Microstructural features and heat flow analysis of atomized and spray-formed Al-Fe-V-Si alloy

Microstructural features of rapidly solidified powders and preforms of Al₈₀Fe₁₀V₄Si₆ alloy produced by spray forming process have been studied. The atomization and spray deposition were carried out using a confined gas atomization process and the microstructural features were characterized using scanning electron microscopy and transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The microstructure of a wide size range of atomized powders invariably revealed cellular and dendritic morphology. The extent of dendritic region and the dendritic arm spacing were observed to increase with powder particle size. The TEM investigations indicated the presence of ultrafine second-phase particles in the intercellular or interdendritic regions. In contrast, the spray deposits of the alloy showed considerable variation in microstructure and size and dispersion of the second-phase particles at specific distances from the deposit-substrate interface and the exterior regions of the deposit. Nevertheless, considerable homogeneity was observed in the microstructure toward the center of the spray deposit. The formation and distribution of a cubic phase α-Al(Fe,V)Si has been characterized in both atomized powders and spray deposits. A one-dimensional heat flow model has been used to analyze the evolution of microstructure during atomization and also during spray deposition processing of this alloy. The results indicate that thermal history of droplets in the spray on deposition surface and their solidification behavior considerably influence the micro-structural features of the spray deposits.     

Further discussions on the solute redistribution during dendritic solidification of binary alloys

After a review over former works about the solute redistribution during dendritic solidification, a new“local solute redistribution equation ”is deduced based on Flemings's model, where lim-ited diffusion in solid during solidification is carefully treated. Because a form parameter is also included, the equation can be used for the solidification processes with different shapes of den-drites. By solving the equation at the condition of directional solidification, more completef _s -C, functions for both needlelike and platelike dendritic solidifications with both linear and parabolic solidification rates are obtained. As examples, the volume fractions of nonequilibrium phase in Al-4.5 pct Cu alloy is evaluated with differentf _s -C _l functions. On the thinking that the dendrites in actual solidification process is usually between needlelike and platelike ones, the volume fraction of the nonequilibrium phase is suggested to be in the region between the one calculated by the model for platelike dendrites and that for needlelike dendrites. The relationship between the region and local solidification time is also presented by figures, which are compared with the data of former researchers.     

Texture and Crystal Orientation in Ti-6Al-4V Builds Fabricated by Shaped Metal Deposition

The texture and crystal orientation of Ti-6Al-4V components, manufactured by shaped metal deposition (SMD), is investigated. SMD is a novel rapid prototyping tungsten inert gas (TIG) welding technique leading to near-net-shape components. This involves sequential layer by layer deposition with repeated partial melting and heat treatment, which results in epitaxial growth of large elongated prior β grains. This leads to a directionally solidified texture, where the prior β grains exhibit only a small misorientation with each other. The β grains grow in \( \left\langle { 100} \right\rangle \) direction with a second \( \left\langle { 100} \right\rangle \) direction perpendicular to the wall surface. During cooling, the α phase transformation follows the Burgers orientation relationship leading to a Widmanstätten structure, with orientation relations between most of the α lamellae and also of the residual β phase. The directionally solidification and the transformation into the α phase following the Burgers relationship results in a texture, where the hcp pole figures look similar to bcc pole figures.     

On the influence of in-situ reactions on grain size during reactive atomization and deposition

A fundamental study of the factors that govern grain size of 5083 Al processed via reactive atomization and deposition (RAD) is reported. Microstructural observation shows that the average grain size in RAD 5083 Al is slightly smaller than that in the material processed via N₂ spray deposition (SDN). A numerical approach, together with measurements of the temperature histories inside the deposited materials, is implemented to analyze the influence of in-situ reactions during RAD process on the evolution of grain size. The numerical results show that RAD 5083 Al possesses a slightly higher density of nuclei relative to that present in SDN 5083 Al on a per unit volume of deposited material basis at the beginning of the slow solidification of remaining liquid phase. Furthermore, the RAD material exhibits a slightly lower coarsening extent during the slow solidification. Grain growth is negligible during the solid-phase cooling. Accordingly, the calculated grain size in RAD 5083 Al is slightly smaller than that in SDN 5083 Al, consistent with the observed results.     

Up‐scaling of a Low Temperature PACVD Process for the Deposition of α/γ‐Nanocrystalline Alumina Coated Tools for Thixocasting of 1.2208 steel

An industrial‐scale pulsed plasma‐assisted chemical vapor deposition (PACVD) process for crystalline alumina growth was developed. To obtain a homogeneous coating thickness distribution over complex geometries and large dimensions, a suitable gas injection system was designed. A phase formation diagram for alumina coatings as a function of pulse length and cathode voltage has been compiled, allowing for the deposition of dense α/γ‐Al₂O₃ coatings at a substrate temperature of 590 °C. Moulds coated with an α/γ‐Al₂O₃ coating were utilized in steel thixocasting at temperatures of ～1400 °C. The coatings were intact after thixocasting and showed significantly improved chemical wear resistance compared to plain steel moulds.     

SOME RELATIONS BETWEEN THE STRUCTURE AND MECHANICAL PROPERTIES OF WC–TiC–Co ALLOYS

Abstract

The relationship between the mechanical properties and the structure of the sintered carbide WC–TiC–Co has been studied. Specimens containing 7 and 15% cobalt were sintered at temperatures between 1350 and 1550°C (1625 and 1825 K), for times varying from to 32 h. The structure was examined by electron. microscopy. Density, coercive force, hardnesss, transverse rupture strength, and energy of crack initiation were determined.

The rate of grain growth is governed by the rate at which the carbide dissolves in the cobalt phase. The activation energy for growth was found to be 120 ± 15 kcal/mol (500 ± 63 kJ/mol).

The coercive force is a linear function of the specific grain surface rather than of the specific cobalt surface. The hardness of alloys with different cobalt contents is a function of a single structure parameter [(1– f_β)/f_β]S_γγ, where f_β is the volume fraction of β phase and S_γγ is the specific grain-boundary surface.

It has been suggested that transverse rupture strength should vary as the square root of the specific grain surface. The present results tend to confirm this suggestion. The energy of crack initiation is highly dependent on the contiguity of the carbide phase.     

Phase evolution of AISI 321 stainless steel during directional solidification

Abstract

The phase evolution of AISI 321 stainless steel was studied by directional solidification and quenching techniques. Two interfaces, solid/liquid and the peritectic reaction interface, were found to exist in the directional solidification structure. With increasing growth velocity the solid/liquid interface changed in the sequence of planar, cellular, dendritic and the primary phase changed from austenite to ferrite. The phase and morphology selection was verified by the interface response functions (IRFs) and the maximum growth temperature criterion. The ferritic island banding structure was observed, not only in the austenite cellular primary growth condition (3 μm s^?1), but also in the dendritic ferrite primary growth one at relatively low growth velocity (5 μm s^?1). It is deemed that the former resulted from the nucleation of ferrite in the continuous matrix of austenite phase, yet the latter is the residual primary ferrite attributed to the growth of austenite. Both of them do not come from the nucleation near the solid/liquid interface.     

Microstructural investigation of as cast and PREP atomised Ti–6Al–4V alloy

Abstract

A microstructure characterisation of Ti–6Al–4V is conducted for cast, extruded and micrometre sized particles. The plasma rotating electrode process is used to produce spherical Ti–6Al–4V powders from an alloy electrode. The process parameters and their impact on the material properties are described. The effects of electrode rotation speed on the particle size distribution, particle shape and crystal structure are investigated in detail. Optical microscopy and scanning electron microscopy are used for microstructural characterisation. The analysis shows that cast and extruded Ti–6Al–4V alloys have equiaxial α and α+β phase structures, while plasma rotating electrode processed powder from the same alloy compositions has an acicular or martensitic (α) structure. The microstructure scale depends on the particle size. Microhardness measurements are used to assess mechanical property dependence on the microstructure of this alloy. The rapidly cooled alloy particles have much higher hardness than cast or extruded bulk alloy.     

Microstructural Evolution and Bonding Behavior during Transient Liquid-Phase Bonding of a Duplex Stainless Steel using two Different Ni-B-Based Filler Materials

Microstructural evolution and bonding behavior of transient liquid-phase (TLP) bonded joint for a duplex stainless steel using MBF-30 (Ni-4.5Si-3.2B [wt pct]) and MBF-50 (Ni-7.5Si-1.4B-18.5Cr [wt pct]) were investigated. Using MBF-30, the microstructure of the athermally solidified zone was dependent on B diffusion at 1333.15 K (1060 °C). Ni₃B and a supersaturated γ-Ni phase were observed in this zone. BN appeared in the bonding-affected zone. However, using MBF-50, the influences of base metal alloying elements, particularly N and Cr as well as Si in the filler material, on the bond microstructure development were more pronounced at 1448.15 K (1175 °C). BN and (Cr, Ni)₃Si phase were present in the bond centerline. The formation of BN precipitates in the bonding-affected zone was suppressed. A significant deviation in the isothermal solidification rate from the conventional TLP bonding diffusion models was observed in the joints prepared at 1448.15 K (1175 °C) using MBF-50.     

Effect of Fluid Convection on Dendrite Arm Spacing in Laser Deposition

Ni superalloys are widely used for hot section components in jet engines because they are very resistant to corrosion and maintain reasonably high strength at elevated temperature. However, the repair cost of the parts is high, partly due to the complexities of process variable optimization and control in laser cladding. In particular, optimizing the process parameters by experiments is time-consuming and costly. The microstructure and properties of the metal deposit are significantly influenced by values temperature gradient G and solidification rate R at the weld pool solidification boundary. Optimized values can help to reduce defects and improve properties of laser deposits. Optimization is hindered by the fact that the clad melt pool is hot and small, making in situ measurement of such solidification conditions difficult. Numerical simulation of the laser deposition process is a possible alternative to experimental measurement to obtain values of clad solidification parameters. In this investigation, G and R values at the weld pool solidification boundary were obtained from a three dimensional numerical simulation of laser deposition process and melt pool. The primary dendrite arm spacing and cooling rate of the deposited material were then correlated to these solidification conditions.     

Analysis of heat flow and microstructure evolution during spray deposition of Fe-3C-1.5Mn alloy

A theoretical model for the concomitant solidification of droplets and preform during spray deposition has been proposed, based on heat-flow analysis. It has been unambiguously demonstrated that cooling rates approaching those in the rapid solidification (RS) regime can only be achieved when the droplets are still in free flight during the deposition process. The cooling rates in the droplets range from 10⁴–10⁶ Ks^?1 depending upon their size for the experimental conditions employed in the present studies. In contrast, the model predicted cooling rates for the deposits in the region of 10³–10⁴ Ks^?1. A hypoeutectic Fe-3C-1.5Mn-0.3Si has been chosen as an experimental alloy for studies relating to microstructural characterization. The microstructure of powder developed fully during solidification of droplets in free flight revealed dendritic morphology of the metastable austenitic phase, whereas the spray-deposited alloy exhibited characteristic homogeneous and refined substructure. The evolution of microstructure during spray deposition as also during atomization has been compared and discussed by invoking the proposed model.     

Solidification of an alloy 625 weld overlay

The solidification behavior (microsegregation, secondary phase formation, and solidification temperature range) of an Alloy 625 weld overlay deposited on 2.25Cr - 1Mo steel by gas metal arc welding was investigated by light and electron optical microscopy, electron microprobe, and differential thermal analysis techniques. The overlay deposit was found to terminate solidification at ≈ 1216 °C by aγ/Laves eutectic-type reaction. The Laves phase was highly enriched in Nb, Mo, and Si. The solidification reaction and microsegregation potential of major alloying elements in the overlay deposit are compared to other Nb-bearing Ni base alloys and found to be very similar to those for Alloy 718. Solidification cracks observed in the overlay were attributed to the wide solidification temperature range (≈170 °C) and formation of interdendritic (γ+Laves) constituent. Reasonable agreement is obtained between the calculated and measured volume percent (γ+Laves) constituent with the Scheil equation by treating the overlay system as a simpleγ-Nb “binary” and using an experimentally determinedk ^Nb value from electron microprobe data.     

The Effect of Bath Parameters on the Electrocrystallisation of Cox–Cu100−x Alloys on Stainless Steel Cathode

The electrocrystallisation of the alloys of Co_x–Cu_100?x onto stainless steel cathode was investigated by performing cyclic voltammetry (CV) to understand the mechanism of deposition. The deposit morphology and crystal structure of deposit were analysed using scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The kinetic parameters were obtained from the cathodic polarisation of the CV to predict the electron transfer mechanism in the process. The transfer coefficient value (α) of the kinetic parameter revealed that both cathodic and anodic processes were unsymmetrical. It was demonstrated that the current efficiency of the deposit increased from 96.8% at pH 4.0 to 99.2% at pH 7, and then it dropped to 89.7% at pH 8. Before the deposition of the Co–Cu alloy, the initial copper deposition occurred at ??0.24 V and peaked at ??0.66 V. This was followed by the deposition of the Co–Cu alloy at ??1.04 V, which occurred after the deposition potential of Cu²⁺ (??0.24 V) and Co²⁺ (??0.89 V). The current then increasesd with a small increment in applied potential due to subsequent diffusion-controlled copper reduction along with the co-deposition of Co. The variation in the kinetic parameters was also reflected in the current efficiencies, the deposit morphologies, the crystallographic orientations and the nucleation overpotential values. The percentage of cobalt content in the alloy was observed to decrease in at.% from 54.35% at pH 4 to 49.86% at pH 6 and further to 20.62% at pH 8. The structure of the deposited alloy confirmed the formation of a single solid solution phase having different planes such as (222), (311), (220), (200) and a sharp peak due to face-centred cubic structure with (111) plane. This strong peak along with other similar peaks were observed in all the XRD of the deposit obtained at pH 4, 6 and 8. The morphology of the deposit characterised by the SEM showed that the deposit changed from a bitter gourd to a regular cauliflower-like structure as the pH value changed from 4 to 8.

     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part II. Containerless solidification experiments

The solidification behavior of undercooled Fe-Cr-Ni melts of different compositions is investigated with respect to the competitive formation of δ-bcc (ferrite) and γ-fcc phase (austenite). Containerless solidification experiments, electromagnetic levitation melting and drop tube experiments of atomized particles, show that δ (bcc) solidification is preferred in the highly undercooled melt even at compositions where δ is metastable. Time-resolved detection of the recalescence events during crystallization at different undercooling levels enable the determination of a critical undercooling for the transition to metastable bcc phase solidifcation in equilibrium fcc-type alloys. Measurements of the growth velocities of stable and metastable phases, as functions of melt undercooling prior to solidification, reveal that phase selection is controlled by nucleation. Phase selection diagrams for solidification processes as functions of alloy composition and melt undercooling are derived from two types of experiments: X-ray phase analysis of quenched samples and in situ observations of the recalescence events of undercooled melts. The experimental results fit well with the theoretical predictions of the metastable phase diagram and the improved nucleation theory presented in an earlier article. In particular, the tendency of metastable δ phase formation in a wide composition range is confirmed.     

A unified microscale-parameter approach to solidification-transport phenomena-based macrosegregation modeling for dendritic solidification: Part I. Mixture average-based analysis

Based on the scale-length comparisons of the microscale heat or solutal-mass transfers with dendrite structures, a general form for the time-differential mixture-averaged composition (TDMAC) term for a macroscale metallic dendrite solidification model with any finite solid backdiffusion (SBD) was shown to be necessary and valid. Starting from such an integral term-included TDMAC expression with a general dendrite geometric modeling, Fick’s second diffusion law and the species massconservation principle were applied to confirm the equivalency of a unified Φ-parameter-involved all-differential TDMAC form to the original TDMAC term. Through the modeling, the unified microscale parameter Φ was found to be a function of the Fourier diffusion number with dendrite geometrical modifications (ϕ) and of another nondimensional parameter, θ, representing the sensibility of the interdendritic-liquid-concentration variation in response to the SBD inside the growing dendrites. In the solved ϕ-parameter function of ϕ= (D _S(T)/R _f)ζ·A _2N , the scalar vector product defines the geometric modification accounting for a general growing dendrite morphology with five basic units of spherical, cylindrical, platelike, inward cylindrical, and inward spherical shapes. Through an approximate solution to the integral equation with respect to the solutal-mass solid/liquid (S/L) interface flux, (Eq. [19]), the unified Φ parameter was proposed to take a function form of Φ=θ·ϕ/(1+θ · ϕ), where θ=(1+β) · k · f _S/f _L ² . The result discussions and ϕ-f _S, and θ-f _S, and Φ-f _S curve sample calculations on an Al-4.5 pct Cu alloy with different constant alloy properties, solidifying phase morphologies, and solidification parameters were carried out to investigate the behaviors of various influential factors on the extent of SBD in a dendrite solidification process.     

Phase decomposition of rapidly solidified Fe-Mn-Al-C austenitic alloys

The phase decomposition of two (Fe_0.65Mn_0.35)_0.83Al_0.17-xC (x=3 and 4 at. pct orx=0.74 and 0.98 wt pct) austenitic alloys prepared by rapid solidification (RS) has been investigated on aging at 823 and 923 K by means of X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Under low bulk carbon supersaturation conditions (823 K aging of low carbon alloy and 923 K aging of high carbon alloy), zones formed preferentially at the cellular boundaries and in the bands in the {100} planes, giving rise to line broadening in the X-ray diffraction patterns. On the other hand, the initial aging under high, carbon supersaturation condition (823 K aging of high carbon alloy) resulted in the sideband formation, resulting from homogeneous structural modulation in the <100>_γ directions throughout the grain. The bulk carbon supersaturation dependence of initial decomposition modes indicates that carbon atom fluctuations are crucial in the initial state of phase decomposition, and that the observed {100} modulated structure corresponds to a structure consisting of alternate carbon-rich and carbon-poor zones. Together with the interstitial clustering process, an fcc-based substitutional ordering reaction concurrently took place. Later on these zones were replaced by a coherent metastable phase in the matrix, which was finally transformed into the cubic carbide (κ carbide) of (Fe, Mn)₃AlC_x chemical formula with the L'1₂ structure. However, at the end, a combined heterogeneous β-Mn and κ carbide precipitation seemed to finalize the decomposition process over the matrix κ carbide precipitation.     

Synthesis of ultrafine WC/Co powder by mechanochemical process

Abstract

A new approach to produce ultrafine WC/Co powder by a mechanochemical process was made to improve the mechanical properties of advanced hardmetals and to cut production costs. For powder preparation, the water soluble salts containing W and Co components were used as starting materials. After synthesis of the precursor powder from an aqueous solution by a spray drying technique, a salt removing heat treatment in air atmosphere was carried out to prepare the oxide powder. The oxide powder was mixed with carbon black by ball milling and this mixture was converted at 800^oC to the nanophase WC/Co powder in H₂ and N₂ atmospheres. The average size of the WC particle was 100-150 nm. The possibility of achieving high density sintered material with an ultrafine and homogeneous microstructure using grain growth inhibitors, such as tantalum and vanadium carbides, has been shown.     

Spray deposition of a Sn-40 Wt Pct Pb alloy with uniform droplets

An apparatus capable of producing and depositing uniform droplets (100 to 200 μm in diameter) was developed and used to study the relationship between spray deposition parameters and the microstructures of Sn-40 wt pct Pb alloy spray deposits. The sprays used in the study consisted of uniform droplets, either 103 or 178 μm in diameter, that were in identical thermal and solidification states as they impacted the substrate. The thermal and solidification states of the uniform droplets were determined as a function of the flight distance (the distance from the metal pouring orifice) by model calculations and calorimetric measurements assuming equilibrium solidification. Although a fair agreement was noted between the model and the calorimetric measurements at small flight distances, corresponding to large liquid fractions, the calorimetric measurements indicated 10 to 20 pct higher liquid fractions at larger flight distances. The resultant microstructures comprised either a mixture of the Pb-rich and Sn-rich phases, both in an equiaxed morphology, or a lamellar eutectic structure with a small amount of the Pb-rich primary phase in a coarse spherical morphology. The mostly lamellar eutectic structure resulted from an excessive enthalpy flux and/or slow heat extraction from the deposit. Fine, equiaxed, two-phase microstructures and high deposit density resulted from optimal combinations of droplet enthalpy, deposition rate, droplet size, and deposit cooling rate which gave short local solidification times.     

Unidirectional solidification of Al-Si eutectic with the accelerated crucible rotation technique

The accelerated crucible rotation technique (ACRT) is applied to Al-Si eutectic to reveal the effect of forced convection on the unidirectional solidification structure. Experimental results show that if the crucible rotation is started from the beginning of the unidirectional solidification, irregular eutectic with short bars and chunks of silicon will be formed. But if a normal Bridgman process is experienced before the crucible rotation is applied, a more satisfactory directional needle-like silicon structure where the silicon is straight, long, and more regular will be formed during the ACRT process. In order to get a satisfactory unidirectional needle-like silicon structure, ACRT must be used with suitable solidification parameters (temperature gradientG and growth rateR). The rotation method of the crucible is also important. Formerly Associate Professor, Department of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China