But, the desire of overcoming the strength problems due to the formation of compounds started the competition for the effective control of the IMCs, where different solutions had the purpose of complete suppression of IMCs using non-conventional joining methods such as laser beam welding (LBW), diffusion welding (DFW), and ultrasonic welding (USW) with promising results by reducing the amount of IMCs , but without complete elimination of them. Other employed techniques with the same objective, involve the solid-state welding by heating the elements usingfriction (frictionwelding, FRW). Rotary frictionwelding (RFW) is a solid-state welding method, where heat is produced by rubbing components together under load. In this process, one of the workpieces is held stationary while the other rotates, making the workpieces surfaces soften and leading them bond together . In RFW there are three phases : i) heat-up stage, ii) burn-off stage, and iii) forging stage. Three parameters control the character of a weld: rotation speed, heating time and axial force . These parameters determine the amount of energy input to the weld and the rate of heat generation at the interface. Some important parameters for RFW include rotation speed (rpm), upset pressure (N), friction time (second), burn-off
Few sound joints have been obtained, owing to the formation of a large amount of brittle intermetalliccompounds in the weld using fusion welding. In the recent years welding of dissimilar metals by conventional welding techniques has become difficult. . The flux used for the welding will create lot of heat which declines the strength of the welded joints. In order to overcome this, Frictionwelding is more effective in joining dissimilar metals when compared with fusion welding, because it is a solid state process. Heat in frictionwelding is generated by conversion of mechanical energy into thermal energy at the interface of work pieces during rotation under pressure. (e.g. [2, 3]).
G. KiranKumar, K. Kishore, and P.V.GopalKrishna  reviewed that because of the existence of intermetallic phase, the welding of non-ferrous metals is very hard. But, continuousdrivefrictionwelding method can suitably be adopted for welding of different ferrous and non-ferrous materials. . Fuji, A., Kimura M., North, T.H., Ameyama, K. and Aki, M  investigated that the frictionweldingprocess was very efficient in the welding of dissimilar materials such as aluminium and stainless steel. It is showed by the results of tension mechanical tests that presented mechanical properties which are not possible to achieve by means of fusion welding processes. Jessop, T.J. and Dinsdale, W.O. 1976 reviewed that as compare to other welding technique, the continuousdrivefrictionwelding is of highly materials saving, low producing time, high quality, high efficiency and high reliability characteristics.
procedures. There are several types, or levels, of UT scanning available for this purpose. Their main categories are A, B, and C scan. A-scanning is primarily used to measure the thickness of a material. B-scanning can measure defects in a plane perpendicular to the surface. C-scanning provides a 2-D presentation of defects in a plane parallel to the surface at any given depth. A C-scan allows the user to determine what types of defects are present, and their position, density, and size. This type of UT test is a very effective way to investigate flaw distribution because the presence and location of the flaw, as well as its severity, can be readily indicated by inspection. These traits, in addition to its flexibility with thick materials, have caused UT C-scanning to become a widespread method for weld quality inspection .
compared to weld, where the tool rotates perpendicular to the workpiece, when placed at an angle facing the direction gave high strength joint. As discussed when tool tilted at an angle of more than 1° void formation is observed and this condition is for a plate not more than 4mm thickness. The tool tilt angle is based on the thickness of plates to be used. It is observed that in some cases of FSW steel weld TMAZ zone is not seen and in other cases HAZ and TMAZ zones are clearly distinct. TMAZ zone is influenced by both mechanical and thermal cycles, while HAZ zone is influenced by only thermal cycles. Peak temperatures can be seen only in the HAZ zone, due to this hardness in the region will be more. Differentiating TMAZ zone from HAZ zone is most preferably difficult and hence in most of the weld, the region is differentiated as two zones in HAZ viz. HAZ zone 1 and HAZ zone 2. Fatigue life and fracture toughness of FSW steel weld is not explained and studied till yet. When compared to aluminum and its alloys, the characterization study on FSW steels are very less. Only based on the tensile strength and impact toughness, the quality of FSW steel welds are evaluated. Hence more characterization study is needed for deeper knowledge of FSW welds. Corrosion in steel and iron is a natural phenomenon, which occurs at a faster rate than any other metal. Hence alloying element is added, to improve corrosion resistance. Study of corrosive nature in the weld region of FSW of steel is not carried out in any of the previous studies. Further tool wear is an important criterion to be considered before planning for FSW of steel. One practical remedy for controlling the tool wear is preheating the tool and work piece and the other is as said before that selecting a suitable alloying element. Corrosion and tool wear is not only the properties for study, there is lots of characterization study that is need for FSW of steels to undergo and understand.
problems, it should weld this joint without melting. There are solid state welding processes which join similar or dissimilar metals without melting such as frictionwelding , ultrasonic joining , FSW [11- 22]. FSW was invented and patented by The Welding Institute (TWI) in1991 . The FSW was only applied in this study to investigate the dissimilar joint. The mechanism of the FSW is thermo-mechanical which needs three elements to complete the operation. These elements are heat generation, plastic deformation, and forging. The heat is generated by the friction between a rotated tool and a base metal, and the heated base metal becomes very soft (plastic deformation state). The tool of the FSW consists of a shoulder which helps to force the plasticized metal and a pin to stir this metal from side to side to make a joint. Few studies were investigated the welding of aluminum to steel by FSW [11-22]. M. Dehghani et al.  welded mild steel to AA3003-H18. They examined the effects of process variables on the mechanical properties. The maximum tensile strength which they achieved was 140 MPa (about 73% of Al base metal) under the variables condition of 12 mm/min traverse speed and 450 rpm rotational speed. They also reported that Al 5 Fe 2 and Al 6 (Fe, Mn) IMC phases were observed at the interface and in the nugget,
EDS. This weld also contained microcracks initiated from TiN particle and also Al, P, S. This elemental precipitation especially TiN in the SZ microstructure is usually the result of a high peak temperature experience exceeds 1200 C [18–20]. Orlando et al.  proved by the aid of optical microscope and SEM that cubic TiN particles can cause voids during the tensile test of IF steels. They interpreted this type of defect as a fragmentation of weak particles coming from stresses generated from TiN particles corners. An extensive study about microphysical process of cleav- age fracture in steel can be found in Chen and Cao  which focuses on the steps of crack initiation, nucleation and propagation under internal residual stresses caused by TiN precipitation. Figure 11a and b shows microcracks Fig. 9 High amount of BN particles found near the void at AS, EH46 steel W 2E (steady state)
There are limited studies thus far for FSW of structural steel in the relevant technical literature, particularly for high welding speeds. In a well cited study from 2003, Reynolds et al.  examine friction stir single sided welds of hot rolled, 6.4 mm thick DH36 steel, produced by four different welding speeds in an inert gas environment, to assess the relationship between varying weld parameters and resultant weld properties. They  observe a bainitic and martensitic microstructure in the bulk of the thermo-mechanically affected zone (weld nugget) of the fast weld (450 mm/min). However, only this weld’s microstructural features are reported therefore no comparison can be made to the intermediate and slower welds. The hardness of all welds demonstrates a continuous increase from parent material to nugget, with a variation of approximately 190 HV up to the peak hardness of the fast weld. The tensile tests reveal significant overmatching of all welds; longitudinal tensile tests show that the yield strength of all welds is higher than the parent material’s UTS, and this is attributed to the weld nugget microstructure being very different from the original ferrite / pearlite microstructure. In all, weld hardness and strength is seen to increase with increasing welding speed. The effect of increasing rotational speed on weld properties is not considered in this study .
the elongated grains in the base material are dynamically recrystallized to a ﬁne-equiaxed grain structure with high angle grain boundaries in the TMAZ. A further heat treatment (1380°C for 1 h) of the HT-ODS/ODS joints results in secondary recrystallization with a large grain structure (>200 µm). This results in a hardness decrease in the TMAZ compared to the base material. There is only a slight decrease of hardness in the base material of about 5% after annealing. It is evident that only recovery has taken place in the ODS/ODS base materials with a sub-grain of low angle grain boundary structures present in this stage. The 430 / 430 joint shows a ﬁ ne duplex structure of ferrite and martensite in the TMAZ. Dynamic recrystallization and grain reﬁnement were also observed in this region, but result in an increase of hardness compared to the base 430 stainless steel material. These results can be attributed to the rapid cooling rate and high plastic deformation during the FSW process. The 430/ODS joint displays a complicated microstructure in the mixture region. The large fraction of the low angle grain boundary in the 430 stainless steel region proves that not only dynamic recrystallization but also recovery took place. The ODS steel is composed of mostly high angle grain boundaries. The initially elongated grains have been trans- ferred to equiaxed grains, implying the formation of dynamic recrystallization.
The steady state welding stage is modeled in the Eulerian formulation, where materials flow into the computational domain with the prescribed welding speed while the tool stays at the same location with only the rotational motion. Solid state metals are treated as non-Newtonian fluids with high viscosities. Accordingly the flow field belongs to laminar regime and the viscosity is a function of temperature and strain rate. Aluminum and steel are treated as different phases. Based on the topology of phase distribution, multiple phase flow can be categorized into two general groups: separated flow and dispersed flow. In the former one, different phases are continuous and separated by a clearly-defined interface. The latter group corresponds to flow of discrete phases, such as bubbles, droplets and particles, in a continuous primary phase. According to experimental observations of weld cross section macrostructure in Chapter 3, both aluminum and steel are basically continuous. Only a small amount of steel or intermetallic compound particles are dispersed in the aluminum matrix. However, the quantity and sizes of these particles are small, which are neglected in the current model for simplification. The dissimilar FSW process is therefore modeled as a separated multiple phase flow problem.
300, 600, 1200 emery paper. After that, they are polished with 3μm, 1μm diamond and 0.03μm colloidal silica. Both optical microscope and scanning electron microscope (SEM) are applied for the characterization of the joint cross section. The energy dispersive X-ray spectroscopy (EDS) is utilized to analyze the elemental distributions at the interface between the steel and aluminum. Since the keyhole refilled FSSW consists of two steps, the operation parameters of each step are summarized in Tables 4-1 and 4-2 respectively. To find out a proper rotation speed for the refilled step, several trials have been made at the beginning stage of the experiment. When the rotation speed is chosen as 1000 rpm, the refilling quality is low due to lots of voids and a rough surface. The reason behind this is that the low rotation speed generates less heat, which in return constrains the material flow and lead to a poor joint quality. Thus, a higher rotational speed helps to enhance the material flow and to improve the joint quality. Therefore, a higher rotational speed is chosen in the refilled step. The traveling radius in Table 4-2 refers to the radius of the circular path during the keyhole refilling step. It is equal to the offset distance that the tool moves away from the center of the original keyhole after the regular FSSW step. In this study, the plunge depth is defined as the distance between the end surface of the tool pin and the original top surface of the aluminum sheet. During welding, the aluminum alloy sheet is placed on top of the TRIP 780 steel sheet and the experimental configuration is shown in Figure 4-5. As indicated in Figure 4-5, the welding tool is operated in the welding region for both the welding and refilling process.
As automotive manufacturers push towards new and more efficient technology utilizing the power of electric motors and batteries, they are faced with a challenge of using lighter weight structural materials to offset the heavier weight of the batteries or reduce the greenhouse emissions. One promising metal to use is aluminum, which exhibits the necessary light weight properties and is available in many different alloy compositions for specific service requirements. A problem arises when two dissimilar metals, such as aluminum and carbon steel, are connected together in a structure. The difference in the potentials of the two metals creates a galvanic couple between them, leading to the anodic dissolution of the metal with lesser nobility. Eventually the part is subject to fail as its structural integrity will be negatively affected over time, the rate by which is controlled by how aggressive the environment is. Experimental testing can be used to model and predict the corrosion process of a galvanic couple in dissimilar metal joints under atmospheric corrosion conditions, lending to a better understanding of the electrochemical behavior of galvanic couples. Using this knowledge, better informed decisions can be made regarding mitigation strategies to decrease the destructive influence of galvanic coupling in dissimilar metal joints, leading to safer and improved structures being developed in society. Better industry understanding of the issues at hand can also result in more effective programs for corrosion management and mitigation.
The effect of weld heat input on the formation of intermetallic compound (IMCs) layer during arc welding–brazing of aluminium and steel dissimilar alloys was investigated through both finite element method (FEM) numerical simulations and experimental measurements. The results of FEM analysis as well as welding experiments indicated that increasing weld heat input increases the thickness of IMCs layer. The thickness of IMCs layers, as calculated from FEM simulations, was approximately equal to that measured from microstructural images in the range of 2-6 μm. The tensile strength of arc welding– brazing joints was dependent on the thickness of IMCs layer and spreading of molten weld metal on the surfaces of steel sheet. The highest mechanical strength of 120 MPa was obtained in the optimized heat input of 420 J/mm. The presence of the Si element in the Al-5Si filler metal led to IMCs layer with the composition of Fe(Al,Si) 3 phase in side of the steel and Al 7.2 Fe 1.6 Si phase in side of the weld
Friction stir welding (FSW) is a solid state joining method in which the base metals do not melt. Its advantages compared to conventional welding methods include producing welds with higher integrity, minimum induced distortion and low residual stress. FSW is used largely for aluminium alloys, but recent developments have focused on higher temperature parent mate- rials such as steel. Modelling of friction stir welding, particularly for high-temperature alloys, is a challenge due to the cost and complexity of the analysis. It is a process that includes material flow, phase change, sticking/slipping and complex heat exchange between the tool and workpiece. A review of numerical analysis of FSW is available in  He et al. Many studies have been carried out on FSW modelling of aluminium alloys; however, FSW modelling of steel is still limited. Nandan et al.  used a 3- D numerical analysis to simulate heat transfer and material flow of mild steel during FSW. In their work, the viscosity was calcu- lated from previous extrusion work where the range in which steel can experience flow was rated from 0.1 to 9.9 MPa.s. Heat was mainly generated from viscose dissipation and frictional sliding in the contact region between the tool and the workpiece and was controlled by a spatial sticking-sliding parameter based on the tool radius. There has also been extensive work done on modelling of DH36 mild steel carried by Toumpis et al. . In their model, the viscoplastic thermo-mechanical behaviour was characterised experimentally by a hot compression test. They established a 3D thermo-fluid model to simulate the material flow, strain-rate and temperature distribution. Micallef et al.  carried out work on CFD modelling and calculating the heat flux of FSW DH36 6-mm plates by assuming full sticking conditions * M. Al-moussawi
In the Ono’s rotary bending fatigue test using smoothed test specimens, the fatigue fractured occurred in the weakest area in joints. Therefore, the cantilever rotary bending fatigue test was carried out using notched test specimens in order to examine the fatigue strength at the weld interface. S-N curves of joints and the A6061 base metal are shown in Fig. 6. The typical sections of fractured paths after fatigue test and appearance of fatigue-fractured surfaces of joints and the A6061 base metal are shown in Figs. 7 and 8, respectively. The S-N curve of the joint A could not be obtained due to the large dispersion of data and the fact that its fatigue strength was considerably lower than that of the A6061 base metal. Also, the joint A fractured rectilinearly along the weld interface and thin A6061 adhered on the fractured surface. It is clear that the weld at the weld interface was poor. In the joints B-E, although the fatigue limit was almost equivalent to the A6061 base metal, the fatigue strength of all joints at high repeated stress was low in comparison with that of the A6061 base metal. This diﬀerence decreased with a reduction
FSW of steel is expected to deliver substantial commercial and technical benefits to the marine manufacturing sector such as reduced pre-weld preparation and re-work, manpower savings, scope for redesign of assembly lines, and low distortion, high quality welds of excellent fatigue properties. In one example from FSW of aluminium, Norwegian shipyard Fjellstrand reported that “using prefabricated FSW panels has enabled a 40% increase in production capacity and turn-over at the yard”. Issues associated with setup, fixture and tooling costs along with reduced health and safety, training, consumable and material usage costs compared to conventional fusion welding techniques should also influence the introduction of this innovative weldingprocess in industry if it can be transferred into weldingsteel. There are a number of currently identified obstacles, deeper scientific understanding of the process on steel, engagement with the shipbuilding industry, use of fillet welds, different thicknesses and joint design, which will be required to overcome before full acceptance of the process is achieved.
In this study, the effect of various welding parameters on the weldability of Mild Steel specimens having dimensions 50mm× 40mm× 6 mm welded by metal arc welding were investigated. The welding current, arc voltage, welding speed, heat input rate are chosen as welding parameters. The depth of penetrations were measured for each specimen after the welding operation on closed butt joint and the effects of welding speed and heat input rate parameters on depth of penetration were investigated.
stir processing (FSP) zone the formation of FSP zone is affected by the material flow behavior under the action of rotating tool. However, the material flow behaviors is predominantly influenced by the material properties such as yield strength, ductility and hardness of the base metal, tool design, and FSW process parameters ,there have been lot of efforts to understand the effect of process
Currently, energy saving and minimizing environmental impact are impor- tant challenges for automotive industries. One efficient solution for these challenges is to reduce vehicle weight, because low vehicle weigh results in the reduction of fuel consumption. Some automobile manufacturers are using thinner steel sheets for car body structure resulting in a maximum reduction of 30% vehicle weight. But further weight reduction is hardly achievable with exclusive dependence on this method . Therefore, an alternative material is needed to replace steel. Aluminium alloys is one of this alterna- tive materials. With good heat transfer, high strength, good formability and weight saving, aluminium alloy are already utilized for aerospace structures, ship building and also automotive application . A number of automo- tive companies have already succeeded in manufacturing aluminium cars . However, some issues exist in these cars, e.g., aluminium alloys have inferior strength compared to steel, as a result, the safety of the cars are lower, and aluminium alloys are more expensive than steel. Consequently, for achiev- ing high safety and a cost-effective product, the combination of steel and aluminium alloys become the most promising method for automotive indus- tries, which has resulted in increased research interest. Nevertheless, joining of aluminium alloys to steel is challenging due to the differences of physi- cal and chemical properties: their melting points are incompatible and they have different thermal conductivity and coefficient of thermal expansion . Furthermore, the low solubility of iron in aluminium promotes the formation of brittle intermetalliccompounds (IMCs) in the weld zone . Although the combination of these two dissimilar materials is desirable, it is extremely hard to obtain a reliable joint by using fusion welding processes.