PNNL-13501 - Pacific Northwest National Laboratory

(Johnson 1999; Colligan 1999; Gould et al. 1998; Kallee
and Nicholas 1998; Thomas and Nicholas 1997; Liu et al.
1997; Rhodes et al. 1997). Most of the efforts have
focused on welding wrought and extruded materials.
Single-pass butt-weld joints with aluminum alloys have
been made in thickness ranging from 1.2 mm to 50 mm
without the need for a weld preparation. Parameters for
butt-welding of most similar aluminum alloys have been
developed in a thickness range from 1.6 mm to 10 mm.
The process has been shown to produce welds of the
highest quality, and has led to the development of a
number of new product designs previously not possible.
The fine microstructure created via friction stir welding
also has shown the ability to create weld materials with
higher strength and resistance to failure during uniaxial
tensile tests. Friction stir is revolutionizing the welding of
aluminum in industry. Friction stir already produces costeffective
welds in aluminum alloy plates with higher
quality and lower distortion than is possible using fusion
welding techniques. Friction stir also can weld aluminum
alloys that are virtually unweldable by conventional
techniques.
Results and Accomplishments
We investigated existing dissimilar joining technologies,
and identified potential research areas at PNNL. After
identifying technologies of interest, research and
development was conducted to investigate the joining of
dissimilar materials using friction stir welding methods.
Friction stir welding was conducted on three main
materials: wrought aluminum sheet materials, wrought
and cast magnesium alloys, and aluminum metal matrix
composites. An illustration of a friction stir welding tool,
dissimilar aluminum sheet butt-welds, and magnesium
sheet butt-welds is shown in Figure 2. These welds were
produced on conventional milling machines. The
aluminum sheet weld consists of several welds produced
between dissimilar aluminum alloys of potential
automotive interest, as well as Sky 5083 alloy
conventionally used for superplastic forming applications.
The weld on the right side of the aluminum coupon is
between a 5754 alloy 2 mm thick and two 6111 alloy
sheets stacked to create a total 2 mm thickness.
We also investigated the ability to friction stir weld
aluminum metal matrix composites. A micrograph of a
typical aluminum-SiC metal matrix composites joint is
shown in Figure 3. Fusion welding of aluminum metal
matrix composites is typically very difficult and/or
undesirable due to particle pushing and subsequent
Figure 2. A friction stir welding tool together with
dissimilar aluminum and magnesium welds
Figure 3. Cross section through an aluminum-SiC metal
matrix composite weld joint processed through friction stir
welding
particle segregation, and formation of undesirable
aluminum carbide and aluminum oxycarbides. The
friction stir welds exhibited no particle segregation and
resulted in continuous welds. However, the tool steel
used to construct the welding tool deteriorated at a
relatively high rate, resulting in tool debris in the
weldment microstructure.
Summary and Conclusions
Solid-state friction stir welding methods were investigated
for potential application to lightweight and hybrid
material automotive structures. The weld methods
produce joints between dissimilar materials not normally
possible using fusion welding techniques. The weld
methods also show promise for potential metal matrix
composite joints not normally possible using fusion
welding methods.
References
Colligan K. 1999. “Material flow behavior during
friction stir welding of aluminum.” Supplement to the
Welding Journal pp.229-s-237-s.
Design and Manufacturing Engineering 187