Results and Discussion
In the three following sections, results obtained from the simulations and casting trials are presented and discussed. First, the numerical and experimental results from the origi- nal casting assembly are analyzed to establish a base case. Then, the numerical results of the mould filling and solidi- fication processes based on the reworked chills and gating system are listed. In the last section, the results from the multi-objective optimization problem (riser volume and chill size optimization) are presented. At the end a compari- son of the casting yield of the original and optimized designs is carried out.
Simulation and Casting Trial Results— Original Layout
The original casting arrangement, Figure 4, was simulat- ed both with respect to filling and solidification using the casting conditions listed in Table 1. The filling analysis is supposed to indicate whether the current gating system will provide uniform filling without any melt aspiration in the downsprue or surface turbulence that would likely lead to excessive oxidation of the propagating melt, caus- ing various filling-related defects, e.g. reoxidation inclu- sions, entrapped air pockets, etc.28
The primary source of
oxygen in reoxidation inclusion formation is air, which contacts the metal stream during pouring as well as the metal free surface in the mold cavity during filling. The oxide mixture that forms during pouring of carbon and low-alloy steel is partially liquid,29
as opposed to the solid oxide films or particles that form during casting of high-
alloy steel or light metals. Figure 6 captures three differ- ent stages of the filling process.
It can be seen in Figure 6(a) that due to a constant cross- section area over the entire downsprue, the melt starts to spire from the mould walls and becomes oxidized. This phenomenon can be explained by the continuity equa- tion. The melt experiences a free fall from the nozzle of the pouring ladle down to the bottom of the gating system. During the free fall, it accelerates due to the effect of gravi- ty and changes its area (the area decreases with the increas- ing velocity). In order to compensate for the area reduction resulting in the aspiration from the mould walls, one has to decrease the area of the downsprue accordingly. The nearly ideal solution would be an application of a stream-lined gating system.30
However, for such a large cast part, this solution is unfeasible. The stream-lined gating system is used mainly in gravity die casting. Another option would be the use of “choke” conical elements at several locations in the downsprue.
Figure 6(b) shows that due to no velocity control during the early stage of the filling process, the melt reaches the mould cavity with a high velocity (approx. 5m/s). A very rapid en- trance naturally leads to a formation of fountains (1.47 m high) inside the mould cavity. When the melt starts to fall down again, it splashes, becoming highly turbulent and dis- integrated. In most of the bottom-filled casting assemblies it is a difficult task to fully avoid this formation. Nevertheless, it should always be the primary objective of a designer to design such a gating system with all necessary attributes to keep this phenomenon at a minimum.
(a) 1% filled International Journal of Metalcasting/Fall 10
(b) 2.5% filled Figure 6. Three different stages of the filling process of the bottom-filled forging ram. 67
(c) 16% filled
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