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Crackin G in Spot Welding Aluminum Alloy AA5754 Cracking and Its Mechanisms During the Resistance Spot Welding of Aluminum Alloys Are Analyzed

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Crackin G in Spot Welding Aluminum Alloy AA5754 Cracking and Its Mechanisms During the Resistance Spot Welding of Aluminum Alloys Are Analyzed Crackin g in Spot Welding 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 arc welding 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 weldability" (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 forming 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 electrode 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.
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