Solidification Sequence of Spray-Formed Steels

Abstract

Solidification in spray-forming is still an open discussion in the atomization and deposition area. This paper proposes a solidification model based on the equilibrium solidification path of alloys. The main assumptions of the model are that the deposition zone temperature must be above the alloy's solidus temperature and that the equilibrium liquid fraction at this temperature is reached, which involves partial remelting and/or redissolution of completely solidified droplets. When the deposition zone is cooled, solidification of the remaining liquid takes place under near equilibrium conditions. Scanning electron microscopy (SEM) and optical microscopy (OM) were used to analyze the microstructures of two different spray-formed steel grades: (1) boron modified supermartensitic stainless steel (SMSS) and (2) D2 tool steel. The microstructures were analyzed to determine the sequence of phase formation during solidification. In both cases, the solidification model proposed was validated.

Influence of Al Additions on the Microstructure and Mechanical Properties of a C and Si-Free High-Mn Steel

“…Metals 2020, 10, x FOR PEER REVIEW 7 of 18 to a highly homogeneous equiaxed grains in the microstructure of the deposit, as described in [23,24] and as can be observed in Figures 5 and 6.…”

“…Some peaks related to oxides are observed in the XRD pattern, which are related to the strong oxidation tendency of these compositions, increased by the high specific surface area of the particles. It is well known from the literature that the Metals 2020, 10, 352 7 of 17 dendritic microstructure observed in the droplets leads to a highly homogeneous equiaxed grains in the microstructure of the deposit, as described in [23,24] and as can be observed in Figures 5 and 6.…”

“…From the nonequilibrium models of conventional solidification processes, it would be possible to predict that the final grain size and the stable phases could be different mainly due to the difference in the solidification range and transformations for all three alloys. However, due to the specific solidification mechanism prevailing during spray forming, the final microstructure of the deposit will be the same, except for the grain size, as will be shown in the sequence [18,23,24]. Metals 2020, 10, Figure 4 presents the droplet microstructure of the overspray, i.e., the droplets that did not attach to the deposit during the spray forming process; Figure 5 presents the as-spray deposit microstructure.…”

“…On the other hand, the 5Al alloy presented no well-defined dendrites and some indication of new grains formed by solid state transformation can be observed. Due to the high cooling rates, in the range of 10 2 to 10 4 K/s, the dendritic structure is expected during the solidification of the droplets [18,23]. The high thermal undercooling leads to instability in the solidification growth of the solid interface in face of a negative thermal gradient in the liquid forming the dendritic structure.…”

Wear resistant coatings of boron-modified stainless steels deposited by Plasma Transferred Arc

“…Comparing literature results of dry sand/rubber wheel [16,17], the worn volumes of specimens obtained by spray forming with the same composition are nearly in the range of PTA. Spray forming led to slightly higher abrasive resistance due to the more homogeneous distribution of boride particles and the finer equiaxed grains achieved [17,25].…”