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 metallurgi-


cal and mechanical changes that may take place during overcasting will help metalcasters determine the optimal method for successful- ly 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. Te adherence along the rod was measured in MPa, or N/mm2


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 locations in the insert. The four conditions (two pour- ing 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 ob- tained when the adherence was plotted against the local solidifica- tion time, i.e., the time elapsed between the beginning and end of solidification. For the range of lo- cal solidification times investigated (from 45 to 65 seconds) the ad- herencevaried 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 tem- peratures of 1,310F (710C) and 1,400F (760C) and copper tube initial temperatures of 77F (25C) and 617F (325C).


Mar/Apr 2014 | METAL CASTING DESIGN & PURCHASING | 37


For the four casting conditions


investigated, the differences in the measured values of the heat trans- fer 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 coef- ficient 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


These are the first four castings of a campaign.


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.


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