Ti-22Al-25Nb合金板材气压胀形实验及数值模拟

通过气压胀形实验和数值模拟研究了Ti-22Al-25Nb合金板材在高温下的热成形行为,并分析了在930和970℃下的胀形球壳形状和壁厚分布,讨论了胀形部分的微观结构和力学性能。结果表明,在胀形初期,胀形球壳接近球面,随胀形高度的增加逐渐椭球化。在930和970℃下,最终胀形高度分别达到46.25和49.85 mm,壳顶的曲率半径分别达到49.33和49.19 mm。胀形部分壁厚不均匀,从底部至顶部逐渐减小。变形温度对球壳形状影响较大,在930℃时胀形局部形状更加不均。在相同的胀形高度下,930℃下的胀形球壳曲率半径较小且壁厚减薄率较高。此外,在930℃胀形过程中,O相析出并球化,导致O型、V型空洞的产生且硬度下降。而在970℃胀形后,微观组织分布均匀,O相在冷却过程中以层片形式析出,强化了胀形件。     

Forming-Limit Diagrams for Magnesium AZ31B and ZEK100 Alloy Sheets at Elevated Temperatures

Modern design and manufacturing methodologies for magnesium (Mg) sheet panels require formability data for use in computer-aided design and computer-aided engineering tools. To meet this need, forming-limit diagrams (FLDs) for AZ31B and ZEK100 wrought Mg alloy sheets were developed at elevated temperatures for strain rates of 10^?3 and 10^?2 s^?1. The elevated temperatures investigated range from 250 to 450 °C for AZ31B and 300 to 450 °C for ZEK100. The FLDs were generated using data from uniaxial tension, biaxial bulge, and plane-strain bulge tests, all carried out until specimen rupture. The unique aspect of this study is that data from materials with consistent processing histories were produced using consistent testing techniques across all test conditions. The ZEK100 alloy reaches greater major true strains at rupture, by up to 60%, than the AZ31B alloy for all strain paths at all temperatures and strain rates examined. Formability limits decrease only slightly with a decrease in temperature, less than 30% decrease for AZ31B and less than 35% decrease for ZEK100 as the temperature decreases from 450 to 300 °C. This suggests that forming processes at 250-300 °C are potentially viable for manufacturing complex Mg components.     

The development of a forming limit surface for 5083 aluminum alloy sheet

A custom mechanical stretching setup based on the Nakazima method was designed and built for testing sheet metals at elevated temperatures. Specimens from a fine-grained 5083 aluminum alloy sheet were deformed at various temperatures, spanning between those associated with warm forming (250°C) and hot forming (550°C). Circle grid analysis of the deformed specimens produced the forming limit curves at each of the covered temperatures, hence revealing the great influences of forming temperature on the material’s formability limits. Finally, all the curves were combined to construct a unique three-dimensional forming limit surface, which we present here as a more comprehensive map for describing material formability limits at wide-ranging temperatures.     

Effect of heating on palygorskite and acid treated palygorskite properties

The behavior of palygorskite as an adsorbent or catalyst support is governed mainly by the magnitude of its surface areas and degree of surface activity. Most heterogeneous catalytic reactions take place at elevated temperatures. It is important and necessary to known the possible changes in the physicochemical properties and hence the catalytic activity due to the thermal treatment. The present work investigates the effect of heat treatment on the surface and textural properties of palygorskite and acid activated palygorskite. Samples were heated at the required temperatures (150°C, 350°C, 550°C and 750°C) for four hours. The chemical analysis, X-ray diffraction, thermal analysis, and textural properties evaluated on the basis of the nitrogen adsorption have been reported for different samples. Acid-treated samples showed a shift of the DTA curves maxima to low temperatures with increasing acid treating time. The weight loss observed between 300–900°C was approximately half of the value observed for the untreated mineral over the same temperature range. On heating, water molecules are removed causing changes in the BET surface area and the porosity. The modification as a function of the temperature differs with the samples. However, at 750°C, a noticeable decrease of the surface area, which is attributed to closure of the mesoporosity, was observed for all the samples. The text was submitted by the authors in English.     

Evolution of springback and neutral layer of AZ31B magnesium alloy V-bending under warm forming conditions

The evolution of springback and neutral layer for AZ31B magnesium alloy sheet was investigated by V-bending tests at temperatures from 50 to 300 °C. Moreover, in order to perceive the influence of the punch radii on springback and offset of neutral layer, the tests with punch radii at 7.5, 8.1, 8.7 and 9.3 mm were conducted at 100 °C. The results show that the neutral layer shifts to the tension zone of the sheets. The coefficient of neutral layer (k-value) decreases with the increase of temperature and punch radii. This is mainly because of the asymmetry between the outer tension layer and inner compression layer during bending. The outer tension region is dominated by slip, while the inner compression region is dominated by twinning. With the increase of temperature, the asymmetry of tension–compression becomes weaker, and the offset of neutral layer decreases. The offset of neutral layer increases as punch radii decreases. The shift of neutral layer of AZ31B sheet results in the calculation of springback bigger than the reality.     

Material Models for Simulation of Superplastic Mg Alloy Sheet Forming

Accurate prediction of strain fields and cycle times for fine-grained Mg alloy sheet forming at high temperatures (400-500 °C) is severely limited by a lack of accurate material constitutive models. This paper details an important first step toward addressing this issue by evaluating material constitutive models, developed from tensile data, for high-temperature plasticity of a fine-grained Mg AZ31 sheet material. The finite element method was used to simulate gas pressure bulge forming experiments at 450 °C using four constant gas pressures. The applicability of the material constitutive models to a balanced-biaxial stress state was evaluated through comparison of simulation results with bulge forming data. Simulations based upon a phenomenological material constitutive model developed using data from both tensile elongation and strain-rate-change experiments were found to be in favorable accord with experiments. These results provide new insights specific to the construction and use of material constitutive models for hot deformation of wrought, fine-grained Mg alloys.     

Effect of thermal exposure on the microstructure,tensile properties and the corrosion behaviour of 6061 aluminium alloy sheet

Sheet material of the Al‐Mg‐Si alloy 6061 in the tempers T4 and T6 was thermally exposed at temperatures ranging from 85 to 120°C for 1000 h. The microstructure, tensile properties and the corrosion behaviour in the different heat treatment conditions were investigated using differential scanning calorimetry and transmission electron microscopy as well as performing tensile tests and various corrosion tests. The additional heat treatments, which should simulate aging during long‐term service usage, caused an increase in strength of 6061‐T4 sheet, associated with changes in the naturally aged microstructure. Thermal exposure at 120°C for 1000 h resulted in tensile and corrosion properties being similar to those obtained for peak‐aged sheet. Alloy 6061 in the T6 temper exhibited microstructural stability when additionally heat treated at 85 and 120°C for 1000 h. No significant alterations in the microstructure, tensile properties, and corrosion performance were observed after exposure to slightly elevated temperatures.     

5A06-O aluminium–magnesium alloy sheet warm hydroforming and optimization of process parameters

The uniaxial tensile test of the 5A06-O aluminium–magnesium (Al–Mg) alloy sheet was performed in the temperature range of 20–300 °C to obtain the true stress–true strain curves at different temperatures and strain rates. The constitutive model of 5A06-O Al–Mg alloy sheet with the temperature range from 150 to 300°C was established. Based on the test results, a unique finite element simulation platform for warm hydroforming of 5A06-O Al–Mg alloy was set up using the general finite element software MSC.Marc to simulate warm hydroforming of classic specimen, and a coupled thermo-mechanical finite element model for warm hydroforming of cylindrical cup was built up. Combined with the experiment, the influence of the temperature field distribution and loading conditions on the sheet formability was studied. The results show that the non-isothermal temperature distribution conditions can significantly improve the forming performance of the material. As the temperature increases, the impact of the punching speed on the forming becomes particularly obvious; the optimal values of the fluid pressure and blank holder force required for forming are reduced.     

Investigations on warm forming of AW-7020-T6 alloy sheet

7000 series aluminium alloys have greater strength than conventional aluminium alloys used in the automotive industry, but little has been reported on their formability. In this paper the strength and formability of age-hardenable AW-7020 alloy sheet in the T6 temper condition was investigated at temperatures between 150 and 250 °C by warm tensile, Swift-cupping and cross-die deep-drawing tests. Differential scanning calorimetry (DSC) investigations were carried out to study the precipitation state of AW-7020 sheet in as-received, warm cross-die deep-drawn and post-paint-baked conditions. Formability was found to improve at temperatures above 150 °C and was sensitive to temperature and strain rate. There was also an onset of dynamic recovery from 150 °C. DSC results showed the presence of η′ precipitates in T6 temper and that these coarsen during the warm cross-die deep-drawing and paint baking processes with ∼30% drop in ultimate tensile and yield strengths. Dynamic recovery and coarsening of η′ precipitates were found to contribute to the increase in formability at elevated temperatures.     

Deformation mechanism temperature-dependence of AZ31 magnesium alloy

The deformation behavior of AZ31 Mg alloy is studied here in relation to the temperature. A rolled plate with a thickness of 50 mm was first homogenized at 400 °C for 4 h before preparing test specimens with the tensile axis parallel to the rolling direction (RD). A series of tensile tests was then carried out at a strain rate of 10⁻²/s together with load relaxation tests to obtain flow curves in terms of the stress and strain rate at room temperature (RT), 100 °C, 200 °C, and 300 °C. The flow curves were found to represent the usual grain matrix deformation (GMD) behavior, consisting of the accumulation and relaxation of glide dislocations at temperatures of less than 100 °C. At temperatures greater than 200 °C, grain boundary sliding (GBS) was found to play an important role, as described in theories related to an internal variable. The GBS could be characterized as a non-Newtonian viscous flow with a power index value of M _g = 0.5.     

Room Temperature Ductility and Microstructure of Magnesium AZ31B Sheet

Formability of wrought magnesium alloys at room temperature or slightly elevated temperatures is modest, reaching about 20% elongation in a tension test and exhibiting poor resistance to strain localization and failure. The hexagonal close packed structure of Mg has few active slip systems at lower forming temperatures, limiting ductility and reducing applications in auto body structures. Much greater levels of ductility can be reached at higher temperatures (typically >300 °C), but this is expensive and inconvenient for a high-volume production environment. Tension testing and biaxial forming of annealed AZ31B magnesium alloy sheets were done at room temperature to various levels of strain. High-resolution electron back scatter diffraction (EBSD) was used to measure twin fraction and dislocation density, in order to find relationships between strain and potential failure locations within the microstructure. Twin fractions were found to have a weak positive correlation to uniaxial and biaxial tensile strain, while dislocation density was found to correlate more strongly with uniaxial tensile strain.     

Characteristics of the warm deep drawability of a transformation-induced plasticity steel sheet

Warm deep drawability in a square cup drawing was investigated using a newly developed high-strength steel sheet with retained austenite that was transformed into martensite during formation. For this investigation, six different temperatures between room temperature and 250°C, and five different drawing ratios ranging from 2.2 to 2.6 were considered. The results showed that the maximum drawing force and the drawing depth were affected by the change in temperature, and a more stable thickness strain distribution was observed at elevated temperatures. However, blue shortness occurred at over 200°C. FEM analysis using the LS-DYNA code was used to compare the experimental results with the numerical results for the thickness strain distribution.     

Direct extrusion of thin Mg wires for biomedical applications

Biodegradable wires, able to provide load-bearing support for various biomedical applications, are the novel trends in current biomaterial research. A thin 99.92% Mg wire with a diameter of 250 µm was prepared via direct extrusion with an extreme reduction ratio of 1:576. The total imposed strain in a single processing step was 6.36. Extrusion was carried out at elevated temperatures in the range from 230 to 310 °C and with various ram speeds ranging from ∼0.2 to ∼0.5 mm/s. The resulting wires show very good mechanical properties which vary with extrusion parameters. Maximum true tensile stress at room temperature reaches ∼228 MPa and ductility reaches ∼13%. The proposed single-step direct extrusion can be an effective method for the production of Mg wires in sufficient quantities for bioapplications. The fractographic analysis revealed that failure of the wires may be closely connected with inclusions (e.g., MgO particles). The results are essential for determining the optimal processing conditions of hot extrusion for thin Mg wire. The smaller grain size, as the outcome of the lower extrusion temperature, is identified as the main parameter affecting the tensile properties of the wires.     

A new technology for the joining by forming of magnesium alloys

Joining by forming of magnesium alloys is restricted by the limited forming capability of magnesium at room temperature. To form acceptable joints without cracks usually heating of the parts to temperatures of 220°C or more is required. The application of state-of-the-art joining by forming methods (such as self-pierce-riveting or clinching with a contoured die) implicates pre-heating times of at least 3–6 s to achieve joints of acceptable quality. A new joining by forming technology, that is working with a flat anvil as a counter tool instead of the contoured die shall be introduced in this paper. This new technology is offering important advantages especially in joining Mg/Mg, Al/Mg or Fe/Mg connections, most remarkably being the reduction of pre-heating times to less than 1 s, thus allowing for the fast and reliable joining of magnesium parts. Parameter influences on the formation of the connections have been investigated and the values for the tensile strength have been determined for a wide range of connections.     

The Effect of Strain-Rate Sensitivity on Formability of AA 5754-O at Cold and Warm Temperatures

Aluminum-magnesium (Al-Mg) alloys have been widely used in diverse applications ranging from automotive bodies to food processing industries because of their excellent high-strength-to-weight ratio, corrosion resistance, and recyclability potential. The formability of these alloys is decreased at room temperature (RT) and is related with the strain-rate sensitivity. This study presents the effect of strain-rate sensitivity on formability of AA 5754-O alloy sheet at a test temperature range of −60 to 250 °C by duplicate tensile test at different strain rates. The test results indicated that the formability change with positive or negative strain-rate sensitivity values. It was observed that the strain-rate sensitivity values increased at negative temperatures with respect to RT. The best formability condition for this alloy in the test ranges was observed at 250 °C and 0.0016 s⁻¹.     

Effects of aging temperature on the precipitation behavior of Ω phase in an Al-Cu-Mg-Ag alloy

Microstructures and mechanical properties of an Al-Cu-Mg-Ag alloy aged for 1 h at temperatures in a range 25 °C to 450 °C were characterized in the present work by means of hardness tests, electrical conductivity measurements, and transmission electron microscopy (TEM). In-situ X-ray diffraction (XRD) was also employed to examine the precipitation behavior of Ω phase in a temperature range of 25 °C to 400 °C The in-situ Xray diffraction peak at 2θ = 26°–28° detected at elevated aging temperatures between 165 °C and 400 °C was attributed to the formation of Ω phase. TEM observations demonstrated the existence of Ω phase in the alloy when aged for 1 h at temperatures in a range 145 °C to 450 °C.     

High temperature EMAT design for scanning or fixed point operation on magnetite coated steel

Bulk thickness measurements were performed at elevated temperatures on magnetite coated low carbon steel pipe and aluminium samples, using a permanent magnet electromagnetic acoustic transducer (EMAT). The design presented here exploits the non-contact nature of EMATs to allow continuous operation at elevated temperatures without physical coupling, sample preparation (in the form of oxide scale removal), or active cooling of the EMAT. A non-linear change in signal amplitude was recorded as the magnetite coated sample was heated in a furnace, whereas a steady decrease in amplitude was observed in aluminium. For a magnetite coated pipe sample, after a dwell time of 3 h, a SNR of 33.4 dB was measured at 450 °C, whilst a SNR of 33.0 dB was found at 25 °C. No significant EMAT performance loss was observed after one month of continuous exposure to 450 °C. EMAT-sample lift-off performance was investigated at elevated temperature on magnetite coated steel; a single-shot SNR of 31 dB for 3.0 mm lift-off was recorded at 450 °C, highlighting the suitability of this design for scanning or continuous fixed point inspection at high temperature.     

Zum Korrosionsverhalten von Nicrofer 3033 in hochkonzentrierter Schwefelsäure

Contribution to corrosion behaviour of Alloy 33 in high concentrated sulfuric acid Investigations on the corrosive behaviour of Alloy 33 were conducted in 93% and 95–99% technical sulfuric acid at temperatures of 70–200°C. 6‐h‐tests with gravimetric evaluation and measurement of the free corrosion potential and 48‐h‐tests with gravimetric evaluation served to determine the influence of the sulfuric acid concentration and the temperature on the resistance of sheet samples at presence and absence of SO₂. The influence of the flow rate was examined in 6‐h‐tests via rotation of pipe samples at circumferential speeds up to 2 m/s. The potential and gravimetric measurements showed that there is a resistance with ≤ 0.1 mm/a for Alloy 33 as from about 98.5% at temperatures from 100°C. However, about 125°C there seems to be a zone at which the resistance down to about 97.5% is kept at least for a long test duration. Examinations on the flow influence indicate that there is a reactivation in the said zone with increasing flow speed. However, the flow influence is low at the other temperatures. This extension of the resistance zone about 125°C proves that the temperature sensitivity of Alloy 33 is not monotonous in highly concentrated sulfuric acid. There are corrosion maxima and minima as a function of the sulfuric acid concentration at different temperatures. If SO₂ is present, there is only a limitation of the resistance zone at low test temperatures, 80°C, 100°C in the 6‐h‐test and 70°C, 80°C in the 48‐h‐test.     

Microstructure evolution of the Ir-inserted Ni silicides with additional annealing

Thermally-evaporated 10 nm-Ni/1 nm-Ir/(poly)Si structures were fabricated in order to investigate the thermal stability of Ir-inserted nickel silicide after additional annealing. The silicide samples underwent rapid thermal annealing at 300 ° C to 1200 ° C for 40 s, followed by 30 min annealing at the given RTA temperatures. Silicides suitable for the salicide process were formed on the top of the single crystal and polycrystalline silicon substrates, mimicking actives and gates. The sheet resistance was measured using a four-point probe. High resolution x-ray diffraction and Auger depth profiling were used for phase and chemical composition analysis, respectively. Transmission electron microscope and scanning probe microscope were used to determine the cross-section structure and surface roughness. The silicide, which formed on single crystal silicon substrate with surface agglomeration after additional annealing, could defer the transformation of Ni(Ir)Si to Ni(Ir)Si2 and was stable at temperatures up to 1200 °C. Moreover, the silicide thickness doubled. There were no outstanding changes in the silicide thickness on polycrystalline silicon. However, after additional annealing, the silicon-silicide mixing became serious and showed high resistance at temperatures >700 °C. Auger depth profiling confirmed the increased thickness of the silicide layers after additional annealing without a change in composition. For a single crystal silicon substrate, the sheet resistance increased slightly due to the significant increases in surface roughness caused by surface agglomeration after additional annealing. Otherwise, there were almost no changes in surface roughness on the polycrystalline silicon substrate. The Ir-inserted nickel monosilicide was able to maintain a low resistance in a wide temperature range and is considered suitable for the nano-thick silicide process.     

Effect of Ca addition on the oxidation resistance of AZ91 magnesium alloys at elevated temperatures

AZ91 magnesium alloys containing 0.27–5.22 wt.% Ca, were melted and cast to study the effects of Ca addition on oxidation resistance at elevated temperatures. An ignition temperature test showed that the ignition of AZ91 alloy occurred at about 350–450 °C below the melting point, whereas that of the Ca-containing AZ91 alloys did so at above 650 °C. Weight gain measurements indicated that the oxidation resistance of the AZ91 alloys improved with Ca addition. The oxidation rate was dependent on the oxidation temperature. In the temperature range of 300–400 °C, the oxidation rate increased linearly. By contrast, the weight of 5 wt.% Ca-containing AZ91 alloy increased slowly due to the formation of a protective oxide layer. The oxidized surfaces were analyzed with low-angle XRD, FE-SEM equipped with EDS and AES. Complex structures were found in the oxide layers of the Ca-containing alloys: the outer layer mainly consisted of CaO, which was of uniform thickness, and the inner layer was a mixture of CaO, MgO, and Al₂O₃. In contrast to the loose and porous MgO formed on the surface of AZ91, the compact and dense oxide layers acted as an effective barrier to the further oxidation of the Ca-containing AZ91 alloys.