Download Aluminum spray forming is a rapid solidification process that results

Aluminum spray forming is a
rapid solidification process that
results in a fine and homogeneous
microstructure with uniform
distribution of micron-size
silicon particles.
SPRAY-
Arvind Agarwal*
Tim McKechnie*
Tungsten
cathode
Plasma Processes Inc.
Huntsville, Alabama
I
44
Inert
gas
Plasma gun
n the vacuum-plasma spray (VPS) forming
process, powders are simultaneously melted
and accelerated onto a rotating mandrel. Hypereutectic Al-Si alloys have been successfully
vacuum-plasma spray formed into near net shapes,
including thin sheets and thick cylinders. These
products have potential for aerospace, automotive,
and electrical applications. The spray-formed structure has high strength and refined microstructure
with a fine and homogeneous distribution of silicon particles. Strength and stiffness can be tailored
by varying the process variables.
Hypereutectic aluminum-silicon alloys have been
chosen for applications in aerospace, automobile,
and electrical equipment because of their low density, high specific stiffness, high-temperature resistance, wear resistance, and low coefficient of
thermal expansion. Some of these applications include high-performance automobile engine parts
such as connecting rods, rocker arms, cylinder
sleeves, piston rings, valve retainers, lightweight
optics, and electronic packaging material for aerospace applications.
Conventionally, hypereutectic Al-Si alloys (such
as A390 containing 16.5 wt% Si) are processed by
casting, but this results in growth of large columnar
silicon grains, which causes problems with machinability. For example, the automotive industry has
experienced difficulties during honing operations
because of the large and angular silicon particles.
Hypereutectic Al-Si alloys processed by powder
metallurgy typically develop deleterious oxides.
To overcome the limitations of casting and powder
metallurgy processing routes, Osprey Metals Ltd.
developed a spray deposition process in which a
stream of molten metal/alloy is gas-atomized
through a nozzle and deposited on a rotating mandrel. Spray forming is a rapid solidification process
that results in a fine and homogeneous microstructure with uniform distribution of micron-size silicon
particles. Homogeneous distribution of fine silicon
particles translates to higher resistance to scuffing
and wear, as well as improved machinability.
In one application of the Osprey technology,
Daimler-Benz has developed low-friction hypereutectic Al-Si cylinder liners that provide reduced
weight and up to 30% lower oil consumption. However, the handling of liquid metal and its atomiza*Member of ASM International
Powder
feeder
Powder
feeder
Main
power
supply
Arc
Copper
anode
Transfer arc
power
supply
Plasma flame with
high velocity molten particles
VPS formed Al-Si
Rotating mandrel
Fig. 1 — Vacuum Plasma Spray (VPS) process illustration. An arc ionizes a gas stream of argon and hydrogen to
form a >10,000 K plasma into which powders are fed. Powder
material is heated and accelerated in either an inert or reactive
environment to form a coating or structure on a rotating mandrel, which is later removed. Cylindrical mandrel shown can
be replaced with a more complex shape.
tion causes some difficulties during Osprey spray
forming.
The vacuum-plasma spray (VPS) technology was
developed to overcome these difficulties, and has
been adopted by Plasma Processes Inc. to fabricate
near net shape structures of hypereutectic Al-Si
alloys. This article explains the VPS process, describes
the microstructural development of the spray formed
alloy, and relates the properties to applications.
Vacuum plasma spray
Conventionally, thermal spray techniques (such
as air plasma, vacuum plasma, high velocity oxyfuel) deposit bulk coatings on engineering components. However, free–standing, near net shape
structures can also be fabricated by spray forming.
Special nozzles have been designed (and patented)
to simultaneously melt powders and accelerate the
molten particles for deposition on a rotating mandrel. As shown in Fig. 1, the nozzle generates an
arc that ionizes a gas stream, forming the plasma
with temperatures above 10,000K. Powders are fed
into the plasma, where they melt and are accelerated to supersonic speeds. A high electric potential
directs the molten particles toward a rotating mandrel, where they deposit and rapidly cool, forming
the correct shape by replicating the mandrel shape.
Sprayed deposit is removed from the mandrel by
mechanical or chemical means.
ADVANCED MATERIALS & PROCESSES/MAY 2001
FORMING
Aluminum Structures
The VPS process can be tailored by
varying the precursor materials, particle dimensions, plasma velocity, and
mandrel temperature. The density of
the deposited materials depends directly on the plasma velocity. The velocity controls the time that the particles are exposed to the heating zone,
and the kinetic energy with which they
impact the rotating mandrel/substrate. a
b
The plasma gun and the mandrel are
Fig. 2 — Optical micrographs of the vacuum plasma sprayed hypereutectic Al-Si alloy: (a) ascomputer controlled, allowing fabrication of complex shapes. The high tem- sprayed and (b) as-sprayed under cooled conditions.
perature of the plasma implies that virtually any material that can be melted is
suitable for spraying to controlled thickness and geometries. The process results
in a very effective use of precursor materials, because waste is minimal, parts
are sprayed to near net shape, and little
final machining is required.
The process is especially appropriate
for materials that are difficult to process
b
by conventional means or are difficult a
to machine. Some of the other advan- Fig. 3 — Optical micrographs of (a) as-sprayed + hot pressed and (b) cast A 390 Al alloy.
tages of spray forming by VPS include:
• Compositional flexibility: Materials include metal Hypereutectic Al-Si alloys with processing histories
matrix composites (MMC), ceramic matrix comPrimary silicon
Microhardness,
posites (CMC), and functionally gradient materials Processing
history
particle size, µm
Density, %
Knoop
(FGM),
• Minimal oxidation: The droplets are entrained As-sprayed
≤5
99.8
151 ±16
in the ionized gas, hence are not exposed to oxgyen. As-sprayed
~2
99.2
136 ±16
• High density: The small size of the silicon par(cooled)
As-sprayed +
20
100
101 ±12
ticles results in high density.
Hot Pressed
• Refined microstructure: Rapid solidification enCast A 390 Al
50
100
102 ±8
ables fine microstructure.
• Potentially lower cost of fabrication: Very little
silicon particles (Fig. 2b), with the result that the
waste means efficient use of materials.
density of the as–sprayed structures is higher than
Properties of spray formed structures
99%. Hot isostatic pressing of the as-sprayed alloy
Rings and flat tensile specimens have been ma- results in coarsening of silicon particles up to 40
chined out of spray formed cylinders, indicating ex- mm (Fig. 3a). Eutectic silicon agglomerates into
cellent machinability. The key innovation is the ability coarse particles and undergoes grain growth during
to keep the silicon particles small and well distrib- the HIP operation.
The microstructure of cast hypereutectic A-390
uted. The microstructure of the as-sprayed hypereutectic Al-Si alloy (Al-23 wt% Si) constitutes a homo- (Fig. 3b) provides a comparison with the vacuum
geneous distribution of fine eutectic silicon and small plasma sprayed alloy and hot pressed alloy.
primary silicon particles (~5 μm) in the aluminum Vacuum plasma sprayed hypereutectic Al-Si alloy
matrix (Fig. 2a). This is possible with VPS processing contains finer and rounded silicon particles (<5 μm),
because of the high temperatures and rapid cooling, which is a significant improvement over the large
which provide a high nucleation rate for particles, and columnar particles (50 to 100 mm) in the A390
alloy, which are detrimental for machining and
yet do not allow time for particle growth.
A proprietary cooling technique further enhances wear resistance. The table lists the microstructural
cooling rates and consequently reduces the size of features of these alloys.
Continued
ADVANCED MATERIALS & PROCESSES/MAY 2001
45
0.02
0.015
As sprayed
A 390
0.01
0.005
5
10
Time, min.
15
20
Fig. 4 — Wear resistance of A390 and spray formed Al-Si.
The tensile strength of as-sprayed hypereutectic
Al-Si alloy was as high as 345 MPa (50 ksi), which is
comparable to the wrought A 390 aluminum alloy.
High tensile strength of the as-sprayed alloy is
caused by fine silicon particles that produce classical second-phase “composite” strengthening, and
impede dislocation movement. Hot pressing of assprayed alloy results in a reduction of tensile
strength because the material fractures at the coarsened silicon particles. Dry sliding wear resistance
of the as-sprayed alloy evaluated by a block-ondisk tribometer suggests a comparable or lower
wear rate than wrought A 390 aluminum (Fig. 4).
Structural applications
Apart from automotive and electrical applications, hypereutectic Al-Si alloys have potential ap-
Fig. 5 — Spray formed ring of hypereutectic Al-Si with a
varying degree of stiffness. Diameter of the ring is 125 mm,
width is 5 mm and thickness is 0.5 mm.
plications for structural parts in the aerospace industry as a replacement for beryllium-based alloys.
Traditionally, beryllium has been selected for truss
structures, optical mirror shells, satellite attach fittings, and solar panel support structures because
of its low density, high stiffness, low thermal expansion, and high thermal conductivity. Despite
its usefulness, beryllium is not an ideal material because it is expensive and too brittle to work.
However, hypereutectic Al-Si possesses properties similar to those of beryllium at lower cost. For
example, a hypereutectic Al-Si based X-ray mirror
can be fabricated by a replication technique. The
“replicated optics” technique is a process for producing multiple thin mirror shells from a single
shaped and polished mandrel. Each shell is built
up on the surface of a correctly figured (negative)
mandrel, usually made of glass or nickel-coated
aluminum. The mandrel is super-polished to a ~5
nm high surface finish that has the negative figure
of the required optic. The spray-formed material is
then removed from the mandrel, usually by taking
advantage of the large difference in thermal expansion between the two components. No additional polishing or shaping of the shells is needed.
The technique has considerable merit in that
many shells can be fabricated for a single shaping
and polishing of the mandrel, and the resulting
shells can be very thin. A 25 mm diameter spray
formed X-ray mirror of hypereutectic Al-Si successfully replicated the optical finish on the inner
surface. Optical-finish mandrels have been fabricated by NASA Marshall Space Flight Center and
Goddard Space Flight Center. This technique can
be adopted to fabricate mirror shells of larger sizes,
such as 62 mm and 125 mm diameter. Stiffness of
the spray formed hypereutectic Al-Si could be controlled by varying the process parameters (Fig. 5). ■
For more information: Arvind Agarwal and Tim McKechnie, Plasma Processes Inc., 4914 D Moores Mill Road,
Huntsville, AL 35811; tel: 256/851 7653; fax: 256/8594134; e-mail: [email protected] ; Web site: www.
plasmapros.com.
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