Dimple Density Research Articles

Optimizing friction stir welding of AA7075 and AA8090 aluminum alloys: a desirability-driven investigation into mechanical and microstructural enhancement

This research explores the multifaceted analysis of a friction-welded joint, employing Central Composite Design of Response Surface Methodology. The study integrates microstructural investigations and fracture analyses to explain the effect of process parameters on mechanical properties. The optimum settings for Friction Stir Welding of AA7075 and AA8090 were determined by assessing desirability indices. These settings comprised a tool rotation speed of 1927.7 rpm, a tool travel speed of 35 mm min−1, and a tool tilt angle of 0.9°. This specific combination yielded a noteworthy combined desirability index of 0.79, considering both Ultimate Tensile Strength (UTS) and Tensile Elongation (TE). Microstructural examinations revealed distinct characteristics in the Heat-Affected Zone (HAZ), Thermo-Mechanically Affected Zone (TMAZ), and Nugget Zone (NZ). Notably, fine grain structure in the NZ was attributed to the stirring effect created by the tool pin. Fracture analyses indicated ductile fractures, with dimple size variation correlating to tensile strength. Lower dimple density in low-strength joints suggested insufficient material mixing during welding. The maximum tensile strength sample exhibited a high dimple density. These findings contribute to a comprehensive understanding of the welding process’s influence on microstructure and fracture characteristics, providing valuable insights for optimizing mechanical properties in friction-welded joints.

Laser-Fabricated Micro-Dimples for Improving Frictional Property of SKH51 Tool Steel Surfaces

Friction involved in metal-forming processes typically leads to the wear of tool and die surfaces, and in turn shortens the tool’s service life. A thriving need for reducing surface friction requires the tool surface to be modified. This paper presents the surface modification of SKH51 tool steel, on which the hexagonal array of micro-dimples is fabricated by a nanosecond pulse laser. Using the average laser power of 25 W can create decent dimples for trapping lubricant and enabling hydraulic pressure at the surfaces in contact. The effect of dimple density and sliding speed on the coefficient of friction was examined in this study through the pin-on-disc test, in which a stainless steel pin was applied against the tool steel disc with a constant load. The laser-textured tool steel surface with a dimple density of 35% had a friction coefficient of 0.087, which was lower than that of the untextured surface by 12.6% when using a sliding speed of 15 cm/s. In addition to friction reduction, there was no substantial wear found on the laser-textured surface compared to the untextured sample. The findings of this study can be a processing guideline and benefit the treatment of tool and die surfaces for friction and wear reduction in metal-forming and related processes.

Effects of Operating Conditions and Pit Area Ratio on the Coefficient of Friction of Textured Assemblies in Lubricated Reciprocating Sliding.

The experiment was carried out in a reciprocating lubricated conformal sliding contact between steel discs of the same hardness. The effects of disc surface texturing on the friction coefficient at various operating conditions (temperature, normal load, and frequency of oscillations) were studied. Under various conditions, surface texturing caused friction reductions of sliding pairs. The largest reduction was 4.6 times at a lower temperature and 2.5 times at a higher temperature. The effect of the pit area ratio on the friction reduction was visible at a higher temperature. The highest dimple density of 25% corresponded to a lower coefficient of friction than the smallest density of 9%. The sliding pair with a dimple density of 17% led to large variation of the friction force. At lower temperatures, the coefficients of friction were lower compared to tests at higher temperatures.

Effect of a micro-textured surface with deposited MoS2-Ti film on long-term wear performance in vacuum

To improve the long-term tribological properties of precision moving parts in aerospace, MoS2-Ti film was applied on a low-density micro-textured surface of the most commonly used 9Cr18 steel. The 1.0, 1.5, 2.0 and 3.0 % dimple densities of micro-textured surfaces were produced by an Nd: YAG femtosecond pulse laser system. The effects of different micro-textured densities surface with MoS2-Ti films on friction properties and wear mechanisms were investigated. Tribological results showed that all the micro-textured surfaces with MoS2-Ti films had a lower friction coefficient and wear rate than the un-textured surfaces with MoS2-Ti films. The wear performance was enhanced by increasing the density of the micro-textured surface. The friction coefficient and wear rate after 3.6 × 105 lap rotation were reduced to below 0.018 and <1.0 × 10−7 mm2/N•m, respectively, when the density of micro-texture was 3.0 %. It was found that the friction coefficient and wear rate increased with increasing sliding laps, but the 3.0 % density surface with MoS2-Ti film after a lap rotation of 9.0 × 105 still had a lower wear rate than the un-textured surface, indicating an excellent long-term wear resistance. The function of micro-textures is not only to capture wear debris and store lubrication films but also to form a moderate transfer layer on the friction pair, all of which could help to prolong the wear life of MoS2-Ti films.

Tribological Properties of Ti6Al4V Titanium Textured Surfaces Created by Laser: Effect of Dimple Density

The loss of energy due to friction is one of the major problems industries are facing nowadays. Friction and wear between sliding components reduce the mechanical efficiency of machines and have a negative impact on the environment. In recent years, surface texturing has shown tremendous ability to reduce friction and wear. Micro-features generated on surfaces act as a secondary reservoir for lubricants and wear debris receptacles to further reduce abrasion. In addition, surface texturing boosts hydrodynamic pressure, which increases the elasto-hydrodynamic lubrication regime of the Stribeck curve, reducing friction and wear. Amongst all different techniques to texture surfaces, laser texturing is the most popular due to its advantages such as high accuracy, good consistency and celerity as compared to other techniques. This study investigated the effect of laser texturing on the tribological properties of Ti6Al4V in contact with a ceramic ball. The effect of varying the dimple density on friction and wear was studied using a ball-on-flat reciprocating tribometer under lubricated conditions. Results show that friction and wear were reduced for all the textured samples as compared to an untextured sample, with important friction and wear reductions for the samples with the highest dimple densities. For samples with intermediate dimple densities, the friction coefficient stayed low until the dimples wore out from the surface and then increased to a value similar to the friction coefficient of the untextured surface. The dimple wear-out time observed in these specimens was greatly influenced by the dimple density.

Micromachined dimples on carbon electrode for enhancing electrochemical reaction

Coating catalyst particles on carbon gas diffusion electrode (GDE) is a common application for fabricating membrane electrode assembly (MEA) where the catalyst layer is formed in between the microporous carbon electrode and the polymer electrolyte membrane (PEM). In this study, we created micro-dimples on the surface of the GDE using laser-plasma to increase the open area of original pores, which leads to substantial enhancement in the electrochemical reaction. Scanning electron microscopy analysis of the surface processed carbon electrode shows that the electrode surface machining increases natural pore size up to ∼ 15 times and facilitates to accommodate most of the geometrically non-uniform Molybdenum disulfide catalyst particles. Electrochemical measurement results indicate that the micro-machined electrode with a dimple density of 0.16 cm2dimple/cm2electrode enhances the electrochemical reaction by 21%. Our PEM electrolysis tests also reveal that the current density can be increased up to 64% using MEA manufactured through the hot-pressing process along with the micro-machining technique. This study opens up the route for improving the performance of MEA in various potential applications such as water electrolysis and fuel cells.

An Experimental Study of Micro-Dimpled Texture in Friction Control under Dry and Lubricated Conditions

Friction control is a vital technology for reaching sustainable development goals, and surface texturing is one of the most effective and efficient techniques for friction reduction. This study investigated the performance of a micro-dimpled texture under varying texture densities and experimental conditions. Reciprocating sliding tests were performed to evaluate the effects of the micro-dimpled texture on friction reduction under different normal loads and lubrication conditions. The results suggested that a micro-dimpled texture could reduce the coefficient of friction (CoF) under dry and lubricated conditions, and high dimple density results in a lower CoF. The dominant mechanism of the micro-dimpled texture’s effect on friction reduction was discussed, and surface observation and simulation suggested that a micro-dimpled texture could reduce the contact area at the friction interface, thereby reducing CoF.

Improving Tribological Behavior of ICE’s Cylinder Surface by Laser Surface Texturing (LST)

This study investigates the effect of laser surface texturing (LST) on the friction and wear behavior of grey cast iron (GCI) of internal combustion engine (ICE) cylinder in lubricated conditions. The dimples having diameter of about (58-60) μm and depth of about 10 μm were created on the surface with various dimple densities ranging from 5 to 50%. A ball-on-disc friction tests were performed for all the specimens under 5W30 and 15W50 oils with different viscosities. The tests were carried out at a load of 5N and speed of 5cm/s. The coefficient of friction of the dimpled specimen was reduced significantly by approximately 32% as compared to the polished speciemen. Specimen with 15% dimples exhibits the lowest coefficient of friction of all the dimpled specimens in both low and high viscous oils. The high viscous oil found to be more efficient regarding coefficeint of friction compared to the low viscous oil. The degree of wear of the specimens was analyzed on the basis of wear scar developed on the counter surface as it supplements the wear during the friction tests. The resistance to wear of the sliding specimens was found to be increased in high viscous oil compared to that in low viscous oil.

Optimization of Tribological Characteristics of a Combined Pattern Textured Surface Using Taguchi Design

Laser surface modification of contact surfaces, which can generate arranged channels or micro-dimples, is an important factor that dictates several improvements in tribological performance, including lubrication lifetime, wear resistance, load-carrying capacity, and reduced friction coefficients. Patterns of combined textures having circular and sine wave shapes were created using an Nd:YAG laser. Pin-on-disk tribometer tests under oil lubrication were conducted over a range of test conditions. Moreover, an L9(34) orthogonal array experimental design was employed to determine the influence of parameters such as the area density, applied load, and sliding speed on the tribological characteristics. The results indicate that the combined textures effectively reduced the coefficient of friction compared with that of a polished surface. Among the investigated patterns, the texture with a 100-μm-groove-width pattern exhibited the lowest friction coefficient and wear rate. Analysis of variance suggested that the dimple density is the most important parameter influencing the friction coefficient at a confidence level of 95%.

Mechanical Response of Stainless Steels at Low Strain Rate

The deformation behavior of the selected stainless steels (UNS S31603, UNS S32205, UNS S32550 and UNS S32760) was evaluated at strain rates ranging between 10-2 and 10-7 s-1. Microstructural evaluation was conducted using optical and scanning electron microscopy, electron backscattered diffraction and x-ray diffraction. The total strain at fracture remained approximately constant as the strain rate increased from 10-7 to 10-3 s-1. Further increase in strain rate above 10-3 s-1 resulted in relatively lower strain. The strain rate sensitivity of each steel decreased with increasing strain. Also, the strain hardening rate and exponent of the alloys increased with decreasing strain rate. Post-deformation microstructure evaluation on UNS S31603 specimens showed slip, twinning and martensitic transformation, while the duplex alloys exhibited slip, twinning with less noticeable martensitic transformation. Fractographic examination after tensile test indicates an increase in dimple diameter with strain rate and subsequent decrease in dimple density.

Laser surface texturing and tribological behaviour under solid lubrication on titanium and titanium alloy surfaces

Surface textures with dimple and groove were fabricated on titanium and its alloy surfaces by a Nd:YAG pulsed laser. The effect of laser parameters on the two types of texture morphologies was investigated. The unidirectional sliding tests were carried out for the Ti6Al4V surface under solid lubrication conditions and the effect of dimple density from 5% to 30% was examined. The results show that laser parameters have a significant effect on the size and depth of texture for titanium and its alloy surfaces. Comparing with the untextured surface, the longevity of solid lubricant film on the textured surface is prolonged by 90% at least. The beneficial effect of dimples is to act as solid lubricant reservoirs under solid lubrication.

Promising dimple technologies of vortex heat and mass transfer enhancement in energy and microelectronics

Turbulent separated air flow and heat transfer at the starting hydrodynamic length of the longitudinal section of the dimpled narrow channel (1 in height, 4 in width, and 84 in length) with inclined one-row oval-trench dimples (OTD) are calculated. Packages of 20 and 31 dimples are compared. Each dimple has a length of 4.5, a width of 1, a depth of 0.2, an edge rounding radius of 0.2, and an inclination angle of 45°. Re=6000 and is determined in terms of the flow velocity at the channel entrance. The effect of abnormal separated flow intensification and heat transfer enhancement in OTDs due to the dimple density is confirmed. (fx/fxpl)min=−4 and (Nu/Nupl)max=7.2. Relative heat transfer grows by a factor of 1.9 and relative hydraulic losses – by a factor of 1.6.

ABRASIVE WEAR BEHAVIOR OF NITRIDED TEXTURED TITANIUM

In order to improve the anti-friction and anti-wear performances of titanium and expand its application in aerospace and aircraft area, a commercially pure titanium grade 2 (TA2) was chosen and treated by compositing surface treatment. Dimple textures were prepared on the titanium surface by laser surface texturing (LST), and then the textured titanium was treated by ion nitriding. Tribological behaviors of the textured titanium and nitrided textured titanium were investigated under abrasive wear on a tribo-tester. The result shows that the anti-friction and anti-wear properties of textured titanium can be greatly improved by 47.1% and 79.3% after nitriding treatment, respectively. In addition, the dimple density has a significant effect on anti-friction and anti-wear behaviors.

Corrosion Wear Performance of Pure Titanium Laser Texturing Surface by Nitrogen Ion Implantation

The poor tribological performances of titanium have significantly limited its applications in the field of artificial joints. In order to solve problems regarding the wear and corrosion of artificial joints in the body, we fabricated the composite materials utilizing the combination of laser surface texturing and nitrogen ion implantation technology, and investigated the effect of laser surface texturing, nitrogen ion implantation, and different dimple area densities on tribological performance. The results show that the textured surface could reduce the friction coefficient and improve the wear resistance, and the optimum dimple density was found to be 25%. After N ion implantation, the wear resistance of the textured sample was further improved, due to the formation of the nitride layer. Moreover, as shown by the electrochemical test results, the corrosion resistance was enhanced significantly. The friction coefficient decreased the most, and the wear resistance increased by 405% with the lowest wear rate of 0.37 × 10−3 mm3/N·m. However, the specimen with a dimple density of 60% had the worst wear resistance. The results of the study provide a basis for the development and application of artificial joint materials.

Numerical Study of the Influence of Geometric Features of Dimple Texture on Hydrodynamic Pressure Generation

Controlling friction and wear are essential for reducing energy loss and lengthening the life span of friction pairs in sliding contacts. Surface texturing is an effective and efficient way to reduce friction and wear, especially under lubricated conditions. Dimple texture on friction pair surfaces has been verified to enhance the lubrication condition and shift the lubrication regime by generating additional hydrodynamic pressure. However, the geometric features and distribution of the microstructures considerably influence the production of additional pressure. Choosing and designing dimple texture with suitable geometric features is necessary and important for texturing applications. In this study, computational fluid dynamics (CFD) are used to investigate the effect of the geometric features of dimple textures on pressure build-up. The influence of dimple shapes, dimple depths, minimum film thickness, dimple densities, and dimple surface angles are presented and discussed. Notably, the influence of the dimple surface angle is introduced and presented.

Friction characterization when combining laser surface texturing and graphite-based lubricants

The present work analyzes the friction capabilities at room temperature of three types of lubricants (denoted as A, B and C) with a graphite concentration of 5%. To do that, the standard pin-on disc test is deployed to study the variation of the friction coefficient when combining these graphite-based lubricants with surfaces made by grinding and different laser surface textures. These lubricants are characterized by measuring the percent of the chemical elements, the average size of the graphite particles and the kinematic viscosity. The experiments show that the lubricant B combined with a higher density of LST presents the lowest friction coefficient of about 0.24. Additionally, assuming a hydrodynamic regime for the textured surfaces, the fluid dynamics simulations carried out as part of the study showed, in agreement with the experimental measurements, the lowest friction coefficient value for a textured surface with the highest dimple density. This seems to be associated to the combined effect of an increase of the hydrodynamic pressure with a weak vortex formation within the dimples, due to the low distortion of the streamlines which, ultimately, attenuates the friction coefficient between the surfaces.

Experimental investigation of flow and heat transfer in a channel with dimpled plate

This study presents the experimental investigation on the effect of dimpled arrangements on flow and heat transfer characteristics. Three plate surfaces were prepared (smooth, evenly distributed spherical dimples and unevenly distributed spherical dimples) and were placed successively in a channel. The unevenly distributed dimpled plate had the same dimple density with the evenly distributed dimpled plate but had varying transverse pitches to concentrate the dimples at mid-plate in flow direction. Data obtained from the experiment were analysed to determine the performance of each dimpled plate channel. It was observed that the average Nusselt number due to the heat interaction with the air-flow increases with the Reynolds number. The evenly and unevenly dimple plate channels had respectively, 75.7% and 91.8% increase in Nusselt number over the smooth channel. The flow friction factors of the evenly and unevenly dimple plate channels were merely more than that of smooth plate channel by 0.59% and 0.67%, respectively. Thus, the unevenly dimple plate channel had the highest overall thermal-hydraulic performance, followed by the evenly dimple plate channel.

Effect of microstructural evolution and fractographic properties on hole expandability of hot rolling complex phase steel with different coiling temperature

Hole expansion is one of the most important properties describing the formability of steel sheets, especially those used in automotive industry. In this paper, the effect of microstructure evolution and fractographic properties on hole expansion ratio has been studied and reported. The hole expansion test has been carried out for complex phase steel with different coiling temperature. Hole expansion ratio is strongly influenced by the microstructure of the steel. With the change of different mechanical properties, regular changes have also taken place in hole expansion rate. Besides, the fractographic factors, such as crack characteristic in the cross section of the vicinity of the hole expansion surface, void formation mechanisms, average dimple size, dimple density affect the hole expansion ratio. Among them the steel coiled at 520 °C has the best hole expansion ratio.

Evaluation of aluminium/alumina/titanium composites produced by continual annealing and roll-bonding process

ABSTRACTAnodising, continual annealing and roll-bonding (CAR) processes were employed to fabricate aluminium/alumina/titanium composites with different amounts of alumina. After each rolling cycle, annealing treatment was performed at 300°C for 1 h. After six cycles of CAR process, the Al/0.22 vol.-% Al2O3/13vol.-%Ti composite showed uniform alumina and titanium particles distribution. The tensile strength of the composites first decreased after the first cycle, then increased up to 200 MPa after the sixth cycle. Conversely, by increasing the alumina content (Al/0.43vol.-%Al2O3/13vol.-%Ti composite), microhardness and tensile strength immediately increased. SEM micrographs demonstrated that by increasing the number of cycles, the dimples density increased from 0.007 to 0.028 µm−2 for the first and the last cycles, respectively.

Friction characteristics of mechanically microtextured metal surface in dry sliding

Improving tribological properties by applying various textured surfaces have been reported by many researchers. In these studies, most utilized surfaces have been textured with lasers. However, such a texturing method contains several problems including thermal effects, formation of pileups, possible dimple shapes, and lower efficiency than mechanical methods. The traditional mechanical texturing methods also have some problems. In this study, an alternative method is developed by using vibration-assisted microcutting with a cube-corner diamond indenter which is vibrated in the depth direction with an amplitude of tens of microns or less. In this technique, the pileups generated around all the dimples in texturing can be removed by supplementally conducting a normal microcutting on the machine. By the developed method, textured surfaces of brass plate (c2680), in which micro dimples transferred tip shape of the indenter were regularly arranged at a period of tens of microns, were successfully manufactured. In order to verify their potentials, dry sliding tests using steel balls (534A99) were conducted on the surfaces textured by both the proposed and the normal cutting methods for comparison. Subsequently, the influence of the removing pileups and the dimple density were also examined through the dry sliding tests. A series of the sliding test results were analyzed through the monitored friction coefficients, wear rates, and observations of worn surfaces and generated debris. As a result, textured surface with a relatively high area density, such as 40% where pileups were completely removed showed the lowest friction coefficient and wear due to the trapping and reducing effects of the generated wear debris and real contact area by the periodically arranged micro dimples. Therefore, the proposed texturing method can be a new candidate to produce the surface textures for better tribological properties. Also, higher texturing efficiency could be realized by just applying higher sample rotation speed and/or vibration frequency.