Page 50 - MSAM-3-2

Materials Science in Additive Manufacturing Heat treatment on bimetallic parts

due to the diffusion of Ni and chromium (Cr) toward the Consequently, this study aimed to investigate 17-4PH/
steel granules. 25 IN625 bimetallic composites fabricated via ES-AM. It
Heat treatment techniques are utilized to enhance presents a comparative analysis elucidating the effects
bimetallic bonding by increasing the thickness of the of heat treatment on the interfacial characteristics of
diffusion zone and refining the intermetallic phase bimetallic composites, with a focus on elemental diffusion,
composition. The bimetallic assembly comprising SS316L transition zone thickness, and microhardness evolution.
and IN625, fabricated through arc welding, was heated to To facilitate the expansion of the diffusion zone, specific
970°C to facilitate the transformation of δ-ferrite into the heat treatment conditions were explored, including a
austenite phase in SS316L, thereby improving the bond homogenization treatment at 1150°C, with varying dwell
times and cooling rates, followed by an aging treatment
strength over the as-fabricated sample. The process of
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normalizing applied to the IN625/carbon steel bimetallic to establish an optimized heat treatment procedure for
joint resulted in the recrystallization of the constituent superior interfacial bonding strength.
materials, along with the emergence of a diffusion zone 2. Materials and methods
and the precipitation within IN625. After heat treatment,
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the bimetallic parts made of Ni-based superalloy and SS 2.1. Materials and fabrication procedures
exhibited a secondary phase (Nb-rich phase) near the fusion In this study, 17-4PH SS and IN625 filaments were procured
boundary in the heat-affected zone on the Inconel side from Markforged Corporation., (USA) consisting of
with higher hardness values. Heat treatment effectively metal powders combined with a consistent binder system
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diminishes residual stresses and improves the toughness of comprised of wax and polyethylene. These filaments were
aluminum (Al) bronze-steel bimetallic structures produced fabricated using a dual-nozzle desktop 3D printer (F350,
through AM, resulting in a sample with reoriented Creatbot, China) with a heated nozzle capable of reaching
grains and a more uniform microstructure. Varying the temperatures up to 450°C. An illustrative diagram of the
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temperature or duration of heat treatment led to different manufacturing process is presented in Figure 1A. The
levels of bonding strength at the interface. A continuous 17-4PH SS filament was on the left and first extruded
layer of titanium aluminide (TiAl ) intermetallic was through a heated nozzle to reach a thickness of 2 mm
3
formed in the interface of Al/Ti bimetallic during specific with a layer thickness of 0.1 mm. Then, IN625 was printed
heat treatment conditions, and the shear strength was using the right nozzle to deposit onto 17-4PH parts with
governed by the strength of Al, with minimal impact from a thickness of 0.1 mm. Finally, the overall size of parts is
changes in the interlayer thickness. The bonding behavior 15 × 15 × 4 mm , as shown in Figure 1D. The material
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3
of a copper (Cu)/Al/Cu clad composite was investigated properties for each filament and specific printing settings
under different heat treatment temperatures, uncovering employed for each material are documented in Table 1.
significant formation of intermetallic layers following The processes of binder removal and solid-state sintering
specific heat treatment cycles, such as annealing at 500°C, were carried out utilizing the Markforged production
™
leading to higher ductility and relatively high strength. equipment (Metal X System, USA). A tailored sintering
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For a bimetallic part consisting of SS and carbon steel, profile for IN625 alloy was applied due to its lower melting
the diffusion transition zone exhibited a rising trend with point compared to 17-4PH, with the entire procedure
increasing annealing temperature, resulting in enhanced lasting 29 h. To achieve a denser structure during sintering,
interfacial shear strength and improved ductility; however, a smaller component with a thickness of 2 mm was placed
it had a less pronounced effect on impeding fatigue on top of the bimetallic part. The printed and sintered
crack propagation along the interface. Heat treatment bimetallic parts are presented in Figure 1B and C. The alloy
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between 800°C and 1100°C for 30 min to 2 h significantly compositions for 17-4PH and IN625 feedstocks are listed
improved the properties of a bimetallic low-carbon steel in Table 2. In addition, the properties of single metals are
and austenitic-SS structure, increasing its ultimate tensile listed in Table 3 for comparison with bimetallic parts.
strength by 35% and elongation by 250%. 33
The heat treatments were conducted at a steady
Nevertheless, there is insufficient research on the effects temperature of 1150°C and a heating rate of 10°C/min,
of heat treatment on the microstructure and mechanical with durations of 1, 4, and 8 h, labeled as HT1, HT4, and
properties of 17-4PH/IN625 bimetals produced through HT8, respectively. The utilization of this high constant
ES-AM. The co-sintering process, conducted at relatively temperature and extended holding time was aimed at
low temperatures, results in pore formation and weaker effectively broadening the transition zone. Typically,
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bonding strength, necessitating heat treatment to enhance a homogenization temperature exceeding 1000°C is
bonding strength and overall material properties. required for 17-4PH to attain complete supersaturation

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