New Possibilities for High Strength Aluminum
Modeling and analyzing core gas evolution and metal solidification behavior can aid in the prediction and prevention of porosity caused by core gas.
Y. Fasoyinu, D. Weiss and J. Shah
(Click here to see the story as it appears in the September issue of Modern Casting.)
The U.S. Department of Energy funded a number of proposals in 2004 under the Energy Saving Melting and Revert Reduction Technology (E-SMARRT) program, which included a project lead by Canmet Materials (CMAT), Hamilton, Ontario, Canada, on permanent mold casting light metal alloys. CMAT focused specifically on alloys that were considered difficult to pour in a permanent mold but offered significant energy savings by reducing revert (or returns).
A206 is one of the highest strength structural alloys. Yield strengths in excess of 45 ksi are routinely achieved in the T71 temper. It has very good high temperature properties with little reduction in strength up to 350F (177C), making it an ideal alloy for cylinder heads and other elevated temperature applications.
When poured into a permanent mold, however, A206 has a tendency for hot tearing, which is caused by thermal contraction in the late stages of solidification. Because of this difficulty, A206 most often is cast in green or chemically bonded sand molds. A recent research project aimed to establish processing parameters for A206 in permanent mold castings that prevent hot tearing for structural components in automotive and other applications.
Overcoming Obstacles in Simulation
When working with the 200 series of aluminum alloys, chemistry control is more important than with 300 series aluminums. Iron and silicon levels need to be minimized to achieve the expected properties. Impurities from insufficiently cleaned crucibles or improperly mixed alloys can lead to unsuitable results. Metalcasters must ensure tools and ladles are clean. Process controls to minimize changes in chemistry also are essential.
In 200-series alloys, mechanical properties depend on controlling the iron and silicon levels in the alloy. In A206, iron levels need to be controlled to less than 0.1% and silicon levels to less than 0.05%. Castings made with specified chemistries and solidification rates typical of the permanent mold process can develop yield strengths of 50 ksi coupled with double digit elongation.
To research the ability to cast A206 in permanent mold, a case study on a mounting bracket casting used in medical equipment was conducted. The component, shown in Fig. 1, is currently produced in A356 in a 4140 steel permanent mold. The current design ensures the A356 alloy can be cast without hot tearing. The A356 component also needed to pass grade C radiographic quality.
The team’s key objectives were to establish and refine casting processing parameters, including:
- Initial mold preheating
- Pouring temperature.
- Mold fill time.
- Total die cycle dwell time.
Because the casting was currently in production, tooling was in place, so the research team could not make any changes to the design that would disrupt production during casting trials. Before initial trials, the team used simulation software to ensure the gating system was properly designed before pouring. Predictive modeling allowed the research team to ensure the casting could be poured at a reasonable mold temperature that will avoid hot shortness but still have enough strength to withstand ejection stresses.
3-D CAD models of the casting with rigging were provided by Eck Industries Inc. to Product Development & Analysis for analysis. The simulation suggested minor shrinkage would be discovered in the areas under risers when the die was opened (shown in Fig. 2). Additionally, hot tearing was likely near the top cross bar and in the transition from the post to the shaft area (Fig. 3). Toward the end of solidification, hot tearing looked to develop where the top post transitioned into the journal areas (Fig. 4). These problematic locations correlated with observed hot tearing on the bracket component.
Hot tearing tended to develop when the fraction solid, the percentage of solid phases in the solidifying metal, ranged from 75-90%. The defects were reduced or eliminated when the fraction solid was reduced to 65-75%, due to the presence of liquid metal that fed the solidification shrinkage.
Shrinkage underneath the top risers and the junctions were eliminated by redesigning the risers by either using cylindrical insulating risers with increased contact or by insulating the current top risers.
Successfully Filling the Mold
The bracket component was selected because of its fairly complex 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.
The mold was preheated by gas torch, and two thermocouples (TC) were placed in the mold to monitor the mold temperature (Fig. 5). The 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 locations. The mold surface temperature between the two risers was measured by IR3, and IR4 measured the temperature of the surface near the flange under the hoop section of the casting. The 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 breakage, 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. The 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.
The 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 microstructures showed minimal shrinkage defects associated with poor feeding 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 management. 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. This project could lay a foundation for future work on developing appropriate charge material and primary/scrap ratio for A206. Achieving that goal will make the casting of alloy 206 in permanent molds more economically viable, which then will lead to wider acceptance within the metalcasting industry.
This 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.