aluminum. Similarly, inserts may be used in aluminum castings to locally enhance their strength, heat transfer properties or wear resistance. Aluminum and magnesium cast-
ings offer significant mass savings when compared with ferrous or copper parts. Hollow sections gener- ally are more efficient in reducing stresses in a mechanical assembly. These sections may be obtained by overcasting tubes of “heavy” materi- als with aluminum, which can ac- commodate the complexity in shape offered by the metalcasting process.
Proving the Process To test a method of overcasting to
make a hybrid metal casting, Tech- nology Magnesium & Aluminum Inc., Trois-Rivières, Québec, Canada, conducted casting runs for a metal- lurgical, mechanical and heat transfer study conducted at the interface of steel rods and copper tubes overcast
resistance at the copper-aluminum interface of copper tubes embedded in aluminum A356. This resistance, expressed by a heat transfer coef- ficient 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 radio-
Fig. 2. These are inserts (red) embedded in aluminum (gray).
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 in- terface of 0.25-in. (6mm) cylindrical steel inserts overcast with aluminum A356 and, likewise, the thermal
graphs and metallographic struc- tures 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 dimen- sions 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 investi- gated 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 coef- ficients for the steel rods and copper tubes, respectively. Metallographic
Tubes and rods, shown here before pre- heating, will have metal cast around them.
36 | METAL CASTING DESIGN & PURCHASING | Mar/Apr 2014
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