Applied Mechanics and Materials Vol. 787

Abstract: Magnesium alloys are used in many applications, particularly in orthopaedic implants are very difficult to machine by conventional processes because of their complex 3D structure and limited slip system at room temperature. Hence there is an inherent need for alternative processes for machining such intricate profiles. Electric Discharge Machining is growing rapidly in tool rooms, die shops and even in general shop floors of modern industries to facilitate complex machining for difficult-to-machine materials and provide better surface integrity. Therefore, the use of electric discharge machining on ZM21 magnesium alloy is attempted in this paper. Nanographite powder is added for machining zone to enhance the electrical conductivity of EDM oil by way it improves the machining performance. Machining parameters such as the current, pulse on time and pulse off time were process parameters to explore their effects on the material removal rate and tool wear rate. It is observed that, an increased material removal rate was due to the enhanced electrical and thermal conductivity of the EDM oil.

Abstract: Empirical relationships were developed between the resistance spot welding process parameters (i.e. power, time and electrode force) and quality characteristics (i.e., tensile shear failure load and nugget diameter) of resistance spot welded DP980 steel joints. Further, the process parameters were optimized to get maximum tensile shear failure load and optimum nugget diameter. It is observed that, the electrode force of 4.25 kg/cm², power of 70 kW and welding time of 3.5 sec yielded a maximum tensile shear failure load of 39 kN with an optimum weld nugget diameter of 4.58 mm.

Abstract: Many advanced machining processes have higher initial investment, maintenance and tooling costs. In these circumstances micro electrochemical machining [µECM] processes meet the requirements with better efficiency and economy. The micro electrochemical machining process parameter such as electrolyte concentration, machine voltage, and pulse on time and duty factor are optimized with considerations of the multiple performance characteristics such as material removal rate (MRR) and overcut. Increase in electrolyte concentration considerably improves MRR but yields a poorer over cut. Increased machine voltage serves to increase both MRR and over cut significantly. Increased pulse on time and duty factor did not have notable impact on MRR and overcut. Less concentration of sodium chloride [NaCl] is sufficient for machining compared with sodium nitrate [NaNO₃]. Higher MRR and better over cut are achieved with sodium chloride and moreover it is more economical than sodium nitrate.

Abstract: An attempt is made to modify the surface metallurgically and enhance the wear resistance of AISI 316LN austenitic stainless steel using friction stir processing. Friction stir welding tools made up of tungsten based alloy with pin and pinless configuration was used. Fine equiaxed grains were observed in the friction stir processed zone irrespective of tool configuration used. Dry sliding wear resistance was evaluated using pin-on-disc wear tester and it is found that, the friction stir processed zone showed superior wear resistance compared to the base metal. Microstructure, micro hardness, and worn surfaces were used to correlate the results obtained.

Abstract: Magnesium alloys are widely used in applications where weight reduction is of primary importance. MgAZ91D is an Mg-Al-Zn alloy and its application in the automotive sector is limited by its poor corrosion resistance. Recent advances in solid state processing techniques have made it easier to modify the mechanical and corrosion characteristics of various alloys. Friction stir processing (FSP) is such a solid-state process for surface and sub-surface modification, which increases the microstructural densification, thereby producing fine and equiaxed grains. Through this work, an attempt was made to analyse the effect of friction stir processing on the corrosion resistance of the alloy in an enclosed salt spray chamber. Micro-analysis tools like FESEM and EDS are used to supplement our results. It is seen that, FSP significantly contributes to the increase in the corrosion resistance by homogenising the distribution of α and β phases and hence making the use of the alloy more practical in moisture rich environments.

Abstract: Titanium Alloys are unique due to their high strength-weight ratio,good fracture and corrosion resistance characteristics.They form a significant metal portion of the aircraft structural and engine components. However, their low thermal conductivity and high reactivity with cutting tools during machining, makes them difficult-to-machine materials. In this work, an investigationof the thrust force, torque and chip morphology in drilling of Ti6Al4V, titanium alloy using tungsten carbide tipped drill has been carried out at varying cutting speeds of 19.4, 27.14, 43.41 and 67.82m/min at a constant feed rate of 0.15mm/rev.The machinability, work hardening and thermal softening effects of titanium alloy, during drilling, has been analysed through the effects of the machining process on the thrust force, torque and chip microstructure.

Abstract: Superplasticity is the ability of the material to produce neck free elongations within a material before fracture. For the past three decades superplastic forming has gained a major development in many industries to produce complex shapes. To perform the superplastic forming at elevated temperatures, the material parameters such as strain rate and strain rate sensitivity index has to be determined. These parameters affect the formability in such a way that higher the strain rate during deformation, lesser will be the percentage elongation and which in turn increases the flow stress of the material there by limiting the formability. Similarly, the strain rate sensitivity index is a measure of resistance to neck formation during deformation. Lesser the strain rate sensitivity value, more will be the neck formation thereby limiting the formability. Hence in this work, an experimental setup is designed to perform the uniaxial tensile testing at elevated temperatures to determine the flow stress, percentage elongation, strain rate and strain rate sensitivity. The determination of these parameters will be helpful in executing the forming at certain temperature and pressure to attain maximum formability. Also the SEM photographs of the fractured specimen were analysed to determine at what temperature and strain rate, the cavitation density increases.

Abstract: Microstructure, micro hardness and wear resistance of friction stir processed cast magnesium alloy are investigated in this work. Image analysis is used to differentiate the amount of phases present in the base metal and friction stir processed regions. Hardness mapping indicates that the frictions stir processed region has 64 % increase in microhardness compared to the base metal. Wear resistance was evaluated using pin-on-disc testing and it is found that the friction stir processed region has superior wear resistance compared to the base metal. Fine grains with uniformly distributed second phase particles are the reasons for improved microhardness and wear resistance of friction stir processed region.

Abstract: While machining a material, it is essential to understand the characteristics of work material for choosing the favorable condition of the material, appropriate cutting tool, machining parameters to achieve desired optimum output. Accordingly, researchers always devise their studies to improve the machining processes resulting in enhanced product quality and increased production rate simultaneously. Such machining studies on super-alloys attract significance, since they are oriented towards overcoming the difficulties involved in machined out a component. Generation of heat in the cutting zone and high cutting forces on the tool being experienced while machining super-alloys, are due to poor thermal conductivity and work hardening characteristics. Above factors contribute to two basic problems viz. the inability of the tool materials to give long tool life and the metallurgical damage to the machined components due to induced stresses. Among the super-alloys, Inconel 718 is the widely used heat resistant super alloy (HRSA), which withstands stringent operating conditions in high and cryo temperature environments for longer duration. Studies on understanding the behavior and the relationship between work piece material, cutting tool materials, cutting conditions and the process parameters is an essential requirement for establishing and optimizing the machining process and the present work is tailored towards this objective. In this work, Inconel 718 alloy was subjected to turning experiments and the experimental data collected was used for constructing empirical models whose performance was assessed through statistical approach. It is established that the models developed could be used for predicting the output parameters for a set of input of parameters through 2-D (surface) and 3-D (contour) plots.

Abstract: The most common industrial needs of coatings are to improve the surface properties such as, wear resistance, corrosion resistance and thermal conductivity/insulation. The use of expensive high strength materials and super alloys can be mitigated by using low grade materials covered with suitable coatings compatible with the media and service. Air plasma spray deposition method is used to achieve dense coating on substrate. The present research work demonstrates the wear properties of Ni-Cr/Micro-ZrO₂ coating prepared through air plasma spray deposition (PSD) method. In this process, ZrO₂ particulates of average particle size (10-40µm) were sprayed on Ni-Cr coated AISI 410 grade steel substrate. Distribution of ZrO₂ particle was found uniform throughout the coating as revealed from SEM microphotographs. Dry sliding wear tests were performed on Pin-On-Disc Tribometer at room temperature with moderate humidity conditions. Significant improvement in wear performance was observed in Ni-Cr/Micro- ZrO₂ coating deposited on steel via PSD which was also evident from worn surface morphology.