Influence of Welding Variables On Indentation Depth in Ultrasonically Welded Al/Cu Dissimilar Joints and Theoretical Fracture Load Estimation
Influence of Welding Variables On Indentation Depth in Ultrasonically Welded Al/Cu Dissimilar Joints and Theoretical Fracture Load Estimation
Influence of Welding Variables On Indentation Depth in Ultrasonically Welded Al/Cu Dissimilar Joints and Theoretical Fracture Load Estimation
Abstract
Despite increasing industrial demand, typical fusion welding of aluminum and copper joints remains difficult due
to the formation of intermetallic compounds. Therefore, ultrasonic welding, which is a type of solid-state welding, is
used for combining aluminum and oxygen-free copper. It has a low dependence on the materials and can secure
large bonding joints in a short duration. However, indentations formed due to the vibration and pressure of ultrasonic
welding greatly impact joint quality by affecting joint strength and fracture location.
This study investigated the formation of indentations in accordance with welding variables, and proposed a theoret-
ical estimation of fracture strength in ultrasonic welded joints of aluminum and oxygen-free copper. Since the frac-
ture occurred along the indentation boundary in aluminum substrate—which has lower strength than copper—failure
strength increased with increasing indentation depth in the copper plate. Average fracture strength was 65% of the
tensile strength of base materials due to the stress concentrated around the indentation. Therefore, product design
needs to consider base material strength and fracture propagation path before deciding on a safety factor.
Key Words : Ultrasonic welding, Welding variable, Indentation depth, Al/Cu dissimilar joint, Theoretical strength
tab. Among them, ultrasonic welding is typically used (a) (b) (c) (d)
when connecting 30-50 sheets of ultra-thin plates. It Fig. 1 Steps in the ultrasonic welding process: (a) Clamping,
has low dependence on the materials and can secure (b) Vibration and welding, (c) Holding, and (d)
large bonding joints in a short duration, since ultrasonic Unloading7)
tion between welding variables and indentation depth. length of the surface indentation formed after ultrasonic
This study also attempts to predict the maximum theo- welding, and indentation depth was quantified via cross-
retical fracture strength. sectional analysis and a non-contact three-dimensional
measuring machine. At least six pieces of data were de-
2. Experimental Method rived from the fabricated specimens and were averaged
to present the width, length, and indentation depth. A
Overlapping plates of 0.2 mm thick non-coated C1020 tensile-shear test was performed with a head speed of 3
and Al1050 material were ultrasonically welded. The mm/min, and the results were presented as averages of
C1020 showed an average tensile strength of 461 MPa, the three specimens for each condition.
and the Al1050-H18 was 181 MPa. As shown in Fig. 2
(a), the materials were machined to a width of 50 mm 3. Experimental results and discussion
and a length of 80 mm. The copper material was placed
on the top, and the aluminum was placed at the bottom. 3.1 Change in indentation shape with experimental
The overlap length was 25 mm. The knurl of the anvil variables
and horn had a height of 0.22 mm. Figs. 2 (b) and (c) The shape of the indentation on the contact surface of
show the experimental equipment and dimensions of the material and ultrasonic tool changed in accordance
the horn and anvil, respectively. with the experimental variables. Fig. 3 shows the in-
Welding time, clamping pressure, and amplitude were dentation images with different welding times at a
varied as shown in Table 1 to investigate the influence clamping pressure of 0.4 MPa and amplitude of 24 μm.
of these process variables on indentation formation. An At short welding time condition, the indentations in the
optical microscope was used to measure the width and oxygen-free copper surface exhibited were relatively
longer in the transverse direction due to transverse
25 mm
vibration. However, as time progressed, the lengths in
50 mm
Time Top plate (Cu) Bottom plate (Al)
Cu 80 mm
Al
0.2 mm
0.20 s
(a)
0.25 s
(b)
70 mm
10 mm
0.30 s
15 mm
10 mm
98 mm
<Horn> <Anvll>
(c)
0.35 s
Fig. 2 Experimental set up. (a) Schematic diagram of fab-
ricated sample, (b) Images of used ultrasonic weld-
ing machine, and (c) Designs of horn and anvil
200
Clamping pressure Cu
0.2 MPa
0.3 MPa
150 0.4 MPa
Time (s)
the longitudinal and transverse directions became similar.
(a) 24 μm
In contrast, the aluminum surface indentations ex- 200
hibited no large changes in shape after 0.25 s. As the Clamping pressure Cu
0.2 MPa
horn and anvil have a quadrangular pyramid shape, the 0.3 MPa
surface indentation area has a tendency to increase in 150 0.4 MPa
0.4 MPa
This proves that the increasing pressure enhanced the
frictional force between the tool and substrate. Therefore,
100
the applied external force was exhausted and the force
transfer in the thickness direction was insufficient. On
the other hand, changes in indentation depth on the alu- 50
200 1000
Clamping pressure Al
0.2 MPa
0.3 MPa
150 0.4 MPa 950
Indentation depth (㎛)
Load (N)
100 900
50 850
Force: 0.2 MPa
Force: 0.3 MPa
Force: 0.4 MPa
0 800
0.20 0.25 0.30 0.35 0.40 0.20 0.25 0.30 0.35 0.40
Load (N)
100 900
50 850
Force: 0.2 MPa
Force: 0.3 MPa
Force: 0.4 MPa
0 800
0.20 0.25 0.30 0.35 0.20 0.25 0.30 0.35
Load (N)
100 900
50 850
Force: 0.2 MPa
Force: 0.3 MPa
Force: 0.4 MPa
0 800
0.15 0.20 0.25 0.30 0.15 0.20 0.25 0.30
Fig. 6 Indentation depth measurement on Cu substrate Fig. 7 Measured tensile-shear load according to the weld-
using non-contact measuring machine according ing variable. At amplitude of (a) 24 μm, (b) 29.5 μm,
to the welding variable. At amplitude of (a) 24 μm, and (c) 33 μm
(b) 29.5 μm, and (c) 33 μm
form next to the knurl, and act as a crack tip inside the
900
Al joint11). In addition, the indentation boundary of the
fracture, which is an interface contiguous with the un-
850 jointed part, can be treated as a sharp crack tip. Plastic
deformation commonly occurs even at low stresses
800
near crack tips. This is called fracture toughness (Fig.
40 80 120 160 11). A concentrated stress near the knurl can be ex-
Indentation depth (mm) pressed as in Eq. 112,13). Since it has a wide bonding in-
Fig. 8 Relationship between failure load and indentation terface compared to its thickness, a plane stress con-
depth depending on the substrates dition can be assumed. The above assumption justifies
the summarization shown in Eqs. 2 and 3 (where θ =
0). It means that when the material is locally under ten-
sion, it can undergo plastic deformation at less than
70% of the base material strength. The measured frac-
ture strength was 900-950 N, which is 62-65% of the
tensile strength of aluminum substrate, and similar to
the value predicted by the equation.
120 https://doi.org/10.1038/nature11475
2. S. S. Lee, T. H. Kim, S. J. Hu, W. W. Cai and J. A.
Abell, Joining technologies for automotive lithium-ion
90
battery manufacturing: A review, in ASME 2010 inter-
Hardness (HV)