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Crackin g in Spot Aluminum Alloy AA5754 Cracking and its mechanisms during the resistance spot welding of aluminum alloys are analyzed

BY J. SENKARA AND H. ZHANG

ABSTRACT. The phenomenon of crack- technique in auto-body assembly, resis- during spot welding. Toyota reported so- ing was observed during resistance spot tance spot welding is used for joining alu- lidification failure in the nugget or liqua- welding a commercial aluminum alloy minum parts, as it has been used for steel. tion cracking in the heat-affected zone AA5754, and mechanisms of cracking The nonheat-treatable AI-Mg alloys (HAZ) for one of the 5000 series alloys and healing are discussed in this paper. (5000 series) in sheet product form are containing above 5 wt-% Mg (Ref. 13). Metallographic study of welded coupons among the most promising aluminum They observed cracking under a wide revealed cracks located on only one side materials, and among them, AA5754 has range of welding parameters and sug- of a weldment in the heat-affected zone been developed especially for the auto- gested that preheating or increasing (HAZ), with respect to the welding se- motive industry. The optimized magne- welding time may decrease thermal quence. Cracks are visible from longitu- sium content in the alloy assures satis- stresses and therefore decrease cracking dinal cross sections only. Some of them factory mechanical properties and low tendency. The possibility of cracking was are partially or fully filled. Crack appear- susceptibility to stress corrosion crack- also indirectly implicated in spot welding ance and orientation are fairly repeatable ing. The AA5754 alloy has good forma- AA5754 by Thornton, et al. (Ref. 10). and their intergranular characteristics bility, good static, impact and fatigue Although there is very limited pub- and dendritic fracture surface morphol- strength and high resistance to pitting lished literature on cracking mechanisms ogy prove they were formed at elevated and intercrystalline corrosion (Refs. 4, 5), during resistance spot welding, cracking temperatures in the presence of liquid as well as a stable microstructure after ex- has been studied relatively extensively metal. The discussion of metallurgical posure in moderate temperatures. The for aluminum alloys of vari- factors considering the AI-Mg equilib- strength of the alloy results from a com- ous working ranges, and high suscepti- rium phase diagram and the possible bination of solid-solution hardening, bility to hot cracking during solidification temperature histories of various zones in cold work and grain-size strengthening, of the liquid pool has been reported. For a weldment during spot welding eluci- as expressed by the HalI-Petch equation. instance, Lippold, et al. (Ref. 14)investi- dated the approximate conditions for The microstructure of this alloy and re- gated hot cracking in two lots of 5083 cracking during spot welding and for sulting features are described in detail by aluminum alloy (4.28 and 4.78 wt-% Mg, mending the structure. A thermome- Burger, et al. (Ref. 5). Because AA5754 respectively) weldments that were gas chanical analysis revealed a high possi- sheet material is annealed and its surface tungsten arc (GTA) welded. They ob- bility for tensile stress buildup on the pre-treated, thereby assuring repeatabil- served crack initiation and propagation cracked side of the weldments as a result ity in resistivity for spot welding, it is gen- in either the fusion zone or the HAZ and of material flow, thermal stress develop- erally regarded as a material with "good found that cracking susceptibility de- ment and localized straining. spot " (Ref. 4). This has been pends on the Mg content in the particu- confirmed by a number of publications lar alloy and weld orientation relative to Introduction with positive experimental results for the rolling direction of the material, in "classical" spot welding (Refs. 6-10), as continuous-wave CO 2 laser beam weld- Aluminum-based alloys have been well as for weld bonding (Refs. 11, 12). ing and pulsed Nd:YAG laser welding widely used in automobile structures due Although it is known that one of the 5000 AI-Mg alloy series, including to their unique properties, such as high major problems in welding AI alloys is AA5754, Jones, et al. (Ref. 15), reported specific strength. Although large-scale cracking, there is very little information fairly low hot-cracking susceptibility. production of aluminum-intensive vehi- in published literature concerning crack They observed the tendency toward cles will not be feasible in the near future, formation in AA5754 or similar alloys cracking increases with Mg content, there is no doubt that the use of alu- reaching a peak value at 2 wt-% Mg; high minum-based materials will increase weld strength and low crack susceptibil- steadily (Refs. 1-3). One attractive area is ity were found when Mg content is above to use aluminum alloys as structural ma- KEY WORDS 4 wt-%. The maximum cracking ten- terials, and this application depends pri- dency in AI-Mg alloys was reported ear- marily on and joining of alu- Resistance Spot Welding lier at approximately 3 wt-% Mg (Ref. 16) minum parts. As the major joining Aluminum Alloy or 1-2 wt-% Mg (Ref. 17). The observa- Hot Cracking tions are consistent with hot-tearing phe- Cracking Mechanism J. SENKARA AND H. ZHANG are with De- nomenon in casting of AI alloys (Ref. 18), Healing in accordance with the established fact partment of Mechanical Engineering and Ap- Heat-Affected Zone plied Mechanics, University of Michigan, Ann that the peak of hot-cracking susceptibil- Stress ity of binary alloys is at about one half of Arbor, Mich. J. SENKARA is on sabbatical Strain leave from Welding Dept., Warsaw University maximum solubility of the second com- of Technology, Warsaw, Poland. ponent in the solid state.

194-s I JULY 2000 Fig. 1-- Microstructure of AA5754 base mater- ial used in the experiment. 5 mm I

It is noteworthy to compare cracking during solidification (Ref. 16). during welding with solidification crack- The lack of practical information ing during casting, although there are dif- on cracking in spot welded AI-Mg al- ferences in the processes. According to loys, the increasing use of AI alloys in the classical works by Pellini and Flem- the automotive industry and the care ings, hot tearing in casting alloys occurs needed for spot welding aluminum al- at the last stage of crystallization (Refs. loys are the driving forces for the study 18-21), during which solid grains are sur- of cracking in resistance spot welding rounded by the liquid; such a structure AA5754 sheets. Because single spot has a very low strength. Tensile stresses welds are rarely used in welded struc- and strains, resulting from nonuniform tures, multispot welds were chosen temperature distribution and cooling, for this investigation. The influence of may cause material failure. A certain metallurgical interaction among spot amount of strain is necessary for crack welds and other factors are empha- initiation, as pointed out by Pellini (Ref. sized to understand the mechanisms Fig. 2--Appearance of a typical button after peel 19). Hot cracking tendencies in casting of crack initiation and propagation. testing. A -- An amplified side view of the button increase with grain dimensions, solidus- wall; B -- cross section of the same button. liquidus gap and solidification shrinkage, Experiments which is especially high for AI alloys. The presence of impurities and grain bound- AA5754 aluminum alloy sheets of meters used were 7 kN force, 3 ary segregation also promote cracking. 1.6- and 2.0-mm gauges and in 0 temper kA preheat current for 3 cycles (50 ms), The mechanism of hot cracking in condition (annealed), produced by 12 cycles (200 ms) of weld delay, 26 kA welding, similar to that in casting, is Alcan Aluminum Co., were used in all welding current for 5 cycles (83 ms) and based on a theory developed by Borland experiments. The sheet surface was pre- 12 cycles (200 ms) of holding time. The (Ref. 22) and Prokhorov (Ref. 23). Oc- treated and prelubricated by the pro- weld pitch was 30 mm. Generally, welds currence of cracking in "coherence tem- ducer. The chemical composition speci- with satisfactory appearance were ob- perature range" (Borland's definition) de- fied by the producer, as well as the tained. Commonly used peel testing con- pends on both critical strain and critical composition of the sample tested inde- firmed good quality and repeatability of strain rate. Hot cracking during welding pendently, are listed in Table 1. The data the spot welding process. A regularly at elevated, near-solidus temperatures in- show that the tested composition is shaped, good sized button is shown in cludes failure of welds (solidification within the specified range. Fig. 2A. cracking) and cracking in the HAZ (li- The sheets were cut into 350 x 25-mm Spot welded samples were then sec- quation cracking) (Ref. 24). Cracking in coupons for multiwelding. Taking into tioned in two perpendicular directions the HAZ is related to liquation at the account the small anisotropy of structure (normal and parallel to the rolling direc- grain boundaries of either the secondary and properties reported by Burger, et al. tion) and were ground, polished and phase or low-melting-point impurities, at (Ref. 5), all coupons were cut out paral- structurally investigated to disclose pos- subsolidus and at supersolidus tempera- lel to the rolling direction. Figure 1 shows sible internal discontinuities and their tures of the primary phase. Existing theo- a typical material microstructure of the natures. Optical and scanning electron ries of formation and solidification of base metal. Slightly elongated grains rep- microscopy techniques were used, as grain boundary liquid films include equi- resenting Mg in AI solid solution are vis- were energy dispersive X-ray (EDX) and librium melting of the vicinity of grain ible as are precipitates of AI~Mg 2, (Fe, wave dispersive X-ray (WDX) micro- boundaries (Ref. 25), constitutional li- Mn)AI 6 and silicides. analyses. Microhardness was measured quation of secondary phases and the ef- In this set of experiments, Alcan's across the sectioned weldment using a fects of segregation (Refs. 26, 27). domed were used. Such an LECO hardness tester. Comparisons of various AI alloys in electrode has a face diameter of 10 mm, casting and arc welding revealed the AI- and the radius for the domed face is Description of Cracks Mg system is second to the AI-Cu system 50 mm. in crack susceptibility among aluminum Multiwelds were made on the Despite the normal appearance of a alloys (Refs. 16, 18, 21, 28), in spite of coupons using a medium-frequency (MF) typical button after peeling, an amplified only a small amount of eutectic formed DC welding machine. The welding para- side view of the button wall revealed

WELDING RESEARCH SUPPLEMENT J 195-s A

Welding Sequence Ib I 10 ~ I

Table 1 -- Chemical Composition of AA5754 (Ref. 4) and of the Sheets Tested (in wt-%)

Mg Mn Cu Fe Si Ti Cr Zn Nominal 2.6-3.6 0.5~.,, 0.1 ,.,~ 0.4,.,, 0.4,.,, 0.15 .... 0.3,.,, 0.2 .... Tested 3.22 0.25 <0.05 0.19 <0.05 <0.05 <0.05 <0.05

(a) Maximum value. Fig. 3 -- Longitudinal cross section of a multispot welded coupon. A -- Cracks cracks -- Fig. 2A. This was confirmed by from fine roots (grain boundaries). Many on the right side of the nuggets; B -- one optical and scanning electron micro- of them are fully or partially filled -- Fig. of its nuggets by higher magnification. C scope (SEM) examination of the sec- 6. The failure surface has a dendritic mor- -- For comparison, the transverse cross tioned specimens (Fig. 2B); these re- phology -- Fig. 7. section of a neighboring nugget is also vealed discontinuities of porosity and Cracks initiate in a zone where the presented. cracks in the weldments. Although a cer- alloy remained in the solidus-liquidus tain volume of porosity could be seen in temperature range during welding at the weld nugget on all the cross sections some distance from the weld interface -- mens, as demonstrated in previous sec- of welded samples, there were no cracks Fig. 5. A "web" of grain boundaries dec- tions. Although discontinuities such as in the nuggets. Optical microscopy in- orated by precipitation is visible around porosity and cracks inside a nugget may spection, however, revealed many cracks the zone. Grain boundary failure can be not influence the strength, as suggested on the sides of nuggets in many speci- clearly seen near the base of the wide- by Thornton, et al. (Ref. 10), and Michie mens. Cracks were all located in the opening cracks. A typical microstructure and Renaud (Ref. 30), the occurrence of HAZ. In many cases, the gaps of cracks of the region in the HAZ close to the cracks outside the nugget as revealed by were filled by the base material, which nugget and at some distance from cracks this study may influence the strength of was detectable only after etching. Cracks is presented in Fig. 8. Precipitates inside the welded components; this needs fur- were found only on one side of the the grains and at grain boundaries (inter- ther study. It is therefore necessary to nugget, with respect to welding se- granular precipitates), where they form study the mechanisms of cracking and quences -- Fig. 3. In this case, they are chains or even continuous layers, are vis- potential structure remediation. Like all clearly visible from longitudinal cross ible. EDX and WDX analyses revealed an other processes of resistance spot weld- sections, whereas there are no visible increased amount of Mg in these regions. ing, the formation, propagation and re- cracks or only very narrow traces of This is most probably due to an AIgMg 2 covery of cracks involve interactions of cracks visible in transverse sections. secondary phase (the presence of which metallurgical, thermal and mechanical The appearance of cracks, their loca- should be about 6% in the AI-Mg3.5 factors. The influence of these factors is tions and their orientations are fairly reg- alloy, according to the AI-Mg equilibrium analyzed in the following sections. ular as revealed by the examination of a phase diagram [Ref. 29]), which exists in large number of specimens. The angles the alloy before welding and serves as the Metallurgical Factors between the main axes of cracks and the source of liquid at elevated temperature. tangents to the weld interface are similar This was confirmed by X-ray diffraction The cracking phenomenon in the and are equal to about 70 deg for the examination. HAZ during spot welding of AA5754 de- specimens examined, as shown by the The hardness in the nugget, the HAZ scribed in this study is consistent with statistics of the orientations of nearly 70 -- both near and far from cracks -- and the observations in published literature cracks in Fig. 4. the base metal were measured, as shown on the susceptibility to cracking of other A photo of higher magnification in Fig. 9. Generally, there is no difference aluminum alloys containing several per- shows intergranular fracture characteris- in hardness between the nugget, the cent of magnesium in casting (Ref. 18) tics -- Fig. 5. Cracks initiate in the vicin- HAZ and the base AA5754 alloy. The and arc welding (Ref. 28). No solidifica- ity of the weld interface in the HAZ and hardness near the crack in the HAZ is tion cracks, however, were observed in propagate away from the nugget into the somewhat higher than the hardness the nuggets in the present study, contrary base metal. A typical cracking trace is not some distance from it, which may sug- to the observations of Watanabe, et al. straight, as shown in Fig. 5. It follows the gest strain hardening. The scatter of the (Ref. 13). grain boundaries while keeping the over- hardness data inside the nugget results Intergranular characteristics of cracks all outward direction. Some of the cracks from microporosity. in the HAZ and dendritic morphology of tilt slightly toward the faying interface as failure surfaces, as shown in Figs. 5-7, they propagate. Most of them are wide at Mechanisms of Cracking are typical features of hot cracking and their bases and become narrower toward and Healing evidence of cracking at elevated temper- the base material. Wide cracks have tree- atures. The dendrite structure inside open like structures, i.e., large trunks (wide Cracking and healing in AA5754 have cracks proves that liquid had to be pre- opening of cracks at the bottom) formed been observed in multiwelded speci- sent at the moment of crack formation,

196-s I JULY 2000 15 1 r I r r 1 r

14

8

6

2

0 45 50 55 60 65 70 75 80 85 Crack Angle (deg.)

Fig. 4 -- Measured crack angles with respect to weld interface. Fig. 5 -- A close look at cracks in the HAZ. and, therefore, it is liquation cracking ac- namic effects in the heating/cooling aries is then extended in a relatively wide cording to the classification by processes, such as overheating and un- temperature range during the cooling Hemsworth, et al. (Ref. 24). The micro- dercooling, were not investigated in this stage. porosity visible in some inclusions of the study due to experimental difficulties, al- High heating and cooling rates, as secondary phase serves as additional ev- though they may contribute to hot crack- well as a high temperature gradient in the idence of the existence of liquid in this ing. For instance, the effective solidus weldment due to the nature of Joule heat- part of the HAZ during spot welding. temperature during cooling may be ing, are the thermal characteristics of spot Generally, there are two possible lower than the equilibrium solidus tem- welding. Based on the experiments con- ways for melting to occur at grain bound- perature because of the high cooling rate ducted in this study of welding AA5754, aries in the HAZ: at supersolidus temper- in RSW. This effectively enlarges the tem- the heating and cooling rates are esti- atures and at subsolidus temperatures. In perature range in which the material is mated to be as high as 8000 and 3000 the heating stage of welding, equilibrium weak and susceptible to cracking. K/s, respectively. Because of this, liquid melting of the material near grain bound- As a result of combined supersolidus films at grain boundaries may exist for an aries occurs in the part of the HAZ heated and subsolidus melting/liquation, large extended time at elevated temperatures, to the temperature range between solidus grains in these parts of the HAZ are sur- as proved by Radhakrishnan and Thomp- and liquidus (partially melted zone). In rounded by liquid during welding. son in their model (Ref. 27). This results addition to partially melting at tempera- Nearly continuous films of liquid are from the concentration gradient in the tures above the solidus, liquation of the formed at grain boundaries -- Fig. 8. liquid due to rapid solidification, which secondary phase may occur at subsolidus Therefore, the overall material structure effectively lowers the solidification tem- temperatures. During rapid heating, close to the nugget is favorable for crack peratures of liquid parts with higher (than which is a characteristic of resistance initiation and growth during the last stage equilibrium) Mg concentration. spot welding, there is not sufficient time of heating in resistance spot welding AI- As described above, metallurgical to dissolve the AI3Mg 2 phase in the c~-so- Mg alloys. factors during spot welding of AA5754 lution matrix, and inclusions of this After the current is switched off, the create favorable conditions for tearing phase still exist after the alloy is heated material cools quickly because of heat the structures in the HAZ. over the solvus line. AI3Mg 2 inclusions transfer through the water-cooled elec- melt in the region (next to the partially trodes. The life of transient liquid films at Thermomechanical Factors melting zone) that experiences maxi- grain boundaries depends on several fac- mum temperature above the eutectic tors, including the cooling rate and com- Besides the metallurgical effect, ther- point but below the solidus temperature. positional segregation. For AA5754, the momechanical factors also play a role in Existence of liquid below the solidus of difference between equilibrium temper- the initiation and subsequent propaga- AA5754 can also be attributed to other atures of liquidus (915 K), solidus (876 K) tion and growth of cracks. This section is low-temperature melting additions/im- and eutectic temperature (723 K) is sig- devoted to describing the mechanisms of purities present in a commercial alloy. nificant, according to the AI-Mg phase crack formation by qualitative thermal The zones around the nugget are diagram (Ref. 29). Decreases of the and mechanical analyses using simpli- schematically shown in Fig. 10. Struc- solidus and eutectic solidification tem- fied assumptions (because of the compli- tures/zones in the HAZ are linked to the peratures can be expected due to kinetic cated mechanical, thermal and metallur- equilibrium phase diagram via assumed effects during cooling. The coexistence gical interactions). temperature history during RSW. The dy- of solid and liquid phases at grain bound- As seen in Fig. 3A, cracks appear on

WELDING RESEARCH SUPPLEMENT I 197-s Fig. 7 -- Intergranular characters of a crack and dendritic morphology of failure surface.

Fig. 6 -- An almost fully filled crack.

I00 '' I .... I .... I .... I .... I ....

: : 8o N~-~. 0 O M.~hy Zo~ 60 ii ~ 4o , O0 ,m ,i I~L~F,~-~T~-. ' , _ . ~..,~ Zvle~s~erl ~e~ t~ ¢r~k Izl 2o Measxu'@. ~r~y from t~ cr~k

Fig. 8 -- Precipitation zone in the HAZ. 0 '' . . I , , , , I , , , . I , . , , I , , , ~ | , , , , -1500 -I000 -500 0 500 I000 1500

Distance (l~m)

Fig. 9 -- Hardness distribution of a spot weldment. the leading sides of nuggets that coincide configuration can be seen in all the weld- ing, there is usually a temperature gradi- with the welding sequence in a multi- ments in Fig. 3A. This phenomenon is di- ent in the solid phase in the HAZ as re- spot welded strip. No significant traces of rectly related to the deformation of the vealed by a finite element analysis (Ref. cracking have been found on the trailing solid phase in the HAZ near the nugget 32). The temperature in the solid near the sides of the nuggets. Figure 11 shows the during welding. liquid nugget is higher than the tempera- outline of a spot weldment (Fig. 3B) that The resistance heating during welding ture farther from it, because that area is includes workpieces, the nugget and expands both the molten metal and the constantly cooled by the electrodes. Be- cracks. It is taken from the middle of a solid phase of the weldment. This expan- cause of this nonuniform temperature multiwelded strip, and the welding se- sion is constrained by the electrodes and distribution in the weldment, the solid quence is from the left side (Side L) to the the work sheets surrounding the weld- near the liquid nugget tends to expand right side (Side R). The crack initiation ment. The loading condition on one more than the solid away from the and propagation can be explained based sheet can be simplified as shown in Fig. nugget; therefore, there is a tendency to- on the physical processes this weld has 12. On top of the sheet, there is an ap- ward sheet separation on the sides of the gone through during welding. plied electrode force, simplified as dis- nugget. This separation, however, is not tributed load. There is also a liquid pres- even on the two sides. On Side L, the Material Flow sure, developed in the nugget, as sheets are confined by the previous weld. demonstrated in a previous work by the No large separation is allowed on this From Fig. 11, it can be seen that the authors (Ref. 31), inserted on the bottom side. The situation is different on Side R. weldment is not symmetric. Specifically, of the solid phase. The boundary condi- Because there is significantly less con- there is no significant deformation near tions shown in the figure reflect the con- straint on the sheets on this side, which the faying surface (sheet-sheet interface) straints imposed by the surrounding solid was free during welding, there is very lit- in the solid phase on Side L, while on phase. Because of the forces inserted by tle resistance to sheet separation. Conse- Side R there is virtually no separation the electrodes and the liquid nugget, the quently, the solid metal around the liquid near the nugget, but a blunt notch can be solid phase between the electrode and nugget and at the faying interface near seen at a distance of approximately 0.7 the nugget is squeezed in a process sim- the nugget on Side R are squeezed out mm from the nugget. This unsymmetrical ilar to a rolling operation. During weld- between the electrode, the nugget and

198-s I JULY 2000 I

& E "

37,J'-AA5754, , lJ

o 10 20 3(~ 4(} AI Atomic% Mg Fig. 11 -- Outline of the weldment in Fig. 3B.

Distributed Electrode Force

Previous SideL Weld

Fig. 10 -- Schematic diagram of the links between the equilibrium Fig. 12 -- Loading and mechanical constraining on a weldment during phase diagram of AI-Mg and the solidified structures after welding multiwelding and schematic material flow. Side L is constrained by the through possible temperature history of various zones. Zone h fusion previous weld. zone; Zone Ih partially molten zone; Zone IIh liquation zone. the other workpiece, as evidenced by the fact, cooling is provided by the elec- electrode force, the workpiece is ideal- geometry of the blunt notch and the elec- trodes during the entire period of weld- ized as a simply supported cantilever trode indentation mark shown in Fig. 1 1. ing, but the rate is much higher after the beam, with consideration given to con- This material flow is made possible by the electric current is shut off. The cooling straining conditions and loading, and high temperature (just below the solidus) rate is so high (very similar to the rate electrode force is simplified as a uni- field surrounding the liquid nugget, found in a quenching process) that ten- formly distributed load. The distance be- which softens the solid by lowering the sile stresses develop in a very short pe- tween the previously made weld (simu- yield strength. riod of time, regardless of their stress lated as a fixed end) and the left side of states after heating. By superimposing the the weld under consideration is approx- Thermal Stress Development stress states during heating and cooling, imately the weld pitch. By a structural the final stress can be found tensile on mechanics derivation, the distribution of Stresses are developed in the solid Side R, and very little (tensile or com- the bending moment in the beam is as during heating and cooling, and they are pressive) on Side L. The tensile stress on depicted in Fig. 13A. It clearly shows closely related to the constraining condi- Side R during cooling is directly respon- that in the solid near the weld interface, tions. The solid phase between the elec- sible for crack initiation in the region. compressive stress is developed due to trodes and the liquid nugget tend to ex- The tensile stress developed during electrode force on Side L, while tensile pand during heating. Because of the cooling is approximately along the tan- stress is developed on Side R. Note that difference in constraint, the stress devel- gent of the isotherm, which is parallel to the distributed load in Fig. 13A is the re- oped during heating is different on the the weld interface, near the nugget. sultant of applied electrode force and two sides. Materials near the weld inter- Therefore, the orientation of the crack is pressure from the liquid nugget. face tend to expand more than those normal to the tangent, or along the tem- The liquid pressure in the nugget, away from the weld interface. On Side L, perature gradient. which acts on the opposite side of the large compressive stress is developed in worksheet, may relax to a certain extent the direction parallel to the weld inter- Stressin~ Due to Bendin~ the bending stress resulting from the face due to constraints from the solid electrode force. However, in most cases, phase of the sheets. Meanwhile, on Side Because of the constraint conditions the force due to nugget pressure is of less R, there is very little compressive stress imposed on one of the sheets as shown magnitude compared with electrode buildup in the vicinity of the nugget, be- in Fig. 12, the workpieces are bent under force, as the applied electrode forces are cause the solid phase can be squeezed the electrode force. The stress induced normally chosen to sufficiently contain out, as shown in the previous section; this by the electrode force, though, is not the the liquid from expulsion. The net tensile effectively releases the stress. same for the two sides due to uneven stress accelerates cracking when com- When the current is cut off, heating is constraining conditions. To understand bined with the thermal stresses analyzed terminated and rapid cooling starts. In the stress state of the workpiece under in previous sections.

WELDING RESEARCH SUPPLEMENT I 199-s 1 stress to the region in the solid near the A liquid nugget. In addition to tensile stress, expulsion also induces a high strain rate ev,ous V,'eld 1 ' t'~' I to the solid near the weld interface, be- cause the side wall of the liquid nugget is nl I pushed outward to the side by an ex- tremely high-speed liquid metal ejection under pressure. This high strain rate is m i Bending detrimental if it falls in the brittle defor- UJI .Moment mation range. During welding and the subsequent processes, all of the factors, both metal- lurgical and mechanical, contribute to the formation and propagation of cracks ~ a in the HAZ. Because of the complexity and changing nature of the spot welding uu I Distributed Electrode Force Uil B process, it is impossible to quantitatively Will ILLILLI[IIIIIIIIILIILILL[IIIILLLLLI[I identify the contribution of each factor. However, the developed understanding of cracking mechanisms can be used to ul I Side L SlOe R formulate guidelines for avoiding crack- ing in resistance spot welding.

Crack Healing Mechanisms I,LI I Axial stress iu I ', I II l,i~ I Depending on the stages in which ;rll,lllllil: ililiilllillill llllill[LIl[[lltl ,,[iuL[iiiliiiiil iL[iiiiiillii41i IIII]]lillllllJ/i IIIlllUllillli/i I IIIIillillll,I ]!lllilli;illiill !111!1111111i1111 ¸ II,!lllll cracks initiate during welding, the crack gaps may or may not be filled. The most Bending Ivioment likely mechanism for filling the cracks is leading liquid metal from the nugget into cracks during welding. This is supported uul by a WDX analysis that revealed slightly Fig. 13 -- Simplified loading and boundary conditions, and resulting bending moment and higher Mg content inside the filled area, force, to understand the influence of electrode force (A) and liquid pressure or expulsion (B) but no significant difference between the li I on cracking. chemical composition of the material fill- B, I I ing the gaps and that of the base metal. If a crack exists during heating it may helps material flow. It be easily filled because of high pressure u i ~ Tensile also increases the tensile ILI I inside the liquid nugget (Ref. 31), and stress/strain in the solid. then the structure is "healed." As shown Take Fig. 12 as an ex- in Fig. 14, under tension, the material ample and assume ex- near the nugget may open up and form pulsion through Side R cracks. A zone of solid with low strength, as is frequently observed or even a partially molten zone, exists be- --" ¢" f t: ,,.. during multispot weld- tween the root of the cracks and the - - -'-,<. :!,'" ing. The liquid metal 0"11 nugget. Because of the high liquid pres- M,il pressure in the nugget sure in the nugget, which can be more ab, i exerts a force to the than 100 MPa (estimated by the authors sides in the direction in Ref. 31), the liquid in the nugget may ul I parallel to the faying in- be pushed at a very high speed through terface; this is illustrated the mushy zone into the opening and --~ Nugget .~~" ~ in Fig. 13B. The value of may follow the extension of cracks. An- ...... -~-~@,~.~2 forces PL and PR can be other source of filling material is the liq- i | IJI approximated as liquid uid eutectic existing at the grain bound- Fig. 14 -- Schematic illustration of crack filling mechanisms. Ar- pressure multiplied by aries near the weld interface. But this rows indicate possible paths for liquid being ejected into the half of the nugget eutectic alone is not sufficient to fill height. Because of the cracking gaps because of its limited ill. I crack. constraint on Side L, PL amount (6% estimated for AA5754 ac- has virtually no influ- cording to equilibrium phase diagram). Contribution of Expulsion I0 Cracking ence on the stress and iill The healing process is not always possi- @'If strain in the solid, while PR induces ten- ble because it depends on the resistance Expulsion stems from liquid metal sile stress and strain by extending and of the mushy zone to liquid metal ~ t pressure (Ref. 31), and its contribution to bending the workpiece-- Fig. 13 B. Once flow/penetration. Ifa crack is formed dur- cracking is mainly mechanical. Expul- part of the liquid metal is pushed out, the ing heating, or close to the nugget, it has sion itself is the ejection of liquid metal solid between the electrode and liquid a high chance of being filled. A crack that from the nugget during heating, so it nugget moves inward, adding tensile forms when cooling starts, or that initi-

2OO-s I JULY 2000 ates far from the weld interface, has less Boomer (Alcan Aluminum Co.) for help- terials, pp. 275-282, and R. A. Patterson, K. W. chance to be filled; or it can be only par- ing with specimen preparation and M. Mahin, eds., ASM International, Materials Kimchi (Edison Welding Institute) for pro- Park, Ohio. tially filled. Further study is needed to un- 17. Anik, S., and Dorn, L. 1991. Metal derstand how welding parameters influ- viding facilities for part of the structural physical processes during welding -- weld- ence cracking in aluminum alloys. Based analyses. ability of aluminum alloys. Welding Research on the cracking mechanisms in multispot Abroad37: 41-44. welding AA5754 discussed in this paper, References 18. Rosenberg, R. A., Flemings, M. C., and methods are proposed to minimize/elim- Taylor, H. F. 1960. Nonferrous binary alloys 1. Cole, G. S., and Sherman, A. M. 1995. hot tearing. Transactions of the American inate cracking, as described in Ref. 33. Lightweight materials for automotive applica- Foundrymen's Society 68:518-528. The conclusions drawn here can be used tions. Materials Characterization, 1 : 3-9. 19. Pellini, W. S. 1952. Strain theory of hot in practice to prevent cracking and, 2. Irving, B. 1995. Building tomorrow's au- tearing. The Foundry 80:125-133. therefore, to make aluminum alloys more tomobiles. Welding Journal 74(8): 29-34. 20. Bishop, H. F., Ackerlind, C. G., and suitable for structural use. 3. Osterman, F. 1993. Aluminum Materials Pellini, W. S. 1957. Investigation of metallur- Technology for Automobile Construction. Me- gical and mechanical effects in the develop- chanical Engineering Publications Ltd. London. ment of hot tearing. Transactions of the Amer- Conclusions 4. Automotive Sheet Specification. Alcan ican Foundrymen's Society 65: 247-258. Rolled Products Co., Feb. 1994. 21. Flemings, M. C., Uram, S. Z., and Tay- Cracking in resistance welding is a 5. Burger, G. B., Gupta, A. K., Jeffrey, P. W., lor, H. F. 1960. Solidification of aluminum complicated subject because of the com- and Lloyd, D. J. 1995. Microstructural control castings. Transactions of the American plex nature of spot welding. The analyses of aluminum sheet used in automotive appli- Foundrymen's Society 68: 670-684. of cracking mechanisms presented in this cations. Materials Characterization 35:23-34. 22. Borland, J. C. 1961. Suggested expla- paper are attempts to provide an under- 6. Keay, B. B. 1992. Welding of aluminum nation of hot cracking in mild and low alloy for automotive body assembly. AWS Sheet standing of cracking phenomenon and to steel welds. British Welding Journal 8: Metal Welding Conference V, Detroit, Mich., 526-540. prevent it from happening. The following Paper No. D4. 23. Prokhorov, N. N. 1968. Theorie und conclusions can be drawn from this study: 7. Newton, C. J., Browne, D. J., Thornton, verfahren zum bestimmen der technologis- 1) Aluminum Alloy AA5754 was M. C., Boomer, D. R., and Keay, B. F. 1994. The chen festigkeit von metallen beim schweissen. found susceptible to cracking during re- fundamentals of resistance spot welding alu- 5chweisstechnik 19:8-11. sistance multispot welding according to minum. Proc. AWS Welding Con- 24. Hemsworth, B., Boniszewski, T., and the welding sequences under certain cir- ference VI, Detroit, Mich., Paper No. E2. Eaton, N. F. 1969. Classification and definition cumstances. 8. Hao, M., Osman, K. A., Boomer, D. R., of high temperature welding cracks in alloys. and Newton, C. J. 1994. Development in char- , Feb., pp. 5-12. 2) Intergranular characteristics of fail- acterization of resistance spot welding of alu- 25. Thompson, R. G. 1990. Inter-granular ure surfaces and their dendritic morphol- minum. Proceedings of 75th AWS Welding liquation effects on weldability. Weldabilityof ogy indicate the formation of cracks at Conference, Philadelphia, Pa., Paper No. Materials, pp. 57-63, R. A. Patterson and K. W. near-solidus temperatures and also the 94089. Mahin, eds., ASM International, Materials possibility of crack filling by liquid metal. 9. Browne, D., Newton, C., and Boomer, D. Park, Ohio. 3) Metallurgical analysis revealed fa- 1995. Optimization and validation of a model 26. Brooks, J. A. 1990. Weld microsegre- vorable conditions for cracking in the to predict the spot weldability parameter lobes gation: modeling and segregation effects on for aluminum automotive body sheet. Proceed- weld performance. Weldability of Materials: HAZ during RSW as the result of a liquid ings of International Body Engineering Confer- 41~-7, R. A. Patterson and K. W. Mahin, eds., layer formation at grain boundaries due ence, Advanced Technologies and Processes ASM International, Materials Park, Ohio. to melting and liquation. Section, Detroit, Mich., pp. 100-106. 27. Randhakrishnan, B., and Thompson, R. 4) A thermomechanical analysis 10. Thornton, P. H., Krause, A. R., and G. 1992. A model for the formation and so- showed that local stress and strain in the Davies, R. G. 1996. The aluminum spot weld. lidification of grain boundary liquid in the HAZ from the unconstrained side of a Welding Journal 75(3): 101 -s to 108-s. heat-affected zone (HAZ) of welds. Metallur- weldment became tensile during weld- 11. McGregor, I. J., Gao, I., Sheasby, P., gical Transactions A 23A: 1783-1799. and Wilson, I. 1995. Weld-bonding: a joining 28. Cross, C. E., and Olson, D. L. 1986. ing and may overcome weakened mate- technology for aluminum structured vehicles. Hot tearing model to assess aluminum weld- rial strengths at elevated temperatures. Proceedings of In ternationa I Body Engineering abil ity. Aluminum AIloys -- Their Physica I and 5) The phenomena of material flow Conference, Automotive Body Materials Sec- Mechanical Properties Ill: 1869-1875, and thermal expansion/shrinking, as tion, Detroit, Mich., pp. 76-84. ELMAS, E. A. Starke and T. H. Sanders, eds. well as expulsion in the process of mul- 12. Wang, P. C., Chisholm, S. K., Banas, 29. T. B. Massalski, ed. 1990. BinaryAIIoy tispot welding, contribute to the devel- G., and Lawrence, F. V. 1995. The role of fail- Phase Diagrams, 2nd Ed., p. 130. ASM Inter- opment of stresses in the solid phase (in ure mode, resistance spot weld and adhesive national, Materials Park, Ohio. on the fatigue behavior of weld-bonded alu- 30. Michie, K. J., and Renaud, S. T. 1996. the HAZ) during heating and cooling. minum. Welding Journal 72(2): 41 -s to 47-s. Aluminum resistance spot welding: how weld 6) There is a high probability of in- 13. Watanabe, G., and Tachikawa, H. defects affect joint integrity. Proc. AWS Sheet stantly filling open cracks with highly 1995. Behavior of cracking formed in alu- Metal Welding Conference VII, Detroit, Mich., pressurized liquid from the nugget, if minum alloy sheets on spot welding. 48th An- Paper No. B5. cracks form and propagate before the nual Assembly of IIW, Stockholm. IIW Doc. 31. Senkara, J., Zhang, H., and Hu, S. J. electric current is shut off. This instanta- No. 111-1041-95. 1998. Expulsion prediction in resistance spot 14. Lippold, J. C., Nippes, E. F., and Sav- welding, submitted to Welding Journal. neous filling is a characteristic of spot age, W. F. 1977. An investigation of hot crack- 32. Zhang, H., Huang, Y., and Hu, S. J. welding aluminum alloys, which has not ing in 5083-0 aluminum alloy weldments. 1996. Nugget growth in spot welding of steel been observed in other welding or crack- Welding Journal 56(6): 171 -s to 178-s. and aluminum. Proc. AWS Sheet Metal Weld- ing systems. A separate preliminary ex- 15. Jones, J. A., Yoon, J. W., Riches, S. T., ing Conference VII, Detroit, Mich., Paper perimental investigation showed that and Wallach, E. R. 1994. Improved mechani- No. B3. cracking has no significant influence on cal properties for laser welded automotive alu- 33. Zhang, H., Senkara, J., and Wu, X. minum alloy sheets. Proc. AWS Sheet Metal 2000. Suppressing cracking in RSW AA5754 static tensile-shear strength. Welding Conference VI, Detroit, Mich., Paper aluminum alloys by mechanical means, sub- No. B2. m itted to Journal of Engineering Manufacture. Acknowledgments 16. Cross, C. E., Tack, W. T., Loechel, L. W., and Kramer, L. S. 1990. Aluminum weldabil- The authors greatly acknowledge D. ity and hot tearing theory. Weldability of Ma-

WELDING RESEARCH SUPPLEMENT I 201-s