B.S. Yilbas

Plasma nitriding of Ti6Al4V alloy to improve some tribological properties

The present study was conducted to investigate the tribological and mechanical properties of plasma-nitrided Ti6Al4V alloy. Specimens were nitrided in an H2N2 (1:8 ratio) plasma. The nitrogen concentration along the nitrided zone was obtained using the nuclear reaction analysis technique. The workpiece temperature was varied from 450 to 520 °C during the nitriding process. Pin-on-disc wear tests were carried out to evaluate the wear properties of the resultant samples and a ball-on-disc experiment was conducted to measure the friction coefficient. Microhardness tests, Scanning electron microscopy and X-ray diffraction were carried out to investigate the phases developed in the nitrided zone. It was found that the wear resistance improved considerably after the nitriding process. Three distinct layers were identified: (i) an inner layer where δ-TiN + e-Ti2N phases formed, (ii) an intermediate layer where α-(TiN) with or without e phase developed and (iii) an outer layer where precipitations were dominant.

Parametric study to improve laser hole drilling process

Abstract In the laser drilling process, the quality of the drilled holes is the main task. A method of studying the influence on the quality of the main process variables needs to be developed, which seeks to improve the quality and explains the drilling mechanism. In the present study, the effect of the laser parameters and the material properties on the hole quality when drilling is examined. A statistical approach, referred to as factorial design, is employed to test the significance level of the factors that affect the hole quality. Three materials, stainless steel, nickel and titanium, are considered. The experimental study yields tables of significance of each factor on the aspects that determine the quality of the holes. The hole geometry is evaluated by assigning marks for each geometric feature, the marking scheme being conducted relevant to the importance of the hole feature.

Friction welding of StAl and AlCu materials

Abstract The present study investigates the mechanical and metallurgical properties of friction-welded steel-aluminum and aluminium-copper bars, a friction welder having been designed and built for this purpose. The effects of the three main parameters: speed of rotation; friction load; and duration of welding; on the metallurgical and mechanical properties of the weld such as the yield, tensile and breaking strengths, are studied experimentally and statistically. The metallurgical properties of the weld are examined using electron and optical microscopy.

Repetitive laser pulse heating with a convective boundary condition at the surface

Repetitive laser pulses deposit sufficient energy to provide uniform-like heating at the surface of a substrate. This improves the surface properties of the substrate so treated. In the present study, an analytical solution for the temperature distribution due to repetitive laser pulse heating with a convective boundary condition at the surface is obtained. A Laplace transformation method is used when obtaining the analytical solution for the heat transfer equation. The effects of the pulse parameter (β/γ) and the Biot number (Bi) on the resulting temperature profiles for the possible attainment of a steady temperature at the surface during repetitive laser pulse heating is explored. The consecutive pulses with decreasing intensities are employed in the analysis while Bi is varied as 2×10-4≤Bi≤0.2. It is found that it is unlikely that the temperature profile follows the pulse profile. The effect of Bi on the temperature profiles resulted from the repetitive pulses becoming significant when Bi≥10-2.

Laser-induced thermal stresses on steel surface

Abstract In laser heat treatment of steels, a thin surface layer of austenite forms during heating and subsequent phase change process in the cooling period. However, thermal stress develops due to high-temperature gradient attainment in the surface vicinity which in turn results in microcrack development at the surface. The present study is carried out to compute the temperature profiles due to step input pulse laser radiation and determine the resulting thermal stresses. The study is extended to include three-step input pulses having the same energy content. This provides the comparison for the influence of the pulse length on the resulting thermal stresses. To validate the theoretical predictions, an experiment is conducted to irradiate the AISI 4142 steel surface by an Nd–YAG laser. Microphotography and EDS analysis of the heated regions are carried out. It is found that considerable thermal stress is eveloped at the workpiece surface due to attainment of high-temperature gradient in this region. In addition, microcracks are observed at the surface of the irradiated spot.

Development of a new moisture transfer (Bi-Re) correlation for food drying applications

In this paper, newly developed Biot–Reynolds (Bi–Re) correlation to determine the moisture transfer parameters is presented. Development of the new correlation is based on the experimental data taken from various sources in the literature. Moisture diffusivity and moisture transfer coefficient are calculated using the previously developed model. The moisture distribution profiles are then obtained for regular objects such as slab, cylinder and sphere. The results obtained from the present study are compared with the experimental data and a correlation available in the literature. It is found that they are in good agreement. Hence, it is believed that the developed correlation is of great significance for design and practicing engineers working in the drying industries.

A study of the corrosion properties of TiN coated and nitrided Ti-6Al-4V

Abstract TiN PVD coating and plasma nitriding are two well-known techniques for improving tribological properties of treated samples. The study examines the corrosion resistance of plasma nitrided and TiN coated Ti-6A1–4V samples. The potentiodynamic polarization technique was used to measure the corrosion rate and corrosion resistance of the substrates. A micro PIXE technique was employed to determine the nitrided zone depth profile. SEM was carried out to investigate the pitting defects. To achieve a comparative study, TiN coated, nitrided and untreated samples were examined. It was found that TiN coating improves the corrosion properties, but nitriding worsens the corrosion resistance of the substrate.

A closed form solution for temperature rise inside solid substrate due to time exponentially varying pulse

Modeling of laser heating process minimizes the experimental cost and enables to optimize the process parameters for improved end product quality. In the present study, an analytical solution for laser conduction limited heating due to time exponentially varying pulse is presented. The governing equation of heat diffusion is solved analytically using a Laplace transformation method. The closed form solution is validated by a solution for a step input pulse intensity presented in the previous study as well as numerical predictions. Temperature rise inside the substrate material is computed for steel. It is found that the present solution reduces to previous solution once the pulse parameter (β=0) are set to zero. Temperatures obtained from the closed form solution agree well with the numerical predictions. Moreover, temperature rises rapidly in the surface vicinity due to time exponentially varying pulse. The pulse parameter (β∗/γ∗) has a significant effect on the temperature rise. In this case, low value of (β∗/γ∗) results in high temperature rise in the surface vicinity of the substrate material.

Local entropy generation in an impinging jet: minimum entropy concept evaluating various turbulence models

Second law analysis techniques have been widely used to evaluate the sources of irreversibility in fluid flow systems. The same technique may be used to evaluate the various turbulence models. In the present study, a local entropy generation rate is computed for a fluid jet impinging on a heated wall. The standard k–e, low-Reynolds number k–e and two Reynolds stress models are introduced to account for the turbulence. A numerical scheme employing a control volume approach is used to solve the governing equations. The predictions are, then, compared with the experimental findings in the literature. The local volumetric entropy generation in the region close to the stagnation point is used to evaluate the turbulence models. The entropy generation gives information about the magnitude of viscous dissipation in the flow field. The minimum energy concept alone may not be used to evaluate the various turbulence models, in which case, the experimental measurements are accompanied with the results of entropy analysis.

Laser treatment and PVD TiN coating of Ti-6Al-4V alloy

Improved adhesion between a coating and the base substrate is necessary for successful engineering applications. This may be achieved through generating intermediate zone between the coating and the substrate. In the present study, laser gas alloying of Ti-6Al-4V alloy was carried out. Nitrogen was introduced co-axially with the laser beam while the workpiece is scanned at a constant speed in the x-direction during the laser heating process. The laser-treated workpieces were then slightly ground and TiN PVD coated. Thickness of the coating layer was limited to 2 μm. The wear tests were conducted on the laser-treated TiN-coated workpieces. To examine the metallurgical changes, microphotographs of workpiece cross-sections were taken. To monitor the nitride compounds in the surface region, XRD was carried out. It was found that three wear stages occur. The steady wear stage is relatively longer for laser-treated TiN-coated workpiece compared to the TiN-coated workpiece.