Page 49 - MSAM-3-2

Materials Science in Additive Manufacturing Heat treatment on bimetallic parts

mechanical strength, and improved economic efficiency spectrum of applications ranging from engine components
compared to components manufactured from a single to structural parts in harsh environments. In addition,
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metal alloy. Additive manufacturing (AM) techniques, the thermal expansion coefficients of IN625 and 17-4PH
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categorized into fusion-state and solid-state AM methods, are relatively close, with IN625 nickel-based superalloy
have been utilized to produce bimetallic parts. Fusion- at approximately 12.8 × 10 /°C and 17-4PH SS at
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based processes such as directed energy deposition about 10.8 – 11.0 × 10 /°C. This similarity in thermal
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(DED) and powder bed fusion (PBF) utilize high-energy expansion coefficients means that these two materials
sources such as electrons, wire arcs, and lasers to rapidly tend to expand and contract at similar rates in response
melt feedstocks, which then solidify on the substrate to temperature changes, thereby reducing interface stress
to form the desired part. However, due to the varying and minimizing the likelihood of cracks and defects. The
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thermal coefficients of bimetallic parts, the intense energy co-sintering process in ES-AM is utilized to establish
input and rapid melting and solidification processes interface bonding, concurrently minimizing thermal
often result in cracks and brittle failures. Hybrid cold stress and the development of intermetallic phases, due to
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spray AM is a solid-state AM technique where micron- its operation at relatively low temperatures. These lower
sized particles are accelerated to high velocities by high- temperatures can impede atomic diffusion, leading to pore
pressure, low-temperature gas, impacting a substrate and formation and reduced interface bonding strength. The
undergoing plastic deformation to form a coating layer interface thus emerges as a critical factor in determining
by layer. Extrusion-based sintering-assisted additive the structural integrity of dissimilar metal materials, acting
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manufacturing (ES-AM) employs a printing-debinding- as an essential conduit for the transfer of electricity, heat,
sintering process, utilizing sintering temperatures lower damping, and other properties. 9,18-20
than the melting point to handle bimetallic parts. This Numerous studies have investigated the microstructure
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approach allows for sufficient element diffusion during the of interfaces and the mechanical properties of bimetallic
sintering process, typically resulting in smoother interface parts made from SS and Inconel alloy, which were fabricated
surfaces and fewer intermetallic phases. In addition, using various AM techniques. In a bimetallic part fabricated
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ES-AM offers benefits such as ease of operation, reduced through hybrid DED and thermal milling, consisting of
risk, and an environmentally friendly manufacturing IN718 and 316L, diffusion layers were observed with a
process. 8-10 significant width of 450 mm, featuring a microstructural
Several researchers have investigated the fabrication of transition from columnar to equiaxed dendrites. Niobium
bimetallic parts with fewer defects and cracks in the interface (Nb) and molybdenum (Mo) precipitates were identified
using ES-AM. Bimetallic components comprising low- and in the Inconel near the interface, with no other phases
high-carbon steel have been synthesized through ES-AM; detected. In the interface of a 304SS/IN738L bimetal
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yet, the quality of the resultant part was compromised fabricated by DED, an architecture featuring soft and stiff
due to suboptimal optimization of process parameters, lamellae with inherent interfacial defects was observed,
particularly in the co-sintering process. Employing resulting from partial solute mixing and intricate fluid
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a copper infiltration technique, a composite structure dynamics in the melt pool during rapid solidification.
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featuring a high carbon steel exterior and a copper channel In bimetallic materials composed of 316L and IN625,
was engineered to fulfill the requirements of an injection printed using DED, two types of interfaces were identified:
mold, necessitating adequate mechanical strength, one showing a gradual compositional change with IN625
stiffness, and wear resistance, coupled with superior grains growing epitaxially on 316L grains, and the other
thermal conductivity. In prior investigations, 17-4PH exhibiting a sudden compositional shift that encourages
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and IN625 were processed through ES-AM to fabricate a bidirectional nucleation and grain growth, leading to a
bimetallic component with a smooth interface and cohesive high susceptibility to cracking. In a bimetallic SS/nickel
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bonding devoid of conspicuous cracks. 17-4PH stainless (Ni) alloy manufactured through PBF, no intermediate
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steel (SS) boasts high strength, hardness, and relatively softening from ferrite phase formation occurred, but a
good corrosion resistance, although it may have reduced decrease in hardness at the 316L side was noted, resulting
elongation and temperature stability. Meanwhile, IN625, from the thermal effects of manufacturing nickel alloy.
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a nickel-based superalloy, excels in corrosion resistance In addition, MC carbides form in Ni-based superalloys
and high-temperature performance with reduced strength, during slow solidification due to the strong affinity of
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commonly employed in aerospace and industrial sectors. carbon for these metallic elements. Bimetallic parts
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By combining these two materials in a bimetallic structure, made of IN718 and high-carbon steel have been produced
the aim is to enhance mechanical properties, corrosion using ES-AM, which revealed a layered microstructure in
resistance, and temperature stability, catering to a broad partially sintered high-carbon steel and IN718 particles

Volume 3 Issue 2 (2024) 2 doi: 10.36922/msam.3281