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Materials Science in Additive Manufacturing Carbon addition in IN738LC

Table 1. Chemical composition of IN738LC powders
wt.% Cr Co Al Ti W Nb Ta Si Zr C B Ni
IN738LC 16.06 8.61 3.54 3.58 2.65 0.97 1.69 0.032 0.031 0.124 0.0086 Bal

A B C

D E

Figure 1. The effect of laser power, 115 W and 130 W, on the density and microhardness of IN738LC samples mixed with different amounts of carbon,
from 0 to 0.4 wt.%. (A) The actual as-built samples. (B and C) The optical image and (D and E) microhardness evolution along the build direction of these
specimens.

followed by Vibratory polishing (MasterMet colloidal silica LPBF-built IN738LC does not have any γ’ in its as-built
suspension). state, as they are kinetically suppressed. Thus, the γ’
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Optical microscopy (OM) examination was conducted dissolution treatment no longer justifies its presence.
using an Olympus BX53M microscope equipped with The three-step HT was conceived in this study
Olympus stream motion 2.3.3 software. Scanning electron to promote grain recrystallization and avoid the
microscopy and electron backscattered diffraction were anisotropic performance of the LPBF-produced
performed using a JEOL JSM-IT500HR equipped with an samples. The three-step HT began with a stress-relief
Oxford Symmetry detector, using an accelerating voltage procedure at 845°C for 24 h, which has shown to be
of 20 kV and a step size of 1 μm. Bruker D8 discover effective in lowering the threshold temperature for
diffractometer was employed for the X-ray crystallography recrystallization during HT. Subsequently, the stress-
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(XRD) study, with a Cu Kα radiation and 2D Vantec detector.
relieved specimens were held at 1200°C for 4 h before
Only specimens built with the 130 W laser power being placed at a temperature of 1120°C for 24 h to
input were subjected to subsequent HT in the QSXL-1616 promote a bimodal γ’ distribution, with the fine γ’
box-type electric furnace due to its higher relative density having a size below 100 nm. All heating processes use
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(to be shown in Section 3.1). For HT studies, specimens a constant heating rate of 10°C/min. Water quenching
with different compositions were machined into smaller was adopted after each HT step.
blocks of 2.0 × 4.5 × 1.0 mm. In total, two HT procedures
were adopted to optimize the grain recrystallization and Microhardness tests were carried out on the Innovatest
gamma prime (γ’) morphology, namely, the two-step HT Falcon 5000 machine, using a 0.1 kg and 1.0 kg load,
and the three-step HT. The two-step HT process mimics respectively, with a holding time of 15 s. At least, ten
the standard industrial practice of post-processing the indents were placed on each specimen. Tensile specimens
conventionally made IN738LC. The material first goes have a gauge length of 20 mm, gauge width of 8 mm, and
through a solution treatment at 1120°C for 2 h, with the gauge thickness of 1 mm. They are machined from the
intention of dissolving any previously present γ’. Then, an larger, as-built rectangular blocks. Tensile studies were
aging process is conducted at 845°C for 24 h to promote conducted on an Instron Mechanical Tester 5569 machine,
γ’ precipitations. However, literature results showed that using a nominal strain rate of 0.3 mm/min.
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Volume 3 Issue 1 (2024) 3 https://doi.org/10.36922/msam.2264