Institute of Fundamental Technological Research

A discrete element thermal conductance model suitable for the modelling of heat flow between sintered particles has been proposed. The model is formulated using the sintering geometry consisting of two spheres connected with a cylindrical neck. The calculation of the neck size is based on the criterion of volume conservation. Therefore the neck obtained is more accurate than that of the popular Coble's model. The thermal conductance is determined for different neck sizes by the finite element simulations of the heat flow in half of the sintering geometry. The numerical results are fitted with a linear relationship which is the basis to determine the equivalent conductance between two sintered particles. The model can be used in the pipenetwork formulation of the discrete element method for simulation of heat conduction problems in powder sintering or in sintered porous materials.

sintering, particles, discrete element method, thermal conductance, pipe-network model, volume conservation, heat conduction

Friction stud welding process is a suitable candidate in joining stud fasteners for steel structure buildings, military vehicles, automobiles, aircraft, ocean liners, bridges, ship buildings, etc., The peak temperature for welding is achieved by converting mechanical energy to thermal energy at the sample interface without the use of electric energy from other sources because it is a solid-state process. The study of the thermal behavior of different metals during friction stud welding is very important since it is a thermal energy process. However, there is no good thermal model for the friction stud welding process. In this work, the generation of heat flux at the interfacial area of two distinct metals, namely aluminum and mild steel, is calculated using a mathematical model. The temperature at the interfacial region, which plays a significant role in the quality and strength of the weld component, is particularly focused on experimentation and analytical modeling. In the experimentation, a noncontact type infrared thermometer is used to measure temperature directly. The temperature profile was determined by the finite difference method based on thermal resistance and capacitance formulation at transient conditions. The obtained mathematical results are compared with the experimental results at the distance of 5 and 10mm from the welded interface. The computed temperature profile is in good agreement with the experimental data on the heating side and with a minimum degree of deviation in the cooling part. The maximum percentage of error for the 5mm interface is 3.349 and for the 10mm interface is 2.857. This deviation is due to the zero-axial shortening assumption in the analytical model. Besides, the temperature characteristics of the welded are analyzed at various time increments by numerical simulation. As a result, the predicted temperature is more on the aluminum side compared to the mild steel due to a change in thermal properties. This proposed thermal model would be helpful to improve the design and manufacture of welding machines.

friction stud welding, thermal modeling, finite difference method, numerical simulation, aluminum, mild steel

In the present scenario, aerospace and automobile industries depend on lightweight materials such as magnesium and aluminum alloys because of their great balance between mechanical properties and weight ratio. Despite these benefits during the joining process of these dissimilar materials by welding, many challenges arises. The prominent one is related to the low melting points of these lightweight metals which make it almost impossible the joining using conventional arc welding techniques. To tackle this challenge, Friction Stir Welding (FSW) can be considered as a promising candidate tool. In this study, to demonstrate the FSW performances of joining two dissimilar materials we have investigated the joining of AA 6061 and Mg AZ 31 B using a built-in house a modified milling machine. The dissimilar combinations of AA 6061 and Mg AZ 31 B joints were successfully joined by embedding different welding conditions and varying the offset distance. The mechanical performances were evaluated by conducting specific mechanical tests such as micro-hardness, tensile, and impact tests, respectively. To explain the mechanical results, we have applied optical microscopy observation on the microstructure associated with the bonding location. The results prove that the strength of the Friction Stir Welded joints is much higher as compared to other techniques especially in terms of dissimilar metals.

friction stir welding, aluminum, magnesium, mechanical properties

In the metal forming process, the understanding of metal flows and the fracture strains are most significant to the failure/damage of the components. Usually, in metalworking, damage occurs because of nucleation, growth and coalescence of the void into a small fracture. These small fractures increased in the circumferential path due to the existence of stresses and the pores which leads to failure at the equatorial position during the upsetting of porous samples. Hence, the fracture of the workpieces strongly depends on the stresses and the pores. Such form of stresses and pores if relieved will give a better damage limit of the material. Therefore, in this research, a novel scheme of localised heating is adopted at the equatorial position of the compressed samples to enhance the critical damage parameter. The powder metallurgy route was used to prepare the required compacts with different relative densities (80%–90%) and 1 aspect ratio by applying suitable powder forming pressures. The upsetting test was performed on the obtained porous samples for various weight percentages of titanium (2%–6%) in the aluminium at the stable strain rate (0.1 s^−1) and the damage location was noticed for various components. After the identification of damage position, various temperatures (100 °C–250 °C) of localised heating were attempted on the failure site of the specimens after some incremental stages of upsetting tests. The experimental results were analysed using various damage criteria and it was found that the initiation of failure is delayed and increased the critical damage value by selectively heating the samples because of relieving the stresses, reduction in porosity and changes in microstructure.

aluminium, failure parameter, localised heating, powder metallurgy, strain path, titanium, upsetting

In the present work, localised heating has been adopted at the damage site of the cold upset materials and the role of this mechanism on the workability has been analysed. Cylindrical specimens containing 96% aluminium and 4% titanium were prepared through powder metallurgy technique with an aspect ratio (height to diameter) of 1 by suitable pressures. A series of cold upsetting test was conducted and the material properties for various preforms initial relative densities (80%, 85% and 90%) were determined under the stable strain rate. The flow of metals was analysed using a finite element tool and it was observed that the metal flow starts from near the centre zone to the equatorial zone and the damage happens in the outer position because of more amount of accumulated stresses and the pores. These stresses and pores decrease the workability of the final component. Hence, the present research is intended to reduce the stresses and minimize the pores by applying a localized heating (100 °C–250 °C) at the equatorial sites of the components and thereby increasing the workability of the material. Also, heating selectively at the equatorial site of the workpiece improves the workability due to change in grain size and it was noticed that the grain size of the developed porous preforms was high for the higher heating conditions due to the growth of the grains. Therefore, the localized heating adopted in this work is a superior method to enhance the workability of the powder samples and this novel technique could be useful in improving the workability of the structural components that have extensive applications in the automobile and aerospace industries.

powder metallurgy, workability, localised heating, aluminium, titanium, upsetting