1. Friction Stir Welding of Aluminum MMC 6061-XX%
Boron Carbide
Han Zhang* and Tracy W. Nelson** T. Haynes*
*Reynolds Metals Company
**Brigham young University
Provo, UT
Aluminum metal matrix composites (MMC) offer increased stiffness, strength and wear
resistance over standard aluminum alloys. However, the weldability of these materials is greatly
compromised with the addition of non-metallic reinforcement. Powder metallurgy products of
METAMIC materials with B4C reinforcement developed at Reynolds Metals Company is being
considered for neutron shielding applications because of the ability of B10 isotope to absorb the
neutrons. Fusion welding studies have demonstrated the ability to join in this type of material
with minimal welding-related defects using carefully controlled parameters. However, the joint
strength is determined by the choice of filler material. Solid state welding method could be used
to join this type of material offering better joint mechanical properties. Friction stir welding
(FSW) was investigated as a possible means of joining this material. Preliminary results
demonstrate that FSW is a viable process for joining producing superior mechanical properties
compared to traditional fusion welding. Test results for both fusion welding and FSW will be
compared, as well as tool wear and the feasibility of joining 6061-21% B4C METAMIC neutron
shielding materials.
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2. 1. Background
Friction stir welding (FSW) is a relatively new joining process that has received
considerable attention since being patented in 1991 by The Welding institute in Cambridge
England (Ref. 1-3). The advantages of friction stir welding (FSW) over conventional fusion
welding have been recognized by many industries, especially for joining aluminum alloys (Ref.
11-17). A few of the advantages that pose significant cost reductions in the manufacturing of
aluminum aerospace structures include the elimination of cracking in both the fusion zone and
heat-affected zone, porosity, filler metals, shielding gases and costly weld preparation (Ref. 1
and 15). Despite these potential benefits and the tremendous development efforts by numerous
industries to implement this process into production, there is still a vast amount of research and
development needed in order to better understand the microstructure and long term service
integrity associated with friction stir welds (FSWs). An understanding of microstructural
evolution during FSW and how the associated microstructures affect such mechanical and
physical properties as strength, fatigue, creep and corrosion is critical. Such an understanding
will bring broader acceptance, and thus, new applications for this new innovative joining
technology.
Although FSW has gained broad acceptance in joining many aluminum and cooper
alloys, there are a number of other materials which could benefit tremendously from this process.
Aluminum metal matrix composites (MMC) are among those materials that could take advantage
of the benefits that FSW offers over traditional fusion welding techniques. MMC materials
suffer from a variety of problems when subject to traditional fusion welding processes. Welds in
alloys like 6061-Al2O3 exhibit poor mechanical properties due to loss in composite material and
porosity formation. During welding, the aluminum oxide particulate dissociates resulting in free
excess oxygen which may for porosity in the weld and a lower composite particulate count in the
matrix. Both of the above lead to reduced mechanical properties.
Likewise, aluminum alloys reinforced with SiC or B4C particulate suffer from similar
problems. SiC and B4C dissociate during fusion welding resulting in excess Si and B in the
matrix. The carbon form these phases combines with aluminum to form an adverse aluminum
carbide (Al4C3) phase which is soluble in water. Again, the loss of reinforcing particulate and the
formation of adverse aluminum carbide and porosity results in reduce mechanical properties of
these alloys when fusion welded.
Despite their problems during welding, these materials offer increased stiffness and
specific strength. Alloys strengthened with B4C also exhibit reduced density with increased wear
resistance and neutron absorption. Alloy 6061-XX%B4C can exhibit an XX% in modulus with
up to XX% B4C. This alloy also exhibits excellent neutron absorption characteristics making
this alloy particularly beneficial in the nuclear industry. ?????
2. Experimental Approach
Both gas tungsten arc welds (GTAW) and friction stir welds (FSW) were made in ??? in. thick
6061-XX%B4C. GTAW were produced using xXX amps alternating current with pure Argon
shileindg gas. ????? FSW were produced at 670 RPM and 4.5 and 5.5 IPM using a tool design
developed at BYU.

3. 3. Results and Discussion
Although possible, it is difficult to produce quality welds in 6061-XX%B4C using GTAW.
There exist a miriad of problems associated with GTAW in this materials such as porosity,
adverse second phase formation and excess eutectic formation in the fusion zone. Optical and
SEM photomicrographs of transverse section of GTAW and FSW in 6061-XX%B4C are shown
in Figures 1-5.
2 mm
Figure 1. Transverse photomicrograph showing presence of porosity in an autogenous GTAW in
6061-XX%B4C.
10 µm
Figure ?. Transverse photomicrograph showing aluminum carbide (black needles) in an
autogenous GTAW in 6061-XX%B4C

4. Al4C3
Figure ?. Chemical analysis of needle phases (black) in Figure ?.
2 mm
Figure ?. Photomicrograph of GTAW with 4043 filler metal in 6061-21%B4C.

5. Shrinkage Porosity
Si
20 µm
a)
1 µm
b)
Figure ?. GTAW with 4043 filler in 6061-21%B4C, a) showing shrinkage porosity in weld, and
b) showing eutectic formation.
a)
b)
Figure ?. Transverse photomicrographs of the weld regions in a) GTAW and, b) FSW.
All the test results indicate that FSW is a superior joining process comparative to GTAW when
joining 6061-XX%B4C. Tensile test results are shown below in Figure ?. From

6. 36.0
Strength (ksi) + Elongation (%)
40.0
35.0
30.4
29.4
30.0
22.9
25.0
19.5
18.0
20.0
19.8
17.4
15.0
12.0
10.0
5.0
5.0
4.0
0.0
BS
FSW-4.5
UTS
4.0
YS
Elongation
FSW-5.25
Conditions
TIG
Figiure ?. Comparison of base metal and weld metal Tensile properties in 6061-XX%B4C.
Steels
1 mm
100 µm
Figure ?. SEM backscatter of surface of FSW indicating deposits of Fe from the tool at the
advancing side of the welding.
4. Conclusions
1. METAMIC neutron shielding materials containing B4C can be welded using TIG method.
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7. 2. Standard welding procedure used to join I/M monolithic 6061 can not be used to join this
type of material due to serve hydrogen-induced porosity and formation of Al4C3 at the fusion
zone.
3. The density of hydrogen-induced porosity can be minimized by controlling heat input and
dilution.
4. No apparent heat affected zone softening was detected.
5. Fusion zone mechanical properties are determined by the choice of filler material.
6. FSW can be used to join this material with similar tensile properties as parent material.
However, the long weld can not be made due to serve tool wear.
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