Low pressure permanent mold overcasting optimizes part characteristics.

Franco Chiesa, Guy Morin, Bernard Tougas, Centre de Métallurgie du Québec, and J.F. Corriveau, Collège de Trois-Rivières, Trois-Rivières, Québec, Canada

(Click here to see the story as it appears in February's Modern Casting.)

Among its potential uses, overcasting light metals onto metal substrate is a key enabling technology for vehicle weight reduction. Overcasting steel or copper with aluminum or magnesium allows one to take advantage of the strength of steel and the corrosion resistance and heat transfer capability of copper without compromising the light weight sought in many applications. Following the substitution of aluminum for ferrous castings in the automotive industry, further innovations involve adopting hybrid solutions where a mix of widely different materials are combined.

For instance, the high mechanical resistance of steel may be allied to the lightness of magnesium, as in the example shown in Fig. 1. Another spectacular example of a hybrid assembly is the BMW inline six-cylinder engine. In this instance, weight reduction was achieved by casting magnesium over aluminum which, unlike magnesium, resists the corrosive aggression of the cooling fluid. Overcasting can be advantageous in reducing machining cost or enhancing heat transfer, such as by embedding copper pipes in aluminum. Similarly, inserts may be used in aluminum castings to locally enhance their strength, heat transfer properties or wear resistance.

Aluminum and magnesium castings offer significant mass savings when compared with ferrous or copper parts. Hollow sections generally are more efficient in reducing stresses in a mechanical assembly. These sections may be obtained by overcasting tubes of “heavy” materials with aluminum, which can accommodate the complexity in shape offered by the metalcasting process.

Proving the Process

A metallurgical, mechanical and heat transfer study was conducted at the interface of steel rods and copper tubes overcast with aluminum A356 by the low pressure permanent mold process. Technology Magnesium & Aluminum Inc., Trois-Rivières, Québec, Canada, participated in the casting runs.

The first objective was to measure the mechanical adherence, expressed in kPa, at the steel-aluminum interface of 0.2-in. (6mm) cylindrical steel inserts overcast with aluminum A356 and, likewise, the thermal resistance at the copper-aluminum interface of copper tubes embedded in aluminum A356. This resistance, expressed by a heat transfer coefficient in W/m2/°C, was done for pouring temperatures of 1,310F (710C) and 1,400F (760C) and for insert initial temperatures of 77F (25C) and 617F (325C).

For each condition, the radiographs and metallographic structures at the interface were observed to assess surface conformity and possible soldering or dissolution of the insert. Filling and solidification modeling allowed the determination of local thermal conditions along the interface. The research attempted to correlate these thermal parameters to the measured properties at the interface, namely, the mechanical adherence for the steel rods and the thermal resistance for the copper tubes. This extends the quantitative results to a variety of insert dimensions and casting shapes.

The 0.2-in. (6mm) diameter steel rods and copper tubes were overcast in the thicker section (1.0 in. [26mm]) of a step casting as schematized in Fig. 2. Trapezoidal holders were fitted at each extremity of the  0.2-in. (6mm) rods and tubes for a precise positioning and easy extraction upon ejection. Figure 3 shows copper tubes and steel rods before preheating and insertion into the mold.

Figure 4 shows the flat face of the step casting after the mold is opened just before ejection. Two aluminum A356 pouring temperatures (1,400F [760C] and 1,310F [710C]) and insert temperatures (77F [25C] and 617F [325C]) were tested. Figure 5 shows step castings 1, 2 and 3 poured to bring the mold to a dynamic thermal equilibrium, along with the first casting (4) poured with an insert.

Thirty-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 adherence 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. The adherence along the rod was measured in kPa, or Newton per mm2 of interface. This 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 locations in the insert.

The steel insert was sectioned into six slices, as illustrated in Fig. 6. A consequence 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 conditions. The solidification time maps shown correspond to the solidification time for a pouring temperature of 1,310F (710C) and an insert at an initial temperature of 77F (25C). The four conditions (two pouring temperatures and two insert initial temperatures) 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. The results are shown in Table 1.

At any time, the numerical solution provides the thermal conditions at each point inside the insert, the casting and at the interface. This 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.
• Casting temperature at the interface when the insert reaches its maximum temperature.
• Time necessary to reach the liquidus.
• Time elapsed between the beginning and end of solidification (local solidification time).

The punch and die used to measure 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). The 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.

The 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.

The 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, the prevailing property required is a good thermal contact at the copper-aluminum interface. This thermal contact is expressed as a surface heat transfer coefficient, hAl-Cu, measured in W/m2/°C; hAl-Cu was determined for pouring temperatures of 1,310F (710C) and 1,400F (760C) and copper tube initial temperatures of 77F (25C)and 617F (325C).

The 1-in. (25mm) thick plate of 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 second. For the four casting conditions investigated, the differences in the measured values of the heat transfer coefficients were very small.

While the mechanical adherence of the copper tube inserts is less important than with the steel rods, it is nevertheless interesting to measure it. This was done on 0.25-in. (6mm) thick slices so the pressure force necessary to extract the insert was not greater than the shear resistance of the copper. The mechanical adherence at the copper-aluminum interface was found to vary between 5 and 9 MPa. It is three times less than what was observed when overcasting steel rod, probably because of the lower coefficient of thermal expansion of steel and the higher resistance opposed by steel to the compressive thermal stresses applied by the surrounding aluminum as it cools to room temperature.

Microscopic Analysis

A typical micrograph at the interface between steel rod and aluminum is shown in Fig. 10 for a pouring temperature of  1,310F (710C) and an insert initial temperature of 77F (25C). The overall cross section porosity is less than 1%. The alloy consists of nearly pure aluminum primary dendrites (white) with a smaller amount of Al-Si eutectic (dark). The secondary dendrite arm spacing (SDAS) is around 35µm with a slightly finer structure at the trailing end of the aluminum flow around the insert.

Some of the Al-Si eutectic was in contact with the insert as a result of inverse segregation. No iron containing intermetallic phases such as AlFeMgSi (Chinese script) or Al5FeSi (acicular) were observed, implying that no significant amount of iron was dissolved in the stream of liquid aluminum.

No modification of the steel structure near the interface was noticed. The macro-hardness of the cold drawn mild steel was 226 HV0.5kgf (average of three readings). The micro-hardness of the white phase (ferrite) was equal to 225 HV10gf while that of the dark constituent (perlite) was 261 HV10gf.

For copper tube overcast with aluminum, typical optical micrographs of the interface at two magnifications are shown in Figs. 11a and 11b, for a pouring temperature of 1,310F (710C) and an insert initial temperature of 77F (25C). The copper tubes have been deformed because of the anisotropy in the compressive stresses resulting from the higher thermal contraction coefficient of aluminum.

Similarly to what was observed with the steel inserts, the two materials match perfectly at the interface (Fig. 11b) without any welding or cross diffusion between the copper and the aluminum alloy.

The spectrographic analysis of eight points in a casting (pouring conditions:  1,400F [760C], 77F [25C]) showed evidence of copper dissolution into the melt, with copper contents varying from 0.25 to 0.27% while the original A356 alloy content was 0.08% Cu. From these results, it can be calculated that an average tube thickness of 80 µm had been dissolved in the aluminum liquid stream. This copper dissolution was much less with preheated inserts due to the protective presence of the copper oxide layer formed at the surface of the tube by the preheating process. This oxide layer, about 2µm thick, is visible in Fig. 12.

Aluminum Overcasting Conclusions

Pouring a series of plate castings in aluminum A356 over steel rods and copper tubes demonstrated the following:

1. The adherence at the aluminum-steel contact is purely mechanical. For local solidification times at the interface varying from 40 to 65 seconds, the adherence decreases from about 25 to 15MPa.

2. No discernible iron pick up is observed in the aluminum when overcasting steel rods.

3. Applying a T6 heat treatment on the aluminum plate decreases by half the adherence of the insert, very probably due to the stress relief brought about by the plastic deformation of the aluminum alloy during the solutionizing treatment.

4. The heat transfer coefficient at the copper-aluminum interface of the copper tube inserts varies little with the pouring and preheating temperatures. Its value is close to 10 kW/m2/°C.

5. Copper is partially dissolved into the aluminum melt, particularly with the room temperature inserts where no oxide is present at the surface.

6. No welding or cross diffusion occurs at the aluminum-copper interface. The mechanical adherence is about three times less than the one measured with the steel rod inserts. 

This article was adapted from “Overcasting Steel Rods and Copper Tubes in Low Pressure Permanent Mold,” presented at the 2013 AFS Metalcasting Congress in St. Louis.