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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 (
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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
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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.
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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.
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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
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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
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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.
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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.
Gould 1-12 corr_Layout 1 12/14/11 3:46 PM Page 32
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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.
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Fig. 35 — Details of a torsional USW made on anominally 0.8-mm
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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
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AWS, Weld-Ed Web SitePromotes Welding
CareersThe American Welding Society
(AWS) and National Center for Weld-ing Education & Training
(Weld-Ed)Web site at www.CareersInWelding.comoffers the following
details: people andcompanies in the welding industry; funfacts;
salary information; industry news;videos; articles; and upcoming
trainingopportunities, seminars, and events.
Additionally, the site features pagesgeared directly for
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