the aluminum alloy during the solutionizing treatment.


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


Fig. 3. This micrograph shows a typical aluminum-steel interface.


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 mate- rials 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 cop- per 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 alumi- num-steel contact is purely mechanical. For local solidi- fication 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.


Mar/Apr 2014 | METAL CASTING DESIGN & PURCHASING | 39


Fig. 4. The micrograph shows a copper tube insert in its surroundings.


5.Copper is partially dissolved into the aluminum melt, par- ticularly with the room tem- perature inserts where no oxide is present at the surface.


3.Applying a T6 heat treatment on the aluminum plate de- creases by half the adherence of the insert, very probably due to the stress relief brought about by the plastic deformation of


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.


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