equilibrium, along with the first casting (4) poured with an insert. Tirty-eight step castings were
investigated in subsequent studies. As a rule, the same casting conditions were applied three times to assess the repeatability of the measured adher- ence and heat transfer coefficients for the steel rods and copper tubes, respectively. Metallographic and SEM microscopy around the interface were performed on some of those castings and radiographic shots allowed for verification of possible voids at the casting-insert interface.
Steel-Aluminum Mechanical Adherence
When overcasting steel rods, the usual property required is the mechanical adherence at the steel- aluminum interface. Te adherence along the rod was measured in kPa, or Newton per mm2
• Casting temperature at the inter- face when the insert reaches its maximum temperature.
• Time necessary to reach the liquidus.
• Time elapsed between the begin- ning and end of solidification (local solidification time). Te punch and die used to measure
Fig. 3. These are tubes and rods before preheating.
of interface. Tis
was done for pouring temperatures of 1,310F (710C) and 1,400F (760C) and insert initial temperatures of 77F (25C) and 617F (325C) at six loca- tions in the insert. Te steel insert was sectioned into
six slices, as illustrated in Fig. 6. A con- sequence of the symmetry is that each casting provides three repeated local solidification conditions. For instance, slices 3L and 3R in Fig. 6 are subjected to the same local solidification condi- tions. Te solidification time maps shown correspond to the solidifica- tion time for a pouring temperature of 1,310F (710C) and an insert at an initial temperature of 77F (25C). Te four conditions (two pouring tempera- tures and two insert initial tempera- tures) were modeled using a value of 1,550 W/m2
/°C for the mold-casting
interface heat transfer coefficient and a filling time of four seconds. Te results are shown in Table 1. At any time, the numerical solu-
tion provides the thermal conditions at each point inside the insert, the casting and at the interface. Tis data is useful to find the best correlation between the insert-casting mechanical adherence and an adequate thermal parameter such as: • Maximum insert temperature at the interface.
52 | MODERN CASTING February 2014
Fig. 4. This is the flat face of a step casting before ejection.
the force necessary to extract the insert from its overcast aluminum is shown in Fig. 7. By dividing the measured force by the surface area of the steel- aluminum interface, a value for the adherence is obtained in MPa (or ksi). Te maximum force is reached as soon as slip occurs at the steel-aluminum interface. Figure 8 shows a steel rod partially pushed out following the adherence test. Te best correlation was obtained when the adherence was plotted against the local solidification time, i.e., the time elapsed between the beginning and end of solidification. For the range of local solidification times investigated (from 45 to 65 seconds) the adherence tends to be higher for shorter solidification time. Te adherence of the insert was
found to be roughly divided by two when applying a T6 treatment to the aluminum casting.
The Copper-Aluminum Interface When overcasting copper tubes,
Fig. 5. These are the first four castings of a campaign.
was determined for pouring tem- peratures of 1,310F (710C) and 1,400F (760C) and copper tube initial temperatures of 77F (25C) and 617F (325C). Te 1-in. (25mm) thick plate of
the prevailing property required is a good thermal contact at the copper-aluminum interface. Tis thermal contact is expressed as a surface heat transfer coefficient, hAl-Cu
, measured in W/m2
/°C; hAl-Cu
Fig. 6. Sectioning the 25mm plate into six samples is illustrated.
the casting was cut off, and thermo- regulated hot water was pumped through the copper tube at a constant rate of 2.1 qt. (2 L)/min. An exposed junction thermocouple was inserted at the very center of the aluminum block (see Fig. 9). As the water flow initiated, the rise in temperature was recorded at one acquisition per sec- ond. For the four casting conditions
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