Production of rapidly solidified metal powders by water cooled rotating disc atomisation

Abstract

In this study production of rapidly solidified metal powders by water cooled rotating disc has been investigated. A rotating disc was cooled by water which was not in contact with the molten metal and the produced powders during atomisation. This method is an effective process to produce rapidly solidified powders and ribbons.

Tin, lead, aluminium, zinc, and AA 2014 aluminium alloy powders and ribbons were produced using different types of discs. Parameters such as disc rotation velocity, disc shape, diameter of liquid metal stream, and superheat were investigated. The results showed that increasing disc rotation velocity, decreasing liquid metal stream diameter, and increasing superheat resulted in finer particles as expected.

Different types of discs such as flat, flat with fins, and cone with fins were used to investigate the effects of the disc geometry. Performance of the flat type disc with rectangular fins was better than that of the others.

Metallographic examinations showed that estimated cooling rates of atomised AA 2014 powders were between 10⁴–10⁵ K s^-1 depending on particle size. With the ribbons cooling rates were relatively higher, of the order of 10⁵–10⁶ K s^-1.     

Generation of small batch high quality metal powder

Abstract

As part of a project to develop a small batch metal atomisation system, based on free fall atomisation and close-coupled atomisation, the influence of the process parameters on various powder quality features has been investigated, using copper–tin alloys as feedstock material. Particle size distribution, appearance of satellite particles, particle circularity and the flowability were recorded as criteria of particle quality. From the data obtained in these experiments, the effects of the main process parameters (atomisation pressure, mass melt flow, and height of the spray chamber as well as the atomiser system) have been evaluated with respect to powder quality features. The atomisation system was optimised to produce high quality powder with narrow particle distributions (d_84·3/d_50·3?=?1·6 for free fall) and high circularity with mass melt flows in the range of 100 kg h^?1 using melt volume between 100 and 1000 mL.     

Characteristics of rapidly solidified Cu–10%Sn alloy powders produced by water jet cooled rotating disc atomisation

Abstract

In the present work, an experimental water jet cooled rotating disc centrifugal atomiser was designed and constructed and used to produce rapidly solidified Cu–10%Sn alloy powders. The characteristics of rapidly solidified Cu–10%Sn alloy powders have been investigated with respect to powder size and disc surface condition. Uncoated and ZrO₂ coated copper discs were used to investigate the effect of disc surface conditions on the microstructure and cooling rate of the powders. The produced powders appeared in the shape of sphere, rounded, ligament, irregular and flaky, depending on the particle size. The powders exhibited fine grained microstructure, cell size increased with increasing powder size and higher cooling rates were obtained using uncoated disc. The results indicated that cooling rates of 20 μm powder produced with uncoated and ZrO₂ material coated discs were estimated as 5·82×10⁵ and 1·44×10⁵ K s^?1 respectively.     

Study of free fall gas atomisation of liquid metals to produce powder

Abstract

A free fall gas atomising unit was designed and constructed to produce metal powder. The effect of focal length, number, and diameter of nozzles and apex angle of atomisers is studied on size and size distribution of the powder, atomisation efficiency, and particle shape by disintegrating the molten stream at various plenum pressures, superheats, and free fall distances. Droplets were found to be solidified and built up around the nozzles and the liquid delivery tube whenever the atomiser was used beyond a particular value of the plenum pressure; this pressure is termed limiting plenum pressure. All the powder collectives produced under a wide range of conditions were found to obey log normal distribution function. The geometric standard deviation was shown to bear a constant value. The mass median size was correlated with the dynamic parameters characterising the atomising field and a correlation is proposed. The atomisation efficiency was correlated with the mass flowrate and velocity of gas.     

Current PM Literature for Engineers and Users

Abstract

Fine tin powders were produced in a pilot plant gas atomiser. Nitrogen gas at 1·56 MPa pressure was used as the atomising agent in a ‘confined design’ nozzle which operated vertically upwards. A range of metal flowrates from 0·864 to 1·425 kg min^?1 was studied at a melt temperature of 450°C. Powders were sized using dry sieving down to 45 μm and wet sieving for smaller sizes. The Sauter mean diameter of the powders varied from 9·01 to 10·28 μm, depending on the rate of production. The size distribution was bimodal (albeit not very well defined) with the peak separation at ～44 μm. In the fine size range, particles were spherical, while those in the coarse range were more elongated or irregular in shape and free of satellites. Comparison of the tin powders with copper powders from another study, AA 2014 aluminium alloy powders, and magnesium and zinc powders from previous work showed that the differences in mean diameter and standard deviation are small among these common metals at a given volumetric production rate. This confirms the overriding importance of liquid metal volume flowrate under fixed gas flow conditions in gas atomisation, while the actual physical properties of the liquid playa secondary role. Although surface tension is secondary to volume flowrate in importance for controlling particle size, the study has shown that a liquid metal with lower surface tension and viscosity than AA 2014 alloy, together with a higher density, yields finer particles. PM/0667A     

Large undercooling,rapid solidification,and nucleation mechanism during multistage atomisation

Abstract

The large undercooling and rapid quenching that can occur during multistage atomisation are studied both theoretically and experimentally. Results show that these two effects are closely related to and promoted by each other. The level of undercooling for droplets is dependent on alloy composition, powder particle size, and atomisation condition. The cooling rates of droplets depend heavily on their particle size. Both experiments and theoretical analysis on Sn-Pb alloys reveal that a certain function of undercooling has a linear relationship with the logarithm of the mean powder particle particle size. Based on the present results, a mechanism of nucleation preferred on surface oxide is proposed to give a quantitative interpretation of the experimental observations.     

Effect of production parameters on cooling rates of AA2014 alloy powders produced by water jet cooled,rotating disc atomisation

Abstract

In this study, a novel type of rotating disc unit was designed and constructed and was used to produce rapidly solidified AA2014 alloy powders. Copper and stainless steel were used as the disc material and the temperature of the cooling water was selected as 0°C and 18°C. Effects of the production parameters, such as disc material, cooling water temperature, superheat of liquid metal and disc speed on the microstructure and the cooling rate of the powders, have been investigated.

The microstructure of the produced powders was cellular and changed to cellular-dendritic with increasing powder size. It was found that cooling rates were relatively higher using a copper disc and 0°C cooling water temperature. The results indicated that cooling rates of 25 μm powders produced with a copper disc were estimated as 1·01×10⁶ K s^-1 and 9·02×10⁵ K s^-1 for 0°C and 18°C cooling water temperatures respectively. Decreasing the superheat of the liquid metal and increasing disc rotating speed also increased the cooling rates. PM/1050     

Sintering and tribological properties of lead-free bronze alloy for friction materials

Abstract

A lead-free bronze alloy powder with a dispersed sulphide solid lubricant phase has been produced by atomisation as a substitute for lead bronze friction materials. The powder has been processed to produce a bronze/steel bimetal strip by a sinter/roll/sinter process route, via both solid state and liquid phase sintering. Friction properties of the developed materials and a sintered bronze were compared in pin-on-disc tests; the Cu–Sn–S material shows excellent properties, equivalent to those of a lead bronze, and is being assessed for use in high pressure bushes.     

A novel crucible-less inert gas atomisation method of producing titanium powder for additive manufacturing

A novel low-cost gas atomisation technology producing spherical titanium powder (wire induction heating gas atomisation, WIGA) has been developed for additive manufacturing. Combined with the gas atomisation principle, the characteristics of WIGA were analysed. The effects of the gas pressure, metal temperature and the wire-feeding speed on the particle size of the titanium powder were studied. The results indicated that the decreases in mass median particle diameter (D₅₀) and the increases in efficiency of fine size powders occurred with the increase in gas atomisation pressure and melting temperature and with the decrease of wire-feed speed. The optimum parameters are that the main gas pressure (P₀) is 4.0?MPa, the degree of superheat of the metal melt is 350°C and the wire-feed speed is 50?mm?s^?1. On the condition, the D₅₀ of titanium powder was 40.2?μm and powder morphology was spherical. Satellites rarely existed on the surface of particles.     

Characteristics and printability of K417G nickel-base alloy powder prepared by VIGA method

Nickel-based K417G superalloy powder is the main material used in laser additive manufacturing (LAM) of aerospace engine turbine blades. In this study, K417G powder was prepared by the vacuum induction melting gas atomisation method (VIGA). Powder characteristics and microstructure were measured, and the properties of fabricated samples were studied. The results show that the powder mean particle size D₅₀ was 74?μm, flowability was 16.6?s/50?g, apparent density was 4.78?g?cm^–3, and the oxygen content of the powder was 0.015%. Powder and LAM sample microstructure consisted of γ, γ′, and carbide. The average microhardness was 410?HV, tensile strength was 1080?MPa, yield strength was 828?MPa and elongation was 13.52%. K417G superalloy powder prepared by VIGA method has potential for application in laser additive manufacturing.     

Effect of processing parameters on copper powder produced by novel hybrid atomisation technique

ABSTRACT

In the present work, a novel design of centrifugal atomiser for producing pure copper powder was studied. The novel complementary hybrid system provides an external stream of gas to increase the cooling rate of the atomised particles. Effects of the operating parameters, such as disc rotating speed and gas flowrate on the morphology, particle size distribution, cooling rate and microstructure, were analysed. It was evidenced from the experimental results that the median particle size in the novel atomisation process is mainly controlled by centrifugal disintegration. The microstructure of the produced powders was equiaxed and the grain size decreased with increasing gas flowrate. The cooling rate experienced by centrifugal atomised Cu powders was studied via numerical formulation estimated to be 10⁴–10^6?K?s^?1. The results show that the cooling rate is a strong function of particle size and increasing the rotating disc speed also increased the cooling rates.     

Preparation and printability of 24CrNiMo alloy steel powder for selective laser melting fabricating brake disc

Spherical 24CrNiMo alloy steel powder used for selective laser melting (SLM) fabricating high-speed train brake disc was prepared by the vacuum induction melting gas atomisation (VIGA) method. Powder morphology, particle size, flowability and microstructure were measured. Part properties fabricated by SLM were investigated via some modern analysis method. The experimental results showed that powder mean particle size D₅₀ was 75?μm, flowability was 16.69?s/50?g and apparent density was 4.71?g?cm^?3. 24CrNiMo alloy steel specimen microstructures prepared by SLM consisted of proeutectoid ferrite and granular bainite. Average microhardness was 346?HV, tensile strength was 1223?MPa, extensibility was 13.1% and the product of strength and elongation was 16.1?GPa%. 24CrNiMo alloy steel powder prepared by the VIGA method had good laser printability and huge potential application value for SLM-fabricated brake disc.     

Synthesis of Fine Titanium Powders Via Solution Route

Abstract

Titanium, because of its light weight, high specific strength, corrosion resistance and biocompatibility is a demanding material for aerospace, chemical processing industries and biomedical applications. Titanium powders produced from titanium sponge, hydride-dehydride processes and by a variety of centrifugal atomisation techniques from liquid metals are relatively coarse. If fine size titanium powders can be produced, then the grain size in the sintered titanium will be small thereby contributing to the high strength of the product. In the present investigation, an attempt has been made to synthesize fine titanium powders from titanium dioxide. The later is allowed to form a complex, titanium catecholate, in the presence of ammonium sulphate and concentrated sulphuric acid. The complex is filtered, washed with cold isopropyl alcohol and dried. Titanium hydride is prepared by heating the titanium catecholate in the temperature range 500-600°C in hydrogen atmosphere. The powder obtained is crushed ground and reheated at temperatures upto 950°C in vacuum. The product is rapidly cooled from this temperature to obtain titanium powders. The powder characteristics such as particle size and crystallization have been evaluated and these results are reported and discussed.     

New insights into influencing variables of water atomisation of iron

Abstract

Trace amounts of surfactants have an acute influence on measured surface tension of melts and may influence viscosity. A water atomisation experiment was performed to investigate if variations of these elements could affect quality. Effects of water pressure, melt superheat, and sulphur content, iron scrap oxygen content, and aluminium content were studied. Responses studied were particle size distribution, apparent density, flow, powder chemistry, morphology, green density, and dimensional change. A large sulphur addition reduced the particle size, as a result of a reduction of surface tension, but the largest effect came from changing water pressure. Higher water pressures also resulted in powders with lower apparent density, lower flowrate, and reduced swelling during sintering. An empirical water atomisation model is proposed. Aluminium additions reduced the powder size standard deviation and increased the carbon content of the powder. A reduced powder size standard deviation was seen also for melts with raised superheating.     

Investigation on metal powder production efficiency of new convergent divergent nozzle in close coupled gas atomisation

Abstract

In the present study, a new designed close coupled annular slit type nozzle is discussed for the most efficient powder production and the availability of the finest particle size. Pure tin powder was produced by using nitrogen gas at the Dumlupinar gas atomisation unit. It was found that the finest particle size with the highest efficiency was obtained at the atomising pressure where the maximum aspiration pressure is formed. Above a certain gas/melt flow ratio, the resultant particle sizes cannot be considerably reduced by further increasing atomising pressure. At that point, the applied nozzle system reaches a capacity limit, which also marks the point of the most economic work for the production of fine powders with that system. The main achievement of the nozzle discussed in the present study is that the most efficient powder product is accomplished at relatively low atomisg gas pressures.     

THE INFLUENCE OF EXTRUSION-CONSOLIDATION VARIABLES ON THE INTEGRITY AND STRENGTH OF THE PRODUCT FROM PREALLOYED 7075 ALUMINIUM POWDER

Abstract

A study has been made of extrusion-consolidation processing variables for the production of sound material from spherical 7075 aluminium alloy powder (median particle size 132 μm) canned in evacuated cylinders at ～60% initial density. Maximum product integrity and tensile properties were obtained by extrusion at 644K (700°F)–700K (800°F) and 6:1–10:1 reduction ratio. At lower reduction ratios (2:1 and 3:1) the product exhibited gross cracking and was not completely dense. At a reduction ratio of 40:1, it had significantly poorer tensile properties, attributable to the formation, during extrusion and heat-treatment, of longitudinal cracks at the particle boundaries and to the microstructure produced within the particle grains by the thermomechanical conditions. In general, processing behaviour and product properties were either inferior to or, in some cases, equal to those of wrought material extruded for comparison under the same conditions. Inferior behaviour of the metal powder during processing and tensile testing results from the presence of a brittle oxide film on the surfaces of the particles. Suggestions for improving the processing behaviour of the metal powder and the properties of the extruded product are made.     

PowderMet2009: prospects improving after automotive debacle

Abstract

A homogeneous powder and binder distribution in the green body in powder injection moulding (PIM) is important. In the present study, the mould filling model of PIM has been developed, based on the multiphase fluid theory, viscosity model of feedstock and powder-binder drag force model. The particle Reynolds number is influenced by the particle size and density, resulting in the different drag force between powder and binder. Furthermore, the varied velocity of binder and powder will be obtained with numerical calculation of the continuity equations, leading to the change of green body homogeneity. CFX was used to simulate the mould filling in PIM. The results showed that the homogeneity of green bodies was relative to the filling patterns, which varied with different powder densities. The powders were not suitable for PIM when the particle size was bigger than 20 μm, and the fine powders were beneficial to improve the homogeneity.     

Water debinding behaviour of water soluble Ti-MIM feedstock

Abstract

A water soluble polyethylene glycol (PEG) based binder system was used to formulate three feedstocks containing prealloyed Ti–6Al–4V, NiTi and blended elemental Ni/Ti powders respectively. The selection of these metal powders was to investigate the effect of powder characteristics such as particle size and particle shape. Various solid loadings were investigated in these feedstocks. The water debinding behaviours were systematically studied in terms of debinding temperature, time, sample thickness, powder characteristics and solid loading. Corresponding debinding rate was calculated from the measurements using a shrinking core reaction kinetic model.     

Preparation and printability of high performance 15Cr13MoY alloy steel powder for direct laser deposition

ABSTRACT

A novel 15Cr13MoY alloy steel powder used for direct laser deposition was produced using the vacuum induction gas atomisation (VIGA) method. The characteristics and laser printability of the 15Cr13MoY alloy steel powder were studied. The flowability of the spherical powder was 18.98?s/50?g^?1, and apparent density was 4.75?g?cm^?3. The fabricated 15Cr13MoY alloy steel samples composed of ferrite and bainite were prepared by DLD. The phases primarily consisted of a Fe–Cr–Mn solid solution M₇C₃, M₂₃C₆ and Y₂O₃. At the optimum laser power of 2200W, the average microhardness of 15Cr13MoY alloy steel sample was 350 HV, the tensile strength was 897?MPa, the yield strength was 704?MPa, the average elongation was 14.5%, and the product of strength and plasticity was 13.0?GPa%. The 15Cr13MoY alloy steel powder produced usingthe VIGA method had good powder characteristics and printability for fabricating the components produced by DLD.     

Nitrogen in SL/RN direct reduced iron: origin and effect on nitrogen control in EAF steelmaking

Abstract

In the steel plant considered here, direct reduced iron (DRI), produced by the coal based Stelco–Lurgi/Republic–National (SL/RN) process, makes up 50% or more of the total iron charge. The SL/RN DRI samples from a kiln cooler had high nitrogen contents (50–250 ppm, depending on particle size), contributing to elevated nitrogen levels in liquid steel produced in the electric arc furnaces. The proposed mechanism of nitriding of SL/RN DRI involves gaseous nitrogen (present within the rotary cooler) diffusing into the solid bed and is supported by a simple diffusion model. A strong correlation was found between the melt-in carbon content of the liquid steel and the final tap nitrogen content, with melt-in carbon of 0·3%C or higher resulting in nitrogen levels below 50 ppm at tap, even when charging DRI material that is high in nitrogen.