31-05-2012, 03:46 PM
fastigue properties of a semi -solid cast
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introduction
It has long been known that the fatigue endurance of castings
is sensitive to the size of casting defects [1]. There is some
evidence that it is also affected, but to a lesser extent, by
solidification time as reflected by dendrite arm spacing and
the size of silicon particles [2, 3]. The porosity of semisolid
castings can be very low and so the process should be capable
of providing castings that have superior fatigue properties.
Surprisingly, however, there are few references to the
fatigue properties of such castings although semisolid
Materials
Thixotropic billets were supplied by Aluminium Pechiney.
The composition of the castings was found by ICP-AES (Inductively
Coupled Plasma / Atomic Emission Spectrometry)
and is shown in Table 1. The composition conforms to the
United States Aluminum Association specification for A356
with the addition of Sr as a chemical modifier.
Design of castings, casting procedure and
heat-treatment
A stepped plate casting (Fig. 1) was used for this work, casting
being carried out in an Ube horizontal die-clamping, vertical
injection, squeeze casting machine, which applied a
squeeze pressure of 100 MPa soon after the cavity was filled.
The operational conditions have been described elsewhere
[7]. The 10 mm and 15 mm steps were cut from the plates
and were given a T6 heat-treatment of 6 hours at 540°C,
followed by quenching into water at 25°C, natural ageing for
20 hours and, finally, artificial ageing for 6 hours at 170°C.
This produced a slightly under-aged condition.
Specimen design, tensile tests and hardness tests
The heat-treated bars were machined into cylindrical tensile
and fatigue test specimens. Tensile specimens had a gauge
length of 20 mm and a gauge diameter of 5 mm. Tensile tests
were made using a screw-driven machine, the strains being
measured with an extensometer. Fatigue specimens had the
same design except that they had a 10 mm gauge length to
avoid buckling in compression. Vickers hardness measurements
were made on the grip ends of selected specimens after testing.
Fatigue tests
Tests were carried out in a servo-hydraulic machine in sinusoidal
force control. All tests were done at a stress ratio, R,
of -1 (i.e., zero mean stress). The ambient temperature varied
between 22 and 26°C and the relative humidity was usually
between 30% to 40%. Care was taken to align the load
train and evidence of the good alignment was that the locations
of the fracture origins with respect to the load frame
were random over the course of these experiments.
Microstructure and fractography
Optical metallographic observations were made of the cast
microstructure. In addition, the fatigue fracture surfaces were
examined in a low-power stereomicroscope for evidence of
defects near the initiation site. Some fracture surfaces were
also examined by scanning electron microscopy using secondary
electron imaging and light-element energy-dispersive
X-ray spectroscopy.
Metallography
The microstructure was typical of semisolid castings [7] and
no differences could be observed between the 10, 15 and 20
mm thick sections of the plates. The 5 mm section did contain
some subsurface oxide films and evidence of unsteady
filling, but in any case was too thin to be used to prepare test
samples. The 20 mm section generally showed some slight
shrinkage porosity in X-radiographs and so these sections
were also not used for fatigue samples.
Tensile and hardness properties
The tensile and hardness properties of the heat-treated castings
are shown in Table 2 and are typical of the properties
for A356 alloy heat-treated to a T6 condition.