**4.1. Effect of bonding time on joint properties**

316 Advanced Aspects of Spectroscopy

completely filled.

given by equation 4:

of a thin foil, powder or coating [27, 28] which is tailored to melt by eutectic or peritectic reaction with the base metal. The liquid filler metal wets the base metal surface and is then drawn into the joint by capillary action until the volume between components to be joined is

The driving force of TLP bonding is diffusion. A process which can be described by Fick's first and second laws. The first law describes diffusion under steady-state conditions and is

*<sup>C</sup> J D*

Fick's second law describes a non-steady state diffusion in which the concentration gradient

*<sup>C</sup> <sup>C</sup> <sup>D</sup> t x* 

Equation 6 shows a general solution for Equation-5 using separation of variables is [29]:

<sup>4</sup> <sup>1</sup> ( ,) ( )

*Cxt Dt fe d D t*

<sup>0</sup> 1 2 ( ,) <sup>2</sup> 4 4 *<sup>C</sup> xx xx C x t erf erf Dt Dt* 

If the following boundary conditions are applied to Equation-7, the concentration as a

4 4

 

TLP bonding can be conducted by one of two distinct methods. Method-I employs a pure interlayer which forms a liquid through eutectic reaction with the base metal and Method-II employs an interlayer with a liquidus temperature near the bonding temperature [2]. Method-II is most commonly used as it reduces the overall process time by decreasing the volume of solute to be diffused from the interface before the liquid is formed. On the other hand, method-I can be considered to be more effective in TLP bonding as the eutectic reaction is able to displace surface oxides during bonding. TLP bonding process was first divided into five discrete stages by Duvall et al. [30]. These stages were: heating, melting,

 

*x x*

( 0) ( , 0) and ( , 0) 0 (0 ) <sup>0</sup>

*<sup>x</sup> kx C x t C erf erf Dt Dt*

*C x o o C x C x C x*

2

Where the error function solution for equation 6 is shown in equation 7:

function of time can be calculated using Equation-8:

0

( ,) 1

changes with time and can be expressed as shown in equation 5:

*x*

2 2

) (

*x*

(6)

(4)

(5)

(7)

(8)

Wavelength dispersive spectroscopic analyses of joints bonded at 600oC using a 5 μm thick Ni-Al2O3 coating as the interlayer as a function of bonding time as shown in Table 1. For joints bonded for 1 minute a large concentration of Ni remained at the interface after bonding. However, when the bonding time was increased to 30 minutes resulted in the elimination of the interface and an increase in the grain size within the joint. This disrupts the band of segregated particles at the interface (see Figure 4) and chemically homogenized the joint zone.

**Figure 4.** Light micrographs of joint bonded with 5μm thick Ni–Al2O3 coating for: (a) 1 min (b) 30 min [23].

The segregation of particles was accredited to the pushing of micro-Al2O3 particles by the solidifying liquid-solid interface. Stefanescu [18] showed that particle pushing can be assumed to be a steady-state condition under which the interface velocity can be assumed to be equal the rate of isothermal solidification. Li et al. [5] suggested that the segregation tendency is dependent on the relationship between the liquid film width produced at the bonding temperature, particle diameter and inter-particle spacing. When the liquid film width is large enough that sufficient particulate material is contained in the melt, particles will be pushed ahead of the solidifying liquid-solid interface resulting in particle segregation at the bond-line. However if the liquid film width is less than some critical value, segregation should not occur.

In the reported studies on transient liquid phase diffusion bonding of Al-MMCs it was shown that the width of the segregated zone at the joint center increased with increasing bonding time. The opposite of this relationship was seen when using the Ni-Al2O3 coating. As the bonding time increased, the width of the segregated region decrease. This can be attributed to the heterogeneous nucleation of grains within the joint zone during solidification and this lead to grain refining at the joint. A high resolution SEM micrograph shown in Figure 5 revealed the presence of a nano-sized alumina particle at the center of a grain. EDX spectra of the particle showed Al and O in high concentrations with traces of Mg [23, 24, 33]. Comparing Gibbs free energy of formation at the bonding temperature for MgO (-1195 kJ/mol) and Al2O3 (-985 kJ/mol) it is found that Al2O3 is unstable in the presence of Mg hence it is like that some of the nano-size Al2O3 will decomposed to form MgAl2O4 compound. WDS analysis across the joint zone as a function of bonding time indicated that the Ni volume at the joint center varied between 3.122 wt% after 1 minute and 0.37 wt% after 30 minutes bonding time (see Table 1).

**Figure 5.** (a) SEM image of nano-particle present in the center of a grain (b) EDS analysis of nano-Al2O3 particle [23]


**Table 1.** WDS analysis of joints made at 600oC as a function of bonding time

#### **4.2. Effect of temperature on joint properties**

318 Advanced Aspects of Spectroscopy

value, segregation should not occur.

after 30 minutes bonding time (see Table 1).

particle [23]

The segregation of particles was accredited to the pushing of micro-Al2O3 particles by the solidifying liquid-solid interface. Stefanescu [18] showed that particle pushing can be assumed to be a steady-state condition under which the interface velocity can be assumed to be equal the rate of isothermal solidification. Li et al. [5] suggested that the segregation tendency is dependent on the relationship between the liquid film width produced at the bonding temperature, particle diameter and inter-particle spacing. When the liquid film width is large enough that sufficient particulate material is contained in the melt, particles will be pushed ahead of the solidifying liquid-solid interface resulting in particle segregation at the bond-line. However if the liquid film width is less than some critical

In the reported studies on transient liquid phase diffusion bonding of Al-MMCs it was shown that the width of the segregated zone at the joint center increased with increasing bonding time. The opposite of this relationship was seen when using the Ni-Al2O3 coating. As the bonding time increased, the width of the segregated region decrease. This can be attributed to the heterogeneous nucleation of grains within the joint zone during solidification and this lead to grain refining at the joint. A high resolution SEM micrograph shown in Figure 5 revealed the presence of a nano-sized alumina particle at the center of a grain. EDX spectra of the particle showed Al and O in high concentrations with traces of Mg [23, 24, 33]. Comparing Gibbs free energy of formation at the bonding temperature for MgO (-1195 kJ/mol) and Al2O3 (-985 kJ/mol) it is found that Al2O3 is unstable in the presence of Mg hence it is like that some of the nano-size Al2O3 will decomposed to form MgAl2O4 compound. WDS analysis across the joint zone as a function of bonding time indicated that the Ni volume at the joint center varied between 3.122 wt% after 1 minute and 0.37 wt%

**Figure 5.** (a) SEM image of nano-particle present in the center of a grain (b) EDS analysis of nano-Al2O3

Wavelength dispersive spectroscopic analyses of the joint as a function of bonding temperature indicated that the Ni concentration at the interface decreased from 4.65 wt% to 0.19 wt% as the bonding temperature is increased from 570 to 620oC (see Table 2). This was attributed to an increase in the diffusivity of Ni from the interface into the base metal as the temperature increased. A review of the scientific literature shows that the diffusivity of Ni increased from D570=4.69 x10-13 m2/s to D620= 1.58 x10-12 m2/s when the bonding temperature was increased from 570 to 620oC [10, 31].

A study of the joint microstructure for a bond made at 570oC revealed the segregation of Al2O3 particles to the bond-line as shown in Figure 6(a). When the bonding temperature was increased to 590oC the width of the particle segregated zone within the joint decreased to approximately 150 μm as shown in Figure 6 (b). Further increase in bonding temperature to 600oC also resulted in a reduction of the width of the segregated zone. A similar result was obtained when the bonding temperature was increased to 620oC (see Figure 6d). This observation was consistent with earlier literature, which suggested that the use of thin interlayers during bonding can help to control the degree of particle segregation taking place within the joint [5]. The micrographs indicate that the width of the particle segregated zone decreases with increasing bonding temperature. This can be attributed to particle pushing by the primary α-phase during solidification as shown in section 4.1 [17, 18].

**Figure 6.** Light micrographs of joint region for bonding temperatures of (a) 570oC and (b) 590oC (c) 600oC and (d) 620oC.


**Table 2.** Wavelength dispersive spectroscopic analyses of joints made at 600oC for 10 minutes composition as a function of bonding temperature (wt %).

### **4.3. Effect interlayer thickness on joint properties**

The effects of interlayer thickness on microstructural development across the joint region and subsequent effect on joint micro-hardness and shear strength were investigated. According to Bosco and Zok [8], there exists a critical interlayer thickness at which pore-free bonds are produced. This critical interlayer thickness should correspond to maximum joint strength. Therefore the objective in this section is to identify the critical interlayer thickness that maximizes joint strength. Joints made without the use of an interlayer resulted in the formation of a "planar interface" due to the presence of a layer of surface oxide, which prevents metal to metal contact (see Figure 7a). This was corroborated by studies on the solid-state diffusion bonding of Al-MMC [15]. The inability to achieve effective bonding in the solid-state highlights the need for low melting interlayers. When a 1μm thick Ni-Al2O3 coating was used as the interlayer a thin joint zone was achieved (see Figure 7b). However a WDS analysis of this region indicated the presence of a higher concentration of Al2O3 when compared to bonds made using a pure nickel coating of the same thickness. This was attributed to the presence of nano-sized Al2O3 particles in the joint zone and the presence of residual surface oxide.

**Figure 7.** Microstructure of joints bonded at 600oC for 10 min using (a) no-interlayer used (b) 1 μm thick Ni–Al2O3 coating (c) 9μm thick Ni –Al2O3 coating [32].

Figures 8 show that the width of the segregated zone increased with increasing coating thickness. At a coating thickness of 2 μm a more chemically homogeneous joint was created however WDS analysis showed pockets of oxide with the following composition 65.56 wt% and 28.15 wt% were still present at the interface. When the coating thickness was increased to 4 μm, a 95 μm wide segregated zone was at the joint center and the concentration of nickel remaining at the interface after bonding increased from 0.47 wt% with 3μm thick coating to 0.58wt% when a 4μm thick coating (see Table 3). Further increase in coating thickness to 5 μm, resulted in the formation of a 110 μm wide segregated zone while the nickel increased to 0.97 wt%.

320 Advanced Aspects of Spectroscopy

residual surface oxide.

Temperature Mg**/** wt% Ni**/** wt% Si**/** wt% Fe**/** wt% Al**/** wt% 570oC 2.53 4.65 0.72 0.35 91.75 590oC 1.73 1.69 0.52 0.28 95.78 600oC 1.52 0.35 0.41 0.21 97.51 620oC 0.94 0.19 0.21 0.19 98.47

The effects of interlayer thickness on microstructural development across the joint region and subsequent effect on joint micro-hardness and shear strength were investigated. According to Bosco and Zok [8], there exists a critical interlayer thickness at which pore-free bonds are produced. This critical interlayer thickness should correspond to maximum joint strength. Therefore the objective in this section is to identify the critical interlayer thickness that maximizes joint strength. Joints made without the use of an interlayer resulted in the formation of a "planar interface" due to the presence of a layer of surface oxide, which prevents metal to metal contact (see Figure 7a). This was corroborated by studies on the solid-state diffusion bonding of Al-MMC [15]. The inability to achieve effective bonding in the solid-state highlights the need for low melting interlayers. When a 1μm thick Ni-Al2O3 coating was used as the interlayer a thin joint zone was achieved (see Figure 7b). However a WDS analysis of this region indicated the presence of a higher concentration of Al2O3 when compared to bonds made using a pure nickel coating of the same thickness. This was attributed to the presence of nano-sized Al2O3 particles in the joint zone and the presence of

**Figure 7.** Microstructure of joints bonded at 600oC for 10 min using (a) no-interlayer used (b) 1 μm thick

Figures 8 show that the width of the segregated zone increased with increasing coating thickness. At a coating thickness of 2 μm a more chemically homogeneous joint was created

**Table 2.** Wavelength dispersive spectroscopic analyses of joints made at 600oC for 10 minutes

composition as a function of bonding temperature (wt %).

Ni–Al2O3 coating (c) 9μm thick Ni –Al2O3 coating [32].

**4.3. Effect interlayer thickness on joint properties** 


**Table 3.** WDS analysis of joints made at 600oC for 10 minutes as a function of interlayer thickness

**Figure 8.** Width of the particle segregated zone formed at within the joint as a function of interlayer thickness for pure Ni coating and Ni–Al2O3 coating [32].

The increase in the width of the segregated zone was attributed to increased liquid formation with increasing coating thickness. As the width of the eutectic liquid increases more Al2O3 particles are immersed in the liquid phase. These particles are pushed by the solid/liquid interface during isothermal solidification [5].

The width of the particle segregated zone was significantly lower than that achieved when pure Ni-coatings are used as the interlayer. The difference in the width of the segregated zone between joint bonded using pure Ni coating and Ni-Al2O3 coating was attributed to the presence of nano-size Al2O3 particle in the joint center and a reduction in the concentration of Ni (81.6 wt%) present in the coating, when Ni-Al2O3 is used (see Figure 13b).
