Technical Article Continued from pg 19


though the material is not 100% solid, it can no longer flow any reasonable length to fill out a section. Flow essentially stops and is frozen in the state that it is left in. An exception that is worth pointing out is when the pressure is sufficiently high. Similarly to a fluid passing through a porous medium, given enough pressure to overcome the pressure drop created over the small length scale of high solid fraction material, the remaining liquid metal can been forced to move. The amount of pressure needed would be dependent on the solid fraction and other external forces that may exist. As the metal continues to cool through the solidification interval it reaches its solidus temperature. The solidus temperature is the


temperature at which the alloy material is 100% solid. At this point, metal has no ability to flow and will remain quite literally frozen in place. Even though the coherency point is the important stop condition to consider when evaluating misrun, knowing the solidus temperature caps off the freezing range and along with solidification rate can help determine how quickly the material will transition through this temperature interval.


When considering solving a misrun


issue using changes in temperature, the first thing that is done is to increase the pouring temperature; the logic being that given a constant heat extraction rate, you will be able to increase the temperature of the advancing melt front when it reaches thin features and that it will be more likely that temperature will be near or above liquidus temperature and thus remain fluid enough to fill out the region. Figure 1 demonstrates this. The turbo wheel depicted in the image on the left has a pouring temperature of 2925° and the image on the right has a pouring temperature of 3000°F. The result shows the temperature at which each region of the casting is filled. The yellow regions are locations that have metal near the pouring temperature and the blue and purple regions are nearer


22 ❘ November 2022 ®


Figure 2: The velocity decreases as it reaches the impeller vanes due to the flow direction of the metal in the casting cavity.


to the liquidus temperature where the material begins to freeze. You can see that the blades fill at higher temperatures for the casting on the right, the higher pouring temperature, than on the left, the lower pouring temperature. However, there are a few problems with increasing the pouring temperature that must be weighed against other factors before committing to this route. Firstly, when you increase the pouring temperature you will also need to increase the tap temperature, and further still, increase the melting temperature. Increasing the melting temperature can have a few detrimental effects such as damaging furnace


lining or crucible


and thus decreasing its life, increasing the damage of the liquid melt due to absorption of gases and oxidation at high furnace temperatures, among others. Secondly, when you increase the pouring temperature you can also have debilitating effects during mold transfer and mold filling. Similar to the furnace, the ladle lining and protection of the liquid melt is needed during metal transfer. Air can be entrained at a higher rate and oxidation more readily when the temperature is higher. Further, the temperature could not be the root cause of the issues. Issues with shell permeability or low velocity


while filling the casting cavity could also be at play. The superheat applied to the melt must be minimized while maintaining sufficient temperature to battle heat extraction during mold filling and so other factors must be consider in addition to filling temperature must be evaluated.


Shell temperature works in tandem with pouring temperature because if you pour hot material into a cold shell, non-fill defects would almost be certain; not to mention cracked shells. What gives investment castings an advantage over sand casting is the ability to heat up the mold. Filling up a shell at elevated temperatures allows for thinner sections to be filled out that wouldn’t be achievable in sand casting. However, there are limits to this. You may be limited by how hot your preheat furnace can operate at consistently and with limited maintenance. You may also have limits in shell transport. The shell needs to come out of the furnace and poured as quickly as possible. The longer there is a delay, the more the shell cools and can contribute to misrun defects.


Velocity The velocity of the melt, or how fast the metal is moving in a given direction, can determine if the casting cavity


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32