Dr Meraj Ahmed

2010-2017
Current Position: Scientist, Advance Material and Process Research Institute (AMPRI), Bhopal.

Importance of light weight alloys in automobile industry has increased tremendously in the recent decades due to their high strength to weight ratio and excellent corrosion resistance. However, aluminum alloys are difficult to be formed into complex sheet metal parts in conventional forming due to their limited formability at room temperature. The conventional forming processes for such materials are becoming extremely energy consuming and often involve multi-stage forming and expensive equipment and tooling. High Energy Rate Forming processes like Explosive Forming, Electromagnetic Forming and Electrohydraulic Forming have gained lot of importance in the recent past and some of them have the potential to replace conventional forming processes for certain applications. Electrohydraulic Forming (EHF) is a high strain rate process in which the energy from a charged capacitor bank is delivered to the workpiece in the form of a shockwave through a fluid medium and this energy is utilized to deform the workpiece into the desired geometry in an open or a closed die. EHF has been a recent interest for some automotive applications as it combines or compliments the advantages of conventional and hydroforming processes. In the present work, formability of 0.5 mm thick annealed AA 5052 alloy sheets in EHF has been studied and compared with conventional forming. An experimental setup has been developed for biaxial stretching (free bulging) with a 5.25 kJ capacitor bank. It has been found that the important process variables are stand-off distance (SOD), wire diameter, electrode gap (EGAP) and the medium through which the energy is transferred. For the given die set-up and capacitor bank, three levels of SOD, EGAP and wire diameter have been used to study the effect of these parameters on formability (in terms of limiting dome height). The compressibility of the medium affects the propagation of shock wave and hence water and oil have been used in this work. Variation in wire diameter and electrode gap influences the circuit pulse characteristics and hence formability. Formability was determined by conducting limiting dome height tests by using all the combinations of the parameters. Taguchi analysis was also carried out to assess the influence of process parameters on formability. Forming Limit diagram (FLD) is extremely useful in the assessment of overall formability of sheet metals and it helps designer to reduce shop floor trials. At very high strain rates, the deformation behavior of Al alloys and the safe forming window could be different from quasi-static conventional forming. Experiments have been conducted at different energy levels to identify the highest safe strains in different modes of deformation. Forming limit diagram has been determined in EHF and compared with formability in conventional forming by punch-stretching experiments. Strain distribution in the deformed samples has also been analyzed in both EHF and conventional processes. Maximum limiting dome height has been obtained with the intermediate SOD and two strain peaks are observed due to obstruction to the shockwave propagation by the electrode in the vicinity of the workpiece at lowest SOD. EGAP and wire diameter affect resistance and inductance of the circuit altering its pulse characteristics. From finite element simulations, it has been found that rate of heating and current density in the wire cross section in solid state are in the suitable range for creation of plasma, superheated vapor bubbles and the associated shockwave. Smallest EGAP and intermediate wire diameter have been found to result in better formability. Taguchi analysis reveals that energy is the most influencing parameter followed by SOD and EGAP for formability in biaxial stretching. Significant improvement in formability has been achieved in EHF. The limit strains increased by nearly 45-50% in all the three regions of the FLD (tension-tension, plane strain and tension-compression) when compared to conventional FLD. In EHF, no clear necking due to strain localization has been observed prior to failure due to very high strain rates of the order of 103 /sec. The strain distribution has been found to be more uniform in the case of EHF with a single strain peak at the pole. Absence of friction in EHF also leads to higher degree of biaxiality leading to higher limit strains in biaxial tension. In the case of EHF, the effective strain is maximum at the pole. While conventional testing shows purely dimple type ductile failure, the fracture mode in EHF appears to have mixed features of both ductile and sudden shear type of failure after large deformation without strain localization.

Publications

Journals

• Meraj Ahmed, D. Ravi Kumar, M. Nabi"Enhancement of Formability of AA5052 Alloy Sheets by Electrohydraulic Forming Process", Journal of Materials Engineering and Performance (Springer), vol 26, Issue 1, pp. 439-452, 2017, (doi: 10.1007/s11665-016-2446-0)
  
  
  

Conferences

• M. Ahmed, S. Panthi, M. Nabi, A. K .Jha. & P. Pawar, "Effect of cross sectional shape of coil used for Electromagnetic forming Process using Numerical simulation", 2nd International Conference on Automation & Information Technology in Iron & Steel Making Processes (AITISM09), Ranchi, India, Dec 2009 (As part of project, prior to PhD)