Testing Welds

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Testing and Verification of Intercell Welds - 1 - Testing and Verification of Intercell Welds Tony Schröer, Jeff R.Snell Digatron / Firing Circuits One of the more critical processes in battery manufacturing is the inter-cell weld. At the same time inconsistencies in lead material and less than reliable testing methods complicate process control and verification efforts. Time proven methods to verify welds such as X-ray, Gamma ray and ultrasonic testing are either not suited for lead welds or impractical when considering the production environment. Only electrical tests are suited for 100% production lot testing of inter-cell welds. This paper attempts to highlight the weaknesses inherent in traditional electrical testing methods when using a tester integrated with the inter-cell welding machine. Unimpeachable data regarding the quality of inter-cell welds cannot be achieved using traditional electrical test methods. More reliable results can be obtained with an improved measurement system, which is separate and installed after the inter-cell welding machine. An optimized inter-cell test process and measurement techniques are discussed. 1. Introduction To achieve a battery voltage of 12V, six cells must be connected in series. Connections from cell to cell for automotive batteries are made within the polypropylene container through holes punched in the partitions. The positive - strap terminal on one side and the negative strap terminal on the other side of the hole are pressed together and welded (Fig.1). Fig.1: Positive and Negative Strap Terminal This intercell weld is one of the more critical and difficult processes used in manufacturing batteries. Thousands of batteries are destined for scrap if any one of the six intercell welds per battery (Fig.2) does not fulfill the specifications. Fig.2: Automotive Battery The following will focus on the resistance welding process of lead, specifically lead-alloys.

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Transcript of Testing Welds

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Testing and Verification of Intercell Welds - 1 -

Testing and Verification of Intercell WeldsTony Schröer, Jeff R.SnellDigatron / Firing Circuits

One of the more critical processes in battery manufacturing is the inter-cell weld. At thesame time inconsistencies in lead material and less than reliable testing methodscomplicate process control and verification efforts. Time proven methods to verify weldssuch as X-ray, Gamma ray and ultrasonic testing are either not suited for lead welds orimpractical when considering the production environment. Only electrical tests are suitedfor 100% production lot testing of inter-cell welds.

This paper attempts to highlight the weaknesses inherent in traditional electrical testingmethods when using a tester integrated with the inter-cell welding machine.Unimpeachable data regarding the quality of inter-cell welds cannot be achieved usingtraditional electrical test methods.

More reliable results can be obtained with an improved measurement system, which isseparate and installed after the inter-cell welding machine. An optimized inter-cell testprocess and measurement techniques are discussed.

1. Introduction

To achieve a battery voltage of 12V, sixcells must be connected in series.Connections from cell to cell forautomotive batteries are made within thepolypropylene container through holespunched in the partitions. The positive -strap terminal on one side and thenegative strap terminal on the other sideof the hole are pressed together andwelded (Fig.1).

Fig.1: Positive and Negative Strap Terminal

This intercell weld is one of the morecritical and difficult processes used inmanufacturing batteries. Thousands ofbatteries are destined for scrap if anyone of the six intercell welds per battery(Fig.2) does not fulfill the specifications.

Fig.2: Automotive Battery

The following will focus on theresistance welding process of lead,specifically lead-alloys.

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There is very little literature availablewhich discusses the behavior of leadmaterials when welded. What literatureexists, only considers welding by openflame.

There are two reasons for the limitedliterature about lead welding:

- Only the battery industry employs theprocess of welding lead tomanufacture it products

- Lead welding is one of the mostdifficult processes to control. Itrequires control of several variablesto achieve reliable results.

Over the past decades the weldingprocess have been more focused onimproving efficiency. It may not bepractical to eliminate all subtle materialand process variations.

In some cases this leck of processcontrol may result in marginal intercellconnections which could lead to batteryfailure.

Why is it difficult to weld lead?

Instead of pure lead (Pb), lead-antimony(PbSb) alloy is used to increase thehardness of lead. But properties andparticulary mechanical properties of lowantimony alloys do vary and dependmainly on material history

Fig.3 to fig.6 will reflect thecharacteristics of lead-antimony alloywith 3% antimony.

Fig.3: Two-Phase-Diagramm

Lead is a very soft metal with a relativelylow melting point at 327 deg.C

With respect to the resistance weldingprocess the melting point can becontrolled by an appropriate amount ofenergy. But the control range is smalland if the energy applied, slightlyexceeds the maximum limit leadbecomes liquid and will squirt out.

Another important parameter is thehardness.

One cannot state the general hardnessof a lead-antimony alloy. The rate ofhardening depends on the coolingprocess and the percentage content ofantimony.

Fig.4 shows hardness versus age.

Fig.4: Hardness vs. Age

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It is obvious that newly casted leadbehaves somewhat differently than leadwhich was stored. Newly casted lead isvery soft and can be easily formedtogether. Stored lead – roughly speakingone day or more – is brittle and breakswith even minor deformation.

Usually welding machines receive newlycasted lead without the necessaryhardness. The welding clamps will pinchthe two cast on strap flanges and justthe force of the clamps will join the partsand create an electroconductivejunction.

Even though welding in this state ispossible the process energy must beincreased to compensate for the lowresistance junction. It is possible toconduct the welding procedure withoutwelding the material. And the internalresistance measurement will displayvalues within the limits and will let thebattery pass.

The electrical resistance curve (Fig.5)typically shows the highest gradient atthe melting point.

Although most welding machines ensurerapid cooling after the high currentwelding process the electrical resistancemay vary significantly due to thetemperature gradient.

Fig.5: Electrical Resistance vs. Temperature

As a result internal resistancemeasurement systems can only be usedto compare resistance data within aproduction batch and do not accuratelymeasure the real resistances of theintercell welds.

The Oxidation curve (Fig.6) indicatesthat oxidation begins immediately aftercontact with air. Once a thin oxidationlayer has been developed furtheroxidation is inhibited. The film has anisolating property.

In accordance with equation

d = m / O x V where

m = oxidation [mg],O = 5400 mm2 andv = 0.252 mm3/g

in our test a thin film of about 0.4µm to0.6µm was developed after a fewminutes.

Fig.6: Oxidation vs. Age

Any measurement system whetherintegrated or separate from the weldingmachine should have probes whichbreak through the oxidation layer toensure accurate measurement.

All these variables make it very difficultto control the welding process for leadalloys and to consistantly producequality welds.

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Contact detectionvia voltagemeasurement

Weld electrodeswith projectionforming tools(thrust adapter)

Water cooling

Strap position beforeforming of the projection

… and after projection forming

Despite these issues the battery industryhas proven that lead welding is possible.

Todays welding machines consist of twohydraulic clamps for interconnection 1,3, 5 and interconnection 2 and 4.These clamps are positioned byautomated x/y-positioning systems [1].

Power electronics are used to providewelding currents in kA range usingprogrammable controllers and a PLC isused to control the conveyor station andto position the battery underneath theclamps.

Fig.7 shows details of the weldingclamps.

The voltage sense connection providesthe feedback for current control and alsodetects the initial contact made with theparent material.It is important to control the timebetween inital contact with the parentmaterial and the start of the weldingprocess. This time period determinesthe size of the contact area and theinitial contact resistance prior to welding.Because this pause time is so criticalspecial contact detection modules areused to precisely measure initial contact.

Fig.8: Projection Forming

Fig.7: Welding Clamp Details

Fig.8 shows the strap connectionsbefore and after projection. If the initialcontact area is too small the resistancewill be too high and the welding processwill generate too much heat, causinglead, to squirt out at the beginning of thewelding process.

If the initial contact area is too large avery low resistance electroconductivejunction will result. This will alwayscreate a condition for cold welds.

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The entire welding sequence (Fig.9) willbe explained in the following.

Fig.9: Welding Sequence

When the battery is positioned, theclamp contacts the strap terminal andcloses the projection forming electrodes(phase 1).

Phase 2 starts with a signal from thecontact detection module. If theprogrammed pause time (phase 2) iscomplete, the weld current starts (phase3). Current amplitude and current timeare controlled by the welding controller.The lead will melt and due to pressureapplied to the strap terminals thematerial will be pressed through and fillthe partition hole.

In phase 4 the hold time allows the leadto cool (phase 4).

After the hold time the clamps will openand the contact detection moduleindicates the end of the welding process(phase 5)

Dynamic regulation is required to controlweld energy and particulary weldcurrent. As a result switched modepower supplies are typically used inmodern welding machines.

Fig.10: 1000 Hz MF-DC Inverter Power Unit

Fig.10 shows the principle schematic ofa 1000Hz DC inverter power moduleusing pulse-width-modulation.

The 3-phase AC signal is rectified toprovide a DC link voltage for the IGBTinverter. The IGBT inverter is controlledby 1kHz frequency and provides 0.5 mscontrol cycle. The welding transformergenerates the high current output andafter rectification by the two diodes atrue DC signal is applied across thewelding electrodes.

Even considering the equipment andprocess improvements developed overthe recent years there remainssignificant variation in both, process andmaterials which can result insubstandard welds.

As difficult it is to weld lead, it is moredifficult to verify lead welds.

The next pages will investigate andanalyse existing methods of qualityinspection for weld processes for use inbattery assembly lines.

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2. Experimentals

Methods used in the industry to verifywelds are:

X-Ray, Gamma-Ray, Ultrasonic,Electric Inspection such asResistance Measurement orResistance vs. Welding Time andDestructive Inspection.

2.1 Destructive Inspection

Fig.11: Adapter to prepare sample weldings

Destructive testing of welds is performedon a periodic or sample from a lotbases. The results can be used to adjustthe intercell welding machine.

A special adapter is used to produceweld samples to avoid wasting entirecell packages (Fig.11).

A fixture keep the strap terminals in theright position for welding. There arecavities in the upper platform to positionthe terminals. Different battery sizes canbe accommodated by the same fixtureusing the telescope.

After welding the operator lowers theupper platform to get access to the strapterminals.

Fig.12: Twist off tool kit with torque display

Strap terminals are twisted off using atorque wrench to measure the maximumtorque value prior to failure (Fig.12). It isassumed that the welding qualitycorrelates to the torque value.

Fig.13 shows torque data, deviation anddistribution of 28 samples

Fig.13: Production lot test results

Another criteria to verify the weldingprocess is optical inspection.

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Destructive Inspection, optical evaluation

Fig.14: Ideal Welding Fig.15: Corona welding Fig.16: Blow holes

Defective areas can be seen clearly as displayed in figures 15 and 16

2.2 Non-Destructive Inspections

There are several non-destructiveinspection methods typically used in theindustry.

As a result the high degree ofabsorption, X-Ray and Gamma-Raycannot be used for inspection of leadwelds. The reason is the attenuationeffect whereby the atomic number iscubing and multplied by the density ofthe material in question. As a result thedegree of absorption for lead is muchhigher when compared to other metals.It is technically inpossible to use X-Rayor Gamma-Ray to find irregularities inthe intercell welds.

Ultrasonic inspection would be anideal method to determine irregularitiesin intercell welds. The main advantage isthe straight-lined propagation ofultrasonic waves through the mediumand very good diffusion [2]. In thisregard ultrasonic waves are comparableto X-Rays or Gamma-Rays.

Unlike X-Rays or Gamma-Rays,Ultrasonic waves accurately detect thesmallest of air gaps, which create anunbridgable barriers completelyreflecting signals.

Fig. 17: Ultrasonic inspection, pause time too short

Fig.17 shows an ultrasonic image of anintercell weld where the pause time wastoo short.

One can see in the image that there isno coherent area of welding. It isassumed that blow holes have beengenerated.

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In general blow holes will be generatedif there is too little material available.This will be the case when pause timesare too short and when the weldingprocess starts immediately after the twoterminals are in contact with each other.

Fig.18: Ultrasonic inspection, pause time too long

If the pause time is too long the materialwill develop an electroconductivejunction and the resistance will be toolow to produce the necessary weldingtemperature. Fig.18 shows a coronawelding at 1000 ms pause time.

The next two figures will show theresults when varying the weldingcurrent.

Fig.19: Ultrasonic inspection, welding current too high

Fig.19 shows the result if the weldingcurrent is too high. The lead squirts outand leaves blow holes.

Welding current below the minimumrequired will result in an incomplete andpoor quality weld.

Fig.20: Ultrasonic inspection, welding current too low

In Fig.20 it can be seen that the areas at1, 4 and 7 o‘ clock are not weldedproperly.

Fig.21: Ultrasonic inspection, welding current with ramp

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Fig.21 shows a substantial result.

Whenever current ramps were usedbetter quality welds resulted.

The major advantage of a current rampis the preliminary warming of lead whichwill create uniform material and processconditions. Thus the influence ofmaterial hardness can be minimized.

The current time was set to 240 mswhereas 180 ms were programmed as aramp.

The next figures show the variation ofcurrent time.

Fig.22: Ultrasonic inspection, current time too long

If the current time is too long the energyapplied to the weld will be too high andmaterial will squirt out. After coagulationblow holes and hair line fractures will befound. As it can be seen in Fig.22 only asmall area between 11 and 12 o‘clock iswelded.

Fig.23: Ultrasonic inspection, current time too short

If the current time is too short the weld isincomplete. Fig.23 shows weldedmaterial in the center only. Outside thecenter the weld is very shallow.

The next figures will show theimportance of correct pressure whenwelding the strap terminals

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Fig.24: Ultrasonic Inspection, Fig.25: Ultrasonic Inspection, Fig.26: Ultrasonic Inspection, pressure 2.5 kN pressure 3.8 kN pressure 4.7 kN

Fig.24 to 26 show results generated at2.5 kN, 3.8 kN and 4.7 kN.

Too little pressure will not create thedesired electroconductive junction,where resistance is low enough so asnot to produce excess heat.The lead will squirt out and will leaveintercell welds with blow holes (Fig.24).

Beyond a certain force applied, theresults will be consistant with Fig.25 andFig.26. This supports the theory thatincreasing pressure should be appliedup to that point just before structualdamage to the partition or hole mightoccur.

The ultrasonic inspection of intercellwelds yields significant andreproducable results without damagingthe test object.

But problems occur with regard to thetestheads used to transmit and receiveultrasonic signals. Testheads are onlyavailable for larger objects. The spacerequired to properly position a testheadis not available. Another problem is to

establish a good ultrasonic junctionbetween the testhead and the intercellweld.

In typical applications water, grease oroil are used as a transfer medium toachieve the necessary contact. It isdifficult to believe that this could beimplemented in an assembly line forbatteries.

Beyond destructive inspectiontechniques, electrical testing is the onlysuitable method to verify intercell welds,in production environments.

Several battery manufacturers haveimplemented two tests:

The first using a tester integrated intothe cell welding machine and also thesecond test after the battery has left theintercell welding machine.

In the following the advantages anddisadvantages of both systems will bediscussed.

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Fig.27:4-Wire Current-Voltage Measurement

The general principle is shown in Fig.27.

The principle is based on the 4-wirecurrent-voltage measurement. A powersource will provide constant current tothe test object and voltage will bemeasured across the test object. A highimpedance voltage input is required tomeasure voltages in the microvolt range.The microprocessor will then calculatethe resistance.

Fig.28: General Assembly of a Welding Clamp

The general assembly of a weldingclamp is shown in Fig.28.The voltage sense leads are connectedto the clamps. After the welding processand a cool down period an additionalmeasurement current is programmed topick up the voltage drop across theclamps.

The internal microprocessor calculatesthe resistance which - at this stage –does not represent the resistance of theintercell welds. First, the parasiticresistances must be eliminated from thecalculation.

Parasitic resistances are:

- electrode resistance- contact resistance- resistance of the lead material

Fig.29: Parasitic Resistances

Fig.30: Real Resistance Model

The real resistance model of theintegrated resistance measurement isshown in Fig. 30.

V

Welding current

R5

Voltage sense

R3 R4 R2R1 R6 R7

V

Welding current

Intercell weld

Voltage sense

R1: electrode resistance

R3: contact resistance

R6: material resistance

R5: contact resistance

R7: material resistance

R4: contact resistance

R2: electrode resistance

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Some machines measure the offsetresistance and store this result for useas a compensating value whencalculating the intercell resistance.

However in production conditions theoffset resistance may increase over timedue to contamination that accumulateson the contact surfaces of the thrustadapter (Fig.31).

Fig.31: Contamination of Thrust Adapter

Contamination of the thrust adapter willresult in increased resistance betweenthe electrode and the lead materialbecause of a layer of contaminationaccumulates between the electrode andlead material acting as an insulator.Additional heat will be generated at theelectrode/thrust adapter, not at the leadmaterial. Therefore this heat will notcontribute to the welding process.

Heat and high pressure join to create athin lead film on the upper thrustadapter. Any accumulation of lead willtend to accelerate this plating process.Poor intercell welds will result.

It is necessary to periodically clean thesurface of the thrust adapter to ensure aconsistant welding process.

The parasitic resistances mentioned onthe previous pages will form the majorpart of the total resistance. Irregularitiesin the intercell weld will not represent asignificant portion of the total resistanceand will typically be within the toleranceband.

Another fact to be considered is theresistance measurement takes placewhen the the test object is subjected topressure from the welding clamp. Sincelead is a soft metal the pressure appliedis capable of creating a low resistanceelctroconductive junction even withoutwelding. Experiments confirmed thatresistances of both welded andunwelded strap terminals can be withinthe tolerance band when using aclamping pressure of above 3.5kN.

Subsequently the integrated resistancemeasurement can only be used ascomparative data. It does not reflect thereal resistance of the intercell weld.

Another option would be to measure andrecord resistance during the weldingprocess. This is commonly done whenwelding steel or sheet metal [3].

Fig.32: Master resistance curve for steel and sheet metal welding

cleaning cleaning

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Welding steel and sheet metal mastercurves are used to control the weldingprocess.

The first part of the curve should beignored as the data will vary significantlydepending on material conditions underpressure.

The important part of the curve is theshaded area. This area indicates theacceptable limits of time and resistancethat will result in an appropriate level ofenergy applied to the weld.

The question is:Can this control method be used for leadwelding as well?

To answer the question 3000 curveshave been recorded.

Fig.33: Expected Resistance Curve

The typical resistance curve is acombination of contact resistance andmaterial resistance vs. welding time. Atthe very beginning in the weldingprocess contact resistance will be veryhigh generating heat up to the meltingpoint of the lead material. Contactresistance will decrease, as materialresistance increases due to risingtemperatures(Fig.33).

The results of all the records are shownin Fig.34.

Fig.34: Intercell Weld Resistance Curves (yellow curve)

The resistance curves for lead arealmost constant throughout the process.There is no significant event during theprocess that could surve as a direct orindirect indicator of a good or bad weld.

Table 1 shows the deviations in voltage,current and resistance as measuredduring welding of 3.000 samples.

Processtime

0.01s 0,02s 0.09s 0.17s

Voltagedeviation

+/- 12% +/- 10% +/- 5% +/- 5%

Currentdeviation

+/- 5.9% +/- 5% +/- 5% +/- 5%

Resistancedeviation

+/- 9.3% +/- 8.2% +/- 5% +/- 5%

Table 1: Deviation of voltage, current and resistance

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The flat resistance curve can beexplained by two variables which tend tocompensate for each other: Althoughthe resistance of lead increases withhigher temperatures, resistance alsodecreases as the distance between theelectrodes is reduced.

3. Results

3.1 Integrated Resistance Measurement

To summarize the investigation of theintegrated resistance measurement onecan say the advantages are:

- High pressure contact allows for hightest currents which generate highervoltages across the clamps improvingsignal to noise ratio measurement.

- No additional equipment investment isrequired

The disadvantages are:

- Resistance measurement takes placewhen the test object is subjected topressure from the welding clamp. Sincelead is a soft metal the pressure appliedis capable of creating a low resistanceelctroconductive junction even withoutwelding

- No real 4-wire measurement. Parasiticresistances will form the major part ofthe total resistance. Irregularities in theintercell weld will not represent asignificant portion of the total resistanceand will typically be within the toleranceband

The integrated resistance measurementsystem cannot reliably determine thequality of strap terminal welds. Inaddition this method will not differentiatebetween welded and non welded strapterminals. It should only be used tocapture possible trend data such as theresistance increase due tocontamination

3.2 Separate Resistance Measurement

To measure the strap terminalresistance more accurately a separatesystem is required. This system ensuresreal 4-wire measurement (Fig.35).

Fig.35: Separate Resistance Measurement

Contact is made by two current probesand two voltage sense probes. Thecurrent probes are spring-loaded toensure safe contact. However, unlikethe integrated resistance measurementsystem, this method does not clamp theterminals together. In contrast theprobes apply force in the oppositdirection attempting to separate a poorweld.

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The voltage probes are also spring-loaded and have tapered contacts whichpenetrate the oxyd film layer toaccurately measure voltages.

Five of these contact couples areconnected in series assembled in onecommon testhead.

The machine interface should allow fastchange over of testheads to adapt toany battery type.

The maximum current is limited by thedesign of the spring-loaded currentprobes.

Fig.36: Testhead of Separate Resistance Measurement

Tests were conducted to determine theoptimum current amplitude required togenerate significant variation inmeasured resistance which thencorrelates to percentage completenessof welds.

Welded terminals were modified bydrilling holes of increasing diameterthrough the weld nuggets.

It is clear that test currents well inexcess of 100A are required formeaningful results (figures 37 to 39).

Fig.37: Effective Welded Area 104 mm2

Fig.38: Effective Welded Area 145 mm2

Fig.39: Effective Welded Area 245 mm2

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Unimpeachable data regarding thequality of intercell welds cannot beachieved using the integrated electricalresistance measurement

More accurate results can be obtainedwith an improved measurement system,which is separate and installed after theintercell welding machine

In general the addition of a separateWelding Resistance Tester would be anenhancement to any battery assemblingline.

Digatron / Firing Circuits has literatureavailable describing the WeldingResistance Tester Product. Pleasecontact [email protected] [email protected]

References:

[1] Oliver Roehl, Bielomatik Leuze GmbH + Co; Intercell Welding Machines August 2004

[2] Burghard Danch, Study of the Intercell Welding process, October 1993

[3] Matuschek Messtechnik GmbH, Adaptive Control for Resistance Welding in the Automotive Industry, August 2001