The effect of galvanizing on the mechanical resistance and fatiguetoughness of a spot welded assembly made of AISI410 martensite

N. Becker a, J. Gilgert a, E.J. Petit b,n, Z. Azari a

a LaBPS-ENIM, 1, rue d'Ars Laquenexy, 57078 Metz Cedex 03, Franceb LEM3, Université de Lorraine, Ile du Saulcy, 57000 Metz, France

a r t i c l e i n f o

Article history:Received 12 September 2013Received in revised form31 October 2013Accepted 3 December 2013Available online 11 December 2013

Keywords:MartensiteSpot weldingGalvanizingFatigueMicrostructureHardness measurement

a b s t r a c t

Welding of high strength steels containing martensite is difficult due to the appearance of brittle grainsafter fast cooling. Galvanizing spot-welded specimens made of AISI410 solves this problem, and stronglyimproves the performances of spot welds. The mechanisms responsible for these achievements arepresently analyzed. The changes of the mechanical strength and fatigue toughness of spot-weldedsamples made of AISI410 after galvanizing have been studied. AISI410 is a chromium-rich martensite.Tensile tests on normalized specimens indicate that the yield stress is close to 1010 MPa and the UTS at�1270 MPa. The fatigue limit of raw AISI410 is at 690 MPa (ΔsL at R¼0.1). Specimens were cut followingthe rolling direction and spot-welded. Metallography reveals that spot welding produces: – hardmartensite at the center of the nugget; – a heat affected zone, which has been forged by the weldingelectrodes and recrystallized at high temperature; – and finally, a ring of grains tempered in a subcriticaltemperature range. Spot welds were tested in tensile–shear mode. Both overload failure and fatiguetoughness were evaluated. Annealing and galvanizing post-treatment have been used to improve themechanical properties of the spot-welded specimens. The improvement of the mechanical performancesof spot-welded specimens after post-treatments are reported as follows. First, we observed significantchanges in the modes of rupture both in overload failure and fatigue tests. The mechanical strength ofuntreated spot welds is controlled by the brittleness of the nugget and the forged heat affected zone.After post-treatment, the ultimate tensile strength is increased, and rupture results from fissurespropagating exclusively into the zone recrystallized in the subcritical range of temperature. This mode isgenuine to spot welds in martensite because of the high strength of the base metal. Second, three modesof rupture depending on the force range control the fatigue toughness. Annealing and galvanizing modifythe ranges. Galvanizing increases the endurance, and the fatigue limit by a factor �3.5. This effectis attributed to zinc brazing that strongly changes the distribution of stresses under tensile–shearsolicitation. Hardness measurements and tensile test on specific samples clarify the mechanical behaviorof the layers around the spot, and shed some light on the mechanisms of rupture. Lifetimes are controlledby the initiation of fatigue cracks.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Our research aims at evaluating the advantage of galvanizingpieces made of advanced high strength steels after forming andassembling, in order to avoid galling of GI and/or GA coatingsduring hot stamping. Presently, we report on the changes of themechanical strength and fatigue toughness of spot-welded cou-pons made of AISI410 martensite after galvanizing.

Previous papers addressed the cases of microalloyed, TRIP andmartensite steels [1–4]. We have found that the optimized bulk

mechanical performances of high strength steels are preserved orimproved after hot-dip galvanizing. On the contrary, surface-related mechanical properties, like fatigue toughness, change.The fatigue limit, which is controlled by: – the microstructure ofthe Fe–Zn alloyed layer, – the microstructure of the zinc top layer,and – eventual corrosion pits produced after pickling, is reducedbetween 10% and 30% [2]. This loss is compensated by theprotection against corrosion, which is a higher benefit.

Resistance spot welding is one of the primary methods to joinsheet metals for automotive components. Two thousand to 5000 spotwelds join parts in a car or a truck [5,6]. Mathematical models helpto optimize the number and location of welds in order to guaranteethe stiffness and lifetime of the welded parts [6–9]. In these models,the spot welds are described by links having a complex non-linearbehavior depending on the load in real situations [7,8].

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Materials Science & Engineering A

0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.msea.2013.12.008

n Corresponding author. Tel.: þ33 3 87 54 73 39, þ33 68 67 67 507;fax: þ33 38 73 15 366.

E-mail address: etienne.petit@univ-lorraine.fr (E.J. Petit).

Materials Science & Engineering A 596 (2014) 145–156

The strength of weld spots has followed the improvementof performances of the new high strength steels. On one hand,the total number of welds to build a structure can be reduced, onthe other hand, the behavior of each part in the whole must beconsidered while modeling the behavior of spot-welded structures[6,8]. Table 1 provides a synoptic view of recent progress in thefield. Only works dealing with tensile–shear tests are listed.Table 1 sketches present trends; it does not pretend to be anexhaustive review of the literature.

Data in Table 1 clearly evidence that the tensile strength of thebase metal is one of the parameters controlling the ultimatetensile strength of the weld spots. Our results involving nearlyfull martensite steel are among the best. This was expectedregarding the mechanical properties of the base metal.

Most reports available in the literature stress the fact thatstrongest welds fail by pulling out the welded button, instead ofbreaking the nugget (i.e., partial interfacial failure, or full inter-facial failure) (see for instance [10–12]). In most cases, experiencehas confirmed that failure by pullout occurs when the diameter ofthe nugget exceeds a critical diameter D given by D¼ 5

ffiffit

p, where t

is the thickness of the metal sheet. Large spots hamper partial orfull interfacial failure because stresses are distributed on theperiphery of the nugget. Experience confirms that overloadstrengths increase linearly versus the diameter. However, thestrength is not exactly proportional to it.

The width and thickness of the plate also determine thestrength of a spot weld, especially in tensile–shear tests. As amatter of fact, a stiff piece limits the rotation of the nugget;favoring shear stress at the nugget and reducing the opening ofthe welded plates (see Section 3.3). This explains the increase ofstrength and lifetime of our galvanized specimens (see below).In relation to this, it is important to remember that Lee et al.have shown that spot-welded samples tested under pure shearloading were 30% stronger than the ones tested under tensile–shear loading [9].

Since the high performance of new steels (like microalloyed,dual phase and TRIP steels) are grain-size dependent or multi-phase composites, the microstructure of the metal in the nuggetand in its surroundings is also critical [12]. Ultra-fine grained steel

can recrystallize at the border of the heat affected zone [13].The high carbon equivalent (from C, Si, Mn, P and S) of modernsteels induces the formation of martensite during the fast quench-ing following the solidification of the melted pool, and potentiallymakes the weld nugget hard and brittle [10]. For this reason,authors often measure and compare the hardness of boththe nugget and the heat affected zone with the one of the basemetal. These measurements confirm expectations and fears (seeTable 1).

On the other hand, hardness measurements also enable to inferabout the local mechanical properties because the yield strength(sy) and the ultimate tensile strength (UTS) are linearly correlatedto hardness [9,12,14]. This information is used to identify the weakpoint(s), locate the place where the cracks selectively open, anddetermine how cracks propagate. To this respect, our study is anew case. As a matter of fact, for the first time, the base metal(martensite) has the same strength as the weld nugget. A genuinemode of rupture could possibly occur. Moreover, since we knewthat hard martensite was brittle, we expected that a post-treatment could improve the performances of the welds. Actually,we found that annealing fulfilled the objective. Our research wasthen partly devoted to understanding the result.

Finally, we compare the mechanical behavior of spot-weldedmartensite plates before and after galvanizing. Galvanizing afterforming and assembling was first considered for anticorrosionprotection, especially in order to level the electrochemicalsurface potential of pieces made with different steel grades, andto prevent alloying, fissuring and galling of the zinc coating ofprecoated steel sheets during hot stamping. In fact, more isprovided since galvanizing the assembly after fabrication specta-cularly improves the strength and the lifetime of spot welds.It increases the stiffness of spot-welded parts, authorizing toreduce the number of welds. This article explains the reason,through the analysis of the mechanisms of rupture.

Galvanizing after fabrication prevents

� the difficulty of controlling spot welding of pre-coated steels(zinc explulsion, variation of electrical resistivity at the spot,reactivity of the electrodes with coating elements) [5,15,16];

Table 1Synoptic view of reports on tensile-shear tests of spot welds.

Author Base metal properties Sheet thickness Spot diameter Maximum strength Hardness (HV) Fatigue limitof the weld

Grade sy (MPa) UTS (MPa) (mm) (mm) (kN) Base metal Weld nugget (kN)

Lin [7] 185 300 1.0–1.5 6.5 5–8 – – –

Langrand [8] 170 300 1.2 6 5 100 300 –

Ozsarac [5] Low C – microalloyed (v) 1.0 5–7.5 1–1.1 (iv – IF) 123 332 –

Aslanlar [15] Low C microalloyed (ii) 1.2 6.5–9.0 6.5–7.5 pull out – – –

Vural [21,22] AISI304 326 667 1.03 4–6 3–5 190 350 2–3 (iii)IF (ii) 160 266 0.95 4–6 3–5 82 350 2.5–3combination 0.95/1.03 4–6 3–5 82/190 350 1.5–2.0

Lee [9] 245 365 1.0–1.4 5–6 7–11 183 323 41.5 (i)Ma [10] DP600 (ii) 370 636 1.24 4.5–6.0 17.5 200 425 1.5Sun [11] TRIP 800 450 820 1.5 4–7.5 27þ/�1.5 250 500 –

DP 800 (ii) 400 875 1.6 4–7.5 27þ/�0.5 250 450 –

Dancette [12] IF 260 260 408 1.5 4–6.5 7.5–13 250 475 –

DP450 (ii) 285 469 1.5 5–8 7.5–17.5 150 310 –

DP590 378 637 1.5 4.5–6 13–17 –

DP980 706 1034 2.0 5.5–7.5 21–35 –

TRIP 780 562 827 1.5 5.0–6.0 17–24 250 485 –

Present work AISI 410 2.0 6–8Raw 900 1270 20 395720 425735 1.570.5Annealed 1050 1270 30 400720 390735 2.570.5Galvanized 1050 1270 40 400725 385735 7.070.5

(i) Estimated from data provided in the paper. Preferred mode of rupture: sheet breaking; (ii) galvanized precoated steel; (iii) fatigue tests were stopped at 1.106 cycles;(iv) interfacial failure; and (v) electrogalvanized.

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156146

� as well as the so-called zinc embrittlement (precipitationof Fe–Zn intermetallic phases into the nugget, voids inducedby the variability of welding conditions, etc.) [10].

2. Experimental

2.1. Materials and treatments

AISI410 is a chromium-rich martensite (Cr: 13.12; Mn:0.39;Si:0.39; C:0.12; P: 0.026; So0.001; V:0.09; Cu:0.121; Mo:0.006;and Ni: 0.09 wt%). Coupons were cut from 2 mm thick sheets. Thecoil underwent continuous annealing following a thermal path foraustenizing and quenching in order to simulate an optimizedheating and quenching during hot stamping, as well as temperingduring cooling [3].

Samples have been job galvanized according to an industrialprocessing route optimized for standard and high silicon steels [3].Galvanized samples were dipped in a zinc alloy containing tracesof Ni, Al, Bi and Sn, for 4 min at 440 1C.

Reference specimens have been annealed in a bath of moltensalt following the same thermal route as the galvanized samples inorder to produce the same bulk metallurgical transformations.A molten alkaline bath was chosen for temperature compatibility.Experience learned that a brand new ultra-dry salt is requiredin order to avoid excessive surface oxidation and oxidation-stimulated chromium diffusion toward the surface. The remainingsalt at the surface after withdrawal was dissolved with water.

2.2. Flat normalized samples

Flat normalized samples were prepared according to therecommendations of NF-EN ISO 6892-1 (2009) norm for tensiletests. Samples were cut in order to align the tensile force duringtests with the rolling direction. These tests were performed inorder to evaluate the bulk mechanical properties of the base metalbefore and after galvanizing (see Section 3.1, Fig. 2), as well as theones after austenizing and fast quenching (see Section 4, Fig. 17).The effective working volume of the sample was 120�40�2 mm3.

Specimens for testing the fatigue toughness of the base metalwere machined according to the norm ISO-DIS 1099 (2006) (seeSection 3.1, Fig. 4). Samples were cut in order to align the tensileforce during tests with the rolling direction. Special attention wasdevoted to the preparation of the sample: they were machinedusing brand new cutting tools, edges were cut, and cut sides weremirror-polished in the longitudinal direction. The effective work-ing volume of the sample was 30�18�2 mm3 [4].

2.3. Spot-welded samples

Geometry and dimensions were the same for the spot-weldedsamples used in tensile tests and tensile-fatigue tests. Specimenswere made from flat plates 40 mm wide�150 mm long with anoverlap of 50 mm. Steel plates were cut and machined so that thetensile direction during the mechanical tests was parallel to the

rolling direction (Fig. 1). The single spot weld was in the middle ofthe overlap at 125 mm from the border in length.

The electrodes of the welding machine that were initially 7 mmin diameter became blunt due to the hardness of the steel. Finally,the diameter of welding mark was 6 mm (see Section 3.2). Wetested a large range of welding currents and forces applied onelectrodes. The selected welding parameters were: force appliedon the electrodes¼5 kN; I¼14 kA; current duration¼0.4 s. Theseconditions are not standard because they are above the thresholdcurrent causing liquid metal expulsion. However, they maximizethe force of the first crack during tensile–shear tests of singlespot welds (before galvanizing). Metal expulsion does not alter thescientific conclusion of the paper. Technological issues will bediscussed below. Spot-welded assemblies were partially brazed atthe overlap during galvanizing.

2.4. Testing the mechanical properties and sample preparation formetallography

Tensile tests on spot-welded specimens were performed atroom temperature using a Zwick Z250, equipped with a charge cellof þ/�250 kN. The stress rate was kept at 200 N/s until the yield;then the displacement was kept at 5 mm/s.

Tensile–fatigue tests were performed using a Schenk Hydropulsequipped with a charge cell of þ/�100 kN. Tests were performedat 30 Hz using a sinusoidal waveform. The load ratio during thecycles was: R¼smin=smax ¼ 0:1 during the tests on flat samplesand R¼ Fmin=Fmax ¼ 0:1 during the tests on spot-welded assem-blies. The Wöhler curves of flat samples give the lifetime ofsample versus the variation of the tensile stress during cycling:Δs¼smax�smin. The Wöhler curves of spot-welded assembliesgive lifetime of sample versus the elongation of the variation ofthe tensile force: ΔF ¼ Fmax�Fmin.

Mechanical behavior and fatigue toughness of welded sampleswere tested in the tensile–shear mode. Since the tensile–shearspecimen are asymmetrical in their plane, we put two spacershaving the same thickness as the welded metal sheets into the gripof the testing machine in order to restore symmetry and reducesheet bending and nugget rotation during both overload failuretests and fatigue tests.

Classical metallographic polishing was performed using SiCpapers down to grid P4000. The microstructure was revealedusing the V2A reagent (10 min at room temperature).

A Duramin microdurometer has been used for microhardnessmeasurements. The specimen microindented at intervals of200 mm using the Vickers scale at an applied load of 300 g (HV0.3).

3. Results

3.1. Properties of the base metal before and after galvanizing

Martensite was austenized for 5 min at 950 1C, quenched andtempered in order to simulate the microstructure of the steel afterhot stamping. This optimized martensite has been galvanized for4 min at 440 1C in a zinc alloy containing nickel, tin, bismuth andaluminum. Fig. 2 displays the engineering curves of the tensiletests performed on flat samples made of this optimized martensite‘as delivered,’ after galvanizing and after annealing in the salt bathat 440 1C for 4 min.

The yield stress of the bare martensite is close to Rp0.2¼1010 MPa. The ultimate tensile strength is about UTS¼1270 MPa.After galvanizing, the yield increases slightly to 1050 MPa, becauseof the appearance of a little plateau at the yield. Bulk mechanicalproperties are not significantly modified.

Fig. 1. Shape and dimensions of the spot-welded samples. Plates have been cut andwelded so that the rolling direction of the steel sheet is parallel to the force duringtensile tests.

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156 147

Fig. 3 presents a cut view of the coating produced aftergalvanizing the chromium-rich martensite. The coating is�30 mm thick, with an intermetallic layer �9 mm thick. Detailsabout the composition and microstructure analysis can be found inRef. [3].

Fatigue tests were performed on normalized flat samples inorder to evaluate the evolution of surface properties after galva-nizing and annealing. Results are presented in Fig. 4.

Galvanizing and annealing do not significantly modify endur-ance because lifetime remains at 750% of the values measuredon the raw steel. On the contrary, fatigue limits are sensitive tosurface changes that modify the probability of crack openingand/or propagation. Fatigue limits (expressed as the variation ofstress range: ΔsL) are, respectively, at �760715 MPa, 690715 MPa and 640715 MPa on annealed, raw and galvanized speci-mens. These results confirm and complete the ones in Ref. [3].The global increase of the measured fatigue limits is due to theimprovement of the specimen finishing.

Variations of the fatigue limits of the three kinds of specimensare significant. First, the reduction of the fatigue limit of galva-nized sample is especially low (�7%) when compared to the effectof galvanizing other steel grades [1,2,4]. The decrease is attributedto the limited fatigue toughness, or defects, of the coating. Second,the increase observed in annealed samples could be due tothe pinning of dislocations that slows down the opening of fatiguecracks. Another explanation could be that annealing relaxesresidual surface stresses.

3.2. Metallographic analysis of the spot weld

Fig. 5 presents the metallographic view of a cut through aspot weld.

The indentation results in molten metal expulsion duringwelding. The density of porosity is very low confirming theefficient forging. As expected, as a consequence, tensile and fatiguetests produced practically neither partial nor full interfacial fail-ures because cracks couldn't propagate through pores. Ma et al.claim that indentation produces surface cracks [10]. Such crackswere not observed in our samples.

The dendrites in the nugget grow toward the center of themelted zone (FZ). The oriented solidification is due to the fact thatsolidification starts at the electrodes, which are water-cooled. Theforged heat affected zone (HAZ), and rings of re-crystallized andtempered martensite (RZ) surround the nugget. The base metal(BM) is further outside the rings. The weld nugget, which spreadsoutside the welding mark, is �8.5 mm in diameter. Due to metalexpulsion, the thinnest section is in the HAZ.

Fig. 6 presents a closer view of metallographic micrographsexamined by optical microscopy.

Image analysis indicates that the relative volume fractionof phases in the base metal were as follows before galvanizing:martensite (brown and dark brown): 85%, and ferrite (white):7%, carbides (black): 8%. The microstructure of HAZ is similar,

Fig. 3. Microstructure of the coating produced after galvanizing the martensite at440 1C for 4 min.

0

300

600

900

1200

1500

Strain (%)

Stre

ss (M

Pa)

Raw

Galvanized

0 5 10 15

Fig. 2. Mechanical behavior of the raw and galvanized martensite.

Fig. 5. Side view of a cut through a spot welding of two martensitic plates. (BM¼base metal; HAZ¼heat-affected zone; RZ¼recrystallized zone; and FZ¼melted zone.)

Δ σ

(MPa

)

600

700

800

900

1000

1,E+04 1,E+05 1,E+06 1,E+07 1,E+08

Number of cycles to failure

Galva 4 minRawAnnealed

R = 0,1RD // F

Fig. 4. Comparison of Wöhler curves measured (R¼0.1) with flat samples, respec-tively,, made of (a) raw steel (optimally tempered martensite); (b) martensitegalvanized; and (c) martensite annealed for 4 min at 440 1C in a salt bath in orderto simulate the annealing resulting from galvanizing. Samples were cut andmachined so that the direction of the tensile force during the tests were parallelto the rolling direction. Fatigue limits are observed at �760715 MPa,690715 MPa et 640715 MPa (þ10%, �7% par rapport au brut).

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156148

but grains have been plastically deformed due to forging. Heatingalso favored the growth of carbide grains (in black) in the HAZ.Domains of martensite are coarse in the fusion zone and moredispersed in the base metal. In the RZ, the martensite domainsseem to be divided by dark grain boundaries, which couldevidence the precipitation of finely dispersed carbides.

Metallographic analysis of a dual-phase (DP600) steel by Maet al. led to quite similar observations of large dendrites in thenugget and of a series of layers surrounding the nugget [10]. Maet al. did not observe an RZ zone. The RZ zone in our samples couldbe the one that Dancette et al. call the SCHAZ (subcritical heataffected zone) [12]. Following Dancette, this zone is expected tohave been annealed at a temperature lower than Ac1. Khodaba-khishi claims that this zone is prone to re-crytallization and graingrowth in fine-grained steels [13]. In our case, short annealing atrelatively high temperature is able to temper martensite and favorthe migration of carbon to the grain boundaries.

Similar observations were performed on weld spots afterannealing and galvanizing. Pictures do not differ significantly forthe eye. Image analysis indicates that the relative volume fractionof phases in the base metal were as follows after galvanizing:martensite: 82.5%, ferrite: 4%, and carbides: 13.5%.

3.3. Overload ruptures of the spot welds

The nuggets were pulled out after all the welded assemblieswere torn in overload rupture tests. Fig. 7 presents typical force-displacement curves recorded during these tensile–shear tests.

Job galvanizing increases the stiffness of the welded coupons.This is due to the fact that brazing by zinc hampers both thebending of the plates and rotation around the weld nugget asillustrated in the numerical simulation presented in Fig. 8.

The numerical simulation has been performed using thecommercial finite element code MARC. The difference betweenthe two cases relies on the extent and resistance of the linked

region between the plates. Calculation shows that brazing partiallytransfers the normal tensile load at some distance from the weldto the alloyed bonding layer. Brazing keeps the metal sheetsparallel, reduces the rotation, and shares the shear stress due tothe moment bends of the metal sheet between the nugget and thebonding layer. Bending is especially localized outside the brazedarea of the ‘galvanized’ sample. The overall result is that stressesare distributed over a larger volume of matter and the apparentstiffness of the assembly of the ‘galvanized’ sample is increased.

Force-displacement curves of Fig. 7 confirm that annealing andgalvanizing improve the mechanical resistance of the weldedassemblies. The force at the first crack is presently considered asthe measure of the mechanical strength of the weld. This choicecontrasts with the practice in the literature, which reports theultimate tensile strength. Arrows in Fig. 7 point out the first cracks.

Fig. 6. Metallographic observations of the microstuctures of steel at different places: (a) base metal (BM); (b) recrystallized zone (RZ); (c) heat-affected zone (HAZ); and(d) melted zone (FZ).

0

10

20

30

40

50

0 0.4 0.8 1.2Elongation (mm)

Forc

e (k

N)

a)

b)c)

Fig. 7. Typical curves of force versus displacements recorded during the overloadrupture of spot welded assemblies: Sample (a) was made of raw steel – notreatment was applied on the spot weld; (b) the sample was annealed for 4 minat 440 1C after spot welding; and (c) the sample was galvanized after spot welding.

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156 149

Rounded curves correspond to continuous displacement andtearing. Force drops are due to the release of stresses produced bysudden crack hopping. These drops are observed during tests onthe three kinds of samples, but they are more frequent and largerwith the annealed samples. A sharp drop is always observed onforce-displacement curves when the brazed area of the galvanizedsample breaks. The drop is then correlated to a rotation of thenugget and plate bending. After the rupture of the brazed area, theforce-displacement curve restarts from the range of force corre-sponding to annealed samples.

Eight samples of each kind were broken in order to evaluatereproducibility. Table 2 summarizes the stiffness and the loweststrengths of these repeated experiments.

The variance of stiffness measurements is 2% of the mean value.The brazing produced by job galvanizing increases the apparentstiffness of the specimens by 30%. The lowest strength of spot-welded AISI410 without post-treatment is 17.5 kN. It increases to26 kN after annealing at 440 1C for 4 min. Galvanizing stillincreases the minimum tensile–shear strength to 50% above theone of the annealed samples.

These values can be compared to those reported by Dancettefor spot welds of 8 mm in diameter on TRIP780 (17–25 kN) andDP980 (21–35 kN) samples (cf. Table 1) [17]. It is, respectively,twice and ten times those reported on microalloyed and lowcarbon steels, by Aslanlar et al. [15] and Ozsarac [5]. None of theauthors cited in Table 1 reported a ‘first crack’ and force dropbefore the deformation proceeds continuously. The ‘first crack’could be a peculiarity of spot welds of martensite. In the case ofthe galvanized samples, the crack related to the failure of thebrazed area merges with the one of the martensite weld.

The statistical character of cracking appears through the dis-tribution of strengths. The Weibull's plot in Fig. 9 presents astatistical analysis of the tests data. It gives the cumulativefrequency of rupture versus the force where the first crack occurs.The cumulative frequency (CF) on the Y-axis is scaled according tothe corresponding reduced variable X ¼ ðs�s0Þ=Δso of a normallaw of probability. The co-ordinate of sample (i) is given byCLðXiÞ ¼ R Xi

�1 expðX2ÞdX= R1�1 expðX2ÞdX. Straight lines on such

plots are called Henry's lines. They reveal a normal probabilitydistribution, and enable to identify the center (so) and variance

(Δsο) of the distribution. The parameters of Henry's lines arereported in Table 3.

Centers and variances confirm that mechanical performances ofthe whole population are improved after annealing and galvanizing.Annealing and galvanizing cumulatively improve the strength.

3.4. Rupture modes in overload failure

Tensile–shear tests have been interrupted in order to observewhere the rupture starts and the path of propagation. Fig. 10presents typical observations.

During the overload failure test of assemblies made of raw steel, acrack initiates at the HAZ at the top of the notch made by the platesat the weld spot (Fig. 10a). The crack grows in the HAZ around thenugget to some extent before the nugget breaks. Frequently, thecrack penetrates the nugget, producing a partial interfacial failure.

During the overload failure tests of annealed (Fig. 10b) andgalvanized (Fig. 10c) samples, the crack grows through the thicknessof the sheet and reaches the outer surface through the RZ. Then, thecrack propagates, circles the nugget following RZ, and breaking iscompleted by pulling out a button that leaves the nugget in one piece.

Dancette et al. have listed three mechanisms of overloadrupture [12].

The mechanism responsible for the pull-out failure of mild steelcannot account for our observation because failures do not presentlyproceed through stretching and necking of the BM [7,9,11,12]. Thestiffness of our plates, which are 2 mm thick and made of a high yieldstrength martensite, prevents this mode of rupture [9].

The rupture of the HAZ of the weld spot of martensite without anypost-treatment is characteristic of ‘semi-brittle’ rupture [12]. Thenugget does not withstand the stress concentration at the tip of thecrack. Without post-treatment, the weld spot behaves in tensile sheartest like a small diameter weld (as if D was less than 5

ffiffit

p). The

electrode indentation is presently partially responsible for the weak-ness of the HAZ during the overload tensile–shear tests.

Ruptures of annealed and galvanized spot welds are similar tothe failure of large weld spots on TRIP780 steel [12]. Dancetteexplains this behavior as a ductile shear slip through the weakestlayer surrounding the nugget. The shear slip is activated by the

Fig. 8. Bending and rotation of spot-welded steel plates under tensile-shear stresscomputed numerically using MARCs. (a) Raw steel plates are linked by amechanical bond at the spot weld and (b) the brazing of galvanized samples ismodeled by a mechanical bond between plates. Strains have been exaggerated forthe sake of clarity.

Table 2Strength and stiffness measured in tensile-shear tensile tests.

Apparent stiffnessat 15 kN (106 N/m)

Lowest tensile-shearstrength (kN)

Untreated AISI 410 4771 17.5Annealed at 440 1C – 4 min 4871 26Galvanized at 440 1C – 4 min 5971 38.5

0 10 20 30 40 50Force (kN)

Cum

ulat

ive

freq

uenc

y

-1.2

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9

1.2

X

Raw LAnnealed LGalva L

50%

90%

10%

Fig. 9. Weibull's plot of forces for the first crack during overload failure tests undertensile–shear stress.

Table 3Parameters of Henry's lines on the Weibull's plot fig. 9.

so (kN) Δso (kN)

Untreated AISI 410 – L 19.5 4.5Annealed at 450 1C – 4 min 34 5.7Galvanized at 450 1C – 4 min 42.5 4.5

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156150

moment that produces the rotation of the nugget. This momentumis at maximum on the borderline of the nugget [10,18]. The ductileshear slip relaxes the stress at the tip of the notch and preventsinterfacial failure. In TRIP steel, the weak layer was the subcriticalheat affected zone (SCHAZ). In our case, it seems to be the RZ.

So, in conclusion, the increase of the strength revealed inFigs. 7 and 9, as well as the changes in the mechanisms of ruptureof the annealed sample (cf. Fig. 10), suggests that annealing andgalvanizing alter the microstructure of the nugget and the HAZ.This topic will be further examined in Section 4.

3.5. Fatigue failure of the spot welds

Fig. 11 presents the fatigue lifetime of spot-welded couponstested in tensile–shear mode. Open symbols and arrows are fortests interrupted before failure. Historical experience has producedthe criterion that infinite endurance is expected for flat steelsamples unbroken after 106 cycles. For this reason, high cyclefatigue tests are usually interrupted after 106 cycles. Our experi-ence is that tests must be extended to 107 cycles for spot-welded

assemblies. The maximum force variation providing infinite life-time (ΔFL) is the fatigue limit. In order to guarantee infiniteresistance to fatigue, the fatigue limit should be used for dimen-sioning structures submitted to cyclic loading and unloading.

The Wöhler curves of raw and annealed samples are similar.One can see as an inconsistency that annealing the spot weldimproves its ultimate tensile strength but not its fatigue lifetime.

The shift of the curve related to galvanized samples reveals animprovement of endurance. For instance, the lifetime of galva-nized samples is improved by one order of magnitude at 8 kN. Themodification of the slope of the Wöhler curve confirms a change inthe mechanism of failure.

Results of Fig. 11 indicate that the fatigue limit of spot-weldedassembly made of raw martensite without post-treatment is ΔFl�1.570.5 kN. This value is comparable to the one of Ma andcollaborators on DP600 (reported as independent of the nuggetdiameter) (cf. Table 1) [10].

After annealing for 4 min at 440 1C, the fatigue limit isincreased to �2.570.5 kN. The fatigue limit of galvanized spot-welded coupons is at ΔFl �7.070.5 kN. This last increase of thefatigue limit is presently the most spectacular benefit of jobgalvanizing in respect to its mechanical performances in the field.

3.6. Rupture modes in fatigue failure

At first sight, the examination of fractured samples after fatiguetests reveals a diversity of situations. We found that simple criteriaare able to reduce the apparent complexity. The following

Fig. 10. Propagation of cracks around the spot weld during overload failure under tensile–shear stress. (Presently, the tests were interrupted before complete breaking.) (a) Asample made of raw steel; (b) the spot-welded sample was annealed after welding; and (c) the spot-welded sample was galvanized after welding.

0

4

8

12

16

1,E+03 1,E+04 1,E+05 1,E+06 1,E+07 1,E+08Number of cycles to failure

Δ Fo

rce

(kN

)

Galva

Raw

Annealed

R = 0,1

Fig. 11. Wöhler curves of tensile–shear fatigue experiments using spot-weldedassemblies. Open symbols are for experiments interrupted before fatigue failure.(The force ratio during cycling was: R¼(Fmin/Fmax)¼0.1).

Fig. 12. Modes of rupture of our samples in tensile–shear tests. Sizes anddeformations are exaggerated for better readability. Arrows represent crack paths.

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156 151

description emerged from the observation of more than 50fractures, ordered after their stress range. The sketch in Fig. 12,which summarizes the modes of rupture, is a side view of a cutthrough a weld spot under tensile–shear test. Arrows representthe crack paths through the metal plates.

Cracks always open at the internal side of the metal sheetwhere the force is applied. There is a symmetrical distributionof stress on plates bearing the load on both sides of the weld spot.Of course, cracks do not start from the side opposite to the spotbecause the stress is low at that place [10,18].

In our samples during fatigue tests under high tensile–shearstress, ductile and fragile ruptures start from the heat-affectedzone (HAZ). The crack starts from the ‘notch’ at the border of theweld point as pointed out by the HAZ arrow in Fig. 12. Often,this mode of rupture leads to pulling out the weld button.However, from the ‘notch,’ the crack can also partially or fullycleave the nugget, depending on sample rotation and on therelative mechanical resistance of the HAZ and the weld nugget.These (exceptional) ‘partial interfacial failure’ and ‘full interfacialfailure’ of the nugget are, respectively, referred to as PIF andFIF in Fig. 12. The vicinity of the force range used in the overload

experiments, and the similarity of the rupture modes, led tocalling this kind of fatigue failure as ‘oligocyclic fatigue’ rupture.

In our samples, and under moderate tensile–shear stress, theso-called high cycle fatigue cracks start in the recrystallized zoneor in the base metal. These are, respectively, referred to as ‘RZ’ and‘BM’ in Fig. 12.

Rotation of the sample, which influences the probability ofoccurrence of PIF, FIF and BM cracking, depends on the steelstrength and the width of the metal sheet (material and samplegeometry; cf. Fig. 8). Galvanizing also hampers rotation.

Fig. 13 is a plane view of the crack paths around the weld spot,which have been observed under overload and fatigue tensile–shear tests. The crack always opens between the metal sheets.The stress is maximum at point 1 [10,18].

In our samples under high tensile–shear stress (‘oligocyclicfatigue’), the front line of the crack is horizontal since it starts fromthe joint of the two steel plates. The rupture propagates throughthe HAZ in a plane nearly parallel to the metal sheet surface.

� Usually, crack splits in two front lines that propagate symme-trically, circling the nugget until pull out (paths 4 and 40).

� PIF and FIF occur when these front lines enter the nugget.� The button can also be pulled out from both sheets when four

crack lines propagate symmetrically on both sides of the piece.This happens when one crack opens simultaneously on eachsheet, and the two cracks split and propagate separately andsymmetrically in the two sheets around the nugget (see Fig. 12).

In our samples under ‘high cycle fatigue tests,’ the crack opensin RZ. The front line propagates vertically in the RZ, perpendicularto the metal sheet surface, until it reaches the opposite side of theplate. Then, the crack front line splits; cracks continue along paths2 and 20; turn around the nugget remaining in RZ; and suddenlyturn into the metal sheet perpendicular to the pulling force or in adirection 451 apart. So, in contrast to overload failure and ‘oligo-cyclic fatigue’ ruptures, ‘high cycle fatigue’ ruptures do not pull outthe weld button.

Fig. 13. Observed fatigue crack paths around the weld spot in our samples duringtensile–shear tests. The size of the weld spot is exaggerated.

Fig. 14. Typical ruptures of spot welds after tensile–shear fatigue tests. (a) rupture in the HAZ due to the so-called oligocyclic fatigue; (b) rupture from RZ due to ‘high cyclefatigue’; and (c) rupture of a galvanized sample in the BM after brazing has failed.

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156152

Fractures mixing cracks produced by ‘oligocyclic fatigue’ and‘high cycle fatigue’ produce complex facies, where one candistinguish characteristics described above.

Finally, ruptures in BM at some distance from the weld spotgenerally produce straight fatigue crack lines perpendicular to themetal sheet surface and to the pulling direction (paths 5 and 50).The momentum associated with the sheet bending causes sheetbreaking in BM. It is due to the stiffness of the plate and the factthat the spot weld and brazing create a cantilever.

In summary, only three elementary rupture modes (starting,respectively, from HAZ, RZ and BM) are enough to analyze andcategorize all the fracture facies.

Fig. 14 shows typical examples of samples broken duringtensile–shear fatigue tests through these elementary mechanisms.

In Fig. 14a, the weld button has been pulled out after fatiguetesting in the oligocyclic range of stress variation. The sample wasmade of raw martensite. The weld was not post-treated; it brokeafter 32,823 cycles at ΔF¼9 kN. There is a hole in the sheet at thebottom.

Fig. 14b shows a typical failure caused by sheet breaking in thehigh cycle fatigue range of stress variation. The sample was madeof raw steel sheets; the weld broke after 2,695,602 cycles withΔF¼3 kN. The integrity of the nugget in the upper plate ispreserved. The dashed line indicates the border of the remainingpart of the broken plate on the opposite side.

In Fig. 14a, arrows point out the tips of the crack simultaneouslyinitiated in RZ, which has crossed the sheet and has startedrunning the paths 2, 20, 3 and 30. This example demonstrates thattwo types of cracks can open simultaneously, mixing the rupturemechanisms in various ways, depending on the relative speed ofcrack propagation. More than two cracks can open simultaneously.

Fig. 14c corresponds to the fatigue failure of a piece galvanizedafter spot welding. The metal sheet broke in the BM after 241,253cycles at ΔF¼9 kN. The rupture of galvanized samples uponfatigue occurs anywhere in the BM between the weld spot(present situation) and the border of the brazed plate. The crackfront line is sometimes curved when path 5 and 50 starts in thevicinity of the spot weld because the spot weld, which is thesupport of the cantilever, produces a stress concentration.

Table 4 summarizes modes of rupture, paths and ranges ofstress variations (at R¼0.1), where they have been observed in ourtensile–shear experiments.

Possible mixing of rupture modes explains that the ‘oligocyclic’fatigue range and the ‘high cycle’ fatigue range overlap in experi-ments performed on samples made with raw steel and aftergalvanizing. In the range overlap (between 9 and 8 kN on rawsteel, and between 12 and 10 kN on galvanized samples), propaga-tion of cracks opened in HAZ and RZ proceed simultaneously,

producing either rupture by pull-out or sheet breaking. Theseruptures leave complex fracture facies. PIF and FIF were exceptionsin our experiments, probably because the HAZ were thin due tometal ejection during welding.

The fatigue limits of samples made of raw steel, as well as theone of annealed samples, are, respectively, limited by the fatiguetoughness of HAZ and RZ. Annealing significantly decreases thespan of forces of the ‘oligocyclic fatigue’ range, as well as the rangeof lifetime. So, fatigue failure of annealed samples is dominated bythe fatigue toughness of RZ. The fatigue limit of annealed samplesis significantly increased.

Brazing resulting from galvanizing increases the fatigue limit ofspot-welded metal sheets. This beneficial effect forces the appear-ance of a new mode of failure (sheet breaking in BM), whichpushes upward the lower limit of force range where crack openingin RZ occurs.

Lee et al. and Ma et al. have reported similar Wöhler curves forspot-welded specimens. Lee pointed out that the initiation offatigue cracks was the lifetime-determining step for high strengthsteels. Ma et al. observed that the endurance of spot-weldedDP600 continuously galvanized in line, was independent of thespot diameter in the ‘high cycle fatigue’ range. Then, theyidentified a series of defects where the cracks start, pointing outinclusions and impurities that degrade the interface cohesion ofthe weld and zinc embrittlement of the grain boundaries in thenugget. Finally, they proposed that the zinc expulsed duringwelding and accumulated at the notch between the sheets couldfix the fatigue toughness after welding of pre-coated galvanizedhigh strength steels.

Ma et al. recognized that fatigue cracks in the ‘high cyclefatigue’ range propagate the following paths: 2 and 20 and 3 and 30.They reported that cracks similar to the one in Fig. 14c occurred.However, the case was not analyzed because the paper was notfocussed on factors limiting the fatigue limit.

Wang and Barkey investigated the opening of fatigue cracksusing X-ray imaging and found the same three modes as we havedescribed in Fig. 13 [19]. Too deep comparison with their resultsmay be cautious since they worked on galvanized and galvan-nealed low strength steels. Nevertheless, they provide finiteelement numerical evaluation of the Von Mises stress distributionsaround the notch, and show that the maximum stress is at somedistance from the nugget border, behind the tip of the notch.

Lee et al., Ma et al., Wang et al., and Vural et al. found fatiguelimits close to 1–2 kN [9,10,19,21,22]. We report that job galvaniz-ing after spot welding increases the fatigue limit by a factor 4.5,and favors fatigue cracks along path 5 and 50 (Fig. 14c and Table 4).This limit at 7 kN may be compared with the one reported in Fig. 4for normalized galvanized samples (ΔsL�650 MPa). Considering

Table 4Mode of rupture observed in shear-tensile experiments according to the range of force and lifetime of the spot weld.

Range: ΔF (kN) Lifetime: N (cycles) Crack opens at Progagationand rupture

Raw metalOligocyclic fatigue 48 o5.104 HAZ Pull-outHigh cycle fatigue 9–2 3.104–1.106 RZ Sheet breakingFatigue limit �1.5þ/�0.5 41.107 – –

Annealed at 440 1C for 4 minOligocyclic fatigue 410 o2. 104 HAZ Pull-outHigh cycle fatigue 9–3 41.106 RZ Sheet breakingFatigue limit �2.5þ/�0.5 41.106 – –

GalvanizedOligocyclic fatigue 410 o1.105 HAZ Pull-outHigh cycle fatigue 12–9 5.104–3.105 RZ Sheet breakingCracking apart from spot 9–7 2.105–3.106 BM Sheet breakingFatigue limit �7.0þ/�0.5 41.107 – –

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156 153

the load, the fatigue limit of a flat sample 40 mm wide and 2 mmthick (cf. Fig. 1) should be close to 50 kN. So, the following must beconsidered:

� on one hand, that the shear stress due to bending adds to thetensile stress (cf. Fig. 8);

� and, on the other hand, the effect of stress concentrations bothat the border of the brazed area (cf. Fig. 8) and at the vicinity ofthe weld spot [18].

Ma et al. observed what they called the mode of rupture IV,which is similar to the one we describe by fatigue rupture in BM(paths 5 and 50) [10]. These fractures were exceptions in Ma'sexperiments because the yield strength of the DP600 base metal islower than the one of martensite. Plastic deformations of the platein Ma's specimens curve the fatigue cracks and give them a similarappearance to the ones observed in the high cycle fatigue range.

4. Mechanisms of failure

In summary, the overload failure of weld spots in martensiteproceeds by pull-out. Galvanizing, and the correlated annealing,increase the ultimate resistance to overload failure. This improve-ment is due to the fact that the mechanism of rupture changesfrom a ‘semi-brittle’ fracture of the HAZ and the nugget tofissuring and shear slip of the RZ.

Galvanizing and annealing increase the lower limit of the forcerange of the ‘oligocyclic fatigue’ rupture mode, as well as the fatiguelimit. Three modes of fatigue failure have been identified. The brazingresulting from galvanizing produces a synergy to spot welding: itshifts upward the lower and upper limits of the ‘high cycle fatigue’force range. Brazing increases significantly the fatigue limit of spot-welded assemblies, and incidentally favors the mechanism of fatiguefailure in the base metal at some distance from the weld nugget.

Additional experiments have been performed in order to shedsome light on the reasons of these changes.

Fig. 15 presents the maps of microhardness filiations performedon cuts through spot welds. The samples were the same as theones used for metallography. Variations of hardness on maps arecorrelated to the BM, RZ, HAZ and FZ in Fig. 5. The values of thehardness along the dashed filiation lines in Fig. 15 are provided inFig. 16. Arrows point out the positions corresponding to the breaksin the line tagged in Fig. 15. Table 5 provides the mean values andvariances.

Hardness is at maximum at the center of the melted nugget(FZ) and in the HAZ on untreated samples because the steel hasbeen rapidly quenched there from the melt and from the

intercritical temperature range. The variances of the hardnessdistribution in FZ and HAZ evidence anisotropy of mechanicalproperties at these places, due to finely dispersed soft and hardphases alternating through dendrites. The hardness is reduced inRZ by about 50 HV after welding. This phenomenon is referred inthe literature as the ‘HAZ softening.’ The phenomenon occurs inhigh strength steel containing significant amount of martensite.Sun et al. reported the phenomenon in DP800, but not in TRIP 800[11]. It is attributed to the tempering of martensite in thesubcritical range of temperature (ToAc1).

After annealing and galvanizing the hardness in FZ and HAZ arebrought back to the value of the base metal. Nevertheless, thevariance remains larger than the one of the BM as expected frommetallographic observation. Annealing at 440 1C and galvanizingare not able to reform the austenite and restore the temperedmartensite. So, RZ remains soft after annealing.

Hardness is said to be linearly correlated to the yield strengthand ultimate tensile strength [9,14,17]. Fig. 17 presents tensile testsperformed on normalized flat AISI410 samples; first, austenized at1000 1C and quenched in water, and second, tempered at 440 1Cfor 4 min. These tests were performed in order to clarify therelationship between the hardness and the mechanical behavior.

Mean hardness on the samples were the following: – beforeaustenizing (raw martensite): 411 HV; – AISI410 austenized andquenched: 472 HV; – after tempering for 4 min at 440 1C: 407 HV.The evolution of hardness corresponds to the observation of theweld nugget after welding and annealing/galvanizing. Tensile testsevidence that the hardness increase is correlated to the formationof a brittle microstructure which makes the material so sensitiveto defects that the yield and ultimate tensile strength are practi-cally reduced. This observation is coherent with the observation ofa ‘semi-brittle’ behavior of the untreated weld spot during over-load tensile tests.

Fig. 17 also demonstrates that tempering restores ductility andmechanical resistance. The strength of the quenched martensiteafter short tempering is superior to the one of the raw martensite(Rm¼1400 MPa) (compared to Fig. 2). Figs. 15 and 16 stress thepeculiarity of the spot weld in martensite. As a matter of fact, bothBM and (FZþHAZ) are stronger than RZ. This explains the changeof the behavior of the spot weld in overload tensile tests afterannealing since fissures are confined and guided in RZ duringoverload tensile test when the samples have been annealed orgalvanized. This explains that overload failure proceeds by pulloutdespite the indentation by the welding electrodes.

The same fatigue lifetime of untreated and annealed sampleduring fatigue tests in the oligocyclic range of force (Fig. 11)(paths 4 and 40 in Fig. 13) contrasts with the improvement of themechanical behavior of the HAZ upon annealing (Figs. 15–17). This

Fig. 15. Mapping of microhardness measurements on the cut of weld spots. (a) Spot-welded raw metal steel sheets; (b) idem after annealing for 4 min at 440 1C; and(c) same as (a) after galvanizing for 4 min at 440 1C.

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156154

apparent contradiction can only be reconciled if the lifetime iscontrolled by the initiation of the crack by a mechanism that is notaffected by the annealing after welding. Opening in the RZ and

propagation of the fatigue crack in the HAZ, as well as startingfrom defects produced by metal expulsion, are not probablebecause conditions for such phenomena are not reproducible inour samples. The most straightforward hypothesis is that thefatigue crack opens at the notch. Surface oxidation of the notchtip could explain that the initiation is not affected by the annealingafter welding. The constant fatigue toughness of martensite afterannealing is consistent with this proposal (Fig. 4). The results ofVural et al. who reported that the lifetime of spot welds were notdependent on the spot diameter, also suggest that the lifetime iscontrolled by the initiation of fatigue cracks on surface defectsat the notch [21,22]. The explanation on the influence of theexpulsed zinc accumulated at the notch of welded pre-coated steelsheet is similar [10].

The same fatigue lifetime of untreated and annealed sampleduring test in the ‘high cycle fatigue’ range of force (Fig. 11) (paths2 and 20 in Fig. 13) is not surprising since the mechanical proper-ties of the RZ are unaffected by annealing (Figs. 15–17). Theconstant fatigue toughness of martensite after annealing is con-sistent with this proposal (Fig. 4). Along paths 2 and 20, crackingstarts in mode I and progressively turns into modes II and III whenthe crack splits in two after it has reached the outer surface of theplate. Bifurcation to paths 3 and 30 occurs because modes I and IIIare combined and work together. Along paths 3 and 30, mode I isactivated both by pulling and bending of the metal sheet.

The increase of the fatigue limit of welded samples after anneal-ing (Fig. 11) is consistent with the crack opening at the surface of theBM, and the increase of the fatigue limit of normalized samples(Fig. 4). This effect is coherent with the relaxation of surface stressupon annealing, and with the pinning of dislocations due to carbonprecipitation, which is revealed by the plateau at the yield in Fig. 2.Pinning of dislocations delays the opening of the fatigue crack.

Brazing after galvanizing prevents the opening of plates(mode I at point 1 of the notch – Fig. 13) (cf. Fig. 8). This changes

Table 5Mean hardness recorded in the maps.

Samples HV microhardness

Base metal Recrystallized FZþHAZ

Untreated AISI 410 395þ/�20 335þ/�20 425þ/�35Spot weld annealed

at 440 1C – 4 min400þ/�20 335þ/�20 390þ/�35

Spot weld galvanizedat 450 1C – 4 min

400þ/�25 335þ/�20 385þ/�35

0

300

600

900

1200

1500

Strain (%)

Stre

ss (M

Pa)

Austenized and quenched

tempered

0 1 2 3 4 5 6

Fig. 17. Mechanical behavior of the AISI410 steel after specific thermal treatments:(a) martensite produced by autenizing the AISI410 at 1000 1C, and quenching inwater; and (b) AISI410 after the martensite has been tempered at 440 1C for 4 min.

Fig. 16. Filiation of hardness measurements through the side of weld spots. (a) A spot weld of an optimally tempered marensite (raw steel); (b) the same as (a) afterannealing for 4 min at 440 1C; and (c) the same as (a) after galvanizing.

N. Becker et al. / Materials Science & Engineering A 596 (2014) 145–156 155

the triaxial stress distribution in the sample during overload andfatigue testing since the plate joint is only solicited in shear modewhen the plates are brazed [18,20]. Fig. 4 suggests that galvanizingcould affect the fatigue limit when the crack starts from BM.

5. Technological aspects

The indentation at the weld spot in our samples was anopportunity to distinguish between the semi-brittle fracture ofthe HAZ and the shear slip rupture of RZ. Decreasing the indenta-tion depth could increase the ultimate tensile strength of thenugget and the HAZ. Decreasing the indentation depth could alsoincrease indirectly the resistance of the RZ because the surround-ing in the ‘composite’ structure will reinforce this layer.

In our fabrication, the welding electrodes fulfilled the twofunctions of gripping the plates and forging the weld. These twofunctions could be separated if an independent clamp is placed inthe vicinity of the weld spot. An improvement of mechanicalperformance is expected since the pressure on the electrode andmetal expulsion could be reduced that way.

Galvanizing is advantageous for anticorrosion protection andbecause it increases significantly the mechanical performance.Nevertheless, galvanizing is an additional processing step thatpeople could prefer to skip. The annealing post-treatment alonecan be performed on-line by a second electrical shot with reducedenergy. This post-treatment moderately reduces the throughput.

Using high strength martensite potentially enables reduction ofthe amount of metal for a given structure by a factor of 3 or 4. Post-treatments enable reduction of the number of required weld spots.Galvanizing after fabrication presents the advantage of providing acontinuous and adherent anticorrosion coating. High-chromiummartensite is not sensitive to hydrogen embrittlement.

New galvanizing alloys could be developed in order to improvethe resistance of the brazed area and the fatigue toughness ofcoated steel. Improving the fatigue toughness of the base metaland improving the fatigue lifetime of the brazing should increasethe fatigue limit and the lifetime of galvanized spot weldedassembly.

6. Conclusions

The strength and fatigue toughness of weld spot on AISI410martensite after annealing or galvanizing have been studied intensile–shear tests. Post-treatment improved the performanceconsiderably.

Annealing restores the mechanical properties of the martensitein the welded nugget, and improves the ultimate tensile strengthof the spot weld by �50%. The fatigue lifetime on the Wöhlercurve is not changed after annealing.

Job galvanizing increases the strength of spot welds by up to100% due to a change of microstructure of the melted nuggetand heat affected zone, and also because the brazing of thesheets modifies the triaxial distribution of stress around the weld.Brazing controls the lifetime of galvanized spot-welded couponsunder tensile–shear cyclic loading. Brazing also increases thefatigue limit by a factor of 3.5 because the ‘high cycle fatigue’mode of rupture is shifted to a higher range of stress. The fatiguelimit is controlled by the rupture of the base metal due to the platebending in the tensile–shear test geometry. The benefit certainlydepends on the sample geometry.

The mechanism of overload failure of galvanized spot-weldedspecimens made of martensite is genuine, because the base metalis nearly as hard as the nugget. Then, fissures are confined andguided in the heat-affected zone tempered in the subcritical rangeof temperature.

Welding of steel sheets galvanized in a continuous line cannotproduce increase in stiffness, resistance and endurance similar tothe one reported here. As a matter of fact, the improvement of thefatigue toughness comes from brazing. Brazing does not replacespot welding. It provides a synergy, which considerably increasesthe mechanical performances of spot weld.

A design of the part that optimizes the brazing around the spotweld after job galvanizing can greatly enhance the spot weldperformance. An interesting question for the future could be tooptimize the resistance of the brazed area. For instance, anadditional study could explore the importance of sample geometryand galvanizing conditions. As a matter of fact, zinc penetrationbetween the plates, and the extent of the metallurgical reaction,could modify the quality and variability of brazing. Presently,we fixed these parameters (Fig. 1); and focussed our effort onguaranteeing the reproducibility of fabrication (all samples weregalvanized simultaneously in the same batch).

Acknowledgments

The authors are grateful to APERAM for providing the AISI410,and to Galva45 – GALVAUNION (Escrennes-France) for galvanizingthe samples. This work has been supported by the FrenchEnvironment and Energy Management Agency (ADEME) and the‘Conseil Régional de Lorraine’.

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