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Linking process and structure in the friction stir scribe joining of dissimilar materials: A computational approach with experimental support
We present that friction stir welding (FSW) is a popular technique to join dissimilar materials in numerous applications. The solid state nature of the process enables joining materials with strikingly different physical properties. For welds in lap configuration, an enhancement to this technology is made by introducing a short, hard insert, referred to as a cutting-scribe, at the bottom of the tool pin. The cutting-scribe induces deformation in the bottom plate which leads to the formation of mechanical interlocks or hook like structures at the interface of two materials. A thermo-mechanical computational model employing a coupled Eulerian-Lagrangian approach is developed to quantitatively capture the morphology of these interlocks during the FSW process. Simulations using this model are validated by experimental observations. In conclusion, the identified interface morphology coupled with the predicted temperature field from this process–structure model can be used to estimate the post-weld microstructure and joint strength.
Frictional properties of Opalinus Clay: influence of humidity, normal stress and grain size on frictional stability
SUMMARY The Opalinus Clay (OPA) is a clay-rich formation considered as a potential host rock for radioactive waste repositories and as a caprock for carbon storage in Switzerland. Its very low permeability (10−19 to 10−21 m2) makes it a potential sealing horizon, however the presence of faults that may be activated during the lifetime of a repository project can compromise the long-term hydrological confinement, and lead to mechanical instability. Here, we have performed laboratory experiments to test the effect of relative humidity (RH), grain size (g.s.) and normal stress on rate-and-state frictional properties and stability of fault laboratory analogues corresponding to powders of OPA shaly facies. The sifted host rock powders at different grain size fractions (<63 μm and 63 < g.s. < 125 μm), at room (∼25 per cent) and 100 per cent humidity, were slid in double-direct shear configuration, under different normal stresses (5–70 MPa). We observe that peak friction, μpeak and steady-state friction, μss, depend on water vapour content and applied normal stress. Increasing relative humidity from ∼25 per cent RH (room humidity) to 100 per cent RH causes a decrease of frictional coefficient from 0.41 to 0.35. The analysis of velocity-steps in the light of rate-and-state friction framework shows that the stability parameter (a–b) is always positive (velocity-strengthening), and it increases with increasing sliding velocity and humidity. The dependence of (a–b) on slip rate is lost as normal stress increases, for each humidity condition. By monitoring the variations of the layer thickness during the velocity steps, we observe that dilation (Δh) is directly proportional to the sliding velocity, decreases with normal stress and is unaffected by humidity. Microstructural analysis shows that most of the deformation is accommodated within B-shear zones, and the increase of normal stress (σn) promotes the transition from strain localization and grain size reduction to distributed deformation on a well-developed phyllosilicate network. These results suggest that: (1) the progressive loss of velocity dependence of frictional stability parameter (a–b) at σn > 35 MPa is dictated by a transition from localized to distributed deformation and (2) water vapour content does not affect the deformation mechanisms and dilation, whereas it decreases steady-state friction (μss), and enhances fault stability.
Modeling and data analysis of the NASA-WSTF frictional heating apparatus - Effects of test parameters on friction coefficient
A theoretical model is being developed jointly by the NASA White Sands Test Facility (WSTF) and the University of California at Santa Barbara (UCSB) to analyze data generated from the WSTF frictional heating test facility. Analyses of the data generated in the first seconds of the frictional heating test are shown to be effective in determining the friction coefficient between the rubbing interfaces. Different friction coefficients for carobn steel and Monel K-500 are observed. The initial condition of the surface is shown to affect only the initial value of the friction coefficient but to have no significant influence on the average steady-state friction coefficient. Rotational speed and the formation of oxide film on the rotating surfaces are shown to have a significant effect on the friction coefficient.
Evaluation of two transport aircraft and several ground test vehicle friction measurements obtained for various runway surface types and conditions. A summary of test results from joint FAA/NASA Runway Friction Program
Tests with specially instrumented NASA Boeing 737 and 727 aircraft together with several different ground friction measuring devices were conducted for a variety of runway surface types and conditions. These tests are part of joint FAA/NASA Aircraft/Ground Vehicle Runway Friction Program aimed at obtaining a better understanding of aircraft ground handling performance under adverse weather conditions and defining relationships between aircraft and ground vehicle tire friction measurements. Aircraft braking performance on dry, wet, snow and ice-covered runway conditions is discussed as well as ground vehicle friction data obtained under similar runway conditions. For a given contaminated runway surface condition, the correlation between ground vehicles and aircraft friction data is identified. The influence of major test parameters on friction measurements such as speed, test tire characteristics, type and amount of surface contaminant, and ambient temperature are discussed. The effect of surface type on wet friction levels is also evaluated from comparative data collected on grooved and ungrooved concrete and asphalt surfaces.
Friction-based recycling: an evaluation of friction extrusion for fabricating Ti-6Al-4 V wire fabricated from machining chip feedstock
Abstract Titanium and its alloys are used in aviation and automobile industries due to their remarkable strength to weight ratio, but machining loss commonly is high with ~ 80 wt% of the material being converted to scrap. Recycling post-consumer Ti scrap directly into solid bulk products is a potential solution for repurposing valuable material. Further, eliminating fresh Ti sponge during recycling might lead to lower energy and greenhouse gas emissions. In this study, a solid-phase process known as friction extrusion was utilized to recycle Ti-6Al-4 V machining chips into solid wires which could be used as feedstock in additive manufacturing. The friction consolidation technique was first used to convert chips with varying degrees of oxygen content into solid billets for its use as feedstock material in subsequent friction extrusion. The extrudates were fabricated above the beta transition temperature, which was achieved by selecting the rotation rate and feed rate, to process the billets near 1000 °C using a tungsten-lanthana extrusion die. This work presents the first occurrence of friction extruded titanium alloy wires. The effect of friction extrusion on microstructural features, tensile properties, and texture are reported. Overall, the friction extrusion method is capable of directly recycling Ti-6Al-4 V scrap into extruded wire.
Joining thick section aluminum to steel with suppressed FeAl intermetallic formation via friction stir dovetailing
A new solid-phase technique called friction stir dovetailing (FSD) has been developed for joining thick section aluminum to steel. In FSD, mechanical interlocks are formed at the aluminum-steel interface and are reinforced by metallurgical bonds where intermetallic growth has been uniquely suppressed. Lap shear testing shows superior strength and extension at failure compared to popular friction stir approaches where metallurgical bonding is the only joining mechanism. High resolution microscopy revealed the presence of a 40-70 nm interlayer having a composition of 76.4 at% Al, 18.4 at% Fe, and 5.2 at% Si, suggestive of limited FeAl3 intermetallic formation.
Acid Treatment of Diamond-Like Carbon Surfaces for Enhanced Adsorption of Friction Modifiers and Friction Performance
Diamond-like carbon (DLC) is a class of amorphous carbon materials used as wear-resistant coatings in tribo-components. The chemical inertness of DLC surfaces, while important in many applications, makes them incompatible with additives in commercial lubricants. Specifically, DLC surfaces do not permit strong adsorption of friction modifier molecules. This study aims at improving the adsorption of these friction modifier molecules on DLC surfaces through an acid treatment. Water contact angle measurements show that such a treatment results in improved hydrophilicity of the DLC. XPS analysis demonstrates 50% increase in the uptake of ArmeenT, an organic friction modifier, on DLC after the acid treatment. This increased ArmeenT adsorption is accompanied by marked decrease in friction in micro-scale friction experiments.
Ultralow Boundary Lubrication Friction by Three-Way Synergistic Interactions among Ionic Liquid, Friction Modifier, and Dispersant
Interactions among antiwear additives (AWs), friction modifiers (FMs), and dispersant in a lubricating oil are critical for tribological performance. This study investigates compatibilities of three oil-soluble ionic liquids (ILs, candidate AWs) with an FM, molybdenum dithiocarbamate (MoDTC), and a dispersant, polyisobutene succinimide (PIBSI) under boundary lubrication. Either synergistic or antagonistic effects were observed depending on the IL’s chemistry. Adding an aprotic phosphonium–alkylphosphate or phosphonium–alkylphosphinate IL into the oil containing MoDTC and PIBSI had detrimental impact on the friction and wear behavior. PIBSI was found to preferably interact/react with the aprotic IL to lose its ability of suspending MoDTC and to partially consume or even deplete the IL. In contrast, a protic ammonium–alkylphosphate IL seemed to be able to coexist with PIBSI and work synergistically with MoDTC, yielding a sustainable, ultralow boundary friction. A three-stage tribochemical process is proposed to explain how this IL + MoDTC pair interacts with the contact surface to form a chemically reacted, wear-protective tribofilm supporting a physically adsorbed, friction-reducing film on top. We report this study provides fundamental insights of the compatibilities among three common lubricant components, antiwear, friction modifier, and dispersant, which can be used to guide future lubricant development.
Friction- and Wear-Reducing Properties of Multifunctional Small Molecules
Nitrogen- and oxygen-containing compounds were designed and synthesized, and their tribological performance as multifunctional base oils (MFBOs) for internal combustion engine applications was evaluated. Two of the compounds displayed good rheology without the addition of polymeric viscosity modifiers, with a high viscosity index (VI) above 200.Additionally, all three MFBOs had lower coefficients of friction when compared to well established and accepted benchmarks, and demonstrated wear improvement by a factor of 5.5 to 70 compared to either of the benchmarks. Moreover, the compounds were easily synthesized in one step from commercial intermediates or starting materials. This work demonstrates that careful design of a single molecule might provide the functions of base oil, viscosity modifiers (or viscosity index improvers), anti-wear and friction reducing additives.
Friction and wear of TPS fibers: A study of the adhesion and friction of high modulus fibers
The adhesional and frictional forces between filaments in a woven fabric or felt, strongly influenced the processability of the fiber and the mechanical durability of the final product. Even though the contact loads between fibers are low, the area of contact is extremely small giving rise to very high stresses; principally shear stresses. One consequence of these strong adhesional and frictional forces is the resistance of fibers to slide past each other during weaving or when processed into nonwoven mats or felts. Furthermore, the interfiber frictional forces may cause surface damage and thereby reduce the fiber strength. Once formed into fabrics, flexural handling and manipulation of the material again causes individual filaments to rub against each other resulting in modulus, brittle fibers such as those used in thermal protection systems (TPS). The adhesion and friction of organic fibers, notably polyethylene terephthalate (PET) fibers, have been extensively studied, but there has been very little work reported on high modulus inorganic fibers. An extensive study was made of the adhesion and friction of flame drawn silica fibers in order to develop experimental techniques and a scientific basis for data interpretation. Subsequently, these methods were applied to fibers of interest in TPS materials.
A vacuum (10(exp -9) Torr) friction apparatus for determining friction and endurance life of MoSx films
The first part of this paper describes an ultrahigh vacuum friction apparatus (tribometer). The tribometer can be used in a ball-on-disk configuration and is specifically designed to measure the friction and endurance life of solid lubricating films such as MoS(x) in vacuum at a pressure of 10 exp -7 Pa. The sliding mode is typically unidirectional at a constant rotating speed. The second part of this paper presents some representative friction and endurance life data for magnetron sputtered MoS(x) films (110 nm thick) deposited on sputter-cleaned 440 C stainless-steel disk substrates, which were slid against a 6-mm-diameter 440 C stainless-steel bearing ball. All experiments were conducted with loads of 0.49 to 3.6 N (average Hertzian contact pressure, 0.33 to 0.69 GPa), at a constant rotating speed of 120 rpm (sliding velocity ranging from 31 to 107 mm/s due to the range of wear track radii involved in the experiments), in a vacuum of 7 x 10 exp -7 Pa and at room temperature. The results indicate that there are similarities in friction behavior of MoS(x) films overs their life cycles regardless of load applied. The coefficient of friction (mu) decreases as load W increases according to mu = kW exp -1/3. The endurance life E of MoS(x) films decreases as the load W increases according to E = KW exp -1.4 for the load range. The load- (or contract-pressure-) dependent endurance life allows us to reduce the time for wear experiments and to accelerate endurance life testing of MoS(x) films. For the magnetron-sputtered MoS(x) films deposited on 440 C stainless-steel disks: the specific wear rate normalized to the load and the number of revolutions was 3 x 10 exp -8 mm exp 3/N-revolution; the specific wear rate normalized to the load and the total sliding distance was 8 x 10 exp -7 mm exp 3/N-m; and the nondimensional wear coefficient of was approximately 5 x 10 exp -6. The values are almost independent of load in the range 0.49 to 3.6 N (average Hertzian contact pressures of 0.33 to 0.69 GPa).
Investigation of Interfacial Layer for Friction Stir Scribe Welded Aluminum to Steel Joints
Friction stir scribe (FSS) welding as a recent derivative of friction stir welding has successfully fabricated a linear joint between automotive Al and steel sheets. However, there is a lack of understanding on its bonding mechanism. In this paper, the microstructures and phases at the bonding interface of FSS welded Al to ultra-high strength steel joints were studied using scanning electron microscopy and transmission electron microscopy. Based on the microstructural analysis, it is found that both mechanical and chemical bonding occurred simultaneously, and the energy-dispersive X-ray spectroscopy revealed that a thin intermetallic compound (IMC) layer was formed at the interface. Owing to the varied material flow and heat generation, the IMC layer thickness gradually decreased from the advancing side to the retreating side, and two IMCs (i.e. Fe2Al5 and Fe4Al13) were confirmed in different weld regions using diffraction pattern (or 1D fast Fourier transform pattern). Also, a Fe/Al solid solution was noticed in the IMC layer, and several Al fringes were found in the Fe-rich side in the scribe stir zone.
A kinetic model of friction in strongly coupled strongly magnetized plasmas
Plasmas that are strongly magnetized in the sense that the gyrofrequency exceeds the plasma frequency exhibit novel transport properties that are not well understood. As a representative example, we compute the friction force acting on a massive test charge moving through a strongly coupled and strongly magnetized one-component plasma using a generalized Boltzmann kinetic theory. Recent works studying the weakly coupled regime have shown that strong magnetization leads to a transverse component of the friction force that is perpendicular to both the Lorentz force and velocity of the test charge, in addition to the stopping power component aligned antiparallel to the velocity. Recent molecular dynamics simulations have also shown that strong Coulomb coupling in addition to strong magnetization gives rise to a third component of the friction force in the direction of the Lorentz force. Here, we show that the generalized Boltzmann kinetic theory captures these effects and generally agrees well with the molecular dynamics simulations over a broad range of Coulomb coupling and magnetization strength regimes. The theory is also used to show that the “gyro” component of the friction in the direction of the Lorentz force arises due to asymmetries associated with gyromotion during short-range collisions. Computing the average motion of the test charge through the background plasma, the transverse force is found to strongly influence the trajectory by changing the gyroradius and the gyrofriction force is found to slightly change the gyrofrequency of the test charge resulting in a phase shift.
Friction and friction-generated temperature at a polymer-metal interface
Results of friction and thermal tests of molded polyimide and pyrrone polymers are presented. The coefficient of sliding friction up to surface velocities of 2 m/sec and the coefficient of thermal expansion from 300 to 500 K were measured. An apparatus was constructed to measure simultaneously the coefficient of sliding friction and the friction-generated temperature. Measurements were made at a nominal pressure-velocity product of 0.25 MN/msec and at temperatures between 300 and 500 K.
Aircraft and ground vehicle friction correlation test results obtained under winter runway conditions during joint FAA/NASA Runway Friction Program
Aircraft and ground vehicle friction data collected during the Joint FAA/NASA Runway Friction Program under winter runway conditions are discussed and test results are summarized. The relationship between the different ground vehicle friction measurements obtained on compacted snow- and ice-covered conditions is defined together with the correlation to aircraft tire friction performance under similar runway conditions.
The Effect of Friction on Penetration in Friction Stir Welding
"Friction stir butt welding," as it was originally termed by Wayne Thomas and Christopher Dawes, in the early 1990s, but now commonly called "friction stir welding," has made great progress as a new welding technique. Commercially, friction stir welding (FSW) is finding many applications. Marine Aluminum of Norway is using the process for welding 20-meter aluminum sheets for ferry construction. Japan is using the technique in development work on bullet trains. Sapa of Sweden utilizes FSW in the landing of refrigeration units for cargo ships. In the United States, Boeing is using the technique in the production of Delta rockets. Eclipse Aviation Corporation plans to use FSW to produce a business jet available in August, 2003. Marshall Space Flight Center has been investigating the use of FSW for assembly of the Shuttle's external fuel tank since the late 1990s and hopes to have the process in use by the summer of 2002. In FSW, a cylindrical pin tool of hardened steel, is rotated and plunged into the abutting edges of the parts to be joined. The tool is plunged into the weldment to within about .050 in of the bottom to assure full penetration. As the tool moves along the joint, the tool shoulder helps produce frictional heating, causing the material to plasticize. The metal of the two abutting plates flows from the front of the tool to the back where it cools and coalesces to form a weld in the solid phase.