New developments in advanced welding

Ultrasonic welding equipment

In reviewing the principles of ultrasonic metal welding, the basic features of two of the most widely used systems, the lateral drive and the wedge-reed, have been described and shown in Fig. 9.1 and 9.4. An example of a 20 kHz,

2.5 kW wedge-reed welder is shown in Fig. 9.7. These various systems produce spot welds whose areas depend on the specifics of materials, thicknesses and welder power capabilities. Thus, 2.5kW-3.0kW, 20 kHz welders can produce spot sizes of the order of 40 mm2 or larger.

Another type of weld that can be achieved with ultrasonics is a seam weld. The basic features of an ultrasonic seam welder are shown in Fig. 9.8, where a continuously turning, lateral drive type of ultrasonic transducer vibrates a circular disk sonotrode that traverses the workpieces, producing a continuous weld. The details of fixturing and moving the rotating transducer system, including bearings and drivers, are not shown in this simplified diagram. The disk is machined as an integral part of the sonotrode, which requires turning the entire transducer-sonotrode assembly. A fixed anvil is shown in the figure, but in practice the anvil may be moving in unison with the turning sonotrode disk. The means of achieving this synchronous motion may be in the form of a turning disk anvil, or simply a laterally moving flat anvil base. In some cases, the rotating ultrasonic transducer is fixed in space, or it may be traversed along the workpiece. As with spot welders, the transducer must be leveraged to apply a static force to the workpieces as it turns. Seam welders find extensive use in joining aluminum and copper foils.

Another means of achieving an ultrasonic weld is to impart a twisting or torsional motion to specially designed horns and tooling, thus producing an ultrasonic torsion weld (sometimes called a ‘ring weld’). The means of

achieving this is shown in Fig. 9.9, where two ultrasonic transducers, operating in longitudinal vibrations as previously shown in Fig. 9.2, are attached (typically they are welded) to ultrasonic boosters and tooling to produce a push-pull or torsional vibration to the booster system. The booster-tooling is specifically designed to be resonant in the torsional mode, versus the longitudinal mode, as used in the previous systems. The torsional resonance produces a circular vibration at the sonotrode and welding tip, which in turn creates a circular weld pattern on the workpiece. Although the vibration is of a circular nature, the motion of the tool surface is still parallel to the workpiece surface, as in

the spot welding action. Hence, the nature of the weld that is created is the same as that produced in spot and seam welding.

The concept of the torsion welder has been illustrated for two transducers. Depending on power requirements, from one to four transducers may be used. A four transducer torsion welder can produce up to 10kW of power at 20kHz.

9.3 Mechanics and metallurgy of the ultrasonic weld

In examining the mechanism of ultrasonic metal welding, it is helpful to start with the weld zone, as first depicted in Fig. 9.5(a), where the workpieces are shown adjacent to each other and Fig. 9.5(b) where they are shown in separation. This separation along the plane of the pre-welding interface also displays the primary forces present in making an ultrasonic weld, namely the shearing force, caused by the transverse ultrasonic vibration of the parts, and the normal force, caused by application of the static clamping force. The shear, normal force vectors are, of course, the result of a distribution of normal and shear stresses over the contacting surfaces of the two parts.

Now consider the condition of the two surfaces that will be in the zone of welding. Examined on a magnified scale, it is realized that the opposing surfaces consist of peaks and valleys whose profile depends on the surface finish of the materials. It is further realized that the surfaces when pressed in contact, will initially only be in contact at intermittent asperities or ‘high spots.’ While the number of contact points will depend on surface roughness

,c>

(c)

9.10 Development of contact surfaces in ultrasonic welding: (a) initial contact through asperities; (b) and (c) progression of shearing, deformation and formation of weld zones as the weld develops.

and clamping force, the general nature of initial static contact between a small region of the surfaces within the weld zone is shown in Fig. 9.10(a). Further, it is realized that the surfaces have oxide coatings, as well as possible surface contaminants, such as finishing or forming lubricants and absorbed moisture, which generally prevents a pure metal-to-metal contact between the surfaces, although at some contact points there may be penetration of oxides and local microwelds might occur.

When the ultrasonic vibrations are started, the top piece moves relative to the bottom piece in a transverse, friction-like motion. Plastic deformation and shearing of the interfering asperities occurs, cutting through surface contaminants and fracturing and dispersing surface oxides, resulting in increased metal-to-metal contact across the surfaces and formation of weld zones (also called ‘microwelds’). Continued vibrations result in increased areas of contact, until complete or nearly complete contact and joining of the surfaces has occurred and a weld between the parts developed.

Numerous studies of the progression of this weld process have been done by stopping the weld cycle at various stages and peeling apart the surfaces. These studies show initial intermittent ‘islands’ of bonded surface. Initial bonding may also occur around the circumference of the weld. This particular feature is a consequence of using a welding tip that has a shallow spherical
curvature, which creates a contact stress distribution that has a maximum at the circumference. Flat weld tips do not exhibit this feature. The short, elongated striations visible at the start of the process correlate with the direction of ultrasonic vibration, but not with the amplitude, which may only be on the order of 10-15 mm at the interface. They instead represent the growth of microwelds from initial contacts. Progressive growth of the microwelds occurs until a point is reached at which a nearly complete weld is achieved.

The nature of the bond that is formed across the interface of the parts is solid state; that is, it has been achieved without melting and fusion of the workpieces, but instead brought about by direct metal-to-metal adhesion of the solid materials. While the bond is solid state, this does not suggest that temperature does not play a role in the process. The plastic deformation that occurs results in a noticeable temperature rise, with this rise varying with materials and welding conditions, but always being below melting temperatures. However, the yield point of materials is temperature sensitive, and it is found that ultrasonic welding temperature increases are sufficiently great to cause reduction in the local yield strength of materials in the weld zone. This reduction in turn enhances further plastic deformation and flow of the materials in the weld zone.

It should be noted that a number of studies have been done on the temperatures of the weld zone and at the weld interface, typically using thermocouple and infrared techniques. Temperatures were generally found to rise very quickly in the initial welding stage, then to remain stable for the remainder of the cycle. Temperature rises varied greatly with the metals and metal combinations being welded. The heat generated by plastic deformation is quite localized at the interface and may be sufficient to cause recrystallization and diffusion. Studies of aluminum welds, for example, showed maximum temperatures reached to be on the order of 400 °C. In general, the temperatures reached during welding will depend on the mechanical properties of the harder/stronger of the materials welded. Thus, temperatures of a copper - Monel weld would be higher than those of a simple copper-copper weld.

This general description of the weld process has been basically a mechanistic one, where local vibration, plastic deformation and heating create the circumstances for a solid-state metallurgical bond to be achieved. Metallurgical examination of the weld zone shows local plastic deformation to be confined to a very small thickness, as brought out by Fig. 9.11 for the case of a weld in 6061T6 aluminum which shows a typical section of a completed ultrasonic weld. The ‘sawtoothed’ upper surface is a result of the weld tool imprint on the top weld part. The thickness of the actual weld zone of deformed material is quite small, approximately 50 mm for this particular case. This layer consists of very fine grain structure resulting from heavy plastic deformation. After 0.1 s of weld time (i. e. approximately a third of a 0.3 s weld cycle), one can

see discrete deformation islands or microwelds separated by unbonded surface. At completion of a 0.3 s weld, one arrives at a continuous interface layer. The grain structure just a short distance from the weld is substantially undisturbed, while an irregular thin zone of fine or even amorphous structure exists in the zone. There is some undulation of the pattern, which is sometimes seen to transfer to near-turbulent patterns of mixing in some regions of the zone.

Given this background information on the mechanism of ultrasonic welding, and some examples of metallurgical features, it is natural to inquire as to what metals are ultrasonically weldable. ‘Weldability’ as defined by The American Welding Society refers to the ease with which materials may be joined to meet the conditions of their intended service. Over the years, a large number of materials and material combinations have been investigated for their ultrasonic weldability. As a starting guideline, the chart shown in Fig. 9.12 may be used. Thus, metals that have been shown to be weldable are listed along the top of the chart and range from aluminum to zirconium. These same materials, or their alloys, are shown along the chart diagonal. Most materials can be joined to themselves or their alloys, that is, ‘monometal’ welds. Exceptions to this are germanium and silicon. It is seen that aluminum is exceptionally weldable, having been joined to all the listed metals. Other easily welded materials include copper alloys and the precious metals (gold, silver, platinum). On the other hand, iron and its alloys including steels, and refractory metals, such as molybdenum and tungsten, while weldable, can typically only be used in thin gages. Welding of softer alloys to these materials is more readily accomplished, however. In general, it is found that ease of weldability is tied to ease of plastic deformation, so that material hardness and yield strength play important roles in ultrasonic welding, with higher strength and hardness materials being increasingly difficult to weld.

While attention here has been on metal weldability, it should be noted that ultrasonic welding has been used to achieve joints between metals and non­metals. In particular, metals have been joined to ceramics and types of glass. Typically, the metals are in the form of foils and are those of more easily bonded metals (e. g. aluminum, copper). In most cases a thin foil transition layer is used, or a thin metallization has been applied to the glass or ceramic substrate.

Al Be Cu Ge Au Fe Mg Mo Ni Pd Pt Si Ag Ta Sn Ti W Zr

Al Alloys

Be Alloys

Cu Alloys

Ge

Au

Fe Alloys

Mg Alloys

Mo Alloys

Ni Alloys

Pd

Pt Alloys

Si

Ag Alloys

Ta Alloys

Sn

Ti Alloys

W Alloys

Zr Alloys

9.12 Ultrasonic material weldability (source: American Welding Society, 1991).

In addition to the limits imposed by materials on the scope of ultrasonic welding, there are limits imposed by the geometry and dimensions of the parts being welded, chief of which are thickness limitations. Thickness limits can be understood in terms of the contact stresses acting between the surfaces being welded and their relationship to weld tip geometry and thickness of the top welded part.

Using Fig. 9.5(b) as a starting point, the general case of a flat welding tip is illustrated in Fig. 9.13(a), where interface shear stresses and the vibration component of the weld tip have been omitted for simplification. Thus, the contact stresses between the flat weld tip and top surface are shown as approximately uniform. These contact stresses are transmitted to the weld interface, spreading out somewhat to cover a wider contact area, with this resulting in a reduced contact stress amplitude and a smoothing out of the edges of the stress distribution.

If the top part is very thin, there will be very little reduction of contact stress between the tip and the interface, as brought out in Fig. 9.13(b-1),

Top part

(3)

(b)

9.13 Contact stresses in weld zone: (a) stresses at weld tip, top and bottom parts; (b) stresses on top part with increasing part thickness (1)-(3).

where just top part stresses are shown. On the other hand, if the top part is quite thick (with ‘thin’ and ‘thick’ being measured relative to the width of the weld tool), then the amplitude and shape of the contact stresses may be greatly changed. These situations are shown in Fig. 9.13(b), where the focus is reduced to just the top part. Thus, in the thin top part of Fig. 9.13(b-1), the contact stresses are little changed in transmission to the interface. Figure 9.13(b-2) is the situation shown in Fig. 9.13(a). The case of a thick top part is shown in Fig. 9.13(b-3), where the form and magnitude of the contact stresses are greatly changed at the interface.

Now, under ultrasonic vibration, the contact stresses directly influence the weld producing interface shearing stresses. If, for a given thickness, the total static clamping force is too small, the resulting contact stresses, and resultant shearing stresses, may be insufficient to create a weld. Obviously an increase of static force will increase contact stresses, but if part thickness is too great, the amount of deformation at the top surface may be excessive, before conditions of welding are reached at the interface. The general relationship of part thickness, weld tip size and contact stresses is important in understanding the basic factors influencing welding. Thus, increased part thickness requires an increased size of weld tip in order to maintain uniformity and level of stresses at the interface. Increased weld tip size in turn demands increased clamping forces to maintain stress levels for welding. These in turn demand greater power of the ultrasonic welding system in order to drive the welding
tip, under high forces, at the vibration amplitudes needed to achieve the necessary shearing stresses and welding action at the interface. While this provides a general description of the issues of part thickness and welding, there are no current formulas that relate these issues of tip size, part thickness and materials to weldability.

Another issue of welding relates to overall lateral dimensions and shape of the parts being welded. It is evident that achieving an ultrasonic weld relies on vibrating the top piece relative to the bottom piece. Some motion is absorbed in the local plastic deformation at the weld zone, but there is also some net motion imparted to the top part. In many applications of ultrasonic welding, the dimensions and mass of the top part are small, i. e. dimensions are of the order of the welding tip (e. g. electrical contacts), or the mass of the part is uncoupled from the weld zone (e. g. as when welding flexible cables and wires), so that the impact of top part motion may be neglected. However, in applications where top part dimensions begin approaching the longitudinal vibrational wavelength in that material, the forces acting in the weld zone may be affected by top part mass and dimensions. In particular, the interface shearing forces may be impacted by these vibration phenomena, dropping dramatically and ceasing to create a weld. In general, when welding larger parts that have dimensions of the order of acoustic wavelengths, attention must be paid to part vibrations as they may affect the weld being made or welds previously made.