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GENERAL PRINCIPLESResistance welding is a thermo-electricprocess in which heat is generated at the in-terface of the parts to be joined by passing anelectrical current through the parts for aprecisely controlled time and under a con-trolled pressure (also called force). Thename “resistance” welding derives from thefact that the resistance of the workpieces andelectrodes are used in combination or contrastto generate the heat at their interface.

Key advantages of the resistance weldingprocess include:• Very short process time• No consumables, such as brazing materi-

als, solder, or welding rods• Operator safety because of low voltage• Clean and environmentally friendly• A reliable electro-mechanical joint is formed

Resistance welding is a fairly simple heatgeneration process: the passage of currentthrough a resistance generates heat. This isthe same principle used in the operation ofheating coils. In addition to the bulk resistances,the contact resistances also play a major role.The contact resistances are influenced bythe surface condition (surface roughness,cleanliness, oxidation, and platings).

The general heat generation formula for re-sistance welding is:

Heat = I2 x R x t x K

Where “I” is the weld current through theworkpieces, “R” is the electrical resistance (inohms) of the workpieces, “t” is the weld time(in hertz, milliseconds or microseconds), and“K” is a thermal constant. The weld current(I) and duration of current (t) are controlled bythe resistance welding power supply. Theresistance of the workpieces (R) is a functionof the weld force and the materials used. Thethermal constant “K” can be affected by partgeometry, fixturing and weld force.

The bulk and contact resistance values of theworkpieces, electrodes, and their interfacesboth cause and affect the amount of heatgenerated. The diagram (above right) il-lustrates three contact and four bulk re-sistance values, which, combined, helpdetermine the heat generated.

BULK RESISTANCE is a function of tempera-ture. All metals exhibit a Positive Tempera-ture Coefficient (PTC), which means that theirbulk resistance increases with temperature.Bulk resistance becomes a factor in longerwelds.

CONTACT RESISTANCE is a function of theextent to which two surfaces mate intimatelyor come in contact. Contact resistance is animportant factor in the first few millisecondsof a weld.

The surfaces of metal are quite rough if theyare examined on a molecular scale. When themetals are forced together with a relativelysmall amount of force, some of the peaks makecontact.

On those peaks where the contact pressureis sufficiently high, the oxide layer breaks,forming a limited number of metal-to-metalbridges. The weld current is distributed overa large area as it passes through the bulkmetal. However, as it approaches the inter-face, the current is forced to flow throughthese metallic bridges. This “necking down” in-creases the current density, generatingenough heat to cause melting. As the first ofthese bridges melt and collapse, new peakscome into contact, forming new bridges andadditional current paths. The resistance of themolten metal is higher than that of the newbridges so that the current flow transfersfrom bridge-to-bridge. This process continuesuntil the entire interface is molten. When thecurrent stops, the electrodes rapidly cool themolten metal, which solidifies, forming a weld.

Exaggerated cross-section of two piecesof metal indicates formation of metallic

bridges that result inhigh current density.

Subsequent meltingand the formation ofnew bridges allow theweld to be formed.

HEAT BALANCE – During resistance welding,part of the heat generated is lost to thesurroundings by conduction (heat transferthrough solids), convection (heat lost from ex-posed surfaces by air-cooling), and radiation(does not require a medium). Heat balance isa function of part material and geometry, elec-trode material and geometry, polarity, and theweld schedule. The goal of good resistancewelding is to focus the heat generated closeto the weld interface at the spot where theweld is desired.

In general, the highest resistance results inthe highest heat assuming that the resistancewelding power supply can produce sufficientenergy to overcome the resistance. Thus,dissimilar parts and electrode combinationsare preferred since their dissimilarity resultsin higher resistance. For example, conductiveelectrodes, e.g. copper, are used to weld re-sistive materials such as stainless steel ornickel, and resistive electrodes, e.g.molybdenum, are used to weld conductivematerials, such as copper or gold.

To force the metals together, electrodepressure (force) provided by the weld head, isequally important. Heat, generated by the re-sistance of the workpieces to the flow of elec-tricity, either melts the material at theinterface or reduces its strength to a levelwhere the surface becomes plastic. When theflow of current stops, the electrode force ismaintained, for a fraction of a second, while theweld rapidly cools and solidifies.

There are three basic types of resistancewelding bonds:

SOLID STATE BOND – In a Solid State Bond(also called thermo-compression Bond), dis-similar materials with dissimilar grain struc-ture, e.g. molybdenum to tungsten, are joinedusing a very short heating time, high weld en-ergy, and high force. There is little meltingand minimum grain growth, but a definitebond and grain interface. Thus the materialsactually bond while still in the “solid state.”The bonded materials typically exhibit excel-lent shear and tensile strength, but poor peelstrength.

Fundamentalsof Small Parts Resistance Welding

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FUSION BOND – In a Fusion Bond, either similar or dissimilar materials withsimilar grain structures are heated to the melting point (liquid state) of both. Thesubsequent cooling and combination of the materials forms a “nugget” alloy ofthe two materials with larger grain growth. Typically, high weld energies ateither short or long weld times, depending on physical characteristics, areused to produce fusion bonds. The bonded materials usually exhibit excellenttensile, peel and shear strengths.

REFLOW BRAZE BOND – In a Reflow Braze Bond, a resistance heating of a lowtemperature brazing material, such as gold or solder, is used to join eitherdissimilar materials or widely varied thick/thin material combinations. Thebrazing material must “wet” to each part and possess a lower melting pointthan the two workpieces. The resultant bond has definite interfaces with mini-mum grain growth. Typically the process requires a longer (2 to 100 ms) heatingtime at low weld energy. The resultant bond exhibits excellent tensile strength,but poor peel and shear strength.

HEAT AFFECTED ZONE (HAZ) is the volume of material at or near the weldwhich properties have been altered due to the weld heat. Since the resistancewelding process relies on heating two parts, some amount of HAZ is in-evitable. The material within the HAZ undergoes a change, which may or maynot be beneficial to the welded joint. In general, the goal in good resist-ance welding is to minimize the HAZ.

Solid State Bond Fusion Bond Reflow Braze Bond

The physical metallurgy of the materials to be welded deter-mines the application of the resistance welding processvariables. In general there are two categories of metals tobe welded: “Conductive” (such as aluminum, copper, silverand gold), and “Resistive” (steel, nickel, inconel, titanium,tungsten, molybdenum) with a third, small, middle groundcategory occupied primarily by brass. In general, electricallyconductive materials are also more thermally conductiveand are softer.

These categories apply equally to both the workpieces tobe joined and to the electrodes. As discussed earlier, higherelectrical resistance produces higher heat and better welds.Thus the “rule of opposites” applies to matching electrodesto workpieces to be welded. The general rule (with a fewexceptions such as aluminum and beryllium copper) is toutilize conductive electrodes against resistive parts andresistive electrodes against conductive parts. By extension,when welding dissimilar materials, the upper and lower (oranode and cathode) electrodes must be of different materi-als to each other in order to apply the“rule of opposites.”

When welding a resistive materialto a conductive material, oneshould use conductive electrodes(copper) on resistive parts (steel)and resistive electrodes (moly) onconductive parts (copper).

ELECTRICAL RESISTIVITY – Low resistancemetals, e.g. copper, require larger currents toproduce the same amount of heat. Low resistancematerials also exhibit low contact resistance.

THERMAL CONDUCTIVITY – Metals withhigh thermal conductivity, e.g. copper, exhibithigh electrical conductivity. The heat gener-ated in high thermal conductivity materials israpidly conducted away from the region ofthe weld. For metallic materials, the electricaland thermal conductivity correlate positively,i.e. materials with high electrical conductivity(low electrical resistance) exhibit high ther-mal conductivity.

THERMAL EXPANSION – Softer metals exhibita high coefficient of expansion (CTE); whereasharder materials, such as tungsten, exhibita low CTE. A CTE mismatch between twoworkpieces can result in significant residualstresses at the joint which, when combinedwith the applied stresses, can cause failureat lower pull strengths.

HARDNESS AND STRENGTH – In seemingcontradiction to the “rule of opposites,” hardmaterial workpieces generally require harderelectrodes (which exhibit lower conductivity)due to the higher weld forces required.

PLASTIC TEMPERATURE RANGE is the tem-perature range in which a material can be de-

formed easily (melt) under the application offorce. Steels and alloys exhibit a wide plastictemperature range and thus are easy to fu-sion weld. The natural elements, copper and alu-minum exhibit a narrow plastic temperaturerange. Accurate control of the weld tempera-ture is critical to avoid excessive melting.

POLARITY should be considered when usingall power supply technologies. If any of the inter-faces of a resistance weld (between elec-trodes and workpieces or between theworkpieces to be joined) is composed of dis-similar materials, that interface will heat orcool depending on the polarity of the appliedpotential. This effect is dominant only in thefirst few milliseconds of a weld. Although it ismore dominant for welds of short duration, itaffects the weld quality and electrode wear oflong welds as well. The effects of polarity canbe minimized or controlled via the use of con-trasting size electrode forces and/or weldpulses of alternating polarity.

Other material related parameters affect theresistance welding process, and must thereforebe controlled. These parameters includeoxide contamination, plating inconsistencies,surface roughness and heat imbalance.

OXIDE CONTAMINATION causes inconsistentwelds by inhibiting intimate contact at theweld joint. Preventive actions include pre-

cleaning the workpieces, increasing the weldforce to push aside the oxide, and/or using acover gas during welding to prevent additionaloxide formation.

PLATING INCONSISTENCIES include varia-tions in plating thickness, degree of oxidecontamination in the plating and the type ofplating. Proper control of workpiece platingreduces the chance of weak or inconsistentwelds and/or electrode sparking or stickingto the workpieces. Electroplating is muchpreferred over electroless plating.

SURFACE ROUGHNESS can also result in lo-calized over/under heating, electrode stickingand/or material expulsion. The same rule ap-plies to all three material parameters: anysurface condition that impairs intimate work-piece contact to each other and to the elec-trodes will inhibit good welding.

HEAT IMBALANCE and heat sinks can resultin unexpected heat loss or misdirection. Heatmust be concentrated at the point of the weldto insure correct and consistent welds.

PROJECTIONS (low thermal mass islands)are one method of insuring proper heat bal-ance in difficult applications when there ex-ists a 5:1 size difference between the parts tobe welded.Another method is to vary the size,shape and/or material of the welding electrode.

MATERIALS

MATERIAL PROPERTIES

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ADVANTAGES OF PROJECTIONS IN MICRO SPOT WELDING

BASIC WELD SCHEDULE

WELD FORCE ENERGY AND TIME

By providing a projection on the surface of one of the workpieces, the current and force can be focused intothe small area of the projection to produce heat at the desired weld location. Projection welding can also extendelectrode life by increasing the electrode contact area and decreasing the current density at the surface ofthe electrode. Projection welding is effective even if the weldments are thick.

A key parameter of all three types of resistancewelding is weld pressure or force. The proper andconsistent application of force improves the matingof the materials increasing the current paths, re-ducing the interface resistance, and insuring thatany oxide barriers between the workpieces arebroken through. Repeatable force control insuresrepeatable weld quality through consistentelectrical contact resistance and consistent heatbalance. Force control can also be used to triggerwelding energy when a pre-determined force levelhas been achieved, often called “force firing.” Op-timum welds are achieved when the applied forceis precise, repeatable, controlled by time schedule,used to fire the power supply, and regulatedboth to reduce the initial impact and not to becomeexcessive after the weld. Weld force control isequally as important as weld energy and timecontrol.

The power supply with either an internal or external transformer both powers andcontrols the application of heat and time in the resistance welding process. In generalterms, resistance welding applies high current with low voltage.

The generic schematic is:

In simple terms the resistance w e l d i n g p o w e r s u p p l y transforms, modulates andcontrols the electrical energy of the power line and applies it to the weld accordingto a user defined or user programmed “weld schedule.” Depending on the complexityand intricacy of the power supply the user can program from one to more than 100 at-tributes and permutations of the welding process, and, using a microprocessor, storethese attributes as a uniquely defined “weld schedule.”

This basic weld schedule forms the basis for all microwelding sched-ules. The amplitude and duration of all force and heating parameterscan be defined in the “weld schedule.” The four critical parametersare: electrode force, squeeze time, weld pulse and hold time. Variationscan also be dual pulse and other sequences shown below.

EXAMPLES OF WELDING SEQUENCES (ALSO CALLED HEAT PROFILES) INCLUDE:

Single Pulse:Use on flat un-plated parts

Quench/Temper

Up Slope: Use on hard,irregular shaped, oxidized parts and aluminum parts

Preheat/Postheat:Use on refractiveparts

Unbalanced:Use on polarity sensitive parts

Roll Spot: Useto make non-hermetic seam welds

Down Slope:Used to reducemarking andembrittlement

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STORED ENERGY(CAPACITIVE DISCHARGE):The stored energy welding power supply,commonly called a Capacitive Discharge or CDWelder, extracts energy from the power lineover a period of time and stores it in weldingcapacitors. Thus, the effective weld energy isindependent of line voltage fluctuations. Thisstored energy is rapidly discharged through apulse transformer producing a flow of electricalcurrent through the welding head and workpieces.

Capacitive discharge power supplies are ratedin accordance with the amount of energy they

store and the welding speed. The energystored, expressed in watt-seconds (joules), isthe product of one-half the capacitance of thecapacitor bank and the square of the appliedvoltage. The energy delivered to the electrodesis considerably less than this value because oflosses in the primary and secondary circuits.

Some power supplies provide a “Dual Pulse”feature which allows the use of two pulses tomake a weld. The first pulse is generally usedto displace surface oxides and plating, and thesecond pulse welds the base materials. Thisfeature also reduces spitting.

PULSE TRANSFORMERS – are designed tocarry high secondary currents, typically up to10,000 amps. Welds made with a capacitivedischarge system are generally accomplishedwith a single, very short weld pulse with a du-ration of from 1 to 16 milliseconds. This pro-duces rapid heating that is localized at thewelding interface. The length of the outputpulse width can normally be modified bychanging taps on the pulse transformer. Po-larity switching is a convenience when themachine is used to weld a wide variety of po-larity sensitive dissimilar metals.

In practical applications, the short pulse isused to weld copper and brass, which requirefast heating; the medium pulse is used toweld nickel, steel and other resistive materialsand the long pulse is also used to weld re-sistive materials and to reduce sparking andelectrode sticking.

The AC welder derives its name from the factthat its output is generally a sine wave of thesame frequency as the power line. It extractsenergy from the power line as the weld isbeing made. For this reason, the power linemust be well regulated and capable ofproviding the necessary energy. Some ACwelders (including all Miyachi Unitek ACwelders) include a line voltage compensationfeature to automatically adjust for power linefluctuations. In its simplest form, the ACwelder consists of a welding transformer thatsteps down the line voltage (normallybetween 480 to 100 volts) to the weldingvoltage (typically 2 to 20 volts). The weldingcurrent that flows through the secondary ofthe transformer, and its connected load, isvery high, ranging from 10 to more than100,000 amps. The welding current isallowed to flow for very short periods of time,typically .001 to 2 seconds. AC welders canoperate at rates up to 5-6 welds per second.

AC Welding Systems are generally composedof the three elements. The Welding

Transformer, the Welding Control, and theMechanical System.

WELDING TRANSFORMERS – are used inAC machines to change alternating currentfrom the power line into a low-voltage, highamperage current in the secondary winding.A combination of primary and/or secondarytaps on the welding transformer arecommonly used to provide a macro adjustmentof the welding current, as well as adjustmentof secondary voltage. Transformer ratings forAC machines are expressed in KVA (kilovolt-amperes) for a specified duty cycle. This dutycycle rating is a thermal rating, and indicatesthe amount of energy that the transformercan deliver for a stated percentage of a

specific time period, usually one minute,without exceeding its temperature rating.The RMS Short Circuit Secondary Currentspecification indicates the maximum currentthat can be obtained from the transformer.Since heating is a function of the weldingcurrent, this parameter gives an indicationof the thickness of the materials that canbe welded.

Recent advances in AC welding technologyhave adapted constant current feedbackcontrol at the line frequency (50 or 60 Hz) whichcan be useful for welds longer than 5 cycles(82-100 milliseconds) by automaticallyadjusting the power supply parameters.

High Frequency Inverter Welders usesubmillisecond pulsewidth modulation(switching) technology with closed-loopfeedback to control the weld energy insubmillisecond increments. Three phase

input current is full wave rectified to DCand switched at (up to) 25 kHz to produce anAC current at the primary of the weldingtransformer. The secondary current is thenrectified to produce DC welding currentwith an imposed, low-level, AC ripple. Thehigh-speed feedback circuitry enables the in-verter power supply to adapt to changes in the

secondary loop resistance and the dynamicsof the welding process. For example, a 25 kHzinverter power supply adjusts the output cur-rent every 20 microseconds after rectification,which also allows the weld time (duration ofcurrent) to be controlled accurately in incre-ments as small as 0.1 milliseconds.

POWER SUPPLY TECHNOLOGIES

HIGH FREQUENCYINVERTER (HFDC)

DIRECT ENERGY (AC)

Functional Diagram of an AC Resistance Welding Machine

Functional Diagram of a Stored Energy Resistance Welding Machine

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POWER SUPPLY TECHNOLOGY COMPARISON

The transistor direct current power supplies(also called “Linear DC”) produce much thesame results as the high frequency inverterby using a high number of power transistorsas the direct energy source. This technologyprovides clean, square wave forms with ex-tremely fast rise time. Used primarily in con-stant voltage feedback control, transistor DCpower supplies are effective in thin foils andfine wire welding applications and for ex-tremely short welds.

Linear DC welders utilize transistor con-trolled feedback enabling total feedback re-sponse times of less than 5 µS. The termLinear DC comes from the waveform that isoutput from the power supply. No trans-former is utilized. The primary limitation toLinear DC technology is the low duty cycles,typically much less than 1 weld per secondat less than rated output.

Typically, constant voltage feedback is

utilized in conjunction with short weldpulses. Because the feedback response is sorapid, high energy welds with extremelyshort duration can be used without weldsplash or arcing. These short pulses limit theheat stress and the size of the heat affectedzone on the weldments. This provides astronger more ductile weld joint, along withless part deformation, less discoloration,and significantly longerelectrode life.

Cons tan t vo l t agefeedback is chosen fortwo reasons: its abilityto prevent arcing andto provide the optimumweld power distribu-tion based on the partresistance. If for somereason the weldmentscollapse faster than the

weld head can fol low up, arcing usuallyoccurs. When constant voltage feedbackis applied with the feedback responsetimes capable by Linear DC welding thisarcing is minimized.

Transistor DC units tend to be larger andheavier than other resistance weldingpower supply technologies.

Typical Typical Repetition Power Supply Cycle Time Bond Type Rate Advantages Limitations Waveform

Capacitor Discharge (CD) provides a 1-16 msec Solid State ≤ 2/sec. Rugged and inexpensive. Open loop. uni-polar fixed duration weld current pulse Suitable for highly Discharge of short duration with a fast rise time. conductive materials. “self-regulating.”

Direct Energy (AC) provides a uni-polar >8 msec Fusion, Reflow, ≤ 5/sec. Rugged and Poor control at or bi-polar, adjustable duration weld current Braze inexpensive. short cycle times. pulse with rise times dependent on the % weld current setting.

High Frequency Inverter (HFDC) provides a 1,000 msec Fusion, Solid ≤ 10/sec. Excellent control Higher cost. uni-polar, adjustable duration weld current State, Reflow, and repeatability. pulse with an adjustable moderate-to-fast, Braze High current capacity; rise time. high duty cycle.

Transistor or Linear DC (DC) provides a 0.010 –9.99 Solid State ≤ 1/sec. Suitable for amorphous Higher cost uni-polar, adjustable duration weld current msec materials, thin foils, maintenance. pulse with a fast voltage rise time, and square fine wires. Limited duty voltage wave. Excellent control cycle. One piece and repeatability. construction.

The high frequency closed loop feedback canbe used to control (maintain constant) eithercurrent, voltage, or power while also monitoringanother of the same three parameters.

Additional benefits of high frequency switchingtechnology include reduced power consumption,smaller welding transformers, and the use ofa very short pre-weld “check pulse” to testelectrode and parts positioning prior to ex-ecuting a weld. The result of this pre-weldcheck can be used to inhibit the weld bysetting check limits.

CONSTANT CURRENT can be used for 65%of all welding applications including those

that exhibit low contact resistance, smallvariability in contact resistance, flat parts, andmultiple part “sandwiches.”

CONSTANT VOLTAGE can be used for ap-plications where the workpieces do not haveflat surfaces, e.g. crossed wires, and wherethe resistance varies significantly, and for ex-tremely short welds (less than 1 millisecond).

CONSTANT POWER can be used for applica-tions with significant variations in electricalresistance from weld to weld, including appli-cations where the plating erodes and builds-up on the face of the welding electrodes.

Due to their extensive programmability,small transformer size, and robustness, highfrequency inverter power supplies are generallythe best choice for automation applications.

TRANSISTOR DIRECT CURRENT (LINEAR DC)

Functional Diagram of an HFDC Resistance Welding Machine

Functional Diagram of a Linear DC Resistance Welding Machine

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Welding electrodes are installed in the weld head to touch andmaintain contact with the workpieces through the full weld schedule.The MATERIALS section (pg. 2) discussed the “rule of opposites” andthe criteria for selecting the electrode material.

The welding electrodes play three different roles in resistance welding:maintaining uniform current density, concentrating current at weldingpoints, and maintaining thermal balance during welding. Electrodesare available in many shapes, with the most common shown at right.Electrode material and shape are determined by considering the forcenecessary for welding and the thermal conductivity of the workpieces.

COMMON ELECTRODE SHAPES:

In conventional macro-welding, e.g. car body assembly, the electrodesare made of copper alloys and usually water-cooled. However, inmicro-welding, the electrodes are made of a wide variety ofconductive and refractory materials depending on the parts to bejoined, and are air-cooled.

WELDING ELECTRODES

As described earlier, the application and control of force during theresistance welding process is extremely important. The mechanicalsystem to do so is generally referred to as the weld head. The weldhead (including the welding electrodes), functions to force theworkpieces together and hold them during the weld. The weld headprovides the current path, welding pressure or force, triggers (initi-ates) the weld current, provides follow-up force as the workpiecesmelt together, and cools the workpieces after the weld. Developmentof a weld head force schedule is equally as important as develop-ment of a power supply schedule. The ideal force schedule insuresthat proper electrical contact resistance and proper heat balance areboth achieved and maintained between the workpieces and the elec-trodes. Force is measured in pounds (lbf), Kilograms (Kgf) or Newtons(N or dN).

In small parts resistance welding the weld heads are of linear motiondesign with linear races or bearings and spring-driven force ad-justment. Low inertia weld heads with low mass electrode holdersand low friction bearings provide fast “follow-up.” “Follow-up” refersto the capacity of the weld head to accelerate and remain in contactwith the workpieces as the workpieces become molten and melt to-gether during the weld.

Recent advances in weld head design include electronic weld headswhere weld head movement and force are electronically controlled,and/or electronically monitored, via a precise schedule. The precisecontrol of an electronic weld head can program the timing of eachelement of the force profile, minimize impact force, duplicate force

profile between weld stations, and pro-vide electronic evidence of the actualweld force profile. The control forelectronic weld heads can be inde-pendent of, or integrated into, resist-ance welding power supplies. The“Electrode Force” diagram, below left,depicts the precisely controlled forceprofile, including follow-up force, of anelectronic weld head.

Today, force sensors, strain gauges, andmotion sensors/transducers can be builtinto a mechanical or electronic weldhead for control and/or monitoringpurposes.The weld head must be designed and operated to precludethese potential problems.

The most typical weld head related problems are depicted in thedrawing below.

Lastly, the use of properly designed fixtures to hold the workpiecesin fixed position during welding is highly desirable. The workpiecesmust be in a fixed rigid position prior to the initiation of the resist-ance welding process. In manual welding, operators should be usedto load workpieces in a fixture, not to hold workpieces during thewelding process. Additionally, the fixtures should be constructed toinsure that the welding surface of the electrodes fit squarely andcompletely against the workpieces.

WELD HEAD TECHNOLOGIES

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COMMON ELECTRODE MATERIALS

WELD QUALITY AND PROCESS VALIDATION

RWMA 1 – COPPER CADMIUM ALLOY – 70BRockwell Hardness, 90% conductivity. Used forwelding aluminum and tin plate. Not availablefrom Amada Miyachi America. GLIDCOP is asubstitute.

RWMA 2 – COPPER CHROMIUM ALLOY – 83BRockwell Hardness, 85% conductivity. Used forwelding steels, nickel alloys and other high re-sistance materials.

GLIDCOP – DISPERSION STRENGTHENEDCOPPER with 0.15% ALUMINUM OXIDE – 68BRockwell Hardness, 92% conductivity. Longer

life, greater thermal stability, higher strengththan RWMA 2. Generally interchangeable withRWMA 2 without changing schedules.

RWMA 3 – COPPER COBALT BERYLLIUMALLOY – 100B Rockwell Hardness, 48% con-ductivity. Used for welding high resistance ma-terials requiring high weld forces.

RWMA 11 – COPPER TUNGSTEN ALLOY – 99BRockwell Hardness, 46% conductivity. Usuallyinserted into an RWMA 2 shank. Used forwelding cuprous and precious metals. Used forlight projection welding dies.

RWMA 13 – TUNGSTEN –70A Rockwell Hard-ness, 32% conductivity. Usually inserted intoan RWMA 2 shank. Cannot be machined butmay be ground to the desired shape. Used toweld non-ferrous metals such as copper andbrass.

RWMA 14 – MOLYBDENUM – 90B RockwellHardness, 31% conductivity. Usually insertedinto an RWMA 2 shank. Machineable. Used forwelding copper, silver, gold and their alloys.

The monitoring of any manufacturing processis essential for achieving the “six sigma” goalsof production quality. Often the cost of monitor-ing equipment is significantly less expensivethan the cost ramifications of the field failureof a single weld.

Destructive testing methods include tensilepull-test, peel tests, shear tests, corrosiontests, optical microscopy, cross-section in-spection, and scanning electron microscopy.These tests are typically used to qualifyprocesses initially as well as periodically. On-line monitoring of key resistance welding pa-

rameters is a more effective method of con-tinuous weld quality.

Weld monitors are devices that measure oneor more specific electrical and/or mechanicalparameters that dynamically change duringthe welding process. These measurementsmay include weld current, voltage drop acrossthe electrodes, workpiece expansion and de-formation, electrode force, electrode move-ment (displacement), size of the electrodeface, acoustic energy emitted while the weldis being formed, and temperature of the work-pieces. Variations in the thickness, tensile

strength, hardness, surface finish and cleanli-ness of the workpieces have a significant ef-fect on weld quality. As discussed earlier, theshape of the electrode face also affects weldquality. Modern measurement techniquesmake it possible to accurately measure theenergy and pressure used to make a resist-ance weld. Weld monitoring is effective to theextent that the electrical and mechanicalmeasurements made during the weldingprocess reflect the variations in the physicalproperties of the workpieces and the weldingequipment.

The size of the weld will not be larger thanthe electrode face. Therefore, it is importantto utilize electrodes of the same tip diameteras the desired weld nugget. The current den-sity at the workpiece interfaces varies as thesquare of the diameter of the electrode face.Electrode positioning is critical: electrodes

should be positioned where the weld is de-sired, should generally not overhang theedges of the part (except in wire and smallterminal welding), should not bend, shouldbe perpendicular to the plane of the work-pieces, should maintain constant diameter(constant area) as they wear, and should be

cleaned and dressed regularly. Electrodesshould be dressed with 600 grit silicon car-bide paper or polishing disk pulled with lightforce in one direction only. Electrodes shouldbe replaced when the tip is damaged or blowsout. It is best to have all electrode tips regroundregularly by a qualified machine shop.

THE FOUR BASIC ELECTRODE CONFIGURATIONS ARE:

The choice of electrode configurations is determined by the geometry of the workpieces, the application, and the desired current path.

Opposed (Direct) Welding is themost commonly used type of resist-ance welding. The welding currentflows directly from one electrode tothe other, through the weldments.

Step (Indirect) Welding is often usedwhen the workpieces are configuredin such a way that only one side ofthe workpiece is accessible with anelectrode, or there is a large thermalimbalance. The welding current flowsfrom the first electrode, through theworkpiece, through the area of theweld, through the other workpieceand into the other electrode.

Series Welding is also used whenonly one side of the weldment is ac-cessible with electrodes. This form ofwelding has the advantage of makingtwo weld nuggets at one time. How-ever, series welding is generally lesscontrollable because of the manyshunt paths available to the weldingcurrent.

Seam Welding is another variationon resistance spot welding. in thiscase, the welding electrodes aremotor-driven wheels rather than sta-tionary rods. The result is a “rolling”resistance weld or seam weld used tojoin two sheets together. Overlap-ping and continuous seam welds canproduce gas- or liquid- tight joints.

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Specifications subject to change without notice. Copyright© 2015 Amada Miyachi America,Inc. The material contained herein cannot be reproduced or used in any other way withoutthe express written permission of Amada Miyachi America, Inc. All rights reserved.

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Amada Miyachi do Brasil Ltda.Av. Ceci, 608 – Bloco 16B – Empresarial Tamboré,06460-120, Barueri, SP, BRT: +55 (11) 4193-1187 • antonio.ruiz@miyachi.com

Amada Miyachi America Mexico Sales Office:5959 Gateway West, Ste 327, El Paso, TX 79925T: (915) 881-8765 mxsales@amadamiyachi.com

Your Local Amada Miyachi Representitive:

1820 S. Myrtle Ave. • P.O. Box 5033Monrovia, CA 91017-7133 UST: (626) 303-5676 • F: (626) 358-8048info@amadamiyachi.com • www.amadamiyachi.comISO 9001 Certified Company • 24/7 Repair Service: 1-866-751-7378

Amada Miyachi America Midwest Sales Office:50384 Dennis Ct., Wixom, MI 48393T: 248 313 3078 • F: 248 313 3031midwestsales@amadamiyachi.com

RES

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ELD

ING

Studies by the Edison Welding Institute haveshown the following probability ratio of causesof poor weld quality:

40% Fixture related20% Weld head related20% Part/electrode geometry20% Weld schedule or power supply related

As with all good manufacturing practices,the welding process must be clearly defined,documented, and validated. The typicalsteps include:

1. Defining weld quality parameters: • Peel, tensile, or shear strength. • Part deformation allowable. • Nugget penetration and diameter. • Cosmetic requirement.

2. Optimizing the weld schedule.

3. Correlating welding and weld monitor withweld quality.

• Peak weld current and electrode voltage. • Displacement (set-down). • Force. • Nugget diameter (if applicable). • Nugget penetration.

• Peel, tensile or shear strength. • Cosmetic acceptability.

4. Establishing process limits.

5. Documenting weld schedule and monitorschedule.

6. Auditing the weld schedule and weldprocess regularly.

7. Establishing a regular equipment inspectionand maintenance.

Weld documentation should address each ofthe following subjects:

• Materials: ❑ Alloys ❑ Dimensions ❑ Surface Conditions ❑ Projections, if applicable

• Power Supply: ❑ Model/Voltage ❑ Time/Pulse width (msec) ❑ Energy (w-s, I, V, or P) ❑ Heat profile ❑ Limit settings

• Weld Transformer: ❑ Model ❑ Tap Setting

• Weld Head: ❑ Weld head model ❑ Weld force (lbf, Kgf, dN) ❑ Weld cable length ❑ Weld cable diameter ❑ Weld force verification frequency

• Electrodes: ❑ Electrode polarity ❑ Electrode alloys ❑ Electrode dimensions ❑ Electrode gap ❑ Electrode cleaning and changing frequency

• Test Parameters: ❑ Pull strength ❑ Cross section depth ❑ Weld monitor parameters ❑ Sampling schedule ❑ Cosmetic requirements

PROCESS VALIDATION

Today’s state-of-the-art resistance weld-ing monitors can measure the following pa-rameters practically and effectively:

• Current• Voltage• Force• Displacement (weld collapse)

Combining these measurements in vari-ous ways can provide the user practical in-formation regarding weld quality.

Pre-weld resistance checks can be used todetect the absence of parts or major irregular-ities in part thickness or fit-up.

Force monitoring can be used as a preventivemeasure to prevent excessive impact or weldforce and as a diagnostic tool. Force moni-toring is generally used as a process control

tool. It is used less often as a quality evalua-tion tool.

Extensive experiments are normally re-quired in order to determine which combi-nation of measurement parameterscorrelates with the quality of their specificparts. Once correlation is verified in a pro-duction environment over a reasonabletime, the weld monitor becomes a vital man-ufacturing tool. If the user carefully controlsthe quality of the workpieces and uses goodmanufacturing process control, a weld moni-tor can provide the necessary electrical datafor statistical process control which in turnshould increase quality and reduce man-ufacturing costs.

Modern weld monitors integrate with or in-

clude statistical process control (SPC) soft-ware. SPC software packages can performstatistical calculations, generate X-bar andR-control charts, and provide summary in-formation of the weld data. A few monitorscan compare multiple weld parametrics forweld analysis.