3.1 Micro hardness profile
Wear resistance of the cast welds improves subsequent to FSW process owing to the fact that stir welding process conceals the micro pores present in the cast weld regime. Reduction of grain size occurs by dynamic recrystallization phenomena resulting in the particle fragmentation of the cast weld. The weld regime is accumulated with particles that are dispersed homogeneously. As a result, the dislocation density in the sight of the weld increases contributing to hardening. This culminates in improved wear resistance.
Finer grain sizes, often associated with higher micro hardness, can contribute to improved wear resistance. The presence of hard phases or constituents in the microstructure, such as carbides or intermetallic can significantly influence both micro hardness and wear behaviour. The presence of eutectic phases and their hardness can affect the overall micro hardness and wear properties of the alloy. Figure 6 shows the detailed survey on the hardness values obtained across the various weld regimes.
Figure 6. Micro hardness profile
The micro hardness for the threaded cylinder tool was comparatively higher (83.75 Hv) than that of the square (71.2 Hv) especially in Nugget zone-NZ. At some locations of heat affected and thermo mechanically affected zone, hardness is lower than cast metal zone as precipitates of the alloy may dissolve if the alloy is subjected to a heat cycle in that area during the welding process. The shorter dwell time of tool at the work-joint interface facilitates improved grain refinement in the area and could be the reason for the higher hardness values measured. The reason for the variance in average hardness levels in the NZ is the uneven mixing and distribution of material compositions at the zone, which may cause strengthening precipitates to dissolve. Hardness drop is noticeable for the square tool across NZ. This can be the result of a tiny gap at the weld centre caused by incorrect grain bonding at that particular location [8]. The weld nugget centre tends to have a lower hardness compared to other weld zones due to the plasticization and subsequent cooling.
High heat generation and poor cooling rate are produced by higher rpm of 2100 and lower feed of 10 mm min− 1, which results in high heat input in welding. Precipitates dissolve at this temperature, and some of them regenerate in the aluminium matrix. High hardness is the outcome of this re-precipitation process at the agitated zone. Wear resistance improves in tandem with hardness thereby minimal wear is noticeable [9].
Typically, the axial hardness distribution in the weld zone created by a threaded cylinder tool shows a gradient. The threaded profile of the tool helps effectively in stirring and material mixing in the course of welding. The severe plastic deformation and continuous recrystallization during FSW lead to smaller grain sizes of the order of 2–4 microns, which can impact the hardness. The increased surface area due to the threads may enhance heat dissipation, preventing localized overheating. Controlled heating and cooling rates contribute to a more desirable microstructure and hardness distribution. The square tool generates a pulsating effect causing adherence of the stirred material to the pin thereby exhibiting the variation in hardness.
3.2 Wear study
Wear is the process by which material is removed from one or both of the solid surfaces where they come into contact. It can also mean the progressive loss of material on a solid surface as a result of two moving surfaces sliding relative to one another. The primary faction for wear phenomena is friction on the commencement of sliding. With the relative displacement of asperities, wear particles are staged from tips of members. The process proceeds at the slower pace. In the nugget zone, the hardness levels gradually declines evidencing the inverse relationship between wear rate and hardness as proposed by Archard (Eq. 1):
$$W{\alpha }\frac{L}{H}$$
1
Where, W = Wear, H = Surface hardness of the wearing material, L = Normal load on contact.
The wear particles are formed by a process of progressive plastic extrusion by an accumulation of plastic strain and in a form of plastic ratcheting. The process proceeds with the pummelling activity on the softer facet by the action of harder asperities. The juncture of the contact at the summit of the asperities reaches a pinnacle developing localized stresses. Repeated sliding, high contact pressures with the surface traversal subjects the material layer to severe contact stresses [19].
Adhesive wear arises when two smooth bodies slide over one another. Abrasive wear is the material loss resulting from hard particles pushing over a solid surface. Abrasive wear usually occurs at high loads and is produced by a rough, hard substrate or a soft surface with hard particles moving on a softer surface and ploughing a sequence of grooves [15]. Either reciprocating or unidirectional relative movement is possible. As a result, pieces separate from one surface and stick to the other; eventually, these pieces may separate from the surface they originated on and return to it, or else they may become loose wear particles.
3.2.1 Effect of Load on Wear-rate and COF
The effect/influence of varying load, sliding velocity and distance on wear-rate and COF are discussed pertaining to experimental sets 1, 2 and 3. The influence of varying load on wear-rate and COF is shown in Fig. 7a and b.
Figure 7. Influence of Load on a Wear-rate, b COF
Wear rate increases with increasing load for both cast alloys, W1 and W2. Lower order of wear rate is largely dependent on the oxides that evolve at the interface. At higher load, wear rate for W1 and W2 (1.89 and 1.26 mgm/min) is less than that of the two cast metals (3.96 and 1.62 mgm/min), as evidenced from the Fig. 7a. This could be related to the weld heat cycle influence, grain refinement, and appropriate material blending and cohesion at NZ. Previous study [20] have offered explanations for increment in wear-rate with load based on a variety of phenomena, including adhesion, sub-surface impairment, thermo mechanical influence, and oxidation. When cast Al alloys slide under high load, the temperature rises, which causes the particles to become coarser. A steady metal wear rate rises as the load increases.
Slope of wear versus load indicates a shift in the wear paradigm. Wear results from metal failure at the conflux of the mating surface due to high contact load. Nevertheless, there is no sign of deformation on the wear surface below the transition point. However, appreciable amount of metal flow and plastic deformation prevails at loads slightly over the transition point [21]. From Fig. 7a, at 15 N the surface below the transition region exhibited lower order of deformation and reduced wear rate. With increased load and due to high contact stress, deformation of the asperities takes place deteriorating the surfaces thereby increased wear rate.
The applied load has a significant ramifications/pay off on the wear rate of the cast alloys and dissimilar stir welds. This could be attributable to the formation of wear debris at higher load resulting from plastic deformation. Also, material removal to a greater extent and subsurface cracking are also linked to the consequences of deformation. More amount of material removal and cracking of subsurface occurrence are due to larger extent of plastic deformation. Under a lower load condition of 10 N for all combinational aspects of welded and non-welded samples, wear rate is less pronounced. Around 15 N steady state of wear is observed. Around 10–15 N condition, where the state of loading on the pin is small, lower order of wear rate is observed. Strain hardening intensity is also less pronounced and accordingly wear rate is not so sensitive. Rising of the normal load increases the wear rate. This trend may be to the fact that the load pronounces an enhanced de-bonding and fracture.
Increase in load causes rise in friction. This is in accordance with the findings of Chowdhary et al. [22]. Increase in load causes frictional rise. Ready oxidation of aluminium-silicon casting in air occurs with the initial rubbing interval. Increment in load leads to increased wear loss. The rubbing interval disintegrates the surface layers, cleans and smoothen the surfaces which aids in increasing the interconnectivity strength and contact between surfaces.
The ascendance of friction coefficient with increasing normal load as referred from Fig. 7b is attributable to the surge in adhesion strength. At lower load condition the oxide film effectually cleaves/ splinters the two metal surfaces leaving behind no base metallic contact, thereby lower order of COF is pronounced. At higher load condition, the film results in attrition/undermining of the intimate metallic contact encouraging higher friction. Further increase in load up to 20 N causes diminishing trend in friction coefficient values due to the subsequent rising pattern of weight loss. Friction between cast metal surface and counter face results in temperature upswing causing micro melting and mechanical descent.
3.2.2 Time versus Wear and COF
The discussion on time (s) versus wear (µ) and COF is explicated based on the readings of Set-1 experiments and the relevant plots are displayed in the Fig. 8a and b.
Figure 8. Time versus a Wear, b COF
In the initial stages of wear test, there might be a period of rapid wear or a break-in period where the wear rate is relatively high. This could be attributable to factors such as surface asperity removal and settling of contact stresses. After the initial period, the wear rate may stabilize or reach a steady state condition where it remains relatively constant over time. This is indicative of the fact that wear has reached a equilibrium condition and the material is experiencing consistent wear under the given condition. Abrupt variation in wear/fluctuations in the wear rate over time may indicate the onset of different wear mode/transition from adhesive to abrasive wear.
It has been reported that the rate of wear lowers as the grain size decreases, suggesting that the wear rate falls as the grain aluminium's dimensions. This can be accounted by the aluminium's grain boundary strengthening, which causes strain hardening. The wear loss/wear versus time shows minor variations in the curve which could be caused by debris particles getting trapped and released across the sliding surfaces. The delamination of the tribo-layers may be another cause of the variations. However, because of the complexity of the dry-sliding system, it is challenging to determine the precise reason for these variations [23]. As noticed from Fig. 8a, the wear for the weld with threaded cylinder tool shape (W1) is comparatively lesser than the other weld (W2) and the cast alloys. This could be due to the more hardness pronounced across the nugget weld regime leading to improved grain refinement.
The initial rise in COF may be ascribed to the increment in frictional force required to subjugate the adhesive contact between the cylinder and tested surface. The variation in the extent of plastic deformation at contact areas may prompt/engender differences in frictional coefficient. FSW prone surfaces are harder, less plastically deformed and hence lower order of friction is observed [24].
The graph between COF and sliding time is shown in Fig. 8b. The frictional coefficient of friction stir welded cast alloy is less compared to other. The fluctuations in COF are higher initially and the value of COF decreases momentarily attributable to crack growth and slip deformation beneath the surface in the material [25]. The examination of wear coefficients suggests that Al alloys were subjected to severe wear conditions. This is known as the plastic ratcheting regime, in which the surface accumulates more plastic strain caused by each asperity contact.
3.2.3 Effect of Sliding velocity on Wear-rate and COF
The varying sliding velocity versus wear-rate and COF is shown in Fig. 9a and b. Figure 9a represents the variation in wear rate with sliding velocity for cast alloys and welded specimens. The sliding velocity for a given higher load 20 N was changed from 0.5–1.5 ms− 1.
Figure 9. Sliding velocity versus a Wear-rate, b COF
Referring to Fig. 9a, the graph indicates that the wear rate for the cast alloys and the cast-welded alloys (W1 and W2) increased as the sliding velocity rises. All the combinations exhibited a substantial increase in wear rate at all the investigated sliding velocity. Lower order of wear rate was observed at low velocity (0.5 m/s). At medium velocities, gradual increasing trend is observed. Higher sliding velocity concatenated with high pressure forces the specimens are put through higher stress levels. Resultantly, the fracturing of oxide layer, subsequent detachment takes place acting as abrasive aggravating the wear rate [26].
The wear rate increases in proportion to the sliding velocity. With the articulation of the sliding velocities, the shear strain shoots up resulting in increased material loss. Also, at higher sliding velocity, thermal softening prevails due to temperature effect at the interface of pin and disc. This softening effect results in weakening/lessening of bond strength between the constituents, further leading to detachment of particles stimulating the wear rate. However, the cast-welded specimen using threaded cylindrical and square pin showcased lower order of wear rate compared to other cast alloys. Wear resistance improves with sliding velocity due to increased oxide layer thickness. Tribological contact triggers bulk metal strengthening processes such as grain refining, solute and particle strengthening.
Sliding velocity of higher magnitude advances the heat generated from friction at the interface of the disc and pin. Consequently, material softening sets in, culminating in the reduction of bond strength in the matrix. As a fact, harrowing/tilling of asperities on the surface of the pin takes place influencing the material removal. However, stir welded samples using threaded cylinder and square shaped tool results in dynamic recrystallization providing grain refinement. Subsequently dislocation densities were levitated in the material bestowing decrement in wear rate [27, 28].
According to Fig. 9b, high frictional force at a high sliding velocity result in harder asperities clinging to the sliding facet, favoring a lubricating layer. Greater velocity causes thickening of the film, followed by breakdown and clogging of the structure on the mating surface, resulting in an elevated frictional coefficient. During rubbing action at higher sliding velocity coalescence, the cast pin material takes place along with the work hardened steel surface serves as a solid lubricant (Formation of MML-Mechanically mixed layer at the sliding boundary) improves the cohesiveness and proliferates/escalates the load withstanding capacity and frictional resistant attribute of the cast welded samples. The COF value decreases considerably with surge in sliding velocity [29].
3.2.4 Effect of Distance on Wear-rate and COF
The varying distance versus wear rate and COF is shown in Fig. 10a and b. During sliding condition as the sliding distance increases, interfacial temperature of the pin surface increases owing to friction heating. As a result, asperities of all compositions soften and deformed easily by asperities of counter face. Hence, wear rate increases when the sliding distance is between 1000–1500 m and the variation is observed in the Fig. 10a. Upsurge in wear rate with sliding distance could be attributed to the ample contact surface between the sliding faces with extended time. Owing to prolonged nature of time heat generated at the sliding pin regime is more and hence the temperature. Thus, the softening of the material commences resulting in plastic deformation and increase in wear rate [30].
Figure 10. Sliding distance versus a Wear-rate, b COF
Aluminium cast alloys consists of asperities softer in nature than the counter disc. These asperities get broken easily under sliding condition resulting in material loss thereby increased wear rate. As sliding distance surges, the broken asperities of Al alloy intersect between the disc and pin and material loss is produced via third body interaction.
At the initial stage, all the specimens showcased a rise in COF owing to greater extent of frictional force required to repress/quash the adhesive contiguity between pin and the sliding disc as referred to Fig. 10b. The rise in COF is associated with continuous scratching of pin. Aluminum cast alloys are softer in nature than steel pins. Scratching occurs due to pin interaction with the hard aluminium oxide layer on the surface. Besides this action, additional metal transfer from the cast welded alloys to the pins (galling) aids in increment in COF. With increase in sliding distance COF showed a decreasing trend. This is due to higher heat generation producing more slipping action.
The COF variation with respect to cast-welds is because of reduced plasticity, lower degree of deformation on the surface of the specimen by fine grained particles formed by effective recrystallization by threaded cylinder/square tool. The tribo-layer film formation reduces the proximity between the specimen and counter disc surface paving the route for COF decrement. Oxide layer formation serves as a self-lubricating agent reducing the shear stress generated on the surface. High interfacial bond between the cast welds creates a resistive action against the compeer perforation and wounding on the surface during the sliding time. At higher and constant velocity of 1.5 ms− 1, increased temperature causes thermal softening underlying the worn surface, and an oxide film accumulates on the pin surface, reducing COF [31].
3.3 Corrosion study
Electrochemical measuring methods can be utilized to calculate the corrosion rate for cast alloys and cast welds. Polarization resistance involves polarizing the weld samples a few millivolts higher and lower than open circuit potential, measuring the current response. This study sought to determine the influence of stir casting and stir welding on cathodic and anodic kinetics of A356 T6 and AA 2014 T6 alloy in an aqueous solution containing 3.5% NaCl. The Tafel polarization experiment was used to establish the aforementioned influence. Figure 11 displays the potentiodynamic polarization curves for cast alloys (A356 T6 and AA 2014 T6) and cast welds (W1 and W2). The abscissa and ordinate represents the current potential and voltage potential. The current potential is a measure of rate of corrosion. More positive the current potential, lower the rate of corrosion.
Figure 11. Corrosion plot
The potentiodynamic corrosion test evaluates the corrosion resistance of materials by determining key parameters including potential and current. These factors are essential for assessing the material's resistance to corrosion. The corrosion potential (Ecorr) of A356, W1, W2 and AA 2014 were found to be -0.65, -0.72, -0.78 and − 0.8 V respectively. A higher negative rating for corrosion potential indicates low corrosion resistance. As a result, A356 and W1 offered more corrosion resistance than W2 and AA 2014.
The ‘Ecorr’ values are shifted in the negative potential direction with increase in concentration of NaCl. The trend is similar for all the pairs. The decrease in corrosion potential (‘Ecorr’ values) indicates loss of passivity due to thinning of the primary oxide by chemical dissolution [32]. A356 cast alloy consists of primary α-Al and eutectic ‘Si’ phases. The morphology and arrangement of ‘Al’ and ‘Si’ phases significantly influence the corrosion performance of the material. Additionally, A356 possesses excellent corrosion properties. For heat-treatable alloy such as AA 2014, decrease in the corrosion resistance across weld regime is common. However, the AA 2014 demonstrate increased corrosion resistance in its weld [33].
High ‘Ecorr’ values represent better resistance to corrosion. The ‘Ecorr’ value of the cast weld (W1) is higher than that of cast alloys and the cast weld (W2). This result is substantially due to the welding area's considerable plastic deformation and the recrystallization during FSW process, resulting in microstructural homogenization. Microstructure homogenization reduces galvanic corrosion and improves corrosion resistance [33]. Moreover, it is noteworthy to emphasize that the use of FSW exhibited an advantageous increase in the resistance of the material to corrosion.
Plastic deformation and thermal cycle of the weld modify the microstructure of the weld zone. Grains in NZ were refined and equi-axed. Following the FSW in weld regime, precipitates dissolved and coarsened, lowering micro hardness. The mechanical characteristics of the weld declined when compared to base cast alloys. The potential difference between them promotes the corrosion process. The precipitates at the base cast alloys' grain boundaries enhanced the corrosion reaction continuously and without interruption. The distribution of precipitates in the weld enhanced the NZ's resilience to corrosion. Consequently, the FSW joint's resistance to corrosion was enhanced [34].
The corrosion characteristic is attributed to galvanic action. Pitting results from local matrix dissolution caused by galvanic interaction between intermetallics and the surrounding matrix. The dissolving of precipitates in the NZ appears to be the responsible variables for both the increased corrosion resistance and the more noble corrosion potentials of this region. The weld nugget appears to have transformed into a cathode, completely shielded from corrosion damage. W1 pointed out more ‘Ecorr’ values towards the positive side of the ordinate owing to the dissolution of the strengthening precipitates in NZ.
It was predicted from Tafel polarization plot that, cast welds by FSW process (W1 > A356 and W2 > AA2014) exhibits higher corrosion potential/resistance. This observation is because of fine-grained welded zones during FSW that are susceptible to lower corrosion. It is also reported from the plot that cast alloy (A356) exhibits increased corrosion potential compared to cast weld (W2) due to changes in the electrochemistry and thermal history during welding procedures [35].
The corrosion rate refers to how quickly a specific material degrades within an environment. The mentioned relationship in Eq. 2 determines this corrosion rate [36]:
$$Corrosion rate=\frac{0.13 * Icorr * W * \rho }{A}$$
2
Where ‘Icorr’ = Corrosion current potential in A/cm2, W = Equivalent weight of the sample in gm, A = Area in cm2 and ρ = Density in g/cm3.
The computed values of corrosion rate for A356, W1, W2 and AA 2014 were 2.08, 1.90, 2.27 and 2.35e− 4 mm/year respectively. The cast alloy A356 by nature possesses superior corrosion resilience. The weld W1 surpasses the other pairs with higher resistance to corrosion owing to more refined weld microstructure with smaller grains.
3.4 Worn surface morphology
Wear due to delamination results from subsurface nucleation of cracks and promulgation of these cracks. Plastic deformation causes microstructural changes on the subsurface creating instability in the material when subjected to local shearing resulting in delamination. Entrapment of the delaminated asperities between the sliding surfaces results in furthering the plastic strain owing to mechanical alloying. Motion of the loose debris particles in the spaces of the sliding boundary causes three body abrasion. Delamination comprises of progression and crack nucleation. Acceleration of the load induces their phenomena and increments/ increases wear.
The worn surface of the cast alloys and cast welds subjugated at higher load condition underwent microstructural study with chemical composition analysis. The microstructure and EDS of the worn surface of A356 T6 and AA 2014 T6 cast alloy are depicted in Fig. 12a-f.
Figure 12. Worn surface microstructure of a, b A356 T6, c, d AA 2014 T6, EDS plot of e A356 T6, f AA 2014 T6
Figure 12a and b demonstrates the worn microstructure of A356 T6 at 35X and 250X magnification respectively. Particles of debris may generate an abrasive wear pattern similar to ploughing. The material is transferred to the sides of the worn groove. Grooving and scratching are more pronounced at higher load. These characteristics are the result of abrasion in which the asperities of steel surface plough or harrow into the pin causing wear by the progressive removal of material fragments. The abrasion initiates with ploughing/pulling of material/dislodging and displacement of material on either side forming a wedge-shaped groove.
Figure 12c and d indicates the worn microstructure of AA 2014 T6 at 35X and 250X magnification respectively where the wear pattern and rubbing of the edges is indicated. The counter body material of AA 2014 is composed of Al and Cu constituents of the material flow and wear/worn surface, requiring complete ploughing and cutting of the micro-grain edges. Exertion of hydrostatic pressure at higher load scenario promotes the deformation further developing cracks and pits on the subsurface. The retreating of the particle are visible on the worn out surface and rupturing of the oxide layer takes place due to loosening of lubricating layer/MML. Removal of material in patches and thin groove formation on the surface clearly indicates the abrasive and delamination mechanism [37].
The EDS spectrum, as indicated in Fig. 12e shows the Al and Si elements from the counter-body pin element material, and Fe and Co show the base body elements of the constituents. Figure 12f represents the EDS with Al, Mg and copper elements from the counter-body pin element. Fe and Mn show the base body elements of the constituents. Cu element is impurities. The carbon shows that the debris particles produced during the adhesive wear mechanism. The presence of oxygen shows the ploughing and river pattern appear during the wear.
Wear is the material being removed Due to counter body’s mechanical action material is removed from solid body surface. The wear pattern is adhesive. Adhesion wear is caused by micro-junctions formed on the contacting surfaces of counter bodies by welding across their opposing asperities. Because of extreme pressure placed on the contacting asperities, they flex and stick to one another, creating micro-joints. The micro-joints break as a result of the rubbing counter bodies moving in motion. In the areas that are not deformed (not cold worked), the welded asperity breaks. Figure 13a-f portrays the microstructure and EDS of the worn surface of W1 and W2 cast welds.
Figure 13. Worn surface microstructure of a, b W1, c, d W2, EDS plot of e W1, f W2
Figure 13a and b reveals the worn surface morphology of W1 cast weld obtained using threaded cylinder tool pin shape. The worn surface microstructure shows the micro debris particles where discontinuous chip and ploughing patterns are observed. The fine fragmented grains have a low wear rate and a higher efficiency in their wear enhancement properties. It is evident that the micro-cracks and ploughing marks are observed clearly. The wear flow grain morphology is very fine. The wear path is a crest and trough pattern. Fragmentation of the surface takes place at higher sliding distance. Amalgamation of fragments with the wear debris from the counter disc generates MML thus improving the wear resistance. Hardness of cast welded sample using threaded cylinder tool pin shape showed improved order of hardness. The harder surface resists the material scouring during sliding contributing to better wear resistance. Microstructural refinement increases alloy hardness, leading to improved wear results. It is known that in abrasive wear situations, increasing material hardness reduces wear rates [38].
Figure 13c and d unveils the worn surface morphology of W2 cast weld obtained using square tool pin shape. The wear surface microstructure shows the discontinuous flow with macro chips and severe scouring. The Al-Si eutectic and aluminium-copper constituents are predominately dispersed. The coarse grains have a moderate wear rate and a low efficiency in their wear properties. The wear flow grain morphology is coarser. Sliding of the pin surface over the disc, material removal takes place due to micro level mechanism namely micro-cutting and ploughing. Micro cutting is a consequence of microchip emergence during chipping of material and micro-ploughing due to material movement. For lower and higher order wear, deformation proceeds by micro-ploughing and micro cutting respectively. Governing of wear resistance is through transition from micro ploughing to micro cutting [39].
On EDS observation from Fig. 13e and f, the oxidative wear is confirmed by peak ‘O’ and ‘Fe’ unveils the mating of steel surface. Cast welded samples collide with the surface of the steel thereby transfer of iron takes place promoting MML. This layer significantly assists in wear declining for cast alloys. Oxidative/mild wear is noticeable initially and later transits from mild to severe oxidative-metallic. The images also indicate ploughing at higher loads. In addition, removal of oxides at the interface serves as third body responding to overall wear rate.
3.5 Corroded surface morphology
The corroded surface is characterized by localized attack in the form of small pits on the surface of the alloy. Pitting corrosion can occur when the protective oxide layer on the surface strikes a balance owing to local variation in composition, microstructure, or exposure to aggressive environments. Micro-galvanic corrosion within grains results in pitting corrosion surrounding tiny precipitates in weld zone. The corroded surface microstructure of cast alloy A356 T6 and AA 2014 T6 with EDS are illustrated in Fig. 14a-f.
Figure 14. Corroded surface microstructure of a, b A356 T6, c, d AA 2014 T6, EDS plot of e A356 T6, f AA 2014 T6
The potentiodynamic polarization is an approach of electrochemistry which alters test samples electrochemical potential (V), and the current density passed at a specific potential is noted. It is utilized with a 3.5% NaCl brine solution. The brine solution reacts with the base material of A356 and AA 2014 to enumerate the ultimate protection of the corrosion resistance role to play Al-Si eutectic and Cu-Al2 precipitation morphology. The grain morphology and orientation are linear. The localized corrosion has only taken place. Figure 14a and b shows the micro pits observed and also oxide formation at the grain boundaries. The presence of minor pits identifies the pronunciation of corrosion. The local corrosion attack is to resist the base material constituents which have excellent corrosion resistance in the alloy. From the Fig. 14c and d, the grain morphology and orientation have a linear and crystalline structure. The crystalline structure attain higher corrosion rate accordingly. The localized corrosion that has affected the region is higher with large pits observed.
The EDS spectrum from the Fig. 14e shows carbon, oxygen, sodium, magnesium, manganese, copper, and the base material of aluminium. The oxygen percentage is very high because of the corroded surface. The major elements are oxygen and the carbon matrix. The remaining elements are base material constituents. The measurement of the sweeping potential range is -250 mV to + 250 mV, and the scanning rate is 0.4 mV/s. Anodic zones typically form in the areas of metal that are under stress or shielded from oxygen. The corrosion range did not exceed a higher range because of the base material constituents that withstand the inductance force. Figure 14f shows carbon, oxygen, sodium, silicon, chlorine and the base material of aluminium. Anodic zones are formed in the areas of metal that are under stress and crystalline structure. The corrosion range exceeds a higher range because of the base material constituents that would not withstand the inductance force.
Figure 15a-f shows the corroded surface microstructure of cast weld W1 and W2 with EDS.
Figure 15. Corroded surface micrograph of a, b W1, c, d W2, EDS plot of e W1, f W2
Figure 15a and b depicts the SEM micrograph of W1 corroded surface. The material constituents are A356 and AA2014 aluminium alloy matrix. This matrix exhibits corrosion resistance in the surface morphology. Because the alloy is made up of different parts and the matrix is mixed, the surface shape showed up without any pits or deep irradiation. The local affecting area could lead to the oxide in onion ring formation to imitate the corrosion taking place. The corroded surface could not penetrate inside the matrix. The micrograph also shows small size dimples and oxide formation at the grain boundaries. The local corrosion attack is to resist the base material constituents. The base material constituents have excellent corrosion resistance in the alloy.
Figure 15c and d shows the SEM micrograph of W2 corroded surface. This Al matrix is mixed, the surface shape showed large pits or deep irradiation. The pitting diameter is 50–65 micro meter. The corroded surface could penetrate inside the matrix. The micrograph also revealed that the corrosion is taking place at only subsurface interacting deep formation. The formation of corrosion could be seen the surface morphology.
Figure 15e and f exhibited the EDS spectrum for W1 and W2 cast weld. Anodic zones typically form in the areas of the metal that are low stress and mono/poly crystalline structure. The corrosion did not exceed a higher range because of the threaded cylinder and square tool effect on the grain orientation. Base material constituents withstand the inductance force at higher level.
From the SEM micrographs and EDS plot, it is inferred that threaded cylinder pose a proper mixing of the weld particles and also when subjected to polarization test, the current density inhibited by the pair for corrosion of the surface is also minimal favouring lesser/nil formation of dimples. Weld joints produced using threaded cylinder and square pin shapes showcased lower order of corrosion rate. The findings of the results could be ascribed to the smallest contact area of both the pins. At the tangency of tool and the specimen exists thermal gradience and varied heat generation. Higher the heat, faster is the cooling rate leading to dynamically recrystallized grains. This aspect results in reduction in cathodic area to anodic promoting a resistive nature against corrosion and the observation is in accordance with previous studies [40].