pour transfer system (and the associated melt treatment) is the main reason for the high level of quality associated with wrought aluminum products.


This suggests that improved metal handling in the foundry would significantly improve aluminum castings; and more- over, produce a consistency of quality and reliability nor- mally associated with wrought alloys. Unfortunately, before realizing this improvement, there is one last hurdle to clear. The metal must enter and fill the mold in a way that does not produce additional oxides, or other defects. The impor- tance of quiescent metal flow was demonstrated clearly by the pioneering research of Campbell and co-workers,29


who


established that the metal gate velocity must be less than a critical value (0.5 m/sec) to avoid the formation of oxide films. Subsequent studies demonstrated the importance of runner design30, 31


on tensile32 and fatigue33 properties of


aluminum castings. In a sense, this problem represents the ‘last frontier’ for aluminum foundrymen. Solving it would allow us to produce oxide-free, net-shaped castings having the same reliability as forgings and machined wrought alloy components.


This objective is probably best accomplished when the metal transfer/treatment system and the casting process are considered and designed together, to avoid the prob- lem of oxide film generation. One example is the Cos- worth low-pressure sand casting process,34


1. Top Poured and Unfiltered: 12 mm bars were poured from the top of the mold, using a pour cup, down sprue and no filtration in the mold. In this casting, the fall of metal was in excess of 150 mm. So, the theoretical maximum metal velocity was 1.7 m/sec. This was in excess of the critical flow velocity of 0.5 m/sec, so there was potential for metal turbulence and oxide production.


2. Bottom Filled and Unfiltered: These castings were bottom filled, but no precautions were taken to eliminate turbulence, and no filters were used in the mold.


3. Bottom Filled and Filtered: A filter was placed in the runner. This ensured that the metal flow into the mold was not turbulent. This was confirmed by real time, high speed radiography of the mold during filling.


4. Bottom Filled, Filtered and Sr Modified: This condition was the same as (3) above, except the melt was modified with Sr.


The quality of each casting was determined by calculating the Weibull modulus associated with the distribution of ulti- mate tensile strength of the bars. The modulus is determined by using the equation:


Equation 13 designed to


eliminate metal turbulence during mold filling. Metal is transferred via an electromagnetic pump into the bottom of the mold, minimizing the generation and entrapment of oxides caused by falling metal. Very low porosity and a high casting quality are obtained. This process was used for many years by Cosworth to produce Formula 1 and CART racing engines. It was later adapted by Ford to produce engine blocks. Another approach is a level pour transfer system with in-line degassing and filtration sta- tions. This type of metal delivery system is now being used by a number of foundries.35


But most cast shops have


not completely solved the challenge of filling the mold without generating oxides. It is common practice to place a holding furnace with a dip-out well at the end of the launder, and ladle metal into the mold with a robot. The ladling operation will generate some oxide films. One so- lution, adapted at Littlestown Hardware and Foundry,36


is


to use a molten metal pump. It is also possible to place a dosing furnace at the end of the level pour transfer sys- tem. This approach was taken by Nemak.37


It will now be instructive to consider an example of how metal transfer and mold design affect the casting quality. What follows is taken from the 1994 study of Green and Campbell.32


a diameter of 12 mm (1/2 inch). These were poured from a heat of Al-7Si-0.42Mg (356) alloy containing 0.30% Fe and 0.17% Mn. They poured sand castings under four dif- ferent conditions:


18


where FX


is the cumulative fraction of tensile failures that occurred at X, a specific value of the mechanical property un- der consideration (in this case, tensile strength), and σ is the value of X at which 37% (1-1/e) of the sam- ples survive, and m is a material constant, which determines the width (or spread) of values. This is called the modulus.


The results obtained for the four different designs of the casting are plotted in Figure 16.


In many respects the Weibull modulus is a better measure of material reliability than conventional statistics.38


In the latter They made simple round bar castings having


a Gaussian distribution is assumed and variation about the mean is determined. This is the approach commonly taken by automotive companies, who specify minimum properties using ±3σ. However, the distribution of mechanical proper- ties is not really Gaussian. It is skewed. On one side we have the ‘maximum’ quality possible, associated with defect-free areas. On the other side there are areas (or volumes) of ma- terial which contain flaws. The Weibull analysis has been used for many years to describe the probability of fracture in ceramics, considering the distribution and size of flaws in the material. Taking the same approach in castings ap- pears to offer several benefits. Firstly, Weibull plots clearly


International Journal of Metalcasting/Winter 11


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