CHAPTER 5 ALUMINUM ALLOYS

Transcription

1 CHAPTER 5 ALUMINUM ALLOYS Advances in metal joining process have led to the development of many innovative approaches towards various technologies. Meshram et al. (2007) revealed that the conventional fusion welding of many dissimilar metal combinations is not feasible due to wide difference in melting point, thermal mismatch..also the joining of dissimilar Al-alloys results in excessive porosity formation and cracking particularly in age hardened grades Cam et al 1997, 1999, 2000 ) and Pakdil et al. (2011). Such results are not encountered in the solid state welding processes. The peak temperature generated during this process is below the melting temperature of the material to be welded. Von Strombeck et al. (2001), Cavaliere et al. (2006), Cavaliere et al. (2008) Cabello Munoz et al. (2008) Feng et al. (2010) revealed that the mechanical properties of the joints produced by this technique in Al-alloys are usually better than those obtained by fusion joining of these materials. Continuous direct drive friction welding is a solid-state welding process where metallic bonding is forged at temperatures lower than the melting temperature of the base metals and prevents weld defects and hazardous fumes, Gupta et al. (2006). Lightweight high strength age hardened aluminum alloys AA7075-T6 are extensively used in aerospace, transport sectors. The specimen is solution treated at 480 C for 2 hours and aged at 120 C for 24 hours to obtain peak strength. This alloy is called extra super duralumin amongst Al-Zn-Mg group 7000 series alloys. It consists of zinc as a primary alloying element, which is capable of producing high strength weld joints and low-melting-point compound. After a solution treatment at 540 C for 1 h and ageing at 160 C for 12 hours, wrought alloys of AA6061-T6 consisting of elements like Al- Mg-Si from the 6xxx series have been found to have good weldability. Hence Al-Mg- Si alloys are widely used for structural components and weld assemblies Cam et al. (2007). Bahemmat et al. (2010) revealed that deterioration of mechanical properties takes place in 6xxx series aluminum alloy due to the generation of heat during the process of welding.

2 5.1 TENSILE STRENGTH The samples were welded in accordance with the parameters incurred through the Taguchi s orthogonal array. Fabricated welded joints are shown in Figure 5.1. The ultimate tensile strength of the welded samples varied from 53 MPa to 203 MPa with the maximum elongation of the strain rate of 7.88 % shown in Figure 5.2 (a, b). It was observed that the increase in the upset and friction pressure and decrease in the rotational speed during the experiment gave rise to better results in terms of tensile strength and ductility. Sample 1 show the very little flash height of 28.5 mm which exhibited lower tensile strength in contrast to the highest tensile strength obtained with a flash height of mm; similar flash study results were identified, i.e., less flash formation tends to give low tensile strength. The results of the weld flash are shown in Table Sample 1 is fractured at the weld zone. This brittle fracture occurred with a lower tensile strength of 53 MPa and a lower strain rate of 2.71%. A sample stressstrain curve of the welded samples is shown in Figure 5.2.The samples 3 and 9 having maximum ductility and low tensile strength. Even though the strength of the weldment is less compared with the base metal, in T6 condition the fracture zone of sample 1 occurred only at the weld while others occurred at thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ), and base metal (BM) which showed acceptability for fitness of purpose. The failure occurred in the base metal AA6061-T6 side because of higher ductility. This could be due to the presence of alloy manganese. Since the joints have dissimilar combination, the joint efficiency can be found by selecting the material in which the failure occurs. By comparing the joint tensile strength with base metal, the base metal ultimate tensile strength was found to be 276 MPa. The obtained joint strength was 203 MPa which was broken at AA6061-T6 side. In this study, maximum joint efficiency was about 74 % of the base metal which was obtained by the ratio of ultimate tensile strength of weld joint to the ultimate tensile strength of the base metal. The flash features of different welding process parameter shows that too low flash thickness produce insufficient bonding with low

3 tensile strength. Higher flash shows good ductility medium flash shows high tensile strength. Figure 5.1 Fabricated Friction Welded Dissimilar Weld Joints Figure 5.2 (a) Tensile Graph for Low Tensile Strength (Friction Pressure 18MPa, Upset pressure 22 MPa rotational speed 1000 rpm, BOL 1mm)

4 Figure 5.2 (b) Tensile Graph for High Tensile Strength (Friction Pressure 32MPa, Upset pressure 44 MPa, Rotational speed 1250 rpm, BOL 1 mm) Figure 5.3 (a) Process Parameters during Welding Process Level 1 Friction Pressure 18 MPa, 22MPa, 1000rpm, BOL 1mm

5 Figure 5.3 (b) Level II FP 22 MPa, UP 32 MPa, N 1250 rpm, BOL 2 mm Figure 5.3 (C) Level III FP 32 MPa, UP 44 MPa, N1500 rpm, BOL 3 mm

6 Table 5.1 Results of Ultimate Tensile Strength for Different Process Parameters Sample ID. Friction pressure (MPa) Upset pressure (MPa) Speed of rotation (rpm) Burnoff Length (mm) Tensile strength (MPa) Table 5.2 Flash Values of Different Parameters Sample no. Length (mm) Flash Thickness (mm) Flash Height (mm)

7 5.2 METALLURGICAL STUDIES The joints were studied under low magnification using a stereo zoom microscope. Generally, the fusion- welded joints are affected by solidification cracking, slag inclusion etc. In CDDFW, the joints are free from solidification cracks due to the welding temperature not exceeding around 80 % of melting point which is sufficient to make a plastic flow between the materials. The confirmative test was performed by the radiography method to determine the defects on the welded joints. The voids and the lack of penetration are obtained which are similar to those already reported by Silva et al. (2010) some earlier researchers and other samples were accepted within the limits. Lack of penetration allows the natural stress to rise from which a crack may propagate and it causes the lamellar cracks. This may be due to low friction pressure and upset pressure which is evident from the radiography tests as shown in Figure 5.4 (a). From Figure 5.4 (b) it can be clearly seen that the metals were not bonded throughout the circumference of the samples due to the lower value of friction pressure (sample 1). This resulted in lower heat generation and creation of voids that led to the fracture with less joint strength of 53 MPa. This seems to indicate that the joint strength will increase as the friction and upset pressures increase due to severe plastic deformation shown in Figure 5.5 (a). Ravikumar et al. (2013) studied the macro structure of friction welding of a similar aluminum alloy AA6061-T6 condition of the welded samples shows that there is discontinuity and lack of fusion at the different zones. The parent metal shows parallel flow of grains along the longitudinal direction on both sides. At the edge both the materials show the change of direction of flow of grains perpendicular to the prior grain direction of the parent metal. The interface zone is free from voids and discontinuity. The plastic zone (no melting occurs) is measured as ranging from 2.0 to3.5 mm in Figure 5.6 (a, b).

8 Figure 5.4 Radiography test samples: (a) with no defects, (b) with defect Figure 5.5 (a) Sample 1 Friction Zone with Less Plastic Deformation (FP 18 MPa, UP 22 MPa, N 1000 rpm, BOL 1 mm)

9 Figure 5.5 (b) Sample 3 Friction Zone with Medium Plastic Deformation (FP18 MPa, UP44 MPa, N1500 rpm, BOL 3 mm) Figure 5.5 (c) Sample 9 Friction Zone with Fine Severe Plastic Deformation. (FP 32 MPa, UP44 MPa, N1250 rpm, BOL 1 mm)

10 Figure 5.6 (a) Macro Structure for Sample (FP18 MPa, UP22 MPa, N1000rpm, BOL 1 mm) (b) Macro Structure for Sample (FP 32 MPa, UP44 MPa, N1250 rpm, BOL 1 mm) 5.3 MICROSTRUCTURE In the BM of AA7075-T6 alloy, precipitation hardening occurs by artificial aging. It consists of acicular eutectic precipitates embedded in the aluminum matrix. The grains thus observed are found to be in an elongated fashion. The second-phase undissolved particles, such as Al 2 Cu Fe or Al 2 Cu Fe 4, are identified in the base metal. The BM of AA6061-T6 consists of elongated grains of the parent metal with fine precipitated particles of Mg 2 Si eutectics, which are evenly present. Microstructures at different regions are shown in Figure 5.7 (a-g). The microstructure of the BM AA7075 alloy is drawn after T6 condition. The grain flow is formed along the direction of the forming/rolling. The microstructure shows elongated grains of aluminum solid solution with finely precipitated eutectics of the strengthening phases. The various precipitated eutectics are Zn-Al 2 Mg 2 Si, MgAl 2, and CuAl 2. The microstructure of BM shows the wrought alloys of AA6061-T6 with grain orientation of banded structure along the direction of extrusion. It reveals that

11 precipitate alpha grain particles are present as in the aluminum matrix distributed in elongated script-like structure with fine spheroids. It is inferred from the heat affected zone AA6061-T6 side that inter-metallic particles are in a dispersive manner which are closely spaced grains and it shows slightly coarse grains which might be due to the lower temperature difference while compared with interface and thermo-mechanical zones. Thermo-mechanical heat affected zone of the AA6061-T6 alloy is recrystallized with fine dispersed spheroids observed with slightly more precipitates than the base metal when the direction of flow is changed perpendicularly. In the thermo-mechanical heat affected zone AA7075-T6, the direction of flow patterns of grains was changed to the raw material flow due to severe plastic deformation. The elongated grains of the parent metal were changed perpendicularly, the presence eutectics were also observed and these particles were of considerable quantity having fine dispersive nature. The heat affected zone AA7075-T6 shows elongated coarse grains with intermetallic particles which are agglomerated in an irregular pattern towards the flow. This is due to the lower temperature difference when compared with interface and thermo mechanical zones. The microstructure shows the interface zone which is dynamically recrystallized, that there is sufficient plastic deformation and good bonding between these alloys which gave high tensile strength. The high strength is also due to the fine equiaxed grains. All the dissolved eutectics of the alloy have re-appeared due to heat. The grain of the eutectic has fragmented and present as agglomeration with fine grains. This zone contains the eutectic particles of both AA7075-T6 and AA6061-T6. Very fine particles, mostly AA7075-T6 diffused in AA6061- T6, are observed.

12 Figure 5.7 (a) Microstructure of the Parent Metal AA7075-T6 (b) Microstructure of the Parent Metal AA 6061-T6 Figure 5.7 (c) Microstructure at HAZ of AA 6061-T6 (d) Microstructure at TMAZ AA6061-T6; Figure 5.7 (e) Microstructure at TMAZ of AA7075-T6 (f) Microstructure at HAZ of AA7075-T6

13 Figure 5.7 (g) Microstructure at the Weld zone. 5.4 CRYOGENIC TREATMENT Mechanism of Cryogenic Treatment By treating the aluminum alloys at cryogenic treatment the aluminum matrix will close the internal defects like micro pores and vacancies and by reducing the lattice parameters and also removing solute atoms and more volume of second phase particles causes strength to be increased. By treating the steel in cryogenic temperature and tempering it increases the properties of the materials by precipitation of fine carbides and by homogeneous carbide distribution. The two main mechanisms in cryogenic treatment of steel is transformation of retained austenite and low temperature conditioning of martensite. For cryogenic treatment the sample were selected based on low, medium and high ultimate strength Tensile Strength of Shallow Cryogenic Treatment The welded samples which were selected were subjected to (SCT) for two different time periods, 48 hours and 72 hours. Tensile tests were conducted for the samples with three repetitions. The tensile tests results are tabulated in Table 5.3. The tensile strength varies from 48 MPa to 224 MPa, depending upon the process parameters used and the treatment done to the specimen between room temperature and sub zero temperature for 72 hours. Sample 4 shows an ultimate tensile strength of

14 224 MPa at 72 hours, with the increase in strain rate of %. The cryogenically treated sample shows improved strength and the failure occurs in the base metal AA6061-T6 side. The reason for higher ductility and failure on the AA6061-T6 side could be due to the presence of manganese alloy. However, the results of the tensile test show that sample 1 break at the weld showing a brittle fracture exhibiting a lower tensile strength of 81 MPa, and a low strain rate of 3.41 %. The Sample 4 which was treated for 72 hours under sub zero temperature, shows improvement in tensile strength and strain rate, due to the presence of ultra fine grains as shown in the microstructures. The fracture location of the specimen occurs at four zones namely, (i) weld (ii) thermo mechanically affected zone (iii) heat affected zone and (iv) parent metal Hardness The micro hardness profiles of samples at different temperatures, in different zones are plotted by varying distances from the center (Weld region) to parent metal are shown in Figure 5.8 High hardness is obtained at the weld region. At HAZ, the hardness is less, and it gradually increases up to 181 VHN towards the parent metal AA7075-T6 side. The maximum hardness value is obtained as 101VHN towards the AA6061-T6 side. Figure 5.8 Micro Indentation of the Micro Structural Specimen

15 Table 5.3 Ultimate Tensile strength for AA7075-T6 to AA6061-T6 Aluminum alloys with different temperature Parameters Tensile strength(ts) (MPa) Room Temperature Tensile strength(ts) (MPa)48 hours Tensile strength(ts) (MPa) 72 hours Repetitions Fracture Location At Weld TMAZ HAZ Parent metal

16 Figure 5.9 Micro Vickers Hardness for Friction welded joints at Different Temperatures Microstructure of weld joints at Shallow cryogenic treatment The microstructures of the friction welded dissimilar metals of AA6061- T6 and AA7075-T6 as shown in Figure 5.10 (a-g) reveals that breakage occurs at the parent metal due to their good ductility so that it was broken at the parent metal. The microstructures of the parent metal in all the samples, including the cryogenic treated samples reveal a banded structure of coarse grains getting finer on their course towards the weld zone. In contrast, the sample which experienced the cryogenic treatment for a longer duration shows ultra fine, equiaxed grains of size range from ASTM-8 to ASTM-6.These finer grains dominates in the weld zone and the comparative differences is shown in Figure 5.10 (d, e). During SCT, it is clearly observed that there is a change in the grain size due to recrystallization but there is no major change in the orientation of grains.

17 In thermo mechanically transformed zone AA7075-T6 with direction of flow of grains changed almost perpendicular to the flow of raw material. The grain growth of the parent metal with the eutectics also observed. The elongated grains of the parent metal are changed due to partial re-crystallization. However the elongated grains have resolved and separated substantially. In interface zone shown in Figure 5.10 (e), reveals that dissolved eutectics of the alloy have reappeared due to frictional heat. The grains of the eutectics have fragmented and present as agglomeration. This zone contains the eutectic particles of both AA7075-T6 and AA6061-T6. In HAZ AA7075 T6 parallel flow of grains has almost re-crystallized and the grain growth could be seen in circular form. The eutectic particles have become coarser. The TMA zone of the T6 is recrystallized and changed its direction of flow diagonally at the edge. The HAZ of AA6061-T6 shows the material structure of heat affected zone in this the grain are oriented in opposite direction of the flow of grain in parent metal. Figure 5.10 (a) Sample at Interface Zone of Dissimilar Aluminum Weld joint of AA7075-T6 to AA6061-T6

18 Figure 5.10 (b) Dissimilar Aluminum Weld Joint of AA7075-T6 to AA6061T6 treated for 72 hours Figure 5.10 (c) Dissimilar Aluminum Weld Joint of AA7075-T6 to AA6061-T6 treated for 48 hours

19 ϭϳϯ Figure 5.10 (d) Dissimilar Aluminum Weld Joint of AA7075T6 to AA6061- T6 Treated for 72 hours (at weld) Figure 5.10 (e) Dissimilar Aluminum Weld Joint of AA7075 to AA6061- T6 Treated for 72 hours (at weld)

20 ϭϳϰ Figure 5.10 (f) Thermo Mechanical Zone for Dissimilar Aluminum Weld joint at AA7075-T6 Figure 5.10 (g) Thermo Mechanical Zone for Dissimilar Aluminum Weld Joint at AA6061 Side

21 Table 5.4 Tensile strength at Deep Cryogenic Treatment SAMPLE ID Treated at 2 hours MPa Treated at 4 hours MPa Low Tensile Strength Medium Tensile Strength High Tensile Strength The tensile test was carried out by deep cryogenic treatment on low, medium, high tensile strength sample. The graph shows the stress Vs strain diagram. The strain rate is low when compare to the sample for high tensile strength sample the sample treated at for 2 hours treated sample this shows that the ductility is reduced when the time period is increased. At the same ultimate tensile strength also is high MPa. The medium UTS sample shows good ductility even though the time period does not shows any drastic change. From this it is concluded that deep cryogenic treatment shows higher tensile strength when compare to shallow cryogenic treatment. Figure 5.11 (a) Tensile Graph between Stress Vs Strain at 4 Hours

22 Figure 5.11 (b) Tensile Graph between Stress Vs Strain at 4 Hours Micro Structure for Deep Cryogenic Treatment The microstructure of the weld joint subjected to DCT for high tensile strength with the duration of 2 hours is shown in Figure Depending upon the nature of secondary phase created in this system it shows the nucleation process should have an inherent characteristic time, so that the highest population density of secondary carbides is observed for a holding time of 2 hrs during deep cryogenic treatment. Finest uniform grains were induced at the weld region to take ultimate tensile strength greater when compare to four hours holding time of the welded samples. In TMAZ region of AA6061-T6 side there is a flow of grains were moved towards the interface as elongated manner. In TMAZ zone of AA7075-T6 side the flow of grain are perpendicular to the faying part and also the grains are finer with small precipitates.

23 Figure 5.12 (a) Weld Zone for Cryogenically Treated at 2 hours Figure 5.12 (b) TMAZ for AA6061-T6 at Deep Cryogenic Treatment for 2 hours

24 Figure 5.12 (c) HAZ for AA7075-T6 Alloy for 2 hours Figure 5.12 (d) TMAZ for AA7075-T6 at Deep Cryogenic Treatments for 2 hours 5.5 HEAT TREATMENT ON DISSIMILAR ALUMINUM WELDED JOINTS The Figure 5.13 shows the microstructure of heat treated sample at 4 hours. Even though the tensile strength was improved when compared to room temperature, coarser grains were observed in the conventional heat treatment process in the form of

25 partially dissolved eutectic particles and the maximum tensile strength were observed at 151Mpa.In Solutionizing and ageing treatment the ultimate tensile strength is lowered for the few samples when compared to room temperature. Figure 5.13 (a,b) Microstructure of Heat Treated Sample SEM and EDS Study The chemical composition of the weld zone shows the presence of all the constituents of both the wrought aluminum alloys of namely AA 6061 & AA7075 both in T6 conditions. The percentage of elements present at the fusion zone is different from each individual alloys. The percentage of elements particularly shows lower value due to dilution caused by fusion of the AA 6061-T6 which does not have zinc. Hence it is evident that the agglomeration has taken place between the

26 constituent elements of two metals at the weld. Similarly the percentage of Si is lowered in the fusion which is higher in AA6061-T6. The copper percentage in AA7075-T6 is lowered in the fusion zone. The percentage of Magnesium is lowered which was higher in AA7075 and lower in AA60061 in T6 conditions. The value obtained is neither of the AA7075 nor of the AA6061-T6 conditions. The thermal effects in the other zones of the process had resulted in loss of certain elements like magnesium and zinc. Both the elements show lower value due to evaporation. The percentage of some of the elements shows lower value as the zone is heat affected zone and the possibilities of loss of certain percentages shown in the Table 5.5. For example the value of magnesium is lower which is susceptible for evaporation. The presence of oxygen is due to oxidation that had taken place during the welding process. Table 5.5 Elements Present in Welded Samples at Low and High Tensile Strength Elements C K O K MgK Al K CuK Si K ZnK Wt.% (Sample3) Atomic% (Low tensile strength) Wt.% (High tensile strength) Atomic% (Sample3)

27 Figure 5.14 (a) EDS for Low Tensile Strength Sample Figure 5.14 (b) EDS for High Tensile Strength Sample

28 Figure 5.15 (a) SEM Image at the Interface Zone for Low Tensile Strength Sample (b) SEM Image at the Interface Zone for High Tensile Strength Sample Figure 5.15 (c) SEM Image at the TMA zone AA6061-T6 for High Tensile Strength sample (d) SEM Image at the TMA zone AA7075-T6 for High Tensile Strength Sample

29 Figure 5.15 (e) SEM Image at Weld Zone for High Tensile Strength Sample The SEM images in Figure 5.15 (a, b) are taken on different regions of AA6061-T6, AA7075-T6 materials. The microstructure at the AA6061 side shows the particles of Al 2 O 3 with dissolved constituents of Mg 2 Si particles while the interface zone consists of constituents of AA6061-T6 & AA7075-T6.The microstructure shows fine re-precipitated particles in AA7075 T6 which are re-crystallized due to the thermal effect. The dark particles are Cu-Al 2 and fine particles are MgAl 2.The parent metal shows re-precipitated particles from AA7075-T6. The SEM image shown in 5.15 (c) depicts the thermo mechanical transformed zone of AA6061material results in grain flow in a direction normal to the longitudinal direction present in the raw material. The plastic deformations of the grains are seen in different direction. The SEM image shown in Figure 5.15 (d) is from the AA6061-T6 TMA zone side.the matrix shows the orientation of the grains of AA6061 which have recrystallized in a direction normal to the extrusion direction. The particles present are the fragmented and re-crystallized eutectic particles of AA6061-T6. SEM image in Figure 5.15 (e) shows the fusion zone to close proximity to the AA7075-T6.The fusion zone shows fine particles of eutectic which are the constituents of both sides.

30 5.5.2 Impact Studies on AA6061-T6-AA7075-T6 Joint To evaluate the impact toughness values of the welded joint, Charpy V-notch tests were performed on friction-welded joints at various temperatures, such as room temperature, cryogenic temperature and critical temperature. From the Table 5.6 it is revealed that aging for at 2 hours shows improvement as more energy is absorbed when compared to all the other process. The reason for reduction in impact strength in cryogenic treatment is due to the formation of finer grain of secondary particles present in the micro structure which was discussed earlier. The scanning electron microscope of the impact fracture at different post weld treatment is shown in Figure 5.17and5.18. During cryogenic treatment intrinsic brittle fractured occurred easily and form a cracking linearly by reducing the deformation energy when strain exceeds the critical value. Ductile failure occurred when healthy population of voids of varying size and dimples of elongated manner indicate ductile fracture was observed during the conventional heat treatment. There is no indications of micro failure mechanism were observed due to overload fracture. Figure 5.16 (a) Impact Specimen at Room Temperature

31 Figure 5.16 (b) Impact Specimens at Cryogenic Treatments Table 5.6 Impact Test Results of AA6061-T6-AA7075-T6 after Various Treatments Energy absorbed (J) Parameters levels RT R SCT 48hrs. SCT 72hrs. 72 DCT 2 hrs. 2hrs. DCT 4 hrs. 4hrs. Aging at 2 hrs. Aging at 4 hrs. Low Medium High

32 Factrography of Impact test specimen Figure 5.17 Scanning Electron Micrograph of the Impact Specimen as Cryo Treated indicating the Cracks with Brittle Failure Figure 5.18 (a) Shallow Voids with Ductile Fracture (b) Homogeneous Voids and cracks

33 Figure 5.18 (c) Fine Dimples with Micro Pores and pits 5.6 QUICK FIELD ANALYSIS OF ALUMINUM WELDED SAMPLES It is inferred that the maximum stress value occurred at a distance of 6 mm towards the right of the interface, which closely agrees with the experimental results. The heat flux values for AA6061 were higher whereas, the Von Mises stress for the same is comparatively lower for the same sample. This proves that the Von Mises stress and heat flux are inversely proportional Structural Analysis Conclusions It is found that the Heat flux values for AA 6061-T6 was higher compared to AA7075-T6.This is because the thermal conductivity of AA6061-T6 was higher compared to that of AA7075-T6. Hence thermal conductivity and Heat flux are directly proportional. The maximum value of heat flux was obtained when friction pressure is 18 MPa upset pressure is 22 MPa and burn of length is 1mm. The minimum value of heat flux was obtained when friction pressure is 32 MPa, upset pressure is 44 MPa and burn of length is 1mm.The fracture of specimen took place at AA6061-T6 side during the tensile testing. Similar results were obtained during simulation of tensile testing. The values of stress were greater in AA6061-T6 side in the three specimens which led to the failure of AA6061-T6.It is found that when the heat flux of AA6061-T6 is higher,

34 the corresponding stress value is lower. Thus heat flux and von-mises stress are inversely proportional. The friction pressure and upset pressure are inversely proportional to the heat flux. Also, greater the values of friction and upset pressure, greater the stress value in AA6061-T6 side. Thus, the friction and upset pressure are directly proportional to stress. The ductility of the sample 3 was found to be greater than the other two samples. This is because when the friction and upset pressure was 22 Mpa and 44 Mpa respectively, and the burn off length was 3mm more fusion and transfer of grains took place near the interface. This improved the elongation of the 6061 side and led to the improvement in ductility of that sample. Shear fracture took place in the sample 9 whose shear plane was found to be 45 C. Since the friction and upset pressure were higher for this sample, and the interface temperature was lower, the heat affected zone was very narrow which led to the shear fracture. AA7075-T6 AA6061-T6 Figure 5.19 Meshing of Two Materials (a)

35 (a) Figure 5.20 (a) Temperature Distribution of Low Tensile Strength Sample (b) Heat Flux Variation of Low Tensile Strength Sample Figure 5.21 (a) Temperature Distribution of medium tensile sample

36 Figure 5.21 (b) Simulation of Medium Tensile strength Sample Figure 5.21 (c) Variation of Stress of Medium Strength Specimen

37 Figure 5.22 (a) Simulation of Tensile Strength on High Tensile Strength Sample Figure 5.22 (b) Variation of Stress of High Tensile Strength Sample 5.7 CORROSION STUDIES David Talbot and James Tabolt (1998) studied that passive oxide film can be easily formed on the surface of aluminum alloys when it exposed to air or water it is due to chloride ions. Kenneth R et al. (1996) also reveals that presence of chloride ions the corrosion rate is very high and also it depends on heterogeneity of their microstructure. Polarization resistance can be related to the rate of general corrosion for metals at or near their corrosion potential, Ecorr. Polarization resistance measurements are an accurate and rapid way to measure the general corrosion rate.

38 Electrochemical polarization test methods are extremely pertinent for understanding and evaluating the corrosion resistance of materials and the effect of changes in the corrosive environment. They can establish criteria for anodic or cathodic protection and susceptibility to several forms of corrosion. Venugopal et al. (2004) (63) studied micro-structural and pitting corrosion properties of friction stir weld of AA7075 Al alloy in 3.5%NaCl solution. Corrosion resistance of weld metal is better than that of TMAZ and base metal. Srinivasa Rao and Prasad Rao (2004) studied the mechanism of pitting corrosion of heat treatable Al Cu alloys and welds. In this study, electrochemical corrosion test by Tafel curve extrapolation method was carried out for low, medium and high tensile strength samples. Samples of base alloy AA6061 and AA7075 and friction welded area in sodium chloride solution of 3.5% to determine corrosion parameters such as 0corrosion potential (Ecorr.) and corrosion current (Icorr.) as shown in Table 5.7. Generally aluminum alloys, AA7075 are susceptible to galvanic attack near precipitates of MgZn 2 MgAlCu. Srinivasan et al. (2007) reported that the general corrosion resistance of the AA6056 parent material is better than the AA7075 parent material due to content of copper. The zinc rich precipitates in the AA7075 alloy causes the formation of micro galvanic cells, leading to higher rates of dissolution. Further, zinc is active when compared to aluminum alloy matrix, hence can enhance the corrosion rate. In this study, electrochemical corrosion test method was carried out for sample low, medium, high tensile strength. Samples of base alloy AA6061 and AA7075 and friction welded area in sodium chloride solution of 3.5% to determine corrosion parameters such as corrosion potential (E corr.) and corrosion current (I corr ) as shown in Table 5.7. The corrosion rate ranges from to mm/year. The low corrosion resistance rate is due to high friction pressure and upset pressure, at these high working the constituents elements in the parent metal of aluminum alloys disperse widely along the weld zone and they are susceptible to galvanic attack near precipitates of MgZn 2 MgAlCu. Due to zinc rich precipitates in the AA7075 alloy formation of micro galvanic cells, leading to higher rates of dissolution.

39 For acquiring the Tafel slope by polarization method a cyclic sweep was chosen at the sweep rate of 100 m V. The curve was determined by stepping the rest potential at a scan rate of 0.5 mv/sec, from -250 mv to +250 m V Vs. SCE. Figure 4 (a, b) shows the Tafel plot of weld metals of two samples S3, S9 and reveals the rest potential of , respectively. This result indicate that the sample 3 of the weld joint act as a cathode and give better resistance to corrosion when compared to high strength sample.in both the cases the weld metal (core zone) region the metal behaves more corrosion resistance compared to Heat affected zone, thermo mechanical affected zone due to homogeneous distribution of elements present in an aluminum matrix. Linear polarization resistance (LDR ohm cm 2 ) is used to find the measurement of corrosion rate for immediate feedback as well as tendency of pitting. The LDR values of medium strength and high tensile strength is ohm cm 2 and ohm cm 2 respectively. From the data it is clearly indicates that sample having high tensile strength has tendency for deep pits and it is validated in micro structural studies. Table 5.7 Corrosion Test Results of AA6061T6-AA7075-T6 Sample I.D Icorr Corrosion current in µa cm 2 Corrosion rate mm/ year Corrosion rate mils per year (mpy) Area of the sample Sq. cm Rest potential (mv) Low Medium High Optical microscopy study was conducted to the weld samples in parent of AA6061, AA7075 side, weld, HAZ region. The Figure 5.23 (a,b) shows the microstructure image of potential-dynamic polarization subjected location of AA 7075 wrought alloy surface. The effect of polarization leads to the formation of large corrosion pits. These pits are shown in black color cylindrical shape and occurred severely. In addition the grains boundaries are ditched and nowhere the

40 ϭϵϰ surface of the metal is free from corrosion pits. Hence the parent metal of AA7075T6 is mainly affected by localized corrosion pits. Figure 5.23 (a) Parent Metal AA7075 (b) Parent Metal AA6061 The Figure 5.24 (b) image shows the micro structure of after potentialdynamic polarization subjected to location of AA 6061 wrought alloy surface. The effect of polarization leads to the formation of large corrosion pits formed by the polarization. The pits formed by the potential applied are isolated and majority of the matrix is free from the effect of polarization effects of corrosion. Figure 5.24 (a) Microstructure Heat Affected Zone of corroded sample (b) Microstructure Heat Affected Zone of corroded sample

41 Figure 5.24 (c) Thermo-Mechanical Zone (sample 3) The Figure 5.25 shows the potential- dynamic polarization of the weld joints of sample (S3 parameters). The effect of polarization leads to the formation of corrosion pits of different gravity. The three corrosion surface images are taken from the fusion zone. Figure 5.25 (a) image is taken from the core of the fusion zone which shows better resistant to corrosion but it shows inter granular corrosion by polarization method. Elatharasan et al. (2014) also reveals that the affected zone of the weld exhibited highest susceptibility to inter-granular corrosion. This zone does not show the presence of deep pits instead, the grain boundaries are marginally ditched. The Figure 5.25 (b) is taken from the fusion zone which is closer to the AA7075 side. The constituents of the fusion zone are more diluted with 7075 alloy. Hence the corrosion behavior is more and follows the pattern of AA7075. The figure 5.25 (c) Shows the zone more which is more diluted with AA6061 and least affected. However the corrosion rate is the consolidated effect of the entire three zones as the corrosion cell covers all the three zones. In S-9 parameter the effect of polarization leads to the formation of corrosion pits of different gravity. The three corrosion surface images are taken from the fusion zone. All the three images shows (Figure 5.25) severe corrosion pits as well as grain boundary ditches but there is more resistance to corrosion while compare to other zones. It is evident that this surface should have shown more corrosion and the experimental values confirm the same. Moreover fusion zone is more diluted with AA7075 alloy constituents.

42 Figure 5.25 (a) Microstructure of High Tensile strength sample at core Zone (b) Microstructure of High Tensile strength sample at Heat Affected Zone Figure 5.25 (c) Microstructure of High Tensile strength sample at Thermo-Mechanical Zone