Summary of process advantages and disadvantages
Having reviewed the principles, key features and applications of ultrasonic metal welding, both the advantages and disadvantages of the process may be evident at this stage. Nevertheless, a summary of these features is in order.
• A solid-state welding process - hence low heat
• Excellent for Al, Cu and other highly thermal conductive materials
• Able to join wide range of dissimilar materials
• Can weld thin-thick combinations
• Welds through oxides and contaminants
• Fast, easily automated
• No filler metals, welding gases required
• Low energy requirements.
• Restricted to lap joints
• Limited joint thickness
• High strength, high hardness materials difficult to weld
• Material deformation may occur
• Noise from part resonance may occur
• The process is often unfamiliar.
These points will be discussed below in more detail.
Solid-state joining process
Many of the advantages of ultrasonic metal welding stem from its basic solid-state nature. The weld zone, where joining of parts occurs, is a region of plastic deformation and flow, resulting in local mechanical distortion of grain structure, but with absence of melting, fusion, or other evidence of high temperatures relative to melting. Little modification of grain structure is seen away from the deformation zone, which is itself thin, of the order of tens of micrometers at most.
Welding of aluminum, copper, etc.
The ultrasonic materials weldability matrix shows ‘in principle’ that nearly every material and most material combinations can be welded. Nevertheless, the softer metal alloys, such as those of aluminum and copper have been shown to be the most weldable. Because of the high thermal conductivity of these alloys, they have often proved the more difficult to weld by more traditional methods, such as resistance spot welding. This benefit of welding high thermal conductivity materials is, of course, tied to the solid-state, non - thermal nature of the process.
Joining of dissimilar materials
Note has been taken of the range of dissimilar metal combinations that are reported to have been welded; this advantageous feature is again due to the solid-state nature of the process. A wide variation in welding conditions, depending on materials may be necessary. Thus, a weld readily achieved with 1.5kW, 0.25 s and 900N of static force in 1mm 6061T6 aluminum, may require 2.5 kW, 0.5 s and 1800 N for 2.5 mm, if the weld is to be made. In general, well-developed weld procedures for a wide range of materials and conditions do not currently exist (as they do, for example, in arc welding), so that each combination must be approached in an exploratory manner. In some cases, joints in widely dissimilar materials are assisted by thin foil transition layers of some intermediate material. Joints have also been made in metal-ceramic, metal-glass combinations, in some cases using transition layers or metallized coatings.
Yet another advantage accruing from the solid-state nature of the process is the ability to join thin sheets and foils (in the ranges of 0.025-0.250mm) to thick parts. Making such joints, especially in materials with high thermal conductivity can be difficult using other procedures due to the large heat sink of the thick material drawing heat from the weld zone. Such joints are readily made with ultrasonic welding, including joining of multiple foils to a thick substrate with a single weld.
Oxides and contaminants
By the very nature of the inherently friction-like ultrasonic welding action, oxides and contaminants are fractured, disrupted and dispersed. For example, any oil-like surface residues are quickly vaporized in the early stages of the cycle, and some oxides will be dispersed into the weld periphery, or into the turbulent micro-volumes of plastically deformed material at the interface. In summary, the ultrasonic welding process can be tolerant of less than ideal surface conditions. Nevertheless, there is sometimes the tendency to assume the process can achieve successful welds through any level of surface contamination, or that control of surface conditions is unnecessary, both of which are unsound assumptions. As a minimum, different conditions will yield variations in the process, such as changeable weld times, and as a maximum, variation in a weld quality such as strength. While special cleaning methods may only be rarely needed, due consideration must be given to consistency of surface conditions.
Fast, easily automated
The typical ultrasonic weld cycle, from start to stop of the ultrasonic vibration, is a fraction of a second, with 0.2-0.5 s being common. Other time components of the weld-to-weld cycle are tied to details of the welder clamping force and retraction mechanism, and are thus subject to being minimized by appropriate machine design and control. Any other aspects of weld-to-weld time are a function of the manufacturing system requirements of the specific application. No demands, such as cooling time or setting time, are placed on total cycle time by the weld process. The basic electromechanical nature of the process and simple mechanical features of the clamping action make it ideal for automation and continued monitoring and control for production and quality assurance.
No filler metals, gases required
Neither filler metals nor shielding nor consumable gases are needed in the usual ultrasonic welding processes. In some cases, in attempting difficult materials combinations, a thin foil transition layer has been placed between the parts. In those cases where welds are made under hazardous conditions (e. g. closure of explosive containers), an inert gas-filled enclosure can be used.
Low energy requirements
Typically, selection of the ultrasonic welding process is based on one or more of the above advantages, most being related to the solid-state nature of the process. If energy use is a factor, it is found that ultrasonic welding is a low energy user compared to other processes; thus, it uses about one-sixth of the energy of resistance spot welding for comparable welds and even less if compared to arc process welding.
In considering some of the process disadvantages, we should note the following:
Restricted to lap joints
The requirement to achieve a friction-like interface vibration and the limits on the amount of mass that can be moved with ultrasonic systems, logically requires the moving welding tool to be close to the interface, which results in the lap joint constraint of the process. Thus, butt, tee and corner joints, easily made in fusion processes, are not yet possible with current ultrasonic systems.
Limited in joint thickness, material hardness
These items have been covered in some detail and appear in both cases to be related to current restrictions on available ultrasonic power. In aluminum alloys of the 5XXX, 6XXX classes, 2 mm joints are close to the outer limits achievable with current 20 kHz systems and drop to the order of 0.1 mm for titaniums and harder alloys.
The ultrasonic weld tip will typically create some deformation of the top surface in the softer alloys. This will be more or less pronounced depending on weld tip surface (whether flat, slightly hemispherical and/or serrated or otherwise roughened or knurled) and welding conditions (forces and powers involved). Deformations depths of 5% to 10% of part thickness may result. Anvil-side deformations can arise from the same circumstances, but typically are far less pronounced. Through special attention to tip design and parameters, it is usually possible to reduce, but seldom eliminate, some part deformation or marking.
Being an ‘ultrasonic’ process, it would seem that audible sound would not be an issue in ultrasonic welding. Two aspects arise, however. The first and most common is that the 20 kHz (or higher) welding frequency may induce subharmonic vibrations in larger parts, which are in the audible range. In such cases, it may be possible to dampen these modes by light clamping of the part at one or more locations. The second aspect is that for some higher power welders, a driving frequency of 15kHz-16kHz may be used, frequencies that are in the high audible range. For these cases, an acoustic enclosure is needed to shield the radiated sound.
The very large majority of welding processes used in production are fusion based, using electric arcs, resistance heating or high energy beams, such as lasers or electron beams. Ultrasonic metal welding, being a solid-state process, and, further, one that is based on high frequency vibration mechanics, is not typically encountered in the education and experience of manufacturing and welding engineers. Dealing with the unfamiliar is not typically a comfortable option, especially when faced with the pressures of modern manufacturing, and can be a disadvantage in considering the ultrasonic process.
The future direction of ultrasonic metal welding is being driven by pushing back the current main boundaries of the process which are (1) joint thicknesses, (2) weldable materials and (3) joint types. The steps being taken in these directions, or that can be envisioned, are briefly summarized below.
Most ultrasonic metal welders operate at 20 kHz and in the 2.5kW-3.5kW range. Such systems have been sufficient to achieve most of the results described previously, but also face limitations in joint thicknesses and materials that can be joined, as have been described. One area of future development will be the introduction of more powerful welding systems, with this achieved by development of more powerful transducers. Thus, 5kW-6kW systems operating at 20 kHz are now becoming available on a select basis. This trend is expected to continue, with powers climbing to yet higher levels, potentially to 10 kW. An additional way of increasing power of welders is to couple two or more additional transducers to the weld head, achieving an additive effect of the individual transducers. An example of this was shown for the case of the torsion welder, Fig. 9.9, where four transducers were harnessed in a push-pull fashion. The design of coupling devices requires the solution of a complex tool vibration problem, but has been done for special cases. Another alternative being explored is to apply vibrations to both top and bottom of the workpieces, using two welding systems.
Over the years, research has significantly advanced understanding of the ultrasonic metal welding process, both from a metallurgical and a mechanics perspective. Still, much remains to be done in several areas; for instance, the full range of the weldability of materials must be better understood, including specific procedure data on the various metallurgical combinations. This latter relates to development of benchmark welding procedures for some of the more common material combinations. A necessary and important development is that of providing a mechanics-based model of the welding process, with such a model relating welding and weld quality (e. g. weld strength) to measurable weld parameters, such as vibration amplitudes, forces and power inputs. Some work has been accomplished here (e. g. Gao and Doumanidis, 2002; de Vries, 2004), but more remains to be done in order to provide methods of real-time monitoring and quality assurance.
Some limited progress has occurred in this area, but progress may be slow in achieving welds in completely new joints. Achievement of butt welds has been reported, for example by Tsujino et al. (2002), who has developed a means of creating a butt weld by clamping one part and creating a bending resonance in the second part, resulting in the friction-like vibration at the interfaces characteristic of the ultrasonic welding action. For small parts, butt welds have been made by attaching the small part to the vibrating weld tip. These developments notwithstanding, it is difficult to envision ultrasonic butt welds becoming routine joint geometry. One may also inquire whether a concept of ultrasonic metal ‘far field’ welding might develop, such as is done in ultrasonic plastic welding, where vibrations are applied to a part at one location and a weld created at a part interface some distance removed. If such were possible for metals, butt and tee joints would be feasible. However, no instances of such welds are known. It is believed that joint types for ultrasonic metal welding will largely remain restricted to lap-type configurations.
In summary, of the three areas of (1) joint thicknesses, (2) weldable materials and (3) joint types, significant progress is expected in the first two, driven by more powerful welding systems and improved understanding of the ultrasonic welding mechanism, but with the process still having its main application on lap-type joints.
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