Page 273 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

252 Chapter 10 Fundamentals of Metal Casting

EXAMPLE l0.2 Casting of Aluminum Automotive Pistons
Figure 10.16 shows an aluminum piston used in higher engine speeds and reduced counterweighting in
automotive internal combustion engines. These prod- the crankshaft, and the higher thermal conductivity
ucts must be manufactured at very high rates with allows for more efficient heat transfer from the engine.
very tight dimensional tolerances and strict material The H13 tool-steel mold is preheated 200° to
requirements in order to achieve proper operation. 450°C, depending on the cast alloy and part size.
Economic concerns are obviously paramount, and it Initially, preheat is achieved with a hand-held torch,
is essential that pistons be produced with a minimum but after a few castings, the mold reaches a steady-
of expensive finishing operations and with few state temperature profile. The molten aluminum is
rejected parts. heated to between 100° and 200°C above its liquidus
Aluminum pistons are manufactured through temperature, and then a shot is placed into the infeed
casting because of the capability to produce near-net section of the mold. Once the molten metal shot is
shaped parts at the required production rates. in place, a piston drives the metal into the mold.
However, with poorly designed molds, underfills or Because ofthe high thermal conductivity of the mold
excess porosity can cause parts to be rejected, adding material, heat extraction from the molten metal is
to the cost. These defects were traditionally controlled rapid, and the metal can solidify in small channels
through the use of large machining allowances before filling the mold completely. Solidification
coupled with the intuitive design of molds based on usually starts at one end of the casting, before the
experience. mold is fully filled.
The pistons are produced from high-silicon As with most alloys, it is desired to begin
alloys, such as 413.0 aluminum alloy. This alloy has solidification at one extreme end of the casting and
high fluidity and can create high-definition surfaces have the solidification front proceed across the volume
through permanent mold casting (see Section 11.4); it of the casting. This results in a directionally solidified
also has high resistance to corrosion, good weldability, microstructure and the elimination of gross porosity
and low specific gravity. The universal acceptance that arises when two solidification fronts meet inside a
of aluminum pistons for internal combustion engine casting. Regardless, casting defects such as undercuts,
applications is due mainly to their light weight and hot spots, porosity, cracking, and entrapped air zone
high thermal conductivity. Their low inertia allows for defects (such as blowholes and scabs) can occur.

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FIGURE |0.|6 Aluminum piston for an internal combustion engine: (a) as
cast and (b) after machining. The part on the left is as cast, including risers,
sprue, and well, as well as a machining allowance; the part on the right is the
piston after machining. Source: After S. Paolucci.