Frictional Heat Input Research Articles

Microstructure and Micro-texture Development of Variable Frictional Heat Input Friction Stir Welded Joints of Laser Powder Bed Fusion-Made AlSi10Mg Plates

Microstructure and Micro-texture Development of Variable Frictional Heat Input Friction Stir Welded Joints of Laser Powder Bed Fusion-Made AlSi10Mg Plates

Investigation on mechanical properties and corrosion behavior of dissimilar joints of 5083 and 6005A alloy welded by friction stir welding

AbstractIn this paper, the microstructure, mechanical properties, and corrosion behavior of friction stir welded (FSW) joints of the two aluminum alloys 6005A‐5083 were investigated and their correlation was discussed. In contrast to FSW joints of the same aluminum material, this results in a “V” shape curve of hardness distributed nonsymmetrically along the weld. The lowest hardness area occurs at the interface between the heat‐affected zone and the thermo‐mechanically affected zone of 6005A (6‐HAZ and 6‐TMAZ) due to the transformation of the β" phase under the influence of heat input during the stir friction process. This also leads to all tensile specimens fracturing in this area. The corrosion behavior of the FSW joint in acidic solution containing Cl− was determined by an exfoliation corrosion (EXCO) test, an intergranular corrosion (IGC) test, and potentiodynamic polarization measurements, based on the potential application in acid rain environment. The results showed that the corrosion resistance order in the acidic solution is: 6‐HAZ > NZ > 6‐BM > 5‐HAZ > 5‐BM. The 5‐BM has the worst corrosion resistance due to the high corrosion sensitivity of Al3Mg2 in acidic solution. However, good corrosion resistances are shown in NZ and 6‐HAZ, which is related to a relatively homogeneous microstructure in NZ and a dissolution or coarsening of β" phases in 6‐HAZ because of frictional heat input.

Influence of pass number on microstructure, mechanical, and tribological properties of cold-rolled Al1050 during friction stir processing

In this investigation, cold-rolled Al1050 sheets underwent various passes of Friction Stir Processing (FSP), namely one, three, and five. Then, microstructural evolution was assessed by focusing on underlying restoration phenomena, including severe dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX), and their corresponding effects on mechanical and surface properties were evaluated. Findings illustrated that initial cold-rolling on Al1050 and its high stacking fault energy are the primary driving forces for accelerating DRV, contributing to the supremacy of the DRV-induced softening over sub-grain strengthening caused by CDRX. Increasing the FSP pass number resulted in a drop in yield stress (YS), ultimate tensile strength (UTS), and microhardness values of the FSP-treated samples such that the UTS of one, three, and five passes FSPed samples decreased by ∼18 %, 32 %, and 35 %, respectively, compared to the cold-rolled Al1050. Conversely, the %elongation of the FSP-treated samples dramatically increased with increasing FSP pass number, which is evident from stress-strain curves and the fracture surfaces. Furthermore, increasing the FSP pass number increased the mean coefficient of friction (MCOF) and weight loss during the pin-on-disk test. Also, conducting various passes of FSP improved corrosion resistance compared to the base metal. However, increasing the FSP pass number reduced the corrosion resistance due to grain growth such that the one-pass FSPed sample illustrated better corrosion resistance than the five passes FSPed sample. Repeating the FSP led to excessive frictional heat input to the matrix, contributing to grain growth after increasing FSP passes, thereby decreasing mechanical properties and corrosion resistance.

Temperature-dependent friction coefficient and its effect on modeling friction stir welding for aluminum alloys

In friction stir welding (FSW), heat input and material flow characteristics are determined by the contact behavior between the tool and workpiece. Accordingly, calibrating the friction coefficient of the tool–workpiece interface is consequential to understand the joining process fully and achieve reasonable modeling. In the present study, the variation of friction coefficient with temperature between a steel tool and three aluminum alloys are obtained by experimental tests. As the temperature increases, the friction coefficient first slightly decreases, rapidly increases, and finally rapidly decreases until it approaches zero. The variation in friction coefficient during the three phases is further analyzed and explained using plowing and adhesive friction models based on examining the microscopic wear morphology and mechanism. A butt FSW test for AA5A06-H plates is also implemented. Subsequently, a fully coupled thermo–mechanical numerical model in which different friction coefficient schemes are used is developed based on the coupled Eulerian–Lagrangian method for simulating the FSW process. The results show that the numerical model considering the temperature-dependent friction coefficient can well reproduce the temperature field and material flow in the welding process.

Deformation-Driven Processing of CNTs/PEEK Composites towards Wear and Tribology Applications

Despite wide applications in mechanical transmission components, sparked with extraordinary wear resistance, polymeric composites face the challenges of reinforcement agglomeration. In this work, deformation-driven processing was proposed to prepare carbon nanotube (CNTs)-reinforced poly-ether-ether-ketone (PEEK) matrix composites with enhancement in wear resistance. Severe plastic deformation contributed to the homogeneous dispersion of the reinforcements without undesirable agglomeration. Low frictional heat input ensured the structural integrity of CNTs. The coefficient of friction and wear rate of 3.0 wt.% CNTs/PEEK were, respectively, 7.32% and 6.71% lower than those of pure PEEK. This strategy provides a high-efficiency approach to preparing high wear-resistance polymeric composites, attributed to its self-heating, low-cost, and high-performance characteristics.

Microstructure and tensile behavior of a Bobbin friction stir welded magnesium alloy

In bobbin friction stir welding (BFSW), frictional heat input is simultaneously applied to the top and bottom surface of a workpiece by the rubbing action of the tool shoulders. Consequently, a relatively homogeneous microstructure is obtained in BFSW welds compared to the conventional FSW. In the current study, we performed BFSW bead-on-plate experiments on Mg alloy AZ31B with the goal to study evolution of microstructure and resulting tensile behavior of the welded material. We used three different combinations of intense process parameters which were intentionally chosen to be outside of the norm for BFSW i.e., tool rotational speeds >1500 rpm and constant translational speed of 10 mm/min. The BFS weldments showed the classic heat affected zone (HAZ), a small but discernible thermomechanical affected zone (TMAZ) and a large, near rectangular stir zone (SZ). The SZ consisted of a bimodal microstructure similar to the base material but some degree of grain refinement was also observed, which was more prominent on the retreating side of SZ. BFS welded specimen showed maximum of 67% and 48% joint efficiency (ratio of weld UTS to base material UTS) at room temperature and 200 °C, respectively. Fracture of specimens occurred in TMAZ/SZ interface and SZ, in room and elevated temperature tested specimens, respectively. Large number of twins were observed near fracture surface of the room temperature tensile tested specimen while coalescence of microcracks led to fracture at elevated temperature.

Electrochemical behaviour of friction stir welded joints: Effect of tool shoulder diameter

In the present research work, the effect of shoulder diameter on the corrosion rate of stir zones (SZs) of dissimilar joints exposed to 3.5 M NaCl solution fabricated via friction stir welding (FSW) was studied in detail. In FSW, the tool shoulder diameter as one of the important process parameters significantly affects frictional heat input and material flow leading to the heterogeneous distribution of second phase particles (precipitates) that alters the electrochemical behaviour of FSWed joints. The influence of shoulder diameter on corrosion rate was analyzed via macro and microstructural characterization and open circuit potentiodynamic testing. The surface morphology and microstructural analysis of specimens reveal the occurrence of both pitting and intergranular corrosion (IGC) in base materials and SZs of welded specimens. Among the welded specimens, the SZ of weld I fabricated under shoulder size of 18 mm revealed the lower corrosion resistance. The potentiodynamic test revealed that weld I having very fine grains as compared to other joints exhibits the highest current density and corrosion rate.

Friction welding of high Cr white cast iron to AISI 1030 steel with Ni interlayer

Abstract In this study, the effect of friction time on microstructure and weldability of AISI 1030 steel with nickel interlayer and high chromium white cast iron welded by the friction welding method were investigated experimentally. The weld joints were produced with 2000 rpm rotational speed, under 80 MPa friction pressure, 150 MPa forging pressure, for 8 s forging time and 8, 10 and 12 s friction times. After the friction welding process, the microstructures of the weld interfaces were analyzed by optical microscopy, scanning electron microscopy, energy dispersive spectrometry, elemental mapping and X-ray diffraction analysis. The results were lateron compared theoretically and experimentally. The increasing friction time led to high frictional heat input. The results indicated that friction time plays a vital role on the microstructure and weldability.

Brush seal contact force theory and correlation with tests

Brush seals are leakage performance systems that are operating against turbine shafts and bucket tips. Flexible bristle structure can compensate the rotor interference during transients, which enables closer clearance control in turbine and turbo-engine applications. During turbomachinery transients, contacting bristles exert force on turbine shaft, which in turn result in wear, frictional heat generation, counter torque and power loss. Brush seal design with proper stiffness has crucial importance since high bristle tip force levels during rotor contact may result in rotor instability, bristle tip melting or thermo-mechanical fatigue due to large amount of local frictional heat input. Counter torque applied by the contacting brush seal with improperly excessive tip force may result in turbine stall and overall efficiency losses. High levels of bristle contact forces may induce elevated wear rates, which degrades the leakage performance of the brush seal during its life cycle. Brush seal contact force evaluation under operating conditions has critical importance and has to be conducted before turbine integration to avoid performance losses and possible turbine failure in some extreme cases. In this paper, ‘Bristle Contact Force Theory’ is developed and closed form bristle tip force (BTF) function is derived where rotor-bristle, inter-bristle and bristle-back plate interactions are included. BTF measurements under pressurized and unpressurized air environment are conducted for the selected test seals by using the custom ‘Rotary Test Rig’. BTF function of bristle contact force theory show high level of correlation with the test data. The effect of cant angle manufacturing tolerance on bristle tip force levels is also examined through bristle tip contact force theory, and compared with the test seal measurements. Methodology for number of bristle row calculation and comparison with the geometric inspection of test seals are also detailed within the content of this paper.

Study on the importance of thermo-elastic effects in FE simulations of linear friction welding

Thermo-elastic effects generally influence the distribution of contact variables in contact pair of two components undergoing dynamic loading. A 3D fully coupled finite element model with a plastic/plastic friction pair was used to evaluate the effects of thermo-elastic deformation. Two workpieces were in conforming contact at the beginning of welding, when workpieces were aligned, after which friction heat input resulted in local thermal expansion which caused a reduction in the real contact area between the two workpieces. Thermal expansion deformation also resulted in non-uniform contact pressure and interfacial temperature distributions. With the softening of materials around the hot region, the initiation of gross plastic deformation resulted in an increase in the real contact area. Thermo-elastic effects at different welding parameters on the minimum real contact area fraction, and the time taken to recover 100 % of apparent contact area, were investigated numerically. Compared to the LFW model with the rigid/plastic friction pair, the LFW model with the plastic/plastic friction pair could better reflect the thermo-elastic effects in LFW. The study provided a support to understand the LFW physical mechanism using simulation.

Investigation of internal defects and premature fracture of dissimilar refill friction stir spot welds of AA5754 and AA6061

The occurrence of internal defects in welded samples of AA5754-AA6061 produced by refill friction stir spot welding was investigated. A design of experiments using Box-Behnken method followed by a statistical examination using analysis of variance (ANOVA) and response surface modeling were utilized as analysis tools, which proved to be a reliable optimization methodology. This optimization successfully produced sound joints with high lap-shear strength. The statistical analysis showed a large influence of linear plunge depth, quadratic rotational speed, and two-way interaction of feeding rate and rotational speed on lap-shear strength of the welds. A quadratic hypersurface model for predicting weld performance was successfully generated. The subsequent investigation was performed by changing welding parameters, one factor at a time (OFAT), which confirmed the high dependence of lap-shear strength on rotational speed by producing an undesirable outlier. Metallographical analysis on the outlier sample pointed out the occurrence of voids and refilling defects, associated in large scale to low friction heat input. The outlier sample also produced a wing-shaped structure that possibly obstructed the flow of softened material toward void closure. The existence of those defects shows evidence of premature crack in the outlier sample. A subtle adjustment in rotational speed to1000 rpm proved to be sufficient to eliminate the defects and produce stronger welds. Although the problem of refilling defects could be solved, the mechanical properties in the weld were worsened compared to that of base material.

Selection of optimal process parameters and assessment of its effect in micro-friction stir welding of AA6061-T6 sheets

Selection of appropriate welding parameters in micro-friction stir welding (micro-FSW) is challenging and a practical problem. In this regard, an assessment of variation in the favourable process parameters due to downscaling of sheet thickness can prove beneficial. In this work, FSW in 1- and 0.5-mm-thick AA6061-T6 sheets was compared for suitable process parameters based on the tensile strength of the weld. Comparatively high tool rotational speed combined with a high speed of tool travel and low plunge depth was found to be suitable welding parameters in micro-FSW (0.5-mm sheet). The effect of using this favourable set of process parameter was also assessed on frictional heat input requirement per unit weld volume, deformation conditions during welding (predicted using FEM simulation) and weld’s mechanical properties and microstructural characteristics in each sheet thickness. A high frictional heat input per unit weld volume was found in micro-FSW. A reduced weld zone temperature, an increased value of strain rate and a higher cooling rate were found in micro-FSW which led to a smaller weld nugget grain size, an improvement in weld’s ultimate tensile strength (UTS) but a drop in its ductility in this case.

Experimental Investigation on the Influence of Geometrical Parameters on the Frictional Heat Input and Leakage Performance of Brush Seals

Because of the superior sealing characteristics compared to labyrinth seals, brush seals found an increased spread in turbomachinery in recent years. Their outstanding sealing performance results mainly from their flexibility. Thus, a very small gap between the rotor and bristle package can be obtained without running the risk of severe detrimental deterioration in case of rubbing. Rubbing between rotor and seal during operation might occur as a result of e.g., an unequal thermal expansion of the rotor and stator or a rotor elongation due to centrifugal forces or maneuver forces. Thanks to the flexible structure of the brush seal the contact forces during a rubbing event are reduced; however, the frictional heat input can still be considerable. Particularly, in aircraft engines with their thin and lightweight rotor structures, the permissible material stresses can easily be exceeded by an increased heat input and thus harm the engine's integrity. The geometry of the seal has a decisive influence on the resulting contact forces and consequently the heat input. The complex interactions between the geometric parameters of the seal and the heat input and leakage characteristics are not yet fully understood. This paper presents the investigation of the influence of the geometric parameters of a brush seal on the heat input into the rotor and the leakage behavior. Two seals with different packing densities were tested under relevant engine conditions with pressure differences ranging from 1 to 7 bar, relative surface speeds ranging from 30 to 180 m/s, and radial overlaps ranging from 0.1 to 0.4 mm. The transient temperature rise during the rub event was recorded with 24 thermocouples in close proximity to the rub contact embedded in the rotor structure. By comparing the temperature curves with the results of a thermal finite element (FE) analysis of the rotor the heat input into the rotor was calculated iteratively. It could be shown that the packing density has a decisive influence on the overall operating behavior of a brush seal. Furthermore, results for the heat flux distribution between seal and rotor are shown.

A Hybrid Method Based on Macro–Micro Modeling and Infrared Imaging for Tool Temperature Reconstruction in Dry Turning

In metal cutting, the ability to model its tool temperature distribution is highly desirable, as it provides an effective means to monitor the tool and workpiece conditions, particularly when machining hard-to-machine materials that have a low heat conductivity. Because of the complex heat generation in the microscale tool–chip interface, the difficulty to infer its temperature distribution is a well-known problem. In the context of dry lathe turning, this paper presents a hybrid method that considers both the macroscale tool heat transfer and microscale machining mechanics to reconstruct the 3-D tool temperature field from nonobstructed infrared (IR) images. The microscale-mechanics model identifies the contact geometry and estimates the frictional heat input to determine the complete boundary conditions to solve the macroscale heat-transfer model for the steady-state 3-D temperature distribution that provides a basis for experimentally fitting the model by comparing the computed surface temperature with actual temperature measurements. Two sets of experimental results validating the reconstructed temperature on a customized orthogonal-cutting testbed with a high-resolution IR imager and evaluating the effectiveness of the hybrid method on a lathe-turning center are presented. The results demonstrate that with the hybrid macro–micro modeling, the 3-D steady-state temperature field of a typical commercial lathe tool insert can be accurately reconstructed from a relatively low-resolution IR image.

Effects of the Back Plate Inner Diameter on the Frictional Heat Input and General Performance of Brush Seals

Reducing losses in the secondary air system of gas and steam turbines can significantly increase the efficiency of such machines. Meanwhile, brush seals are a widely used alternative to labyrinth seals. Their most valuable advantage over other sealing concepts is the very small gap between the sealing package and the rotor and thus reduced leakage mass flow. This small gap can be achieved due to the great radial flexibility without running the risk of severe detrimental deterioration in case of rubbing. Rubbing between rotor and seal during operation might occur as a result of e.g., an unequal thermal expansion of the rotor and stator or a rotor elongation due to centrifugal forces or manoeuvre forces. Thanks to the flexible structure of the brush seal, the contact forces during a rubbing event are reduced; however, the frictional heat input can still be considerable. Particularly in aircraft engines with their thin and lightweight rotor structures, the permissible material stresses can easily be exceeded by an increased heat input and thus harm the engine’s integrity. The geometry of the seal has a decisive influence on the resulting contact forces and consequently the heat input. This paper is a contribution to further understand the influence of the geometrical parameters of the brush seal on the heat input and the leakage during the rubbing of the seal on the rotor. In this paper, a total of three seals with varied back plate inner diameter are examined in more detail. The experimental tests were carried out on the brush seal test rig of the Institute of Thermal Turbomachinery (ITS) under machine-relevant conditions. These are represented by pressure differences of 1 to 7 bar, surface speeds of 30 to 180 m/s and radial interferences of 0.1 to 0.4 mm. For a better interpretation, the results were compared with those obtained at the static test rig of the Institute of Jet Propulsion and Turbomachinery (IFAS) at the Technical University of Braunschweig. The stiffness, the blow-down and the axial behaviour of the seals as a function of the differential pressure can be examined at this test rig. It could be shown that the back plate inner diameter has a decisive influence on the overall operating behaviour of a brush seal.

Friction Torque and Heat Input Characteristics During Continuous Drive Friction Welding of Aluminum to Steel

Friction Torque and Heat Input Characteristics During Continuous Drive Friction Welding of Aluminum to Steel

Modelling the Effect of Steady State Wheel Temperature on Rail Wear

A moving train subjects a rail cross section to many wheel passes and heat generation at the interface from bulk friction and microslip. Frictional heating at the interface causes the temperature of the contacting rail surface and wheel surface to rise. After the train has passed, the rail temperature drops to ambient till the next train arrives. Wheels, on the other hand, pick up the frictional heat input continuously and hence their steady state temperature could be much higher than that of the rail. A hot wheel provides an additional temperature rise at the rail surface. The accompanying thermal stresses and thermal softening may enhance the rail wear rate. This has been investigated in the present study. A ratchetting failure-based computer simulation has been employed to assess the wear rate of a pearlitic rail steel. It is found that the thermal stress effect on wear rate is only the modest, but thermal softening can enhance the wear rate by up to an order of magnitude for the conditions considered.

Numerical analysis of interfacial deformation and temperature rise during ultrasonic Al ribbon bonding

The plastic deformation, thermal behaviour and interfacial state during ultrasonic bonding of Al ribbon with substrate were analyzed based on numerical simulations. The numerical simulations were carried out by finite difference and element methods. The change of bond interface temperature with the bonding time t was simulated. It is suggested that the interfacial frictional behaviour between ribbon and substrate does not only affect the temperature rising behaviour but also the stress situation and plastic deformation behaviour, i.e., the ribbon deformation and the frictional slip behaviours influence each other. It is also found that partial constraint (frictional slip) at the bond-interface increases interfacial shear stress drastically and controls the frictional heat input and temperature rise of the Al ribbon.

Performance of a two-phase natural circulation in a rectangular loop

Abstract An analytical solution for the loop mass flow rate for a two-phase natural circulation in a rectangular loop is derived. The cases with saturated inlet flow and sub-cooled inlet flow are considered. The loop mass flow rate was obtained as a function of geometric parameters of the loop, frictional loss coefficient, pressure, heat input, and inlet sub-cooling. An optimal configuration for the natural circulation was suggested in terms of geometric parameters and heat input for a stable inherent safe operation of the two-phase natural circulation loop.

Interface Microstructure and Mechanical Properties of Dissimilar Friction Stir Welded Joints between Zr<sub>55</sub>Cu<sub>30</sub>Ni<sub>5</sub>Al<sub>10</sub> Bulk Metallic Glass and Pure Al

Dissimilar friction stir welding of Zr55Cu30Ni5Al10 bulk metallic glass (BMG) to pure Al was carried out at different welding conditions. Sound BMG/Al joints can be obtained at a rotation speed of 350 and 400rpm. However, defects or crack may form in the stir zone when the rotation speed was either too low or too high. In the sound joints, a clear and straight BMG/Al interface was formed and no intermetallic compounds caused either by chemical reaction or crystallization from the amorphous phase can be detected within the entire stir zone. The hardness of Al side in the stir zone decreases with increasing of the rotation speed and is lower than that in the base material due to frictional heat input. The sound BMG/Al joints fractured in the heat affect zone of Al side and the tensile strength can reach about 91% of that of the base material. [doi:10.2320/matertrans.M2011376]