Microstructure and Mechanical Properties of the Interface of Aluminum-Brass Bimetals Produced via Vertical Centrifugal Casting (VCC)

Abstract

Bimetal materials are composed of dissimilar metals, which are increasingly used to fabricate components that withstand harsh thermal and mechanical environments. In this work, aluminum-brass bimetallic hollow cylinders were produced using the vertical centrifugal casting process, and their interface was studied. Aluminum melt, with two different liquid-to-solid volume ratios of 1.5 and 2.5, was cast into brass hollow cylinders preheated to 100–400°C and rotated at 800, 1600, and 2000 rotation-per-minute. The sector-shaped samples were then studied using X-ray diffraction analysis, optical microscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. It was found that the interface consisted of three discernible layers. These included the chill zone (Al₂Cu₅Zn₄ + Al₃Cu₃Zn) near the brass side, platelet precipitate zone (Al₂Cu precipitates scattered in α-Al solid solution matrix), and finally anomalous (or divorced) eutectic grains (α-Al/Al₃Cu) near the aluminum side. Mechanical tests were carried out, in particular Brinell, Vickers and compression tests. The findings revealed that the adhesion of the interface was reduced by increasing the thickness of the interface. Fractography of fractured surfaces illustrated the presence of flat faces (Al₂Cu precipitates) locked together and deep depressions associated with cup-shaped dimples (α-Al/Al₃Cu eutectic).

Appendix

Governing equations

Forces analysis

Mold rotation around its centerline not only causes a centrifuge field but also intensifies the gravitational force, which is exposed on both filling and solidifying liquid metal. By supposing the liquid metal to behave like an incompressible Newtonian fluid, the governing mass and momentum equations can be derived using the Navier-Stokes equations:

$$F-\frac{1}{\rho }\nabla P+\mu \Delta U=\frac{\partial U}{\partial t}+\left(U.\nabla \right)U$$

(a1)

$$\nabla .U=0$$

(a2)

where F, ρ, P, µ, U <u, v, w>, and t represent, respectively, acceleration, fluid density, pressure, viscosity, velocity vector in Cartesian coordinates, and time. The acceleration components are affected by the centrifuge and Coriolis forces (Fig.

Fig. 21

Cell accelerations in different directions.

21). They are applied in three different directions ${a}_{x}$, ${a}_{y}$, and ${a}_{z}$:

$${a}_{x}={\omega }^{2}x+2\omega {v}_{r}$$

(a3)

$${a}_{y}={\omega }^{2}y-2\omega {u}_{r}$$

(a4)

$${a}_{z}=g$$

(a5)

where ω, u_r, v_r, and g, respectively, relate to angular velocity, velocities relative to X and Y directions, and gravity acceleration.^56,57 Moreover, centripetal acceleration is also involved in this process. The acceleration responsible for uniform circular motion is called centripetal acceleration, and its value can be calculated by:

$$a=\frac{{s}^{2}}{r}$$

(a6)

In which r is the mold radius and s represents a tangential velocity of a particle.⁵⁸

Heat transfer

The thermal energy transfer equation in the VCC process can be derived as follows:

$$\frac{\partial T}{\partial t}+\nabla \left(vT\right)=\alpha {\nabla }^{2}T+\frac{h}{{c}_{p}}\frac{{\partial f}_{s}}{\partial t}$$

(a7)

where T and f_s represent temperature and solidified phase fraction, respectively; α also indicates thermal diffusivity and equals to:

$$\alpha =\frac{\uplambda }{{c}_{p}{\rho }_{L}}$$

(a8)

In which λ, ρ_L, h, and c_p are thermal conductivity, fluid density, latent heat, and specific heat, respectively.³² Actually, heat transfer increment at the centrifuge field alleviates the temperature gradient and accelerates the solidification rate so that directional solidification occurs.⁵⁹

Solidification

Mold rotation improves alloy texture and reduces defects by pressurizing the melt.⁶⁰ Generally, rapid heat extraction at the centrifuge field promotes directional solidification.⁵⁹ The continuity and Navier-Stokes equations in a control volume of a fluid with v_x, v_y, and v_z velocity components are as follows:

$$\frac{\partial {v}_{x}}{\partial x}+\frac{\partial {v}_{y}}{\partial y}+\frac{\partial {v}_{z}}{\partial z}=\nabla v=0$$

(a9)

$$\rho \frac{\mathrm{d}v}{\mathrm{d}t}=-\nabla p+\eta {\nabla }^{2}v+\rho g$$

(a10)

Equation (10) can be rewritten for cylindrical coordinates:

$$ \begin{gathered} \rho \left( {\frac{{\partial v_{x} }}{\partial t} + v_{x} \frac{{\partial v_{x} }}{\partial x} + v_{y} \frac{{\partial v_{y} }}{\partial y} + v_{z} \frac{{\partial v_{z} }}{\partial z}} \right) = - \frac{\partial p}{{\partial x}} + \eta \nabla^{2} v + \rho {\text{g}}_{{\text{x}}} \hfill \\ \rho \left( {\frac{{\partial v_{y} }}{\partial t} + v_{x} \frac{{\partial v_{x} }}{\partial x} + v_{y} \frac{{\partial v_{y} }}{\partial y} + v_{z} \frac{{\partial v_{z} }}{\partial z}} \right) = - \frac{\partial p}{{\partial y}} + \eta \nabla^{2} v + \rho {\text{g}}_{{\text{y}}} \hfill \\ \rho \left( {\frac{{\partial v_{z} }}{\partial t} + v_{x} \frac{{\partial v_{x} }}{\partial x} + v_{y} \frac{{\partial v_{y} }}{\partial y} + v_{z} \frac{{\partial v_{z} }}{\partial z}} \right) = - \frac{\partial p}{{\partial z}} + \eta \nabla^{2} v + \rho {\text{g}}_{{\text{z}}} \hfill \\ \end{gathered} $$

(a11)

where ρ, p, $\eta {\nabla }^{2}v$, and ρ_g are, respectively, stood for density, pressure, viscous force, and gravitational force.^60,61