CHAPTER 4 EXPERIMENTATION ON WELDING OF AA 6063 &AISI 1030 STEEL

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1 63 CHAPTER 4 EXPERIMENTATION ON WELDING OF AA 6063 &AISI 1030 STEEL 4.1 INTRODUCTION The main advantage of friction stud welding is the ability to join similar and dissimilar metals. Usually, welding of dissimilar metals has great difficulties because of the formation of intermetallic phase during the arc welding methods. For this reason, dissimilar metals cannot be joined using ordinary welding process. For the welding of dissimilar metals, friction welding process has been gaining importance in the last two decades (Tracie 2011). The welding of aluminium to low carbon steel is of particular interest to engineers, because the resulting products join the very different but favourable properties of each metal. The resulting products have a desirable combination of high thermal conductivity and low density of aluminium, and low thermal conductivity and the high tensile strength of steels (Erol 2006). The demand for aluminium-low carbon steel and especially aluminiumstainless steel joints has therefore increased in many areas including cryogenic applications, spacecraft, and high vacuum chambers and cooking utensils owing to their superior properties. In these structures, aluminium has been partially replaced by stainless steel and it becomes necessary to join stainless steel to aluminium alloys. Aluminium-mild steel friction welding has resulted in considerable cost saving, in the production of down hanger

2 64 assemblies. Kong et al (2009) experimentally investigated the joining of aluminium alloy with stainless steel by friction welding. Sahin (2008) carried out experimentation to determine the tensile properties of aluminium-stainless steel friction welded components. In the present work, dissimilar metals AA 6063 and AISI 1030 steel are joined by friction stud welding process and the resulting mechanical and metallurgical properties were investigated. Quality of any friction welded components depends on the proper selection of the process parameters. Hence, the effect of the process parameters such as friction time, friction pressure and speed of rotation on the weld strength of the specimen were studied by keeping forging time constant. 4.2 EXPERIMENTATION Figure4.1 Schematic representation of friction stud welding arrangement Experiments were carried out on a programmable logic controller based direct drive friction stud welding machine. The schematic

3 65 representation of friction stud welding arrangement is shown in the Figure4.1. AISI 1030 steel samples were prepared by machining down AISI 1030 steel rods to a dimension of 12 mm diameter and 45 mm length. AA 6063stud samples of equal diameter and length with thread having a 1.75mm as pitch are prepared as shown in Figure4.2. The successfully friction stud welded AA 6063/AISI 1030joints are shown in Figure 4.3. Figure 4.2 Preparation of specimens Figure 4.3 Friction stud welded AA 6063/AISI 1030 joints Chemical composition of AA 6063 and AISI 1030 were analyzed using Inductively coupled plasma spectrometer (Optima 7000 DV ICP-OES, USA) and are given in Tables4.1 and 4.2 respectively.

4 66 Table 4.1 Composition of AA 6063 Copper 0.25 Manganese 0.09 Magnesium 0.85 Zinc 0.03 Silicon 0.59 Nickel 0.01 Iron 0.22 Aluminium remaining Table 4.2 Composition of AISI 1030 steel Carbon 0.30 Molybdenum 0.5 Manganese 0.78 Phosphorous 0.02 Silicon 0.22 Sulphur 0.02 Nickel 0.10 Copper 0.44 Chromium 0.12 Iron Remaining Table 4.3Selection of process parameters Sl.No. Process Parameters Levels 1 Rotational speed in rpm Friction time in sec Friction pressure in kpa In the present work, rotational speeds of 800 rpm, 1150 rpm and 1600 rpm were used to produce the dissimilar welded joints. During experimentation, speed of rotation, friction pressure and friction time are varied to study their influence on impact strength of the welded specimen. Forging time is kept constant throughout experimentation, since it has least effect on the joint strength (Noh et al 2007). The selection of process parameters is given in Table 4.3.

5 67 Table 4.4 Tabulation of readings Sl. No Rotational speed Friction time Friction pressure Axial Shortening Length Impact Strength rpm sec kpa mm kj/m

6 68 The mechanical characteristics of friction welds were evaluated from impact strength measured by impact testing machine at room temperature. According to ASTM A370, the specimen was prepared for impact test. The sub size dimension of the specimen is 10 x 7.5 x 50 mm as shown in the Figure groove is cut in the specimen. Using charpy impact tester, the impact strength of the AISI 1030/AA 6063 joints are determined under different welding conditions. The experimental readings are tabulated in Table 4.4. For micro structural investigation and measurement of micro hardness, the friction welded specimen were prepared as per ASTM E384. The sample was sectioned perpendicular to the weld surface and variations in micro hardness across the welded joint, is studied using Vickers hardness tester. Using a load of 2 kg and a holding time of 16 seconds, indents were made on the welded specimen. The indenter is then removed from the surface of the specimen and the indentation dimensions were measured using an optical microscope. High performance non-contact type infra red thermometer (Rev.E2, Raytek, USA) was employed to measure temperature at the joint interface during the welding process. Figure 4.4 Dimensions of impact test specimen (ASTM A370)

7 69 Figure 4.5 Preparation specimens for microscopic investigation Micrographs on the interfacial region of friction welded AA 6063/AISI1030 steel specimen are taken using Scanning Electron Microscopy (Hitachi, SU1510-Japan).The specimen is cut as shown in the Figure 4.5 for microscopic examination. Grinding is done to remove the scratches from the weld surface and then it is mechanically polished by using a series of emery papers. In order to make specimen, scratch free surface, smooth polishing is done. A wheel covered with a special cloth that is charged with carefully sized abrasive particles is used for polishing. Using a 6 micron diamond compound impregnate oil lubricant, grit scratches are removed. Etching is then carried out to reveal many of structural characteristics of the joint. The composition of the etchant is 190 ml Distilled water, 5 ml Nitric acid, 3 ml Hydrochloric acid and 2 ml Hydrofluoric acid. The etchant was applied for 10 to 15 seconds by immersion.

8 MICROSTURAL EVALUATION AT JOINT INTERFACE Macro Structural Inspection Figure 4.6 AA 6063/AISI 1030 friction welded joint Figure 4.7 Macrograph of AA 6063/AISI 1030 joint Figure 4.6 shows the formation of flash, heat affected region and weld zone of the welded specimen. In friction welding of dissimilar metals the formation and amount of flash depends on the mechanical properties of both the base metals (Lee et al 2004). The amount of flash is more on AA 6063 side because of severe plastic deformation of aluminium at the interface. Temperature at the peripheral region is maximum whereas the temperature at the centre is minimum. This is due to the change in angular velocity from centre to outer periphery of the interfacial region. The changes in the physical morphology of the welded specimen depend on this change in temperature (Yilmaz 2003). Within the first 3 seconds the peripheral temperature reaches its peak value and AA 6063 becomes viscoplastic. It readily flows to cover the mild steel because of the effect of friction pressure. Figure 4.7shows the macrograph of AA 6063/AISI 1030 joint with weld interface.

9 Micro Structural Investigation Figure 4.8 Optical micrograph of the interfacial region of AA 6063/AISI 1030 joint at a magnification of 45X Figure 4.9 SEM micrograph showing interfacial region of AISI 1030/AA 6063 joint at a magnification of 1000X Figure 4.8 shows the optical micrograph obtained at the interfacial region of the friction welded joint. AA 6063 matrix appears as dark grey and AISI 1030 steel looks with dim grey colour in disparity. Adjacent to the interfacial region on the AA 6063 side, a narrow region of about 240µm is found as a black band. This black band is referred as DRX region because it contains fine dynamic recrystallized (DRX) grains. The width of this fully plasticized deformed zone decreases from circumferential region to the central region. This can be attributed to the fact that the sliding velocity increases from the middle to the circumferential region. Consequently, the rate of heat generation is higher at the circumferential region than at the centre (Lee et al 2004). This results in severe plastic deformation at the peripheral region. Hence, the thickness of the DRX zone decreases towards the centre. SEM micrograph (Figure 4.9) reveals the formation of new phases, presence of oxides and diffusion of elements across the interface. AA 6063 side consists of both inter metallic compound and porous oxides. It is noticed

10 72 that iron oxide particles are drawn into the AA 6063 side from the weld interface. Fully plasticized deformed zone, containing dynamic recrystallised grains appears in all the welded samples near the interface (Figure 4.10). In the dynamically recrystallized region very fine recrystallized grains are observed through scanning electron microscopy (Figure 4.11). Figure 4.10 SEM micrograph showing recrystallized region of AISI 1030/AA 6063 joint Figure 4.11 SEM Micrograph showing (DRX) dynamically recrystallized region Different Zonesin the Interfacial Region Scanning electron microscopy and optical microscopy were performed at the interfacial region of friction welded AA 6063/AISI 1030 joint specimen. Distinctive zones are found in SEM micrographs (Figures 4.12 and Figure4.13) and grain size reduction is revealed in the observation. The dissipation of frictional heat from the weld centre results in temperature gradient that would cause different zones with different microstructures. The unaffected zone has similar grain structure as that of the parent metal. But, micro structural changes were found in the partially deformed zone (Figure 4.14) and fully plasticized deformed zone (Figure 4.15). The fully

11 73 plasticized deformed zone contains very fine recrystallized grains in the order of 9 microns whereas; grains in the partially deformed zone are in the order of 30 microns. Figure 4.12 SEM image showing interfacial region of AISI 1030/AA 6063 joint Figure 4.13 SEM image showing different zones at the interfacial region Figure 4.14 SEM image showing deformed zone at the interfacial region Figure 4.15 SEM image showing fully plasticized deformed zone at the interfacial region According to the Hall-Petch relation, this would result in higher strength of the welded joint (Lee et al 2004). The grains in the partially deformed zone are of smaller size than the grains in the unaffected base metal

12 74 zone. Grains are partially deformed in the partially deformed zone, because of the applied forging pressure in the final stage of friction stud welding process Micro Hardness Distribution Figure 4.16 Micro hardness distribution of AISI 1030/AA 6063 friction welded joint Micro structural investigation at the interfacial region shows that there is an increased plastic deformation in the interfacial region due to the rise in heat input during the friction stage. The plastic deformation causes a decrease in the grain size. This in turn, leads to hardening in the region of the welding interface (Fu & Duan 1998). Therefore, there is an increasing trend in hardness value in the interfacial region. Dynamic recrystallization has taken place in the fully plasticized deformed zone. The hardness value increases after this recrystallization zone because of the work hardening caused by the applied upsetting load during the upsetting stage. A little further away from this zone, the frictional heat and work hardening effects are low. Hence, the hardness values are closer to that of the base metal (Seli et al 2010).

13 75 The micro hardness values measured across the welded joint have been shown in the Figure It shows the distribution of micro hardness measured perpendicular to the interface of friction welded joints. The micro hardness measured near the weld centre was about 214HV2 at the AISI 1030 side and it is gradually decreases to 209HV2. Similarly, the micro hardness at the fully plasticized region at the AA 6063 side is 140HV2 and it gradually decreases to 133HV2. The minimum micro hardness value is closer to that of the parent material as it remains in the unaffected zone EDX Analysis Energy Dispersive X-ray (EDX) analysis (Figures 4.17 & 4.18) was done at 3 points in the region of heat affected zone of two specimen of welded joints. Keeping rotational speed as 1600 rpm and friction pressure 300 kpa the friction time is varied in two levels. Sample 1 is made with friction time as 2 seconds and sample 2 is made with friction time as 8 seconds. In both the cases, AA 6063 appears dark grey and mild steel appears bright. Homogeneous interfacial layer is observed with a dim grey colour in disparity. Figure 4.17 shows the EDX analysis of sample 1 and Table 4.5 gives the EDX analysis data. In the interfacial region the EDX analysis gives 29.14% Al, 31.93% Fe, 18.95% O. This composition corresponds to the presence of inter metallic compound FeAl. The occurrence of oxygen in higher amount indicates thick iron oxide in the weld zone. It is possible that due to frictional heating the material becomes viscous and there is diffusion of iron atoms towards the interface. Taking up oxygen from the atmosphere, metal oxides are formed in the interfacial region of AISI 1030/AA 6063 joint. Taban et al (2010) friction welded 6061-T6 aluminium and AISI 1018 steel and observed that a thin,

14 76 discontinuous inter metallic layer formed at the weld interface due to inter diffusion between iron and aluminium. Figure 4.17 EDX analysisat three points of sample 1 Table 4.5EDX analysis data at three points of sample 1 Location Atom % C O Al Si Mn Fe SAMPLE-1_pt SAMPLE-1_pt SAMPLE-1_pt Figure 4.18 shows the EDX analysis of sample 2 and Table 4.6 gives the EDX analysis data. In the interfacial region the EDX analysis gives 36.61% Al, 19.44% Fe and 33.31% C. This composition corresponds to the

15 77 presence of inter metallic compound FeAl 3. It is to be noted that intermetallic compound FeAl 3 has higher hardness value that FeAl. Figure 4.18 EDX analysis at three points of sample 2 Table 4.6EDX analysis data at three points of sample 1 Location Atom % C O Al Si Mn Fe SAMPLE-2_pt SAMPLE-2_pt SAMPLE-2_pt For sample 2, the friction time is set as 8 seconds. Increase in friction time leads to increase in axial shortening distance and more flash formation. But the presence of oxygen is eliminated since the entrapped oxide

16 78 layer comes out of the joint in the form of flash. The intermetallic compound is found as FeAl 3 which is more brittle than the intermetallic compound FeAl formed in sample 1. Further frictional heating with increase in friction time favours a tendency for the formation brittle intermetallic compound which detrimental to the joint strength. 4.4 EVALUATION OF JOINT STRENGTH Sathya et al (2007) observed that the impact strength for the friction welded component is comparable in strength with that of the parent metal. In the present work, the impact strength of the friction welded AISI 1030/AA 6063 dissimilar joint is much lower than that of the base metal. The impact test results are much closer to the values predicted by Dunkerton (1986). In the present work, several experimental runs were conducted to study the effect of significant process parameters such as speed of rotation, friction time and axial shortening distance on the impact strength of the friction welded components. a b Figure 4.19 (a)impact test specimen after testing (b) SEM micrograph of fractured surface (1000 X)

17 79 The macrograph of impact test specimen is shown in Figure 4.19a. Unbound region is seen on the fractured surface of the specimen. This is due to difference in rotational velocity from the outer surface to the centre of the components. Fracture occurred at or very near to the weld interfaces usually. The fractured surface of the joints is further subjected to metallographic examination. SEM observation (Figure 4.19b) reveals the combination of flat plane and shear lip with jagged edges. Shear flow of material and presence of few dimples is also seen. Hence, it is evident that the fracture is of ductilebrittle failure mode Effect of Speed of Rotation Figure 4.20 Effect of spindle speed on Impact strength Ozdemir et al (2007) found that the rotational speed of the spindle had significant influence in the interfacial properties of friction welded joints. It is essentially the spindle speed that determines the coefficient of friction and the depth of the heat affected zone. Local heating in the interfacial region

18 80 reaches higher temperature at a short span of time because of high rotational speed. Hence, the speed of rotation of the spindle affects the quality of the weld and the joint strength. Figure 4.20 shows that the impact strength increases as the speed of rotation increases. Keeping all other parameters fixed, the speed of rotation of the spindle had been varied. When the spindle rotates at 800 rpm, the impact strength of the welded joint is measured as 100 kj/m 2. But, the impact strength increases to 181 kj/m 2, when the spindle rotates at 1600rpm. This can be attributed to the fact that the increase in the speed of rotation minimizes the formation of inter metallic compound (Ozdemir 2007). Figure 4.21 shows the effect of speed of rotation on axial shortening distance under frictional pressure of 500 kpa. It is found that the axial shortening distance increases with increase in rotational speed. This is due to the increased heat generation rate at the interface when the rotating speed is increased (Wang 2011). Figure 4.21 Effect of spindle speed on axial shortening distance for 500kPa friction pressure

19 Effect of Axial Shortening Distance Figure4.22 Effect of axial shortening distance on Impact strength for 1600 rpm spindle speed Figure4.23 Effect of axial shortening distance on Impact strength for 1150 rpm spindle speed Figure 4.24 Effect of axial shortening distance on Impact strength for 300kPa friction pressure Figure 4.25 Effect of axial shortening distance on Impact strength for 400kPa friction pressure Hazlett & Gupta (1963) indicated that the axial shortening distance or burn off is a significant factor in the evaluation of joint strength. Axial shortening distance can be calculated as the difference in the specimen length before and after friction welding. In the present study, the calculated axial

20 82 shortening distance is plotted against the impact strength as shown in the Figures 4.22, 4.23, 4.24 and In the experimentation, the friction time was varied under different welding conditions. When the friction time had been varied as 3 seconds, 5 seconds, and 7 seconds with a constant axial pressure as 300 kpa, the axial shortening distance was measured as 8.73mm, 8.86mm and 9.06mm respectively. The impact strength increases from 200 kj/m 2 to 231 kj/m 2. Figures 4.24 and 4.25 also show similar trend when the friction pressure is kept fixed as 300 kpa and 400 kpa respectively. Though axial shortening distance increases with friction time in both the metals, significant amount of shortening is observed in AA 6063 rather than AISI 1030 steel. More amount of shortening is observed in AA 6063 side rather than AISI 1030 steel because AA 6063 reaches visco plastic state at about 70% of its melting temperature. It is found that the axial shortening distance increases with friction time in both the metals. This is due to the increase in the plastic deformation at longer friction time. Similar trend was observed in the work done by Sathya et al (2007) Effect of Friction Time Figure 4.26 Effect of friction time on Impact strength for 1600 rpm spindle speed

21 83 Figure 4.27 Effect of friction time on Impact strength for 1600 rpm spindle speed and 400 kpa The relationship between impact strength and friction time under constant rotation speed is shown in Figure4.26. Impact test results show that the joint strength increases with increasing friction time. Sathya et al (2007) have shown that the plastic deformation in the friction welded joints tends to increase at longer friction time. Hence, an increase in friction time, leads to wider heat affected zone and narrower fully plastically deformed zone (Noh et al 2007). Thus, greater volume of viscous material is transferred out at the interfacial region and this attributes to the increase in joint strength. Figure 4.27 shows the relationship between impact strength and friction time. Keeping all other parameters fixed, friction time varied in 5 levels. Impact test results show that the joint strength increases with increase in friction time. When the friction time is set as 2 seconds and 8 seconds, the impact strength of the joint is measured as 150kJ/m 2 and 225kJ/m 2 respectively. Hence, an increase in friction time, leads to wider heat affected

22 84 zone and narrower fully plastically deformed zone (Noh et al 2007). Since, more amount of viscous material is shifted at the interfacial region of the welded joint; there is an improvement in the joint strength. But, when the friction time is increased to 10 seconds, the impact strength decreases to 200kJ/m 2. It is found that more heating takes place in friction phase and aluminium becomes more viscous when the friction time is increased. AA 6063forms a cover over the AISI 1030 steel component and there is more plastic deformation. During plastic deformation coarsening of grains occur that decreases the impact strength at longer friction time. Besides, with longer friction time, the axial shortening distance increases and results in more amount of material consumption. Similar trend between friction time and axial shortening distance had been previously reported by Sathya et al (2007) Effect of Friction Pressure In friction stud welding process the friction pressure used is in the range of 200 kpa to 800 kpa. It is very low when compared to friction welding process where friction pressure is in the order of 200 MPa. Figures 4.28 and 4.29 show that friction pressure in friction stud welding process has least effect on the strength of the welded joint. Figure 4.28 Effect of friction pressure on Impact strength for 1600 rpm spindle speed Figure 4.29 Effect of friction pressure on Impact strength for 1150 rpm spindle speed

23 85 Figure 4.30 Effect of friction pressure on axial shortening distance for 1600 rpm spindle speed Figure 4.31 Effect of friction pressure on axial shortening distance for 1150 rpm spindle speed However, the friction pressure controls heating and displacement during friction stage. Increasing the friction pressure reduces the friction time needed to reach the level of forgeability required for welding. When welding dissimilar metals, the softer material determines the welding parameters. As the friction pressure increases the axial shortening distance also increases (Figure 4.30 and 4.31) Impact Strength Figure 4.32 Impact strength of welded sample and parent metals

24 86 In the experimentation, the impact strength of the friction welded AISI 1030/AA 6063 joint is found to be lesser than that of the parent metal (Figure 4.32). The impact strength of AISI 1030 steel and AA 6063 are 21joules and 32joules respectively. But the impact strength of the friction stud welded AISI 1030/AA 6063 steel joint is 14.3% and 43.75% lower than that of mild steel and aluminium respectively. The decrease in impact strength may be attributed to the presence of oxides, formation of brittle inter metallic compound and incomplete bonding due to low rotational speed in the interfacial region. 4.5 SUMMARY In the present work, joining of dissimilar metals, AISI 1030 steel and aluminium alloy AA 6063 was achieved successfully using friction welding technique. The following conclusions could be drawn from the study. 1. Optical micrograph shows distinct dynamically recrystallized region of about 240µm on AA 6063 side very near to the joint interface. 2. In the dynamically recrystallized region very fine recrystallized grains are observed through scanning electron microscopy inspection. This would enhance the strength of the joint. 3. SEM micrograph reveals the presence of three distinctive regions in the heat affected zone namely, fully plasticized deformed zone, partially deformed zone and unaffected base metal zone. The dissipation of frictional heat from the weld centre results in temperature gradient across the welded joint, causing different zones with different microstructures.

25 87 4. SEM-EDX analysis confirms the presence of oxides and the formation of inter metallic compound (FeAl) in the interfacial region. When the friction time is increased more heating takes place in the interface and metal oxides are forced to come out in the form of flash. However, higher friction time leads to more material consumption and tendency to form brittle intermetallic compound which is detrimental to the strength of the joint. 5. Hardness value at the interfacial region is found to be higher than the hardness of the parent metals. 19.5% increase in hardness in the AA 6063 side and 18.7% increase in the AISI 1030 steel side are observed in the micro hardness profile. This is due to increased plastic deformation and grain refinement. 6. The impact strength of the friction welded AISI 1030/AA 6063 steel joint is found to be 14.3% and 43.75% lower than that of AISI 1030 steel and AA 6063 respectively. The decrease in impact strength may be endorsed to the occurrence of oxides and formation of brittle inter metallic compound. 7. The impact strength increases as the speed of rotation increases. This can be attributed to the fact that the increase in the speed of rotation minimizes the formation of inter metallic compound. It is found that the axial shortening distance increases with the increase of rotational speed. This is due to the increased heat generation rate at the interface when the rotating speed is increased.

26 88 8. Impact test results show that the joint strength increases with increasing friction time. This is due to the increase in plastic deformation in the friction stud welded at longer friction time. Hence, an increase in friction time, leads to wider heat affected zone and narrower fully plastically deformed zone. Thus, greater volume of viscous material is transferred out at the interfacial region and this attributes to the increase in joint strength. However, with longer friction time above 10 seconds results in more amount of material consumption and decrease of joint strength. 9. In friction stud welding process much lower friction pressure is applied when compared to friction welding process. Experimental results show that friction pressure has least effect on the strength of the welded joint in friction stud welding process.