Thermo-mechanical modelling of the Friction Stir Spot Welding process: Effect of the friction models on the heat generation mechanisms

Friction Stir Spot Welding involves complex physical phenomena, which are very difficult to probe experimentally. In this regard, the numerical simulation may play a key role to gain insight into this complex thermo-mechanical process. It is often used to mimic specific experimental conditions to forecast outputs that may be substantial to analyse and elucidate the mechanisms behind the Friction Stir Spot Welding process. This welding technique uses frictional heat generated by a rotating tool to join materials. The heat generation mechanisms are governed by a combination of sliding and sticking contact conditions. In the numerical simulation, these contact conditions are thoroughly dependent on the used friction model. Hence, a successful prediction of the process relies on the appropriate selection of the contact model and parameters. This work aims to identify the pros and cons of different friction models in modelling combined sliding-sticking conditions. A three-dimensional coupled thermo-mechanical FE model, based on a Coupled Eulerian-Lagrangian formulation, was developed. Different friction models are adopted to simulate the Friction Stir Spot Welding of the AA6082-T6 aluminium alloy. For these friction models, the temperature evolution, the heat generation, and the plastic deformation were analysed and compared with experimental results. It was realized that numerical analysis of Friction Stir Spot Welding can be effective and reliable as long as the interfacial friction characteristics are properly modelled. This approach may be used to guide the contact modelling strategy for the simulation of the Friction Stir Spot Welding process and its derivatives.

Study of wear losses and frictional heat dissipation during dry sliding wear of ilmenite reinforced Al-alloy composite

In this investigation, ilmenite mineral reinforced Al–Si alloy matrix composite (AMC) has been developed. The wear behavior of the developed composites has been studied for their end application as break drum material to replace cast iron used in automobile industries. Ilmenite is one of the major sea beach mineral. AMC has been prepared through a low-cost stir-casting method in which 1 wt.% graphite (Gr)/tin (Sn) as a solid lubricant has been added during the fabrication of composites itself. The optical micrographs of AMC revealed uniform distribution of ilmenite particles throughout the matrix. The wear rate of the base LM30 alloy containing 17% Si and the developed composites has been studied at different normal loads at a constant velocity of 1.6 m.s−1. Optimized data revealed a significant wear rate reduction due to solid lubrication provided by Gr/Sn (∼32%). The wear rate of composites has been compared with traditional cast iron used in brake drums under similar experimental conditions. Composites exhibit nearly identical wear behavior throughout the test. The microstructural study of wear track and debris revealed that Sn and Gr used as solid lubricants played a vital role in reducing the wear loss of the prepared composites. A theoretical study of frictional heat generated during dry sliding and its dissipation has been done to establish the operative wear mechanism in the composites.

Genesis of the Xifeng Low-Temperature Geothermal Field, Guizhou, SW China: Constrains From Geology, Element Geochemistry, and D-O Isotopes

The Xifeng geothermal field is located in the Yangtze Craton, SW China, and is one of the most representative low-temperature geothermal fields in China. Widespread thermal anomalies, hot springs, and geothermal wells have been reported by previous studies. However, the nature and forming mechanisms of the field remain poorly understood. Element geochemical (ions, rare earth elements) and stable isotopic (D, O) composition of hot springs, geothermal fluids, rivers, and cold springs from different locations of the Xifeng geothermal field were analyzed in this study. The ions studies revealed that most samples featured the Ca-Mg-HCO3 type, except Xifeng hot springs, and which were characterized by the Ca-Mg-HCO3-SO4 type. Based on quartz geothermometers, the estimated reservoir temperature was 77°C. The results of stable isotopes (D, O) manifest that the Xifeng geothermal system was recharged by meteoric water at an elevation of 1,583 m from SW to NE. The research of rare earth elements (REE) revealed that their accumulation characteristics and obvious positive Eu anomaly were inherited from host feldspar-bearing reservoir dolomites through water-rock interactions. Combined with these observations, geological setting, and previous studies, it was concluded that the formation of the Xifeng geothermal field resulted from recharge, deep circulation, and secondary rising of the meteoric water along the faults. First, meteoric water infiltrated to depth through faults and crack zones. Second, the deep-infiltrated water was heated by radioactive heat, deep heat, and tectonic frictional heat. Finally, as the warmed-up waters underwent considerable deep circulation in the reservoir, it rose again along the main faults, and mixed with groundwater near the surface. Taken together, we suggest that the Xifeng geothermal system should be assigned as a faults-controlling, and deeply circulating meteoric water of low-temperature category.

Wear Based Lifetime Estimation of a Clutch Facing using Coupled Field Analysis

Repetitive use of the clutch, over a period of time, causes the friction material at the contact surfaces (clutch facing and flywheel/pressure plate) to wear, thus deteriorating its performance and usable life. The working life of a rigid clutch is the limiting factor when it comes to extracting maximum performance from a dual mass flywheel system, which is used in a lot of modern vehicles nowadays to lower fuel consumption and improve ride quality. In this study, we investigate the influence of different groove patterns on wear in rigid clutch facings and estimate their life using a comprehensive finite element model. The wear is calculated and analysed for five different groove patterns across two different inorganic materials, namely FTL180 and TF1600-MC2, using Archard’s Adhesive Wear Model. Coupled multi-physics elements are employed in the analysis to capture the effect of frictional heat generation on wear. We found that the Waffle pattern offered a decrease of 10.4% in volumetric wear loss, a 5.78% decrease in maximum wear thickness and an increase of 11.51% in the average working life is used in city like conditions with frequent engagements. This work sheds light on the impact of groove patterns on clutch facing wear and opens a new path for the design and development of more resilient rigid clutches.

Investigation on Friction Stir Weldability Characteristics of AA7075-T651 and AA6061-T6 Based Nanocomposites

Friction stir welding (FSW) is an emerging solid-state process and alternative to fusion welding, wherein frictional heat is generated between a nonconsumable rotating steel tool and the work substrate. The present study focuses on the influence of the operating attributes like tool pin contact geometry, welding speed, and tool rotational speed on dissimilar aluminum matrix nanocomposites. AA6061-T6 and AA7075-T651 aluminum alloy plates were joined via double-pass FSW with the inclusion of 5 vol. % of nanoscale h-BN particles. Welding was performed with four rotational speeds (600, 800, 900, and 1000 rpm), three traversing speeds (30, 40, and 60 mm/min), and three distinct tool pin geometry (cylindrical, threaded cylindrical, and square), respectively. Besides, unreinforced and reinforced weldments were analyzed for mechanical properties like tensile strength and microhardness. Microstructural characterization was also carried out using FESEM and XRD techniques. The findings concluded that the reinforced samples welded using a cylindrical tool and double-pass strategy showcased homogenous distribution of nanoparticles with grain refinement, thereby exhibiting improved strength and hardness.

Effect of Pin Shape on Thermal History of Aluminum-Steel Friction Stir Welded Joint: Computational Fluid Dynamic Modeling and Validation

This article studied the effects of pin angle on heat generation and temperature distribution during friction stir welding (FSW) of AA1100 aluminum alloy and St-14 low carbon steel. A validated computational fluid dynamics (CFD) model was implemented to simulate the FSW process. Scanning electron microscopy (SEM) was employed in order to investigate internal materials’ flow. Simulation results revealed that the mechanical work on the joint line increased with the pin angle and larger stir zone forms. The simulation results show that in the angled pin tool, more than 26% of the total heat is produced by the pin. Meanwhile, in other cases, the total heat produced by the pin was near 15% of the total generated heat. The thermo-mechanical cycle in the steel zone increased, and consequently, mechanical interlock between base metals increased. The simulation output demonstrated that the frictional heat generation with a tool without a pin angle is higher than an angled pin. The calculation result also shows that the maximum heat was generated on the steel side.

Thermal Analysis of Herringbone Gears Based on Thermal Elastohydrodynamic Lubrication Considering Surface Roughness

To predict the temperature distribution of the tooth surface of a herringbone gear pair, a numerical method for the determination of frictional heat generation was proposed by establishing a thermal elastohydrodynamic lubrication (TEHL) model in the meshing zone taking surface roughness into account. According to the real micro topography of the tooth surface measured by a non-contact optical system and loaded tooth contact analysis, the friction coefficient was obtained by a TEHL analysis and then the heat generation in the contact zone was determined. With the combination of heat generation and heat dissipation analysis, the single tooth model of the herringbone gear pair due to the finite element method (FEM) was proposed and the steady-state temperature distribution of the tooth surfaces was predicted by FEM simulations. The simulation and the experimental results demonstrated good agreement, which verified the feasibility of the present numerical method.

Revisiting mechanics of ice–skate friction : from experiments at a skating rink to a unified hypothesis

The mechanics underlying ice–skate friction remain uncertain despite over a century of study. In the 1930s, the theory of self-lubrication from frictional heat supplanted an earlier hypothesis that pressure melting governed skate friction. More recently, researchers have suggested that a layer of abraded wear particles or the presence of quasi-liquid molecular layers on the surface of ice could account for its slipperiness. Here, we assess the dominant hypotheses proposed to govern ice– skate friction and describe experiments conducted in an indoor skating rink aimed to provide observations to test these hypotheses. Our results indicate that the brittle failure of ice under rapid compression plays a strong role. Our observations did not confirm the presence of full contact water films and are more consistent with the presence of lubricating ice-rich slurries at discontinuous high-pressure zones (HPZs). The presence of ice-rich slurries supporting skates through HPZs merges pressure-melting, abrasion and lubricating films as a unified hypothesis for why skates are so slippery across broad ranges of speeds, temperatures and normal loads. We suggest tribometer experiments to overcome the difficulties of investigating these processes during actual skating trials.

Evidence that abrasion can govern snow kinetic friction

The long-accepted theory to explain why snow is slippery postulates self-lubrication: frictional heat from sliding melts and thereby lubricates the contacting snow grains. We recently published micro-scale interface observations that contradicted this explanation: contacting snow grains abraded and did not melt under a polyethylene slider, despite low friction values. Here we provide additional observational and theoretical evidence that abrasion can govern snow kinetic friction. We obtained coordinated infrared, visible-light and scanning-electron micrographs that confirm that the evolving shapes observed during our tribometer tests are contacting snow grains polished by abrasion, and that the wear particles can sinter together and fill the adjacent pore spaces. Furthermore, dry-contact abrasive wear reasonably predicts the evolution of snow-slider contact area and sliding-heat-source theory confirms that contact temperatures would not reach 0°C during our tribometer tests. Importantly, published measurements of interface temperatures also indicate that melting did not occur during field tests on sleds and skis. Although prevailing theory anticipates a transition from dry to lubricated contact along a slider, we suggest that dry-contact abrasion and heat flow can prevent this transition from occurring for snow-friction scenarios of practical interest.

New Catalyst-Free Polycrystalline Diamond with Industry-Record Wear Resistance

PDC drill bits are the primary drilling tools for oil and gas in most of formations. In a PDC drill bit, PDC cutters are key cutting components to engage with these formations. However, there is often a big challenge for today's PDC drill bits when drilling very hard and abrasive formation. The main weakness in the PDC cutter is due to the unavoidable use of metallic catalyst which is used to bond the diamond grains in the PDC cutters. The thermal expansion of the metallic catalysts resulting from high frictional heat at the cutter/rock interface during drilling operation is higher than that of diamond grains, causing the thermal stress between the metallic catalyst and diamond grain which can break the PDC cutter. Therefore, development of catalyst-free PDC cutters would be a game-changing technology for drill bit by delivering significant increase in performance, durability, and drilling economics. In this study, an innovative ultra-high pressure and ultra-high temperature technology was developed with ultra-high pressures up to 35 GPa, much higher than current PDC cutter technology. We report a new type of catalyst-free PDC cutting material, synthesized under one of conditions using ultra-high pressure of 16 GPa. The new material breaks all single-crystal-diamond indenters in Vickers hardness testing which sets a new world record as the hardest diamond material as of today. Also, the material shows the highest thermal stability in the family of diamonds in air at 1,200°C, which is about 600 °C higher than current PDC cutters. As a consequence of these superior properties, this new material exhibited industry-recorded wear resistance, which is four times of that of current PDC cutters. All of these achievements demonstrated a breakthrough in PDC cutter technology development and presented a feasibility for the goal of "One-Run-To-TD" game-changing drilling technology.