Electrical Modelling and Investigation of Laser Beam Welded Joints for Lithium-Ion Batteries

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Article Electrical Modelling and Investigation of Laser Beam Welded Joints for Lithium-Ion Batteries

Sören Hollatz 1,* , Sebastian Kremer 1,2, Cem Ünlübayir 2, Dirk Uwe Sauer 2,3,4,5 , Alexander Olowinsky 1 and Arnold Gillner 1,6 1 Fraunhofer Institute for Laser Technology ILT, Steinbachstr. 15, 52074 Aachen, Germany; [email protected] (S.K.); [email protected] (A.O.); [email protected] (A.G.) 2 Chair for Electrochemical Energy Conversion and Storage Systems, Institute for Power Electronics and Electrical Drives (ISEA), RWTH Aachen University, Jägerstrasse 17-19, 52066 Aachen, Germany; [email protected] (C.Ü.); [email protected] (D.U.S.) 3 Institute for Power Generation and Storage Systems (PGS), E.ON ERC, RWTH Aachen University, Mathieustrasse 10, 52074 Aachen, Germany 4 Jülich Aachen Research Alliance, JARA-Energy, Templergraben 55, 52056 Aachen, Germany 5 Helmholtz Institute Münster (HI MS), IEK 12, Forschungszentrum Jülich, 52425 Jülich, Germany 6 Chair for Laser Technology, RWTH Aachen University, Steinbachstr. 15, 52074 Aachen, Germany * Correspondence: [email protected]; Tel.: +49-241-8906-613

Received: 13 March 2020; Accepted: 10 April 2020; Published: 21 April 2020

Abstract: The growing electrification of vehicles and tools increases the demand for low resistance contacts. Today’s batteries for electric vehicles consist of large quantities of single battery cells to reach the desired nominal voltage and energy. Each single cell needs a contacting of its cell terminals, which raises the necessity of an automated contacting process with low joint resistances to reduce the energy loss in the cell transitions. A capable joining process suitable for highly electrically conductive materials like copper or aluminium is the laser beam welding. This study contains the theoretical examination of the joint resistance and a simulation of the current flow dependent on the contacting welds’ position in an overlap configuration. The results are verified by examinations of laser-welded joints in a test bench environment. The investigations are analysing the influence of the shape and position of the weld seams as well as the influence of the laser welding parameters. The investigation identifies a tendency for current to flow predominantly through a contact’s edges. The use of a double weld seam with the largest possible distance greatly increases the joint’s conductivity, by leveraging this tendency and implementing a parallel connection. A simplistic increase of welded contact area does not only have a significantly smaller effect on the overall conductivity, but can eventually also reduce it.

Keywords: resistance measurement; contact quality; laser beam welding; aluminium; copper; lithium-ion batteries; battery systems; spatial power modulation; single mode ﬁbre laser

1. Introduction Over the past years, the demand for large battery packs for electric vehicles (EV) has steadily increased with the ongoing electriﬁcation of the transportation sector and a growing demand for greater ranges. State of the art EV battery packs consist of a large quantity of cells connected in series to achieve the desired voltage level and in parallel in order to enable higher charge- and discharge-currents. For example, the EV Tesla Model S comprises of total count of over 7000 type 18,650 battery cells inside its battery pack [1]. A single defective connection can lead to failure or a reduction in performance. The quality of the joint has a decisive inﬂuence on the already discussed sustainability and safety of

Batteries 2020, 6, 24; doi:10.3390/batteries6020024 www.mdpi.com/journal/batteries Batteries 2020,, 66,, 24x FOR PEER REVIEW 22 of of 12

performance. The quality of the joint has a decisive influence on the already discussed sustainability electric vehicles [2] Increased resistance at a welded joint causes more heat loss at this spot and leads to and safety of electric vehicles [2] Increased resistance at a welded joint causes more heat loss at this an increased electrical and thermal load on the individual cells, which in turn can lead to failure or spot and leads to an increased electrical and thermal load on the individual cells, which in turn can accelerated aging. Laser beam welding is a promising technology to contact battery cells enabling lead to failure or accelerated aging. Laser beam welding is a promising technology to contact battery automated, fast and precise production of conductive joints. In comparison to other conventional cells enabling automated, fast and precise production of conductive joints. In comparison to other welding techniques, such as resistance spot welding, the laser beam welding has a reduced thermal conventional welding techniques, such as resistance spot welding, the laser beam welding has a energy input [3]. Compared to ultrasonic welding, the laser beam welding technique does not induce reduced thermal energy input [3]. Compared to ultrasonic welding, the laser beam welding technique a mechanical force [4]. The resulting transition resistances are in the range of the basic material does not induce a mechanical force [4]. The resulting transition resistances are in the range of the resistances. The overall performance of the battery pack is therefore improved by the reduction of the basic material resistances. The overall performance of the battery pack is therefore improved by the ohmic resistance of the joints and heat loss inside the battery cell. reduction of the ohmic resistance of the joints and heat loss inside the battery cell. Furthermore, laser beam welding produces a small heat-affected zone. In the context of production, Furthermore, laser beam welding produces a small heat-affected zone. In the context of laser beam welding is well suited to be integrated into almost fully automated production lines in the production, laser beam welding is well suited to be integrated into almost fully automated manufacturing process of battery packs and EVs. The joining of aluminium and copper is particularly production lines in the manufacturing process of battery packs and EVs. The joining of aluminium challenging in laser welding as the metal pair forms intermetallic phases, which can yield lower weld and copper is particularly challenging in laser welding as the metal pair forms intermetallic phases, qualities [5–7]. These phases can be identified in cross sections, see Figure1. For the investigation, which can yield lower weld qualities [5–7]. These phases can be identified in cross sections, see Figure the different colours in the mixing zone (dark grey and yellow areas) give a first indication on the 1. For the investigation, the different colours in the mixing zone (dark grey and yellow areas) give a concentration of the metals. For further investigation an energy dispersive X-ray spectroscopy can be first indication on the concentration of the metals. For further investigation an energy dispersive X- performed, but will not part of this study. ray spectroscopy can be performed, but will not part of this study.

Figure 1. CrossCross section section of of a alaser laser welded welded aluminium aluminium and and copper copper joint joint (P (=P 294= 294 W, W,v = v120= mm/s,120 mm A/ s,= A0.15= 0.15mm, mm, f = 1000f = 1000Hz). Hz).

This paperpaper showcasesshowcases an an evaluation evaluation of of various various laser laser welded welded joints joints for for the the connection connection of pouch of pouch cell terminalscell terminals to the to the battery battery pack pack in anin an overlap overlap configuration. configuration. The The specimen specimen design design is is relatedrelated toto pouch cells. Due Due to to the the focus on on the connection quality, quality, no functional cells are used for this investigation. First, an ohmic-resistance model for the joints is introduced. Wi Withth the help of this model the current flowflow across the overlap transition is analysed. Last Lastlyly different different geometries of welds were chosen and compared in terms ofof theirtheir conductivity.conductivity. 2. State of the Art 2. State of the Art 2.1. Measurement of Electrical Resistance of Laser-Welded Joints 2.1. Measurement of Electrical Resistance of Laser-Welded Joints A current passing through a conductor encounters an electrical resistance, analogous to an A current passing through a conductor encounters an electrical resistance, analogous to an opposing force by mechanical friction. This resistance is defined in Ohm’s law as the proportion opposing force by mechanical friction. This resistance is defined in Ohm’s law as the proportion of voltage across and current through the same conductor. It is dependent on the specific electrical resistance of the conductor’s material and its dimensions according to [8]. Batteries 2020, 6, 24 3 of 12

Batteries 2020, 6, x FOR PEER REVIEW 3 of 12 of voltage across and current through the same conductor. It is dependent on the specific electrical resistance of the conductor’s material and its dimensions according to [8]. 𝑙 𝑅=𝜌⋅ (1) l 𝐴 R = ρ (1) 𝑅: resistance (Ω); 𝜌: specific resistivity (Ω·m2/mm);· A𝑙: conductor's length (mm) and A: conductor's 2 Rcross-sectional: resistance (Ω area); ρ: (m specific). resistivity (Ω m2/mm); l: conductor’s length (mm) and A: conductor’s · cross-sectionalThe specific area electrical (m2). resistance is furthermore dependent on the material’s temperature and exactThe chemical specific composition. electrical resistance Hence impurities is furthermore can dependenthave an effect on theon material’sthe resistivity. temperature and exact chemicalThe measurement composition. Henceof an electrical impurities resistance can have ca ann be eff ectexecuted on the by resistivity. a combined measurement of the voltageThe and measurement current, as ofsuggested an electrical by Ohm’s resistance law. canA popular be executed method by afor combined this combined measurement measurement of the voltageis the so and called current, four-terminal as suggested sensing. by Ohm’s The law. four-terminal A popular methodsensing for describes this combined the introduction measurement of isa thedefined so called current four-terminal through the sensing. conductor The four-terminal and a separa sensingted voltage describes measurement. the introduction By separating of a defined the currentsensing throughwires the the measured conductor voltage and a separateddoes not falsely voltage include measurement. the voltage By separating across the thecurrent sensing carrying wires thewires. measured As a voltage voltage measurement does not falsely usually include has a the high voltage impedance across the the current current through carrying the wires. voltmeter As a voltagecan be neglected measurement for significantly usually has alower high impedancemeasured resistances. the current through the voltmeter can be neglected for significantlyIn view of laser-welded lower measured joints resistances. of battery contacts, the analysis of the electrical resistance might presentIn viewa suitable of laser-welded indicator for joints the weld of battery quality. contacts, This postulation the analysis is ofbased the electricalon the effect resistance of impurities might presentin the material a suitable and indicator the dimension for the weld of the quality. actual This contact postulation area on is the based joints on theconductivity. effect of impurities However, in thesolely material the quantity and the of dimension the resistance of the might actual result contact in incorrect area on theresults joints when conductivity. comparing However, different solelylaser- thewelded quantity joints. of For the instance, resistance a mightjoint of result worse in quality incorrect can results show when a significantly comparing higher different conductivity laser-welded due joints.to a larger For instance, cross-sectional a joint of area worse of quality the conducto can showr. aTo significantly allow valid higher comparisons conductivity between due to joints a larger of cross-sectionaldifferent dimensions, area of thethe conductor.electrical resistance To allow valid must comparisons be further processed between jointsto yield of dithefferent so-called dimensions, contact thequality electrical index. resistance must be further processed to yield the so-called contact quality index. TheThe contactcontact qualityquality indexindex (CQI, or resistance equivalenceequivalence factor)factor) describesdescribes the proportion ofof thethe joint’sjoint’s resistanceresistance inin respectrespect toto itsits basebase materialsmaterials andand dimensionsdimensions [[9,10].9,10]. To calculate the CQI of a laplap jointjoint anan additionaladditional measurementmeasurement ofof thethe material’smaterial’s resistancesresistances isis necessary,necessary, besidesbesides determinationdetermination ofof thethe actualactual jointjoint resistance.resistance. Assuming a constantconstant cross-sectionalcross-sectional area of thethe conductorsconductors andand constantconstant measurementmeasurement distances,distances, thesethese cancan bebe acquiredacquired asas perper [[9,11]9,11] shownshown inin FigureFigure2 .2.

Figure 2.2. Schematic illustration for the calculation of the contact quality indexindex (CQI).(CQI). The base resistance for the joint is derived by assuming a seamless, resistance-less joint from one The base resistance for the joint is derived by assuming a seamless, resistance-less joint from one material into the other over its length. The expected resistance for this geometrically optimal case material into the other over its length. The expected resistance for this geometrically optimal case would equal half the sum of the measured material’s resistances, namely the average. The CQI can would equal half the sum of the measured material’s resistances, namely the average. The CQI can now be calculated by dividing the actual joint’s resistance by the base joint’s resistance. The equation now be calculated by dividing the actual joint’s resistance by the base joint’s resistance. The equation is shown in (4). is shown in (4). U sJ = Ci = R0Ci and RCi R0Ci for i 1, 2 (2) Imeas · s ∈ { } 𝑅 = and 𝑅 =𝑅 ⋅ for 𝑖∈1,2 (2) UJ s sJ R0J = and RJ = R0J (R0C1 + R0C2) − (3) Imeas𝑈 − · 2𝑠𝑠s 𝑅 = and 𝑅 =𝑅 𝑅 +𝑅 ⋅ · (3) 𝐼 2⋅𝑠 Batteries 2020, 6, 24 4 of 12

2 RJ CQI = · (4) RC1 + RC2

R0Ci: measured conductor resistance (Ω); RCi: conductor resistance for length sJ (Ω); R0J: measured joint resistance (Ω); RJ: corrected joint resistance (Ω); UCi: voltage across conductor i (V); UJ: voltage across joint (V); IMeas: measuring current (A); s: measuring distance (mm); sj: joint distance (mm); CQI: contact quality index (1). A CQI value of 1 can be interpreted as a joint of similar conductivity as the base materials; a value less than 1 indicates a higher, a value higher than 1 a lower conductivity. With the CQI joints of diﬀerent materials and dimensions can now be compared for its joining method and eﬀect on electrical resistivity.

2.2. Laser Beam Welding with Spatial Power Modulation For the joining of materials with high thermal conductivity, laser beam welding is a suitable process. Using small focus diameters of a few 10 µm, the resulting high intensities are able to melt and vaporize the material to achieve deep and narrow weld seams. The process is defined with two process stages, the heat conduction welding and the deep penetration welding. For heat conduction welding the material is molten due to the absorption of the laser beam’s energy on the surface. This process significantly depends on the absorptivity of the material. Significantly higher welding depths can be achieved by exceeding a characteristic intensity threshold using a deep penetration welding process. Therefore the material is vaporized and a keyhole is formed inside the molten pool. Multiple interactions of the laser beam inside this keyhole lead to an increase of the energy input resulting in a higher weld seam depth [12]. To manipulate the shape of the weld seam cross section and to stabilize the process during a deep penetration weld, a spatial power modulation can be used. Therefore the linear feed is superposed with a circular oscillation movement. The path is then characterized by an oscillation amplitude A, frequency f and feed rate v f . In [13] a change from a v-shaped to a rectangular shaped weld seam cross section has been seen. Furthermore, an influence on the hardness, on the mixing of the materials in overlap configurations and on the roughness of the weld seam surface have been identified [13,14].

2.3. Laser Beam Welding of Electrical Contacts For laser welding of electrical contacts, the contact resistance is the most important index, particularly indicated by the previously defined CQI. Therefore [15] has investigated similar material joints and reached a CQI of 0.55 for a copper connection and 0.57 for aluminium. In this case the joining partners have been connected by two parallel weld seams in an overlap configuration. By using two lines with the highest distance possible, the material in the overlap area is connected in parallel and shows a reduced transition resistance due to the higher current-carrying cross-section. The investigation of [16] focuses on welding of dissimilar materials using a pulsed Nd:YAG laser. Weld seam depth and temperature gradient in the melt pool are controlled by a temporal power modulation. The investigations show a reduced mixing of the materials, higher process stability and higher seam quality. Depending on the application, aluminium or copper is usually used to conduct electricity. Due to the reduced density, aluminium is used for lightweight applications, while copper with its higher conductivity is used when space is limited. Joining aluminium to copper, leads to numerous challenges. The differences in melting point, thermal conductivity and expansion cause tensions during the solidification, which can lead to cracks in the weld seam. Furthermore, the materials are soluble in the liquid state and form intermetallic phases inside the mixing zone of the weld seam. Besides the increase of hardness and crack sensitivity, the intermetallic phases increase the electrical resistance [5–7,17].

3. Electrical Equivalent Model for Joint The investigation focused on laser-welded lithium ion pouch cells. The relatively broad contacts of these cells consist of aluminium and copper and oﬀer a large contact area. Hence, in the joining Batteries 2020, 6, x FOR PEER REVIEW 5 of 12

3. Electrical Equivalent Model for Joint Batteries 2020, 6, 24 5 of 12 The investigation focused on laser-welded lithium ion pouch cells. The relatively broad contacts of these cells consist of aluminium and copper and offer a large contact area. Hence, in the joining of lithiumof lithium ion ion pouch pouch cells cells the the overlap overlap joint joint represents represents a asuitable suitable joining joining method. method. The The joint joint in in this investigation was aligned along the current direction, producing a straight junction with an expected homogenous current density in the conductors’conductors’ cross-sectional areasareas (Figure(Figure2 2).). The given lap joint can be modelled by using electrical equivalent circuit diagram from [[4].4]. Each joining partner,partner, M M and and m, m, was was subdivided subdivided into equallyinto equally sized stretchsized stretch elements elements resistances. resistances. The partners The werepartners joined were by bridgejoined elementsby bridge (indexed elements with (indexed J) representing with J) the representing current-carrying the interconnections.current-carrying interconnections.The resulting equivalent The resulting circuit equivalent is shown incircuit Figure is3 shown. The dimensionsin Figure 3. of The each dimensions resistance of in each the resistanceequivalent circuitin the couldequivalent be mapped circuit according could be to regardedmapped materials,according joining to regarded methods materials, and joint areasjoining to methodsrepresent and the equivalent,joint areas to real represent lap joint. the All equivalent, simulation real results lap werejoint. basedAll simulation on basic electricalresults were equations based onimplemented basic electrical in Python. equations implemented in Python.

Figure 3. Model of the lap joint using an electrical equivalent circuit diagram. Figure 3. Model of the lap joint using an electrical equivalent circuit diagram. The simulation of an electrical current through the equivalent diagram of a joint with homogenous The simulation of an electrical current through the equivalent diagram of a joint with resistance revealed a significant phenomenon. The current had an inherent tendency to flow along the homogenous resistance revealed a significant phenomenon. The current had an inherent tendency to edges of the joint, represented by the first and last bridge elements of index 0 and n. The tendency flow along the edges of the joint, represented by the first and last bridge elements of index 0 and n. increased with decreasing bridge resistance in comparison to the joining partners’ resistances (indexed The tendency increased with decreasing bridge resistance in comparison to the joining partners’ with C). This phenomenon was explained by comparing the given equivalent circuit to a cascaded resistances (indexed with C). This phenomenon was explained by comparing the given equivalent bridge circuit. With decreasing bridge resistance, the model approximated a cascaded bridge circuit, circuit to a cascaded bridge circuit. With decreasing bridge resistance, the model approximated a which by definition did not carry any current through the bridges when balanced (of equal resistance cascaded bridge circuit, which by definition did not carry any current through the bridges when ratio). In fact, it is theoretically impossible for the current to be equal across all bridges in the equivalent balanced (of equal resistance ratio). In fact, it is theoretically impossible for the current to be equal circuit with n > 1 of a given joint, if no variance in the resistances is introduced. This can be proven across all bridges in the equivalent circuit with n > 1 of a given joint, if no variance in the resistances mathematically by assuming equal bridge currents and overall equal resistances in the model (n > 1) is introduced. This can be proven mathematically by assuming equal bridge currents and overall and yielding a contradiction by calculating the overall joint resistance via circuit diagram simplification equal resistances in the model (n > 1) and yielding a contradiction by calculating the overall joint and the mesh current method. resistance via circuit diagram simplification and the mesh current method. For the simulation of laser-welded seams on the joint, the electrical values for the model were For the simulation of laser-welded seams on the joint, the electrical values for the model were determined experimentally. Considering the limitations of the two-dimensional model, the bridge determined experimentally. Considering the limitations of the two-dimensional model, the bridge elements modelled a contact line across the whole conductor’s width. The model was set to be of order elements modelled a contact line across the whole conductor’s width. The model was set to be of n = 110, hence having 111 distinct bridge resistances (indices 0 through 110). The bridge elements order n = 110, hence having 111 distinct bridge resistances (indices 0 through 110). The bridge RJ,i, representing either a weld or a purely frictional contact across the joint, could be determined elements RJ,i, representing either a weld or a purely frictional contact across the joint, could be by combining the measurements of a representative laser-welded joint, a purely frictional contact determined by combining the measurements of a representative laser-welded joint, a purely frictional joint and a joint consisting solely of a laser welded seam. The produced bridge resistances show an contact joint and a joint consisting solely of a laser welded seam. The produced bridge resistances improvement in conductivity for laser-welded bridges as compared to frictional contacts by a factor show an improvement in conductivity for laser-welded bridges as compared to frictional contacts by of 9800, 8750 and 178,000 for aluminium–copper, copper–copper and aluminium–aluminium joints a factor of 9800, 8750 and 178,000 for aluminium–copper, copper–copper and aluminium–aluminium respectively. The significantly higher factor in the pure aluminium joint can be explained by high joints respectively. The significantly higher factor in the pure aluminium joint can be explained by resistances of its frictional contact due to surface oxidation [18]. A simulation of current through a joint high resistances of its frictional contact due to surface oxidation [18]. A simulation of current through with simplified resistances yields the proportional current through each bridge element RJ,i presented a joint with simplified resistances yields the proportional current through each bridge element RJ,i in Figure4 (model of order n = 110 model with RM,i = Rm,i = RM/n). presented in Figure 4 (model of order n = 110 model with RM,i = Rm,i = RM/n). Batteries 2020, 6, 24 6 of 12

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FigureFigureFigure 4. 4.Simulation4. SimulationSimulation of of currentof currentcurrent through throughthrough a simpliﬁed aa simplifiedsimplified model modelmodel with withwith a central aa centralcentral weld weldweld seam. seam.seam.

TheTheThe figure figurefigure clearly clearlyclearly shows showsshows the thethe higher higherhigher conductivity conductivityconductivity of of theof thethe laser-weld, laser-weld,laser-weld, spanning spanningspanning over overover the thethe central centralcentral ten tenten bridgebridgebridge elements. elements.elements. Additionally, Additionally,Additionally, the thethe previously previouslypreviously explained explexplainedained tendency tendencytendency of of theof thethe current currentcurrent flowing flowingflowing through throughthrough the thethe joint’sjoint’sjoint’s edges edgesedges could couldcould be bebe identified identifiedidentified in inin the thethe rising risingrising current currentcurrent rates ratesrates towards towardstowards the thethe bridge bridgebridge indices indicesindices of ofof 0 and00 andand 110. 110.110. ModifyingModifyingModifying the thethe identical identicalidentical model modelmodel to toto have havehave the thethe laser-welded laser-welaser-weldedlded bridges bridgesbridges to toto be bebe divided divideddivided to toto the thethe joint’s joint’sjoint’s edges, edges,edges, yieldsyieldsyields simulated simulatedsimulated current currentcurrent rates ratesrates as as shownas shownshown in inFigurein FigureFigure5. 5.5.

FigureFigureFigure 5. 5.Simulation5. SimulationSimulation of of currentof currentcurrent through throughthrough a simpliﬁed aa simplifiedsimplified model momodeldel with withwith weld weldweld seams seamsseams on onon the thethe joint’s joint’sjoint’s edges. edges.edges.

TheTheThe first firstfirst five fivefive and andand last lastlast five fivefive slots slotsslots were werewere now nownow assigned assiassignedgned to to theto thethe laser-welded laser-weldedlaser-welded resistances resistancesresistances and andand hence hencehence representedrepresentedrepresented a joint aa jointjoint of of overallof overalloverall similar similarsimilar cross-sectional cross-sectionalcross-sectional surface surfacesurface area areaarea to to theto thethe central centralcentral weld weldweld example. example.example. Due DueDue to toto thethethe tendency tendencytendency of of edgeof edgeedge currents currentscurrents the thethe overall overalloverall carried carriedcarried current currentcurrent through throughthrough the thethe weld weldweld was waswas significantly significantlysignificantly higher higherhigher thanthanthan in theinin thethe example exampleexample of the ofof centralthethe centralcentral weld; weld;weld; the current thethe currentcurrent through throughthrough the frictional thethe frictionalfrictional contact contact wascontact negligibly waswas negligiblynegligibly small. Assmall.small. a result, AsAs the aa implementationresult,result, thethe implemimplem of aentationentation “double of weld”of aa “double“double not only weld”weld” effectively notnot doubledonlyonly effectivelyeffectively the conductivity doubleddoubled of thethe theconductivityconductivity joint (by connecting ofof thethe jointjoint the (by(by materials connectingconnecting in parallel, thethe materialsmaterials increasing inin parallel,parallel, the conductor’s increasingincreasing cross-sectional thethe conductor’sconductor’s area cross-cross- in sectionalsectional areaarea inin directiondirection ofof mainmain currentcurrent flow)flow) butbut alsoalso complementedcomplemented thethe currentcurrent toto predominantlypredominantly flowflow throughthrough thethe laser-weldedlaser-welded seamsseams onon thethe jointsjoints edges.edges. Batteries 2020, 6, x FOR PEER REVIEW 7 of 12

A variance of the resistances for the modelled materials introduced a shift in the bridges’ current densities, whereas higher densities were found at the side of the lower conductive material (and accordingly the end of the material with higher conductivity). The distribution roughly resembled the rate of currents found in parallel resistances of different magnitude. Overall the simulations yield an understanding of current distribution in the overlap joint and its variance introduced by weld

Batteriesplacement2020, or6, 24different materials. 7 of 12

4. Metrological Investigation of Resistances of Laser Welded Joints direction of main current flow) but also complemented the current to predominantly flow through the The experimental part of the investigation focused on the comparison of different welding laser-welded seams on the joints edges. characteristics and geometries on overlap joints. The equipment consisted of a laser welding machine, A variance of the resistances for the modelled materials introduced a shift in the bridges’ current a micro-ohmmeter and a custom test bench. The laser machine was an IPG YLR 1000 SM, single-mode densities, whereas higher densities were found at the side of the lower conductive material (and fibre laser with a maximum emission power rating of 1 kW. The ohmmeter was a LoRe precision accordingly the end of the material with higher conductivity). The distribution roughly resembled micro-ohmmeter from Werner Industrielle Elektronik and had a resolution of 1 nΩ in low the rate of currents found in parallel resistances of different magnitude. Overall the simulations yield measurement ranges starting at 10 nΩ. The custom test bench was designed to measure overlap joints an understanding of current distribution in the overlap joint and its variance introduced by weld in a manner to obtain both the joints resistance and CQI in respect to the materials, while retaining a placement or different materials. maximum standard deviation of ±45 µm in the measurement tips’ placement. 4. MetrologicalThe investigation Investigation included of the Resistances survey of of different Laser Welded welding Joints parameters and of different weld geometries altogether. The specimens were fabricated from aluminium Al99.5 and copper Cu-ETP metalThe strips experimental of dimensio partns 20 ofmm the width, investigation 85 mm length focused and on0.3 themm comparison strength. The of specimen different geometry welding characteristicsand material andwere geometries based onon overlap a connection joints. The of equipment pouch consistedcell batteries. of a laser As welding per Figure machine, 6 a micro-ohmmeter and a custom test bench. The laser machine was an IPG YLR 1000 SM, single-mode fibre laserProbe with 1 a maximumProbe emission 2 power ratingProbe of 1 kW.3 The ohmmeterProbe was 4 a LoRe precision micro-ohmmeter fromM1 Werner Industrielle ElektronikJ and had a resolutionM2 of 1 nΩ in low measurement ranges starting at 10 nΩ. The custom testsJ = bench10.5 mm was designed to measure overlap joints in a manner to obtain both the joints resistance and CQI in respect to the materials, while retaining a maximum standard deviation of 45 µm in the measurement tips’ placement. ± The investigation included the survey of different welding parameters and of different weld RM1; s = 11 mm RJ; s = 11 mm RM2; s = 11 mm geometries altogether. The specimens were fabricated from aluminium Al99.5 and copper Cu-ETP metal strips of dimensions 20 mm width, 85 mm length and 0.3 mm strength. The specimen geometry the joints and material were based on a connection of pouch cell batteries. As per Figure6 the joints overlap s J wasoverlap dimensioned sJ was dimensioned to be of 10.5 to mmbe of length. 10.5 mm length.

Probe 1 Probe 2 Probe 3 Probe 4 M1 J M2

sJ = 10.5 mm

RM1; s = 11 mm RJ; s = 11 mm RM2; s = 11 mm

Figure 6. Schematic set-up of measuring the specimen.

The measuring sectionsFigure span 6. over Schematics = 11 set-up mm; theof measuring deviation the between specimen. measurement and actual joint distance (s and s respectively) was eliminated mathematically post measurement. The produced joints The measuringJ sections span over s = 11 mm; the deviation between measurement and actual included Al–Al, Cu–Cu and Al on Cu joining; Cu on Al joints were averted due to high instabilities in joint distance (s and sJ respectively) was eliminated mathematically post measurement. The produced the welding process. The diﬀerences in material properties and the occurrence of intermetallic phases joints included Al–Al, Cu–Cu and Al on Cu joining; Cu on Al joints were averted due to high led to weld defects such as cracks. instabilities in the welding process. The differences in material properties and the occurrence of The investigation of diﬀerent welding parameters was conducted on the geometry of a central intermetallic phases led to weld defects such as cracks. weld. Variances were introduced in respect to weld length across the joint, weld width along the joint The investigation of different welding parameters was conducted on the geometry of a central and for the case of Al–Cu joints the induction of resistive intermetallic phases by altering the laser’s weld. Variances were introduced in respect to weld length across the joint, weld width along the joint power. The joint’s resistance progressively increased with a reduction of weld length (Figure7). and for the case of Al–Cu joints the induction of resistive intermetallic phases by altering the laser’s power. The joint’s resistance progressively increased with a reduction of weld length (Figure 7). Batteries 2020, 6, x FOR PEER REVIEW 8 of 12 BatteriesBatteries 20202020, 6, ,x6 FOR, 24 PEER REVIEW 8 of8 12 of 12

Figure 7. Resistance and CQI of the central weld joints with varying length. Figure 7. Resistance and CQI of the central weld joints with varying length. Figure 7. Resistance and CQI of the central weld joints with varying length. Although the variation of weld widths introduced instabilities in the welding process, an overall Although the variation of weld widths introduced instabilities in the welding process, an overall slightAlthough resistance the variationreduction of was weld also widths measured introduced with instabilities the increase in theof weld welding width. process, The anintentional overall slight resistance reduction was also measured with the increase of weld width. The intentional slightintroduction resistance of intermetallicreduction was phases also wasmeasured to test withthe measurability the increase ofof such.weld The width. measured The intentional resistances introduction of intermetallic phases was to test the measurability of such. The measured resistances introductiondid indeed showof intermetallic a dependency phases to thewas introduced to test the lasermeasurability power and of hintsuch. a Thelocal measured minimum resistances for optimal did indeed show a dependency to the introduced laser power and hint a local minimum for optimal didparametrization indeed show a (Figure dependency 8). to the introduced laser power and hint a local minimum for optimal parametrization (Figure8). parametrization (Figure 8).

FigureFigure 8. 8. ResistanceResistance and and CQICQI ofof central central weld weld joints joints with with varying varying laser laser powers powers and and hence hence intermetallic intermetallic phases (v = 100 mm/s, A = 0.2 mm, f = 1000 Hz). Figurephases 8. (Resistancevf =f 100 mm/s, and ACQI = 0.2 of mm,central f = weld1000 jointsHz). with varying laser powers and hence intermetallic

phases (vf = 100 mm/s, A = 0.2 mm, f = 1000 Hz). The weld geometries and their measured CQIs are presented in Table1. They were categorized The weld geometries and their measured CQIs are presented in Table 1. They were categorized by central, long-side and double geometries; additionally, asymmetrical geometries for Al–Cu joints by central,The weld long-side geometries and anddouble their geometries; measured CQIsadditionally, are presented asymmetrical in Table geometries 1. They were for categorizedAl–Cu joints were examined. The values for each configuration indicate the CQI for each material combination bywere central, examined. long-side The and values double for each geometries; configuration additionally, indicate asymmetrical the CQI for each geometries material for combination Al–Cu joints Al– Al–Al (A), Cu–Cu (C) and Al–Cu (M). The long-sided and the sawtooth welds were only tested with wereAl (A), examined. Cu–Cu The(C) valuesand Al–Cu for each (M). configuration The long-sided indicate and the the sawtooth CQI for each welds material were onlycombination tested with Al– a a copper–copper connection as they were not expected to yield significantly different results across Alcopper–copper (A), Cu–Cu (C) connection and Al–Cu as (M). they The were long-sided not expected and theto yieldsawtooth significantly welds were different only tested results with across a material combinations. The double weld with pattern and the asymmetrical configurations were copper–coppermaterial combinations. connection The as doublethey were weld not with expected pattern to andyield the significantly asymmetrical different configurations results across were likewise only implemented as an aluminium–copper connection. materiallikewise combinations. only implemented The doubleas an aluminium–copper weld with pattern connection. and the asymmetrical configurations were likewise only implemented as an aluminium–copper connection. BatteriesBatteries 20202020,, 66,, xx FORFOR PEERPEER REVIEWREVIEW 99 ofof 1212 BatteriesBatteriesBatteries 20202020 2020,, ,66 6,, ,xx x FORFOR FOR PEERPEER PEER REVIEWREVIEW REVIEW 999 of ofof 12 1212 BatteriesBatteriesBatteriesTableTable 2020 20202020, ,, , 6 1.61.6,,, , xx xx CQIs FORCQIsFOR FORFOR PEER PEERPEER PEERofof different different REVIEWREVIEW REVIEWREVIEW weldweld geometriesgeometries forfor AlAl–Al–Al (A),(A), Cu–CuCu–Cu (C)(C) andand Al–CuAl–Cu (M).(M). LaserLaser999 of ofof 121212 BatteriesTableTableparameters:parameters:Table2020 , 1.1.61., 24CQIsCQIsCQIs amplitudesamplitudes ofofof differentdifferentdifferent AAAA = = weld weldAAweldCC == A A geometriesgeometriesMgeometriesM == 0.20.2 mmmm for unlessforunlessfor AlAlAl–Al –Al specified;–Alspecified; (A),(A),(A), Cu–CuCu–Cu Cu–Cufrequenciesfrequencies (C)(C)(C) and andffandAA == f fAl–CuCAl–CuCAl–Cu == ffMM == 1(M).1(M).(M). kHz;kHz; LaserLaserLaser feedfeed 9 of 12

parameters:Tableparameters:TableratesTableratesparameters: v v f,A1.f,A1.1. = =CQIs CQIsCQIs vv f,C amplitudesf,C amplitudesamplitudes == vofvofoff,Mf,M different =different=different 100100 AA A mm/smm/sAAA = = = Aweld AweldweldAC C unless Cunless = == A A A geometriesMgeometriesMgeometriesM = = specified= specified 0.2 0.20.2 mm mmmm unless forunlessandforunlessandfor AlAl powerAlpower –Alspecified;–Al specified;–Alspecified; (A), (A),(on(A),(on specimen) specimen) Cu–Cu Cu–CufrequenciesCu–Cu frequenciesfrequencies (C)(C)(C) PP AA fand f=andAand=fAA 243= 243== f fC fCAl–Cu Al–CuCAl–Cu =W, =W,= f fMfM MP P = =CC= 1 = 1=(M).1(M). (M). kHz; 498 kHz;498kHz; W,LaserW,Laser Laser feed feedfeed PPMM ratesparameters:ratesparameters:=parameters:rates= 294294 v v W.vf,AW.f,Af,A = = = v vvf,C f,C amplitudesf,C amplitudesamplitudes = == v vvf,Mf,Mf,M = == 100 100100 A Amm/sA mm/smm/sAAA = === A AA CunlessCC unlessC unless = === A AAMMM ==specified= specified= specified 0.2 0.20.20.2 mm mmmmmm andunless unless andunlessunlessand power powerpower specified; specified;specified;specified; (on (on(on specimen) specimen)specimen) frequencies frequenciesfrequenciesfrequencies PP PAAA =f f=fAf= A A243 243 = =243== f ffCfC C C W, W,= =W,== f ff Mf MPM P P C= =C== C = 1 =11=1 498 498 kHz; kHz;kHz;498kHz; W, W,W, feed feedfeedfeed PP PMMM Table 1. CQIs of different weld geometries for Al–Al (A), Cu–Cu (C) and Al–Cu (M). Laser parameters: =ratesrates=ratesrates= 294294 294 v W.vW. vf,AW.f,Af,Af,A = == = v vvf,Cf,Cf,Cf,C = === v vvf,Mf,Mf,Mf,M = === 100 100100100 mm/s mm/smm/smm/s unless unlessunlessunless specified specifiedspecifiedspecified and andandand power powerpowerpower (on (on(on(on specimen) specimen)specimen)specimen) P PPAAA = === 243 243243243 W, W,W,W, P PPCCCC = === 498 498498498 W, W,W,W, P PPMMM JoinJoinamplitudes AA = AC = AM = 0.2 mm unless specified; frequencies fA = fC = fM = 1 kHz; feed rates vf,A = === 294 294294 W. W.W. CentralCentral Long-Long-SideSide DoubleDouble AsymmetricalAsymmetrical JoinJoinJointt vf,C = vf,M = 100 mm/s unless specified and power (on specimen) PA = 243 W, PC = 498 W, PM = 294 W. CentralCentralCentral Long-Long-Long-SideSideSide DoubleDoubleDouble AsymmetricalAsymmetricalAsymmetrical JoinJoinJointt t CentralCentralCentral Long-SideLong-SideLong-Side DoubleDoubleDouble AsymmetricalAsymmetricalAsymmetrical Jointtttt Central Long-Side Double Asymmetrical

SingSinglele SingleSingle DoubleDouble ShiftedShifted doubledouble AA SingSing0.90.9Sing924924lelele SingleSingleSingle-- DoubleDoubleDouble00.5.5250250 ShiftedShiftedShifted - doubledoubledouble AACAC 1.0.9Single1.0.9SingleSing0.900100010924924924le 1 1 1.Single1.SingleSingle18311831--- 3 3 Double0.Double0Double0.00.5.552205220.52502502503 3 ShiftedShiftedShiftedShifted- -- doubledoubledouble CACAAC 1.1.0.99241.0.99240.9001000100010924 1 1 221 1.1.1.183118311831- -- 333 0.0.0.52500.0.5250 052205220.552202503 3 3 - -- MM 1.1.005100511 -- 3 00.5.53213213 0.50.5809809 C 1 3 3 - MMCCMC 1.1.00101.1.00101.0051005100100051 21 121 2 1.18311.18311.18311.1831--- 333 0.52200.52200.522000.00.5.55220.5321321321 333 0.50.50.5-809--809- 809 M 1.0051 2 - 0.5321 0.5809 MMM 1.00511.00511.0051 2 222 ---- 0.53210.53210.53210.5321 0.58090.58090.58090.5809

CloseClose dodoubleuble DoubleDouble Double Double withwith patternpattern ShrunkShrunk doubledouble AA CloseCloseClose0.90.9 do double do713do713ubleubleuble DoubleDoubleDouble-- Double Double DoubleDouble with withwith-- pattern patternpattern ShrunkShrunkShrunkShrunk - -double doubledouble AACAC CloseCloseClose1.0.91.0.90.97130.922712271 double713 double713do713uble 33 1Double1DoubleDouble.1778.1778- -- 11 33 DoubleDouble Double with withwith- -- - pattern patternpattern ShrunkShrunkShrunk-- - -double doubledouble 3 3 CACAAC 1.1.22711.0.97131.0.97130.9227122712271713 3 3 3 111.177811.1778.1778.1778--- 111333 ------44 MM 00.9.9824824 -- 0.50.5436436 0.55500.55504 M 0.98243 -3 0.5436 0.5550 MMCCMC 1.22711.227101.00.9.92271.9824824824 3 33 1.177811.177811.177811.1778--- 1 333 0.50.50.5-436--436- 436 0.55500.55500.5550 - - -- 444 MMM 0.98240.98240.9824 --- 0.54360.54360.5436 0.55500.55500.5550 4444

SawSawtoothSawtoothtooth ArrowArrow AA SawSawSawtooth-tooth-tooth- ArrowArrowArrow-- AACAC SawtoothSawtoothSaw0.99460.90.9--tooth946-946 ArrowArrowArrow-- - M CAMCAMAC 0.90.90.9-946--946- 946 0.90.94140.9---414- 414 1 2. 3. CC 11A = 0.1 mm, v0.99460.9f = 946120 mm/s. A2.2.= 0.15 mm and vf = 120 mm/s. Process3.3. produced unexpectedly high resistances. -- MMCM AA == 0.10.1 mm,mm,0.9946 vvf-- f -== 120120 mm/s.mm/s. AA == 0.150.15 mmmm andand vvf f == 120120 mm/s.mm/s. ProcessProcess producedproduced unexpectedlyunexpectedly0.90.90.9 -414414 414 highhigh Hypothesis: Higher thermal impact affected material deformations and hence elevated frictional contact resistances. M11 1 - 2.2.2. 3.3.3. 0.9414 MM 4resistances. resistances. A AValuesA = == 0.1 0.10.1 after mm, mm,mm, correctionHypothesis:Hypothesis: v vvff - -f=- = = 120 120120 mm/s. formm/s.mm/s. inhomogeneous HigherHigher AA A = = = thermal thermal0.15 0.150.15 mm mm currentmm impact impact and andand flow. v vvf f f=a =a= ffected Rawffected120 120120 mm/s. mm/s.measurementsmm/s. materialmaterial ProcessProcess Process deformations deformations show produced producedproduced higher resistance and andunexpectedly unexpectedlyunexpectedly hencehence for0.94140.94140.9414 single elevatedelevated high high high and 11 2.2. 3.3. resistances.11resistances.lowerfrictional frictionalresistances. A AA = === 0.1 0.10.10.1 resistance mm, mm,mm,mm, contactcontact Hypothesis: Hypothesis:Hypothesis: v vvff forf f = === 120 120120 double120 resistancesresistances mm/s. mm/s.mm/s.mm/s. Higher long-sideHigherHigher 2. 2. A .A.A = ==4thermal= 4thermal thermalwelds.0.15 0.150.15Values0.15Values mm mmmmmm impact impact impact after andafter andandand v vv f correctionf facorrection f a= ==affected=ffected ffected120 120120120 mm/s. mm/s.mm/s.mm/s. material materialmaterial forfor 3. 3. Process ProcessProcessiProcessinhomogeneousnhomogeneous deformations deformationsdeformations produced producedproducedproduced and and current andcurrentunexpectedly unexpectedlyunexpectedlyunexpectedly hence hencehence flow.flow. elevated elevatedelevated high highhighRawhighRaw frictionalresistances.resistances.frictionalresistances.measurementsresistances.frictionalmeasurements contactcontactcontact Hypothesis:Hypothesis:Hypothesis:Hypothesis: showshow resistancesresistancesresistances higherhigher HigherHigherHigherHigher resistanceresistance... 4 4 thermal4thermal thermalthermal ValuesValuesValues forfor impactimpact impact impactafter aftersinglesingleafter correction correction correctionaandaaandaffectedffectedffectedffected lowerlower materialmaterial materialmaterial for forresistanceresistancefor iinhomogeneousinhomogeneousnhomogeneous deformationsdeformationsdeformationsdeformations forfor doubledouble currentandandlong-sidecurrentandlong-sideandcurrent hencehencehencehence flow.flow.flow. welds. welds. elevatedelevatedelevatedelevated RawRawRaw measurementsfrictionalfrictionalmeasurementsfrictionalAsfrictionalmeasurements presented contactcontactcontactcontact show inshowshow theresistancesresistancesresistancesresistances higherhigher higher table, resistanceresistance aresistance...single 444 4 ValuesValuesValuesValues central forfor for singleafteraftersingle afteraftersingle weld correctioncorrection correctionandcorrectionand and could lowerlower lower reach resistanceforforresistance forforresistance iiinhomogeneousinhomogeneousnhomogeneous suffi forfor cientfor doubledouble double contact long-sidecurrentlong-side currentlong-sidecurrent quality flow. flow.flow. welds.welds. welds. with RawRawRaw respect to themeasurementsmeasurementsAsmeasurementsAs reference presentedpresented materials. show inshowinshow thethe higher higher higher table,table, All material resistance resistancearesistancea singlesingle combinations centralcentral for forfor single singlesingle weldweld and andand could could lower lowerlower reach achievereach resistance resistanceresistance sufficientsufficient values for forfor double doubledouble close contactcontact long-side tolong-sidelong-side a qualityquality CQI of welds. welds.welds. with1,with emulating respectrespect thetoto thethe conductanceAsAsAs reference presentedreferencepresented presented ofmaterials. materials.inin in the thethe the materials.table,table, table, AllAll aa a singlesingle singlematerialmaterial All centralcentral central long-sided combinatcombinat weldweld weld jointscouldcould ionscouldions could showcouldreachreach reach sufficient asufficient achieveachievesufficient higher values CQIvalues contactcontact contact compared closeclose qualityquality quality toto with towith awitha theCQICQI respectrespect respect single ofof 1,1, As presented in the table, a single central weld could reach sufficient contact quality with respect totocentralemulatingtoemulating thethetheAsAs referencereference referenceline.presentedpresented thethe This conductanceconductance materials.materials. materials. in mayin thethe result table,table, of AllofAllAll dueatheathe single singlematerialmaterialmaterial materials.materials. to inhomogeneous centralcentral combinatcombinatcombinat AllAll weldweld longlong ionscouldionscouldions-sided-sided current couldcould couldreachreach jointsjoints densities,achieve sufficientachievesufficientachieve showshow a avaluesvalues values highercontact nothighercontact utilizing close close CQIclose CQI qualityquality comparedcompared tototo the withawithaa fullCQICQICQI respectrespectextent to toofofof the the 1,1,1, to the reference materials. All material combinations could achieve values close to a CQI of 1, emulatingtotoemulatingtoofsinglesingleemulating the thethe availablecentralreferencereferencereferencecentral thethe the conductance conductance line.conductanceline. materialmaterials.materials.materials. ThisThis maymay as of ofofAllAllAll shown the the resulttheresult materialmaterialmaterial materials.materials. materials. induedue Figure tocombinatcombinattocombinat All All inhomogeneousAllinhomogeneous9 longlong below.longionsionsions-sided-sided-sided Thecouldcouldcould jointsjoints joints inhomogeneous currentcurrent achieveachieveachieve showshow show densities,densities, a a avalues valuesvalues higherhigher higher current closeCQIclose closeCQI notCQInot comparedutilizing compared utilizing comparedtototo flow aaa CQICQICQI could thethe to totoofofof thefullthe befull the1,1,1, emulating the conductance of the materials. All long-sided joints show a higher CQI compared to the singleemulatingsingleemulatingcorrectedextentextentsingle central centralofofcentral the bythe thethe recalculating available availableconductance line.conductanceline.line. ThisThisThis materialmaterial maymaymay the ofof inner resulttheresulttheresult asas materials.materials. shownshown two dueduedue measurement to to toinin Allinhomogeneous inhomogeneousAll FiguinhomogeneousFigu longlongrere 9-sided9-sided-sided point.below.below. jointsjointsjoints Those The currentThecurrentcurrent showshow showinhomogeneousinhomogeneous measurement densities,densities,densities, aaa higherhigherhigher CQInotCQInotCQI pointsnot currentcurrent utilizing utilizing comparedcomparedutilizingcompared were flowflow expected the thethe to totocouldcould full fullthethethefull single central line. This may result due to inhomogeneous current densities, not utilizing the full extentsingleextentsingletobebeextent becorrectedcorrected a ofcentral ofcentralffof ected the thethe available available byavailable byline. byline. recalculatingrecalculating the ThisThis current material materialmaterial maymay result flowresult the theas asas shown showninnershowninner as duedue presented twoto twotoin inin inhomogeneousFiguinhomogeneous FiguFigu measmeasre inrere urement9urement 9Figure9 below. below.below.8. Thepoint. Thepoint.currentThe current inhomogeneous inhomogeneousinhomogeneous recalculation ThoseThose densities,densities, measurementmeasurement not wasnot current currentcurrent utilizingutilizing done points points flow flow byflow thethe taking could could couldwere were fullfull extent of the available material as shown in Figure 9 below. The inhomogeneous current flow could beextentbeextenttheexpectedbeexpected correctedcorrectedcorrected measured ofof the the toto be beavailable byavailablebyby total affected affected recalculatingrecalculatingrecalculating resistance materialmaterial byby thethe thebetweenthe current thecurrentasas shown innershowninnerinner flowflow the twotwo twoinin outer Figuas Figuas measmeasmeas prespresrere measurementurement urement 9entedurement9ented below.below. inin point. point.FigureTheFigurepoint.The points inhomogeneousinhomogeneous ThoseThose 8.Those8. TheThe and measurementmeasurement recalculationmeasurementrecalculation subtracting currentcurrent thewas pointswaspoints points flowflow expected donedone could could werewerewere byby be corrected by recalculating the inner two measurement point. Those measurement points were expectedbeexpectedberesistancestakingtakingexpected correctedcorrected thethe to toto measuredofbemeasured be bebyby the affected affected affected recalculatingrecalculating metals totaltotal by byby on the the theresistanceresistance both thecurrent thecurrentcurrent sides.innerinner flow betweenflowbetweenflow two Thetwo as as as meas expectedmeas pres pres pres thetheuremententedurementented ented outerouter resistances in in in measurementmeasurement Figurepoint. Figurepoint.Figure Thosewere8.Those 8.8. The TheThe calculated points measurementpointsrecalculationmeasurement recalculationrecalculation andand by subtractingsubtracting averaging was waspointswaspoints done donedone werewere theby thebyby expected to be affected by the current flow as presented in Figure 8. The recalculation was done by takingexpectedtakingexpectedrespectiveexpectedtaking thethethe to to resistances resistances measurements measured measured measuredbebe affectedaffected totaloftotaloftotal byby the ofthe the the resistanceresistance theresistance metals metals currentcurrent remaining, on on betweenflowbetweenflowbetween bothboth una asas pres sidepresff side thetheectedthe enteds. enteds. outerouter outer TheThe specimens. in in measurementmeasurementexpectedmeasurementexpected FigureFigure The 8.8. resistances resistancesTheThe resulting pointspoints pointsrecalculationrecalculation substitute andandwereandwere subtractingsubtractingsubtracting calculated calculatedwaswas resistances donedone the thebythebyby taking the measured total resistance between the outer measurement points and subtracting the expectedtakingtakingexpectedweretakingaveragingaveragingexpected expected thethethe resistancesresistances resistancesmeasuredmeasuredthemeasuredthe torespectiverespective deliver oftotaloftotaltotalof thethethe more measurementsmeasurementsresistanceresistanceresistance metalsmetalsmetals resembling onon onbetweenbetweenbetween bothbothboth ofof and thesidetheside sideththth comparable ees.remaining,s.remaining, s.outer outer TheTheThe expectedmeasurementexpectedmeasurementexpected estimatesunaffectedunaffected resistancesresistancesresistances inpointspoints specimens.specimens. cases andwereandwerewere of inhomogeneoussubtractingsubtracting calculatedcalculatedTheThecalculated resultingresulting the thebybyby expected resistances of the metals on both sides. The expected resistances were calculated by averagingexpectedaveragingexpectedcurrentsubstitutesubstituteaveraging flow. resistances resistances resistancestheresistancesthethe respectiverespectiverespective of ofwerewere thethe measurementsmeasurements measurements expected expected metalsmetals on ontoto deliverdeliverbothboth ofofof thethesidetheside moremore remaining,remaining,s.remaining,s. The Theresemblingresembling expectedexpected unaffectedunaffectedunaffected andand resistancesresistances comparablecomparable specimens.specimens.specimens. werewere estimatesestimates ThecalculatedThecalculatedThe resultingresultingresulting inin casescases byby substituteaveragingsubstituteaveragingofaveragingofsubstitute inhomogeneousinhomogeneous resistances resistancestheresistancesthethe respectiverespectiverespective currentcurrent were werewere measurements measurements flow.measurementsexpected flow.expectedexpected to toto deliver deliverdeliver ofofof thethethe more moremore remaining,remaining,remaining, resembling resemblingresembling unaffectedunaffectedunaffected and andand comparable comparablecomparable specimens.specimens.specimens. estimates estimatesestimates TheTheThe resultingresulting resulting in inin cases casescases ofsubstituteofsubstitutesubstituteof inhomogeneous inhomogeneousinhomogeneous resistancesresistancesresistances current currentcurrent werewerewere flow. flow. expectedflow.expectedexpected tototo deliverdeliverdeliver moremoremore resemblingresemblingresembling andandand comparablecomparablecomparable estimatesestimatesestimates ininin casescasescases ofofof inhomogeneous inhomogeneousinhomogeneous current currentcurrent flow. flow.flow. Batteries 2020, 6, 24 10 of 12 Batteries 2020, 6, x FOR PEER REVIEW 10 of 12

line of equal potential current flow line weld seam measurement probe

FigureFigure 9. 9. SchematicSchematic of of proposed proposed inhomogeneous inhomogeneous current current densities affecting affecting voltage measurements. By using double welds positioned on the joint’s edges, the CQI was reduced to the near theoretical By using double welds positioned on the joint’s edges, the CQI was reduced to the near optimum value of 0.5. With this configuration, the available material was utilized as a parallel theoretical optimum value of 0.5. With this configuration, the available material was utilized as a connection, effectively doubling the conducting cross sectional area. As seen with the double weld parallel connection, effectively doubling the conducting cross sectional area. As seen with the double with pattern, a further increase of the connection area by an additional weld seam did not lead to a weld with pattern, a further increase of the connection area by an additional weld seam did not lead further reduction of the CQI for an aluminium copper connection. to a further reduction of the CQI for an aluminium copper connection. Due to the materials’ differing resistivity, the current was predominantly through the more Due to the materials’ differing resistivity, the current was predominantly through the more conductive material, in this case copper. To determine the influence of the different material properties conductive material, in this case copper. To determine the influence of the different material and the weld seam geometry, three asymmetrically configurations were investigated. By shifting the properties and the weld seam geometry, three asymmetrically configurations were investigated. By weld seam closer to the copper edge, an increase of the CQI was measured. A slight increase was also shifting the weld seam closer to the copper edge, an increase of the CQI was measured. A slight seen with a reduced weld seam length on the aluminium edge. By using an arrow geometry, the CQI increase was also seen with a reduced weld seam length on the aluminium edge. By using an arrow was increased significantly due to the use of just one weld seam instead of two, but was still lower geometry, the CQI was increased significantly due to the use of just one weld seam instead of two, compared to a single central weld seam. but was still lower compared to a single central weld seam. 5. Discussion 5. Discussion During the investigation the model could be verified to represent the single and double joints, During the investigation the model could be verified to represent the single and double joints, when initialized with representative data. However, it cannot represent more complex joints, where the when initialized with representative data. However, it cannot represent more complex joints, where conductor’s cross-sectional areas did not predominantly carry strictly perpendicular currents. For the the conductor’s cross-sectional areas did not predominantly carry strictly perpendicular currents. For simulation of such joints a more complex three-dimensional model is required. The phenomenon of the simulation of such joints a more complex three-dimensional model is required. The phenomenon current distributions within the joint, as seen in the model, provides implications for the manufacturing of current distributions within the joint, as seen in the model, provides implications for the of battery joints. manufacturing of battery joints. For applying a laser welding process in electrical applications, the results lead to following design For applying a laser welding process in electrical applications, the results lead to following guide lines. To achieve a CQI of 1, it is sufficient to apply a single weld seam along the whole width design guide lines. To achieve a CQI of 1, it is sufficient to apply a single weld seam along the whole of the joining partners. Requirement is a stable welding process for contacting the joining partners. width of the joining partners. Requirement is a stable welding process for contacting the joining The width of the weld seam, rectangular to the current flow direction did not have a significant partners. The width of the weld seam, rectangular to the current flow direction did not have a influence (compare central slim double line in Table1). The measured values indicate a slight CQI significant influence (compare central slim double line in Table 1). The measured values indicate a reduction for the similar aluminium and the aluminium copper connection. slight CQI reduction for the similar aluminium and the aluminium copper connection. By greatly increasing the distance between the two weld seams the CQI was reduced to nearly 0.5 By greatly increasing the distance between the two weld seams the CQI was reduced to nearly leading to the assumption that the position of the weld seams had a greater effect on the resistance than 0.5 leading to the assumption that the position of the weld seams had a greater effect on the resistance increasing the connection width and accordingly seam area with additional weld seams. That concludes than increasing the connection width and accordingly seam area with additional weld seams. That that weld seams should be positioned as wide apart as possible to use the parallel connection of the concludes that weld seams should be positioned as wide apart as possible to use the parallel two conductors. This measurement result is further supported by the simulation, which identified a connection of the two conductors. This measurement result is further supported by the simulation, predominant current flow through the joint’s edges. which identified a predominant current flow through the joint’s edges. The identified dependency of the laser power on the resistance, leads to the assumption that The identified dependency of the laser power on the resistance, leads to the assumption that an an increased weld depth is not improving the CQI. The measurements showed higher values with increased weld depth is not improving the CQI. The measurements showed higher values with increased laser power and therefore weld depths. The reason for this behaviour might be an increased increased laser power and therefore weld depths. The reason for this behaviour might be an increased mixing of the copper and aluminium, which leads to an increased occurrence of intermetallic phases. mixing of the copper and aluminium, which leads to an increased occurrence of intermetallic phases. These phases inhibit the current flow in the weld seam and lead to an increase of the measured These phases inhibit the current flow in the weld seam and lead to an increase of the measured resistances. Furthermore, besides affecting the weld seam’s resistance, it can supposedly reduce the resistances. Furthermore, besides affecting the weld seam’s resistance, it can supposedly reduce the materials’ conductivity. As seen with the joint “Double with pattern” from Table1, the addition of a materials’ conductivity. As seen with the joint “Double with pattern” from Table 1, the addition of a pattern to the normal double joint increases the CQI. The introduction of further intermetallic phases pattern to the normal double joint increases the CQI. The introduction of further intermetallic phases deals a greater effect on conductivity, than the increase of welded area, in the joint’s central area, less deals a greater effect on conductivity, than the increase of welded area, in the joint’s central area, less significant to the bridge-currents. significant to the bridge-currents. Batteries 2020, 6, 24 11 of 12

6. Conclusions To reduce the electric loss in the connection of battery cells for electric vehicles, the joining process and the resulting transition resistance are essential. By introducing a model representing the joints’ partial resistances, the current flow through the connection could be investigated. As a consequence, various joint geometries were investigated using a laser welding process to leverage and examine the observed edge current phenomenon. Naturally the current density along the material edges of the overlap, which are perpendicular to the current flow, was predominantly higher compared to the inner contact area. By measuring a CQI for each proposed connection, the influence of different weld seam geometries could be identified. Using double welds close to the edges of the overlapped materials yielded the investigations minimum CQI of about 0.52 and should hence be considered for manufactural purposes. The sheer increase of the welded contact area should be critically assessed, as it had a significantly smaller influence to the CQI and could even reduce the joint’s conductivity.

Author Contributions: Idea, study design and laser welding were done by S.H. The development of the measurement system and measurements have been performed by S.K. The electrical modelling and theoretical background was contributed by C.Ü. A.O. contributed to the laser welding process and study design. A.G. and D.U.S. contributed to all parts. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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