Some investigative studies on dissimilar metals by friction stir welding

The aim of the current work is to develop weldments and to investigate the microstructure and mechanical properties of the aluminium alloy 6061-T6 and magnesium alloy AE42 plates (120x100x3 mm) which are welded through FSW. H13 conical thread tool pin was selected to prepare the welded joints. Two plates that were co-ordinated vertically according to the welding directions were well joined. In this study, two parameters such as weld speed at 60,150 mm / min and tool rotation speed at 800, 1200 RPM are taken into account as defect free. By changing these parameters similar and dissimilar alloys are welded together. Tensile specimens (according to ASTM standards) were developed to estimate mechanical properties including tensile strength, % elongation, yield s trength and joint efficiency. It was found that an increase in the tool rotational speed or welding speed of the tool led to an increase in the tensile strength, which reached a maximum value and then decreased. (LOM) Light optical Microscopy research was accustomed examine and study the weld zone properties. Dynamic recrystallization was discovered within the weld region in addition as within the thermo-mechanical heat-affected zone (TMAZ). There’s a transparent reduction within the quantity of precipitate through the TMAZ and from the BM (Base Material) into the weld zone. Observed precipitates from similar and dissimilar welded joints -Mg2Si, Al2O3, SiC, Mg17Al12, Mg2Al3, Al11Ce3 and Al2Ce. Welds are without porosity. XRD with the EDS characterization was executed in the NZ (Nugget Zone) demonstrated the existence of SEM intermetallic phases and their weight in percent. Rapid phase structures of welded joints were detected using X-ray diffraction (XRD), however, the SEM fractograph indicated a flexible ductile fracture mechanism. Vickers micro-hardness test was performed on the thickness of the plate within the weld space to review and perceive the variation of hardness with thickness. An interrelationship between size of precipitate and micro hardness was ascertained. The corrosion behaviour of the base alloy and welded joints was measured in 3.5% NaCl solution using the salt spray test and by investigating the corrosion parameters (corrosion rate, exposure time and weight reduction). Corroded products were analysed and classified using optical microscopy and SEM. The corrosion rate decreases with increasing exposure time, but remains uniform as the corrosion time increases. Finally, a three-dimensional finite element phases was developed for temperature evolution in the friction-stir welded joint plunge depth, dwell, moving phases, and the heat conduction effect is arbitrary in the LAGARAGIAN-EULERIAN formulation in the ABAQUS / EXPLICIT, Johnson-Cook (JC) elastic-plastic model Will be used. The JC model defines the strength of the material as a function of three parameters, i.e. the strength of the material depends on the strain hardening effects, strain rate effects and temperature and verifies the s imulated results with the experimental results.


Introduction
After two decades of development, Friction Stir Welding (FSW) has become a viable and important manufacturing alternative, especially in the automotive industry (alloy wheels) with aluminium alloys and magnesium alloys. Although FSW was started as an alternative to dissimilar materials, it is now advancing to higher temperature metals, including aluminium alloys and magnesium alloys. Interest has also increased for FSW of dissimilar metals and alloys, especially when welding by conventional welding [1][2][3][4][5] is difficult or impossible.

Problems in traditional welding dissimilar metals -Problem Identification
Although magnesium alloys are lightweight and do not have sufficient strength to use in structural material, the combination of aluminium alloy and magnesium alloy is a good construction. However, conventional method on unequal joints is not appropriate because the large coefficient of expansion of the Mg12Al17 intermetallic compound structure is in the stir zone. The use of arc welding is not possible because it produces large HAZ by melting the base metal. It produces error due to high heat input and slow cooling rate and is difficult to joint uneven material due to different chemical composition such as melting point. To overcome the disadvantages, friction stir welding is an alternative method and this process takes place below the melting point of the alloy. The intermetallic compound layer in the stir zone is shorter than the conventional welding process. Improving efficiency by enhancing the intermetallic compound by using filler rod is not required in the FSW. There is a less study in joining asymmetric AE42 magnesium alloy and aluminium AA6061-T6.

Material and Specimen preparations
Mg AE42 is collected in 3mm thick slabs. AA6061-T6 belongs to the heat-treatable 6xxx series, of that aluminium, magnesium and silicon are the main composition. Dimensions for weldments are (120x100x3mm 3 ).  FSW depends on design of tool, process parameters. Design of tool and geometry of it's made. For current research on aluminium alloy AA6061-T6 and magnesium alloy conical tapered threaded tool pin profile is selected. The determined material for high speed steel material is hardened 60HRc [6][7][8]. Dissimilar FSW joints Infra-red technique could be a smart activity methodology for temperature. However, the device is extremely expensive and also the temperature below the shoulder of the tool can't be measured at once. K-type grounded thermocouples with a sheath diameter of one metric linear unit were used. The digital measuring system, the TM-747D, was wont to connect four thermocouples to a private computer, with an information acquisition system put in to record temperature histories throughout FSW. Tiny holes with a diameter of one metric linear unit were trained on either side of the work in accordance with the thermocouples. The positions of the thermocouples within the work area unit shown in Fig. 4. Every drilling hole is formed to stop the thermometer from being crushed by a little indented clamp at the doorway. The sensing head of the thermometers is one metric linear unit long and also the holes wont to place the thermocouple area unit eleven metric linear unit deep. Therefore, thermocouples are securely embedded in the holes and the temperature can be measured accurately without external disturbances. The direction of rotation and the direction of movement of the tool are shown in Fig.4. As further described, two of the four thermocouples (TC1 and TC3) are placed on the advancing side, and the remaining two (TC2 and TC4) are placed on the reverse side. The distances between the thermocouples and the tips of the joint line are all 6 mm. This type of layout has been used to measure temperature histories of developing and backward sides [13][14][15][16][17][18].   Fig.5. Microhardness provides uneven distribution. BM average hardness values of dissimilar metals were 105HV and 75HV. NZ microhardness values supported phase elements and local recrystallization. The higher NZ, values will be high because of the presence of hard and brittle intermetallic compounds [19][20][21][22][23][24][25][26]. NZ grains are recrystallized and smaller when compared to base material. NZ hardness will increase within the FSZ and depends on the strength of the grain size hardening metal alloys. There's no report on careful analysis of precipitate distribution across the Al / MG weld joint. However, the FSW study on Al-MG-Si compounds are studied [27][28][29] indicates that the end of sharp needle deposits to a lower hardness within the stir zone. 8 modifying the thermo-mechanically affected zone (TMAZ), in which the material was deformed into plastic and also had some thermal inhalation. Finally, welding zone is called the nugget; NZ is a redesigned fine, identical grains containing a few micrometres of original grains and sub-grain borders.
BM contains a larger grains because of more temperature when compared to 1.3 micro meter of base material. The grain size is 25 to 30 micrometre and is stable. Silicon precipitate also can be observed they are 10-15 micrometres long. Thermal and mechanical effect add each other so that the structure is remarkably distorted and the grain starts smashing in TMAZ and the morphology of the grain does not change but the dimension is reduced up to 10 micron. The silicon precipitates rise a nd are spread in the structure. In addition to recrystallization, both Al-enrichment and β phase dissolve in the interior of α-Mg grains in the nugget area, leading to an increase in Al concentration and corrosion resistance improvement has been noted in the nugget area.

Scanning Electron Microscopy:
SEM with EDS analysis performed at middle of NZ on weldments.    Table 8 shows the corrosion attack on specimens with a rotational speed of 800 and 1200 rpm. These data shows the estimated weight reduction with a solution of NaCl from the salt spray test. The corrosion attack on the magnesium alloy side is much higher compared to the aluminium alloy side. Corrosion testing is performed at 30 h and 90 h intervals. The surface for corrosion evaluation (for salt mist) is mechanically ground with 1200 grit SiC paper, rinsed with distilled water and dried with warm running air [45][46][47][48].

CORROSION STUDIES
Salt spray corrosion testing involves exposing samples in the salt spray as per ASTM B117 standards and methodically evaluating the corrosion-tested sample according to ASTM G1-03. The weight loss is more due to the high rotational speed and high corrosion time as shown in fig 14. The difference in weight loss before and after conducting the experiments are shown in table 8.   Fig. 15 shows the SEM image of the polished sample after 30 and 70 hours of salt spray corrosion testing. The exposed sample surface was prepared for microscopic examination with a small test. Corrosion test specimens for scratch-free surfaces were polished on a disc-polishing machine. To determine the depth and diameter of the pits, the exposed specimens were cut in the cross-sectional direction, the corrosion products were removed, and then the specimens were covered with hot setting resin and the surface was observed at 200 X magnification. Here, it can be seen that the weld joint is attacked towards the magnesium and the other part of the region is related to the distribution of the βphase performance in Fig. 15. The aluminium in the weld joint is not attacked which shows corrosion is higher for aluminium alloys and lower for magnesium alloys [44].

NUMERICAL STUDIES
Numerical simulation of FSW is highly complex due to non-linear contact interactions between tool and work piece.

Geometry, boundary conditions and the FE mesh
The 3-D numerical model is based on the C3D8RT element type, which is a thermo-mechanically coupled hexahedral element with 8 nodes, each with a trilinear displacement. Plate dimensions 120mmx100mmx3mm in numerical pattern in plunging stage. The mesh contains 23608 nodes and 20972 elements. The numerical pattern of the welding plate, the tool is shown in the figure 6 [40, 41].

Thermal model
FSW heat generation has two sources: friction heating at the tool work piece interface and plastic energy dissipation due to shear deformation in the nugget zone. When heat is dissipated by circulation into the workpiece, tool and backing plate, and also by convection and radiation from the surfaces.

CONCLUSION
The temperature fields, plunge force associated plastic deformations of dissimilar weldments completely different rotating speeds throughout the FSW method were analysed employing a couple thermo-mechanical model and therefore the observations associated with an intense plastic deformation influence of the warmth generated because of surface friction plastic deformation.
Observations of macrostructure and microstructure shows different zones of weldments. Coarse grains were found to be in HAZ and fine grains within the nugget. Tensile strength of the parent material found was 84%. This behavior is said to the plastic deformation result of warmth generated by surface friction plastic deformation. Maximum temperature made by weldments will vary from 85% -95% to the melting temperature of the BM. Speed of rotation will increase, the temperature will increase. The temperature field is symmetrical. The contact between the rotating pin and therefore the fastening plate begins to extend, and reaches maximum value at 1200 rev and 800 rev to the fabric malleability and is softened and the plunge force may be reduced. The results show that software ABAQUS is particularly important in simulating mechanical properties in this process. Of the two rotational speeds and welding speeds used to fabricate dissimilar joints, the joint fabricated at a tool rotational speed of 800 rpm and welding speed of 60mm/min yielded superior tensile properties. The above joint showed a maximum joint efficiency of 76% compared to other joints. During high speeds, the high heat generation causes the grain growth in the stir zone. Moreover, a higher tool rotational speed causes excessive release of stirred materials to the upper surface, leads to voids in the stir zone. On the other side, at lower speeds the area of the stir zone decreases with decreasing and affects the temperature distribution in the stir zone. Macrostructure observations have shown that joints formed at low welding speeds (60 mm / min) have defects such as piping defect in the tile or stair zone and as a result have low tensile properties. Low welding speed (60 mm / min) leads to high temperature and slow cooling rate in the weld zone, leading to high grain growth, which in turn can lead to tensile properties of the joints. The joint made at a welding speed of 150 mm / min exhibited high tensile strength and this may be due to insufficient heat output, which in this condition allows the material to flow plastic with appropriate mechanical work.