Low pressure permanent mold overcasting optimizes metal casting 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 March/April 2014's Metal Casting Design & Purchasing.)

As vehicle weight reduction continues to be a driver in part design and development, various new strategies are being developed to provide strength at reduced weight. One strategy is the process of casting a light metal such as aluminum or magnesium onto a heavier metal substrate. 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

To test a method of overcasting to make a hybrid metal casting, Technology Magnesium & Aluminum Inc., Trois-Rivières, Québec, Canada, conducted casting runs for a metallurgical, mechanical and heat transfer study conducted at the interface of steel rods and copper tubes overcast with aluminum A356 by the low pressure permanent mold process. 

The first objective was to measure the mechanical adherence, expressed in MPa, at the steel-aluminum interface of 0.25-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 measured 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. [25mm]) of a step casting as schematized in Fig. 2. 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. 

Understanding the metallurgical and mechanical changes that may take place during overcasting will help metalcasters determine the optimal method for successfully producing hybrid metal castings to reduce weight in vehicles. 

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 MPa, or N/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 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 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 adherencevaried between 15 and 25 MPa (2.1 to 3.6 ksi); it was higher for shorter solidification times.

The Copper-Aluminum Interface

When overcasting copper tubes, the prevailing property required is a good thermal contact at the copper-aluminum interface. A surface heat transfer coefficient  was determined for pouring temperatures of 1,310F (710C) and 1,400F (760C) and copper tube initial temperatures of 77F (25C)and 617F (325C). 

For the four casting conditions investigated, the differences in the measured values of the heat transfer coefficients were very small. 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, hence the higher resistance opposed by steel to the contraction of the surrounding aluminum as it cools to room temperature. 

Microscopic Analysis

Fig. 3 shows a typical micrograph at the interface between the steel rod and the aluminum. 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.

Some of the Al-Si eutectic was in contact with the insert as a result of inverse segregation. No iron containing intermetallic phases 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 macrohardness of the cold drawn mild steel was 226 HV0.5kgf. The micro-hardness of the white phase (ferrite) was equal to 225 HV10gf while that of the dark constituent (perlite) was 261 HV10gf.

The copper tubes overcast with aluminum were 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. 4) without any welding or cross diffusion between the copper and the aluminum alloy.

The spectrographic analysis of eight points in a casting 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 a copper oxide layer formed at the surface of the tube during preheating. 

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 45 to 65 seconds, the adherence decreases from 25 to 15 MPa (3.6 to 2.1 ksi).
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.