Spray formed and rolled aluminium‐magnesium‐scandium alloys with high scandium content

Aluminium‐magnesium‐scandium alloys offer good weldability, high corrosion resistance, high thermal stability and the potential for high strength by precipitation hardening. A problem of aluminium‐scandium alloys is the low solubility of about 0.3 mass‐% scandium when using conventional casting methods. The solution of scandium can be raised by higher cooling rates during solidification. This was realised by spray forming of Al‐4.5Mg‐0.7Sc alloys as flat deposits. Further cooling rates after solidification should also be high to prevent coarse precipitation of secondary Al₃Sc. Therefore a cooling device was designed for the spray formed flat deposits. The flat deposits were rolled at elevated temperatures to close the porosity from spray forming. Microstructures, aging behaviour and tensile properties of the rolled sheets were investigated. Strength enhancements of about 100 MPa compared to conventional Al‐Mg‐Sc alloys were achieved.     

Spray forming of high density sheets

Porosity is one of the most important quality criteria of spray‐formed materials in the as‐sprayed condition. Typically, spray‐formed sheets have a porous rim close to the substrate and depending on the spray conditions cold or hot porosity may also be present in the core of the deposit. This porosity has to be removed or minimized to make further processing steps such as rolling, forging or extrusion possible. In this paper, the influence of both substrate temperature and deposit surface temperature on porosity in spray‐formed sheets is studied. For this purpose spray forming experiments (sheet size 1000 mm × 250 mm) were carried out using three different materials: aluminium‐bronze, tin‐bronze and a nitriding steel. For the copper‐base alloys preheated steel‐substrates with different temperatures were moved through a scanning spray cone. In the case of steel a ceramic substrate at room temperature was used. In addition to the variation of the substrate temperature, the gas to metal mass flow ratio (GMR) was varied to achieve different deposit surface temperatures. During the run the surface temperature in the deposition zone was measured using a scanning, multi‐wavelength pyrometer. Samples of the deposits were polished and rasterized by light microscopy. The local porosity was characterized by digital image analysis. The influence of the substrate temperature and the GMR on the porosity in the vicinity of the substrate is evaluated and discussed in detail. The impact of the deposit surface temperature on the porosity was analyzed and is discussed as well. It was found that the deposit surface temperature has a strong impact on porosity for spray‐formed sheets. Finally, experimental results were used to develop a new approach to predict the porosity in spray‐formed sheets. The results clearly show the dependence on material properties. This approach can be used to identify process parameters to generate high density sheets in the future.     

Modeling the shape of flat deposits during spray forming using a scanning gas atomizer

A three‐dimensional shape model, tracing the coordinates of the moving surface of a growing flat deposit spray‐formed using a scanning gas atomizer, has been developed in this study. Mass flux distributions in the spray cones generated by the scanning atomizer under a typical spray forming condition have been revealed for the shape modeling. Geometrical evolution of flat deposits in spray forming has been investigated based on analysis of the scanning mechanism of the atomizer. The influence of processing conditions on the shape and dimensions of flat deposits have been simulated and discussed. Finally, the shape modeling has been validated by experimental investigations.     

Droplet velocity and thermal state from hot gas atomization of steel melt: Impact on the quality of the spray-formed tubular deposit

The velocity and thermal behavior (temperature, enthalpy, solid fraction) of atomized droplets in a metal spray play the most important role in the spray forming process. These properties mainly determine the materials yield and the final product quality (e.g., porosity, microstructure) of the as-sprayed materials. Changing the gas temperature in the atomization process directly influences these droplet properties in the spray. To understand the droplet behavior in the spray at various atomization gas temperatures (i.e., room temperature RT 293 K, 573 K, 873 K), numerical simulations using computational fluid dynamics (CFD) techniques have been performed and validated by experiments. A series of atomization runs (powder production and spray-forming with AISI 52100 steel) has been conducted at different atomization gas temperatures and pressures with a close-coupled atomizer (CCA). The in-situ temperature detection of the deposit surface (pyrometer) and in the substrate (thermocouples) has been performed to observe the effect of particle properties on the deposit. The result shows that hot gas atomization provides smaller droplets with faster velocity in the spray, affecting the droplet impact and deformation time in the deposition zone. A higher solid fraction of the smaller droplets by hot gas atomization also reduces the deposit surface temperature. Increasing the substrate diameter further decreases the deposit surface temperature without compromising the deposit quality (i.e., porosity) and also refines the grain size. Pre-heating of the substrate up to 573 K results in lower porosity in the vicinity of the substrate.     

热等静压对喷射成形TiAl基合金孔隙率的影响

着重研究了气体辅助喷射成形和离心喷射沉积形成的Ｔｉ－４８Ａｌ－２Ｍｎ－２Ｎｂ金属间化合物的孔隙率以及热等静压工艺对合金孔隙率的影响，利用Ｑｕａｎｔｉｍｅｔ图象分析仪定量测试了各种状态样品的孔隙率。结果表明，气体雾化成形合金的孔隙率约３．２９％，离心喷射盛开有合金的孔隙率约２．２０％，孔隙率的大小与喷射成形工艺密切相关。     

High yield spray forming of small diameter tubes using pressure‐gas‐atomization

The economy of the spray forming process is restricted by the generation of overspray, which in many cases cannot be re‐introduced into the process by re‐melting or co‐injection. Especially for small deposits, such as small diameter tubes (diameter <100 mm), the amount of overspray can become large in conventional spray‐forming processes. In this work, an alternative process with a pressure‐gas‐atomizer operating at low melt flows is presented. Tubes with diameters of 50 mm and 90 mm were spray‐formed and analyzed regarding yield and porosity. It was found that yields up to 96% can be achieved with porosities below 1% if proper process parameters are identified and used. An evaluation of the yield and the corresponding achievable porosity is conducted to identify resource‐efficient sets of parameters.     

Gefüge und mechanische Eigenschaften eines durch Sprühkompaktieren hergestellten Stahles 100Cr6

Microstructure and mechanical properties of a high strength bearing steel SAE 52100 produced by spray deposition Spray depositing allows the production of steels having a structure which is free of any macro‐segregation, homogeneous and equiaxed. Nevertheless the spray deposition of steels shows some effects that have to be regarded as disadvantages. A porous region at the billet border and a rather coarse‐grained microstructure can be noticed. As a result, in the presented work the influence of a subsequent hot rolling and a following heat treatment on microstructure and mechanical properties has been investigated. For this purpose the bearing steel SAE 52100 has been used as master material. The initial microstructure has been transformed to a homogeneous equiaxed fine‐grained pearlitic microstructure without any remaining porosity. To get use of the characteristic properties of this high‐strength steel an isothermal bainitic hardening has been carried out. Information about the mechanical properties of this material has been gained by tensile and fatigue tests. For fatigue tests in the longitudinal and cross direction of the hot rolled bars a special miniature specimen has been developed.     

Advanced cold rolled steels for automotive applications

Advanced multiphase steels offer a great potential for bodies‐in‐white through their combination of formability and achievable component strength levels. They are first choice for strength and crash‐relevant parts of challenging geometry. The intensive development of high‐strength multiphase steels by ThyssenKrupp has led to hot dip galvanizing concepts with an outstanding forming potential. Hot rolled, hot dip galvanized complex phase steels are currently produced in addition to cold rolled DP and RA steels. New continuously annealed grades with tensile strength levels of up to 1000 MPa in combination with sufficient ductility for applications mainly in the field of structural automobile elements make use of the classic advantages of microalloying as well as the principles of DP and TRIP steels. Further improvement of properties will be reached by the new class of high manganese alloyed steels.     

Micromechanical Modelling of Time-Dependent Stress-Corrosion Behaviour of High-Strength Steel

This paper provides micromechanical bases to explain the time-dependent stress corrosion behaviour of high-strength prestressing steel wires. To this end, two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to slow strain rate tests in aqueous environments in corrosive conditions corresponding to localized anodic dissolution and hydrogen assisted cracking. While a tensile crack in the hot rolled bar always propagates in mode I, in the cold drawn wire an initially mode I crack deviates significantly from its normal mode I growth plane and approaches the wire axis or cold drawing direction, thus producing a mixed mode propagation. In hydrogen assisted cracking the deviation happens just after the fatigue precrack, whereas in localized anodic dissolution the material is able to undergo mode I cracking before the deflection takes place. Therefore, a different time-dependent behaviour is observed in both steels and even in the same steel in distinct environmental conditions. An explanation of such behaviour can be found in the pearlitic microstructure of the steels. This microstructural arrangement is randomly oriented in the case of the hot rolled bar and markedly oriented in the wire axis direction in the case of the cold drawn wire. Thus both materials behave as composites at the microstructural level and their plated structure (oriented or not) would explain the different time-dependent behaviour in a corrosive environment.     

Delamination fracture of prestressing steel: An engineering approach

This paper provides a composites engineering approach to explain the stress corrosion behaviour of high-strength prestressing steel wires. To this end, two eutectoid steels in the form of hot rolled bar and cold drawn wire were subjected to slow strain rate tests in aqueous environments in corrosive conditions corresponding to localized anodic dissolution and hydrogen assisted cracking. While a tensile crack in the hot rolled bar always propagates in mode I, in the cold drawn wire an initially mode I crack deviates significantly from its normal mode I growth plane and approaches the wire axis or cold drawing direction, thus producing a mixed mode propagation. In hydrogen assisted cracking the deviation happens just after the fatigue pre-crack, whereas in localized anodic dissolution the material is able to undergo mode I cracking before the deflection takes place. Therefore, a different behaviour is observed in both steels and even in the same steel under distinct environmental conditions. An explanation of such behaviour can be found in the pearlitic microstructure of the steels. This microstructural arrangement is randomly-oriented in the case of the hot rolled bar and markedly oriented under the wire axis direction in the case of the cold drawn wire. Thus both materials behave as composites at the microstructural level and their plated structure (oriented or not) would explain the different time-dependent behaviour in a corrosive environment.     

Analysis of spray formed tool steels

The technique of atomising liquid metal and compacting the micro‐droplets to a billet on a rotating support combines advantages from cast, wrought and powder metallurgical materials. A production rate of several tons per hour requires precisely controlled processing routines. Important process parameters based on processing, manufacturing and application of spray formed material are now developed by fundamental investigations established in projects focused in a special co‐operative research program (SFB 372) at the University of Bremen which is funded by the Deutsche Forschungsgemeinschaft (DFG). By this method advanced alloys of high hardness can be formed without inhomogeneities like segregations and pores in the center of the ingot. By example of tool steels the influence of spray forming on microstructure and homogeneity of the elemental distribution is shown.     

贝氏体区等温时间对TRIP钢残奥及力学性能影响

为研究贝氏体区等温时间对热轧TRIP钢残余奥氏体和力学性能的影响,采用金相显微镜、X射线衍射、拉伸实验等方法对3种不同贝氏体区等温时间下制备的热轧TRIP钢进行分析.结果表明:随着贝氏体等温时间的延长,残余奥氏体量减少而残余奥氏体碳含量增加,残余奥氏体晶粒尺寸及残余奥氏体形貌变化不大;热轧TRIP钢的力学性能随着贝氏体...     

Microstructural features of spray-formed AZ31 magnesium alloy

Abstract

Mg alloy AZ31 was spray-formed using an indigenously developed spray atomisation and deposition unit under protective atmosphere and various processing parameters were optimised. The microstructural features of the bell shaped AZ31 spray-formed deposit were characterised using optical microscope, scanning electron microscope/energy dispersive spectrometer, X-ray diffraction and high resolution transmission electron microscope. It was observed that the microstructural features are critically dependent on location in the spray-formed deposits. Under optimised processing conditions, the central region of the bell shaped deposit exhibited minimal porosity and a uniform fine grained equiaxed microstructure with fine Mg₁₇Al₁₂ intermetallics preferably located at the grain boundaries. However, the peripheral regions of the spray-formed deposit indicate higher porosity with distinct microstructural characteristics different from those in the central region. These microstructural features, observed at different locations in the spray-formed deposit, have been analysed and their evolution is discussed in the light of variations in thermal and solidification conditions of the droplets in flight, during impingement as well as those of the deposition surface.     

Spray deposition of an iron aluminide

High velocity oxyfuel (HVOF) spray forming of an iron aluminide [Fe–12.5 Al–2.93 Ni–0.02 B (wt%), containing 300 p.p.m. oxygen], followed by heat treatment for 24 h at 500°C, 18 h at 600°C and 20 min at 800°C, and multipass hot rolling at 800°C has been studied. Three different thicknesses (0.43, 0.93 and 1.33 mm) of sprayed deposit were produced by spraying for different times (approximately 10, 20 and 30 min). The spray-deposited layers exhibited some oxide and some porosity. This porosity was reduced by heat treatment. The as deposited layer had a high degree of B2 order, and a B2 antiphase domain size of 4.5 nm. On hot rolling this material to a reduction of 38%, it was found to be more susceptible to edge cracking than similar material processed by an ingot–extrusion–hot rolling route. In heat treatment, the aluminide-sprayed layer formed a non-protective Fe2O3 oxide, rather than the usual Al2O3 that forms on the binary alloy. This is attributable to the Ni content of the iron aluminide powder employed. © 1998 Kluwer Academic Publishers     

Integrated gray-level gradient method applied for the extraction of three-dimensional velocity fields of sprays in in-line digital holography

In this study, an integrated gray-level gradient method is applied to extract the three-dimensional (3D) velocity fields of sprays. This method consists of a conventional edge-sharpness method and a new method, namely, the overall-sharpness method, which is an efficient supplement of the former. And then the synchronization system is designed and assembled to record double-exposure spray holograms in a short time interval. Finally, using the integrated gray-level gradient method and some image processing techniques, the 3D coordinates of droplets are easily obtained, which can be used to evaluate the 3D velocity fields and the size features of spray droplets in different spray injection pressures. It proves that the integrated gray-level gradient method is well applied to measure the characteristics of sprays in in-line digital holography.     

Friction stir welding of AISI 304 austenitic stainless steel

The objective of this work is to demonstrate the feasibility of friction stir welding (FSW) AISI 304 austenitic stainless steels. The tool used was formed of a tungsten‐based alloy. The specimens were welded on an 11 kW vertical milling machine. Defect‐free welds were produced on 2.5 mm plates of hot‐rolled AISI 304 austenitic stainless steels at travel speeds ranging from 40 to 100 mm/min with a constant rotating speed of 1000 rpm. Tensile strengths and hardness values of the weld interface were determined and microstructure features of these samples were investigated.     

Microstructure of a recycled AA7050 alloy processed by spray forming followed by hot extrusion and rotary swaging

The aim of this study is to evaluate the microstructure of a recycled AA7050 alloy processed by spray forming followed by hot extrusion and swaging. Machining chips from an aircraft manufacturer were used as raw material. The microstructure was investigated by optical microscopy (MO) and scanning electronic microscopy (SEM). The spray formed deposit was homogenized at 743 K, 8 h, extruded at 693 K with a deformation rate of 0.1 s^–1 and an extrusion rate of 156:1, producing a bar with 8.0 mm in diameter. Due to a specific combination of high extrusion temperature and deformation rate, partial recrystallization occurred and different grain sizes were obtained through the extruded cross section. After extrusion, the 8.0 mm bar was processed by rotary swaging in several steps up to a 2.0 mm final diameter. The resulting microstructure revealed a cold worked structure, with no recrystallization. Tensile tests were performed in both cases and the slant type of fracture reveals that Portevin Le‐Chatelier effect took place in this alloy. Moreover, the efficacy of extrusion and rotary swaging parameters to reduce the porosity, intrinsic to the spray form process, was analyzed, as well the distribution of intermetallic particles.     

喷射成形高合金化材料沉积坯内部疏松成因分析与改善

本文主要研究了大尺寸喷射成形高合金化材料高温合金沉积坯内部疏松成因及其改善工艺。研究认为,高温合金沉积坯中最后凝固部位液相金属补缩不足和断裂应变低是产生疏松的必要条件,而沉积坯的局部收缩变形产生的热应力是造成疏松的重要原因。通过有限元计算,提出一种热控喷射成形工艺,并得到试验验证。该工艺有效减少了坯体内部的疏松等冶金缺陷,改善了沉积坯的质量。     

煤油蒸气为前驱体热梯度CVI法制备C/C复合材料

以炭毡为预制体,煤油蒸气为前驱体,利用两个热源分别加热预制体的上下表面,形成两个热梯度和双沉积面,第三个热源加热前驱体保证反应气体的供给,采用这种改进的化学液相气化渗入法快速制备了炭/炭(C/C)复合材料.对C/C复合材料的密度和气孔率进行了表征,并通过XRD,SEM等方法对其石墨化程度、显微结构进行了研究.结果表明:C/C复合材料的密度随沉积温度的升高呈线性增加,而气孔率逐渐减小,体积密度为0.2g/cm3的预制体在1100℃沉积3h后密度达到1.72g/cm3.2200℃热处理后,C/C复合材料的d002显著降低,具有较高的石墨化程度.C/C复合材料中的炭纤维被环状的热解炭所包围.沉积过程中前驱体较短的对流和扩散路径以及预制体中存在的上下热梯度和相应的双沉积面是材料快速制备的主要因素.     

Microhardness profile prediction of a graded steel by strain gradient plasticity theory

In the present study, the Vickers microhardness profile of functionally graded steel austenitic steel produced by electroslag remelting process has been investigated. To produce functionally graded steels, two different slices from plain carbon steel and austenitic stainless steels were spot welded and used as electroslag remelting electrode. Functionally graded steel containing graded layers of austenite may be fabricated via diffusion of alloying elements during remelting stage. Vickers microhardness profile of the specimen has been obtained experimentally and modeled with mechanism-based strain gradient plasticity theory. In this regard, the density of the statistically stored dislocations and that of geometrically necessary dislocations was related to the Vickers microhardness profile of each layer. The experimental results are in good agreement with those obtained from the theory.