I would like to thank the former and current head of the mechanical engineering department Prof. There is also a significant increase in the FLCs of the TWBs formed by EMF process compared to a quasi-static process.
Forming
Defects Caused by Conventional Forming
High Velocity Forming
High-speed forming technique is one of the processes to increase the formability of aluminum alloy. High kinetic energy associated with the high speed forming process brings inertial effect to the deforming workpiece which led to a plastic deformation.
Advantages of HVF
Motivation
Objective of the Research Work
Contribution of the Research Work
Determination of optimal process parameters for difficult-to-weld thin aluminum alloy sheets of different quality combinations (ie AA 5052 with AA 6061) by friction stir welding. Determination of optimal process parameters for successful welding of thin aluminum alloy sheets of various thickness combinations (i.e. 1 mm by 1.5 mm) using friction stir welding.
Outline of the Thesis
- Explosive Forming
- Electrohydraulic Forming
- Electromagnetic Forming
- EMF Process Characteristics
- Applications of EMF
- Advantages and Disadvantages of Electromagnetic Forming
Two considerations are key in the design of the busbar, it must have low inductance (in general, the majority of the system inductance should be at the forming coil), and long-lasting non-sparking electrical contacts are required [16]. Just like the punching tool in a conventional forming process, the coil in the electromagnetic process also plays an important role in the electromagnetic forming process.
Tailor Welded Blanks
Advantages of Tailor Welded Blanks
The high investment costs for welding processes associated with the manufacture of custom-welded blanks are its disadvantages. However, aluminum has poor welding properties due to its high reflectivity, low melt viscosity and the presence of oxide layers.
Friction Stir Welding
FSW Process Overview
The tool inclination angle is the angle between the tool axis and the normal to the surface of the plates being welded. The area affected by both heat and plastic deformation is known as the thermomechanically affected zone (TMAZ), which is defined by a trapezoid bounded by the diameter of the shoulder and the diameter of the probe at the weld root.
FSW Applications
Another term for the 'clone' often referred to in the literature is the 'dynamically recrystallized zone'. Apart from FSW weld types involved in pipe welds, fillet welds, hemispherical welds, double T-joint welds and fillet welds, the most common types of weld configuration are the square butt weld, lap weld and the T weld.
Advantages and Disadvantages of FSW
Electromagnetic Sheet Forming
70] analyzed the formability of a low-conducting metal sheet by electromagnetic forming process using a new method to generate data for the forming boundary diagram.
History of Tailor Welded Blank
The attention is shifting to friction stir welding technique for joining difficult to weld aluminum TWB over fusion based welding processes. 84] accordingly reported the advantages of friction stir welding technique to join aluminum alloys over laser welding process. The next part of literature review will be more related to friction stir welding of aluminum alloys, especially 5xxx and 6xxx series.
Friction Stir Welding of thin Sheets
Process Principle
For example, during extrusion of the material, a mixture of recrystallization phenomena and recovery of the grain structure takes place in the welding zone [100]. [124] studied the effect of natural aging after welding on the tensile and hardness properties of the FSWed joint of AA 6063.Cavaliereet al.[125] and Scialpi et al. 146] observed the formation and enlargement of wormholes near the bottom of the weld as the tool traverse speed increases while the rotational speed remains constant.
Work Done on FSW of Dissimilar Aluminium Alloys
166] to simulate the correlation between the applied FSW parameters with the resulting mechanical properties of the FSW of AA 7075-O to AA 5083-O aluminum alloys. Palanivel et al. investigated the influence of tool rotational speed and pin profile on the microstructure and strength of the dissimilar FSWed AA5083-H111 to AA 6351-T6 aluminum alloys. 174] investigated the effect of base material location during FSW of dissimilar AA 5052-H32 and AA 6061-T6 aluminum alloys.
Forming of FSWed Aluminium Blanks
191] also studied the effect of weld line movement on the formability of welded blanks. The results showed the influence of weld line orientation on the formability along with decrease in formability compared to base materials. 195] studied the influence of shoulder diameter and indentation depth on the formability of FSWed AA 6061-T6 sheets.
Conclusions
In addition, 80% of the yield stress of the base material was achieved in friction stir welded aluminum alloys in stir welding, with failure usually occurring in the heat-exposed zone, while in friction stir welded steel, it was observed overmatching the location of the defect in the base material. The fatigue life of friction stir welds is lower than that of the base material, but significantly higher than that of laser welds and MIG welds. In addition to aluminum alloys, friction stir welding is successfully used to join different materials and raw materials of different thicknesses.
Friction Stir Welding Machine
Fixture Work-piece
The diameter of the circular tool pins was made equal to one third of the shoulder diameter size as suggested by Arora et al. The plate was used has an upper plate above the work plates as a clamping arrangement to stop all degrees of freedom of the thin blank plates to the current FSW fitting. With the absence of the mentioned upper plate, it was difficult to weld with the current plate thickness, and the occurrence of defects such as tearing along the welding direction and cracks due to bending were found to happen.
Formability Test Setup
Therefore in order to build high speed formed aluminum alloy FLC, we will focus on the biaxial stretching area of FLD. The Nakazima test compound is made of a hemispherical punch, a trowel, a blank holder and a pull bead that limit the slippage of the blanks. Similar approach and setup was used to draw FLD of TWBs for high speed forming.
Electromagnetic Forming System
Capacitor Bank
Measuring Unit
Actuator and Fixture
For the tensile test, the friction stir welded specimens were cut to size according to ASTM E8M standard specifications [202] and the weld line was maintained in the transverse direction.
Friction Stir Welding of Dissimilar Grade of Aluminium Alloys AA 5052 with AA 6061
Results and Discussions
However, there is a wavy deformation on the outer surface of the welded blanks due to the surface tension. It is found that the strength of base material is higher than that of the welded specimens as shown in Fig. We can therefore conclude that in the middle of the stirring zone uniform mixing of AA 5052-H32 and AA 6061-T6 alloys took place.
Summary
The ultimate tensile strength (UTS) of a welded joint with a thickness of 1.5 mm (210.4 MPa) was found to be 22% more than that of a joint with a thickness of 1 mm (172.4 MPa) under the same process parameters as the average hardness value at a weld zone of 1.5 mm. thickness was more than that of a joint with a thickness of 1 mm. The average elongation before failure of 1.5 mm thick welds was approximately 47% more than that of 1 mm thick weld specimens. This characteristic is due to the higher ductility due to the higher hardness in the weld zone/UTS of weld pieces with a thickness of 1.5 mm. At the center of the weld there was a uniform mixing of AA 6061 and AA 5052 alloy constituents.
Friction Stir welding of Thin Dissimilar Thickness Aluminium Alloys of AA 5052 and AA
Summary
Successful bonding with different blank thicknesses of 1 mm to 1.5 mm of each AA 5052 and AA 6061 respectively were obtained by the FSW process. Both AA 5052 and AA 6061 exhibited similar average microhardness values in the weld region, however there was an uneven distribution of the microhardness profile. Both AA 5052 and AA 6061 alloys have undergone dynamic recrystallization showing fine, even grain structures.
Electromagnetic Forming of Friction Stir Weld Blank of Dissimilar Grade AA 5052-AA
Summary
The percentage increase in cube heights for AA 5052 and AA 6062 base by EMF is 115% and 83% respectively against the conventional process. The FLC of the voids formed by the EMF process shows significant increase in both major and minor strain from the conventional process. This is also the same for AA 5052 and AA 6061 base where the FLCs are increased with the EMF process versus the conventional process.
Electromagnetic Forming of Different Thickness Tailor Welded AA 5052 Blank
Results and Discussions
Due to the inertial effect of the deforming TWB at high speed, there is a bulge in the center of the dome when it is formed by EMF process. The FLC of EMF is more than that of the conventional molding as expected due to higher dome height. The increase of FLC by EMF in terms of large voltage is about 28.6 % more than that of the FLC built from conventional process.
Summary
The forming curve of the welded blanks (from the EMF process) is lower than the FLC base material curve (from the quasi-static process). The forming limit curve obtained from the welded blanks formed by the EMF process shows a significant increase in size compared to the quasi-static process. Therefore, the high-speed forming process increases the forming ability of both base and welded blanks (various thicknesses).
FEM Simulation
A major advantage of adopting this strategy is the immediate strong coupling of the multiphysics equation solvers. The explicit mechanical solver calculates the deformation of the workpiece using the new geometry to calculate the EM field in a Lagrangian way. To evaluate the constitutive material response of the aluminum alloy workpiece, the yield stress is determined as a function of the plastic strain; strain rate and temperature using Johnson Cook's constitutive equation [209].
Simulation Work
Initially, at about 55 microseconds (Fig. 6.2), the EM pressure is maximum in the center of the workpiece. Around 265 microseconds, when the workpiece has reached full dome height, the pressure is moved to the perimeter of the dome, i.e. the interpretation is further rationalized with the result of von Mises stress representation in the subject.
Experimental Work
The power levels are chosen in terms of the percentage of output corresponding to the total power output of the machine, as it is easier to categorize the power level than the specific choice of voltage in the existing EMF machine (Table 6.4). Further increasing the voltage resulted in the gap cutting at the bottom of the dome, which can be seen at 12.75 kV. Finally, at 12.88 kV the aluminum blank suffered a defect or complete shear from the dome base area (Fig. 6.9).
Summary
Conclusions
Furthermore, in the EMF process, the height achieved by TWB is greater compared to the conventional process. There is a significant increase in the FLC of TWB formed by the EMF process over that of the conventional process. Also, the FLC obtained from welded blanks formed by the EMF process shows a significant increase in size compared to a quasi-static process.
Future Scopes
Effect of welding parameters on the microstructure and mechanical properties of friction stir welded 2219 Al-T6 joints. Influence of shoulder geometry on microstructure and mechanical properties of friction stir welded 6082 aluminum alloy. Effect of material locations on properties of friction stir welded joints of dissimilar aluminum alloys.
