**1. Introduction**

Transient liquid-phase (TLP) bonding is a material joining process that depends on the formation of a liquid at the faying surfaces by an interlayer that melts at a temperature lower than that of the substrate [1, 2, 3, 4, 5].

The TLP bonding is distinguished from brazing processes by the isothermal solidification of this liquid. This is accomplished since the interlayer is rich in a melting point depressant (MPD). Upon heating through the eutectic temperature, the interlayer will either melt or react with the base metal to form a liquid. During the hold time above the melting temperature, the melting point depressant (solute) into the base metal (solvent). This resulting solid/liquid interfacial motion via epitaxial growth of the substrate is termed ''isothermal solidification.'' A homogeneous bond between the substrates is formed when the two solid/liquid interfaces meet at the joint centerline marking the end of the isothermal solidification stage.

TLP bonding provides an alternative to fusion welding and has been extensively studied for joining particle-reinforced Al-MMCs [1]. The advantage of using this process is that, reinforcing particles are incorporated into the bond region either by using a particle reinforced insert layer or by the melt-back due to a eutectic reaction between the interlayer and the aluminum alloy [6]. It has been shown in the scientific literature that melt-back can be controlled by using thin interlayer materials. It has also been suggested that heating rate, interlayer composition and thickness are most important in reducing melt-back during TLP bonding. These parameters also determine the width of the liquid phase, removal of surface oxide film and particulate redistribution in the bonded region [16, 7,5].

According to Bosco and Zok [8] there exists a critical interlayer thickness at which pore-free bonds are produced. The thickness of the interlayer must exceed that which is consumed

© 2012 Cooke, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

through solid-state diffusion; otherwise, no liquid is formed at the bonding temperature. This sets a minimal requirement for the interlayer thickness, which can be determined using equation 1:

$$h\_c = h\_\eta \mathbf{C}\_\eta \left(\frac{\rho\_\eta}{\rho\_i}\right)^2 \tag{1}$$

Where, *2h<sup>η</sup>* is the total thickness of the reaction layer formed at the interlayer/MMC interface when the eutectic temperature is reached, *C<sup>η</sup>* is the mass fraction of Ni in the reaction layer. The mass densities of the reaction layer and the Ni interlayer are *ρη* and *ρNi* respectively. On the other hand, Li et al. [5] suggested that a maximum thickness exists that minimizes particle segregation and maximises joint strength. The composition of the interlayer has also been shown by Eagar and McDonald [9] to be equally as important as the interlayer thickness. In the published literature, it has been shown that pure metal interlayers such as nickel [10, 11], silver [12,13] and copper [13, 15] can react with the metal surface by means of eutectic or peritectic reaction to displace surface oxides and regulates the bonding temperature. In joints made using these pure interlayers, the existing problems can be summarized as: parent metal dissolution particulate segregation [13, 14,15], void formation in particle segregated region [12] and low strength micro-bonds at particle-metal interfaces resulting from the poor wettability of molten interlayer on ceramic reinforcement [12,16]. Particle segregation has been shown by Stefanescu et al [17, 18] to be promoted by the slow movement of the solid-liquid interface during isothermal solidification. Li et al. [5] showed that during TLP bonding of Al-MMCs containing particulate reinforcements with diameters less than 30 μm, particle segregation to the interface occurred when a copper foil thickness between 5 and 15 μm was used. Earlier research used alloyed interlayers such as; Zn–Al, Cu–Al and Cu/Ni/Cu systems to decrease bonding temperature in air and to prevent particulate segregation [19]. While composite interlayers such as Al–Si–W mixed powder and Al–Si–Ti–SiC mixed powder were used to improve the densification of a thick reaction layer which formed at the joint. This layer was reinforced with a metallic phase and a ceramic phase [20].

Recent studies into joining Al-MMCs have focused on decreasing bonding temperature by using Sn-based interlayers reinforced with silicon carbide. The results show that joint strength of Al6061 + 25% (Al2O3)p improved by approximately 100% when compared to unreinforced Sn-based joints formed by ultrasonic assisted soldering [21]. Yan et al. [22] developed a SiC particle reinforced Zn-based filler which was used to join SiCp/A356 composite. The results indicated that with the use of ultrasonic vibration suitable particle distribution and reduced void formation were achieved. Cooke et al. [23, 24] used an electrodeposited Ni-Al2O3 nano-composite coatings to join Al-6061 MMC. The results showed that the use of Ni-Al2O3 nano-composite coatings can be used successfully to increase joint strength when compared to TLP bond produced using pure Ni-coating. The results also showed that coating thickness of 5μm can be used to control particle segregation during TLP bonding of Al-6061 MMC

In order to study the kinetics of TLP bonding, techniques such as wavelength dispersive spectroscopy, energy dispersive spectroscopy and x-ray diffraction spectroscopy are normally used, since the compositional changes across the joint region is the mechanism by which the process progresses to completion. In previous studies EDS, WDS and XRD has been used extensive for studying materials due in part to the flexibility that the techniques afford. In most papers, a choice is made between the two processes depending on the information that is required. The difference between these processes are that, the energydispersive (ED) type records X-rays of all energies effectively simultaneously and produces an output in the form of a plot of intensity versus X-ray photon energy. The detector consists of one of several types of device producing output pulses proportional in height to the photon energy. Whereas the wavelength-dispersive (WD) type makes use of Bragg reflection by a crystal, and operates in 'serial' mode, the spectrometer being 'tuned' to only one wavelength at a time. Several crystals of different interplanar spacing are needed in order to cover the required wavelength range. Spectral resolution is better than for the ED type, but the latter is faster and more convenient to use. X-ray spectrometers attached to SEMs are usually of the ED type, though sometimes a single multi-crystal WD. This chapter examines the application of spectroscopic analyses such as EDS, WDS and XRD to the evaluation of nanostructure TLP bonds using Ni-Al2O3 nano-composite thin film as a filler material during TLP bonding of Al-6061MMC. The effects of process parameters on the mechanical and microstructural microstructural changes in the joint region will also be studied.

#### **1.1. Spectroscopic analysis techniques**

310 Advanced Aspects of Spectroscopy

equation 1:

ceramic phase [20].

during TLP bonding of Al-6061 MMC

through solid-state diffusion; otherwise, no liquid is formed at the bonding temperature. This sets a minimal requirement for the interlayer thickness, which can be determined using

*c*

*h hC*

 

Where, *2h<sup>η</sup>* is the total thickness of the reaction layer formed at the interlayer/MMC interface when the eutectic temperature is reached, *C<sup>η</sup>* is the mass fraction of Ni in the reaction layer. The mass densities of the reaction layer and the Ni interlayer are *ρη* and *ρNi* respectively. On the other hand, Li et al. [5] suggested that a maximum thickness exists that minimizes particle segregation and maximises joint strength. The composition of the interlayer has also been shown by Eagar and McDonald [9] to be equally as important as the interlayer thickness. In the published literature, it has been shown that pure metal interlayers such as nickel [10, 11], silver [12,13] and copper [13, 15] can react with the metal surface by means of eutectic or peritectic reaction to displace surface oxides and regulates the bonding temperature. In joints made using these pure interlayers, the existing problems can be summarized as: parent metal dissolution particulate segregation [13, 14,15], void formation in particle segregated region [12] and low strength micro-bonds at particle-metal interfaces resulting from the poor wettability of molten interlayer on ceramic reinforcement [12,16]. Particle segregation has been shown by Stefanescu et al [17, 18] to be promoted by the slow movement of the solid-liquid interface during isothermal solidification. Li et al. [5] showed that during TLP bonding of Al-MMCs containing particulate reinforcements with diameters less than 30 μm, particle segregation to the interface occurred when a copper foil thickness between 5 and 15 μm was used. Earlier research used alloyed interlayers such as; Zn–Al, Cu–Al and Cu/Ni/Cu systems to decrease bonding temperature in air and to prevent particulate segregation [19]. While composite interlayers such as Al–Si–W mixed powder and Al–Si–Ti–SiC mixed powder were used to improve the densification of a thick reaction layer which formed at the joint. This layer was reinforced with a metallic phase and a

Recent studies into joining Al-MMCs have focused on decreasing bonding temperature by using Sn-based interlayers reinforced with silicon carbide. The results show that joint strength of Al6061 + 25% (Al2O3)p improved by approximately 100% when compared to unreinforced Sn-based joints formed by ultrasonic assisted soldering [21]. Yan et al. [22] developed a SiC particle reinforced Zn-based filler which was used to join SiCp/A356 composite. The results indicated that with the use of ultrasonic vibration suitable particle distribution and reduced void formation were achieved. Cooke et al. [23, 24] used an electrodeposited Ni-Al2O3 nano-composite coatings to join Al-6061 MMC. The results showed that the use of Ni-Al2O3 nano-composite coatings can be used successfully to increase joint strength when compared to TLP bond produced using pure Ni-coating. The results also showed that coating thickness of 5μm can be used to control particle segregation

2

(1)

*i*

 

The characterization of materials using spectroscopic techniques such as energy dispersive spectroscopy or x-ray diffraction spectroscopy techniques is dependent on the generation of a beam of electrons which interacts with the sample to be analyzed. When electrons strike a anode with sufficient energy, X-rays are produced. This process is typically accomplished using a sealed x-ray tube, which consists of a metal target and a tungsten metal filament, which can be heated by passing a current through it resulting in the "boiling off" of electrons from the hot tungsten metal surface. These "hot" electrons are accelerated from the tungsten filament to the metal target by an applied voltage The collision between these energetic electrons and electrons in the target atoms results in electron from target atoms being excited out of their core-level orbitals, placing the atom in a short-lived excited state. The atom returns to its ground state by having electrons from lower binding energy levels make transitions to the empty core levels. The difference in energy between these lower and higher binding energy levels is radiated in the form of X-rays. This process results in the production of characteristic X-rays. X-rays are generated when the primary beam ejects an inner shell electron thus exciting the atom. As an electron from the outer shell drops in to fill the vacancy and de-excite the atom it must give off energy. This energy is specific to each individual element in the periodic table and is also specific to what particular electron dropped in to fill the vacancy. The conversion between energy, frequency, and wavelength is the well-known de Broglie relationship: E = hν = hc/λ, where ν is the frequency, h is Planck's constant (6.62 x 10-34 joule-second), c is the speed of light (2.998 x 108m/sec), and λ is the wavelength of the radiation (in m). Based on this relationship, two distinct types of x-ray detector systems are used. These two types of detector systems are called Energy-Dispersive x-ray Spectrometry (EDS) and Wavelength-Dispersive x-ray Spectrometry (WDS).

EDS spectrometer are most frequently attached to electron column instruments such as SEM or (EPMA). As the name implies is a method of x-ray spectroscopy by which x-rays emitted from a sample are sorted out and analyzed based on the difference in their energy level. An EDS system consists of a source of high-radiation; a sample, a solid-state detector (usually from lithium-drifted silicon (Si(Li)); and a signal processing electronics. When the sample atoms are ionized by a high-energy radiation, they emit characteristic x-rays. X-rays that enter the Si(Li) detector are converted into signals (charge pulses) that can be processed by the electronics into an x-ray energy histogram. This x-ray spectrum consists of a series of peaks representative of the type and relative amount of each element in the sample. The number of counts in each peak can be further converted to elemental weight concentration either by comparison standards or standardless calculations. In general, three principal types of data can be generated using an EDS detector: (i) x-ray dot maps or images of the sample using elemental distribution as a contrast mechanism, (ii) line scan data or elemental concentration variation across a given region, and (iii) overall chemical composition, both qualitative and quantitatively.

As the name implies, WDS is a detection system by which x-rays emitted from the sample are sorted out and analyzed based on differing wavelength (λ) in the WDS, or crystal spectrometry. As in EDS or imaging mode, the beam rasters the sample generating x-rays of which a small portion enters the spectrometer. As the fluorescent x-rays strike the analyzing crystal, they will either past through the crystal, be absorbed, be scattered, or be diffracted. Those which satisfy Bragg's Law; *n dSin* 2 .

(where n = an integer, *d* = the interplanar spacing of the crystal, θ = the angle of incidence, and λ = x-ray wavelength) will be diffracted and detected by a proportional counter. The signal from this detector is amplified, converted to standard pulse size in the single channel analyzer and counted with a scalar or displayed as rate vs time on rate meter. By varying the positioning crystal one changes the wavelength that will satisfy Bragg's Law. Therefore one can sequentially analyze different elements. By automating crystal movements one can dramatically speed up the analysis time. Typically the WDS analysis is used to gain the same type of information that the EDS is used for, qualitative and quantitative and quantitative information, line scan and dot maps for elemental distribution.
