Guilherme Zepon

Microstructure and wear resistance of spray-formed supermartensitic stainless steel

Since the early 90’s the oil industry has been encouraging the development of corrosion and wear resistant alloys for onshore and offshore pipeline applications. In this context supermartensitic stainless steel was introduced to replace the more expensive duplex stainless steel for tubing applications. Despite the outstanding corrosion resistance of stainless steels, their wear resistance is of concern. Some authors reported obtaining material processed by spray forming, such as ferritic stainless steel, superduplex stainless steel modified with boron, and iron-based amorphous alloys, which presented high wear resistance while maintaining the corrosion performance

Solidification Sequence of Spray-Formed Steels

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.

Mechanochemistry and H-sorption properties of Mg2FeH6-based nanocomposites

Abstract Mg2FeH6 is a promising material for hydrogen storage applications, since it presents the highest known volumetric capacity of 150 kg m−3 of H2 and its metallic constituents are inexpensive. The major drawbacks for its application are the difficulties associated with its synthesis and also its high thermal stability. In this paper, Mg2FeH6-based nanocomposites were prepared from the elements via reactive milling. A high-yield of the complex hydride synthesis was obtained after a systematic processing study, and the best conditions were successfully extended to the mechanochemical synthesis of Mg2CoH5. The influence of different additives such as transition metals, transition metal fluorides and graphite on the H-sorption behavior of the Mg2FeH6-based nanocomposites was evaluated. Mixtures rich in both MgH2 and Mg2FeH6 hydrides present lower temperature ranges of hydrogen release than those which are rich in only one of these hydrides. The MgH2–Mg2FeH6-based nanocomposite presents ultra-fast H-sorption kinetics at 300°C with partial reversible formation of Mg2FeH6.

Microstructural Evolution in Spray Forming

Spray forming is a casting process in which the molten metal is directly converted to a solid bulk with unique characteristics. When processed under optimum conditions, spray formed materials typically present microstructures composed of refined polygonal (non-dendritic) grains, uniformly distributed with low levels of micro- and macro-segregation. This set of characteristics is achieved regardless of the alloy system, making spray forming an attractive process to produce alloys where processing by conventional casting techniques is complicated. This chapter is dedicated to presenting the mechanisms that take place when the atomized droplets arrive at the deposit surface, and how the spray-formed microstructures evolve during deposition. It will be seen that spray forming is a self grain-refining casting process and cannot be considered a rapid solidification technique. Section 7.6 will address the main differences between the microstructural evolution in spray forming and other spray deposition or “thermal spray” processes. These processes include plasma spraying, high velocity oxy-fuel, wire arc spraying, detonation gun spraying, etc. In this way, it will show why spray forming is such a unique process. This chapter is also dedicated to presenting how the porosity—an intrinsic feature of spray-formed microstructures—is generated and how the processing parameters affect its type, size and distribution. Furthermore, the generation of other defects related to the solidification and/or to the cooling of the spray formed product after deposition—such as residual stresses and hot cracks—and their influence on the product quality and material properties will be presented. Finally, this chapter will also discuss the effect of the atomization gas (Ar, N2 or He) on the final product quality in terms of porosity and chemical composition of steels, superalloys, and copper alloys.

Processing Aspects in Spray Forming

Several atomization techniques have been used to spray form a bulk material. Detailed information about those techniques is described in Chapter 2. This chapter focuses on differences in process conditions in spray forming using single fluid and gas atomization as well. Specially, the conditions (e.g. mass flux and enthalpy distribution) in the spray cone of a free fall atomizer are reported in detail. Furthermore, the deposition process is described including topics like overspray, yield, sticking efficiency and temperature history of the deposit. These are important issues with relevance to processes similar to spray forming, such as current thrusts in Additive Manufaturing (AM).