Measurement of friction under sheet forming conditions

Among the tests used to evaluate friction in sheet forming processes, bending-under-tension (BUT) tests have received considerable attention. A new test was developed incorporating a smooth increase of wrap angle during deformation, even at high deformation rates, thus replicating typical conditions at die radii. A corresponding data analysis procedure was introduced to separate the effects of bending and friction and to account for the strain-rate sensitivity of the test material. The new test and analytical procedure were used to investigate the frictional behavior for four sheet alloys: interstitialfree (IF) steel, high-strength galvanized (HSG) steel, 2008-T4 aluminum, and 70/30 brass. A range of testing rates, pin radii, and lubrication conditions were employed, and the variation of friction coefficient (μ) as a function of contact radius, contact angle, and punch speed was measured. Complex variations ofμ with respect to tool radius and wrap angle were observed, consistent with previous studies. No significant, systematic variation ofμ was found for punch speeds ranging from 1.7 to 170 mm/s, contrary to reports in the literature. Failures in 2008-T4 aluminum shifted from tensile localization to bending as the tool radius was reduced. The other materials consistently failed by tensile localization.     

Critical assessment of the local cleavage stress σ ƒ * in notch specimens of C-Mn steel

An elastic-plastic finite element method (FEM) was used to calculate the stress and strain distributions ahead of notches with various root radii in a bending specimen of C-Mn steel with grain sizes of 10 and 30 μm. By accurately measuring the distance of the cleavage initiation site from the root of the notch, the local cleavage stress σ _ƒ ^* was measured. When the notch radius increased from 0.25 to 1.0 mm, the distribution of high stress had a definite variation but the σ _ƒ ^* remained relatively constant. In notch specimens with different root radii, the critical fracture event is identical,i.e., propagation of a ferrite grain-sized crack into the neighboring matrix. Therefore, the σ _ƒ ^* is mainly determined by the length of the critical microcrack, here, the size of ferrite grain instead of the high stress volume for finding an eligible brittle particle. The critical strain for initiating a crack was about 1 pct. The cleavage site ahead of a notch was related to the relative distributions of stress and strain and the random distribution of the weakest grains. The higher fracture load of the fine-grain material can be attributed to its higher value of σ _ƒ ^* /σ_o as compared with the coarse-grain. The σ _ƒ ^* /σ_o is a potential engineering parameter for toughness assessment in notch specimens.     

Specimen Size Effects on Zr-Based Bulk Metallic Glasses Investigated by Uniaxial Compression and Spherical Nanoindentation

Specimen size effects on the mechanical behavior of Zr-based bulk metallic glasses (BMGs) were investigated by compression and nanoindentation tests. In compression, even at the 1- to 10-mm scale, stable shear band propagation and extensive plastic deformation can be achieved in small (2 mm) specimens, in contrast to large (6.5 mm) specimens, which fail catastrophically after limited plastic deformation. The yield strength is independent of specimen size in this range, and plastic deformation remains highly localized in a few shear bands even in those specimens that exhibit stable shear sliding. The fracture surfaces of small specimens are smooth, without the vein patterns normally observed as characteristic features on the fracture surfaces of BMGs. During spherical nanoindentation, it is found that the upper bound of the maximum shear stress to initiate plasticity (yielding) in a Zr-based BMG is almost constant for indenter radii smaller than ~90 μm. However, the lower bound of this maximum shear stress decreases with increasing indenter radius, probably due to the increased probability of finding defects underneath larger indenters.     

The Influence of Notch Root Radius and Austenitizing Temperature on Fracture Appearance of As-Quenched Charpy-V Type AISI4340 Steel Specimens

Charpy-V type samples either step-quenched from 1200 °C or directly quenched from the usual 870 °C temperature, fractured by a slow bend test procedure, have been fractographically examined. Their notch root radius,ρ, ranged from almost zero (fatigue precrack) up to 2.0 mm. The fracture initiation process at the notch differs according to root radius and heat treatment. Conventionally austenitized samples withρ values larger than 0.07 mm approximately (ρ _eff) always display a continuous shear lip formation along the notch surface, whereas specimens with smaller notches do not exhibit a similar feature. Moreover, shear lip width in specimens withρ >ρ _eff is linearly related to the applied J-integral at fracture. In high temperature austenitized samples similar shear lips are almost nonexistent. The above findings, as well as overall fractographic features, are combined to explain why blunt notch AISI 4340 steel specimens display a better fracture resistance if they are conventionally heat treated, whereas fatigue precracked samples show a superior fracture toughness when they are step-quenched from 1200 °C. Variations of fracture morphologies with the notch root radius and heat treating procedures are associated with a shift toward higher Charpy transition temperatures under the combined influence of decreasing root radii and coarsening of the prior austenitic grain size at high austenitizing temperatures. D. FIRRAO, J. A. BEGLEY were both formerly with the Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210.     

Modeling Viscosities of CaO-MgO-FeO-MnO-SiO2 Molten Slags

A model for estimating the viscosity of silicate melts is proposed in this article. The structural characteristics of a silicate slag can be described by the numbers of the bridging oxygen, nonbridging oxygen, and free oxygen present in the slag. A method of calculating the numbers of the different types of oxygen ions is presented in this article, which involves a simple approximation of “complete bridge breaking.” With just a few parameters, the model provides both the temperature and composition dependencies of viscosity for the pure component: SiO₂; the binary systems: MgO-SiO₂, CaO-SiO₂, FeO-SiO₂, and MnO-SiO₂; the ternary systems: CaO-MgO-SiO₂, CaO-FeO-SiO₂, MgO-FeO-SiO₂, and CaO-MnO-SiO₂; and the quaternary systems: CaO-MgO-MnO-SiO₂ and CaO-FeO-MnO-SiO₂. It was found that the ability of different basic metal oxides to decrease viscosity varies and is in the following hierarchy: FeO > MnO > CaO > MgO. Two factors influence the viscosity: The first is related to the mutual interaction among different ions, and the stronger the interaction, the higher the viscosity. The second factor is the size (radius) of basic oxide cation, with viscous flow becoming increasingly more difficult (i.e., viscosity increases) as the cation size increases. However, there is a paradox in the effect of cation radius (of the basic oxide) on the two factors. Thus, varying cation size causes competitive effects; smaller cationic radii give stronger interactions among ions but less hindrance to viscous flow (and vice versa for large cation radii).     

Effect of strain rate on damage evolution in a cast Al-Si-Mg base alloy

An important aspect of damage evolution in cast Al-Si-Mg base alloys is fracture/cracking of Si particles. This microstructural damage is quantitatively characterized as a function of strain rate in the range 10⁻⁴ to 3.7 × 10⁺³, at an approximately constant uniaxial compressive strain level (20 to 25 pct). It is shown that the fraction of damaged silicon particles, their average size, and size distribution do not vary significantly with the strain rate, and at all strain rates studied, larger Si particles are more likely to crack than the smaller ones. However, the stress-strain curves are sensitive to the strain rate. These observations have implications for modeling of deformation and fracture of cast components under high strain rate crash conditions.     

Mechanical Behavior and Microstructural Change of a High Nitrogen CrMn Austenitic Stainless Steel during Hot Deformation

The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900 to 1200 °C) and strain rate range 0.01 to 10 s⁻¹ was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing maps, flow instability was observed at higher strain rate regions (1 to 10 s⁻¹) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain size on Z value was obtained.     

Micromechanical Behaviors of Fe20Co30Cr25Ni25 High Entropy Alloys with Partially and Completely Recrystallized Microstructures Investigated by In-Situ High-Energy X-ray Diffraction

This work conducts a processing strategy to obtain the partially and completely recrystallized microstructures by cold rolling and annealing at different temperatures for a near-equiatomic Fe₂₀Co₃₀Cr₂₅Ni₂₅ high entropy alloy (HEA). The in-situ synchrotron-based high-energy–X-ray diffraction (HE-XRD) technique was adopted to investigate the mechanical behaviors and microstructural evolution of Fe₂₀Co₃₀Cr₂₅Ni₂₅ HEAs with heterogeneous (partially recrystallized) and homogeneous (completely recrystallized) microstructures during tensile deformation. The heterogeneous and homogeneous microstructures were obtained by annealing for 1 hour at 600 °C and 650 °C, respectively (hereinafter 600 and 650 A, respectively). No obvious phase transformation was found during the tensile deformation. The partially recrystallized HEA has a higher initial dislocation density (2.30 × 10¹⁵ m⁻²) than that in the completely recrystallized HEA (9.57 × 10¹⁴ m⁻²). The grains orientated with [200] parallel to the loading direction (LD) yield before the macroyielding (under ~ 951 MPa) in 600 A with the incomplete recrystallized microstructures. The results of transmission electron microscopy (TEM) and HE-XRD confirm that the deformation in the partially recrystallized microstructures mainly relies on dislocation slip, leading to a small number of deformation twins and very high-density dislocations at fracture, while a large number of deformation twins occurred in the completely recrystallized structures during deformation, leading to the obvious strain hardening.

     

Flow and fracture of a multiphase alloy MP35N for study of workability

The flow and fracture of MP35N (35 Co, 35 Ni, 20 Cr, 10 Mo) has been studied by uniaxial com-pression and plane strain bending in the temperature range 1000 to 1200 °C and strain rate range 0.01 to 10 s^•1. This covers the normal bar rolling production conditions (∼1100 °C and 1 to 5 s“^•1). The strain to fracture in plane strain bending was found to increase with increasing strain rate, roughly coinciding with the increase of the strain to the peak stress in the flow curves. Within most of the temperature and strain rate ranges investigated and under plane strain bending deformation conditions, microvoid nucleation was found to be concurrent with or greatly enhanced by the onset of dynamic recrystallization. Under these deformation conditions, flow concentration or localization along the soft layers of newly recrystallized grains oriented along the maximum shear stress directions near the surface generated microvoid nucleation and damage, in effect overriding the stress relieving and crack isolation effects normally associated with the occurrence of dynamic recrystallization. As the tem-perature was decreased toward 1000 °C and the strain rate was increased toward 10 s^•1, an apparent transition to a microvoid nucleation mode by wedge cracking was observed, even at the maximum rate of 10 s^•1. A further decrease in deformation temperature to 900 °C at a strain rate of 10 s^•1, however, removed all evidence of microvoid nucleation (of the wedge type or otherwise) as well as any trace of dynamic recrystallization within the maximum strain imposed in the plane strain bending tests.     

High-Temperature (1023 K to 1273 K [750 °C to 1000 °C]) Plastic Deformation Behavior of B-Modified Ti-6Al-4V Alloys: Temperature and Strain Rate Effects

A minor addition of B to the Ti-6Al-4V alloy, by ~0.1 wt pct, reduces its as-cast prior β grain size by an order of magnitude, whereas higher B content leads to the presence of in situ formed TiB needles in significant amounts. An experimental investigation into the role played by these microstructural modifications on the high-temperature deformation behavior of Ti-6Al-4V-xB alloys, with x varying between 0 wt pct and 0.55 wt pct, was conducted. Uniaxial compression tests were performed in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C) and in the strain rate range of 10^–3 to 10⁺¹ s^–1. True stress–true strain responses of all alloys exhibit flow softening at lower strain rates and oscillations at higher strain rates. The flow softening is aided by the occurrence of dynamic recrystallization through lath globularization in high temperature (1173 K to 1273 K [900 °C to 1000 °C]) and a lower strain rate (10^–2 to 10^–3 s^–1) regime. The grain size refinement with the B addition to Ti64, despite being marked, had no significant effect on this. Oscillations in the flow curve at a higher strain rate (10⁰ to 10⁺¹ s^–1), however, are associated with microstructural instabilities such as bending of laths, breaking of lath boundaries, generation of cavities, and breakage of TiB needles. The presence of TiB needles affected the instability regime. Microstructural evidence suggests that the matrix cavitation is aided by the easy fracture of TiB needles.     

Deformation Mechanism in the Crack-Tip Region of Fine-Grained Magnesium Alloy

The deformation mechanism in the crack-tip region of a fine-grained Mg-2.4 at. pct Zn binary alloy was investigated by focused ion beam (FIB) and transmission electron microscopy (TEM) observation and finite element analysis (FEA) at the beginning of the fracture toughness test. The deformed microstructure observations showed the formation of subgrains instead of deformation twins in the fracture toughness tested sample, which was performed at a conventional crosshead speed of 1 mm/min. By preventing the formation of deformation twins at the beginning of the test, the crack tip of the fine-grained magnesium alloys became blunted, and thus, the alloys obtained high fracture toughness. Finite element results showed that the temperature increased 50 to 110 K, and the strain rate became two orders of magnitude higher; however, this temperature increment was not sufficient to form high-angle grain boundaries, i.e., a complete occurrence of dynamic recrystallization. On the other hand, the deformed microstructure observations in the sample, which was tested at a crosshead speed of 50 mm/min, showed the formation of nano-order {10-12} deformation twins and subgrains. The formation of deformation twins was caused, in part, by the severe strain from the operation of a high strain rate in the crack-tip region.     

Tensile properties and fracture toughness of a Ti-45Al-1.6Mn alloy at loading velocities of up to 12 m/s

A γ-base TiAl alloy with duplex microstructure of lamellar colonies and equiaxed γ grains was prepared with a reactive sintering method. Tensile tests and fracture toughness tests at loading velocities up to 12 m/s (strain rate for tensile tests up to 3.2×10²/s) were carried out. The micro-structure of the alloy before and after tensile deformation was carefully examined with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fractography of the tensile specimens and fracture toughness specimens was studied. The experimental results demonstrated that the ultimate tensile strength (UTS) and yield strength (YS) increase with increasing strain rate up to 10/s and subsequently level off. The UTS and YS exhibited similar strain rate sensitivity. The strain rate sensitivity exponent at strain rates lower than 10/s is about 1.5×10⁻² and at higher strain rates is almost zero. In this study, fracture toughness was found to be less sensitive to the loading velocity, having values of around 25 MPa √m, which is believed to be attributed to the high strain rate experienced at the crack tip. The predominant deformation mechanism for the strain rates used in this study was found to be twinning. However, in the low strain rate range, the dislocation motion mechanism was operative at the initial deformation stage and twinning dominated the later stage of the deformation process. In the high strain rate range, the entire deformation process was dominated by twinning. The interaction between deformation twinning and grain boundaries resulted in intergranular fracture in the γ grains and delamination of α ₂/γ interfaces in the lamellar colonies.     

Influence of strain rate and temperature on the deformation behavior of a metastable high carbon iron-nickel austenite

Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10⁻⁴ s⁻¹ and 10⁻¹ s⁻¹ over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred, the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation. Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms that the substructure following deformation will depend on the proximity of the deformation temperature to theM _s ^σ temperature. At temperatures much greater thanM _s ^σ , austenite twinning will occur, while at temperatures close toM _s ^σ , bcc martensite will form.     

Deformation and fracture behavior of beams composed of aluminum foam core and ceramic Al<Subscript>2</Subscript>O<Subscript>3</Subscript> under monolithic bending

Deformation and fracture mechanisms of sandwich and multilayer beams composed of aluminum foam core and ceramic face sheets under four-point bending condition were investigated in situ by surface displacement analysis (SDA) software. The toughening mechanism of the beams was discussed and a model was given for the computation of the fracture energy of the beams. Beams containing foam core with 5-, 10-, and 20-mm thickness and Al₂O₃ face sheets of 0.5- and 1-mm thickness were prepared. The results show that collapse of the beams is by two basic modes, indentation (ID) and face plate failure (PF). The SDA results illustrated that indentation is localized compression on the portion of the beam adjacent to the loading rollers, where displacement and strain are at the maximum. In PF, the beam entirely bends. It is also found that before collapse of the beams with pure PF mode, the foam core undergoes uniform compressive deformation, which contributes most to the fracture energy of the beams. As for the beams with ID characteristic, the localized compressive deformation plays a key role rather than the uniform compressive deformation in the fracture energy of the beam. The total fracture energy W of a beam under bending condition is proposed as

Finite Element Analysis for Effect of Roll Radius on Metal Deformation of Hot Rolling Plate

Theeffectofrollradiusonrollingforcehasbeenstudiedlargely ,buttheeffectofrollradiusondeforma tionofrollingpieceandontexturewasmentionedlit tle[1] .Therollradiusmayhaveaspecialinfluenceonthedeformationofrollingpiecebyaffectingtheformofdeformationzoneandaddi…     

Responses of damage and energy of sandwich and multilayer beams composed of metallic face sheets and aluminum foam core under bending loading

This present article deals with bending deformation and failure behavior of sandwich and multilayer beams composed of aluminum foam core and metallic face sheets analyzed byin-situ surface displacement analysis (SDA). The effect of beam structure on the failure mode of beam and the energy absorbed by beam failure were investigated and discussed. The SDA results revealed that collapse of the sandwich beams is by two basic modes, indentation (ID) and core shear (CS). The ID is localized deformation on the beam adjacent to the inner or outer roller in four-point bending, where displacement and compressive strains are at the maximum. As for CS mode, failure occurs in the core between inner and outer rollers, which corresponds to the maximum shear strain; discontinuous displacements in both the vertical and horizontal directions are the primary factors for shear crack initiation, growth, and broadening. The failure of the multilayer beams depends on whether the face sheets show ID mode or otherwise. If a single layer core sandwich fails in ID mode, the multilayer beams with similar face sheets show mixed ID + CS modes. If a single layer core sandwich fails fully in CS mode, no ID characteristic appears in the similar face sheet multilayer beams. The deformation energy of the beams relates strongly to the structure and geometry of beam. The predication of the bending fracture workW _x of a beam is given by