DECEMBER 200836

The use of martensitic-type advancedhigh-strength steels (AHSS) provides po-tential benefits for safety and weight re-duction in the automotive industry. Withthe application of thinner AHSS, mate-rial savings and improved crash energymanagement can be achieved.

The automotive industry has beensearching for materials that help reducethe weight of vehicles with the objectiveof reducing fuel consumption as well asmeeting the new safety regulations incrash tests. There is, thus, the need to usemore resistant materials with less thick-ness than those traditionally used, such ascarbon steels and high-strength low-alloy(HSLA) steels. The development ofAHSS has achieved this result; however,with new materials and complex struc-tures, they present a great challenge forcurrent welding technology. Thus, greaterunderstanding and knowledge is requiredof how these materials can be welded,while retaining the properties of the orig-inal base materials and thus securing thehold of the welds established in the basedesign (Ref. 1).

The martensitic-type advanced high-strength steels currently being used in theautomotive industry are joined using theresistance spot welding (RSW) process.Therefore, it is necessary to know the vari-ables involved as well as how they inter-act and what welding procedures shouldbe validated to obtain acceptable weldjoints that meet the design requirements

laid out by the client. It is also necessaryto know the mechanical properties of theapplied welds (Ref. 2).

Steels in automotive industry can beclassified in different forms. One is by ametallurgical designation. Common des-ignations include low-strength steels (in-terstitial-free and mild steels); conven-tional high-strength steels (HSS) (C-Mn,bake-hardenable, high-strength intersti-tial-free, and HSLA); and AHSS (dual-phase, transformation-induced plasticity,complex phase, and martensitic steels)(Ref. 3).

Another method for classifying steels isby their strength — Fig.1. This method de-fines high-strength steels (HSS) as yieldstrengths from 210 to 550 MPa and tensilestrengths from 270 to 700 MPa, while ad-vanced high-strength steels have yieldstrengths greater than 550 MPa and tensilestrengths greater than 700 MPa (Ref. 3).

The difference between HSS andAHSS is their microstructure. HSS aresingle-phase ferritic steels. AHSS are pri-marily multiphase steels, which containferrite, martensite, bainite, and/or re-tained austenite in quantities sufficient toproduce unique mechanical properties.Some types of AHSS have a higher strain-hardening capacity resulting in a strengthductility balance superior to conventionalsteels. Other types have ultrahigh yieldand tensile strengths and show a bakehardening behavior (Ref. 4).

The terminology varies considerablythroughout automotive and steel compa-nies. Each steel grade is identified by met-allurgical type, yield strength (YS) in MPa,and tensile strength (TS) in MPa. In thisarticle, the material used is MS900T/700Y, which a martensitic-type steel

with 900 MPa minimum ultimate TS and700 MPa minimum YS (Ref. 3).

The metallurgy for low- and high-strength steels is generally well known. Themetallurgy and processing of AHSS gradesare relatively new compared to conven-tional steels. All AHSS are produced bycontrolling the cooling rate from theaustenite or austenite plus ferrite phase, ei-ther on the hot mill or in the cooling sec-tion of the continuous annealing furnace.

In martensitic steels, the austenite thatexists during hot-rolling or annealing istransformed almost entirely to martensiteduring quenching on the hot mill or in thecooling section of the continuous anneal-ing line. The martensitic steels are char-acterized by a martensitic matrix contain-ing small amounts of ferrite and/or bainite. Within the group of multiphasesteels, MS steels show the highest tensilestrength level. This structure can also bedeveloped with postforming heat treat-ment. Martensitic steels provide the high-est strengths, up to 1700 MPa TS. Marten-sitic steels are often subjected to post-quench tempering to improve ductility,and can provide adequate formabilityeven at extremely high strengths. Marten-sitic steels use different combinations ofMn, Si, Cr, Mo, B, V, and Ni to increasehardenability and C is added to increasehardenability and for strengthening themartensite. The chemical composition ofmartensitic steel MS 900T/700Y is 0.08%C, 0.005% S, 0.54% Mn, 0.009% P, 0.04%Si, 0.04% Cr, 0.018% Ni, 0.02% Mo,0.011% Cu, 0.004% V, 0.002% Nb, 0.036%Ti, and 0.003% W.

Martensitic-type AHSS differs frommild steels by their chemical compositionand microstructure. In martensitic-typeAHSS, higher strengths are achieved bymodifying the steel microstructure. The as-received microstructure is changed duringresistance spot welding of AHSS marten-sitic type due to the heat input applied. Thehigher the heat input, the greater the effecton the microstructure. At different heat in-puts and cooling, we can obtain differentmicrostructures in the weld metal and heat-

Understanding ResistanceSpot Welding of Advanced

High-Strength SteelsBY V. H. LÓPEZ-CORTÉZ AND

F. A. REYES-VALDÉS

V. H. LÓPEZ-CORTÉZ and F. A. REYES-VALDÉS

(areyes@comimsa.com) are with Corporación Mexicana de Investigación

en Materiales, Saltillo Coahuila, México.

Experiments were conducted to better understandweldability issues when joining martensitic-type

advanced high-strength steels for the automotive industry

Lopez Cortez Feature Dec 08:Layout 1 11/10/08 8:57 AM Page 36

37WELDING JOURNAL

affected zone (HAZ).Resistance spot welding is a process in

which faying surfaces are joined at one ormore spots by the heat generated by resist-ance to the flow of electric current throughworkpieces that are held together underforce by electrodes. The contacting sur-faces in the region of current concentra-tion are heated by a short pulse of low-volt-age, high-amperage current to form a fusednugget of weld metal. When the flow ofcurrent ceases, the electrode force is main-tained while the weld metal rapidly coolsand solidifies. The electrodes are retractedafter each weld, which usually is completedin a fraction of a second (Ref. 5).

The rate of heating must be sufficientlyintense to cause local electrode/workpieceinterface melting. The opposed electrodesapply pressure and, when sufficient melt-ing has been achieved, the current is in-terrupted. Electrode force is maintainedwhile the molten metal solidifies, produc-ing a sound, strong weld. If both the localresistance of the workpiece and the weld-ing current magnitude were constant, thenthe total quantity of heat input Q devel-

oped in the workpiece would be given (injoules) by the following:

Q = I2Rt

where I is the effective value of current inamperes, R is the resistance of the work-piece in ohms, and t is the duration of flowof current in seconds (Refs. 5, 6).

Experimental DevelopmentTo carry out this project, a 1.5-mm-

thick martensitic steel GMW 3399M-ST-S CR 900T/700Y MS of 900 MP minimumtensile strength and 700 MPa minimumyield strength was used. This steel is cur-rently used for the manufacture of carbody structural parts.

The main variables of the process thatwere considered in the experiment werecurrent (A) and time (cycles). Evaluatedwere how these selected variables affectthe welding of martensitic-type advancedhigh-strength steels and what interactionexists between these variables in the per-formance of the spot weld, including the

variation of the mechanical properties andmetallographic structures due to appliedheat input. The following experiment de-sign was used and is shown in Table 1.

• Design, 32

• Level, 3 (–1, 0 +1) Low, Nominal, High• Factor, 2 (Weld Time, Welding Current)• Constant, Welding Pressure

To produce the weld tests, practices in-dicated in AWS D8.9, Recommended Prac-tices for Test Methods for Evaluating the Re-sistance Spot Welding Behavior of Automo-tive Sheet Steel Materials, were consideredand evaluated according to the criteria inGM 4488, Automotive Resistance SpotWelds Steel. The following two types oftests were conducted:

1. Test samples for peel test (D), mi-crohardness and metallographic test (M)— Fig. 2. Sample ID P1a and P1b throughP9a and P9b. Samples totaled 18.

2. Test samples for shear tension testT1a and T1b through T9a and T9b — Fig.3. Total samples 18.

The welding tests were carried out with

Fig. 1 — AHSS steels compared to low-strength steels and tradi-tional HSS.

Fig. 2 — Weld location, spacing, and coupon overlap for peel test samples.

Fig. 3 — Shear tension test samples.

Fig. 4 — Peel test results regarding diameter of the spot welds. Fig. 5 — Peel test results regarding depth of indentation.

Lopez Cortez Feature Dec 08:Layout 1 11/10/08 9:00 AM Page 37

DECEMBER 200838

Fig. 6 — Correlation of the diameter-indentation results. Fig. 7 — Shear tension test results.

the following equipment:• ARO resistance automotive portablegun• MEDAR control panel (time, current)• Control valves (pressure)• Electrodes Class 2 Cu-Cr

Table 2 shows the general conditionsused to perform the weld samples in theexperimental procedure.

The experiments included the follow-ing tests:• Peel test• Shear tension test• Microhardness• Metallography

Results and DiscussionIn the peel test, a caliper was used to

tear down the spot welds to determine thediameter and evidence of fusion of the

welded metal sheets. Figure 4 shows theresults of those tests. It can be seen thatas heat input was increased, the diameterof the welded spot also increased. This isdue to the fact that as we increase the cur-rent (A) and the time (cycles), the quan-tity of heat increases according to theequation Q = I2Rt. As far as the automo-tive industry is concerned, the bigger thediameter of the weld spot, the strongerthe weld.

Another important characteristic toevaluate is the depth of the indention,which is defined as the depression that theelectrodes leave on the surface of the metalsheets. Figure 5 shows that as we increasedthe heat input, the depth of the indentionalso increased. This provides a quality char-acteristic that should be limited to a mini-mum in accordance with the quality re-quirements of the automotive industry.

In correlating the test results of thespot diameter against those on the depthof the indentation, as shown in Fig. 6, itcan be observed that they show a directlyproportionate behavior. In other words,as we increased the diameter of the spotwith a given amount of heat, a greater in-dentation was produced, which is a qual-ity condition of the weld spot that is re-stricted in the automotive industry. There-fore, the optimal parameters of weldingwould be those that produce the greatestdiameter of spot with a lower value of in-dentation, allowed by the applicable qual-ity requirements.

In the shear tension test, the metalsheets joined by the spot weld are submit-ted to testing in a universal mechanicaltesting machine. Figure 7 shows that asthe heat input increases, the resistance ofthe shear tension increases to a maximum

Table 1 — Experimental Design

Process Control ParametersLimits

Parameter Units Notation Low Nominal High–1 0 1

Pressure Pounds P 670 670 670Current Ampere C 8000 9500 12000Time Cycle T 12 18 24

Design of Experiments (DOE)Input Output

Sample Design Matrix Spot Shear Tension Microhardness Heat Input kJNo. Diameter Q=I2Rt

P C T (mm) (kg/mm2) (HV 500)

1 670 8000 12 P1 T1 P1 16642 670 8000 18 P2 T2 P2 24963 670 8000 24 P3 T3 P3 33284 670 9500 12 P4 T4 P4 23465 670 9500 18 P5 T1 P5 35196 670 9500 24 P6 T2 P6 46937 670 12000 12 P7 T3 P7 37448 670 12000 18 P8 T4 P8 56169 670 12000 24 P9 T1 P9 7488

Lopez Cortez Feature Dec 08:Layout 1 11/10/08 9:00 AM Page 38

point (Test P6), whereas in these condi-tions the increase in resistance is not sig-nificant compared to the increase in ap-plied heat. Rather, it reduces the resist-ance of the spot weld.

In correlating the diameter of the spotagainst the shear tension obtained in thetension test, it is shown in Fig. 8 that thereis a maximum shear tension value and thatafter that value (Tests P7–P9), even thoughthe diameter of the spot increases, its re-sistance remains without any greatchanges. This is because applying thegreatest amount of heat to the welding spotcauses the latter to be most affected by theincrease in temperature, which modifiesthe response in its mechanical properties.

To evaluate the effect of heat input onthe spot welding properties, the variationsin microhardness were measured in thebase metal, HAZ, and weld.

The test data were evaluated with vari-ables of low (P1, 1664 kJ), medium (P5,3519 kJ), and high heat input (P9, 7488kJ). It can be seen in Fig. 9 that the HAZis shown to have diminished in hardnesscompared to the original hardness of themetal base and the hardness obtained inthe spot weld. This indicates that the ma-terial is overheating because of the weld-

ing process. Figure 10 shows the microstructures of

the base metal, HAZ, and weld, where thebase metal presents a microstructure oftempered martensite and, in the HAZ, thematerial appears to be heated at a hightemperature between A1 and A3. It isgiven that the speed of cooling is not highenough to make the material pass throughthis zone of two-phase stability (α y γ).This is because of the carbon content,where the ferrite and the austenite coex-ist at this temperature. While the austen-ite transforms the martensite during thecooling down process, the ferrite remains.This indicates the presence of bothmartensite and ferrite phases in the HAZ.Therefore, the formation of the ferritephase causes the hardness in this zone todiminish, and as heat input is increased,the HAZ size also increases. This is re-flected in the drop in resistance with sheartension at high heat levels.

During the welding process, the mate-rial is heated above temperature A3.Therefore, it is totally austenized during

the heating process and newly trans-formed to martensite due to the high hard-enability of the martensitic steels. Conse-quently the microstructure and hardnessof the weld is similar to and even, in someways, stronger than the metal base itself.

ConclusionsIn accordance with the analysis out-

lined in this article and the evaluation ofthe test results in the performed tests, thefollowing can be concluded:

1. The diameter and the depth of thespot weld have a behavior directly propor-tional to the increase in heat input (cur-rent (A) and time (cycles)).

2. Resistance to shear tension(kg/mm2) at the welding point has a reac-tion directly proportional to the increasein heat input (current and time cycles).However, it can be observed that at highlevels of heat input (Tests T7,–T9) this in-crease is minimal.

3. In comparison with high levels ofhardness (301–420 HV) in the base and

39WELDING JOURNAL

Fig. 8 — Correlation diameter-shear tension test. Fig. 9 — Microhardness test results.

Fig. 10 — Metallographic test P5, 3519 kJ. A — Weld; B — HAZ; C — base metal. (Nital2%, 500×.)

Table 2 — Welding General Conditions

Welding Machine Type TransgunTransformer 135 kVAController MEDAR 500SElectrode Face Diameter 5 mmSqueeze Time 10 CyclesWeld Time 12, 18, 24 CyclesCurrent 8000, 9500, 12000 AHold Time % CyclesPreheat NonePostheat NoneCooling Water 12 L/minTemperature Water 22°C

Lopez Cortez Feature Dec 08:Layout 1 11/10/08 9:01 AM Page 39

DECEMBER 200840

weld metals, low levels (250–282 HV) ofhardness were observed in the HAZ.

4. The microstructural analysis showsthe presence of soft phases (ferrite) in theHAZ, which reduces its hardness in thisarea due to the effects of the heat input.

5. From the statistical analysis of the ob-tained results from the performed tests, itcan be observed that the optimal values forthe process are found among those valuesfrom tests P5 and P6 9500 (A), 18 and 24(cycles), where the greatest spot diametersand resistance to shear tension can be foundwith the least acceptable indentation.◆

Works Consulted

1. Tumuluru, M. D. 2006. Resistancespot welding of coated high-strength dual-phase steels. Welding Journal 85(8): 31.

2. Jones, T. B. et al. 2000. Optimizationof both steel properties and auto bodystructural designs for axial, side and off-set impact loading. Final Report, ECSCSponsored Research Project No. 7210.PR/052, July 1997–December 2000.

3. Beenken. 2004. Joining of AHSS ver-sus mild steel. Processing state-of-the-artmulti-phase steel. E A S Conference,Berlin.

4. Cuddy et al. 2004. Manufacturingguidelines when using ultra high strengthsteels in automotive applications. EU Re-port (ECSC) R585.

5. American Iron and Steel Institute.Advanced high-strength steel reparabilitystudies: Phase I final report and Phase IIfinal report. www.autosteel.org.

6. Bode, R., Meurer, M., Schaumann, T.W., and Warnecke, W. 2004. Selection anduse of coated advances high strength steelsfor automotive applications. Galvatech ‘04Conference Proceedings, pp. 107–118.

7. Agashe, W., and Zang, H. 2003. Se-lection of schedules based on heat balancein resistance spot welding. Welding Joumal82(7): 179-s to 183-s.

8. Dickinson, D. W. 1984. Welding inthe automotive industry: State of the art.Republic Steel Corp., Report on AISIProject No. 1201–409C, pp. 1–400.

9. Resistance Welding Manual, 4th edi-tion. 1999. Resistance Welder Manufac-turers’ Association. Philadelphia, Pa. pp. 1, 2, 7, 8, 22.

10. Kaiser, J. G., Dunn, G. J., andEagar, T. W. 1982. The effect of electrica1resistance on nugget formation during spotwelding. Welding Journal 61(6): 167-s to173-s.

References

1. Dinda, S., and Diaz, R. 1995. Thepartnership for a new generation of vehi-cles (PNGV) and its impact on body en-gineering. Proc. IBEC 95, Advanced Tech-nologies and Processes, IBEC, Ltd., 5.

2. Maurizio Mini, M. AHSS: AdvancedHigh-Strength-Steels; GED.

3. International Iron and Steel Insti-tute. 2002. Ultra Light Steel Auto Body -Advanced Vehicle Concepts (ULSAB –AVC) Report/AISI.

4. Gould, J. E., Khurana, S. P., and Li,T. 2006. Predictions of microstructureswhen welding automotive AHSS steels.Welding Journal 85(5): 111-s.

5. ASM Handbook, Vo1. VI, Welding,Brazing and Soldering. 1993. Proceduredevelopment and process considerationsfor resistance welding. Materials Park,Ohio: ASM Intemationa1, pp. 226 –230,416, 1073–1080.

6. Welding Handbook, 8th edition, Vol.2. Welding processes. 1991. Chapter 17,Spot, Seam and Projection Welding.Miami, Fla.: American Welding Society,pp. 531.

Weld Smarter With Total Welding ManagementImplementing the principles andconcepts in this book could saveyou $15,000 to $25,000 annuallyper welder.

Drawing on more than 50 years of welding experience, author JackR. Barckhoff, P.E., gives you a solidstep-by-step plan to manage yourwelding operations for maximumproductivity and cost efficiency.Specific recommendations andreal-life production examples illustrate how your welding teamcan realize productivity gains of 20 percent to 50 percent.Total Welding Management explains the management principles, structure, and details you need to transform yourwelding operations from a cost center into a profit center. Amust-read for supervisors, managers, and executives whoseek to make their welding operations more efficient andmore productive. 185 pages, 35 figure, 20 tables, hardbound.

Order yours today: 888-WELDING (935-3464).Outside North America: 305-824-1177. Online: www.aws.org/standardsOrder code: AWS TWM, $49.50

© American Welding Society 2006 WJS1235

For info go to www.aws.org/ad-index

Lopez Cortez Feature Dec 08:Layout 1 11/10/08 9:01 AM Page 40