Sintering behaviour of Al-6061 powder produced by rapid solidification process

Abstract

Complex aluminium alloy components fabricated by powder metallurgy (P/M) offer the promise of a low cost and high strength-to-weight ratio, which meets the demands of the automotive sector. This paper describes the die compaction and sintering response of an atomised Al-6061 alloy powder containing Mg and Si produced by rapid solidification. A design of experiments is used involving three levels for each of the die compaction pressure, sintering temperature, peak temperature hold time and heating rate. Three trials were used to obtain the optimum press sinter processing conditions. Besides the mechanical properties, phase transformation and microstructure are investigated. Supplemental insight is gained through thermogravimetric analysis, differential scanning calorimetry and SEM with energy dispersive spectroscopy. Analysis of variation is used to quantify the contribution of each design variable to the mechanical properties.     

Effect of Surface Tension on Gas Atomization of a CrMnNi Steel Alloy

The aim of the current research is the experimental investigation of the mass median particle size d₅₀ as a function of surface tension for liquid Cr–Mn–Ni steel alloy with 16% Cr, 7% Mn, and 9% Ni. To modify the liquid steel design sulfur was add to the Cr–Mn–Ni steel in five steps up to a 1000 mass ppm. The surface tension of the liquid steel alloy was measured using maximum bubble pressure method and yttria stabilized capillary in a temperature range from 1701 to 1881 K. In addition, the same steel charges were sprayed to steel powder using a vacuum inert gas atomization using pure argon gas. The increase of sulfur in Cr–Mn–Ni steel will decrease the surface tension to 0.91 N m^?1. The temperature coefficient of surface tension is positive for all investigated Cr–Mn–Ni alloys due to a sulfur content ≥100 mass ppm. The final mass median particle size d₅₀ decreases from 54.3 µm for AISI 304 reference steel alloy to 17.1 µm for Cr–Mn–Ni steel alloy (16‐7‐9 S10) with the highest sulfur content and the lowest surface tension of all investigated liquid steels. It is concluded from the present work that surface tension is the decisive factor in adjusting d₅₀ at a constant spraying parameters.     

Determination of the forging-heating schedule or a large “HASTELlOY” alloy X ingot

The thermal stresses generated by nonuniform temperature distribution during the heating of cold large-diameter HASTELLOY alloy X ingots were determined; the stress figures were obtained by way of temperature profiles calculated during representative heating cycles. The thermal stress along the longitudinal axis at the center of the ingot (in tension) was selected as the critical parameter, and a maximum allowable stress of 90 pct of the 0.2 pct yield strength was imposed to avoid fracture. From this analysis, a heating practice which maximizes the in-furnace heating rate and still avoids ingot fracture can be designed.     

Thermophysical Properties of a Fe‐Cr‐Mo Alloy in the Solid and Liquid Phase

Results of a thermophysical characterization of a Fe‐Cr‐Mo alloy in the solid and liquid phases are reported. Methods applied include calorimetry, dilatometry; the laser flash technique for thermal diffusivity measurement and ultrasound pulse echo for the measurement of the room temperature sound velocities and elastic constants. Density in the liquid phase and surface tension were measured by optical dilatometry and by the oscillating drop method on electromagnetic levitated specimen. In addition, surface tension and viscosity were measured by the oscillating drop method on board parabolic flights under reduced gravity conditions. The methods applied and results obtained are presented. This work represents a collaborative effort, including round robin measurements in different laboratories for a characterization of the basic thermophysical properties needed for process simulation.     

Influence of phosphorus addition on the surface tension of liquid iron and segregation of phosphorus on the surface of Fe-P alloy

This article presents a study of the surface tension and phosphorus surface segregation in Fe-P alloys. The surface tension was measured by the sessile drop technique. The result of the dynamic surface tension for the low phosphorus content alloys shows that the alloy surface vaporization has a clear effect on the surface tension and causes a positive surface tension temperature coefficient. However, from this article, it is evident that phosphorus in liquid iron acts as a surface active element similar to arsenic. The surface segregation was determined using Auger electron spectroscopy. The result on the surface analysis of as-solidified sample indicates that the adsorption of impurity elements, such as oxygen, carbon, and nitrogen, can conceal phosphorus segregation on the free surface. Phosphorus segregation was also examined in the samples as-cleaned by Ar⁺ and then treated 30 minutes at 650°C. Phosphorus was found to segregate extensively on the surface of the alloys. On the basis of the analysis of the published data, the surface active intensity sequence of some nonmetallic elements was arrayed, and the surface active intensity of fluorine and boron in liquid iron was estimated.     

Enhancement of Superplastic Properties in Ultrahigh Carbon Steel through Silicon Addition

The superplastic behaviour of ultrahigh carbon steels (UHCS) has been greatly improved by silicon additions and thermomechanical processing. A UHC steel containing 3 wt% Si shows superplastic behaviour in a wide temperature range from 650 to 900°C. This behaviour is observed even at high strain rates, i.e. 10^‐2 s^‐1, in the temperature range between 800 and 825°C. Furthermore, the flow stress required for superplastic deformation is reduced drastically, i.e. σ=12 MPa at a strain rate of 10^‐4 s^‐l. Finally, it is found that the flow stress at a given strain rate is relatively constant over a wide range of temperatures (750‐900°C) due to a unique transformation behaviour in the UHCS‐3Si alloy.     

Parameters of the polymorphic transformation in VT1-0 alloy in heating and cooling cycles

The polymorphic transformation in VT1-0 alloy in heating and cooling cycles are studied experimentally. Pulsed resistive introduction of thermal energy is used to provide heating rates of 10³–10⁴ K/s and current interruption on reaching a given temperature. After an electric current is switched off, a heated sample is cooled owing to heat removal by radiation. The temperature is controlled by contactless high-speed optical pyrometry. The experimental technique and the results obtained are described in details. It is shown that a change in the direction of thermal action on the metal is accompanied by a hysteresis in the polymorphic transformation temperature in VT1-0 alloy. The studies carried out permit the conclusion that the polymorphic transformation temperature in VT1-0 alloy depends on the thermal action intensity and direction.     

Low-cycle fatigue of nickel superalloy single crystals at elevated temperatures

Single-crystal samples of nickel superalloys containing rhenium or rhenium plus ruthenium are subjected to low-cycle fatigue (LCF) tests under rigid cycle conditions at temperatures of 850 and 1050°C. It is found that the single crystals of the alloy containing rhenium with ruthenium have higher LCF resistance at 10⁴ cycles as compared to the alloy containing only rhenium. At a test temperature of 850°C, volume stress concentrators in the form of pores or their clusters represent fatigue crack nucleation zones; at 1050°C, surface corrosion cracks are the main fracture zones. The fatigue microcrack growth rate is anisotropic: it is higher in the [001] direction and lower in the [011] direction.     

Study of a splat cooled Cu-Zr-noncrystalline phase

By rapid quenching from the melt, using the splat forming gun technique, a noncrystalline phase has been obtained in a Cu-Zr alloy containing 60 at. pct Cu. Upon heating, rapid crystallization of the samples takes place at 477°C with a heat release of about 700 cal per mol. The variation of the electrical resistivity of the samples with temperature confirms the transformation. Very high resolution electron microscopy studies of the structural changes of the samples, at 3 × 10⁶ magnification, upon heating are presented and show the gradual crystallization of the amorphous structure. A. REVCOLEVSCHI was Visiting Scientist at Massachusetts Institute of Technology on leave from CNRS-Paris, France     

Formation of metallic phase on reducing-gas injection in multicomponent oxide melt. Part 2. Density and surface properties

The density and surface tension of melts of ferronickel (0–100% Ni) and oxidized nickel ore are measured by the sessile-drop method, as well as the interface tension at their boundary in the temperature range 1550–1750°C. The composition of the nickel ore is as follows: 14.8 wt % Fetot, 7.1 wt % FeO, 13.2 wt % Fe₂O₃, 1.4 wt % CaO, 16.2 wt % MgO, 54.5 wt % SiO₂, 4.8 wt % Al₂O₃, 1.5 wt % NiO, and 1.2 wt % Cr₂O₃. In the given temperature range, the density of the alloys varies from 7700 to 6900 kg/m³; the surface tension from 1770 to 1570 mJ/m²; the interface tension from 1650 to 1450 mJ/m², the density of the oxide melt from 2250 to 1750 kg/m³; and its surface tension from 310 to 290 mJ/m². The results are in good agreement with literature data. Functional relationships of the density, surface tension, and interphase tension with the melt temperature and composition are derived. The dependence of the alloy density on the temperature and nickel content corresponds to a first-order equation. The temperature dependence of the surface tension and interphase tension is similar, whereas the dependence on the nickel content corresponds to a second-order equation. The density and surface tension of the oxide melt depend linearly on the temperature. The results may be used to describe the formation of metallic phase when carbon monoxide is bubbled into oxide melt.     

Effect of Water Vapor During Secondary Cooling on Hot Shortness in Fe-Cu-Ni-Sn-Si Alloys

Residual Cu in recycled steel scrap can cause hot shortness when the iron matrix is oxidized. Hot shortness can occur directly after the solid steel is formed from continuous casting as the steel undergoes a cooling process known as secondary cooling where water is first sprayed on the surface to promote cooling. This is followed by a radiant cooling stage where the steel is cooled in air to room temperature. This investigation examines the roles of water vapor, Si content, temperature, and the presence of Sn in a Fe-0.2 wt pct Cu-0.05 wt pct Ni alloy on oxidation, separated Cu and Cu induced-hot shortness during simulations of the secondary cooling process. The secondary cooling from 1473 K (1200 °C) resulted in a slight increase in liquid quantity and grain boundary penetration as compared to the isothermal heating cycles at 1423 K (1150 °C) due to the higher temperatures experienced in the non-isothermal cycle. The addition of water vapor increased the sample oxidation as compared to samples processed in dry atmospheres due to increased scale adherence, scale plasticity, and inward transport of oxygen. The increase in weight gain of the wet atmosphere increased the liquid formation at the interface in the non-Si containing alloys. The secondary cooling cycle with water vapor and the effect of Sn lead to the formation of many small pools of Cu-rich liquid embedded within the surface of the metal due to the Sn allowing for increased grain boundary decohesion and the water vapor allowing for oxidation within liquid-penetrated grain boundaries. The presence of Si increased the amount of occlusion of Cu and Fe, significantly decreasing the quantity of liquid at the interface and the amount of grain boundary penetration.     

Determination of the Composition of the Surface Layer of Binary Metal Alloys

We have analyzed the effect of various parameters (surface tensions of the components, molar volumes of the components and the solution, compression and depression in alloy formation, activity coefficients of the components in the melt) on the shape of the surface tension isotherm as a function of the surface layer composition. We compare the analysis results with experimental data. We propose a method for estimating the composition of the surface layer based on a model assuming an additive variation in the surface tension of the solution as a function of the composition of the surface layer. We have calculated the concentration of the surface-active component in the surface layer of melts in the systems Ge - Sn, Ge - Pb, and Ge - Bi.__________Translated from Poroshkovaya Metallurgiya, Nos. 1–2(441), pp. 60–67, January–February, 2005.     

Comparison of Surface Tension Measured Values for Molten Tin at Different Oxygen Potentials

The surface tension of molten tin has been determined by a set of a self‐developed digital equipment with the sessile drop method at an oxygen partial pressure of 1.0×10^?6 MPa at different temperatures. The dependence of surface tension of molten tin on temperature was discussed as well. Based on the summarized relationships of the surface tension of molten tin to temperature and oxygen partial pressure reported in the literature, the reasons for the differences in those reported data were analysed. The emphasis was placed on the comparison of surface tension of the same molten tin sample measured by using different equipments according to the sessile drop method. Results of the comparison indicate that the measurement results obtained with the sessile drop method under similar experimental conditions are coincident, and the self‐developed digital equipment for surface tension measurement has a higher stability and accuracy. The relationships of surface tension of molten tin and its temperature coefficient with temperature and oxygen partial pressure were also elucidated from the thermodynamic analysis in this paper.     

Effect of silicon on oxidation behavior of GH3230 alloy

The high temperature oxidation behavior of GH3230 was investigated at 1100?? in air by means of TGA, XRD and SEM. The precipitation process of oxides is visually described. The results show that: the effect of Si element on high temperature oxidation of alloy is mainly reflected in two aspects. On the one hand, Si slows down the diffusion of Mn element in the oxidation process of alloy and causes difference in the composition of oxide scale after high temperature oxidation of GH3230 alloy; the main components of the oxide scale on the surface after oxidation at 1100?? of the alloy that exclude Si elements are MnCr2O4 and Cr2O3, and of the alloy that contain Si element are Cr2O3. On the other hand, SiO2 is formed in the early stage of the oxidation of the alloy, which plays an important role in the further oxidation of the alloy and contributes to suppress the volatilization of chromium oxide in the later stage of oxidation.     

Fe3Al基合金的高温耐冲蚀磨损性能

利用通以含高浓度ＳＯ２和ＳｉＯ２细颗粒气流的专门装置，研究了Ｆｅ３Ａｌ基金属间化合物的耐高温腐蚀磨损性能。试验结果表明，Ｆｅ３Ａｌ基金属间化合物的抗高温磨损性能优于钴基合金（ＣｏＦｅ１５Ｃｒ２５Ｓｉ７）、高铬镍铁基耐热合金（ＦｅＣｒ２８Ｎｉ７）以及表面喷涂Ｎｉ６０的１８－８不锈钢。     

Localized Recrystallization in Cast Al-Si-Mg Alloy during Solution Heat Treatment: Dilatometric and Calorimetric Studies

During heat treatment, the work piece experiences a range of heating rates depending upon the sizes and types of furnace. When the Al-Si-Mg cast alloy is heated to the solutionizing temperature, recrystallization takes place during the ramp-up stage. The effect of heating rate on recrystallization in the A356 (Al-Si-Mg) alloy was studied using dilatometric and calorimetric methods. Recrystallization in as-cast Al-Si alloys is a localized event and is confined to the elasto-plastic zone surrounding the eutectic Si phase; there is no evidence of recrystallization in the center of the primary Al dendritic region. The size of the elasto-plastic zone is of the same order of magnitude as the Si particles, and recrystallized grains are observed in the elasto-plastic region near the Si particles. The coefficient of thermal expansion of Al is an order of magnitude greater than Si, and thermal stresses are generated due to the thermal mismatch between the Al phase and Si particles providing the driving force for recrystallization. In contrast, recrystallization in Al wrought alloy (7075) occurs uniformly throughout the matrix, stored energy due to cold work being the driving force for recrystallization in wrought alloys. The activation energy for recrystallization in as-cast A356 alloy is 127 KJ/mole. At a slow heating rate of 4.3 K/min, creep occurs during the heating stage of solution heat treatment. However, creep does not occur in samples heated at higher heating rates, namely, 520, 130, and 17.3 K/min.     

Spheroidization of Eutectic Silicon in Direct-Electrolytic Al-Si Alloy

The spheroidization process of direct-electrolytic Al-Si alloy (DEASA) containing Si content in the range of 7 to 12 pct heated at temperatures of 778 K to 803 K (505 °C to 530 °C) was studied. The width, length, and aspect ratio of Si particles were measured to quantitatively analyze the microstructural variety of Si phase during the heating process in terms of chemical composition and remelting. Compared to existing Al-Si alloy, the lower soaking temperature of 778 K to 783 K (505 °C to 510 °C) is required to obtain the full spheroidization of the Si phase of DEASA. When remelting DEASA, a satisfactory granulation rate can be achieved at a higher soak temperature of 788 K to 803 K (515 °C to 530 °C). The origin of the high spheroidizaton rate is attributed to the microstructural characteristic relative to the electrolysis process. It would be expected that high crystallographic defects of Si grain result in the complete spheroidization of Si phase at lower temperatures for a short period.     

差示扫描量热法测试非晶合金居里温度

从坩埚选择、试样称取量、升温速率等方面摸索了差示扫描量热法(DSC)测试非晶材料 Fe78 Si9 B13居里温度的最佳试验条件,分析了不同升温速率对测试结果的影响,同时与该材料晶化温度、In(标准物质)的熔点随升温速率的变化情况进行了对比分析.研究结果表明：该材料的 DSC 曲线上居里转变过程对应的热信号表现为一个抖动.不同条件下测得的居里温度为399~402℃,与 Hall 效应法测试结果基本一致.最佳的试验条件是：Al坩埚、试样称取量6 mg 左右、20 K/min.在升温速率不小于10 K/min 时可以得到居里转变信号,与该材料晶化温度受热滞后影响而明显随升温速率增大而增大的情况不同,由于居里转变过程相变潜热较小,居里温度测试结果基本不受热滞后的影响.     

Wear resistant sintered aluminium parts for automotive applications

Abstract

The automotive industry's focus on weight saving has increased interest in optimised process routes and alloy compositions for light alloy PM components. Conventional pressing, sintering and sizing of aluminium alloys containing about 16%Si has been applied to produce components with high wear resistance and mechanical strength. CISIZE^®, a novel continuous isostatic pressure sintering process, combined with sizing, produces aluminium alloys with finely dispersed Si particles having excellent ductility. Combining parts obtained by these two process routes can give interesting tribological systems. Complete sintered aluminium cam phaser systems, including the sprocket wheel, are being produced in series using this approach, which also shows promise for automotive parts such as oil pumps and rotors.     

The segregation of phosphorus to the free surface of a ferritic iron alloy at 723 to 823 K

The isothermal segregation of phosphorus to the free surface of an annealed polycrystalline Fe-Ni-Cr-C alloy, containing 0.06 wt pct P, has been studied by Auger electron spectroscopy between 723 and 823 K using anin situ heating technique. It is found that after high temperature argon-ion bombardment the kinetics of isothermal segregation are consistent with bulk (lattice) diffusion of phosphorus as the rate determining mechanism. An approximate value for the adsorption isotherm constant is deduced to be 8×10² per wt pct by a limited analysis of the kinetic data in terms of linear bulk diffusion with a boundary condition given by an ideal adsorption isotherm.