adhesive placement in weld-bonding multiple stacks of steel sheets

Introduction

The weld-bonding process, which is acombination of resistance spot welding(RSW) and adhesive bonding, producesmore desirable joint performance whencompared to either spot welding or adhe-sive bonding (Refs. 1–3). It not only im-proves the crashworthiness, stiffness, fa-tigue behavior, and corrosion resistance(Refs. 4, 5), but also enables a reduction inthe number of welds used in vehicle struc-tures. Therefore, this joining technologyhas been widely applied in vehicle bodiessuch as front longitudinal rails; A, B, andC pillars; and the bulkhead to inner wing.However, due to the complexity of themultiple stacks of steel sheets often en-

countered in vehicle assembly, the effectof the adhesive placement is not well doc-umented (e.g., one adhesive layer betweentwo steel sheets, two adhesive layers inthree sheets). Therefore, the benefits ofweld-bonding multiple stacks of steelsheets for vehicle assembly applicationshave not been fully maximized.

Many studies have been reported onthe weld-bonding of two steel sheets(Refs. 6, 7). Studies have shown that theadhesive increased the weld size andstrength in weld-bonded joints comparedto resistance spot welds under the same

welding parameters (Ref. 8). Although ex-tensive studies have been done on weld-bonding two steel sheets, little work hasbeen reported on weld-bonding multiplestacks of steel sheets. Because there aremany joint configurations and designs ona body-in-white structure, it is conceivablethat weld-bonding multiple stacks ofworkpieces is inevitable. Therefore, it isessential that an understanding of weld-bonding of multiple steel sheets be ob-tained. The present study was undertakento experimentally and analytically evalu-ate the effect of the adhesive placementon the weld quality in weld-bonding mul-tiple stacks of workpieces.

Because the adhesive is placed be-tween the steel sheets, the initial contactstate and contact resistance between themwould be changed and conceivably, thequality of the weld nugget might be af-fected. Previous studies have shown thatthe contact state and contact resistancebetween the steel sheets significantly af-fect the weld-bonding process (Refs.9–12). During the weld-bonding process,the contact resistance distribution influ-ences the current density pattern, whichaffects the temperature field throughJoule heating. The temperature field theninfluences the mechanical pressure distri-bution through thermal expansion of theworkpieces. Therefore, the formation ofthe weld nugget is indeed strongly de-pendent on the phenomena at the fayinginterfaces. Meanwhile, the placement ofthe adhesive between the workpieceschanges the contact state between thembecause of the viscosity and heat insula-tion, and consequently affects the weldformation and weld quality. To investigatethe weld formation process, the dynamicresistance, which is a comprehensive re-sistance of the welding joint containing thebulk and contact resistance of the steelsheets, is used as one of the most widelyaccepted procedures (Refs. 13–16). We

Adhesive Placement in Weld-BondingMultiple Stacks of Steel Sheets

An understanding of and guidelines for weld-bonding multiple stacks of steel sheets were developed

BY J. SHEN, Y. S. ZHANG, X. M. LAI, AND P. C. WANG

KEYWORDS

Weld-Bonding ProcessMultiple StacksNugget FormationAdhesive Placement

J. SHEN, Y. S. ZHANG ([email protected]), and X. M. LAI are with the Shang-hai Key Lab for Digital Auto-Body Engineering,State Key Lab of Mechanical System and Vibra-tion, Shanghai Jiao Tong University, Shanghai,China. P. C. WANG is with the ManufacturingSystems Research Lab, General Motors Research& Development Center, Warren, Mich.

ABSTRACT

Weld-bonding technology has been widely used in vehicle manufacturing to im-prove structural crashworthiness, fatigue performance, and corrosion resistance. Thisstudy focused on the weld quality in weld-bonding multiple stacks of steel sheets com-posed of 0.8-mm-thick SAE1004, 1.4-mm-thick DP600, and 1.8-mm-thick DP780 withvarious adhesive placements. To investigate the effect of the adhesive placement onthe weld-bonding process, the static contact and dynamic resistances between thesteel sheets with adhesive were measured. Then, the weld formation, weld size, andhardness of weld-bonded joints with various adhesive placements were studied bymetallographic tests. Results showed that the static contact and dynamic resistancesbetween the steel sheets increased significantly due to the presence of the adhesive.The weld initiation time and weld size for weld-bonded joints were about 40 ms ear-lier and 10% larger than that without adhesive under the same welding parameters.Weld qualities of weld-bonded joints with two adhesive layers at both faying interfacesbetween multiple stacks of steel sheets produced the best. This study provides theguidelines to the application of adhesive in weld-bonding multiple stacks of steelsheets for vehicle manufacturing.

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apply the same approach to assess the ef-fect of the adhesive placement on weld-bonding multiple stacks of steel sheets.

There are three main parts in this re-port. The first presents the experimentalprocedure including material, experimen-tal setup, sample fabrication, and weldcharacterization. In the next section, theexperimental results where the effect ofthe adhesive placement on the static anddynamic resistances during weld bondingare reported. The calculated effect of ad-hesive placement on the temperature dis-tribution is then presented. Finally, the ef-fect of the adhesive placement on the weld

size and hardness of the weld-bondingjoint with various adhesive placements isdiscussed. This work provides valuableguidelines to the application of adhesive inweld-bonding multiple stacks of steelsheets for vehicle manufacturing.

Experimental Procedure

Materials

To investigate weld-bonding multiplestacks of steel sheets, 0.8-mm-thick, hot-dipped galvanized (HDG), low-carbonsteel SAE1004, 1.4-mm-thick (HDG)

DP600, and 1.8-mm-thick (HDG) DP780steels were used in this study. All have thecoating thickness of 60 g/m2. The chemicalcomposition and mechanical properties ofthese steels are measured and listed inTable 1.

An epoxy resin-based adhesive(Terokal 5089) manufactured by Henkelwas used in this study. It is a one-compo-nent, hot-cured adhesive with low viscos-ity before curing and high performanceafter curing. The material properties ofTerokal 5089 from the Henkel data sheetare listed in Table 2.

Experimental Setup and Sample Fabrication

The weld-bonding process is realizedby use of a servo gun welding system hav-ing a medium-frequency direct current(MFDC) welding machine. The servo guncan precisely control the welding force of

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Fig. 1 — Weld nugget configuration in weld-bonding multiple stacks of steels.

Fig. 3 — Experimental setup for measuring the static contact resistance between the steel sheets and adhesive. A — Test setup; B — resistances between the steelsheets.

Fig. 2 — Various adhesive locations in weld-bonding three sheets.

Table 1 — Chemical Composition and Mechanical Properties of Various Steels

Chemical Composition (%) Mechanical PropertiesSteel Yield Strength Tensile Strength Elongation

C Mn P S Si Al (MPa) (MPa) (%)

SAE1004 0.037 0.21 0.01 0.02 0.018 0.04 152 278 66DP600 0.08 1.74 0.012 0.003 0.016 0.041 316 607 29DP780 0.15 1.80 0.004 0.016 0.010 0.048 508 834 26

Table 2 — Adhesive Properties of Terokal 5089

Solids Content Specific Gravity Viscosity @ 50°C (Pa.s)

>98.5% 1.05∼1.20 30–50

BA


the electrode to squeeze out the adhesive.The advantage of the MFDC technologywith an operation frequency of 1000 Hz isto enable a very fast reaction of weld cur-rent with respect to any variation in theweld-bonding process. A typical weld-bonding process contains the followingthree stages: squeeze cycle, weld cycle,and hold cycle. During the squeeze cycle,the electrode force is applied to squeezeout the adhesive between the steel sheets.Then, the weld current can pass throughthe steel sheets to form the weld nugget inthe weld cycle. A weld-bonding joint couldfinally be completed after the cooling inthe hold cycle.

In this study, the multiple stacks ofthree sheets is composed of 0.8-mm-thickSAE1004 as the top sheet, 1.4-mm-thickDP600 as the middle sheet, and 1.8-mm-thick DP780 as the bottom sheet — Fig. 1.The adhesive layer of Terokal 5089 first islaid on the steel sheets, and then the weld-ing current is applied through the adhesiveand steels to get a weld-bonding joint.Class II copper alloy with chromium andzirconium electrode is used in the experi-ment, and the welding parameters arelisted in Table 3.

For a multiple stack weld-bonding jointwith 0.8-mm-thick SAE1004, 1.4-mm-thickDP600 and 1.8-mm-thick DP780 from topto bottom sheet, there would be three dif-ferent types of weld bonding according tothe different adhesive locations as given inthe schematic of Fig. 2. The joint withoutadhesive is traditional resistance spot weld-ing. Weld-bonding type I represents a jointwith one adhesive layer on the interface be-tween SAE1004 and DP600. Similarly,weld-bonding type II represents a joint withone adhesive layer between the DP600 andDP780 sheets and weld-bonding type IIIrepresents a joint with two adhesive layersbetween three steel sheets.

Weld Characterization

Because of the stack-up joint’s com-plexity, the traditional weld dimension (di-ameter in the center of the weld nugget)cannot clearly characterize the quality andshape of the weld nugget in weld-bondingof three sheets. Hence, a new weld config-uration with a typical dimensional param-eter for a joint with multiple stacks is pre-

sented in Fig. 1. There are two critical di-mensions of the weld nugget — weld sizesA and B. Weld size A can represent thejoint quality between the top and middlesheets. Weld size B represents the connec-tion between the middle and bottomsheets. These two weld sizes can be meas-ured from the micrographs of the jointcross section, which is prepared for meas-urement using Nital 4% etch applied aftermechanical grinding and polishing. Thethicknesses of the top and middle sheetsare 0.8 and 1.4 mm, respectively; the min-imum dimension for weld size A is set at4.0 mm (Ref. 17). Similarly, the minimumweld size B is 5.0 mm. After the metallo-graphic test, the Vickers hardness testswere conducted using a 200-g load andhold time of 10 s. Five replicates were pre-pared for the metallographic and hardnesstests.

Results and Discussion

Static Contact Resistance during SqueezeCycle

The first stage of the weld-bondingprocess is the squeeze cycle. During thesqueeze cycle, an electrode force is ap-plied to clamp the sheets together, whichhas the effect of squeezing out the adhe-sive between the steel sheets and creatingintimate contact making it easier for thewelding current to pass through. In orderto investigate the contact state (includingthe adhesive distribution and static con-tact resistance) between the steel sheets

with the adhesive layer, a measurementsystem in Fig. 3A was set up with a microresistance measuring instrument and theservo gun. This system is capable of meas-uring the static contact resistance betweenthe steel sheets with different contactforces in the precision of 0.1 μΩ. Themeasurement principle, presented in Fig.3B, is based upon RM, which is the total re-sistance between two fixtures installed at40 mm from the centerline of the elec-trode and can be directly measured by thesystem. RS is the bulk resistivity of the steelsheets from the contact region to the fix-ture, which is a constant value at ambienttemperatures. RC is the static contact re-sistance between the steel sheets. Accord-ing to the setup in Fig. 3B, the static con-tact resistance RC can be calculated byEquation 1.

RC = RM – 2RS (1)

During the squeeze cycle, an electrodeforce is applied to clamp the sheets to-gether, which has the effect of squeezingout the adhesive between the steel sheetsand creating intimate contact. Figure 4Ashows the contact area between the steelsheets after squeezing. Figure 4B and Dare the state of the adhesive on the inter-face between two 0.8-mm-thick SAE1004steel sheets after the squeeze cycle with anelectrode force of 5.5 kN. As shown in Fig.4B, there is a distinct difference in adhe-sive distribution between the area of con-tact and outside of the contact area.

Area 1 is the contact surface where

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Fig. 4 — Effect of adhesive on the contact state in weld-bonding 0.8-mm-thick galvanized SAE1004 steel. A— Contact area between workpieces after squeezing; B — residual adhesive between the workpieces; C —contact area without adhesive; D — enlarged view of area 1 in B; and E — enlarged view of area 2 in C.

A

D

B C

E


there is some residual adhesive sticking tothe steel surface after the squeeze cycle. Acloser examination of Fig. 4B is given inFig. 4D and shows evidence of granularsolids in the adhesive layer in area 1 eventhough an electrode force of 5.5 kN is applied.

Experimental observations showedthat the amount of residual adhesive lefton the contact surfaces depends on thecomposition and viscosity of the adhesive,electrode force, and ambient tempera-ture. On the contrary, Fig. 4C and E ex-hibit a clear contact surface of a zinc-coated SAE1004 steel (HDG60) withoutadhesive. Comparing the steel surfaces

with and without ad-hesive shown in Fig.4, it is clearlydemonstrated thatthe adhesive inter-feres with the con-tact state betweenthe steel sheets,which could influ-ence the static con-tact resistance inweld bonding theworkpieces.

Figure 5 shows theeffects of the coat-ing and electrodeforce on the staticcontact resistanceof two 0.8-mm-thickSAE1004 steelsheets with adhesiveTerokal 5089. To as-

sess the effect of the coating, steels withvarious coatings were used in the experi-ments. It could be seen that without thepresence of the adhesive, the hot-dippedgalvanized (HDG) and electrogalvanized(EG) steel sheets have smaller static con-tact resistance than does bare steel sheet.This is attributed to the fact that the zinccoating has a relatively high electrical con-ductivity, and consequently contributes tothe reduction of the total contact resist-ance between the steel sheets. However,when the adhesive is placed between thesteel sheets, the static contact resistancesignificantly increases as exhibited by thebare SAE1004 with adhesive having the

largest contact resistance. Within the se-lected range of surface coating and adhe-sive, the change in static contact resistancecaused by surface coating is negligiblecompared to the presence of the adhesive.These results suggest that the adhesive hasthe greatest effect on the static contact re-sistance between the steel sheets by stick-ing to the sheet surface.

To investigate the effect of the adhesivethickness on the static contact resistancebetween the steel sheets, experimentswere performed using 1.8-mm-thickDP780 steel, and the results are shown inFig. 6. The results show that the static con-tact resistance without adhesive (bondlinethickness = 0 mm) is small up to an elec-trode force of approximately 0.5 kN. How-ever, the presence of the adhesive be-tween the steel sheets significantlyincreases the static contact resistance. Thestatic contact resistance increases with theadhesive bondline thickness; however, thiseffect diminishes as the electrode force in-creases. As the electrode force reachedabout 6.0 kN, the static contact resistancevirtually becomes a constant. These re-sults suggest that a large electrode force isrequired for the thick adhesive to conquerthe viscosity of the adhesive sandwichedbetween the steel sheets. Once the exces-sive adhesive was squeezed out, the staticcontact resistance between steel sheetsreached a stable state.

A minimum electrode force is neededto squeeze out the adhesive and thenmaintain the workpieces in intimate con-tact. Figure 7 shows the static contact re-sistances of 0.8-mm-thick SAE1004, 1.4-mm-thick DP600, and 1.8-mm-thickDP780 used in this study. The adhesive isTerokal 5089 with a thickness of 0.4 mm.As can be observed for a given electrodeforce, the 1.8-mm-thick DP780 steel hasthe largest static contact resistance withadhesive, which is attributed to the largebulk resistivity of DP780 compared toSAE1004 and DP600 steels (Ref. 18). For


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Fig. 5 — Effect of the coating on the static contact resistance in resistance weld-ing and weld-bonding 0.8-mm-thick SAE1004 steel.

Fig. 7 — The static contact resistances of 0.8-mm-thick SAE1004, 1.4-mm-thick DP600, and 1.8-mm-thick DP780 in weld bonding.

Fig. 6 — Effect of adhesive bondline thickness on the static contact resistancesin weld-bonding 1.8-mm-thick DP780 steel.

Table 3 — Welding Parameters for Stack-Up Joint

Electrode Diameter Electrode Force Welding Current Time (ms)(mm) (kN) (kA) Squeezing Welding Hold

5.0 5.5 9.0 200 420 100


the three steel sheets in this study, it is ef-fective to decrease the static contact re-sistance by increasing the electrode force.The minimum electrode force to keep thestatic contact resistance stable is differentbecause of the various thicknesses andmoduli of the steel sheets. An electrodeforce of 1.1 kN for SAE1004 (0.8-mm-thick), 3.1 kN for DP600 (1.4-mm-thick),and 4.6 kN for DP780 (1.8-mm-thick) arerequired for weld-bonding.

Dynamic Resistance during Weld Cycle

After the squeeze cycle, the weld cur-rent is applied to the workpieces throughthe electrodes to start the weld cycle. Dur-ing this stage, the properties of the steelsheets and adhesive change with the tem-perature rise resulting from the genera-tion of the Joule heat. When the temper-ature goes up to about 1500°C, a moltenweld nugget forms. The dynamic resist-ance has been used to study the weld for-mation during the welding process. A typ-ical dynamic resistance curve duringwelding contains three stages (Ref. 17). In

the first stage during the beginning ofwelding, the dynamic resistance drops dueto the decrease of the static contact resist-ance between the steel sheets because ofthe temperature rise. After that, the dy-namic resistance goes up with the rise ofthe bulk resistivity of the steels by temper-ature in the second stage. Finally, the dy-namic resistance becomes stable and de-creases slightly in the third stage becauseof the formation of the weld nugget.

In our study, we measured the dynamicresistance to investigate the weld forma-tion in weld bonding. Figure 8A is a photoof the experimental setup used to measurethe dynamic resistance. The weld currentand voltage data between the steel sheetswere collected using the welding monitor(MM-370A) made by Miyachi. To assessthe effect of the adhesive placement, thedynamic resistances on two interfaces be-tween the steel sheets were measured. Fig-ure 8B is a schematic of the measurementmethod where R1 is the dynamic resist-ance between the top and middle sheets,and R2 is the dynamic resistance betweenthe middle and bottom sheets. Five repli-

cates were performed for each type ofweld bonding, and the average dynamicresistances were reported.

During the weld cycle, a welding cur-rent of 9.0 kA is applied to start the join-ing process. The measurements of dy-namic resistance (R1) between the top andmiddle sheets during welding are provided

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Fig. 8 — Measurement of dynamic resistance between the workpieces. A — Test setup; B — dynamic resistance R1 and R2 between the workpieces.

Fig. 9 — Dynamic resistances between workpieces during weld-bonding. A — R1 between the top and middle sheets; B — R2 between the middle and bottomsheets.

Fig. 10 — Weld formation for different types of weldbonding.

A B

A B


in Fig. 9A. Because there is no adhesivelayer at the interface between the top andmiddle sheets in resistance spot weldingand weld-bonding type II, the dynamic re-sistance results for this specific interfaceduring the whole weld-bonding processare similar because of comparable contactconditions. However, when the adhesive isplaced in this interface as defined by weld-bonding types I and III, the dynamic re-sistance results during the first and secondstage are larger than those without adhe-sive, especially during the welding timefrom 50 to 250 ms. This is attributed to thefact that the residual adhesive left be-tween the top and middle sheets aftersqueezing increased the static contact re-sistance. Under the same welding currentduring the weld cycle, additional Jouleheat would be generated, thereby causingthe temperature of the steel sheets to risefaster than if no adhesive is present. Therapid rise of temperature would, in turn,increase the bulk resistivity of the steelsheets. As a result, the dynamic resist-ances between the top and middle sheetsof weld-bonding types I and III would belarger than those of spot welding andweld-bonding type II under the same

welding conditions.Similar results in Fig. 9B were found

for the dynamic resistances between themiddle and bottom sheets (R2) duringwelding. The spot welding and weld-bond-ing type I had similar dynamic resistancesbecause there is no adhesive at this inter-face between the steel sheets. The dy-namic resistances R2 of weld-bondingtypes II and III are larger than those ofspot welding and weld-bonding type I be-cause of the application of the adhesive onthe interface between the middle and bot-tom sheets. It could also be seen in Fig. 9that weld-bonding type III has the largestdynamic resistance on both interfaces dur-ing the welding time from 50 to 150 ms.This is attributed to the fact that the heatgeneration at the two interface influencescombine to cause the greater rise in tem-perature compared to the others duringthe weld cycle.

Weld Formation during Welding

The variations in dynamic resistancecould indicate a change in weld formationduring the weld-bonding process. Figure10 is a plot of the weld size at each of the

two interface locations in the three-sheetstack-up (represented by weld sizes A andB shown in Fig. 1) for different types ofweld bonding. As can be seen in Fig. 10,weld size A of weld-bonding types I and IIIinitiates at 230 ms, which is slightly earlierthan that for spot welding and weld-bond-ing type II (270 ms). Similar results werealso found for the formation of weld sizeB at the interface between the middle andbottom sheets with adhesive (Terokal5089), which initiated approximately 40ms earlier than the case without adhesive.As a result, the final weld sizes A and B areaffected by the increase of static contactresistance between the sheets caused bythe application of the adhesive. It couldalso be seen that for all types of weld-bonding, the weld nugget on the interfacebetween the middle (DP600) and bottomsheets (DP780) initiates earlier and is big-ger than that on the other interface. Thisis because the greater bulk resistivity ofthick DP600 and DP780 steels leads to arapid temperature rise by the Joule heat ascompared to SAE1004.

To understand the early initiation ofthe weld nugget for the case where the ad-hesive is present between the workpieces


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Fig. 11 — Initial temperature distributions with different adhesive locations. A — Spot welding; B — weld-bonding I; C — weld-bonding II; and D — weld-bonding III (welding time = 20 ms).

A B

C D

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during welding, a finite element model hasbeen employed to model the weld-bond-ing process (Ref. 19). The presence of theadhesive was modeled as additional elec-trical contact resistance between the steelsheets (Ref. 20). Figure 11 shows the cal-culated initial temperature field for differ-ent types of weld-bonding at a weldingtime of 20 ms during the weld cycle. Asshown in Fig. 11A, an elevated tempera-ture has built up at the two faying inter-faces between the multiple stacks of steelsheets during spot welding. More Jouleheat is generated around the interface be-tween the middle-to-bottom sheet inter-face due to the large static contact resist-ance and bulk resistivity of DP600 andDP780, compared to SAE1004. However,when the adhesive is placed on the top-to-bottom sheet interface in weld-bonding

type I (Fig. 11B), the maximum tempera-ture location has switched from the mid-dle-to-bottom sheet interface to the top-to-middle sheet interface. Figure 11Cshows that the heat generation for theweld-bonding type II where the heat hasbuilt up at the middle-to-bottom sheet in-terface with less heat at the top-to-middlesheet interface. Similar results were ob-served in Fig. 11D for the weld-bondingtype III. One can conclude from these re-sults that the presence of the adhesive in-creases the static contact resistance be-tween the steel sheets, and consequentlygenerates more Joule heat at the inter-faces with adhesive. The Joule heat gener-ation and localized temperature distribu-tion resulting from the application of theadhesive leads to the rapid rise of the tem-perature of the steel sheets to the melting

point (approximately 1500°C). As a result,the weld nugget on the interface with ad-hesive initiated early.

Weld Size and Hardness

The placement of the adhesive changesthe weld initiation and formation so thatthe weld quality might differ for eachweld-bonding type. Weld size is an impor-tant index to evaluate the quality of weld-bonding multiple stacks of steel sheets.Figure 12 is a series of photos of the met-allographic cross sections taken from dif-ferent types of weld-bonded joints pro-duced with the welding parameters listedin Table 3. It can be observed that the weldsizes A and B of weld-bonded type IIIjoints are the largest. Comparing the re-sults of spot welded and weld-bonded type

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Fig. 12 — Effect of adhesive placement on the weld size. A — No adhesive (spot welding); B — weld-bonding type I; C — weld-bonding type II; and D — weld-bonding type III.

Fig. 13 — Effect of adhesive location on the hard-ness in weld-bonding multiple workpieces.

Fig. 14 — Effect of adhesive location on the weld size. A — Weld size A; B — weld size B.

A

A

B

B

C D


I joints, the weld size A increases from4.20 to 4.57 mm because of the presenceof the adhesive, while the weld size Bchanges little, which is to be expected be-cause there is no adhesive in either case.The opposite phenomenon occurred inthe weld-bonded type II joint, which isconsistent with the previous results thatthe placement of adhesive increases thestatic contact resistance between the steelsheets and initiated the weld nugget ear-lier. Therefore, resistance welding of theworkpieces with adhesive generates moreJoule heat.

Beside the weld sizes, the microstruc-ture of the weld nugget was also examinedto assess the weld quality of the weld-bonded joint. To investigate the effect ofthe adhesive on the microstructure of theweld nugget in weld bonding, the Vickershardness along an oblique line throughthe top to bottom sheet was measured,and the results are presented in Fig. 13.The results show that the hardness of theSAE1004 is approximately 150 HV, referto distance 0–2 mm. The hardness of theDP780 steel is approximately 300 HV,refer to distance 10–12 mm, which is sig-nificantly greater than that of the ferriticstructure of the SAE1004 because of theDP780 microstructure containing marten-site and ferrite. The center portion of themeasurements is the hardness of the weldnugget, whose hardness is significantlygreater than either of the base metals be-cause it is fully martensitic. It should benoted that the hardness of the weld nuggetis nearly the same in all kinds of weldbonding, which means that the weldnugget microstructure is not a function ofthe adhesive presence. However, the dis-tribution of hardness in weld-bonding typeIII is the widest of the various types ofweld bonding, which suggest that the weld-bonded type III joints have the largestweld size.

To determine the increase in weld size bythe presence of the adhesive in weld bond-ing, five duplicates for each weld-bondingtype were measured metallographically.Figure 14 is a graph of the average weldsizes for the various types of weld bondingunder identical welding parameters. It canbe observed that the presence of adhesive inweld-bonding types I and III results in anapproximately 9% increase in weld size A ascompared to spot welding. The weld-bond-ing types II and III exhibit an increase of12.4 and 15.0%, respectively, as comparedto spot welding for weld size B. For bothweld sizes, weld bonding without adhesiveat the interface between the sheets is com-parable to spot welding (i.e., weld size A inweld-bonding type II and weld size B inweld-bonding type I). By comparing theweld sizes with various placements of theadhesive layer, the weld quality of weld-bonding type III is the best.

Conclusions

1. The static contact resistance be-tween the steel sheets during the squeezecycle in weld bonding is higher than that inresistance welding due to the presence ofthe adhesive.

2. To achieve the intimate contact be-tween the steel sheets, a minimum elec-trode force is required in resistance weld-ing of steels. The forces of 1.1, 3.1, and 4.6kN are required for 0.8-mm-thickSAE1004, 1.4-mm-thick DP600, and 1.8-mm-thick DP780, respectively.

3. The temperature distribution is lo-calized at the faying interface between thesteel sheets with the placement of the ad-hesive during the welding cycle. The dy-namic resistance between the steel sheetsin the weld-bonding process increasescompared to the resistance spot weldingbecause of the rapid rise of temperature.

4. The weld initiation time and weldsize for weld-bonded steels composed of0.8-mm-thick SAE1004, 1.4-mm-thickDP600, and 1.8-mm-thick DP780 areabout 40 ms earlier and 10% larger thanthat without adhesive under the samewelding parameters.

Acknowledgments

This research was supported by theGeneral Motors Collaborative ResearchLaboratory at Shanghai Jiao Tong Univer-sity, the Research Fund of State Key Labof MSV, China (Grant No. MSV-2010-04),and Project 50905111 supported by theNational Natural Science Foundation ofChina. The authors gratefully acknowl-edge the financial and technical supportprovided by GM-Research and Develop-ment to carry out the present work.

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adhesive placement in weld-bonding multiple stacks of steel sheets

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