Materials characterization and mechanical properties of graded tool steels processed by a new co‐spray forming technique

In order to meet the requirements of micro cold forming tools, a new co‐spray forming process has been applied to produce graded materials from two different tool steels in this study. The two steel melts were atomized and co‐sprayed simultaneously onto a flat substrate, resulting in a flat graded deposit when the two sprays were overlapped. To eliminate porosity and break up carbide network, the graded deposits were further hot rolled. The resultant graded tool steels were investigated with respect to porosity, element distribution, microstructure, hardness, strength, and toughness. The degree of overlapping of the two sprays determined the concentration gradient of the chemical elements in the deposits. The overlapping of the spray cones also contributed to low porosity in the gradient zone of the deposits. The porosity in the graded deposits could be essentially eliminated by means of hot rolling. The carbides and grain structures of the hot rolled tool steels were fine and homogeneous. By means of combining different tool steels in a single deposit, different microstructures and properties were combined.     

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.     

Bulk synthesis of amorphous and nano‐crystalline materials by spray forming

Bulk amorphous and nano‐crystalline metallic materials have been observed to possess excellent mechanical and physical properties. The conventional process routes, to synthesize such materials, are restricted by their ability to achieve rapid solidification, which limits the dimensions of the materials produced. In the last 10–12 years, spray forming has been employed to avoid these limitations by using its capability of layer by layer deposition of undercooled droplets. The current literature indicates that the opportunities provided by this process can be effectively utilized to produce bulk materials in a single step. In this paper, an attempt has been made to bring out the developments in the synthesis of bulk amorphous and/or nano‐crystalline materials by spray forming. The effect of process parameters, droplet size distribution in the atomized spray, the thermal conditions of droplets prior to deposition and the deposition surface conditions have been discussed. It has been demonstrate that a layer by layer deposition of undercooled droplets of glass forming alloys on a relatively cold deposition surface is the suitable condition to achieve bulk amorphous/nano‐crystalline materials.     

Laser arc hybrid weld microstructure in nickel based IN738 superalloy

Abstract

A study of the microstructural characteristics of laser arc hybrid welded nickel based IN738 superalloy was performed. Laser arc hybrid welding produced a desirable weld profile in the alloy, similar to what is usually obtained during laser beam welding, and no cracking occurred exclusively in the fusion zone. Elemental partitioning pattern in the fusion zone was studied by electron probe microanalysis and calculating the volume fraction of the weld metal that resulted from the consumption of the filler wire. The result showed that Ti, Ta, Nb, Mo, Al and Zr partitioned into the interdendritic regions of the fusion zone. SEM and TEM examination of the fusion zone showed the presence of secondary solidification reaction constituents, which consists of MC type carbides. The study further revealed that non-equilibrium liquation of various second phases that were present in the alloy prior to welding contributed to intergranular liquation in the heat affected zone (HAZ), which consequently resulted in extensive HAZ intergranular cracking during welding. Although laser arc hybrid welding appears promising for improving the weldability of nickel based IN738 superalloy, a suitable weldability improvement procedure is required in order to minimise HAZ intergranular cracking and thereby enhance the applicability of this technology to the joining of the superalloy.     

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.     

Microstructure and elevated temperature mechanical properties of Al–8Fe–4RE alloy fabricated by spray forming

Al–8Fe–4Ce alloy is currently manufactured by consolidating the atomized powders. With the aim to reduce the cost, spray forming process was applied in manufacturing with misch metal as raw materials. Spray forming (SF) as well as casting were employed to prepare Al–8Fe–4RE alloy, followed by hot‐press to compact the samples. The mechanical properties of SFed and cast Al–8Fe–4RE alloys are characterized at a temperature of 350 °C. The results show that the Al₃Fe phases contained in SF alloy is comparatively refined, forming needle‐shaped phases embedded in the Al matrix, and the SF alloy also showed lower degree of preferred orientation in (111) plane. Although both factors might explain the superior performance of the SF sample, the fracture appearance after tensile test at 350 °C shows that the contribution from crystallographic feature might be predominant. Spray forming is proved to be a very promising technique for manufacture of Al–Fe–Ce alloys of high strength at an elevated temperature.     

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.     

Dissolution of the γ′ phase in a nickel base superalloy at equilibrium and under rapid heating

Dissolution of the γ′ phase in a PM superalloy Astroloy was studied at equilibrium and under rapid heating. The ‘solvus’ temperature of primary and secondary γ′ precipitates was determined experimentally. The most prominent feature concerns the large departure from equilibrium for the dissolution of the γ′ precipitates during rapid heating and this departure depends on the initial size of the precipitates. A model for γ′ dissolution during fast thermal cycling is proposed.     

喷射成形管坯形状演变过程模拟

为了实现喷射成形管件的精确控制和深入理解喷射成形过程,建立了描述喷射成形管坯形状演化过程的数学物理模型,并对沉积坯上一点的生长速度进行了修正,使模型更符合实际情况.模拟结果表明:在喷射成形过程中,接收管水平移动速度、接收管外径和起始沉积距离等是喷射成形管坯形状和尺寸的重要参数,它们对管坯的形状演变将产生重要影响.稳定生长的管坯大致可分为3个区域,后部过渡区、直径均匀的中间区和前部过渡区.模拟结果与实验结果吻合得较好.     

Microstructure and wear resistance of spray formed and conventionally cast high‐chromium white iron

A billet of hypoeutectic high‐chromium white iron (19% Cr, 2.5% C) was spray formed using Gas‐to‐Metal Ratios (GMR) of 0.9, 1.0, and 1.1. Microstructural studies and dry sand rubber wheel abrasion tests were carried out, on the one hand, to compare between the spray formed and conventionally cast material and, on the other hand, to investigate the relationship between gas‐to‐metal‐ratio, eutectic carbide morphology and abrasion resistance. The spray formed material was characterized by a considerably finer carbide morphology (max. ?30 μm) than the conventionally cast material (max. 100–200 μm). The coarser carbide morphology is believed to be responsible for the superior abrasion resistance of the conventionally cast material. Although the carbide morphology of the spray formed material was only moderately influenced by the changes in the gas‐to‐metal‐ratio, there was a clear improvement in the abrasion resistance with decreasing gas‐to‐metal‐ratio. The improvement correlated with a decrease in the fraction of very fine (<1.5 μm) carbides, rather than with an increase in the mean carbide size.     

Spray Forming of Homogeneous 20MnCr5 Steel of Low Distortion Potential

Spray forming is an advanced technology for the manufacture of homogeneous material from the melt. In this study, 20MnCr5 steel, a typical case hardening steel for gear and shaft production, has been spray formed and hot worked to semi‐finished bars. Shaft‐like specimens have been made from the spray‐formed material for distortion investigation. Material characteristics of the spray‐formed 20MnCr5 steel have been studied. In cooperation with other projects of the Collaborative Research Center SFB570 ‘’Distortion Engineering’’, evaluation of distortion behavior of the shaft specimens after machining and heat treatment has been made in comparison with continuous cast material. Spray‐formed 20MnCr5 steel shows lower distortion potential due to improved material homogeneity.     

A simple route to electrophoretic deposition of transition metal-coated nickel oxide films for electrochemical capacitors

Nickel hydroxide films coated with transition metals such as nickel and cobalt were fabricated directly by a one-step electrophoretic deposition (EPD) in the presence of charging additives (transition metal salts). A nickel hydroxide particle with a weakly charged surface in an isopropanol solution was found to be detrimental to EPD and dispersion. When a small amount of charging additive was added to the suspension, the adsorption of dissolved metal ions on the nickel hydroxide resulted in a more positively charged particle surface, facilitating EPD and dispersion. When nickel hydroxide particles migrated to the negative electrode during the EPD process, the metal ions adsorbed on the particle were reduced electrochemically to form a metal layer. The as-deposited nickel hydroxide film converted to nickel oxide following heat treatment at 300 °C. Our results revealed that nickel oxide films coated with nickel and cobalt showed better capacitive behavior than the bare film. The improved capacitive behavior was attributed to the co-deposition of transition metals, which provided additional active sites on the nickel oxide surface for the electrochemical reaction to occur.     

Development of the heating and forming strategy in compound forging of hybrid steel‐aluminum parts

In times of increasing energy costs automotive light weight construction is gaining more importance. The production of hybrid compounds by forging is a promising method for manufacturing functional parts by applying resource‐saving process steps. The mechanical properties of these parts can be specifically adapted to the requirements. In compound forging of steel‐aluminum parts the two materials need to be heated to different forming temperatures. In this paper, the challenges and their methods for the development of a heating and forming strategy based on different material characteristics are presented.     

A simplified approach for incorporating thickness stress in the analysis of sheet metal forming using shell elements

A simplified scheme for considering the thickness stress of shell elements induced by contact is presented which improves the accuracy of sheet metal forming analysis. The yield function formulated on the basis of plane stress conditions is modified to incorporate the effect of transverse normal stress induced by contact forces acting on shell elements and return mapping routine is used to update in‐plane stresses at each time step. The transverse normal stress distributions in the thickness direction are determined using the analytic solution of the cylindrical tube under the internal pressure. As numerical examples, uni‐axial compression, bi‐axial tension and bending tests are treated. The problem of cylindrical cup drawing is also calculated. Each result is compared with the results obtained by the analysis using ABAQUS continuum elements. Copyright © 2002 John Wiley & Sons, Ltd.     

Palladium (+ 2) reduction: A key step for the electroless Ni metallization of insulating substrates by a tin-free process

The tin-free electroless metallization process developed in our laboratory consists in attaching the Pd(+ 2)-based catalyst on surfaces which were previously grafted with nitrogen-bearing groups. The so-functionalized surfaces lead to the formation of true covalent bonds between palladium and nitrogen-based species. In this work, the different steps of the process are studied, namely: (i) the grafting of nitrogen-bearing groups on an insulating surface by various processes already used in our laboratory, (ii) the seeding of the surface with Pd(+ 2) species, (iii) the reduction of Pd(+ 2) to Pd(0) by different routes (this step is needed due to the use of industrial plating baths which contain strong stabilizers that prevent or delay plating), (iv) the Ni metallization using low- and high-phosphorus plating baths. Phosphorus which originates from the plating bath reducer is partly incorporated in the Ni films and creates some stresses. Adhesion of the resulting films is evaluated by the Scotch® tape test. The main parameters responsible for adhesion and governing the characteristics of the Ni film/substrate interface are: the nature of the substrate, the Pd(+ 2) reduction route and the phosphorus content of the Ni plating bath. All these parameters influence the amount of stresses generated both at the metal/substrate interface and within the film.     

Characterization of the alumina oxide,copper and nickel powders and their processing intended for fabrication of the novel hybrid composite: A comparative study

The aim of this study was the fabrication and characterization of the novel hybrid composite from the Al₂O₃/Cu/Ni system. The research included the production of composite specimens by uniaxial pressing and their further sintering. The influence of the ceramic and metallic powders and their sintering temperature on the microstructure of composites was investigated. The firing process was conducted in reducing the atmosphere at three different temperatures, selected on the basis of the copper-nickel phase diagram: 1100 °C, 1260 °C, 1400 °C. Reference samples of Al₂O₃/Cu were also produced in the same way for comparative purposes. The addition of the third component, which together with copper will form a Ni−Cu solid solution, was intended to improve the wettability of ceramic matrix through a liquid metallic phase, and thus to reduce the phenomenon of the liquid metal flow on samples surface during sintering. However, it had an impact on physical and mechanical properties.     

A novel hybrid artificial neural network technique for the early skin cancer diagnosis using color space conversions of original images

In this study, an innovative hybrid machine learning-technique is used for the early skin cancer diagnosis fusing Convolutional Neural Network and Multilayer Perceptron to analyze images and information related to the skin cancer. This information is extracted manually after applying different color space conversions on the original images for better screening of the lesions. The proposed architecture is compared with standalone architecture in addition to some other techniques by commonly used evaluation metrics. HAM10000 dataset is used for training and testing as this data contain seven different skin lesions. The novelty of the proposed hybrid model is the structure of the network which handles structured data (patients' metadata and other useful features from different color spaces related to the illumination, energy, darkness, etc.) and unstructured data (images). The results show an overall 86%, 95% top-1 and top-2 accuracy respectively, and 96% area under the curve for the seven classes. The study demonstrates the superiority of the proposed hybrid model with a 2% improvement in the accuracy over the standalone model and a promising behavior as compared to the ensemble techniques. The follow-up research will include more patient data to develop a skin cancer detection device.     

A new three-dimensional finite element model for the simulation of powder forging processes: application to hot forming of P/M connecting rod

In powder metallurgy (P/M) the forming of industrial artifacts requires consolidation of loose powder into dense material leading to near-to-net shape components. In order to realize the economic advantages of the near-to-net shape formation, it is essential to understand the mechanical behaviour of powder deforming domain. The conventional P/M forming process consists of different stages such as closed cold die compaction, sintering and hot/cold forging. In the present study a finite element based computational model has been formulated to study the hot forging stage with particular reference to forging of P/M connecting rods. In order to achieve this purpose, a new finite element formulation has been developed to model the powder deformation under a given thermo-mechanical loading. Essentially, the computational model is formulated based on a visco-plastic Green type material model considering an independent idealization of strain rates into a total deformation part and a dilatational part, and the yield criterion takes into account the pressure sensitivity. The model is set in a perturbed Lagrangian functional, leading to a three field mixed formulation with velocity, Lagrangian multiplier/pressure and volumetric strain rates as three basic unknowns in the finite element domain. The developed finite element model can be used right from the compressible domain to the incompressible domain with the Lagrangian multiplier becoming then the pressure. A relative density-dependent visco-plastic type of friction law is used for characterizing the friction behaviour between the powder preform and the die. The required various material parameters are determined from experiments on aluminium powder preforms. In order to facilitate the non-isothermal deformation study, a powder-based transient thermal analysis has been developed. A computational scheme has been used to couple the mechanical and thermal calculations. Using the developed three-dimensional code, hot forging of automotive components can be simulated and which in the present study is exemplified by simulation of hot forging of a P/M connecting rod. © 1997 John Wiley & Sons, Ltd.