Fig. 6. This microstructural image shows the progression of hot tearing.
Fig. 7. The distribution of shrinkage in an unetched microstructure sample is shown in a 1.3-in. section (left) and a 0.51-in section (right).
were eliminated by redesigning the risers by either using cylindrical insu- lating risers with increased contact or by insulating the current top risers.
Successfully Filling the Mold
Te bracket component was selected because of its fairly com- plex geometry that could be typical of structural components. Given its dimensions and geometry, the bracket is difficult to produce by traditional permanent mold casting techniques, especially when using alloys that are prone to hot tearing. Te mold was preheated by gas
torch, and two thermocouples (TC) were placed in the mold to monitor the mold temperature (Fig. 5). Te thermocouples were located at the top (TC1) and bottom (TC2) of the moving half of the mold. Infrared (IR) pyrometers also measured mold surface temperatures at two loca- tions. Te mold surface temperature between the two risers was mea- sured by IR3, and IR4 measured the temperature of the surface near the flange under the hoop section of the casting. Te thermocouples
and pyrometers were placed 0.75 in. (19mm) from the surface of the die cavity. Because TC1 and IR3 were closer to the pouring cups than TC2 and IR4, the mold temperatures at TC1 and IR3 usually were higher. When the mold temperature in
certain sections of the mold rose above 932F (500C), some castings broke during ejection. To lower the mold temperature and reduce break- age, engineers turned the cooling water flow rate to its highest level. When the die cycle dwell time
was greater than 290 seconds, the extended metal-mold constraint time tended to create more severe hot tearing. Te microstructure image in Fig. 6 shows the progression of hot tearing. Maintaining a die cycle dwell time of 200-230 seconds and maintaining the mold temperature between 860-896F (460-480C) provided the best results. Increasing the mold’s temperature above 896F (480C) tended to lead to mechanical hot cracking due to ejection stresses. Te casting trials performed
at the partner foundry using the production tooling designed to
be used with A356 were successful. Many of the A206 castings were free of hot tearing. Unetched microstruc- tures showed minimal shrinkage defects associated with poor feed- ing during solidification (Fig. 7). Additionally, the sections A and B showed relatively fine grain structure in etched samples (Fig. 8). There appears to be minimum and maximum mold temperatures allowed in particular areas of the mold (related to casting design) to prevent hot tearing. High spikes in mold temperature in locations close to hot spots in the mold can result in casting breakage during ejection. But reducing hot tearing is possible through effective thermal manage- ment. Properly locating temperature sensors and controlling heating and/ or cooling in casting hot spots can reduce conditions that lead to hot tears. Additionally, proper grain refinement and control of casting cycle times can minimize hot tears. Simulation and computer modeling
programs can help streamline design efforts and identify potential problem areas, including those that could lead to hot tearing. While this study was able to produce quality castings, more research must be done. Tis project could lay a foundation for future work on developing appropriate charge mate- rial and primary/scrap ratio for A206. Achieving that goal will make the cast- ing of alloy 206 in permanent molds more economically viable, which then will lead to wider acceptance within the metalcasting industry.
Fig. 8. The distribution of shrinkage in an etched microstructure sample is shown.
Tis article was based on the presentation “Permanent Mold Casting of a Structural Component from Al Alloy 206” from the 2014 Metalcasting Congress in Schaumburg, Ill.
September 2014 MODERN CASTING | 33
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