A. Zambon

Microstructure and phase selection in containerless processing of Fe-Ni droplets

Abstract Fe–Ni droplets (Ni content 0–30xa0at.%) were containerlessly processed by electromagnetic levitation, in a drop-tube or by atomization. The droplet diameter varied from 10xa0 μ m to 7xa0mm. The competition between the formation of b.c.c. and c.c.p. phases is elucidated by studies of the recalescence behaviour and the dendrite growth velocities as a function of undercooling. For levitation the results are summarized in a phase-selection map. For droplets produced in the spray methods, a selection map was constructed which relates the predominant phases and microstructures (coarse-dendritic or grain-refined) to the composition and the droplet size. Links are established between the final microstructures, the crystal phases and the processing conditions. Thermodynamic modelling (CALPHAD) and an analysis of dendrite growth velocities are used to analyse the nucleation and growth behaviour.

Application of microstructure-selection maps to droplet solidification: A case study of the Ni-Cu system

Abstract Ni–Cu droplets were processed by electromagnetic levitation, in a drop-tube and by atomization. The droplet diameters varied from 30xa0 μ m to 7xa0mm, and the entire composition range was studied. Most microstructures observed were coarse-dendritic or refined-equiaxed. Microstructure-selection maps have been constructed for the drop-tube and atomization techniques showing the predominant microstructure as a function of composition and droplet size. From the results on levitated droplets, a selection map has been constructed which relates the microstructure to the composition and the melt undercooling reached prior to nucleation. The results are analysed within a recently developed model for grain refinement which is based upon the fragmentation of primary dendrites. The influence of impurities on the microstructure formation in material of commercial purity is investigated.

Rapid solidification in laser welding of stainless steels

Abstract The microstructural characterization of both weld beads and heat affected zones (HAZ) was carried out on austenitic (AISI 304, 316) and duplex (UNS 31803) stainless steels, laser welded under various working parameters (power, traverse speed, shielding gas), by means of light microscopy, SEM, TEM, and image analysis, with the aim of pointing out changes in the amounts of the present phases, with respect to those predicted by equilibrium diagrams. Moreover, an analytical thermal model of laser beam welding was employed in order to evaluate the cooling rates involved in the process. The thermal field analysis, checked by comparing the calculated and the actual weld beads, has been used as a tool aimed at correlating cooling rates and microstructural characteristics.

Development of solidification microstructures in atomized Fe-Ni alloy droplets

Abstract Inert-gas atomization, using nitrogen as the atomizing medium, has been used to process a series of Fe-Ni alloys. The weight distributions of the powders were bi-modal with relative maxima in the 38 and 60–90 μm ranges. Droplet cooling rates were calculated and the final solidification microstructures were investigated by optical microscopy, X-ray diffraction and transmission electron microscopy (TEM). The smallest droplets were predominantly dendritic with the largest droplets being a mixture of dendritic and grain-refined. For additions of 2 at.% Ni, a number of droplets were found to show solid-state transformations, having a characteristic martensitic microstructure. The primary phase to nucleate was the equilibrium b.c.c. δ-Fe phase and the transformation path was determined to be δ-Fe->-γ-Fe,Ni→α-Fe. TEM examinations of the transformed microstructure showed an interlocked structure with a number of different orientations. In these particles, some duplex b.c.c.-c.c.p. structures were also present. For higher Ni contents, the metastable b.c.c. δ-Fe phase nucleated in preference to the equilibrium γ-Fe,Ni phase.

A simplified model for gas atomization

A simplified model describing the cooling behaviour and the solidification of undercooled droplets in gas atomization has been developed. The resulting computer code can predict the cooling behaviour both in the liquid and in the solid state for any powder size. The droplet velocity in the collecting chamber as well as its temperature, solid fraction and cooling rate can be predicted as a function of the characteristics of the processed alloy, the superheating of the melt, the atomizing gas, its velocity at the nozzles and the droplet diameter. The model overcomes the problem of the undetermined value of the undercooling and evaluates the effects of the simplified approach.

The application of micro structure-selection maps to droplet solidification

Abstract The techniques of electromagnetic levitation, drop-tube processing and atomization are used to study containerless solidification of the Cu-Ni and Fe-Ni systems. Microstructure-selection maps are developed for the case of droplet solidification where droplet diameter and undercooling are the process parameters. The microstructure varies systematically with undercooling and composition, such that grain-refined microstructures are produced at both low and high undercoolings and dendritic microstructures are produced at intermediate undercoolings. The results are analyzed within a recently developed model for grain refinement which is based on the fragmentation of primary dendrites. The role of cooling rate in determining whether the microstructure transforms in the solid state or is retained as a primary structure is explored. Keywords: Cu-Ni; Dendrites; Droplet solidification; Fe-Ni; Grain refinement; Microstructure-selection maps

Characterization of surface chromium and molybdenum alloying on gray cast iron obtained by the plasma-transferred arc technique

Abstract Chromium and molybdenum alloying of gray cast iron to obtain surface wear-resistance coatings by means of the plasma-transferred arc (PTA) technique has been studied. Disk-shaped specimens for mounting on a pin-on-disk wear-testing machine were obtained. The disks were then heat treated to develop different hardness values and wear resistance behavior. Besides an untreated sample, samples treated at 900°C for 10 min, at 1000°C for 10 min and at 1100 °C for 20 min were studied. The specimens showed defect-free coatings with the complete absence of porosity, cracks and segregation. The obtained coatings were stable as regards thermal cycles, which could affect workpieces subject to wear conditions. Microhardness profile, optical microscopy and scanning electron microscopy observations were performed together with qualitative and quantitative energy-dispersive spectroscopy microanalysis, as well as X-ray diffraction on the extracted carbides.

Morphologies in gas-atomized Fe50Ni30Si10B10 amorphizable alloy powders

Abstract In rapid solidification processes, as well as in processes involving high undercooling extents, different solidification structures can result as a consequence of the thermodynamical and thermal parameters involved. Gas atomization can produce both high cooling rates and relatively high undercooling extents. As the cooling rate experienced by the atomized particles depends on their size, both amorphous and well-developed microstructures can be found in an atomization batch if an amorphizable alloy is processed. The solidification morphologies obtained in gas-atomized Fe50Ni30Si10B10, investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) examination, are related with computed cooling rates.

Retained austenite variation in dual-phase steel after mechanical stressing and heat treatment

Abstract Retained austenite changes in a dual-phase steel have been studied after mechanical and thermal treatments. In order to determine the quantitative variations of retained austenite, whose amount in the examined steel is of the order of 5%, Mossbauer spectroscopy has been used. Retained austenite undergoes a martensitic transformation during deformation, but does not transform under the heat treatments performed on the sheet during anticorrosion and painting processes.

Through ribbon cooling rates and related nanostructures in melt spun Fe/Ni base hyperquenched alloys

Abstract An evaluation of the cooling rates and of the speed of the solidification front through the ribbon thickness during the processing of melt spun Fe 40 Ni 40 B 20 and Fe 50 Ni 29 Si 12 B 9 glass forming alloys was carried out by an analytical approach. The characterisation of the ribbons was accomplished by means of light microscopy, X-ray diffraction and TEM examination. The observed structures are related to the computed cooling rates, which are different on the wheel and on the gas side of the ribbons. Heat treatments performed at different temperatures affected both the hardness on the wheel and gas side of the ribbons and the extent of the passive behaviour in potentiodynamic polarisation tests.