On the influence of workpiece material on friction in microforming and lubricant effectiveness

The frictional behaviors between metal forming tool and three different metallic materials were evaluated using the microforming T-shape test. A mathematical function is proposed to describe the calibration curves for different friction coefficients. Round bars of copper, aluminum and silver of diameter 1 mm and length 5 mm were used as the workpieces to study the material influence on friction factor, m, during unlubricated microforming process through comparison between simulation and experimental results. Furthermore, various lubricants were used with the aluminum and copper to examine their performance in microforming. The results have shown that the workpiece materials not only determine the friction factor, m, during unlubricated microforming, but also influence the performance of lubricants. Lubricant can be completely ineffective and may not produce discernible friction reduction in microforming, unlike in conventional metal forming. By considering the influence of contact pressure on lubricant effectiveness, a novel pressure dependent frictional model and a lubricant evaluation method are proposed.     

Deformation inhomogeneity due to sample–anvil friction in cylindrical compression test

Finite element (FEM) analysis was used to systematically evaluate the inhomogeneity of deformation in cylindrical samples with various sample?anvil friction coefficients, m. It was found that the level of friction strongly influences the deformation homogeneity, which increases significantly with the friction coefficient although the overall geometry of the samples almost remains the same when m>0.4. The position, at which the effective strain along the maximum radial direction in a compressed sample is equal to the equivalent strain of the sample, does not vary greatly with respect to both equivalent strain of the sample and m. Hardness measurements of compressed cylindrical 5056B Al alloy samples revealed a change of effective strain distribution similar to that revealed by FEM analysis. There exists a quantitative relationship between the hardness and the effective strain if no recrystallization or recovery occurs during the compression process.     

Performance of W-TI-(N) coated pins in lubricated pin-on-disk tests

W-Ti-(N) thin films were deposited on polished bearing steel balls by dc magnetron sputtering varying the partial pressure ratio, pN₂/pAr. The tribological behaviour was accessed by pin-on-disk testing with contact geometry of uncoated and coated 100Cr6 balls sliding against uncoated different disk materials used as stamping sheet. Different types and amounts of lubricants were used in the tests.In non-lubricated tests, friction coefficients (μ) as high as 0.8 were achieved. For the more ductile sheet materials (Al alloy and Zn-coated steel) strong adhesion was observed. The best compromise between low wear rate and low friction coefficient was achieved for N-containing coatings deposited without ion gun assistance.In lubricated conditions, a significant decrease of the friction coefficient down to 0.05 and a reduction of the wear coefficient in more than one order of magnitude down to < 10^− 16 m²N^− 1 were reached in relation to non-lubricated tests. Very good tribological results were achieved using the corrosion protection oil as lubricant, with amounts usually applied for protection of sheet materials (2 g/m²). It was found that the wear coefficient of the coated ball decreased linearly with increasing hardness of the coating, being the best that deposited with N contents in the range from 35 at.% to 40 at.%. The tribological performance of the coated samples was approximately constant even when the amount of used lubricant was reduced to only 25% of the initial value (0.5 g/m²).     

Friction Factor Evaluation Using Experimental and Finite Element Methods for Al-4%Cu Preforms

In this study, ring compression tests and finite element (FE) simulations have been utilized to evaluate the friction factor, m, under different lubricating conditions for powder metallurgical (P/M) Al-4%Cu preforms. A series of ring compression tests were carried out to obtain friction factor (m) for a number of lubricating conditions, including zinc stearate, graphite, molybdenum disulfide powder, and unlubricated condition. FE simulations were used to analyze materials deformation, densification, and geometric changes, and to derive the friction calibration curves. The friction factor has been determined for various initial relative densities and different lubricating conditions, and a proper lubricant for cold forging of P/M Al-4%Cu preforms is found. Studies show that the use of lubricants has reduced the friction. However, increase in the number of pores in the preforms leads to excessive friction. The FE simulation results demonstrate a shift in the neutral plane distance from the axis of ring specimen, which occurred due to variations in the frictional conditions and initial relative densities. The load requirement for deformation, effective stress, and effective strain induced, and bulging phenomena obtained by FE simulations have a good agreement with the experimental data.     

Tribological behaviors of DLC film deposited on female die used in strip drawing

With the miniaturization of parts, dimensions of parts have obvious effects on friction in microforming processes when liquid lubricants are applied. To eliminate size effects of friction, diamond-like carbon (DLC) film was deposited on the surface of female die and blank holder for its superior tribological characteristics. Strip drawing tests were carried out on conditions of different lubricants, and tribological behaviors of DLC film at female die surface were evaluated using punch load. The results reveal that the specimen dimensions and surface topography of specimen have a little effect on tribological behaviors of DLC film than castor oil. DLC film shows lower coefficient of friction (COF) in large deformation and small dimension. With the increasing of test times, wear of DLC film occurs. Raman spectra indicates that the ratio between integrated intensities of D and G peaks (I_D/I_G) increases with the increasing of tests times, which means that graphitization of DLC film occurs for the rise of temperature induced by the friction at contact surface. The interesting thing is that H content increases with the times of tests especially for high contact load, which is evaluated by the ratio of the slope m of fitted linear background to the intensity of the G peak (m/I_G). The increase of H content means that the number of free hydrogen atoms becomes larger for the breakage of CH bonds induced by the friction work. The graphitization and breakage of CH bonds may have an obvious effect on the tribological behaviors of DLC film in microforming.     

Pressure influence on crystallization kinetics during injection molding

This work presents the measurement of the temperature at the center of the cavity during the molding of PP material using an embedded thermocouple probe. The measured temperature curves show a plateau shape related to the crystallization phenomenon. Measurements, done at various pressures show an increase in the crystallization plateau temperature depending on the cavity pressure. The change in the measured crystallization temperature allows the identification of the pressure dependence on the Hoffman-Lauritzen crystallization kinetics parameters: T_m and T_g. The T_m pressure dependence leads to a good agreement with the data of reported studies that used other experimental devices. The obtained results also show a decrease in cavity pressure duration related to the increase in the gate freezing speed as a result of the pressure increase. The identified pressure dependence laws for T_m and T_g introduced in the crystallization kinetics and the thermal model, allow to describe a temperature change in good agreement with the temperature measured down to the crystallization plateau.     

Effect of nitrogen on tribological properties of amorphous carbon films alloyed with titanium

The article reports on the dependency of friction and wear of a-(Ti,C,N) films on the nitrogen content. The amount of nitrogen N in the film was controlled by partial pressure of nitrogen p_N2 in the Ar + N₂ sputtering gas mixture. It is shown that the incorporation of N in the film results in the increase of (i) the coefficient of friction μ (increases from 0.12 to 0.37), (ii) the coefficient of wear k (increases from 0.16 × 10^?6 to 0.93 × 10^?6 mm³/N m) and the decrease of (i) the film hardness H, (ii) effective Young's modulus E^?, (iii) the elastic recovery W_e of film and (iv) the ratio H/E^?. The changes of μ and k of the a-(Ti,C,N) film correlate well with changes of the film mechanical properties (H and E^?) and its mechanical behavior (W_e, H/E^? and the ratio H³/E^?2) characterizing the film resistance to plastic deformation.     

Slipline field modeling of orthogonal machining for a worn tool with elastic effects and adhesion friction at the contact regions

In this paper, two slipline field models are presented for orthogonal machining with a worn tool with a finite flank wear land. Friction at tool–chip and tool–work piece interfaces are assumed to be governed by the adhesion friction law as suggested by [Maekawa, K., Kitagawa, T., Childs, T.H.C., 1997. Friction characteristics at chip–tool interface in steel machining. 23rd Leeds Lyon Symposium in Tribology] It is further assumed that the natural contact length consists of a zone of elastic contact and a zone of plastic contact such that the forces in the elastic contact region and those at the rigid-plastic chip boundary keep the chip in a state of static equilibrium. The relation between the angular range of α- and β-lines within the secondary shear zone is assumed to be given by the linear equation β = m₀α. The fields are analyzed by the matrix operational procedure developed by [Dewhurst, P., Collins, I.F., 1973. A matrix methods for constructing slipline field solutions to a class of plain strain plasticity problems. Int. J. Numer. Methods Engineering 7, 357–378; Dewhurst, P., 1984. The Coulomb friction boundary value problem in plain-strain slipline field theory. In: First Int. Conf. on Technology of Plasticity (ICTP) Proc. (Advanced Technology of Plasticity), Tokyo, Japan, pp. 1085–1090; Dewhurst, P., 1985. A general matrix operator for linear boundary value problems in slip-line field theory. Int. J. Numer. Methods engineering 21, 169–182]. The limits of the validity of the proposed fields are established using Hill's overstressing criteria. Effect of flank wear on ploughing and cutting forces is studied and their variation with friction parameters examined. It is shown that the peak tool tip pressure for a worn tool is much lower than that for a sharp tool. The results of computation further demonstrate that the force ratio and average tool–chip Coulomb coefficient of friction for a sharp tool decrease with rake angle. But presence of even a small flank wear land may reverse this trend. The results from theoretical analysis are also compared with experiment.     

Numerical analysis of thermal process in continuous drive radial friction welding

A heat source model for radial friction welding was proposed, which was determined by friction pressure, friction coefficient, material properties and extrusion speed of material. A 3 D model was established to analyze the continuous drive radial friction welding temperature field of 45 steel pipe. The influences of friction pressure, friction time and rotation speed on the temperature of the friction interface were analyzed. The results showed that the temperature on the friction inter face rapidly rose to a peak temperature in initial friction stage and kept constant in the stable friction stage. Welding parameters of friction pressure, friction time and rotation speed had few influences on the peak temperature, while the increase of friction pressure and rotation speed could shorten the time to reach the peak temperature.     

搅拌摩擦焊AA7075-T651铝合金接头的疲劳裂纹扩展(英文)

研究12 mm厚AA7075-T651铝合金板搅拌摩擦焊接头的疲劳裂纹扩展行为。从搅拌摩擦焊接头以及母材中截取试样,对试样进行疲劳裂纹扩展实验。对搅拌摩擦焊接头以及母材的横向拉伸性能进行评估。用光学显微镜和透射电镜分析焊接接头的显微组织。用扫描电镜观察试样的断裂表面。与母材相比,焊接接头的ΔKcr降低了10×10-3 MPa·m1/2。搅拌摩擦焊AA7075-T651接头的疲劳寿命明显低于母材的,其原因可归结于焊缝区的析出相在搅拌摩擦焊接过程中的溶解。     

Effects of TiO2 particles size and heat treatment on friction coefficient and corrosion performance of electroless Ni-P/TiO2 composite coatings

The effects of the titanium dioxide (TiO₂) particles size on the friction coefficient and corrosion performance of the Ni-P/TiO₂ composite coatings before and after heat treatment at 400°C for 1h have been investigated. Pin-on-disc analysis results have revealed that the highest and the lowest friction coefficients belonged, respectively, to the simple Ni-P coating and the Ni-P/TiO₂ composite coating containing TiO₂ particles of the average size of 0.1 μm (μ ～ 0.62 against 0.52). Eventually, a relative reduction in the corrosion resistance and the friction coefficient (as low as μ ～ 0.38) have been observed after heat treatment of Ni-P and Ni-P/TiO₂ composite coatings.     

Compressibility study of UIr2

UIr₂ was investigated using high pressure x-ray diffraction technique up to 55 GPa. The compound remains stable in its cubic structure (Space Group: Fd-3m) up to 40 GPa. Above 40 GPa x-ray diffraction pattern indicates the emergence of a yet unidentified phase. The large structural stability of cubic MgCu₂ type structure, predicted empirically, is not exhibited in this compound. Zero pressure bulk modulus and its pressure derivative are found to be 284 GPa and 7.9 respectively. The high bulk modulus value obtained is indicative of itinerant nature of 5-f electrons which is also supported by Hill limit criteria. Electronic structure calculations indicate that uranium tetrahedral network is less rigid as compared to iridium tetrahedral network. Valley in density of states at Fermi level deepens with pressure vindicating observation of a more stable phase at high pressure.     

Electrical properties of amorphous carbon films

Aromatic polyimide films, Upilex S partially carbonized between 700°C and 1000°C. Electrical conductivity is higher at higher temperatures. The electrical conductivity s could be expresses as s = s_o exp (−E/kT), where k is the Boltzmann constant, t is the absolute measuring temperature. s_o and E are found to be 4 × 10⁻¹ Ω⁻¹ m⁻¹ and 0.02eB, respectively. The experimental data show that the Hall coefficient R is negative, and this implies that the carriers are negatively charged, i.e. electrons. The specimens are n-type semiconductors. The carrier density η is given by η = 1/(|e|R) and the mobility μ is μ is s/(η|e|), where |e| is the absolute value of the electron charge and s is the electrical conductivity. Fitting the data, η = A₁ exp (−E₁/κT) and μ = A₂ exp (E₂/κT). E₁ and E₂ depend on carbonized temperature. The polyimide films are not completely carbonized but partially carbonized at 700°C. The partially carbonized polyimide is an n type donor. It is concluded that the “impurity level” lies about 0.36eV below the conduction band.     

Anomalous plastic flow of cerium near the isomorphic phase transformations under high hydrostatic pressure

Compression tests have been carried out on cerium specimens at room temperature (0.27T_m) under high hydrostatic pressures up to 1.2 GPa. A strong increase of the yield strength was observed for both isomorphic γ and α phases at pressures approaching the γ↔α isomorphic phase transformations. That increase was in good agreement with the theory of dislocations when the dependence of elastic properties and a lattice parameter of cerium on pressure was applied to calculate the effect of pressure on the yield stress controlled by the edge dislocations. An anomalous strong decrease of the yield stress was observed in both γ and α phases in the vicinity of both γ↔α phase transformations. That phenomenon was explained as an effect of pressure induced new phase atoms through spreading the cores of edge dislocations. A complete disappearance of work hardening in both γ and α phases was also observed in the wide range of pressures. The influence of hydrostatic pressure on the energy of grain boundaries of both phases was considered to be responsible for that property. The ratio of the grain boundary energy to the Peierls energy is suggested to be a criterion of the work hardening ability of f.c.c. polycrystals.     

Experimental study of friction from cutting forces in orthogonal milling

The present work shows the procedure used for the study of friction in orthogonal milling. The equipment employed to measure cutting forces includes a rotating dynamometer and a multichannel portable data acquisition system. Signal filtering is applied to reduce the dynamic system problems. The analysis of the relation between cutting force (F_c) and radial force (F_r) allows to assess the dependence of friction and chip thickness on cutting speed and feed per revolution. Also, the study of the influence of workpiece material on milling process shows that cutting force (F_c) decreases and friction increases with improving workpiece material machinability. Hence, a significant contribution to knowledge in a research field with a little previous literature, such as the friction in orthogonal milling, is made in the present work through a comparative analysis of cutting forces (F_c and F_r) that allows to study the influence of cutting speed and feed per revolution on friction and to propose a new approach for machinability associated to F_c rather than to friction coefficient.     

Evaluation of friction condition in cold forging by using T-shape compression test

Friction plays an important role in metal forming, and numerical simulation of forging processes requires precise informations about the material properties and the value of the friction factor m or coefficient μ. This paper describes the T-shape compression, a new friction testing method by combined compression and extrusion of a cylinder between a flat punch and a V-grooved die. It can realize actual cold forging condition and allows measuring the friction on the cylindrical surface of the billet during forging process. The results of experiments and simulations show that the stroke–load curve and the height of the extruded part are both sensitive to friction. In order to obtain the highest sensitivity to friction, a FE parametric study of this test has been performed: it indicates that small corner radius and V-groove angle in the die should be chosen. Two commercial FE codes, FORGE 3D and ABAQUS, were used and provided very similar results for a given friction condition. Low carbon steel drawn bar with phosphate and soap coating was chosen as specimens. Friction tests with three different lubrication conditions (solid coating, oil and oil + solid coating) were carried out, and then friction factor m and friction coefficient μ were determined by using experimental results and the calibration by numerical simulation of T-shape compression test.     

Friction role in bending behaviors of thin-walled tube in rotary-draw-bending under small bending radii

For contact dominated rotary-draw-bending (RDB) of thin-walled tube, friction role should be focused to achieve precision bending under small bending radii ratio (R_d/D < 2.0, R_d-bending radius, D-tube diameter). By using explicit FE simulation combined with physical experiment, underlying effects of the friction on bending behaviors are explored from multiple aspects such as wrinkling, wall thickness variation and cross-section deformation. The results show that: (1) By using a simulative twist compression test (TCT), the dynamic contact conditions of RDB with large slipping are reproduced, and the coefficients of the friction (CoFs) under various tribological conditions in RDB are estimated, which provides physical basis for understanding friction role and boundary conditions for FE simulation. (2) Both positive and negative effects of friction role are observed since the friction on each interface affects the multi-defect with different or even contrary tendencies. The effect sensitivity on wrinkling is less obvious than that on wall thinning and cross-section deformation. Under smaller R_d/D, the bending becomes more sensitive to the friction conditions. (3) Considering the knowledge about friction role on individual interface of RDB, by changing two decisive parameters affecting the CoFs such as lubricant types and tube/tool materials, an optimal strategy is proposed to apply the tribological conditions and thus the stable and accurate bending conditions are established for precision forming of RDB under smaller R_d/D.     

含氧环境中航空发动机用Ti40阻燃钛合金的摩擦点燃

采用摩擦点燃的方法,研究摩擦压力p和氧浓度xO对Ti40阻燃钛合金的抗点燃性能的影响,建立了定量描述Ti40钛合金抗点燃性能的p-xO关系曲线,并结合SEM、XRD和EDS等测试手段分析Ti40钛合金的抗点燃机理。结果表明：Ti40钛合金的p-xO关系曲线符合抛物线规律,p在0.1~0.25 MPa变化时,xO的变化范围约为25%。当 xO≥70%时,Ti40钛合金在室温条件下即会被点燃,并迅速发展为持续的稳定燃烧,且时间长于10 s。Ti40钛合金的抗点燃性能优于TC4钛合金的,Ti40合金被点燃时所对应的xO至少比TC4高40%。当xO较低时,Ti40钛合金的抗点燃性能对p较为敏感,而随着xO的增大,抗点燃性能对xO更加敏感。摩擦表面形成的具有强相互作用的V2O5、TiO2和Cr2O3氧化物融合物是Ti40钛合金的抗点燃性能提高的根本原因。     

Phase relationships and stability of the μ and ζ phases in the ag-a1-x (x = zn,ga, ge) systems

Details of phase relationships in three ternary systems, Ag-Al-X (X = Zn, Ga, Ge), near the Ag-rich corner at 400 °C are presented. Metallography, XRD, scanning electron microscopy, and X-ray spectroscopy have been used to establish the respective isothermal sections at 400 °C. In each case, the μ phase and the ζ phase of the Ag-Al binary system extend into the ternary system and terminate at a three-phase region involving the Ag-rich primary solid solution (α₁ phase) in the Ag-Al-Zn and Ag-Al-Ga systems and the Ge-rich primary solid solution (α₂ phase) in the Ag-Al-Ge ternary system. The stability ranges of the μ and ζ, phases follow approximately constant electron concentration lines. The solid solubilities of Zn, Ga, and Ge in the μ phase are relatively small compared with those in the ζ, phase (up to 18 at.%). No ternary phase appears to exist in the Ag-rich portions studied in the three ternary systems.