Microstructure and Mechanical Properties of an Al-Mo in situ
Nanocomposite Produced by Friction Stir Processing
I. S. Lee*, P. W. Kao and N. J. Ho
Department of Materials and Optoelectronics Science; Center for Nanoscience and
Nanotechnology, National Sun Yat-Sen University, Kaoshiung 804, Taiwan.
* Correspondence author.
Keywords: friction stir processing (FSP), Al-Mo intermetallic, Al12Mo
Abstract
In this work, friction stir processing (FSP) was applied to produce aluminum based
nanocomposites from powder mixtures of Al-Mo. This technique combines the hot working
nature of FSP and the exothermic reaction between Al and Mo. Fully dense Al-matrix
composites with large amounts of nanometer sized reinforcement particles, formed in-situ,
can be fabricated by FSP without further consolidation. The microstructure was characterized
using TEM, SEM and XRD. The Al-Mo intermetallic particles were formed in situ during
FSP and were identified as Al12Mo. These particles have an average size of ~200nm. Due to
the fine dispersion of Al12Mo particles, the aluminum matrix has an ultrafine-grained
structure (~1μm). In addition, the reaction mechanism, and microstructural evolution during
FSP, as well as the mechanical properties of the Al-Mo in situ composites, will be presented.
Introduction
The mechanical properties of particulate-reinforced metal–matrix composites (MMCs) are
controlled by the size and volume fraction of the reinforcements as well as the nature of the
matrix-reinforcement interface [1]. Superior mechanical properties can be achieved when fine
and stable reinforcements with good interfacial bonding are dispersed uniformly in the matrix.
In conventionally processed power metallurgy composites, the reinforcing particles are
formed prior to being added to the matrix metal. Therefore, the scale of the reinforcing phase
is limited by the starting powder size; typically on the order of microns to tens of microns and
rarely below 1 m. However, it is possible to synthesis fine reinforcing particles in-situ in the
matrices of MMCs [2]. The advantages of in-situ MMCs include a more homogeneous
microstructure and strong interfacial bonding between the reinforcement particles and the
matrices [2]. Recently, other investigators fabricated in-situ metal matrix composites by
friction stir processing (FSP) [3-7].
Friction stir processing (FSP) was developed by Mishra et al. [8] for microstructural
modification based on the principle of friction stir welding (FSW); a solid state welding
technique invented in 1991 at The Welding Institute [9]. In FSP, a rotating tool, with pin and
shoulder, is inserted and moved along the material providing localized heating and plastic
deformation. The material experiences very large plastic strain (εeff > 40) by being extruded
around the rotating tool [10]. A comprehensive literature review on friction stir welding has
been given by Mishra and Ma [11], and a review on the recent developments in FSP was
provided by Ma [12].
Aluminum reinforced with large amounts (up to ~50 vol. %) of nanometer-sized Al3Ti
particles was fabricated from Al–Ti elemental powder mixtures via friction stir processing
(FSP) [5]. These authors suggested that the rapid Al-Ti reaction in FSP is a combined result
of the large plastic strain and high temperature introduced during FSP and the exothermic
reaction between Al and Ti. In MMCs, high reinforcement volume fractions are typical,
giving rise to excellent properties. Furthermore, Al–TM (TM: transition metals) alloys are
expected to show superior elevated temperature mechanical property because of the
microstructure stability originating from the low diffusivity of TM in Al. In this work, the
Al-Mo binary system was investigated because Mo has very low diffusivity and solubility in
Al [13]. The objective of the present study is to produce ultrafine-grained aluminum
reinforced by large amounts of nanometer-sized Al–Mo intermetallic particles formed in-situ
and dispersed in the aluminum matrix during FSP.
Experimental
In this work, a powder mixture of Al-5 at. % Mo (denoted as Al-5Mo) was prepared from
pure aluminum powder (99.7% purity, -325 mesh), and pure molybdenum powder (99.99%
purity, 4μm). After proper mixing, the Al–5Mo powder was cold compacted to a billet of
12×12×88 mm3 in a steel die set by using a pressure of 225 MPa. To improve the billet
strength for easier handling during FSP, the green compact was sintered in Ar at 823°K for 20
min. The tool pin used during FSP was the standard M1.2*6 (diameter of 6 mm and pitch
height of 1.2 mm). The tool spindle angle (angle between spindle and workpiece normal) was
3o. The rotating tool was traversed along the long axis of the billet. Based on trial studies, a
tool-rotation speed of 1400 rpm, traverse speed of 30-45 mm/min, and four FSP passes were
applied to give defect free specimens consistently. For multiple FSP passes, the rotating tool
was moved along the same line for each pass and was applied after the workpiece had cooled
to room temperature from the previous pass.
X-ray diffraction (XRD, CuK, 40 kV, 30 mA) was used to identify the phases present in the
specimens andcanning electron microscopy (SEM, JSM-6330) used to study the distribution
of second phase particles. Thin foils prepared by ion-milling were examined using
transmission electron microscope (Tecnai F20 G2 Field-Emission TEM) operated at 200 kV.
The Vickers microhardness was measured with a 300 gm load for 15 sec. Mechanical
properties of specimens machined from the stirred zone (SZ) were measured on an Instron
5582 universal testing machine with an initial strain rate of 1×10-3 s-1. The dimensions of the
gauge section of the tensile specimens were 3 mm in diameter and 14 mm in gauge length.
For tension tests, the loading axis was parallel to the FSP tool traversing direction.
Results and Discussion
The X-ray diffraction pattern indicates no reaction between the Al and Mo during sintering,
figure 1, line C. After four FSP passes, diffraction peaks corresponding to the Al12Mo
intermetallic phase appear in the XRD pattern (Fig. 1, lines A and B). In addition, the lower
tool traveling speed (30mm/min) produces a larger amount of Al12Mo intermetallic phase.
However, the diffraction peaks corresponding to Mo still exist in the sample produced by 4
FSP passes. This indicates that the Al-Mo reaction was not completed after 4 FSP passes.
3000
Al12Mo #
Mo
Al
Al
1400/30 (A)
1400/45 (B)
550oC sinter (C)
#
Intensity
#
Al
2000
#
#
Mo
##
##
Al
Mo
# ##
Al
Mo
(A)
1000
(B)
0
(C)
20
40
60
80
2
Figure 1. XRD patterns for (a) specimen processed by 4 FSP passes (1400rpm-30mm/min),
(b) specimen processed by 4 FSP passes (1400rpm-45mm/min), and (c) as-sintered specimen.
The cross-section of the stirred zone in specimens produced by FSP was examined using SEM.
The microstructure of the as-sintered condition is shown in Fig. 2a, and can be characterized
as a dispersion of clusters of fine Mo particles in an aluminum matrix. The typical
microstructure in the stir zone after 4 FSP passes is shown in Fig. 2b, and shows a uniform
dispersion of second phase particles of submicrometer size. Three phases, each differing in
contrast can be characterized in the BEI images (Figs. 2b and 2c). The brightest phase is Mo,
the gray phase is Al12Mo, and the dark gray matrix is Al. The reaction between Al and Mo
is not complete even after 4 FSP passes because Mo particles still exist in the stirred zone.
Figure 2. SEM backscattered electron image (BEI) showing the microstructure of (a)
as-sintered condition, and in the stirred zone of specimens produced by 4 FSP passes (b) 1400
rpm - 45 mm/min, and (c) 1400 rpm – 30 mm/min.
(
The microstructure of the SZ in specimens produced by FSP was also examined by the use of
TEM. Typical microstructures are shown in Fig. 3. As shown in Fig. 3a, large amounts of fine
second phase particles are uniformly distributed in the aluminum matrix. The average size of
the aluminum grains, as revealed by the dark field image, is about 2m (Fig. 3b). In general,
the second phase particles have an equiaxed shape as shown in Fig. 3(c), and the average size
of second phase particles is about 200 nm.
Figure 3 The microstructure of FS processed specimens revealed by TEM.( 1400 rpm – 30
mm/min, 4 FSP passes) (a) Bright field image showing nanometer size particles distributed in
the aluminum matrix, (b) dark field image showing fine Al12Mo particles dispersed in an
aluminum grain, and (c) Al12Mo particle revealed by dark field image.
During FSP, the time that the material is subjected to the thermomechanical action is very
short, i.e., only on the order of tens of seconds. The reaction between the Al and Mo must
proceed very fast during FSP. The large plastic strain in FSP can shear the metal powders and
break the oxide film surrounding Al and Mo powders causing intimate contact between the Al
and Mo accelerating the reaction. The reaction product can be effectively removed from the
interface by the large plastic strain realized during FSP such that direct contact between the Al
and Mo can be maintained, and consequently, the reaction can proceed rapidly at the interface.
In addition, as suggested by Hsu et al.,, the heat release associated with the Al-TM reaction at
the interface might cause local melting of Al, possibly further enhancing the reaction between
Al-TM [3,4].
Decreasing the FSP traveling speed will enhance the Al–Mo reaction, resulting in more
reinforcement particles, and higher hardness. The average hardness in the stirred zone of
specimens produced by 4 FSP passes can be raised from 62 Hv to 74 Hv by decreasing the
tool traveling speed from 45 to 30 mm/min. The tensile stress-strain curve of the specimen
produced by 4 FSP passes with a tool travel speed of 30 mm/min is shown in Fig. 4, where
the results of Al-10Ti [4] and Al-10 Fe [6] are also shown for comparison. The tensile
properties of Al-5Mo, Al-10Ti [4] and Al-10 Fe [6] are summarized in Table 1. All the
Al-TM composites shown in Table 1 have enhanced Young’s modulus as compared to that of
aluminum (70 GPa). Such improvement in the bulk modulus can be attributed to the presence
of large amounts of the intermetallic phases in the composite, each having high moduli, i.e., E
= 216, 130, and 116 GPa for Al3Ti, Al13Fe4, and Al12Mo, respectively. Due to the presence of
large amounts of nanometer sized reinforcing particles and the UFG structure of the
aluminum matrix, the Al-5Mo composite exhibits high strength. As compared with Al-10Ti [4]
and Al-10 Fe [6], Al-5Mo also has good tensile ductility (~16.5%). However, the strength of
Al-5Mo is inferior to that of Al-10Ti. This may be attributed to the coarser particles (~200
nm) in the Al-5Mo as compared to 83 nm in Al-10Ti [4]. The lower strength in the Al-Mo
system also may be affected by the incomplete reaction in the Al-5Mo. If Mo in the Al–5Mo
composite is fully reacted, the volume fraction of Al2Mo can reach 0.69. This should
provide a considerable strengthening effect.
Based upon this work and previous studies [3-6], it is suggested that more research work is
needed in the following two aspects: (1) to enhance the rate of the in-situ reaction during FSP,
and (2) to reduce the size of the reinforcement particles formed by the in-situ reaction.
Besides the processing parameters of FSP, several factors may be considered such as: (a)
thermodynamic characteristics of the reactants such as heat release in the reaction, (b) particle
size of the reactants, and (c) the stability of the reinforcement phase at elevated temperature,
which may be affected by the melting point, and the diffusivity and solubility of the
constituent elements in the aluminum matrix.
Sample
Table 1. Tensile properties of Al–TM composites produced by FSP
σy (MPa)
E (GPa)
UTS (MPa)
Elongation (%)
Al-5Mo
78
160
177
16.5
Al-10Fe [6]
91
177
217
3.7
Al-10Ti [4]
86
316
366
7.2
450
Al-5Mo
Al-10Fe [6]
Al-10Ti [4]
True stress (MPa)
400
350
300
250
200
150
100
50
0
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18
True strain
Figure 4. Tensile stress-strain curves of specimens produced by FSP.
Summary
In this work, an Al-Mo in-situ composite was successfully produced by using a technique
combining friction stir processing (FSP) and the exothermic reaction between Al and Mo. The
Al-Mo intermetallic particles, identified as Al12Mo, were formed in-situ during FSP. The
microstructure of the composite can be characterized as a fine dispersion of nanometer sized
Al12Mo particles (~200nm) in an aluminum matrix with an ultrafine-grained (UFG) structure
(~2μm). Due to the presence of large amounts of nanometer sized reinforcing particles and the
UFG structure of aluminum matrix, the composite exhibits both high strength and good
ductility.
1.
2.
3.
4.
5.
6.
7.
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