• U-GROOVE WELD

    A type of groove weld. 

  • ULTRA-SPEED WELDING

    A nonstandard term for COMMUTATOR CONTROLLED WELDING. 

  • ULTRASONIC COUPLER, Ultrasonic Soldering and Ultrasonic Welding

    Elements through which ultrasonic vibration  is transmitted from the transducer to the tip. 

  • ULTRASONIC SOLDERING (USS)

    A soldering process variation in which high-frequency vibratory energy is transmitted through molten solder to remove undesirable surface films and thereby promote wetting of the base metal. This operation is usually accomplished without flux. 

  • ULTRASONIC TESTING (UT)

    A nondestructive test (NDT) method in which beams of high-frequency sound waves are introduced  into a test object to detect and locate internal discontinuities. A sound beam is directed into the test object on a predictable path, and is reflected at interfaces or other interruptions in material continuity. The reflected beam is detected and analyzed to define the presence and location of discontinuities.

    The detection, location and evaluation of discontinuities is possible because (1) the velocity of sound through a given material is nearly constant, making distance measurements possible, and (2) the amplitude of the reflected sound pulse is nearly proportional to the size of the reflected discontinuity.  Ultrasound wave is electronically collected and presented on a cathode ray tube (CRT) screen for evaluation by a qualified and certified ultrasound technician.

    Ultrasonic testing can be used to detect cracks, laminations, shrinkage cavities, pores, slag inclusions, incomplete fusion or bonding, incomplete joint penetration, and other discontinuities in weldments and brazements. With proper techniques, the approximate position and depth of the discontinuity can be determined, and in some cases, the approximate size of the discontinuity can be determined.

    Advantages

    The principal advantages of UT compared to other NDT methods are the following:

    (1) Discontinuities in thick sections can be detected.

    (2) Relatively high sensitivity to small discontinuities is exhibited.

    (3) Depth of internal discontinuities can be determined; size and shape of discontinuities can be estimated.

    (4) Adequate inspections can be made from one surface.

    (5) Equipment can be moved to the job site.

    (6) Process is nonhazardous to personnel or other equipment.

    Limitations

     

    The following limitations apply to ultrasonic testing:

    (1) Set-up and operation require trained and experienced technicians, especially for manual examinations.

    (2) Weldments that are rough, irregular in shape, very small, or thin are difficult or impossible to inspect; this includes fillet welds.

    (3) Discontinuities at the surface are difficult to detect.

    (4) A coupler is needed between the sound transducers and the weldment to transmit the ultrasonic wave energy.

    (5)  Reference standards are required to calibrate the equipment and to evaluate the size of discontinuities.

    (6) Reference standards should describe the item to be examined with respect to design, material specifications, and heat treatment condition.

    Equipment

    A block diagram of a pulse-echo flaw detector is shown in Figure U-1. Most ultrasonic testing systems use the following basic components:

    (1) An electronic signal generator (pulser) that produces bursts of alternating voltage.

    (2) A sending transducer that emits a beam of ultrasonic waves when alternating voltage is applied.  The sound wave frequencies used are between 1 and 6 MHz, which are beyond the audible range. Most weld testing is performed at 2.25 MHz. Higher frequencies, Le., 5 MHz, will produce small, sharp sound beams useful in locating and evaluating discontinuities in thin wall weldments.

    (3) A coupler to transmit the ultrasonic energy from the transducer to the test piece and vice versa.

    (4) A receiving transducer to convert the soundwaves to alternating voltage. This transducer may becombined with the sending transducer.

    (5) An electronic device to amplify and demodulate or otherwise change the signal from the receiving transducer.

    (6) A display or indicating device to characterize or record the output from the test piece.

    (7)An electronic timer to control the operation.


    There are three basic modes of propagating sound through metals: longitudinal, (sometimes called straight or compressional), transverse (also called shear wave), and surface waves (sometimes referred to as Rayleigh waves). In the longitudinal and transverse modes, waves are propagated by the displacement of successive atoms or molecules in the metal.

    Longitudinal wave ultrasound is generally limited in use to detecting inclusions and lamellar-type discontinuities in base metal. Transverse wave ultrasound is most valuable in the detection of weld discontinuities because of its ability to furnish three-dimensional coordinates for discontinuity locations, orientations, and characteristics. The sensitivity of shear waves is also about double that of longitudinal waves for the same frequency and search unit size.

    The zones in the base metal adjacent to a weld should be tested with longitudinal waves first, to ensure that the base metal does not contain discontinuities that would interfere with shear wave evaluation of the weld.

    In the third mode, surface waves are propagated along the metal surface, similar to waves on the surface of water. These surface waves have little movement below the surface of a metal, therefore they are not used for examination of welded and brazed joints.

    Coupling- A liquid material is used for transmission of ultrasonic waves into the test object. Some of the more common coupling agents are water, light oil, glycerine, and cellulose gum powder mixed with water.

    A weldment must be smooth and flat to allow intimate coupling. Weld spatter, slag, and other irregularities should be removed. Depending on the testing technique, it may be necessary to remove the weld reinforcement.

    Calibration- Ultrasonic testing is basically a comparative evaluation. The horizontal (time) and the vertical (amplitude) dimensions on the CRT screen of the test unit are related to distance and size, respectively. It is necessary to establish a zero starting point for these variables, and to calibrate an ultrasonic unit to a basic standard before use.

    Various test blocks are used to assist in calibration of the equipment. Known reflecting areas can simulate typical discontinuities. Notches substitute for surface cracks, side-drilled holes for slag inclusions or internal cracks, and angulated flat-bottomed holes for small areas of incomplete fusion. The test block material must be similar in acoustic qualities to the metal being tested.

    The International Institute of Welding (IIW) test block is widely used as a calibration block for ultrasonic testing of steel welds. This block and other test blocks are used to calibrate an instrument for sensitivity, resolution, linearity, angle of sound propagation, and distance and gain calibrations.

    Standard test blocks are shown in ASTM E164, Standard Practice for Ultrasonic Contact Exarnination of Weldrnents, latest edition, West Conshohocken, Pennsylvania: American Society for Testing and Materials.

    Test Procedures- Most ultrasonic testing of welds is done following a specific code or procedure. An example of such a procedure is that contained in AWS D1.I, Structural Welding Code-Steel for testing groove welds in structures.

    ASTM E164, Standard Practice for Ultrasonic Contact Examination of Weldments covers examination of specific weld configurations in wrought ferrous and aluminum alloys to detect weld discontinuities. Procedures for calibrating the equipment and appropriate calibration blocks are included. Other ASTM standards cover testing procedures with various ultrasonic inspection methods for inspection of pipe and tubing.

    Procedures for UT of boiler and pressure vessel components are given in ASME Boiler and Pressure Vessel Code, Section V Nondestructive Examination. Section XI, Insewice Inspection Requirements for Nuclear Power Plants, gives methods for locating, sizing, and evaluating discontinuities for continuing service life and fracture mechanics analysis.

    Operator Qualifications- The reliability of ultrasonic examination depends greatly on the interpretive ability of the ultrasonic testing technician. In general, UT requires more training and experience than the other nondestructive testing methods, with the possible exception of radiographic testing. Many critical variables are controlled by the operators. For this reason, most standards require ultrasonic technicians to meet the requirements of ASNT-TC- 1A, Personnel Qualification and Certification in Nondestructive Testing.

    Reporting- Careful tabulation of information in a report form is necessary for a meaningful test. Reporting requirements are included in ANSUAWS D1.1, Structural Welding Code-Steel. The welding inspector should be familiar with the kinds of data that must be recorded and evaluated so that a satisfactory determination of weld quality can be obtained. Standards for testing have been published by the American Society for Nondestructive Testing, the American Society of Mechanical Engineers, and the American Welding Society. 

  • ULTRASONIC WELDING (USW)

    A solid-state welding process that produces a weld by the local application of high-frequency vibratory energy as the workpieces are held together under pressure.  

    Ultrasonic welding produces a sound metallurgical bond without melting the base metal. The basic force in ultrasonic welding is high-intensity vibrational energy. High- frequency electrical energy is converted to mechanical vibration, and a coupler (sonotrode) transmits the vibration to the work. An anvil counters the clamping force.

    This process involves complex relationships between the static clamping force, the oscillating shear forces, and a moderate temperature rise in the weld zone. The magnitudes of these factors required to produce a weld are functions of the thickness, surface condition, and the mechanical properties of the workpieces.

    Typical components of an ultrasonic welding system are illustrated in Figure U-2. The ultrasonic vibration is generated in the transducer. This vibration is transmitted through a coupling system or sonotrode, which is represented by the wedge and reed members in Figure U-2. The sonotrode tip is the component that directly contacts one of the workpieces and transmits the vibratory energy into it. (The sonotrode is the acoustical equivalent of the electrode and its holder used in resistance spot or seam welding). The clamping force is applied through at least part of the sonotrode, which in this case is the reed member. The anvil supports the weldment and opposes the clamping force.

    Applications

    Ultrasonic welding is used to join both monometallic and bimetallic joints. The process is used to produce lap joints between metal sheets or foils, between wires or ribbons and flat surfaces, between crossed or parallel wires, and for joining other types of assemblies that can be supported on the anvil.

    This process is being used as a production tool in the semiconductor, microcircuit, and electrioal contact industries, for fabricating small motor armatures, in the manufacture of aluminum foil, and in the assembly of aluminum components. It is receiving acceptance as a structural joining method by the automotive and aerospace industries. The process is uniquely useful for encapsulating materials such as explosives, pyrotechnics, and reactive chemicals that require hermetic sealing but cannot be processed by high-temperature joining methods. 

    The most important application of the USW process is the assembly of miniaturized electronics components. Fine aluminum and gold lead wires are attached to transistors, diodes, and other semiconductor devices. Wires and ribbons are bonded to thin films and microminiaturized circuits. Diode and transistor chips are mounted directly on substrates. Reliable joints with low electrical resistance are produced without contamination or thermal distortion of the components.

    Electrical connections, both single and stranded wires, can be joined to other wires and to terminals. The joints are frequently made through anodized coatings on aluminum, or through certain types of electrical insulation. Other current carrying devices, such as electric motors, field coils, harnesses, transformers and capacitors may be assembled with ultrasonically welded connections.

    Broken and random lengths of aluminum foil are welded in continuous seams by foil rolling mills, with almost undetectable splices after subsequent working operations. Aluminum and copper sheet up to about 0.5 mm (0.020 in.) can be spliced together using special processing and equipment.

    In structural applications, USW produces joints of high integrity within the limitations of weldable sheet thickness. An example is the assembly of a helicopter access door, in which inner and outer skins of aluminum alloy are joined by multiple ultrasonic spot welds.

    Ultrasonic welding has reduced fabrication costs for some solar energy conversion and collection systems. An ultrasonic seam welding machine, operating at speeds up to 9 dmin (30 ft/min), joins all connectors in a single row in a fraction of the time require for hand soldering or individual spot welding. Solar collectors for hot water heating systems consisting of copper or aluminum tubing can be welded at significantly lower energy cost than soldering, resistance spot welding, or roll welding.

    Other applications include continuous seam welding to assemble components of corrugated heat exchangers, and welding strainer screens without clogging the holes. Beryllium foil windows for space radiation counters have been ring welded to stainless steel frames to provide a helium leak-tight bond. Pinch-off weld closures in copper and aluminum tubing used in refrigeration and air conditioning are produced with special serrated bar tips and anvils.

    Process Variations

    There are four variations of the process, based on the type of weld produced. These are spot, ring, line and continuous seam welding. In addition, two variants of ultrasonic spot welding are used in micro electronics.

    Spot Welding- In spot welding, individual weld spots are produced by the momentary introduction of vibratory energy into the workpieces as they are held together under pressure between the sonotrode tip and the anvil face. The tip vibrates in a plane essentially parallel to the plane of the weld interface, perpendicular to the axis of static force application. Spot welds between sheets are roughly elliptical in shape at the interface. They can be overlapped to produce an essentially continuous weld joint. This type of seam may contain as few as 2 to 4 welds/cm (5 to 10 welds/in.). Closer weld spacing may be necessary if a leaktight joint is required.

    Ring Welding- Ring welding produces a closed loop weld which is usually circular in form but may also be square, rectangular or oval. In this variation, the sonotrode tip is hollow, and the tip face is contoured to the shape of the desired weld. The tip is vibrated torsionally in a plane parallel to the weld interface. The weld is completed in a single, brief weld cycle.

    Line Welding- Line welding is a variation of spot welding in which the workpieces are clamped between an anvil and a linear sonotrode tip. The tip is oscillated parallel to the plane of the weld interface and perpendicular to both the weld line and the direction of applied static force. The result is a narrow linear weld, which can be up to 150 mm (6 in.) long, produced in a single weld cycle.

    Continuous Seam Welding- In this variation, joints are produced between workpieces that are passed between a rotating, disk-shaped sonotrode tip and a roller type or flat anvil. The tip may traverse the work while it is supported on a fixed anvil, or the work may be moved between the tip and a counter-rotating or traversing anvil. Area bonds may be produced by overlapping seam welds.

    The flow of energy through an ultrasonic welding system begins with the introduction of 60 Hz electrical power into a frequency converter. This device converts the applied frequency to that required for the welding system, which is usually in the range of 10 to 75 kHz. The high-frequency electrical energy is conducted to one or more transducers in the welding system, where it is converted to mechanical vibratory energy of the same frequency. The vibratory energy is transmitted through the sonotrode and sonotrode tip into the workpiece. Some of the energy passes through the weld zone and dissipates in the anvil support structure.

    For practical usage, the power required for welding is usually measured in terms of the high-frequency electrical power delivered to the transducer. This power can be monitored continuously and provides a reliable average value to associate with equipment performance as well as with weld quality. The product of the power in watts and welding time in deconds is the energy, in watt-seconds or joules, used in welding. The energy required to make an ultrasonic weld can be related to the hardness of the workpieces and the thickness of the part in contact with the sonotrode tip.

    Process Advantages and Limitations.

    Ultrasonic welding has advantages over resistance spot welding in that little heat is applied during joining and no melting of the metal occurs. This process permits welding thin to thick sections, as well as joining a wide variety of dissimilar metals. Welds can be made through certain types of surface coatings and platings. Ultrasonic welding of aluminum, copper and other high-conductivity metals requires substantially less energy than resistance welding. As compared to cold welding, the pressures used in USW are much lower, welding times are shorter, and thickness deformation is significantly lower.

    A major disadvantage is that the thickness of the component adjacent to the sonotrode tip must not exceed relatively thin gauges because of the power limitations of present ultrasonic welding equipment. The range of thicknesses of a particular metal that can be welded depends on the properties of that metal. Ultrasonic welding is limited to lap joints. Butt welds cannot be made in metals because there is no effective means of supporting the workpieces and applying clamping force. However, ultrasonic butt welds are made in some polymer systems.

    Safety

    The welding machine operator should be provided with eye and ear protection.  Most ultrasonic welding equipment is designed with interlocks and other safety devices to prevent personnel from contacting high voltages in the equipment. Nevertheless, consideration must be given to operating personnel and all personnel in the area of the welding operations. There must be strict conformance to the manufacturers’ operating instructions and safety recommendations as well as requirements in ANSI/ASC 2-49.1 (latest edition), Safety in Welding and Cutting and applicable requirements of the Occupational Safety and Health Administration (OSHA). 

  • ULTRAVIOLET RAYS

    Light rays which are outside of the visual spectrum at the violet end. These rays are comparatively intense in arc welding; eye protection must be worn during welding operations. See EYE PROTECTION. 

  • UNAFFECTED ZONE

    The area of the base metal outside of the zone of a weld in which no changes in grain size have occurred due to the effects of welding. 

  • UNBALANCED FLAME

    An oxyacetylene flame with an excess of either oxygen or acetylene; a flame that is oxidizing or carburizing. 

  • UNDERBEAD CRACK

    A crack in the heat-affected zone generally not extending to the sugace of the base metal. 

  • UNDERCUT

    A groove melted into the base metal adjacent to the weld toe or weld root and le$ unfilled by weld metal.  

    Causes: Excessive welding current; improper electrode technique; mismatch between electrode design and weld position.

    Corrections: use a moderate welding current and proper welding speed; use an electrode that produces a puddle of the proper size; proper weaving technique; proper positioning of the electrode relative to a horizontal fillet weld.

    The term undercut is used to describe either of two situations. One is the melting away of the sidewall of a weld groove at the edge of the bead, thus forming a sharp recess in the sidewall in the area in which the next bead is to be deposited. The other is the reduction in thickness of the base metal at the line where the beads in the final layer of weld metal tie into the surface of the base metal (e.g., at the toe of the weld).

    Both types of undercut are usually due to the specific welding technique used by the welder. High amperage and a long arc increase the tendency to undercut. Incorrect electrode position and travel speed are also causes, as is improper dwell time in a weave bead. Even the type of electrode used has an influence. The various classifications of electrodes show widely different characteristics in this respect. With some electrodes, even the most skilled welder may be unable to avoid undercutting completely in certain welding positions, particularly on joints with restricted access.

    Undercut of the sidewalls of a weld groove will in no way affect the completed weld if the undercut is removed before the next bead is deposited at that location. A well-rounded chipping tool or grinding wheel will be required to remove the undercut. If the undercut is slight, however, an experienced welder who knows just how deep the arc will penetrate may not need to remove the undercut.

    The amount of undercut permitted in a completed weld is usually dictated by the fabrication code being used, and the requirements specified should be followed because excessive undercut can materially reduce the strength of the joint. This is particularly true in applications subject to fatigue. Fortunately, this type of undercut can be detected by visual examination of the completed weld, and it can be corrected by blend grinding or depositing an additional bead. 

  • UNDERFILL

    A condition in which the weld face or root surface extends below the adjacent surface of the base metal. 

  • UNDERWATER CUTTING

    Underwater cutting is used for salvage work and for cutting below the water surface on piers, dry docks, and ships. The two methods most widely used are oxyfuel gas cutting (OFC) and oxygen arc cutting (AOC).

    Technique- The technique for underwater cutting with OFC is not materially different from that used in cutting steel in open air. An underwater OFC torch embodies the same features as a standard OFC torch with the additional feature of supplying its own ambient atmosphere. In the underwater cutting torch, fuel and oxygen are mixed together and burned to produce the preheat flame. Cutting oxygen is provided through the tip to sever the steel. In addition, the torch provides an air bubble around the cutting tip. The air bubble is maintained by a flow of compressed air around the tip. The air shield stabilizes the preheat flame and at the same time displaces the water from the cutting area.

    Special Equipment- The underwater cutting torch has connections for three hoses to supply compressed air, fuel gas, and oxygen. A combination shield and spacer device is attached at the cutting end of the torch. The adjustable shield controls the formation of the air bubble. The shield is adjusted so that the pre-heat flame is positioned at the correct distance from the work. This feature is essential for underwater work because of poor visibility and reduced operator mobility caused by cumbersome diving suits. Slots in the shield allow the burned gases to escape. A short torch is used to reduce the reaction force produced by the compressed air and cutting oxygen pushing against the surrounding water.

    Gases- As the depth of water at which the cutting is being done increases, the gas pressures must be increased to overcome both the added water pressure and the frictional losses in the longer hoses. Approximately 3.5 kPa (1/2 psi) for each 300 mm (12 in.) of depth must be added to the basic gas pressure requirements used in air for the thickness being cut.

    Methylacetylene-propadiene(MPS), propylene, and hydrogen are the best all-purpose preheat gases, because they can be used at any depths to which divers can descend and perform satisfactorily. Acetylene must not be used at depths greater than approximately 6 m (20 ft), because its maximum safe operating pressure is 100kPa (15 psi).

    No great difficulty is experienced in underwater severing of steel plate in thicknesses from 13 mm (1/2 in.) to approximately 101 mm (4 in.) with the oxyfuel gas cutting torch. Under 13mm (1/2 in.) thickness, the constant quenching effect of the surrounding water lowers the efficiency of preheating. This requires much larger preheating flames and preheat gas flows. Cutting oxygen orifice size is considerably larger for underwater cutting than for cutting in air. A special apparatus for lighting the preheat flames under water is also needed.

    Oxygen Lance Cutting (LOC)

    The LOC process can also be used underwater. The lance must be lighted before it is placed underwater; then piercing proceeds essentially the same as in air. The process produces a violent bubbling action which can restrict visibility.

    Oxygen Arc Cutting (AOC)

    This is another underwater cutting process used to cut ferrous and nonferrous metals in any position. Underwater electrodes for AOC are steel tubes with a waterproof coating. A fully insulated electrode holder equipped with a suitable flash-back arrester is required. See OXYFUEL GAS CUTTING, OXYGEN LANCE CUTTING, and OXYGEN ARC CUTTING. 

  • UNDERWATER WELDING

    Underwater welding (wet welding) is described as welding at ambient pressure with the weldeddiver in the water with no physical barrier between the water and the welding arc. Although it is a complex metallurgical process, wet welding closely resembles welding in air in that the welding arc and molten metal are shielded from the environment (water or dir) by gas and slag produced by decomposition of dux coated electrodes or flux cored wire. Underwater dry welding is done at ambient pressure in a chamber from which water has been displaced. Depending on the size and configuration of the chamber, the weldeddiver may be completely in the chamber, or only partially in the chamber, and may work in conventional welder’s attire, dive gear, or a combination of both.

    Underwater welding has been used during the installation of new offshore drilling structures, sub-sea pipelines and hot taps, docks and harbor facilities, and for modifications and additions to underwater structures. However, underwater welding is most often required for repairs to existing structures. Maintenance and repair applications include:

    (1) Replacement of damaged sub-sea pipeline sections and pipeline manifolds

    (2) Replacement of structural members damaged by corrosion and fatigue

    (3) Damage occurring during installation, boat collisions, or other accidental damage.

    Specifications for underwater welding are published by American Welding Society, Miami, Florida; in ANSUAWS D3.6-93, Specification for Underwater Welding. 

  • UNDERWRITERS LABORATORIES STANDARDS

    Rules formulated by the Underwriters’ Laboratories to assure the safe construction of industrial equipment, including welding apparatus. 

  • UNDERWRITERS LABORATORIES, INC.

    A not-for-profit organization chartered to maintain and operate product and safety certification programs.

    Underwriters Laboratories carries out safety examination and testing of devices, systems, and materials against reasonably foreseeable risks. Success in the testing results in a UL label. Founded in 1894, UL representatives make unannounced visits to factories which make products bearing the UL label to check correct maintenance of product integrity. 

  • UNFIRED PRESSURE VESSELS

    Unfired pressure vessels are containers for the containment of pressure either internal or external. Section VI11 of the ASME Boiler and Pressure Vessel I Code (BPVCI) covers unfired pressure vessels. These include towers, reactors and other oil and chemical refining vessels, heat exchangers for refineries, paper mills, and other process industries, as well as storage tanks for large and small air and gas compressors. See BOILER CONSTRUCTION CODE. 

  • UNIDIRECTIONAL CURRENT

    An electrical current that flows in one direction only. 

  • UNIFIED NUMBERING SYSTEM (UNS)

    A method for cross referencing the different numbering systems used to identify metals, alloys, and welding filler metals. With UNS, it is possible to correlate over 4400 metals and alloys used in a variety of specifications, regardless of the identifying number used by a society, trade association, producer, or user.

    UNS is produced jointly by the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM). It cross references the numbered metal and alloy designations of the major organizations and systems, including Federal and military. Over 500 of the listed numbers are for welding and brazing filler metals that are classified by deposited metal composition. See Table U-1. 

  • UNIPHASE

    A single-phase alternating current. 

  • UNMIXEDZONE

    A thin boundary layer of weld metal, adjacent to the weld interface, that solidified without mixing with the remaining weld metal. See MIXED ZONE. 

  • UPHILL, adv.

    Welding with an upward progression. 

  • UPSET

    Bulk deformation resulting from the application of pressure in welding. The upset may be measured as a percent increase in interface area, a reduction in length, a percent reduction in lap joint thickness, or a reduction in cross wire weld stack height. 

  • UPSET DISTANCE

    The total reduction in the axial length of the workpieces from the initial contact to the completion of the weld. In flash welding the upset distance is equal to the platen movement from the end of flash time to the end of upset. 

  • UPSET FORCE

    The force exerted at the faying surfaces during upsetting.