A Study of Friction Behavior in Ultrasonic Welding (Consolidation) of Aluminum

C.B. Zhang1, X.J. Zhu2 and L.J. Li3

Mechanical & Aerospace Engineering Utah State University

Abstract: In the present study, the friction behavior of the ultrasonic welding process is studied. Studies show that the friction coefficient of Al 3003-H18 thin foils during ultrasonic welding has a non-liner relationship with test temperature. Studies also show that the sliding speed has very limited influence on the friction coefficient of the alloy. A coupled-field finite element model is developed to predict the frictional heat generation at different stages of the ultrasonic consolidation process. The effect of friction conditions on the joint formation has been simulated with the coupled-field FE model. Results show the following coupled interactions during ultrasonic welding: changes in process parameters affect the friction condition - friction affects the heat generation - heat affects the local plastic deformation and material flow (for joint formation) - plastic deformation affects the friction condition. Friction has been shown to be the most significant mechanism that controls the ultrasonic bonding and the quality of the joints. Key Words: ultrasonic welding, friction coefficient, Al alloys, 1 Introduction Ultrasonic Welding (UW) method is widely used in many industrial areas such as microelectronic [1], automotive and aerospace [2]. Investigation on the ultrasonic welding mechanics by analytical and experimental methods, including photoelasticity and microscopy [3, 4], and heat transfer by analytical and calorimetric techniques [5] has been carried out by many researchers. However, there is still a need for fundamental understanding and quantitative description of ultrasonic welding under rapid prototyping process conditions. The most important behavior of the UW process is believed to be the friction behavior. Unfortunately, studies in this area are limited [6]. It is clear that characterization of the friction conditions at the interface of thin foils is essential for further understanding of UW. Generally speaking, the friction influences the temperature of the bond region and the substrate. Different friction coefficients will generate different amount of heat during the ultrasonic welding process. On the other hand, the increased temperature will in return influence the friction coefficient. During the UW process, both the temperature of the substrate and the friction coefficient µ are not constant parameters. 1 Graduate Research Assistant 2 Research Associate 3 Faculty member

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The objective of this study is to investigate the relationship between the friction coefficient and the temperature of the substrate. A numerical model will be set up for this purpose. This simulation will be used to study the effect of friction on dynamics, stress distribution, and propagation of plastic deformation in forming an ultrasonic weld.

2 Experimental Procedures Commercially 3003-H18 thin aluminum foil with a thickness of 0.1mm was used in this study. Specimens were machined to dimensions of 110 mm long, 15 mm wide. In order to remove the contamination on the specimen surfaces, all the samples were cleaned by acetone immediately before the test. The schematic representation of the apparatus is given in Fig. 1. The experimental apparatus consisted of a pair of die-sets with flat surfaces. On the surface of each die halve, a layer of aluminum thin foil was firmly fixed by screws. The test specimen was set between the two die halves and a 10kg load was applied on the top die halve. All tests were conducted on a Gleeble™ 1500D system. During the test, the lower die halve was first heated to the desired temperature at a heating rate of 10°C/min, followed by a 30-minute hold time to ensure thermal equilibrium. Then the specimen foil was pulled out at different speeds and the friction coefficient was calculated by the following formula:

µ = F/2P where P is the total load applied on the thin foil and F is the pull force.

Fig. 1. Schematic representation of apparatus for pulling test.

3. Result and discussion The effect of temperature and pulling speed on the friction coefficient is shown in Fig. 2. It can be seen that the pulling speed has little influence on friction coefficient. As the pulling speed varies from 0.80 to 800 mm/s, the coefficient remains at the same level of 0.27. Fig. 2b also shows that the temperature has a dramatically influence on the friction coefficient. At room temperature, the coefficient is about 0.274. With increasing test temperature, the coefficient increases steadily and reaches maximum point of 0.40 at 150°C and 200°C, then decreases with further increasing of temperature. The experimentally obtained friction coefficient was employed for the modeling of the ultrasonic welding process.

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Fig. 2 Variation of friction coefficient against pulling speed (a) and temperatures (b).

Fig. 3 shows the 3-D coupled-field dynamic model involving the sonotrode that is in close contact with and vibrates against the substrate. Because the foil is very thin, it is not shown in figure assuming that the top layer of substrate has the same behavior as the foil. A temperature dependent friction coefficient of the contact surface is included in this model. This contact and coupled field problem was solved numerically with the multi-physics modeling package ANSYS. The meshed model in ANSYS is also shown in Fig. 3.

Fig. 3-D Coupled-field dynamic model for ultrasonic consolidation

The simulated results consist of changing temperature and strain fields. Time snap shots of these fields are presented here. In Fig. 4 and Fig.5, the 2-D projections are given for the 3-D distributions of von Mises strain at the 20th and 150th vibration cycle since the start of simulation. Comparing the top views, it can be seen that at an earlier stage of joining (20th cycle), the von Mises strain concentrates at the two edges of the top layers of substrate, while the middle area is in a lower strain condition. After 150 cycles, the peak von Mises strain becomes more uniformly distributed in the top view. While the side views do not show significant changes, the front views provide the information similar to the top views. At 20th vibration cycle, the front view shows the concentrated stains at the two edges near the bonding region. At 150th vibration cycle, the front view

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shows the maximum strain distributed to the center of the bond region, with the entire contact area under peak strain.

Fig. 4. Distribution of von Mises strain at the 20th vibration cycle from the start of the simulation.

Fig. 5 Distribution of von Mises strain at the 150th vibration cycle from the start of the

simulation.

The temperature distribution in ultrasonic consolidation is shown below for the study of heat generated by contact friction which play an important role in foil bonding and is closely related with plastic deformation and diffusion bonding mechanism. In Fig. 6 and Fig.7, the 2-D projections are given for the 3-D distributions of temperature at the 20th and 150th vibration cycle since the start of simulation. At an early stage of bonding formation (20th cycle), the peak temperature is located at the edges of the bond area. At a later stage (150th cycle), the peak temperature has moved to the center of the bond region. Based on these simulation results, ultrasonic bonding is believed to first start at the two edges of the foil and gradually move to the central part with increased vibration cycles.

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Fig. 6. Distribution of temperature at the 20th vibration cycle from the start of the simulation.

Fig. 7. Distribution of temperature at the 150th vibration cycle from the start of the simulation.

The friction force at the contact area also has its peak value distributed at the two edges of the bond region. Larger friction force will generate more heat and cause a higher temperature at the edge areas. The reason for the peak temperature and strain to move to the center of the bond area can be explained by the coupling effect between the thermal and mechanical fields. The friction force dominates the bonding process because it determines the distributions of strain and temperature as both a primary force causing large deformation and a primary heat source causing high temperature in early stages of bond formation. However, as bonding is progressing, the temperature in central region of the bond area will rise due to conduction, while the temperature at the edge regions will not keep rising due the heat transfer boundary conditions (radiation and convection heat loss). As the bulk of the bond area becomes hot, the mechanical properties of the material start to drastically decrease. Under this condition, even a small strain can cause plastic deformation. Heat generated by such plastic deformation starts to further raise the

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temperature at the central region. Such interaction between the thermal and mechanical fields will start a “self-propagating”, chain-reaction process. 4 Conclusions In summary, the friction coefficient of Al 3003-H18 thin foils shows a nonliner relationship with the temperature. The friction coefficient increases with increasing test temperature, and reaches the maximum at 150°C before decreases to a level similar to that at room temperature. However, sliding speed has very little influence on the friction coefficient at room temperature. A new FEM model using ANSYS has been built for a better understanding of the effects of friction in ultrasonic consolidation bonding mechanism. Based on the simulation results, the ultrasonic vibration generates heat at the interface through friction; the heat lowers the mechanical properties of the material, which enhances localized plastic deformation; more plastic deformation generates more friction and heat. It is the friction force that dominates the bonding process in early stages; the friction effect is believed to be overtaken by the heat generated by large deformation and high diffusion rates in later stages of bonding. Reference:

1. Harman, G.G., and Albers, J., 1997, “Ultrasonic welding Mechanism as Applied to Aluminum-Wire and Gold-Wire Bonding in Microelectronics,” IEEE Trans. Parts, Hybrids Packaging, 13, No. 4, pp. 406-412.

2. Aro, H., Kallioniemi, H., and Aho, A. J., 1980, “Ultrasonic Welding of Experimental Osteotomies,” Acta Orthop. Scand., 51, No. 4, pp703.

3. Devine, J., 1984, “Joining Metals with Ultrasonic Welding,” Mach. Design, 56, No. 21, pp.91-95.

4. Kirzanowski, J.E., 1989, “A Transmission Electron Microscopy Study of Ultrasonic Wire Bonding,” Proc. 39th IEEE Electronic Components Conf, Houston, TX, pp.450-455.

5. Mozgovoi, I.V., 1984, “The Role Played by Shear Strains in Heat Generation in the Ultrasonic Welding of Polymers,” Welding Production, 31, No. 8, pp. 11-14.

6. Gao, Y., and Doumanidis, C., 2002, “Mechanical Analysis of Ultrasonic Bonding for Rapid Prototyping,” J. of Manufacturing Science and Engineering, Trans. of the ASME, 124, No. 5, pp. 426-434.

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