Characteristics of Inconel 625—copper bimetallic structure fabricated by directed energy deposition

Abstract

Inconel 625 (In625) is preferred under the circumstances where high strength and corrosion resistance at elevated temperatures are required. However, the restricted thermal conductivity constrains the application of In625 in high heat flux cases. The joining of materials with high thermal conductivity to In625 is capable of improving upon this limitation so as to adapt for temperature-sensitive requirements. In this study, a bi-metallic structure was fabricated by joining In625 on Copper 110 (Cu) substrate with directed energy deposition. Material examination indicated no crack and minor porosity were detected through and along the interface of two materials and the following as-deposited In625. Mechanical performances were characterized at both as-deposited and heat-treated conditions. The resultant yield strength and ultimate tensile strength of as-deposited In625 was 670.97 ± 5.43 MPa and 925.36 ± 9.90 MPa, respectively, with a mean maximum elongation of 0.416 mm/mm. Slightly decreases (by ~ 5%) in tensile strength were observed after heat treatment under 500 °C for 24 h with an enhancement in elongation by ~ 6%. Ductile fracture mode was observed on the fracture surfaces of broken tensile specimens. The impact toughness for as-deposited In625 and heat-treated In625 (under 600° for 24 h) was 118.058 ± 2.285 and 112.045 ± 5.755 J, respectively. A significant improvement in the thermal diffusivity of ~ 100% was experimentally measured when comparing the bi-metallic structure to pure In625. The thickness fraction of Cu played a significant role in the measured thermal diffusivity result.

Appendices

Appendix A

1.1 Derivation of the thermal diffusivity and thermal conductivity equations for Cu-In625 bi-metallic structures

Thermal diffusivity α is defined as:

$$ \alpha =\frac{k}{\rho c} $$

(7)

where k is the thermal conductivity, ρ is the density, and c is the specific heat capacity.

For the Cu-In625 bi-metallic structures as shown in Fig. A1, the thermal diffusivity is:

$$ {\alpha}_b=\frac{k_b}{\rho_b{c}_b} $$

(8)

where b means the Cu-In625 bi-metallic structures.

Fig. A1

Schematic of the Cu-In625 bi-metallic structures

Thermal resistance of a single material is:

$$ R=\frac{x}{kA} $$

(9)

where R is thermal resistance, x is the thickness of the material, k is the thermal conductivity, and A is the area of the cross-section that is perpendicular to the heat flow.

The thermal resistance of the Cu-In625 bi-metallic structures is

$$ {R}_M=\frac{x_c}{k_cA}+\frac{x_i}{k_iA} $$

(10)

where R_M is the thermal resistance of the overall structures, x_i and x_c is the thickness of In625 and copper. K_i and k_c is the thermal conductivity of In625 and copper, and A is the area of the cross-section.

Considering the Cu-In625 as a single structure, then:

$$ \frac{{\mathrm{x}}_b}{k_bA}=\frac{x_c}{k_cA}+\frac{x_i}{k_iA} $$

(11)

$$ \frac{1}{k_b}=\frac{x_{c\prime }}{k_c}+\frac{x_{i\prime }}{k_i} $$

(12)

where ${x}_{c\prime }=\frac{x_c}{x_b}$, and ${x}_{i\prime }=\frac{x_i}{x_{b.}}$

Therefore, from Eq. (12), Eq. (13) can be obtained:

$$ {k}_b=\frac{k_c{k}_i}{x_{c\prime }{k}_i+{x}_{i\prime }{k}_c} $$

(13)

The density of the Cu-In625 bi-metallic structures can be obtained as Eq. (14):

$$ {\rho}_b=\frac{M_{\mathrm{total}}}{V_{\mathrm{total}}}=\frac{M_c+{M}_i}{V_{\mathrm{total}}} $$

(14)

${\rho}_b={V}_{c^{\prime }}\left({\rho}_c-{\rho}_i\right)+{\rho}_i$ where ${V}_{c^{\prime }}=\frac{V_c}{V_{\mathrm{total}}}$ (15)

For the specific heat of the Cu-In625 bi-metallic structures,

$$ {H}_b={m}_b{c}_b{T}_b={m}_c{c}_c{T}_c+{m}_i{c}_i{T}_i $$

(16)

$$ {c}_b=\frac{m_c{c}_c}{m_b}\frac{T_c}{T_b}+\frac{m_i{c}_i}{m_b}\frac{T_i}{T_b} $$

(17)

If assume T_b = T_c + T_i,

$\frac{T_c}{T_b}=\frac{c_c}{c_c+{c}_i}={c}_{c\prime }$ and $\frac{T_i}{T_b}=\frac{c_i}{c_c+{c}_i}={c}_{i\prime }$ (18)

$\frac{m_c}{m_b}=\frac{\rho_c{V}_c}{\rho_b{V}_b}={\rho}_{c\prime }{V}_{c\prime }$ and $\frac{m_i}{m_b}=\frac{\rho_i{V}_i}{\rho_b{V}_b}={\rho}_{i\prime }{V}_{i\prime }$ (19)

Combine Eq. (17) – (19):

$$ {c}_b={\rho}_{c\prime }{V}_{c\prime }{c}_c{c}_{c\prime }+{\rho}_{i\prime }{V}_{i\prime }{c}_i{c}_{i\prime } $$

(20)

Based on Eqs. (8), (13), (15), and (20), the thermal diffusivity of the Cu-In625 bi-metallic structures can be obtained.

Theoretical calculation results

Table 5 Thermal conductivity of the Cu-In625 bi-metallic structures (k_b)

Table 6 Density of the Cu-In625 bi-metallic structures (ρ_b)

Table 7 Specific heat capacity of the Cu-In625 bi-metallic structures (c_b)

Table 8 Thermal diffusivity of the Cu-In625 bi-metallic structures (α_b)