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ment of the mold before solidification is complete can pro-
mote oxide entrapment.
Casting —
This process usually leads to melt turbulence
to a degree dependent on the pouring technique and gating
system. During casting, melt turbulence caused by poor
gating design is generally associated with high surface tur-
bulence and greater entrainment of oxide films. Entrained
oxides are typically double oxide film defects (bifilms) com-
prised of folded MgO films
[9]
. A fresh melt surface is also
exposed to the atmosphere, further increasing the possibil-
ity of inclusions being entrained within the melt.
Assessment of inclusions in Mg alloys
A summary of methods used or considered to assess
metal cleanliness in the Mg industry is shown in Fig. 1, al-
though none of these methods are considered standard for
determining Mg melt cleanliness. Leading inclusion assess-
ment techniques include:
Metallographic techniques —
This relatively simple
method uses a sectioned sample of the cast product,
which is ground, polished, and examined using metallo-
graphic techniques. Metallography is usually combined
with image analysis to determine particle shape, size,
number, and distribution, or with scanning electron mi-
croscopy (SEM) and energy dispersive x-ray spectroscopy
(EDS) to determine the nature of inclusions and possible
sources of melt contamination
[12]
. Drawbacks associated
with classic metallographic techniques include small sam-
ple sizes, which may not be representative of the entire
casting, and the fact that inclusion content measurements
are done “offline” after the casting has been produced.
HMIAM —
With the hydro magnesium inclusion as-
sessment method (HMIAM), a known volume of the melt
is drawn through a stainless steel filter by vacuum. Inclu-
sions accumulate within or on top of the filter. Once the
melt has solidified and cooled, the filter and its contents
are sectioned parallel to flow direction and examined with
an optical microscope. The amount of inclusions in the
melt is measured as the volume of particles per unit weight
of metal drawn through the filter. Unlike metallographic
techniques, HMIAM is performed “online”
[7, 8]
. One disad-
vantage to this method is that the quantity and morphol-
ogy of oxides in ingots or die cast components may be
different from those observed on the filter due to a differ-
ent melt temperature
[10]
.
Light reflectance —
This technique is based on differ-
ences in optical characteristics between Mg and MgO and
involves fracturing a gravity cast sample and examining it
under the aperture of a brightimeter. The sample is illumi-
nated at a 45° angle to the fracture surface and reflected
light intensity is measured. Because inclusions have differ-
ent optical properties compared to the matrix, incident
light will be scattered at the specimen surface due to mul-
tiple reflections and refractions. In Mg alloys, MgO inclu-
sions absorb more light than the matrix, and as a result,
specimen reflectance is reduced. The reflectance should
therefore correlate with the oxide inclusion content in the
specimen
[11]
. However, this technique, while simple and in-
expensive, is unable to distinguish between different inclu-
sion types and sizes
[7, 10]
. Both HMIAM and light reflectance
techniques are shown in Fig. 2.
LiMCA —
The liquid metal clean-
liness analyzer (LiMCA) instrument
offers online measurement of noncon-
ducting particles as shown in Fig. 3.
The unit includes a probe, current
source, and signal processing system.
The probe consists of two electrodes
and an electrically insulating sampling
tube with an orifice through its sur-
face. Molten metal is drawn through
the aperture in the presence of a large
dc current. Nonconducting inclusions
as small as 20 μm are detected by
measuring the change in electrical
conductivity as they pass through the
aperture
[8]
. However, because this
technique operates by detecting
changes in electrical conductivity, it
cannot detect conductive inclusions
such as intermetallic particles and fails
to provide information on the chem-
istry, shape, or physical state of the in-
clusions. There are also difficulties in
finding appropriate nonconductive
materials for sampling tubes that are
nonreactive with Mg alloys, and equip-
ment cost is high as well
[8]
.
ADVANCED MATERIALS & PROCESSES •
MARCH 2014
16
Fig. 1 —
Methods for inclusion assessment used or considered for use in the Mg industry
[6-8, 10-12]
.
HMIAM—hydro magnesium inclusion assessment method; LiMCA—liquid metal cleanliness
analyzer; NSD—number size distributions.