Functional Coatings: Research Summary
Y.T. Pei and J.Th.M. De Hosson
TABLE OF CONTENTS
Al-40Si functionally graded coatings produced by a one-step laser powder cladding process on cast aluminum-alloy substrate is a possible solution for avoiding the interfacial problems often present in laser coatings. The microstructure of the coatings consists of a large amount of silicon-primary particles surrounded by a-aluminum dendritic halos and Al/Si eutectic. The silicon particles exhibit a continuous increase in both size and volume fraction, from 8.5 mm and 22.7% at the bottom to 52 mm and 31.4% at the top of the coating layer, respectively. The morphology of the silicon particles changes accordingly from a small polygon shape to a coarse, branched, equiaxial shape. The a-aluminum halos and eutectic areas show less change over the same distance. From a qualitative analysis of the temperature field of the laser pool, silicon particles heterogeneously nucleate on incompletely melted silicon particles. The number density of silicon particles is most likely controlled by the nonhomogeneous temperature field of the pool that determines the decomposition of original phase in the Al-40Si powder. The final size of the silicon particles is mainly affected by the growth rate and time available at different depths of the laser pool.
The laser-surface processing of aluminum alloys has drawn a lot of interest
for enhancing the mechanical and chemical resistance of aluminum-alloy components.1
Examples of laser-surface processing techniques include laser-surface melting
(LSM), laser-surface alloying (LSA), and laser-surface cladding (LSC).2
In LSM, the surface performance of the aluminum-alloy substrate is modified
mainly by the homogenization and refinement of the microstructure. In addition,
different precipitates may be formed and the supersaturation of the a-aluminum
phase increased due to nonequilibrium solidification.3-5
However, the modification is limited, because the composition of the melted
layer is the same with respect to the substrate.
The employment of appropriate alloying additions with LSA can lead to the formation of hard phases that reinforce and enhance the properties of the alloyed layer.6-8 Elements used to date have been restricted to the transition metals, such as Ni, Cr, Mo, W, Ti, and Zr, that react with aluminum to form intermetallic aluminide.1 The LSA technique works on the principle of mixing an alloying element in the melt pool of the substrate created by the laser beam. The amount of alloying addition is usually minor compared with the total amount of material that has melted. Therefore, local thermal distortions may be introduced to the substrate, resulting in severe residual stresses.
Previous studies9,10 have shown that LSC is the most flexible technique. In this process, only a thin surface layer of aluminum-alloy substrate melts together with the additive material to form a coating. By minimizing the thickness of the melted substrate layer, the desired properties of the coating are realized while minimizing the changes to the substrate. Consequently, advanced coatings produced by LSC can be designed independent of the composition of the substrate material. Therefore, LSC has the potential to be an effective and economic technique to improve the surface properties with the view of significantly extending the performance of the material as a whole.
A major drawback in laser cladding is that a sharp interface usually forms between the coating and the substrate that is often a potential source of weakness. For example, if the coating material has a very different thermal-expansion coefficient than the substrate, there is the possibility of severe stresses building at the interface and resulting in a crack. A common way to circumvent this problem is to optimize the coating thickness or to introduce a compliant interlayer for the reduction of the thermal stress. Unfortunately, most compliant films also melt at lower temperature. A recent development by Jasim et al.11 is a functionally graded coating (FGC) built up by three overlaid laser tracks in which the proportion of SiC reinforcement increased in steps from 10 vol.% to 50 vol.%. Their work showed the possibility of laser processing to deposit a thick multilayer of essentially discrete composition rather than a gradual composition change.
The term "functionally graded materials" (FGMs) is now widely used by the materials community for a class of materials exhibiting spatially inhomogeneous microstructures and properties. Graded materials in themselves are not something new, but what is exciting about them is the realization that gradients can be designed at microstructural level to tailor specific materials for the functional and performance requirements of an intended application. Therefore, a possible approach for eliminating the sharp interface present in LSC is to introduce the concept of FGMs into the design of coating structure.
Substrates were cut from cast rods
of commercial aluminum alloy with a nominal composition of Al-6.3Si-4.0Cu-1.0Fe-0.3Mg
(in wt.%). Standard face milling finished the surfaces of the flat
substrate specimens in dimensions of 100 mm3
× 50 mm3 × 10 mm3.
The specimen was mounted on a cooling block with the underside in
direct contact with flowing water or oil maintained at a constant
temperature. In this way, the substrate was kept at a constant temperature
as much as possible with a change of less than 5°C during laser cladding.
This ensured that the geometry and dilution degree of laser-clad tracks
depended only on the processing parameters. In addition, setting different
substrate temperatures from 15°C to 200°C made it possible to adjust
the thermal gradient of the specimens that affected the solidification
of the laser pool.
alloy powder was used as the coating material for three reasons. First,
using the same alloy system results in similar thermal properties.
Second, even a high degree of local dilution can create a composition
gradient for the desired FGCs. Third, the primary silicon particles
may serve as hard reinforcement to FGCs, and the solidification process
can control their size. This is very important for the in-situ formation
of FGCs during laser cladding. The powder produced with spray-atomizing
technology exhibited a globular geometry with a particle size of 50
mm to 125 mm
The powder feeder used a Perkin-Elmer-Metco
Co (MFP-I type) commercial instrument. The cladding set-up was
equipped with a specially developed powder-addition module.12
The powder nozzle had an integrated coaxial shielding gas flow to
buffer the laser-molten pool from the atmosphere. The shielding gas
was also used to converge the powder stream into the laser-molten
pool, leading to a powder efficiency higher than 90% as well as a
more homogeneous cladding track with a smooth surface.
|Figure 1. Optical micrographs of Al-40Si FGCs clad at 3,000 W laser power and different beam speeds of (a-left) 10 mm/s, (b-center) 20 mm/s, and (c-right) 26.7 mm/s.|
A large amount of primary silicon particles (Sip)
is observed over the cross sections of FGC tracks. These fine silicon particles
act as reinforcement and are expected to improve the tribological properties
of the FGCs significantly. Note that the primary silicon particles gradually
increase in size with distance from the bottom of FGC tracks. Moreover, the
scanning speed of the laser beam exerts an obvious influence on the distribution
of silicon particles. The difference in Sip
size between the top and the bottom of the tracks decreases with decreasing
|Figure 2. The (a-left) top (b-center) intermediate, and (c-right) bottom of a graded microstructure of Al-40Si FGC produced at 3,000 W laser power and 26.7 mm/s beam speed. The arrows indicate five-branch silicon particles.|
The graded microstructure of Al-40Si FGC is more clearly shown in Figures 2a, 2b, and 2c. The FGC layer consists of primary silicon crystals surrounded by a-aluminum dendritic halos and branched Al/Si eutectic adjacent to the a-aluminum halos. From the bottom to the top surface of the clad layer, the discrete silicon particles increase in apparent size by a factor of five. In contrast, the a-aluminum halos and the adjacent eutectic cells exhibit less change in size over the same distance. These observations suggest that the primary silicon particles were nucleated from the liquid state rather than from a solid-state precipitate. In addition, a thin Sip-free zone composed of a-aluminum dendrites and Al/Si eutectic can be seen at the melted boundary. The thickness of this Sip-free zone reduces from 30 mm to 10 mm with increasing beam speed. It is predicted that this zone will be a beneficial feature by being a region where possible thermal stresses may be relieved.
|Figure 3. Quantitative metallography results of the FGC track revealed in Figure 2 on the graded change of silicon particles as a function of depth about (a-left) apparent size and mean intercept length, (b-center) volume fraction and interparticle spacing, and (c-right) areal density (NA) and volumetric density (NV).|
The changes in apparent size, volume fraction, and number density of silicon
particles as a function of the distance (z) from the molten boundary are given
in Figures 3a, 3b,
and 3c, using quantitative metallographic
analysis.13 The size
of the silicon particles is evaluated in Figure
3a in two terms (i.e., apparent size [d] and mean intercept length [L3]).
The apparent size of the Sip is taken as the
tip-to-tip distance of the star-shaped particles and represents the maximum
distance of growth for a particle along certain preferred directions. The mean
intercept length denotes the average size of particles and is a unique assumption-free
value that is valid for particles of any size and configuration. Both parameters
exhibit an obvious increase over the thickness of the FGC (i.e., six times in
d and 2.7 times in L3), and their difference
is due to the development in shape of Sip.
Accordingly, the volume fraction of Sip varies
continuously from 22.7% at the bottom to 31.4% at the top of the track (Figure
3b). Note that the numbers of Sip observed
per unit area and subsequently calculated per unit of volume decreases obviously
with z (Figure 3c). In other words,
the interparticle spacing (s) increases significantly
with z according to
|Figure 4. An SEM micrograph showing the growth feature of the five-branch silicon particle and the surrounding a-aluminum dendritic halos as well as the eutectic adjacent to a-aluminum.|
Figure 5. The hardness distribution of laser-clad Al-40Si FGCs produced with different powder feeding rates.
In addition to the change in size with depth, silicon particles also display
a morphological transition from polygon at the bottom to an equiaxially branched
shape at the top region of the FGC. An example of the equiaxially branched shape
can be seen as the five-fold branched grains indicated by arrows in Figures
2a and 2b.
Such five-fold geometry of silicon crystals has been previously reported as
a rare morphology that forms at a slow cooling rate,14
but in our case it seems to be very often observed in the upper region of the
FGC. It has been known that they grow from a twinned decahedral nucleus, which
is an assembly of five silicon tetrahedrons in twinned orientation.
The typical growing feature of the five-fold silicon particles is shown in
Figure 4. The obtuse facet angles at
the ends of each branch, as well as the twin planes along the branches and through
the intersection points of the facets, suggests a twin-plane re-entrant edge
The typical hardness profile of the FGC tracks is presented in Figure 5. The graded microstructure leads to a gradual hardness distribution of the FGC from HV0.2 180 down to HV0.2 80. Sometimes it was impossible to avoid making indentations close to the primary silicon particles, the result of which is a much higher hardness. This resulted in some fluctuations on the hardness curves, despite that an average of five measurements was taken. It is interesting to note that the transition from the coatings to the substrate exhibits a gradual change in the hardness, which indicates the absence of a sharp demarcation in materials properties across the interface.
Vs = Vb · cosq
|Figure 6. The temperature field of the laser pool in the case of cladding Al-40Si FGCs. Isotherms I1 and I2 represent the liquidus of Al-40Si alloy and the Al/Si eutectic temperature, respectively; Vb is the beam scanning speed; and Vs is the speed of eutectic growth front.|
DT = Kva
The Netherlands Institute
for Metals Research and the Foundation
for Fundamental Research on Matter (FOM-Utrecht) are acknowledged for their
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Y.T. Pei and J.Th.M. De Hosson are with the Department
of Applied Physics, Materials Science Center and Netherlands
Institute for Metals Research, University
For more information, contact J.Th.M. De Hosson, Department of Applied Physics, Materials Science Center and Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands; e-mail email@example.com.
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