Shaik Shajahan, Swati Ghosh Acharyya
Keywords: Al5052, Galvanized mild steel, intermetallic phases, dissimilar weld joints, welding parameters, Microstructure, Corrosion
The current study investigated the effect of welding parameters such as wire feed rate and welding speed on the corrosion behavior of dissimilar alloy welds of Al 5052 and Galvanized mild steel. Al 5052 alloy and galvanized mild steel plates welded in the form of lap joint by Cold Metal Transfer (CMT) welding process and Pulsed Arc Metal Inert Gas (PAMIG) welding process using 4043 Aluminum alloy filler. Welding conducted at different welding parameters viz., welding speed (0.8 mm/min and 1.0 mm/min) and wire feed rate (5.8 mm/min and 6.5 mm/min). The microstructure and phase determination of the weld joints analyzed by Field Emission Scanning electron microscopy and X-Ray Diffraction respectively. Resistance of crevice and inter granular corrosion of the welds were studied as a function of different welding parameters as per ASTMG67-Al-alloys. The effect of welding parameters on the corrosion resistance of the joints and correlation of microstructural features such as formation of intermetallics at the interfaces etc. throughout specimens studied in detail.
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If you wish to view the human-readable version of the preprint, then please Register (if you have not already done so) and Login. Registration is completely free.Effect of Welding Parameters on Corrosion Behavior of Dissimilar Weld joints of Al5052 and Galvanized mild steel Shaik Shajahan PhD scholar, Department of Metallurgical & Materials Engineering, NIT Rourkela, Rourkela, India, 769008. firstname.lastname@example.org Swati Ghosh Acharyya Assistant Professor, Department of Materials Engineering, School of Engineering sciences and Technology,University of Hyderabad,Hyderabad, India, 500046. email@example.com Abstract: The current study investigated the effect of welding parameters such as wire feed rate and welding speed on the corrosion behavior of dissimilar alloy welds of Al 5052 and Galvanized mild steel. Al 5052 alloy and galvanized mild steel plates welded in the form of lap joint by Cold Metal Transfer (CMT) welding process and Pulsed Arc Metal Inert Gas (PAMIG) welding process using 4043 Aluminum alloy filler. Welding conducted at different welding parameters viz., welding speed (0.8 mm/min and 1.0 mm/min) and wire feed rate (5.8 mm/min and 6.5 mm/min). The microstructure and phase determination of the weld joints analyzed by Field Emission Scanning electron microscopy and X-Ray Diffraction respectively. Resistance of crevice and inter granular corrosion of the welds were studied as a function of different welding parameters as per ASTMG67-Al-alloys. The effect of welding parameters on the corrosion resistance of the joints and correlation of microstructural features such as formation of intermetallics at the interfaces etc. throughout specimens studied in detail. Keywords: Al5052, Galvanized mild steel, intermetallic phases, dissimilar weld joints, welding parameters, Microstructure, Corrosion. Introduction: Steel is widely used in automobile industries, construction industries, ships, aircrafts etc., due to having good strength, high formability and low production cost as compared to other materials. Nevertheless, the disadvantage with Steel is high weight and more fuel consumption. Currently, the concentration of automobile industries to reduce the weight of components or to employ alloys having low weight. Al alloys have been popular in automobile industries, aircrafts etc. due to its low weight and good corrosion resistance. However, Al alloys have less strength compared to steels and other alloys. Hence, an optimum of weight reduction and strength retention is the solution and a hybrid structure of Steel and Aluminum alloys would be ideal for this purpose. The components of the vehicle where strength is the primary requirement constituted of steel and the other components made up of Al alloys. In order to make vehicles more fuel efficient as well as to sustain its required strength, the automobile industries have shifted the focus towards partial replacement of steel with Al alloys J.L. Song , M.Sonia , M. Watanabe , J.Lin , H.T. Zhang . However, this introduces challenges in the fabrication of good quality ‘joints’ or in other words dissimilar alloy welds which would be the key in determining the strength of the automobile by ensuring the safety issues. There are different welding technologies involved in producing this kind of hybrid structure. The primary challenges in producing dissimilar weld joints of Al alloys and steel arise due to the difference in thermal conductivity, electrical conductivity and solid solubility, large variance in melting points chemical composition etc. H.G. Dong , Y. Shi , H.G. Dong , H. Springer . The dissimilar weld joints contribute many advantages such as low manufacturing cost, reduction of processing steps, production of controlled low weight to high strength ratio objects, high fuel efficiency etc. R. H. Wagoner , B. Kinsey , J. Tusek  They also have few disadvantages such as formation of brittle intermetallic phases, precipitates, welding defects etc. The welding defects occur due to the difference in melting points of joining metals. In case of aluminum alloy and steel, they have large variance in melting point that leads to formation of blowholes, porosity, pits etc. in the weld zone. In addition to melting point, introduction of residual stresses in the weld zone comes from the differences in specific heat, conductivity, thermal expansion etc. R. Qiu , J. Song , D. Lohwasser . The weld zone also may effected by the selection of filler metal. The filler selected will determine mechanical properties of the weld metal. The formation of intermetallic phases during welding process also depends on selection of filler metal. There are several researches in the progress to determine the effect of different filler metals on the formation of intermetallic phases and welding process. The corrosion behavior of the dissimilar metal weld joint effected by the formation of intermetallic phases, selection of filler metal, selection of welding process, welding parameters etc. H. Dong , A. M. Saeed , S.G. Shiri . The present work focuses on determining the effect of welding parameters on corrosion rate of Al5052-Galvanized mild steel lap joint by immersion corrosion test. To determine the effect of intermetallic phases and precipitates on dissolution of metal at the interfaces of Al5052-Galvanized mild steel by analyzing the microstructures that led to corrosion at the interfaces. Materials and Methods: In this study, four Al-5052 aluminum alloy and Galvanized mild steel (GMS) plates were lap welded by CMT welding process and PAMIG welding process with different welding parameters at ARCI, Hyderabad as per ASTMG67-Al-alloys G. Padmanabham . The filler metal used was 4043 Aluminum alloy with diameter of 1.2 mm. The compositions of Metal alloys, filler metal and welding parameters presented in Table 1 and Table 2 respectively. Table 1: Composition of metal alloys and filler wire (wt%). Al5052 GMS 4043 C S - - Si 0.069 0.35 0.039 0.011 0.010 - 4.5-6 Fe - P - Cu Mn Mg Cr Zn Ti Al 0.055 0.057 2.28 0.23 0.008 0.018 Bal. Bal. 0.04 0 0.21 0 0 3.7 0 - 0.8 - 0.30 0.05 0.05 - 0.4 0.20 Bal. Table 2: Welding parameters. Designation Sample Welding Speed Feed rate [mm/min] [mm/min] Sample 1 Al5052-GMS, CMT 0.8 5.8 Sample 2 Al5052-GMS, PAMIG 0.8 5.5 Sample 3 Al5052-GMS, CMT 1.0 6.5 Sample 4 Al5052-GMS, PAMIG 1.0 6.5 According to ASTM standard G67-04 5xxx aluminum alloys , the immersion corrosion test of Al-5052-Galvanized mild steel lap joints were conducted in 5% NaOH solution at 800C for 1 min, HNO3 for 30 sec and HNO3 for 24 h at 300C. Silicon Carbide cutting blade was used to cut the samples of required size (15mm×6mm) and initial polishing was done with grits of 100,200,600,800,1200,1/0,2/0,3/0,4/0 g and followed by 1μm diamond paste final polishing. Polished samples alternatively etched with Nital solution and Keller’s reagent to get a clear microstructure of the samples under FESEM (Field Emission Scanning Electron Microscope) to analyze the metallographic changes in the samples. The immersion corrosion test of Al-5052-Galvanized mild steel lap joints were conducted in 5% NaOH solution at 800C for 1 min, HNO3 for 30 sec and HNO3 for 24hrs at 300C. The weight of the samples taken at every step before and after immersion. The difference between weight before and after immersion test of samples was calculated and observed the weight reduction of samples due to effect of welding parameters and formation of intermetallics. Moreover, the reduction in surface area and volume fraction of the samples were calculated. Crevice corrosion and intergranular cracking observed at the interfaces. Results and Discussion: Microstructural Characterization: Figure 1 represents the basic microstructure of the cross-section of Al5052Galvanized mild steel lap joint. It shows interface between galvanized mild steel and weld pool at different welding parameters. CMT weld at welding speed 0.8 mm/min and feed rate 5.8 mm/min [Fig 1 (a)], CMT weld at welding speed 1 mm/min and feed rate 6.5 mm/min [Fig 1 (b)], PAMIG weld at welding speed 0.8 mm/min and feed rate 5.5 mm/min [Fig 1 (c)], PAMIG weld at welding speed 1 mm/min and feed rate 6.5 mm/min [Fig 1 (d)]. The formation of intermetallic phases at the interfaces clearly indicated in all cases with two different welding processes at two different welding parameters. The intermetallics formed at the interface of galvanized mild steel and weld pool identified as Al-Fe based (Fig 1). The intermetallic phases expanded zig-zag manner towards weld pool side with plate like shape. The EDS analysis shown in Table 3 was confirmed that the samples having intermetallc phases which were formed at the interface of weld pool and steel are Al-Fe based viz., Al5Fe2, AlFe, Al13Fe4, AlFe3 in all the samples  Y. Shi , L. H. Shah . Among the all the samples volume fraction of intermetallic phases was more in sample Fig 1 (a) and sample Fig 1 (c). which were welded with welding speed: 0.8 mm/min, feed rate: 5.8 mm/min by CMT and welding speed: 0.8 mm/min, feed rate: 5.5 mm/min by PAMIG respectively. Figure 1: Interface between GMS and weld pool under FESEM at different welding parameters (a) Sample 1 (b) Sample 3 (c) Sample 2 (d) Sample 4. Table 3: EDS analysis at interface of steel and weld pool (welding speed/feed rate) (wt%) Element CMT 0.8/5.8 CMT 1/6.5 PAMIG 1/6.5 PAMIG 0.8/5.5 C 0.16 1.85 1.48 1.49 Al 62.29 57.76 48.73 56.29 Si 1.30 1.58 1.44 1.19 Fe 36.25 38.81 48.35 41.03 Figure 2 shows the microstructure of the cross-section of Al5052-Galvanized mild steel lap joint representing the interface between Aluminum and weld pool at different welding parameters. CMT weld at welding speed 0.8 mm/min and feed rate 5.8 mm/min [Fig 2 (a)], CMT weld at welding speed 1 mm/min and feed rate 6.5 mm/min [Fig 2 (b)], PAMIG weld at welding speed 0.8 mm/min and feed rate 5.5 mm/min [Fig 2 (c)], PAMIG weld at welding speed 1 mm/min and feed rate 6.5 mm/min [Fig 2 (d)]. The evolution of secondary precipitates took place at the interface of Aluminum and weld pool in all the four samples with different parameters. Inter-dendritic structure and eutectic formation of Al-Si observed in all the samples at the weld pool region. The EDS analysis presented in Table 4 depicting the type of precipitates, which formed at Aluminum, side near the interface of Aluminum and weld pool. By EDS analysis, it was pointed that the precipitates formed were Mg5Si6, Mg2Si, Al4Si and Al-Mg-Si based precipitates, which were observed at Al side and Al-Si eutectic formations were found at weld pool side  Y. Shi , L. H. Shah . Figure 2: Interface between Al-weld pool under FESEM. (a) Sample 1, (b) Sample 3, (c) Sample 2, (d) Sample 4. Table 4: EDS analysis at interface of Al and weld pool (welding speed/feed rate) (wt%). Element CMT 0.8/5.8 CMT 1/6.5 PAMIG 1/6.5 PAMIG 0.8/5.5 C 0.62 4.23 3.13 0.00 Al 80.46 64.75 31.56 91.67 Si 17.49 8.33 11.37 2.35 Fe 1.43 10.28 10.02 5.98 Immersion corrosion test: The weld joints of Al5052-Galvanized mild steel with 4043 filler metal at different welding parameters were immersed in 5%NaOH solution at 800C for one min and observed the microstructure changes under FESEM to investigate the corrosion effect on weld joints. Dissolution of intermetallic phases occurred after the corrosion test that shown in Fig 5. However, the dissolution of intermetallic phases was higher than anticipated which comes from the effect of crevice corrosion on the weld joint. Corrosion on steel side took place due to formation of oxides, as Galvanized steel is highly prone to oxidize. Reduction in surface area that leading to weight loss of the samples has encountered due to the effect of corrosion. Weld pool Figure 5: Interface of Galvanized steel and weld pool after 5% NaOH immersion corrosion test for 1 min at 800C under FESEM. (a) Sample 1, (b) Sample 3, (c) Sample 2, (d) Sample 4. The Al5052-Galvanized mild steel welded with different welding techniques and different welding parameters to determine the effect of feed rate and welding speed on corrosion behavior of the welded joint. The samples welded with welding speed: 0.8 mm/min, feed rate: 5.8 mm/min by CMT and welding speed: 0.8 mm/min, feed rate: 5.5 mm/min by PAMIG have less dissolution of intermetallic phases at the interface that shown in Fig.5 (a), (b) respectively. Reduction in volume of intermetallic phases was also became lower that quantitatively shown in Table 6. Hence, the corrosion effect also was lower in sample 1 and sample 3 when compared with sample 2 and sample 4. Whereas, the dissolution of intermetallic phases in the sample 2 and sample 4 was higher as shown in Fig 5 (c), (d) that led to high reduction in volume fraction of intermetallic phases. Weight loss and surface area reduction were calculated which shown in Table 6. The complete corrosion rate calculated after 5% NaOH test for all the samples as shown in Table 4 and 5. Fig 6 shows the interface of Aluminum and weld pool of the four samples. However, Notable changes did not occur at the interface region. The main site of corrosion attack in each sample depicted as precipitate-matrix interface. The corrosion effect was higher in sample 2 and sample 4 as shown in Fig 6 (c), (d) respectively. Figure 6: Microstructure of aluminum and weld pool interface after 5% NaOH immersion corrosion test for 1 min at 80oC through FESEM. (a)Sample 1, (b) Sample 3, (c) Sample 2, (d) Sample 4. Fig 7 and 8 shows that the microstructure of the interface between Galvanized steel and weld pool and the interface between Al alloy and weld pool respectively after HNO3 immersion corrosion test. High dissolution of intermetallic phases took place at the interface of Galvanized mild steel and weld pool. Almost all intermetallic phases dissolved after 24 hr exposure to HNO3 acid as shown in Fig 7. After HNO3 immersion corrosion test, volume fraction of intermetallic phases reduced drastically for all the four samples as shown in Table 4, 5 and 6. The dissolution of 95% intermetallic phases has seen in sample 2, sample 3 and sample 4, on other hand in sample 1 it was only 90% volume reduction. The gap (pits) in the microstructure was observed after the dissolution of intermetallic phases (at the place of dissolution of intermetallic phases) in the weld joint which led to higher dissolution of the metal further led to crevice corrosion in the area of weld joint for all the four samples. Amongst of all the samples, sample 1 has less dissolution of intermetallic phases in the solution thus shown less crevice corrosion when compared to the remaining samples. Corrosion rate also was very low in case of sample 1 because of less dissolution of intermetallic phases in volume fraction when compared to the remaining samples as shown in Table 4, 5 and 6. In addition to the crevice corrosion, in sample 1 inter granular corrosion took place due to presence of residual stresses in the steel region as shown in Fig 7(a). The intergranular cracking was first initiated near the interface of steel and weld pool further propagated towards the interface of steel-weld pool that clearly indicated in Fig 7 (a). The requirement of optimization of the parameters in case of CMT welding process noticed. Figure 7: Microstructure of Galvanized steel and weld pool interface after 24hrs HNO3 immersion test. (a) Sample 1, (b) Sample 3, (c) Sample 2, (d) Sample 4. Figure 8: Microstructure of Aluminum and weld pool interface after 24hrs HNO3 immersion test. (a) Sample 1, (b) Sample 3, (c) Sample 2, (d) Sample 4. The interface of Aluminum alloy and weld pool after HNO3 immersion corrosion test was highly damaged due to presence of precipitates and dissolution of the metal at the interface. Pits nucleated at the precipitate-matrix interface. The size of the pits further increased that contributed to overlapping of pits. The formation of pits resulted in crevice corrosion at the interface as shown Fig 8. Fig 8 also describes, crevice corrosion was very low in sample 1 [Fig 8 (a)], whereas other samples were fully effected by crevice corrosion as shown in Fig 8 (b),(c) and (d). Corrosion rate calculated comparatively for all the samples presented in Table 10. Table 4: Corrosion rate of Al5052-Galvanized mild steel welded samples. Sample W1 A1(in2) (mg) W2 A2 ΔW W3 A3 ΔW W4 A4 (mg) (in2) (mg) (mg) (in2) (mg) (mg) (in2) ΔW Total (mg) weight loss (mg) PAMIG 3410 0.67 3387 0.63 23 3372 0.62 15 3334 0.52 38 80 3300 0.67 3268 0.61 32 3242 0.60 26 3183 0.47 59 117 3350 0.67 3316 0.61 34 3294 0.60 22 3245 0.45 49 105 3310 0.67 3301 0.66 9 3289 0.65 12 3259 0.60 30 52 0.8/5.5 CMT 1/6.5 PAMIG 1/6.5 CMT 0.8/5.8 (W1=Initial weight of the sample, A1=Initial Area of the sample, W2= Weight of the sample after 1min NaOH test and corresponding area is A2, ΔW=Weight loss, W3= Weight of the sample after 30 sec HNO3 test and corresponding area is A3, W4= Weight of the sample after 24 hr HNO3 test and corresponding area is A4, Table 5: Corrosion rate of Galvanized mild steel and Al5052 weld joint with different parameters. Sample id 𝟓𝟑𝟒 𝑾 𝑫𝑨𝑻 Corrosion rate (mpy) for 24hr HNO3 test 0.8/5.5 PAMIG 405 1/6.5 CMT 696 1/6.5 PAMIG 603 08/5.8 CMT 277 Table 6: Volume fraction of intermetallic phases. Sample id V1 V2 V3 ΔV 0.8/5.8 CMT 6.5 % 2.25% 0.7% 90% 1.0/6.5 CMT 6.3% 2% 0.5% 93% 0.8/5.5 PAMIG 6.1% 2% 0.4% 94% 1.0/6.5 PAMIG 6.3% 1.8% 0.35% 96% V1=Initial volume fraction of intermetallic phases, V2=Volume fraction of intermetallic phases after 5% NaOH test for 1 min, V3=Volume fraction of intermetallic phases after HNO3 test for 24hr, ΔV=Total percentage of reduction in volume fraction of intermetallic phases. XRD analysis: Figure 9: XRD analysis of Al5052-GMS weld joint (a) Sample 1, (b) Sample 3, (c) Sample 2, (d) Sample 4. The XRD analysis of the four samples are shown in the Fig 9. It was shown that a number of phases were formed after the welding process and the analysis of XRD confirmed that the Fe and Al based intermetallic phases such as AlFe 3, Al13Fe4, Al5Fe2,AlFe were formed at the interfaces of each sample. Some of the possible precipitates such as Mg5Si6, Fe2Si, Fe5Si3, FeSi etc. were also observed at the interfaces. In all four samples, Mg5Si6 is the most common precipitate which is formed at aluminum and weld pool interface region , Y. Shi . Conclusions: The corrosion behaviour of Al5052-Galvanized mild steel lap joint with 4043 filler metal at different welding parameters by Cold metal transfer welding process and Pulsed arc welding process was investigated by immersion corrosion test. The conclusions drawn as follows: I. Crevice and intergranular corrosion occurred when Al 5052-Galvanized mild steel lap welded joints immersed in 5% NaOH solution for 1 min at 80oC and 30 sec & 24 hr in HNO3 solution. The corrosion rate varied with welding parameters. 0.8/5.8 CMT sample showed least corrosion rate (277 mpy) as compared to 0.8/5.5 PAMIG (405 mpy), 1/6.5 CMT (696 mpy), and 1/6.5 PAMIG (603 mpy). II. High volume fraction of intermetallic phases detected at the interfaces of weld joints rigorously reduced the corrosion resistance of the joints in each case. III. The intermetallics present at the interface of aluminum and weld pool were highly prone to corrosive attack. The dissolution of intermetallics resulted in initiation of pitting corrosion. IV. Intergranular cracking observed in 0.8/5.8 CMT sample due to presence of residual stresses and requirement of optimization of the CMT parameters suggested. V. Crevice corrosion observed at the interface of galvanized mild steel and weld pool due to dissolution of intermetallic phases and at the interface of Al-alloy and weld pool due to formation of overlapped pits, those created by precipitates-matrix interface. VI. Immersion corrosion test indicated that the corrosion resistance of Al5052Galvanized mild steel sample, which is lap welded by CMT with welding parameters: welding speed 0.8 mm/min, feed rate 5.8 mm/min is superior for dissimilar weld joint than remaining samples. Acknowledgement: Authors would like to thank ARCI,Hyderabad for the processing and provision of materials. 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