Metalcasters often talk about castability, but what does that mean? Castability is the ease of forming a quality casting. A very castable part design is one that is easily developed, incurs minimal tooling costs, requires minimal energy, and has few rejections. Castability can refer to a part design or a material property. Castability is also relative. Almost all alloys can be cast with the proper techniques and part design. You may need to be a more careful with part design in more difficult to cast alloys than simpler alloys because it has a big impact on the success of the casting. Figure 1 is an example of a part AFS Corporate Member Eck Industries (Manitowoc, Wisconsin) was asked to produce out of 206 aluminum copper alloy. A natural solidification on it was performed to better understand where the problems may occur in the casting and whether all the problem points could be solved through gating and risering techniques with the current casting design. The simulation showed a number of areas in the casting that were prone to porosity. With this geometry, there was not a good way to chill them or feed them. It was clear some design changes were needed to make an acceptable part. The original casting might be producible in the more common and castable A356 alloy (left), but in this case a design change was needed to be cast in the more difficult aluminum copper alloy (Fig. 2). It was easily produced in aluminum copper alloy (right) with the design change (right). Table 1 lists properties of various aluminum alloys. Castability is usually referenced to the most castable alloys such as 356. Resistance to hot tearing, pressure tightness, fluidity and shrinkage tendency are included in the basic measurement of an alloy’s castability. Higher numbers in the table indicate a castability normally considered not as good. Multiple mistakes in the 300 series of alloys can be made while still producing an acceptable part, but metalcasters don’t have that luxury with the harder-to-cast alloys. The data in Table 1 is historical and does not reflect modern casting techniques. Metalcasters today have a better understanding of how to chill castings, pour cleaner metal, and use grain refinement and strontium modification (in silicon-containing alloys) to improve casting characteristics. So, the numbers given are based on old techniques and not what the industry knows today. It is likely the castability of these alloys are better than Table 1 indicates. Advantages of Uncommon Alloys Most aluminum alloys are castable given the following restraints: A gating system that can deliver metal as needed given the fluidity of the alloy. An established thermal gradient to feed shrinkage. Clean metal. Velocity that is low enough to avoid formation of oxide defects in the gating system that end up as casting defects. Why bother with harder-to-cast alloys? Why not just pour 356 all the time and make it easier on everyone? Demand for higher performance cast alloys is increasing. More structural castings are being used commercially in everything from automotive to aerospace applications, where soundness is important. Stronger alloys and alloys with unique properties such as wear resistance, stiffness, and high temperature strength tend to require more care during casting, but they offer properties that make it worthwhile. Better alloys also drive better designs. If a designer uses a stronger alloy, they can make a more efficient, lightweight design. However, growth of these alloys is constrained by foundry capabilities to pour them. When converting heavier castings or structurally-based castings from forgings or weldments, most of the easy-to-pour aluminum alloys do not develop tensile or yield strength properties in the required range (Table 2). However, the 206 alloy in different tempers can produce tensile strength in the 50s and yield strengths in the 30s to 50s. Elongation is also very good. For aluminum casters who want to gain new business through conversions or weight reductions, alloys with higher properties are good to have in your back pocket. Metal matrix composites are even less common than 206 but they offer very good wear and abrasion resistance, elastic modulus for stiffness and creep properties. Understanding and Using the Niyama Criteria Al-Si alloys without copper are considered to have short freezing ranges, making them easier to feed. Generally, long freezing range alloys contain copper, large amounts of magnesium or tin. Silicon improves feeding characteristics, so having silicon makes it easier to pour. Not having any or a low amount makes it more difficult. Table 3 shows alloy and freezing ranges of various aluminum alloys, along with their percent silicon. Producing sound, high-quality castings is fundamentally about controlling the velocity of the metal in the system and establishing enough of a thermal gradient so proper feeding can occur. Thermal gradients for proper feeding is required in all alloys but need to be higher in long-freezing range alloys. The Niyama criteria is an equation that provides a good way to think about how to cast long freezing range alloys and gives you a relationship between the cooling rate and thermal gradient. Different alloys have different Niyama criterion. The equation is helpful for figuring out how to chill a casting to create a thermal gradient. The equation for the Niyama criteria is: Ny=G/ √T It is derived from Darcy’s law which relates interdendritic feeding-flow velocity to the pressure drop across the mushy zone. The Niyama Criteria (Ny) has a threshold value. If the Ny is below that value, shrinkage is typically present. The criteria is sensitive to alloy type, casting conditions and applied pressure (i.e. low pressure casting). So, for instance, the Ny of a 200 series alloys needs to be higher than a 300 series alloy in order to be successfully cast. This can be applied across all alloys, including a lot of steels. The absolute number is less important than trying to understand where it comes from and how to use the information. Table 4 shows examples of castings and the level of measured gradient needed to avoid shrinkage porosity. A greater gradient is needed in nonferrous alloys in general, and more in a high copper alloy than a 356-type alloy. Those thermal gradients are fairly large, so a casting engineer can’t just put one riser on a casting and expect it to feed properly. From a practical application, more risers are needed in long freezing range alloys to be able to better control the thermal gradient. Grain Size and Grain Refining Yield strength is inversely proportional to grain size. When trying to develop high mechanical properties, chilling as much as possible is tempting. But those two things are at odds because grain size is reduced at higher cooling rates. The goal is to maximize the thermal gradient as much as possible so high cooling rates can be applied without causing high porosity levels. Without a large enough thermal gradient, over chilling the mold will inhibit the metal from feeding the entire casting. For example, Fig. 3 shows a flat plate casting with a riser on one side and chill on another to establish a thermal gradient. Adding a chill increases the thermal gradient and cooling rate. When a casting does not develop the desired mechanical properties in the chilled area, often the temptation is to add a bigger chill. That may help slightly with the thermal gradient but probably has a bigger effect on the cooling rate. This causes the Niyama criteria to be a smaller, meaning porosity could develop near the chill. Another way to improve castability and hot tearing is grain refining. Grain refining is a potent tool that metalcasters can use to control hot tearing in alloys prone to it. Most uncommon alloys cannot be poured successfully without good grain refining practice in your foundry. Grain refining is almost always a good idea and it improves how an alloy feeds. That will results in a sounder casting and higher mechanical properties than without using grain refining. Composite Velocities Reduction of velocities to less than 20 in./sec. benefits the quality of all castings. Damage due to high metal velocities always occurs but the amount of damage is very high in composite type alloys. Composites behave differently than most metal alloys. In composites, hard particles tend to stabilize bubbles that can occur, which tends to make them visible on the surface of the casting after blast cleaning. Figure 4 shows 20% SIC composite castings poured at 50 inches per second. Simulation tools help to understand velocity developments. Velocities above 20 inches per second create significant casting defects in composites. The gating was reworked, and filters were added to control the metal velocity to under 20 inches per second. Without any other changes, a good casting was produced solely through velocity control (Fig 5). Good casting designs and control of melt quality are essential for the manufacturing of high quality castings. Understanding and controlling thermal gradients and metal velocity make it possible to manufacture most complex castings in virtually any alloy.