joining aluminum sheet in the automotive industry – a 30...

Vehicle Lightweighting and theNeed for Aluminum in Body-in-White Construction

For more than 30 years, body-in-whitedevelopment has been dominated by theneed to achieve weight reduction while im-proving crash worthiness (Refs. 1, 2).Weight reduction is considered a primarykey to improvements in fuel economy.One area of considerable development hasbeen through materials substitution.Thirty years ago, automotive structural el-ements were made almost exclusively fromrelatively low-strength steels. Today, how-ever, a variety of materials can be foundwithin the vehicle structure. This includesa range of steels (interstitial free grades upto martensitic grades), magnesium alloys,plastic composites, and of course, alu-

minum (Refs. 3, 4). Aluminum has been ofparticular interest for these applicationsfor a number of reasons. First and fore-most, the aluminum alloys under consid-eration today offer strength-to-weightratio improvements over mild steel on theorder of 3:1. This suggests that for anequivalent design, body-in-white weightreductions on the order of 70% could beachieved simply by this direct substitution.Even given strength and stiffnesses be-tween aluminum and steel, weight reduc-tions of 40 to 60% can still be realized.

Additionally, aluminum sheets typicallyoffer considerable corrosion benefits overeven galvanized steels. This is of consider-able advantage when addressing increasedreliability requirements on newer genera-tions of vehicles. A recent survey assessingtrends in the automotive industry clearlyshowed an increase in aluminum usage(Ref. 5). This has also been reflected in thenumbers of aluminum-intensive vehiclesthat have been either developed or areunder evaluation. These include the Mercedes-Benz CL Coupe (Ref. 6), theAudi A-2 (Ref. 7), Audi A-8 (Ref. 8), andthe General Motors EV-1 (Ref. 9), just toname a few.

It is well understood that vehicle man-ufacturing is dominated by threedesign/assembly strategies. These includethe unitized vehicle, body on frame, space-frame, and check (Refs. 1–4). The unitizedvehicle approach is most commonly usedfor higher volume production vehicles.Unitized vehicle manufacture typically in-corporates stamped components as struc-tural elements. Stamped components arethen assembled into the unibody assembly,generally incorporating subframes for sus-pension attachments. Structural load pathsare then through the vehicle (unitized)body itself. For steel designs, unitized bod-ies are assembled almost exclusively by re-sistance spot welding. Resistance spotwelding offers a number of advantages, in-cluding low cost, minimal fixturing, appli-cation flexibility, and high processrobustness. Body-on-frame as well asspaceframe approaches incorporate astructure (frame) that acts as a loadpath indesign. Body-on-frame approaches arecommonly used for truck and SUV appli-cations. Spareframe applications are lesscommon, and more generally associatedwith low production run vehicles. In bothcases, body panels are designed to be at-tached to this frame, and are not consid-ered significant to the structuralperformance of the vehicle. Manufacture

Joining Aluminum Sheet in the AutomotiveIndustry – A 30 Year History

Resistance welding, mechanical fasteners, and ultrasonic welding are examined in this overview of joining technology

BY J. E. GOULD

KEYWORDS

Aluminum AlloysBody-in-WhiteResistance WeldingMechanical FasteningUltrasonic WeldingJ. E. GOULD ([email protected]) is Technology

Leader for Resistance and Solid-State Welding atEdison Welding Institute, Columbus, Ohio.

ABSTRACT

Aluminum has been used in the automotive industry for more than half a cen-tury. During the last 30 years, however, interest in the use of aluminum has beencoupled with needs for improved vehicle performance. This has largely focused onimproved fuel economy, and by extension, weight reduction. Aluminum generallycannot be implemented in vehicle construction without an associated manufacturinginfrastructure. A key element of that infrastructure is welding. Conventionally, re-sistance spot welding is the dominant joining technology for unitized vehicle con-struction. This paper reviews the impact of aluminum implementation on the joiningtechnologies used in body-in-white construction. This review includes a discussionof the specific aluminum alloys used (nominally 5XXX and 6XXX sheet products,and 3XX cast products) as well as a discussion of candidate joining technologies. Itis noted that assembly of aluminum sheet products is still dominated by resistancespot welding. Basic requirements for aluminum spot welding are discussed, as wellas key manufacturing challenges. The state-of-the-art technology is described, in-cluding the impacts of new generation approaches that are entering the industry. Adescription is also provided of alternative resistance welding approaches that are ap-plicable to aluminum automotive fabrication. The use of mechanical fasteners foraluminum construction is discussed. On emerging technologies, this paper also pro-vides an overview of ultrasonic spot welding. This method is being investigated as acandidate replacement technology for resistance spot welding.

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of such vehicles is dominated by assemblyof the frame itself. Such frames have beenassembled in a number of ways, includingdirect welding of tube or channel ends,and even the use of nodes for inter-tubeattachment. Such manufacturing is domi-nated by direct metal deposition practices(predominantly gas metal arc welding[GMAW]). Gas metal arc welding offersconsiderable manufacturing flexibility forcomplex frame designs. However, cycletimes for GMAW are relatively slow (withcorresponding increases in component

costs) largely restricting design ap-proaches to lower-volume production ve-hicles.

Designs intent on increasing relativecontents of aluminum (composed to otherstructural materials) have largely paral-leled those for steel vehicles. As a result,aluminum welding approaches parallelthose already demonstrated for steels.This paper addresses our understanding ofthose processes as applied to aluminum inan automotive context. As the discussionbelow describes, the welding aluminum al-

loys offer unique challenges different thanthose seen on steels. As a result, researchand development associated with weldingthese materials goes on to this day. In ad-dition, the challenges seen with weldingautomotive grades of aluminum alloys hashelped foster a range of new joining tech-nologies. These, as have been investigatedin an automotive context, are also de-scribed in this paper. Of note, this paperfocuses on sheet metal construction,rather than joints made in heaver sections.As a result, the discussion provided belowaddresses technology appropriate for thinsheet materials (body-in-white), ratherthan those used for heaver section frameand suspension applications.

Aluminum Alloy Sheet and Its Usein Vehicle Construction

Aluminum alloys for automotive con-struction are largely dominated by threeclasses of materials. These include bothsheet and casting grades. Sheet materialsinclude both 5XXX and 6XXX alloyclasses (Ref. 10). 5XXX materials arenominally solid solution strengthened/work hardenable grades. These materialsare typically alloyed with magnesium(2–5%), and are also applied where corro-sion resistance is required. These materi-

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Fig. 1 — Electrode life test results of a 2-mm-thick 5754 aluminum alloy.Note that electrode wear is characterized by increasing numbers of no-weldconditions through the test. All data are taken from Ref. 23.

Fig. 3 — Weld microstructure variations during electrode life testing for 6111. All data are taken from Ref.24. A — Weld Number 2; B — weld Number 246; C — weld Number 749; D — weld Number 2003.

Fig. 2 — Electrode life test results of a 2-mm-thick 6111 aluminum alloy.Note that partial weld failures have replaced no-welds throughout the ma-jority of the test. All data are taken from Ref. 24.

Fig. 4 — Results showing the average numbers of in-terfacial failures during electrode life testing 0.9-mm6111 Al sheet. Aluminum Association guidelines referto radiused electrodes with larger face diameters. Alcoaguidelines refer to truncated cone electrodes withsmaller face diameters. Figure taken from Ref. 25.

Table 1— Representative Physical Properties for Iron and Aluminum

Melting Specific Density Thermal Electrical Latent HeatPoint Heat (g/cm3) Conductivity Resistivity of Fusion(K) (J/kg-K) (cal/cm3-s-°C) (μΩ-cm) (csl/g)

Iron 1809 460 7.87 0.18 9.17 65.5Aluminum 933 900 2.7 0.53 2.65 95.5

Ratios are provided showing the relative variation of each property between the two materials.

A B

C D

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als are primarily used in under-body ap-plications. Most common alloys here are5754, 5182, and more recently 5083 (Refs.11, 12). 5083 is considered a quick plasticforming alloy, and is under considerationby General Motors. 5XXX alloys arelargely used for underbody applications.6XXX materials are precipitation harden-ing type alloys, containing additions ofboth Mg (0.5 to 1%) and Si (0.5 to 1.5%).Specific variants under consideration in-clude 6111 and 6022 alloys. Materials aregenerally supplied in the T4 (natural aged)condition, and then subjected to formingand subsequent welding. Peak strengthsare then obtained as part of the paint bake

aging cycle. This class of alloys is attractivefor dent resistance applications (skin pan-els) due to the high-tensile strengths ob-tainable (

today is resistance spot welding. A typicalbody-in-white constructed today containsas many as 6000 resistance spot welds. Arecent automotive roadmap (Ref. 5) sug-gests that trend is not likely to change anytime soon. Not surprisingly, the majorityof the materials joining research with re-gard to aluminum in automotive assemblyconcerns resistance spot welding. Resist-ance spot welding of steel body assemblieshas been commonplace in the industrydating back to the 1950s (Ref. 18). Resist-ance spot welding of steels has been char-acterized by high reliability, low costs, andconsiderable manufacturing robustness.There has also been considerable experi-ence resistance spot welding aluminumsheet. Many design aspects from resist-ance spot welding steels are transferable

to aluminumsheet. These in-clude flange di-mensions andweld sizes. Spotwelding of alu-minum sheet forautomotive appli-cations has tradi-tionally focused onclosure panels(hoods, deck-lids)and is transition-ing into body-in-white.

Development of basic practices for re-sistance spot welding of aluminum dateback to roughly the 1940s (Ref. 19). Thispractice was based on welding of aero-space components, and has largely beenembodied today in resistance spot weldingguidelines made available by the Alu-minum Association (Ref. 20). Resistancespot welding requirements of aluminumdiffer greatly from those of steel. This islargely based on differences in materialphysical properties. These differences arehighlighted in Table 1. Aluminum showsroughly one- third the electrical resistivityand three times the thermal conductivityof steel. As a result, aluminum generallyrequires considerably higher welding cur-rents and shorter times compared to steels

(Ref. 19). The basic welding practices asdefined in the Aluminum Association rec-ommended guidelines (Ref. 20) also in-clude relatively large face diameter andshallowly radiused electrodes (comparedto steels). These electrodes are used pri-marily to avoid excessive indentations dur-ing welding.

Early on, it was identified that practicessuch as defined in the Aluminum Associa-tion guidelines would be problematic forautomotive applications (Ref. 21). Withinan automotive context, application ofpractices initially defined for aerospacecomponents resulted in a number of man-ufacturing issues. These included processinstability, poor robustness, excessive elec-trode wear, and poor weld microstruc-tures. Much of the early activity focusedon variations in aluminum surface condi-tion (Refs. 13, 22). In particular, it wasshown that following surface cleaning,contact resistances could change markedlyover short durations in time. A key aspectof adding pretreatments to the aluminumwas to stabilize the aluminum surface, re-ducing this process instability (Ref. 10).

Electrode life, however, has been themajor issue affecting resistance spot weld-ing aluminum in automotive applications.Original practices for spot welding alu-minum (Ref. 20) allowed frequent re-dressing of the electrodes, mitigating


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Fig. 7 — The use of a dressing tool to add ridges to the surface of faced electrodes. The ridges provide thesame function as other surface roughening techniques. Figures taken from Ref. 28. A — Cap dressed witha ridged tool; B — ridges resulting from dressing.

Fig. 9 — Schematic power configuration for a capacitive discharge resistancewelding system. Figure taken from Ref. 24.

Fig. 11 — Current waveform for the CD process.

Fig. 10 — Current waveform for the FCDC process.

Fig. 8 — Schematic power configuration for a fre-quency converter direct current (FCDC) resistancewelding system. Figure taken from Ref. 10.

A B


electrode life influences. In an automotivecontext, however, this is not practical, soelectrode life became a concern. Examplesof typical electrode life tests on aluminumsheets are provided in Figs. 1 and 2. Theseresults are for 5754 and 6111 sheets, re-spectively (Refs. 23, 24), and provide datashowing the variation in weld size follow-ing 100% peel testing. Results are definedby two characteristics. Most notably, thetests are characterized by periodic “drop-outs,” that is, where the apparent peel but-ton size falls dramatically, often to zero.The second is that the non-drop-out but-ton size appears to continuously increasethroughout the test. Metallurgical charac-terizations done through these electrodelife tests provide some insights into this be-havior. Sample sections through one ofthese tests are presented in Fig. 3. Theseresults demonstrate that drop-outs duringelectrode life testing are largely related tometallurgical phenomenon with the weld

nuggets themselves. These include the for-mation of shallow welds early in the elec-trode wear cycle, increasing weld size(leading to reduced drop-outs), and thenfinally expulsion and coarse defects towardthe end of the wear cycle.

Through this and similar sets observa-tions, it was identified that stability, ratherthan cleanliness of the electrode face, wasthe key to consistent welding behavior.This, in turn, offered potential for im-proved electrode life. Work by Spinellaand Patrick (Ref. 25) showed that by mod-ifying resistance spot welding practicesthemselves (primarily by reducing elec-trode face diameters), improvements inboth electrode life and weld consistencycould be achieved. This improvement isshown in Fig. 4. Later work from the samegroup (Ref. 26) showed that by steppingthe welding force and current could addadditional improvements in electrode life.This was related to taking advantage of

roughened surfaces, and maintaining spe-cific pressures and current densities as theelectrodes wore. Related works by otherauthors provided confirmation to this ap-proach. Work by Chan and Scotchmer(Ref. 27) showed that pretexturing theelectrode faces with TiC added the neces-sary roughness to create stability and ex-tend electrode life. The impressions takenthroughout a sample electrode life test(Fig. 5) demonstrate consistency of sur-face roughness throughout the trial. Fi-nally, work done by Sigler et al. (Ref. 28)examined several variations of electrodeface surface roughening on spot weldprocess robustness. Work included exam-ining grit blast electrodes (shown in Fig. 6)and radially ridged electrodes (shown inFig. 7). The studies done did not includeelectrode life testing. However, both vari-ants of artificially roughnened electrodesshowed improved weld consistency andtolerance to variations in fitup conditions.

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Fig. 12 — Schematic representation of a medium-frequency direct current (MFDC) resistancewelding power supply. In this approach, AC power is rectified to DC, electronically switched to cre-ate MFAC, and then finally rectified following transformer voltage step-down. Figure taken fromRef. 10.

Fig. 13 — ARO electric servo-gun with MFDC powersupply.

Fig. 16 — Macrosection of a projection weld between6111 (top) and 6061 (bottom) aluminum alloys.

Fig. 14 — Schematic diagram showing the programming relationships between current and forcewith the ARO electric servo-gun.

Fig. 15 — Relationship between dressing schedule and weld size throughout an electrode lifecycle. Work was done on a nominally 2.5-mm-thick 5083 aluminum alloy. Figure was takenfrom Ref. 32.


Process consistency and electrode lifehave also been affected by advances inpower supplies and equipment. As hasbeen suggested from other reviews (Refs.10, 19) and recommended practice docu-ments (Ref. 20), aluminum sheets havetraditionally been welded using DC sys-tems, often with postweld forge capability.Historically (largely in an aerospace con-text), two types of power supplies havedominated resistance welding of alu-minum. These included frequency con-verter (FCDC) and capacitive discharge(CD) systems. The basic power supplyarrangements for FCDC and CD systemsare provided in Figs. 8 (Ref. 10) and 9(Ref. 29), respectively. Resulting process

waveforms are shown in Figs. 10 (Ref. 19)and 11 (Ref. 29), respectively. Both powersupply configurations result in DC currentflowing into a low-impedance secondary.Typical secondary impedances for suchsystems are typically measured in 10s ofμohms. This allows the necessary high

currents for resistance welding of alu-minum sheet to flow at relatively low sec-ondary voltages. It has also been suggestedthat a key factor for aluminum spot weld-ing is the implicit rise time of the currentwaveform (Ref. 19). Faster rise times re-duce conduction effects and allow spotwelds to be made at shorter times with lessthermally related effects. This implieslower currents and reduced levels of elec-

trode wear.Two-stage forcing systems have also

been employed for welding aluminumsheet (again, largely in the aerospace in-dustry), generally paralleling the develop-ment of FCDC and CD power supplies.Two-stage forcing systems are typicallypneumatically driven, employing a “buck-ing cylinder” that works against the mainwelding cylinder force. Weld force thenbecomes the difference between the mainand bucking cylinder forces, while theforge force is applied by rapidly ventingthe bucking cylinder. Venting the buckingcylinder allows full application of the weldcylinder force, providing a consolidatingspike at the end of the cycle.


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Fig. 17 — Physical layout of the HYPAK® system used for projection weld-ing aluminum alloys. Figure taken from Ref. 36.

Fig. 18 — Weldability lobe for RPW of 0.8-mm 6111-T4 aluminumsheet. The lobe shown provides weld strengths as a function of peak cur-rent and applied force. Figure taken from Ref. 36.

Fig. 20 — Cross section of a CHRW on a 2-mm-thick 7075-T6 aluminumalloy.

Fig. 19 — Schematic representation of the conductive heat resistance weld-ing (CHRW) process. Note steel coversheets providing heat generation andconstraint of the Al weld metal.

Fig. 21 — Schematic representation of a single side conductive heat re-sistance spot welding (CHRSW) application.


As suggested, such systems have been ef-fectively used since the 1940s (Refs. 10, 19,25) in the aerospace industry. However,they are both cost prohibitive and physicallyinflexible for automotive applications. As aresult, much of the original work on resist-ance spot welding of aluminum for auto-motive applications focused on AC powersupplies (though some 1- and 3-phase DCsystems are used) and single force systems.Two recent equipment innovations, how-ever, have changed this outlook. These in-clude the development ofmedium-frequency DC (MFDC) power sys-tems, and electric-servo controlled forcingsystems. The medium-frequency power sys-tem is diagramed in Fig. 12 (Ref. 10).MFDC power essentially takes 3-phase cur-rent, rectifies it to DC, electronicallyswitches this power to create single-phaseAC power at frequencies ranging from 300Hz to 20 kHz, achieving welding voltagescurrents through a transformer, and finallyrectifies that output to provide DC on thesecondary. This system allows significant re-duction in transformer sizes while deliver-ing the necessary high currents for weldingaluminum alloys, and is seen as a vehicle foreconomic joining in an automotive context(Refs. 10, 19, 25). More recently, these sys-tems have been coupled with electric-servounits for providing welding force. A typicalwelding system is shown in Fig. 13, and a di-agram detailing the relationships betweencurrent and force is provided in Fig. 14.These combined current/force cycles havebeen recently demonstrated to provide ben-efits for welding complex stack-ups of steel(Ref. 30), and offer advantages mirroringcombined current/forge cycles typical of

previous FCDC and CDsystems of the past.

A primary draw-back of using DCpower for resistancespot welding of alu-minum, however, iselectrode life. Variousresearchers have com-pared electrode livesusing AC and DCpower, and consistentlyfind electrode lives re-duced by ½ to ¾ (Refs.19, 21, 31). This reduc-tion in electrode lifehas been related to po-larity effects during re-sistance welding (Refs.10, 19). Polarity effectsare known to cause ex-cessive heating andpreferential wear on the anode side of thestackup. For this reason, electrode life isconsidered enhanced by frequency con-verter DC machines due to the polarityswitching that occurs between pulses (Ref.10). Generally, however, the economicand manufacturing flexibility advantagesof MFDC (in combination with electric-servo guns) outweigh concerns with elec-trode life. Current strategies are to employdressing to maintain weld integrity whileusing these systems (Ref. 32). Dressingfrequencies are known to vary from 10s to100s of welds depending on the applica-tion. An example of the relationship be-tween frequent dressing (every 20 welds)and electrode life is presented in Fig. 15.These results demonstrate that the use of

MFDC power combined with such fre-quent dressing can achieve effective elec-trode lives in access of 1000 welds.

Other Resistance WeldingProcesses

A range of other resistance weldingprocesses has also come under considera-tion for aluminum auto body construction.A major effort has been to develop pro-jection welding for aluminum. The use ofresistance projection welding on steels iscommonplace (Ref. 33). However, thehigh thermal conductivities and tenacioussurface oxides make projection weldingproblematic. Here, the high conductivitiesrequire extremely rapid current rise times

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Fig. 22 — Cross section of a single-side CHRSW lap joint on 2-mm-thick7075-T6 Al.

Fig. 23 — Schematic representation of the self-piercing riveting process.

Fig. 24 — Morphological details of a self-piercing rivet between sheet ma-terials. A — Rivet cross section; B — rivet head; C — rivet root.

A

B

C


to allow localization of forging (Ref. 29).In addition, work on mechanisms of bond-ing has demonstrated that thermal disso-ciation of Al2O3 particles present on thealuminum surfaces is nearly thermody-namically impossible (Ref. 34). As a result,bonding occurs chiefly from two mecha-nisms; deformation associated with forg-ing and potentially surface destructionassociated with localized melting. An ex-ample cross section of an aluminum pro-jection weld is shown in Fig. 16. In thiscase, bonding occurred through the dis-ruption of the contact surfaces as is clearlyevident from the micrograph. Applicationsfor projection have been described as early

as the mid 1980s (Ref. 35). More recently,projection welding has been demonstratedwith a system is marketed under the trade-name HY-PAK®. This system combines a½ cycle AC power supply with a fast me-chanical follow-up mechanism. The sys-tem itself is shown in Fig. 17. That systemhas been used to develop aluminum hemflange attachments (Ref. 36). A plot ofweld strength as a function of applied cur-rent is provided in Fig. 18. Here, it canseen that for a 0.8-mm 6111 aluminumalloy, joint shear strengths on the order of1 kN were observed over a range of pro-cessing conditions.

Another novel resistance welding ap-proach for aluminum alloys was developedby Edison Welding Institute. This processis termed conductive heat resistance weld-ing (CHRW). The process essentially is re-sistance welding of an aluminum stack-upwith one or more cover sheets employed(Refs. 37–39). A schematic representation

of the process is provided in Fig. 19. In thisprocess, the cover sheets are made fromsteel and consumables with two intendedfunctions. First, the high resistivity of thesteel provides the necessary heat genera-tion for the process. Second, the steelcover sheets are of relatively high thermalstability and actually act to constrain themelting aluminum which occurs during theprocess. A representative cross section ofa conductive heat resistance weld is pro-vided in Fig. 20. It is of note that welds aretypically hour-glass shaped (responding toheat generation in the cover sheets) andfree of internal porosity. In this latter case,it has been shown that the cover sheetstransmit sufficient stress to the weld poolto suppress hydrogen gas evolution duringsolidification (Ref. 37), minimizing any gasrelated porosity. The process has also beenadapted to single side welding (Ref. 40).Here, welding is done with a push-pullarrangement, with the cover sheet locatedonly where the location of the weld is de-sired. The welding configuration as well asthe resulting cross section is shown in Figs.21 and 22, respectively. The morphologyof the weld clearly indicates the locationof the cover sheet, and again shows noporosity in the joint.

Mechanical Fastening

Mechanical fastening is perhaps the old-est of joining technologies, accomplishingattachment by mechanical interferencerather than metallurgical bonding. Me-chanical fastening covers a broad range ofapproaches, including screwing, folding,clamping, riveting, and clinching (Ref. 41).Mechanical fastening offers the opportunityto assemble aluminum sheet structures atrelatively high speeds and low cost (compa-rable to resistance processes), without thethermal effects associated with welding. Themost attractive mechanical fastening ap-proaches for aluminum body assembly haveincluded clinching, self-piercing riveting,and blind riveting (Refs. 41–44).

The self-piercing riveting process usesa rivet to join two or more pieces of sheetmetal in a lap configuration. The process isillustrated in Fig. 23. Essentially, a hollowrivet is forced through the sheets intendedfor joining. On the back side of the sheets,there is typically a mandrel or die. This dieprimarily facilitates spreading of the hol-low rivet, accomplishing the joint. In addi-tion, the geometry of the mandrelguarantees adequate penetration of therivet, and facilitates formation of an ade-quate profile on the back side of the joint.A cross section of a self-piercing rivet onsimilar thickness sheet steels is shown inFig. 24. The configuration of this equip-ment is relatively simple, consisting of aforcing system and a relatively heavy back-up system for the mandrel. Most recent


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Fig. 26 — Morphological details of a clinch joint be-tween sheet materials. A — Clinch joint cross sec-tion; B — top view of the clinch joint; C — root viewof the clinch joint.

Fig. 27 — Schematic representation of the blind riv-eting process. Figure from Ref. 41.

Fig. 28 — Cross section of a blind rivet made be-tween sheet materials. Figure from Ref. 44.

Fig. 25 — Schematic representation of the sheet metal clinching process.

A

B

C


equipment incorporates electric servos foraccomplishing necessary riveting forces.The configuration is in many ways quitesimilar to electric-servo resistance spotwelding guns. One major difference is theabsence of a welding transformer. This re-sults in substantial weight reduction of thegun, making the system considerably moreadaptable to robotic applications. Gener-ally, self-piercing riveting guns use a feed-ing tape to deliver the rivets to the head ofthe gun assembly. More recently, air-dri-ven feed systems have been used to deliverrivets to the gun.

Clinching is a somewhat related tech-nology in which the materials are pressedand drawn to form an interlocking, inter-ference type of a joint. The clinchingprocess is illustrated in Fig. 25. Essentially,a local die set is used. The top die (orpunch) basically extrudes the top and bot-tom sheets into a lower die assembly. Theresulting forming operation leaves mate-

rial from the top sheetformed to a larger di-ameter than the ap-parent hole in thebottom sheet, affect-ing an interlock typeof joint. A macro-graph of a clinch jointis shown in Fig. 26.Apparent in this mi-crograph are the di-ameters of the punchand lower die as wellas the change in diam-eter of metal extrudedfrom the top sheet.Clinching is againdone with small press-

type machines. These are commonly con-figured as clinch-type guns, with similarappearance to the self-piercing rivet gunsdescribed above. Two-side access is againrequired for clinch joining applications.

Blind riveting allows attachment ofsheets through preformed holes. The rivetitself has a head for seating on one side ofthe joint, and a built-in mechanical ex-pander to form the rivet on the back sideof the joint. This mechanical expander isconnected through the body of the rivetwith a tension ligament. The joiningprocess is shown in Fig. 27. As a load is ap-plied to the ligament, the expending ele-ment is drawn up into the rivet, causingexpansion. Once the expanding region ofthe rivet bottoms out on the lower sheet,the tension ligament fails, and joining iscomplete. A typical cross section of a com-pleted blind rivet is shown in Fig. 28. Themost recent adaptations of this process in-clude improved designs for expansion of

the rivet itself into the formed hole (Ref.44). These are referred to as form-fit orFF® style blind rivets. This variant on theprocess has been reported to improve slip-page loads on aluminum joints by greaterthan a factor of 5, as well as enhance fa-tigue strengths. Mechanistically, blind riv-eting differs from self-piercing rivetingand clinching in that the primary defor-mation required for joining is in the rivet,rather than the attached sheets. Blind riv-eting also offers considerable advantagesin that only one-side access is required.However, the requirement of a pre-formed hole is a major drawback com-pared to other mechanical fasteningtechnologies.

Mechanical behavior of mechanicallyfastened joints has also been studied ex-tensively. An example of these results ispresented in Fig. 29. In this figure, datahave been collected for lap joints between1.6-mm aluminum sheets using adhesivebonding, resistance spot welding, self-piercing riveting, and clinch joining. Datahave also been collected for tensile shearand direct tension (cross tension) configu-rations. It is of note that self-piercing riv-ets show comparable performance toresistance spot welds in both shear and di-rect tension. The performance of theclinch joints is generally somewhat poorerthan for the spot welds, averaging roughly70% of resistance spot weld strengths inboth the shear and direct tension configu-rations. It is further of interest that theself-piercing rivets minimal reduction instrength between shear and direct tensionconfigurations. Generally, for resistancespot welds, performance is substantiallypoorer in the direct tension configuration.

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Fig. 29 — Representative mechanical properties for adhesive bonds, resistancespot welds, self-piercing rivets, and clinch joints on 1.6-mm/1.6-mm lap joints.

Fig. 31 — Schematic layout of a lateral drive ultrasonic spot welding (USW)system with details of reactions at the workpiece itself.

Fig. 32 — Cross section of an ultrasonic spot weld made on 0.8-mm-thick1100 aluminum alloy.

Fig. 30 — Schematic representation of the flow-drilling process for apply-ing mechanical fasteners. Figure from Ref. 45.


An additional mechanical fasteningtechnology receiving attention in the auto-motive industry is the use of flow-drillingscrews (Refs. 41, 45). Flow drilling impliesthe use of self-tapping screws with a hard-ened (unthreaded) end effector. The fas-tener is applied to undrilled workpieces athigh rotational speeds. The hardened endeffector generates heat within the work-pieces, allowing plastic deformation andflow. This then results in a self-pierced hole,which on further feeding engages thethreads on the screw portion of the fastener.The result is a single-side mechanical jointwithout the need for a preprepared whole.The process is diagrammed in Fig. 30. Fas-teners for aluminum sheet are generally ofstainless steel, applied with dedicated designC-guns (Ref. 45). The approach is beingused on a number of automotive platformsin Europe, with cycle times ranging from 1.5to 5s.

Ultrasonic Welding

Ultrasonic metal welding is a technol-ogy that employs translational motion be-tween opposing sheet workpieces togenerate the necessary heat and deforma-tion for bonding. The process functions atfrequencies on the order of 20 kHz (Refs.46–48) with displacements on the order of100s of microns. The vibratory action isdeveloped by electrical excitation of apiezoelectric element, which is then am-plified through a booster arrangement. Aschematic representation of this welding

process is provided in Fig. 30. The exactmechanism of bonding during ultrasonicwelding is still the subject of much debate.There are questions regarding the tem-peratures achieved, as well as the specificmetallurgical behaviors that facilitatebonding. It has been suggested that bothmelting (Ref. 47) and solid-state deforma-tion (Ref. 49) might both play a role. Themicrostucture of a typical ultrasonic spotweld in aluminum is provided in Fig. 31.This micrograph provides good insight tothe transitions occurring in aluminumsheets during ultrasonic spot welding. Ev-ident are the indentations on the sheetsurfaces, as well as the characteristic wavybond line indicative of proper welding.The major limitation of ultrasonic weldingappears to be the power delivery capabil-ity of the ultrasonic power source itself(Ref. 50). State-of-the-art power suppliesare limited to roughly 5500 W. At these

power levels, the process is restricted torelatively thin gauge materials or weldingtimes in excess of several seconds. To ad-dress these power limitations, so called“tandem systems” are now in use. Thesesystems may be of either “push-pull” or“torsional” types. Push-pull systems usetwo ultrasonic elements configured on op-posite ends of the driving element. An ex-ample is shown in Fig. 32. Thepiezoelectric elements are then excited180 deg out of phase, resulting in a dou-bling of the deliverable power to the work-pieces. Torsional systems function in asimilar fashion, though with the ultrasonicpower units located on opposite sides of arotary-oscillating mechanism. The powersystems are again excited 180 deg out ofphase, resulting in a short arc displace-ment being transferred to the workpieces.The system is diagramed in Fig. 33, and atorsional ultrasonic spot weld is presentedin Fig. 34. Such systems now allow ultra-sonic power levels on the order of 10 kWto be achieved for spot welding purposes.

Considerably more efforts have beenexpended understanding process-relatedeffects for ultrasonic spot welding. Ultra-sonic spot welding is typically done at con-stant power with the total energy inputdefined by the weld time itself. Given thelimitation of power available from currentsystems, ultrasonic spot welding typicallyemploys weld times on the order of sec-onds. Generally, it is understood that weldstrengths increase with higher levels ofpower, time (total energy), and force. An-other major factor affecting weld quality isthe area of the sonotrodes themselves. Asdemonstrated in Fig. 35, larger contactareas between aluminum sheets generallyresult in higher shear loads (Ref. 50). Thisis consistent with similar data on resist-ance spot welding. There has also beenconsiderable work assessing manufactur-ing robustness associated with ultrasonicspot welding of aluminum sheets (Refs. 46,51). Process factors studied have includedthe relationships between power,sonotrode amplitude, weld time, clampingforce, etc. Generally, higher energies (ex-pressed predominantly by larger ampli-tudes and longer times) are advantageous.However, there appears to be an upperlimit where excessive softening and inden-tation reduce the strength of these joints.The process also tends to benefit fromhigher welding (or clamping) forces. Con-siderable work has also been done evalu-ating the influence of material conditionson the quality and reliability of jointsmade. Variations in surface finish have re-ceived the most attention. Generally, theprocess is unaffected by levels of surfaceoxides (Refs. 44, 51), the presence of lu-bricants (Ref. 51), or substrate texture(Ref. 51). The data do suggest, however,that increases in the degree of residual


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Fig. 33 — Configurations of push-pull systems forultrasonic spot welding. Figures are from Ref. 50. A— Schematic representation of a direct actingpush-pull system; B — an actual push-pull systemconfigured for USW of sheet aluminum.

Fig. 34 — Configurations of torsional weldingsystems for ultrasonic spot welding. A —Schematic representation of the torsional ultra-sonic welding configuration; B — actual 10-kWtorsional welding system at Edison Welding Institute.

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cold work in the aluminum substrate cor-respond to higher weld strengths (Ref. 44).Finally, there appears to be little correla-tion (outside of simple contact area) be-tween the profile of the sonotrode itselfand resulting joint strengths (Refs. 44, 47).

Summary

The last 30 years have seen a progres-sive increase in the use of advanced mate-rials for automotive construction.Aluminum and its alloys have played heav-ily into this mix. Sheet and wrought prod-ucts as well as castings have beenconsidered for various subsystems of ad-vanced material vehicles. A key to the im-plementation of aluminum alloys forautomotive construction is the identifica-tion/development of cost-effective joiningtechnologies. This paper is focused on anumber of key developments that have en-abled these technologies. The generalfocus is on body-in-white applications, orsheet metal construction. It is understoodthat such construction is dominated by re-sistance spot welding (RSW). To that end,developments in resistance spot weldingtechnology (and associated weld bonding)are addressed first. These included theprogression from AC to DC to MFDC

current, development of proper electrodedesigns, and use of dresser systems tomaintain electrode profiles. Also discussedare recent capabilities enabled by electric-servo guns for production spot welding.

A discussion is provided on alternativeresistance welding technologies appropri-ate for aluminum sheet materials. Theseinclude resistance projection welding(RPW) and conductive heat resistancewelding (CHRW). These are widely dis-parate technologies, but they are being in-vestigated for specific applications withinthe body-in-white framework.

This overview then examines use ofvarious forms of mechanical fastening.These technologies are currently beingused as alternatives to resistance spotwelding for both steel and aluminumbody-in-white structures. The methods de-scribed include self-piercing riveting,clinching, blind riveting, and flow drilling.These technologies are all basic mechani-cal interference joining approaches, alter-nately with (riveting, screws) or without(clinching) a third body element. The basicapproaches are described with some base-line mechanical properties data. Of par-ticular interest are new generations ofblind riveting. These approaches havebeen shown to increase the mechanicalstability of the joint itself, and facilitateone-side application.

Finally, developmental efforts on ul-trasonic spot welding are described. Basicconfigurations are outlined, including di-rect action, push-pull, and torsional ap-proaches. Joints are generally solid-statein character, showing joint strengths thatgenerally increase with energy inputs. Akey limitation of this technology is deliv-erable power to the workpiece. A singlepower element is limited to roughly 5500W, driving tandem based (push-pull andtorsional) systems to weld heavier sectionsand incorporate shorter weld times. Nu-merous studies have shown the process tobe robus, with little sensitivity to alu-

minum surface condition, the presence oflubricants, substrate texture, and evenvariations in sonotrode geometry.

The review represents a sampling of thetechnology innovations that have occurredfor assembling aluminum body structures inan automotive context. Exploitation ofthese technologies will, of course, requirecontinued development, enabling improvedreliability and reduced costs of such weldedaluminum structures. Such requirementswill carry aluminum joining researchthroughout the next several decades.

References

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Fig. 35 — Details of a torsional USW made on anominally 0.8-mm 6111 aluminum alloy. Figuresare from Ref. 50. A — Top surface of the torsionalUSW indicating the gripping marks resulting fromthe process; B — interfacial failure of a sheared tor-sional USW showing the apparent bond area.

Fig. 36 — Relationship between sonotrode contact area and resulting joint strength. Thedata here cover several different substrates, equipment configurations, and sonotrode pro-files. Figure is from Ref. 50.

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40. Gould, J. E., Bernath, J., and Albright,C. 2008. Application of conductive heat resist-ance spot welding (CHRSW) to single side join-ing of aluminum sheet. Sheet Metal WeldingConference XIII, AWS Detroit Section, Detroit,Mich. Paper 5-2.

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piercing riveting. Sheet Metal Welding Confer-ence X, AWS Detroit Section, Detroit, Mich.Paper 4-3.

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47. Weare, N. E., Antonevich, J. N., andMonroe, R. E. 1960. Fundamental studies of ul-trasonic welding. Welding Journal 39(8) : 331-sto 341-s.

48. Hetrick, E., Jahn, R., Reatherford, L.,Skogsmo, J., Ward, S., Wilkosz, D., Devine, J.,Graff, K., and Gehrin, R. 2005. Ultrasonic spotwelding: a new tool for aluminum joining. Weld-ing Journal 84(2): 26–30.

49. De Vries, E. 2004. Mechanics and mech-anisms of ultrasonic metal welding. PhD diss.,The Ohio State University.

50. Graff, K., Eastman, J., Devine, J., Walsh,J., Ghaffari, B., Hetrick, E., Reatherford, L.,Ward, S., Rokhlin, S., and Sheehan, J. 2006. De-velopments in processes and quality proceduresfor ultrasonic welding of aluminum structures.Sheet Metal Welding Conference XII, AWS DetroitSection, Detroit, Mich. Paper 9-5.

51. Hetrick, E. T., Baer, J. R., Zhu, W.,Reatherford, L. V., Grima, A. J., Scholl, D. J.,Wilkosz, D. E., Krause, A. R., and Fatima, S.2008. Ultrasonic metal welding process robust-ness. Sheet Metal Welding Conference XIII, AWSDetroit Section, Detroit, Mich. Paper 3-6.

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