Metal Powder Particles Research Articles

Automated 3D EBSD for metallic powders

Metallic powders are commonly used in additive manufacturing processes. While their post-process consolidated properties are widely studied, there is little research on the properties of the powders prior to consolidation. Understanding the powder characteristics before use in additive manufacturing processes could lead to fine-tuning properties of additively manufactured materials. The three-dimensional grain structure of metals can be useful in predicting their properties and microstructure. Powder particles are much smaller and more difficult to fixture and polish than their bulk counterparts, hence typical protocols are difficult to use when serially sectioning them. This method describes a recommendation as to how to fixture, mill, and image metallic powder particles using a Xe P-FIB to mill and take EBSD measurements. It is based on milling and imaging techniques used for bulk materials, but with the specific advantage of how to fixture the powder sample. Our modifications include:•the method of fixturing the specimen to the holder.•the method of protecting the sample during milling.

Thermal efficiency of plasma-jet deposition of powder materials

The article describes the results of investigations of the thermal efficiency of the process of plasma-jet deposition of materials resulting in a higher rate of thermal saturation and melting of metallic powder particles in comparison with the monolithic metal of the component. The results are used to determine the relationships governing the effect of the technological parameters of the process on the formation of the layer of the coating on the components reconditioned by plasma-jet surfacing.

Influence of processing parameters on the evolution of melt pool, porosity, and microstructures in Ti-6Al-4V alloy parts fabricated by selective laser melting

Selective laser melting involves melting and solidification of metal powder particles in a track-by-track and layer-by-layer method to fabricate 3D parts. The present investigation focuses on understanding the effect of laser power and scan speed on the evolution of melt pool, porosity and multiple thermal cycling effects on the microstructure in parts fabricated using selective laser melting. In this study, Ti-6Al-4V pre-alloyed powder was used to produce single-track deposits and bulk parts. Using different combinations of laser power and scan speeds, single-track deposits and bulk parts were produced. The cross-sections of the single-track deposits and bulk samples were prepared for metallographic observations and the melt pool shape and size and porosity were evaluated. When a low energy density was applied the un-melted powder particles produced irregularly shaped porosity, and a high energy density resulted in rounded porosity, which was due to keyhole effects. The samples produced with a proper combination of power and speeds were fully dense. Further, microstructural development under the influence of process condition was highlighted. Overall, the study demonstrates a good correlation between the single-track melt pool geometries, porosity in bulk parts and also demonstrates the microstructural inhomogeneity during deposition.

Microstructure and morphological study of ball-milled metal matrix nanocomposites

Due to the difficulty of preparation and beneficial properties achievable, copper and iron matrix nanocomposites are materials for which fabrication via the powder metallurgy route is attracting increasing research interest. The presence of ceramic nanoparticles in their matrix can lead to considerable changes in the microstructure and morphology. The effects of the type of metallic matrix and ceramic nanoparticle on the distribution of nano reinforcements and the morphology of ball-milled composite powders were evaluated in this study. For this purpose, 25 wt % of Al2O3 and SiC nanoparticles were separately ball-milled in the presence of iron and copper metals. The SEM, FESEM, and XRD results indicated that as-received nanoparticles, which were agglomerated before milling, were partially separated and embedded in the matrix of both the metals after the initial stages of ball milling, while prolonged milling was not found to further affect the distribution of nanoparticles. It was also observed that the Al2O3 phase was not thermodynamically stable during ball milling with copper powders. Finally, it was found that the presence of nanoparticles considerably reduce the average size of metallic powder particles.

Branched morphology of Nb powder particles fabricated by calciothermic reduction in CaCl2 melt

A new branched morphology of metallic niobium powder particles was produced by calciothermic reduction of niobium oxides in molten CaCl2. The influence of some reaction parameters on the powder particle morphology was studied, and the formation mechanism was investigated. It was found that the formation of intermediate complex niobium oxides (CaxNb2Oy) was crucial in the formation of niobium powder particles with the branched morphology. Based on this finding, the formation mechanism of the branched morphology in four stages was proposed. These Nb powder particles with microscopic branches can prevent particle sintering during the fabrication process of electric capacitors, allowing the latter to exhibit a higher capacitance.

Selective Laser Melting Manufacturing of Microwave Waveguide Devices

Additive manufacturing technologies are currently envisaged to boost the development of a next generation of microwave and millimeter-wave devices intended for, among others, satellite telecommunications, navigation, imaging, radio-astronomy, and cosmology. Due to their excellent electromagnetic and mechanical properties, all-metal waveguide components are key building blocks of several radio frequency (RF) systems used in these application domains. This article reports on the prospects originating from the application of all-metal 3D printing to the manufacturing of high-performance microwave waveguide devices. The technology investigated is the selective laser melting process, where a laser beam is used to fuse metal powder particles spread over a building platform. The complete parts are built by overlapping several constant-thickness layers. An overview on process parameters, material properties, and design rules is reported for this technology. The electromagnetic properties of test samples built in Al and Ti alloys have been experimentally characterized. A robust design of Ku/K-band filters aimed at satellite telecommunications has been implemented in several prototypes manufactured in Al. The corresponding measured performance confirm the applicability of the laser selective melting process to the intended applications.

Densification Mechanism for Different Types of Stainless Steel Powders in Selective Laser Melting

Selective laser melting is a powder based additive manufacturing process where the metallic powder particles are melted by a high power laser beam. Different types of stainless steel powders made by gas and water atomization were analyzed before processing, in particular regarding their particle size distributions and morphology. Particle analysis was carried out using laser diffraction technologies and digital image analysis.A suitable designed experiment has been carried out and the specimen density has been measured and linked to the properties of the powders.Eventually the possibility to reach high density specimen by adjusting process parameters is discussed.

Кинетика возникновения трещин в осаживаемых цилиндрических образцах

Introduction. The article is devoted to the crack occurrence analysis in a powder body, and identification of the basic laws of this process in the freely upset material. The work objective is to study kinetics of crack initiation and propagation in freely upset powder materials, and microstructure of alloy and iron powders in a variety of areas in order to determine the conditions for defect-free deformation of the material under free upsetting. The analysis of the crack development in a powder body is of great practical importance, and therefore, the investigation of this problem is essential for the fabrication of accepted parts. Materials and Methods. New powder materials from the Swedish company Höganäs which are characterized by the improved deformation characteristics are considered. Besides, these materials differ in occurrence of alloy elements, such as Ni and Mo which affect cracking. Microstructures of the upset samples are analyzed using the metallographic microscope. Research Results . Cracks in the upset materials are classified as cleavage cracks. The crack origin is determined by the absence of factors for the development of the sample metal component plasticity due to the occurrence of defects and pores in its body. It is established that rapid cracking is confirmed by the nature of the crack when it scarcely has extension into the sample material, and, moreover, ends at an angle to the area for development. Discussion and Conclusions. The analysis of the microcrack development allows identifying morphological structural differences of the central and fringe regions of the powder material. The peripheral area is characterized by different values of grains and increased content of micropores, and the directivity of clusters of micro-discontinuities and non-metallics, in the line of the former physical interface of the metal powder particles.

Effect of Strain Rate and Relative Density on the Compressive Deformation of Open Cell Ti6Al Alloy Foam through P/M Route

Open cell Ti6Al alloy foams of varying porosity fractions have been made using ammonium bicarbonate as space holder through powder metallurgy route. In order to provide the sufficient strength in green compacts 2% of wt.% PVA solution was mixed with elemental metal powder and NH4(HCO3) particle prior to cold compaction. Green pallets were sintered at three stages at three different temperatures 600°C, 800°C and 1100°C. XRD and EDX analyses confirmed that no residue of space holder remained in the sintered foam samples. The compressive deformation behavior of Ti6Al alloy foams with varying relative densities was conducted under different strain rates (0.01, 0.1, and 1.0 s-1). The plateau stress, the Young’s modulus and the energy absorption of the foam increase with increase in the relative density following power law relationships. But the densification strain decreases with increases in relative density following a linear relationship. The strain rate sensitivity and the strain rate sensitivity parameter of these foams were also examined, and it was found that the strain rate sensitivity parameters were varying in the range 0.034 to 0.078.

Damage development and damage tolerance of structures manufactured by selective laser melting – a review

The additive manufacturing technology of Selective Laser Melting (SLM) experiences a rapid development within an increasing marked of quite different application fields. The properties of SLM materials and structures are influenced by a number of tech-nological parameters such as the metal powder (particle size, homogeneity, cleanliness), the laser tool (power, beam diameter, pulse lengths), the scanning operation (speed, sequence and orientation of melting paths), parameters of the over-all equipment (design and preheating of the base plate, currents and turbulence in the protective gas atmosphere) and, last not least, the hatching strategy including the build-up direction of the structure with respect to the loading direction of the component.For the perspective use of SLM structures as load carrying, safety-relevant components the knowledge of their mechanical properties is necessary. It is essential to understand these in the context of the manufacturing-related features and at the back-ground of the basic characteristics of metallic materials: crystal lattice, microstructure and material defects. The paper provides an overview on factors which affect the mechanical parameters stiffness, strength, ductility, toughness, fatigue crack propagation and fatigue strength in the context of selective laser melting.

Effect of Alloying on the 316L and 321 Steels Samples Obtained by Selective Laser Melting

The main objective is to study the structure and mechanical properties of the samples based on 316L and 321 stainless steel powders with different particle size distribution, produced by selective laser melting. Despite the fact that the SLM technique involves almost complete melting of the metal powder particles, the difference in particle size and a fused layer thickness leads to differences in mechanical properties due to incomplete melting of large particles. Therefore, for each type of steel it's necessary to experimentally select fusion mode. Samples for impact strength, tensile strength and density tests were grown. The studies revealed that samples from a larger steel powder 316L, obtained at a larger thickness of 40 microns, with increasing energy input don’t reach the characteristics that have been obtained at a thickness of 20 micron layer. Samples of fine steel powder 321, constructed with a layer thickness of 40 microns, have the same mechanical properties as the samples constructed with a layer thickness of 20 microns, with an increase in the energy input of 1.5. On the basis of the data obtained it's shown that the distribution of powder particle size has a significant impact on the quality of the samples.

The deformation of metal powder particles: hardness and microstructure

Abstract The modelling of powder densification requires knowledge about the material properties of the particles. Some models use the material properties of the material in bulk as inputs for modelling the densification of assemblies of powder particles. It is important to understand how these properties differ when the material is in powder form. Existing works tend to analyse bulk material or powder exclusively and rarely compare properties such as hardness between the two. This work compares the microstructures and hardness of metal powder with bulk material with the aim of guiding the assumptions made in the modelling of metal powder densification. It is expected that the rapid solidification typical in the atomisation of metals results in significant differences in behaviour when compared with material produced via casting and hot/cold working.

Considerations on the construction of a Powder Bed Fusion platform for Additive Manufacturing

As the demand for moulds and other tools becomes increasingly specific and complex, an additive manufacturing approach to production is making its way to the industry through laser based consolidation of metal powder particles by a method known as powder bed fusion. This paper concerns a variety of design choices facilitating the development of an experimental powder bed fusion machine tool, capable of manufacturing metal parts with strength matching that of conventional manufactured parts and a complexity surpassing that of subtractive processes. To understand the different mechanisms acting within such an experimental machine tool, a fully open and customizable rig is constructed. Emphasizing modularity in the rig, allows alternation of lasers, scanner systems, optical elements, powder deposition, layer height, temperature, atmosphere, and powder type. Through a custom-made software platform, control of the process is achieved, which extends into a graphical user interface, easing adjustment of process parameters and the job file generation.

Denudation of metal powder layers in laser powder bed fusion processes

Understanding laser interaction with metal powder beds is critical in predicting optimum processing regimes in laser powder bed fusion additive manufacturing of metals. In this work, we study the denudation of metal powders that is observed near the laser scan path as a function of laser parameters and ambient gas pressure. We show that the observed depletion of metal powder particles in the zone immediately surrounding the solidified track is due to a competition between outward metal vapor flux directed away from the laser spot and entrainment of powder particles in a shear flow of gas driven by a metal vapor jet at the melt track. Between atmospheric pressure and ∼10 Torr of Ar gas, the denuded zone width increases with decreasing ambient gas pressure and is dominated by entrainment from inward gas flow. The denuded zone then decreases from 10 to 2.2 Torr reaching a minimum before increasing again from 2.2 to 0.5 Torr where metal vapor flux and expansion from the melt pool dominates. The dynamics of the denudation process were captured using high-speed imaging, revealing that the particle movement is a complex interplay among melt pool geometry, metal vapor flow, and ambient gas pressure. The experimental results are rationalized through finite element simulations of the melt track formation and resulting vapor flow patterns. The results presented here represent new insights to denudation and melt track formation that can be important for the prediction and minimization of void defects and surface roughness in additively manufactured metal components.

Application of thermal analysis techniques to study the oxidation/reduction phenomena during sintering of steels containing oxygen-sensitive alloying elements

For the consolidation of steel parts manufactured by powder metallurgy (PM) techniques, removal of the surface oxides covering metallic powder particles is a necessary prerequisite. In PM steels with conventional compositions, reduction of the oxides is easily achieved in traditional sintering furnaces. However, processing steels containing alloying elements with a high oxygen affinity represents a big challenge that requires a deeper understanding of the chemical processes occurring during sintering. In the present work, thermogravimetry analysis coupled with mass spectrometry is used to describe the oxidation/reduction phenomena that take place when sintering steel powders and how these processes are modified by the addition of admixed particles containing oxygen-sensitive elements. Carbothermal reduction processes are studied using pure oxides (Fe2O3, MnO2, Cr2O3 and SiO2) as well as water-atomized Fe powders mixed with small amounts—4 mass/%—of Cr, Mn and Si powders or Fe–Mn–Si–(Cr) master alloy powders. The results show that there is an oxygen transfer from the base iron particles to the oxidation-sensitive elements—“internal getter effect”—taking place mostly through the gas phase. Different alloying elements (Cr, Mn, Si) show different temperature ranges of susceptibility to oxidation. Combination of these oxygen-sensitive alloying elements in the form of a master alloy powder reduces their sensitivity to oxidation. Also, the use of master alloys promotes the concentration of the oxides on the surface of the alloying particles and not in the grain boundaries of the surrounding iron particles—as occurs when using Mn carriers—which should have a beneficial impact on the final mechanical performance.

Rapid Solidification: Selective Laser Melting of AlSi10Mg

Rapid movement of the melt pool (at a speed around 1 m/s) in selective laser melting of metal powder directly implies rapid solidification. In this work, the length scale of the as-built microstructure of parts built with the alloy AlSi10Mg was measured and compared with the well-known relationship between cell size and cooling rate. Cooling rates during solidification were estimated using the Rosenthal equation. It was found that the solidification structure is the expected cellular combination of silicon with α-aluminum. The dependence of measured cell spacing on calculated cooling rate follows the well-established relationship for aluminum alloys. The implication is that cell spacing can be manipulated by changing the heat input. Microscopy of polished sections through particles of the metal powder used to build the parts showed that the particles have a dendritic-eutectic structure; the dendrite arm spacings in metal powder particles of different diameters were measured and also agree with literature correlations, showing the expected increase in secondary dendrite arm spacing with increasing particle diameter.

Fabrication of Al7075 / B4C Surface Composite by Novel Friction Stir Processing (FSP) and Investigation on Wear Properties

Friction stir processing (FSP), a process, derived from the friction stir welding (FSW) process, is an emerging novel, green and energy efficient processing technique to fabricate surface composite. In the present investigation, FSP technique has been used for fabrication of surface composites, using aluminium 7075 as parent metal and Boron Carbide (B4C) powder particles as reinforcement., Aluminium 7075 has been selected as matrix phase, as being widely used by automotive and aerospace application and having the highest strength among all commercial Al alloys. In present paper, details about the fabrication of Al 7075-T651- B4C surface composite for various combination of tool rotation, tool travel speed and number of passes have been discussed. The same being intended to improve hardness and thereby wear resistance. The fabricated surface composites are examined for microstructure using image analyser, and found friction stir processed zone with a few defects. It is also observed that the average hardness of friction stir processed surface composite was 40 – 70% higher than that of parent metal (75 – 80 HV). Wear Resistance is found to be improved by 100% compared to parent metal. The increase in same is attributed to B4C particles dispersed in aluminium matrix and grain strengthening mechanism.

鉄粉酸化反応層の熱特性に関する研究

This paper presents an experimental and analytical investigation of the thermal characteristics in a layer of exothermic powder mixture that is utilized in body warmers, hot compresses, thermotherapy, etc. The research objective is to develop a manufacturing method that enables the exothermic temperature to be controlled in order to prevent cases of low-temperature burns. The exothermic powder mixture generally comprises metal powder, catalyst powder, and vermiculite particle absorbing some saline solution. The exothermic powder mixture is put into a mini-autoclave, and then placed in a constant temperature bath after the atmosphere inside the mini-autoclave has been replaced with pure oxygen. The degradation of oxygen pressure inside the mini-autoclave shows that the reaction rate of corrosion depends on the reaction temperature, the volume of the saline solution, and the conversion of iron powder into oxide, in addition to the ambient oxygen concentration; a correlation is suggested for the reaction rate. The variation over time of the temperature distribution in a layer of the exothermic powder mixture is evaluated analytically using this empirical correlation for the reaction rate, and compared with the experimental results. In the analysis, the mass, momentum, energy and oxygen component equations are solved numerically for a layer of the exothermic powder mixture, which is regarded as a porous medium. The analysis does not predict the temperature distribution accurately, but the analytical and experimental results are in accordance qualitatively.

Selective laser melting (SLM) and topology optimization for lighter aerospace componentes

Additive Manufacturing (AM) is a manufacturing process through which a 3D component is produced by consecutively adding material. One of the most promising AM processes is SLM. In SLM a laser completely melts metallic powder particles together forming a 3D component. SLM is known for its freedom of manufacturing constraints allowing complex geometries and high material efficiency. Topology Optimisation (TO) is an optimisation type that calculates the optimal material distribution for a given problem. The combination of SLM with TO is being developed to create lightweight components. In this work, the whole development process, from optimisation to design, production and testing is addressed. Initially, an aircraft bracket topology was optimised to be produced by means of SLM. The TO solution was interpreted and designed for AM. During the interpretation and design process, a design methodology was defined in order to facilitate and make more accurate the TO solution design and make it ready for AM. After the optimised component was produced, metrological and mechanical tests were performed in order to validate the final design and the computer analysis. The optimised component showed considerable weight reduction with an increase of the factor of safety. The experimental tests revealed a good relation to the computer analysis evidencing, however, room for improvement, both in the computer model and the experimental tests.

An Approach to Numerical Modeling of Selective Laser Melting

Technological parameters of selective laser melting determine the quality of the parts produced: porosity, residual stresses and so strength and plasticity. Numerical modeling can help understand this dependence. The modeling process must account for the fact that typical sizes of metal powder particles, used for selective laser melting, are comparable (near equal) to the diameter of the laser beam, so considering powder a continuum is not fully correct. It is shown that LS-DYNA finite element code has enough capability to model important features of the process – heat exchange, melting, contact interaction of the particles.