity, and eliminate surface de- fects (i.e. sinks, hot cracks, laminations, etc.). The key pro- cess parameters for SVDC magnesium alloys were reported in a previous publication,12
and is summarized as follows:
Slow shot velocity: <0.5 m/sec Fast shot velocity: 3.0 – 4.0 m/sec Die temperatures: 125 – 250C (257 – 482F) Die spray (a Chem-Trend die lubricant) and blow off pattern: robotic application with a total spray time of 12 – 15 sec
Die lube ratios: 60:1 – 175:1 Metal temperature; 640 – 690C (1184 – 1274F) Shot pressure: 70 – 110 bar Dwell time: 2.5 – 5.5 sec
Table 1. Chemical Composition (Weight %) of GPMC Samples
about 80C (90F) above the liquidus temperature of the respec- tive alloys, ranging from 595C (1103F) for AM100 to 638C (1184F) for AM20 alloy. A mild steel permanent mold with boron nitride coating was heated to about 280-300C (536-572 F), and molten metal was poured under the SF6
protec-
Magnesium high-pressure die castings are normally not heat-treated due to the following two reasons:7
(1) The loss in strength due to grain coarsening during solution treatment can be larger than the strength gain from subsequent age-hardening; and
(2) The formation of blisters due to expansion of gas porosity upon heat treating can be a problem.
It is expected that more demanding structural applications of magnesium in the future will be produced by alternative casting processes (vacuum die casting, low pressure, squeeze or semi-solid casting) or using wrought alloy products such as extrusions. Grain size in these components will be more controllable and blisters will be essentially eliminated due to much lower porosity.
Various heat treatment schedules were explored in this study. For solution treatment (T4), an anti-germination schedule, 6 hours at 413C (775F), 2 hours at 352C (665 F) and 10 hours at 413C (775F),14
was used to minimize
grain growth during solutionizing. Solution treatments were conducted in a CO2
-circulated furnace (minimizing
oxidation), followed by a quench in warm water at about 70C (158F) to minimize distortion while maintaining a fast cooling rate. For the aging treatments, both T5 (artificial
All three alloys (AM60, AM80 and AZ91) showed excellent castability in the die casting trials.
Heat Treatment
* A: aluminum; Z: zinc; and M: manganese
Table 2. Chemical Composition (Weight %) of SVDC Castings
Figure 1. Tophat die casting with major dimensions. 52 International Journal of Metalcasting/Fall 10
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