Understanding ultrasonic welding - What it is; what it does; where ...

Tag : structural steels

A number of parameters can affect the welding process, such asultrasonic frequency, vibration amplitude, static force, power,energy, time, materials, part geometry, and tooling.
Ultrasonic Frequency. Ultrasonic welding transducers are designedto operate at a specific frequency from 15 to 300 kHz for differentsystems and applications. Most metal welding systems operate at 20to 40 kHz, with 20 kHz being the most common frequency.
Vibration Amplitude. The vibration amplitude of the welding tip istied directly to the energy delivered to the weld. Ultrasonicvibration amplitudes are quite small—10, 30, or 50 microns atthe weld, and seldom exceed 100 microns (approximately 0.004 inch).In some welding systems, the amplitude is a dependent variable;that is, it is related to the power applied to the system. In othersystems the amplitude is an independent variable capable of beingset and controlled at the power supply through a feedback controlsystem.
Static Force. The force exerted on the workpieces via the weldingtip and anvil creates intimate contact between the opposingsurfaces as the weld vibrations begin. The magnitude of the force,which depends on the materials and thicknesses, as well as the sizeof the weld produced, may be from tens to thousands of newtons. Forexample, producing a weld of 40 mm2 in a 6000 series aluminum mayuse force of 1,500 N, while 10 mm2 welds in 0.5-mm-thick softcopper sheet may require only 400 N.
Power, Energy, and Time. While listed as separate weld parameters,power, energy, and time are best examined together since they areall closely related. When a weld is made, the voltage and currentfrom the power supply result in electric power that flows to thetransducer during the weld cycle. The energy delivered is the areaunder the weld power curve. Most welding power sources are rated bythe peak power they can deliver, with this varying from a fewhundred watts to several kilowatts. Most weld times are found to beless than one second. Based on constant power output, a 0.4-secondweld from a 2-kW welder would deliver 800 joules of energy.
Materials. This encompasses a wide range of issues and parametersrelating to ultrasonic metal welding. First is the type of materialor material combination. Most materials and material combinationshave been found to be weldable in some fashion, although specificweld parameter and performance data is generally lacking for mostof them. The properties of the material, including modulus, yieldstrength, and hardness, are a key consideration.
Generally speaking, soft alloys like aluminum, copper, nickel,magnesium, gold, silver, and platinum are most easily weldedultrasonically. Harder alloys such as titanium, irons and steels,and nickel-based aerospace alloys and refractory metals (molybdenumand tungsten) are more difficult.
Material surface characteristics is another parameter, with theseincluding finish, oxides, coatings, and contaminants.
Part Geometry. The shapes of the welded parts play an importantrole, the dominant factor being part thickness. Generally speaking,thin parts have a better chance of achieving a successfulultrasonic weld. Increasing the part thickness, in particular thepart contacting the welding tip, requires a larger welding tiparea, more static force, and higher weld power. Maximum achievablethicknesses will depend on the material and the welding powersource's available power.
Tooling. Comprised of the sonotrode/welding tip and the anvil,tooling serves to support the parts and to transmit ultrasonicenergy and static force. In most cases, the tool tip is machined asan integral part of a solid sonotrode (see Figure 2A ), but in some cases detachable tool tips are used. The toolingcontact surfaces typically have machined knurled patterns ofgrooves and lands or other surface roughening to improve workpiecegripping.
While the weld tip and anvil contact surfaces are usually flat, theweld tip may be designed with a slight convex curvature in order tochange the contact stresses. Ultrasonic Welding Applications