OTHER PROCESSES Projection Welding Seam Welding Butt Welding Flash Welding High Frequency Capacitive Discharge Electro-Brazing.
Post on 20-Jan-2016
OTHER PROCESSESProjection WeldingSeam WeldingButt WeldingFlash WeldingHigh FrequencyCapacitive DischargeElectro-Brazing
Other Resistance Welding ProcessessLesson ObjectivesWhen you finish this lesson you will understand: Projection Welding & Applications Seam Welding & Applications Butt Welding & Applications Flash Welding & Applications High Frequency Welding & Applications Capacitive Discharge Welding & Applications Electro Brazing & ApplicationsLearning ActivitiesView Slides; Read Notes, Listen to lectureDo on-line workbookKeywords: All Processes Above, Pulsing, Roll Spot Weld, Overlap Spot Weld, Continuous Seam Weld, Mash Seam Weld, Metal Finish Seam Weld, Percussion Welding
Principal Types of Resistance WeldsElectrodesor WeldingTipsElectrodesor WeldingWheelsElectrodesor DiesProjectionWeldsElectrodes or DiesSpot WeldSeam Weld Projection WeldUpset WeldFlash Weld
After WeldingAfter Welding[Reference: Resistance Welding Manual, RWMA, p.1-3]
Basic Single Impulse Welding CycleElectrode ForceWelding CurrentWelding CycleSqueeze TimeWeld TimeHoldTimeOffTime[Reference: Welding Handbook, Volume 2, AWS, p.538]
Enhanced Welding CycleSqueezeTimePreheatTimeUpslopeTimeCool TimeWeldTimeCool TimePreweldIntervalWelding CycleWeld IntervalPostweld IntervalDownslopeTimeQuenchTimeTemperTimeHoldTimePulseImpulseTemperingCurrentWelding CurrentElectrodeForceForge Delay TimeForge Force[Reference: Welding Handbook, Volume 2,AWS, p.539]
PulsingCool TimePulse 1Pulse 2Pulse 3PulseTime
Definition of Projection WeldingA resistance welding process that produces coalescence by the heat obtained from the resistance to the flow of the welding current. The resulting welds are localized at predetermined points by projections, embossments, or intersections.FixedMovingworkpiecestransformerProjection welding setup.Definition
Link to Projection Welding video
Process FundamentalsTwo parts, one projected, are placed upon one another between two electrodes.They make contact at the projected point.High current starts to flow through projection.Force is applied to cause the heated projection to collapse, and help fusion.A.B.Formation of a projection weld.Process Fundamentals
Introduction to Projection Welding(a)(b) (c) (d)[Reference: Welding Handbook, Volume 2, p.566, AWS]
Examples of Various Projection Designs(a)(b)(c) (d) (e)[Reference: Welding Handbook, Volume 2, p.562, AWS]
Examples of Various Projection Designs (CONT.)(f) (g) (h)(i)(j)[Reference: Welding Handbook, Volume 2, p.562, AWS]
Projection DesignProjection should besufficiently rigid to support the electrode force.have adequate mass to heat a spot.collapse without metal expulsion.be easy to form.cause little distortion to the part.Spherical radiusWall thicknessshould be at least70% of sheetthicknessDProjectionshould blendinto stocksurface withoutshoulderingGeneral design of a projection steel sheetProjection Design
Advantages of Projection WeldingA number of welds can be made simultaneously in one welding cycle of the machineLess overlap and closer weld spacings are possible1 < Thickness ratio < 6Smaller in size than spot weldingBetter appearance on the side without projectionLess electrode wear than spot weldingOil, rust, scale, and coatings are less of a problem than spot welding
Limitations of Projection WeldingRequire an additional operation to form projections
With multiple welds, require accurate control of projection height and precise alignment of the welding dies
Thickness limitation for sheet metals
Require higher capacity equipment than spot welding
Process AdvantagesMultiple welds can be made simultaneously.Can be located with greater accuracy than spot weld.Electrode wear is much lower because of flat faced electrode.Oil, rust, scale and coatings are less of a problem compared with spot welding.Advantages
Process LimitationsForming of projection may require an additional operation.With multiple welds, accurate control of projection height and precise alignment of welding dies are necessary.Higher capacity equipment requires to make multiple weld simultaneously.
Definition of Seam WeldingResistance Seam Welding (RSEW): A resistance welding process which produces coalescence at the faying surface by heat obtained from resistance to electric current through the work parts held together under pressure by electrodes. The resulting weld is a series of overlapping resistance spot welds made progressively along a joint by rotating the electrodes.
Introduction to Resistance Seam WeldingUpper Electrode WheelWorkpieceLower Electrode WheelThroatKnurl or FrictionDrive WheelRoll Spot WeldOverlapping SeamWeldContinuous SeamWeld[Reference: Welding Handbook, Volume 2, p.553, AWS]
Lap Seam WeldElectrodesOverlappingWeldNuggetsTravelFront viewSide View[Reference: Welding Handbook, Volume 2, p.554, AWS]
Mash Seam WeldSlightly LappedSheetsWide, FlatElectrodesWeld NuggetsBefore weldingAfter Welding[Reference: Welding Handbook, Volume 2, p.554, AWS]
Metal Finish Seam WeldChamferedElectrodeBroad, FlatElectrodeFlashFinishSideBefore WeldingAfter Welding[Reference: Welding Handbook, Volume 2, p.554, AWS]
Definition of Flash Welding A resistance welding process in which coalescence is produced simultaneously over the entire abutting surfaces.
Flash Welding Process
Two parts to be joined are clamped in dies.The dies are connected to a transformer.A voltage is applied as one part approaches other.A. Position and clamp the parts.B. Apply flashing voltage
Flash Welding ProcessUpon contact, resistive heating occurs.High amperage causes rapid melting and explosion of the metal known as flashing.Finally an upsetting force is applied to forge the parts together.
Link to Flash Weld Video
Common Types Of Flash Welds
MeterweldAxially aligned weld. Flash Welding
Common Types of Flash WeldsRing weldFixed platenMovableplatenTransformerX-sectionafter welding
Flash Welding ApplicationsWheel rims in the automotive industry Motor and generator frames in the electrical industry.Landing gear, control assemblies and hollow propeller blades in the aircraft industry.Typical metals used are stainless steel, aluminum, copper, and nickel alloys.
Introduction to Upset WeldingFinished Upset WeldHeated ZoneTo Welding TransformerClamping DieUpsettingForceMovable PartClamping DieStationary Part[Reference: Welding Handbook, Volume 2, p.598, AWS]
Resistance Butt Welded Spike for a Baseball ShoeOgawa, M et al, Spike For Baseball Shoe US Patent 6,041,461 Mar 28, 2000Abrasion-Resistant Cemented Tungsten Carbide Tip Resistance Butt Welded to Carbon Steel Sole Attachment
High Frequency Welding Applications [Reference: Welding Handbook, Volume 2, p.653, AWS]Tube Butt SeamTube Butt SeamTube Mash SeamHFHFHFInduction Coil
High Frequency Welding Applications (CONT.)Strip ButtT-JointSpiral TubeSpiral Tube FinHFHFHFHF[Reference: Welding Handbook, Volume 2, p.653, AWS]
High Frequency Welding Applications (CONT.)Projection SeamPipe ButtBar ButtHFHFHFInductionCoil[Reference: Welding Handbook, Volume 2, p.653, AWS]
Advantages of High-Frequency WeldingProduce welds with very narrow heat-affected zonesHigh welding speed and low-power consumptionAble to weld very thin wall tubesAdaptable to many metalsMinimize oxidation and discoloration as well as distortionHigh efficiency
Limitations of High-Frequency WeldingSpecial care must be taken to avoid radiation interference in the plants vicinityUneconomical for products required in small quantitiesNeed the proper fit-upHazards of high-frequency current
Some Products of High-Frequency Welding[Reference: Welding Handbook, Volume 2, p.665, AWS]
Percussion Welding (PEW): A resistance welding process which produces coalescence of the abutting members using heat from an arc produced by a rapid discharge of electrical energy. Pressure is applied percussively during or immediately following the electrical discharge.
Metals Handbook, ASM, 1983
Metals Handbook, ASM, 1983
Resistance Brazing/Soldering (RB): A brazing/soldering process in which the heat required is obtained from the resistance to electric current in a circuit of which the work is a part.Resistance Brazing
Electro-brazingW. Stanley, Resistance WeldingMcGraw-Hill, 1950
Resistance Soldering Flexible Braided Buss to Automotive Rear WindowRear Window with Silver Ceramic Material Silk Screened onto SurfaceFlat Braided Wire with Contact Pad AttachedGlassSilver CeramicBraidedWireContact PadWith Ball of SolderCurrent Passed, Melts Solder, Bond MadeIngles, G et al Braided Buss Bar with Selectively Clad Solder Pad Attachments US Patent 6,042,932 Mar 28, 2000
Spot, seam, and projection welding operations involve a coordinated application of electric current and mechanical pressure of the proper magnitudes and durations. The welding current must pass from the electrodes through the work. Its continuity is assured by forces applied to the electrodes, or by projections which are shaped to provide the necessary current density and pressure. The sequence of operation must first develop sufficient heat to raise a confined volume of metal to the molten state. This metal is then allowed to cool while under pressure until it has adequate strength to hold the parts together. The current density and pressure must be such that a nugget is formed, but not so high that molten metal is expelled from the weld zone. The duration of weld current must be sufficiently short to prevent excessive heating of the electrode faces. Such heating may bond the electrodes to the work and greatly reduce their life.The heat required for these resistance welding processes is produced by the resistance of the workpiece to an electric current passing through the material. Due to the short electric current path in the work and limited weld time, relatively high welding currents are required to develop the necessary welding heat.The four phases welding cycle for spot, seam, and projection welding are: (1) Squeeze time - the time interval between timer initiation and the first application of current. The time interval is to assure that the electrodes contact the work and establish the full electrode force before welding current is applied. (2) Weld time - the time that welding current is applied to the work in making a weld in single-impulse welding. (3) Hold time - the time during which force is maintained to the work after the last impulse of current ends. During this time, the weld nugget solidifies and is cooled until it has adequate strength. (4) Off time - the time during which the electrodes are off the work and the work is moved to the next weld location. The term is generally applied where the welding cycle is repetitive.The above slide shows a basic welding cycle. The use of one continuous application of current to make an individual weld is called single impulse welding. One or more of the following features may be added to this basic cycle to improve the physical and mechanical properties of the weld zone: (1) Precompression force to seat the electrodes and workpieces together. (2) Preheat to reduce the thermal gradient in the metal at the start of weld time. (3) Forging force to consolidate the weld nugget. (4) Quench and temper times to produce the desired weld strength properties in hardenable alloy steels. (5) Postheat to refine the weld grain size in steels. (6) Current decay to retard cooling on aluminum.In some applications, the welding current is supplied intermittently during a weld interval time. The next slide shows the sequence of operation in a more complex welding cycle.With direct energy machines, the rate of current rise and fall can be programmed. The current rise period is commonly called upslope time, and the current fall period is called downslope time (see the above slide). These features are available on machines equipped with electronic control system.Upslope is used to avoid overheating and expulsion of metal at the beginning of weld time, when the base metal interface resistance is high. Downslope is used to control weld nugget solidification to avoid cracking in metals that are quench-hardenable or subject to hot tearing.Prior to welding, the base metal can be preheated by using a low current. Following the formation of the weld nugget, the current can be reduced to some lower value for postheating of the weld zone. This may be part of the weld interval, as shown in the above slide, or a separate application of current following a quench time period.Multiple impulse welding consists of two or more pulses of current separated by a preset cool time. This sequence is used to control the rate of heating at the interface while spot welding a relatively thick steel sheet.Pulsing is employed primarily when resistance welding heavier gauge materials. Pulsing has two functions: preventing overheating of the electrode faces, and locating the weld nugget. Pulsing is particularly effective for preventing overheating of the electrode faces. Under continuous heating, the electrode-sheet contact resistance will promote considerable heat generation in that area. Where long weld times or high power levels are required, the steel will often soften or melt at the electrode face. This condition promotes electrode skidding, loss of weld quality and accelerated electrode wear. Pulsing, however, allows the electrode face to cool. Since the thermal conductivity of the copper electrode is considerably greater than the steel being welded, heat will be retained in the weld during the cool portion of the cycle, while the electrode will cool appreciably. As such, pulsing allows spot welding of heavier gauge materials without electrode face overheating.In a similar fashion, pulsing allows the weld nugget to be properly located between the sheets to be jointed. During the cool cycle, the electrodes will preferentially cool the surfaces of the weld in contact with the electrodes. This effectively concentrates the heating at the proper location between the sheets.The beneficial effects of pulsing vary with gauge. When there is sufficient steel thickness for heavier gauge materials pulsing is often beneficial, if not required. For lighter gauge materials, the temperature of the entire weld can fluctuate with even the fastest pulsing. Therefore, for light gauge materials pulsing is generally not considered beneficial.In projection welding, two parts - one of them have projections - are placed between two electrodes, which are connected through a transformer. The parts make contact at the projected point. When current starts flowing through the electrodes, it passes through the projected point. A tremendous amount of heat is produced at the projected end, and it gets melted. Now, a forging force is applied to make the projected part to weld with the other one. Resistance projection welding is a variation on resistance spot welding. Basically, a protrusion is placed on one of the two materials to be welded. This projection is then brought into contact against the second material.The welding sequence is similar to that for resistance spot welding. The welding electrodes are used to apply both force and current across the configuration. The point of contact acts to constrict current flow (and is a point of high resistance in the welding circuit), and heating occurs preferentially at this point. As the material heats it becomes soft, and the projection collapses under the force applied by the welding electrodes. Due to the amount of plastic flow involved, melting is not always necessary to form a sound joint.Projection welding is not limited to sheets. Any joint whose contact area is small compared to the thickness of the parts being welded is a candidate for projection welding.The sequence of events during the formation of a projection weld is shown in the above slide. In Figure (a), the projection is shown in contact with the mating sheet. In Figure (b), the current has started to heat the projection to welding temperature. The electrode force causes the heated projection to collapse rapidly and then fusion takes place as shown in Figure (c). The completed weld is shown in Figure (d).The means of producing projections depends upon the material in which they are to be produced. Projections in sheet metal parts are generally made by embossing, as opposed to projections formed in solid metal pieces which are made by either machining or forging. In the case of stamped parts, projections are generally located on the edge of the stamping.The purpose of a projection is to localized the heat and pressure at a specific location on the joint. The projection design determines the currency density. Various types of projection designs are shown in the above and the following slides.In general, projection welding can be used instead of spot welding to join small parts to each other and to larger parts. Selection of one method over another depends upon the economics, advantages, and limitations of the two processes. The chief advantages of projection welding are listed in the above slide.The limitation on the number of welds is the ability to apply uniform electrode force and welding current to each projection.Less overlap and closer weld spacings are possible, because the current is concentrated by the projection, and shunting through adjacent welds is not a problem.Thickness ratios of at least 6 to 1 are possible, because of the flexibility in projection size and location. The projections are normally placed on the thicker part.Projection welds can be located with greater accuracy and consistency than spot welds, and the welds are generally more consistent because of the uniformity of the projections.The most deformation and greatest temperature rise occur in the part with the projection, leaving the other part relatively cool and free of distortion, particularly on the exposed surface.Because large, flat-faced electrodes are used, electrode wear is much less than that with spot welding, and maintenance costs are reduced.
The most important limitations of projection welding are shown in the above slide.The forming of projections may require an additional operation unless the parts are press-formed to design shape.With multiple welds, accurate control of projection height and precise alignment of the welding dies are necessary to equalize the electrode force and welding current.With sheet metal, the process is limited to thickness in which projections with acceptable characteristics can be formed and for which suitable welding equipment is available.Multiple welds must be made simultaneously, which requires higher capacity equipment than does spot welding. This also limits the practical size of the component that contains the projections.Resistance seam welding is another variation on resistance spot welding. In this case, the welding electrodes are motor driven wheels rather than stationary caps. This results in a rolling resistance weld or seam weld. There are three independent parameters in configuring seam welding machines: power supplies and control, welding wheel configuration and sheet configuration.The major concern with power supplies and control is the frequency with which current is applied to the workpiece. Depending on this frequency and the speed with which the material is being welded, the weld will be either a continuous seam weld, an overlapping seam weld or a roll spot weld.Seam welds are typically used to produce continuous gas- or liquid-tight joints in sheet assemblies, such as automotive gasoline tanks. The process is also used to weld longitudinal seams in structural tubular sections that do not require leak-tight seams. In most applications, two wheel electrodes, or one translating wheel and a stationary mandrel, are used to provide the current and pressure for resistance seam welding. Seam welds can also be produced using spot welding electrodes. This requires the purposeful overlapping of the spot welds in order to obtain a leak-tight seam weld. Overlapping spot welding requires an increase in power after the first spot weld to offset the shunting effect in order to obtain adequate nugget formation as welding progresses.Lap joints can be seam welded using two wheel electrodes or one wheel and a mandrel. The minimum joint overlap is the same as for spot welding, i.e. twice the minimum edge distance (distance from the center of the weld nugget to the edge of the sheet).Mash seam welding is a resistance welding variation that makes a lap joint primarily by high-temperature plastic forming and diffusion, as opposed to melting and solidification. The joint thickness after welding is less than the original assembled thickness.Mash seam welding requires considerably less overlap than the conventional lap joint. The overlap is about 1 to 1.5 times the sheet thickness with proper welding procedures. Wide, flat-faced wheel electrodes, which completely cover the overlap, are used. In order to obtain consistent welding characteristics, mash seam welding requires high electrode force, continuous welding current, and accurate control of force, current, welding speed, overlap, and joint thickness. Overlap is maintained at close tolerances, by rigidly clamping or tack welding the pieces.The exposed or show side of the welded component is placed against a mandrel which acts as an electrode and supports the members to be joined. A welding wheel electrode is applied to the side of the joint that does not show. The show surface of the joint must be mashed as nearly flat as possible in order to present a good appearance. Proper positioning of the wheel with respect to the joint is required to obtain a smooth weld surface. Some polishing of the weld area may be required before painting or coating, when the appearance of the finished product is important.Mash seam welding produces continuous seams which have good appearance and are free of crevices. Crevice-free joints are necessary in applications having strict contamination or cleanliness requirements, such as joints in food containers or refrigerator liners.Lap and mash seam welds differ with respect to the amount of forging, or, as the name implies, mash down. The lap weld has practically no mash down, while the thickness of a mash seam weld approaches that of one sheet thickness. In metal finish seam welding, mash down occurs on only one side of the joint and is a compromise between lap and mash seam welding. The amount of deformation, or mash, is affected by the geometry of one electrode wheel face and the position of the joint with respect to that face. Two parts to be joined are clamped in dies (electrodes) connected to the secondary of a resistance welding transformer. Voltage is applied as one part is advanced slowly towards the other. When contact occurs at surface irregularities, resistive heating occurs at these locations. High amperage causes rapid melting and vaporization of the metal at the points of contact, forming minute arcs. This action is called flashing. As the parts are moved together at a suitable rate, flashing continues until the faying surfaces are covered with molten metal and a short length of each part reaches forging temperature. A weld is then created by the application of an upset force to bring the molten faying surfaces in full contact and forge the parts together. Flashing voltage is terminated at the start of upset.Upset welding (UW) is a resistance welding process that produces coalescence over the entire area of faying surfaces, or progressively along a butt joint, by the heat obtained from the resistance to the flow of welding current through the area where those surfaces are in contact. Pressure is used to complete the weld.With this process, welding is essentially done in the solid state. The metal at the joint is resistance heated to a temperature where recrystallization can rapidly take place across the faying surfaces. A force is applied to the joint to being the faying surfaces into intimate contact and then upset the metal. Upset hastens recrystallization at the interface and, at the same time, some metal is forced outward from this location. This tends to purge the joint of oxidized metal.Upset welding has two variations:(1) Joining two sections of the same cross section end-to-end (butt joint).(2) Continuous welding of butt joint seams in roll-formed products such as pipe and tubing.The first variation can also be accomplished by flash welding and friction welding. The second variation is also done with high frequency welding.The general arrangement for upset welding is shown in the above slide. One clamping die is stationary and the other is movable to accomplish upset. Upset force is applied through the movable clamping die or a mechanical backup, or both.This weld relates to a spike which is to be secured to the sole of a baseball shoe, the end of which has an abrasion-resistant cemented carbide tip weld on it to reduce wear during use. The objective is to provide a means for securing the tip to the edge of a shoe attachment in such a manner to ensure and extremely high weld strength and durability. The resistance butt welding process was chosen.High frequency welding includes those processes in which the coalescence of metals is produced by the heat generated from the electrical resistance of the work to high frequency current, usually with the application of an upsetting force to produce a forged weld.There are two processes that utilize high frequency current to produce the heat for welding: high frequency resistance welding (HFRW), and high frequency induction welding (HFIW), sometimes called induction resistance welding. The heating of the work in the weld area and the resulting weld are essentially identical with both processes. With HFRW, the current is conducted into the work through electrical contacts that physically tough the work. With HFIW, the current is induced in the work by coupling with an external induction coil. There is no physical electrical contact with the work.The above and the following two slides show some basic applications of high frequency welding.
With low frequency (50 Hz - 360 Hz), direct current or square wave resistance welding, much higher currents are required to heat the metal, and large electrical contacts must be placed very close to the desired weld area. The voltage drop across the weld is very low, and the current flows along the path of least resistance from one electrode to the other. With high-frequency welding, by contrast, the current is concentrated at the surface of the part. The location of this concentrated current path in the part can be controlled by the relative position of the surfaces to be welded and the location of the electrical contacts or induction coil. Heating to welding temperature can be accomplished with a much lower current than with low-frequency or direct current welding.Although the welding process depends upon the heat generated by the resistance of the metal to high frequency current, other factors must also be considered for successful high-frequency welding. Because the concentrated high-frequency current heats only a small volume of metal just where the weld is to take place, the process is extremely energy efficient, and welding speeds can be very high. Maximum speeds are limited by materials handling, forming and cutting. Minimum speeds are limited by material properties and weld quality requirements.The fit of the surfaces to be joined and the manner in which they are brought together is important if high-quality joints are to be produced. Flux is not usually used but can be introduced to the weld area in an inert gas stream. Inert gas shielding of the welding area is generally needed only for joining reactive metals such as titanium and certain stainless steel products.High-frequency welding processes offer several advantages over low frequency and direct current resistance welding processes. One characteristic of the high-frequency processes is that they can produce welds with very narrow heat-affected zones. The high-frequency welding current tends to flow only near the surface of the metal because of the skin effect and along a narrow controlled path because of the proximity effect. The heat for welding, therefore, is developed in a small volume of metal along the surfaces to be joined. A narrow heat-affected zones is generally desirable because it tends to give a stronger welded joint than with the welder zone produced by many other welding processes. With some alloys the narrow heat-affected zone and absence of cast structure may eliminate the need for postweld heat treatment to improve the metallurgical characteristics of the welded joint. The shallow and narrow current flow path results in extremely high heating rates and therefore high welding speeds and low-power consumption. A major advantage of the continuous high-frequency welding process is its ability to weld at very high speeds. High-frequency welding can also be used to weld very thin wall tubes. Wall thickness down to less than 0.005 in. are presently being welded on continuous production mills. The process is adaptable to many metals including low carbon and alloy steels, ferric and austenitic stainless steels, and many aluminum, copper, titanium, and nickel alloys. Because the time at welding temperature is very short and the heat is localized, oxidation and discoloration of the metal as well as distortion of the part are minimal. High-frequency welding power sources have a balanced three-phase input power system. Using conventional vacuum tube welding power sources, as much as 60 percent of the energy is converted into useful heat in the work.As with any process, there are also limitations. Because the equipment operates in the radio frequency range, special care must be taken in its installation, operation, and maintenance to avoid radiation interference in the plants vicinity.As a general rule, the minimum speed in carbon steel is about 25 feet/min. For products which are only required in small quantities, the process may be uneconomical unless the technical advantages justify the application.Because the process utilizes localized heating in the joint area, proper fit-up is important. Equipment is usually incorporated into mill or line operation and must be fully automated. The process is limited to the use of coil, flat, or tubular stock with a constant joint symmetry throughout the length of the part. Any disruption in the current path or change in the shape of the vee can cause significant problems. Special precautions must be taken to protect the operations and plant personnel from the hazards of high-frequency current.Examples of a few products that can be fabricated by high-frequency welding are shown in the above slide.The attachment of the current conductors on rear window defoggers can be done by using resistance soldering as shown here. The solder bead is placed on a contact pad and current passed through the assembly to melt the solder during manufacture. The solder bonds with a silver ceramic silk screened pattern placed on the window glass.