Page 43 - ESAM-1-2

Engineering Science in
Additive Manufacturing Multi-material additive manufacturing of metals

A B An irregular elliptical island-like heterogeneous phase
appeared in both material-rich regions due to the high-
energy laser beam penetrating through several layers to
re-melt the solidified SS316L. 56,58,142 The rapid movement
of the laser beam caused the molten pool to be unstable.
However, CuSn10/SS316L exhibited fewer cracks than
SS316L/CuSn10. The crack formation was attributed
C D to liquid metal embrittlement, driven by differential
elemental diffusion rates at high temperatures, consistent
with the Kirkendall effect, similar to observations by
Liu et al. Similarly, in SS316L/C18400 and SS316L/
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C25400 bimetallic structures, cracks were observed at
the transition region and extended toward the SS-rich
region in both alloy combinations due to the high residual
stress in SS316L owing to its low CTE. A similar behavior
E was observed in the SS316L/Cu/Ti-6Al-4V MMAM
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structure using a Cu-alloy as an intermediate bonding
layer (IBL).
From these studies, it is evident that in SS/Cu bimetallic
structures, crack formation is primarily driven by dissimilar
thermal and mechanical properties between metal alloys.
Compared to SS/Ni bimetallic structures, which generally
F G exhibited good metallurgical bonding, SS/Cu combinations
were more prone to cracking, largely due to the significantly
higher thermal conductivity of Cu relative to SS. In SS/Cu
structures, cracks propagated in the Cu-rich region due to
the deeper penetration of Cu into the SS alloy. Conversely,
in Cu/SS bimetallic, a small amount of Cu penetrates the
interface of SS316L, and a brittle Cu film with low strength
is formed. In the subsequent cooling process, due to the
difference in the thermophysical properties between Cu
Figure 5. Interfacial meso- and microstructural characteristics of and SS, the degree of shrinkage between Cu and SS is not
stainless steel and copper bimetallic alloys fabricated through multi- consistent due to high residual stress, resulting in thermal
material-laser powder bed fusion. (A-D) SS316L/C18400 and (E-G)
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140
SS316L/C52400 . Scale bars: 20 μm, 50 μm, and 100 μm, magnifications: penetration cracking on the SS side.
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(E and G) 300×. Reprinted with permission from Liu et al. (Copyright © In addition to the SS/Cu bimetallic structure, SS316L/Zr
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2014, Elsevier Inc.) and Bai et al. (Copyright © 2020, Elsevier Inc.). The
images reveal the interfacial characteristics associated with each copper and SS410/Zr bimetallic structures exhibited vertical cracks
alloy composition, highlighting variations in metallurgical bonding, during fabrication due to the differences in CTE and
porosity formation, elemental diffusion, and phase transformation. dissimilar thermal properties. In contrast, SS316L/17-
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4PH structures displayed a smooth transition without
SS316L-rich region, attributed to the high content of Cu intermetallic phase formation, attributed to the similarities
(Figure 5E-G). Further analysis revealed that the cracks in thermal conductivity and CTE and a narrow thermal heat
formed within the SS-rich region were visible from high- gradient between the two materials. Finally, SS316L/W
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resolution optical microscopy (Figure 5F and G; indicated bimetallic structures exhibited larger, irregular pores in the
by the yellow arrows). SS-rich region, which were closer to the transition region.
Extensive research on SS316L/CuSn10 bimetallic These pores and cracks were formed due to inadequate
structures has concluded that good metallurgical bonding process optimization, excess thermal stress gradient from
between SS/Cu is achievable. 58,139,142,183 The width of the high heat input from the molten pool, and mismatch of
transition zone between SS316L/CuSn10 and CuSn10/ CTE values between the two materials. From all the above
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SS316L was about 400 μm and 160 μm, respectively, miscellaneous studies, it was evident in SS316L/Zr, SS410/
indicating a higher bonding strength on the SS316L/ Zr, and SS316L/W that thermal properties such as thermal
CuSn10 side compared to the CuSn10/SS316L side. conductivity, CTE, and temperature gradient played a
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Volume 1 Issue 2 (2025) 11 doi: 10.36922/ESAM025180010