probably because of the lower coeffi- cient 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 tempera- ture of 77F (25C). Te overall cross section porosity is less than 1%. Te alloy consists of nearly pure alumi- num primary dendrites (white) with a smaller amount of Al-Si eutectic (dark). Te 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
were observed, implying that no sig- nificant amount of iron was dissolved in the stream of liquid aluminum. No modification of the steel
contact with the insert as a result of inverse segregation. No iron containing intermetallic phases such as AlFeMgSi (Chinese script) or Al5
FeSi (acicular)
Fig. 11a. The micrograph shows a copper tube insert in its surroundings.
structure near the interface was noticed. Te macro-hardness of the cold drawn mild steel was 226 HV0.5kgf (average of three readings). Te 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). Te copper tubes have been deformed because of the anisotropy in the compressive stresses resulting from the higher thermal contraction coef- ficient 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. Te spectrographic analysis of eight points in a casting (pouring
Fig. 10. This micrograph shows a typical aluminum-steel interface.
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 calcu- lated that an average tube thickness of 80 µm had been dissolved in the alu- minum liquid stream. Tis copper dis- solution 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. Tis 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 cop- per 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 sec- onds, 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.
Fig. 11b. A typical aluminum-copper inter- face is shown.
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 adher- ence is about three times less than the one measured with the steel rod inserts.
Fig. 12. This micrograph shows the copper oxide layer at the Al-Cu interface.
Tis 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.
February 2014 MODERN CASTING | 55
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