Eutectic Ag Research Articles

Vacuum brazing of mild steel using eutectic CuSil brazing alloy

Need for joining similar and dissimilar materials with various thickness and geometry increasing rapidly, inferable from their bit of leeway of light-weighting in the automobile and aviation industries. Brazing is such method expanding quickly with the approach of reasonable high-temperature furnaces incorporated with high vacuum systems. Negligible distortion and restraint of thermal stresses with steady heating and cooling rates make this procedure amiable for joining materials of different size and thickness. In the present study, mild steel brazing was carried out in butt and lap joint configurations using a foil composed of CuSil eutectic (72%Ag28%Cu) to check for reliability in piping and automotive components. Brazing process was performed with varying overlap for lap joints, and butt joint at a constant temperature of 835 °C in a vacuum furnace. Microstructure and elemental composition of the joints were examined using Optical microscope, SEM and EDS. Mechanical properties were investigated at the joint region using micro hardness and tensile strength at room temperature. Results showed that microstructure composed of eutectic Ag and Cu microstructure with no signs of porosity and cracks in the brazed joint. The average hardness of 95 HV in the brazed region is less when compared to the base metal of average hardness 135 HV. It has been found that due to more brazing area, failure occurred away from joints in lap joint configuration, and in brazing joint in case of butt configuration due to micropores and voids.

On the solid/liquid interfacial energies of metals and alloys

The solid/liquid interfacial energies of pure metals and metallic alloys are modelled in this paper. A simple model is offered for pure metals, showing that their solid/liquid interfacial energy (sigma) slightly increases with temperature. Sigma for metallic alloys is considered for the interface between solid and liquid solutions being in thermodynamic equilibrium, calculated by the CALPHAD method. The Butler equation is extended to find the equilibrium composition of the solid/liquid interfacial region and the solid/liquid interfacial energy at fixed temperatures. This method takes into account the segregation of low-interfacial energy components to the solid/liquid interfacial region. It is shown how the new method can be extended to multi-component alloys. The method is applied to calculate the solid/liquid interfacial energy of Al-rich solid solutions in equilibrium with eutectic liquid alloys of Al–Cu, Al–Ni, Al–Ag and Al–Ag–Cu systems. Good agreement was found with experimental values. For the Al–Ag–Cu system, the modelled value allows to select the more probable experimental value from the two contradicting experimental values published in the literature. The solid/liquid interfacial energy is calculated for the eutectic Ag–Cu system as function of liquidus composition (which determines both the equilibrium solidus composition and the equilibrium temperature). Finally it is claimed that using solely bulk thermodynamic data (melting enthalpy and molar volumes of pure components and molar excess Gibbs energies of equilibrium solid and liquid solutions) it is possible to provide meaningful values for the temperature and concentration dependence of solid/liquid interfacial energies of alloys. The method can be applied for simulation of solid/liquid phase transformation and also to solid/liquid equilibrium calculations of nano-alloys.

Low temperature and low pressure bonding of plateless Cu–Cu substrates by Ag-based transient liquid phase sintering

Bonding silicon carbide/gallium nitride (SiC/GaN) based power modules, particularly epoxy-molded modules to heat-substrate and/or heat sink, requires low processing temperature preferably lower than 250 °C, and low pressure as low as 0.1 MPa to prevent damage to the modules. In addition, due to the impracticality of depositing metal-plating to the epoxy-molded module, bonding of plateless Cu-substrates is in great demand. Furthermore, post-processing residual flux cleaning, which is costly and unfavorable to industry need to be avoided as possible by opting out the usage of flux. Up to authors’ knowledge, our study is the first to fulfill all requirements stated above. Transient liquid phase sintering (TLPS) of mixed fluxless Sn–Bi (tin–bismuth) eutectic alloy and Ag (silver) particles was applied to bond plateless Cu–Cu substrates. Sintering temperature of 250 °C, sintering pressure of 0.02 MPa, and reducing environment were applied during processing. The effects of addition amount of Sn–Bi and sintering holding time to the shear strength and microstructure were investigated. The remelting temperature after sintering was also examined. Shear strength of 30 wt% added Sn–Bi was over than 20 MPa, which qualify the requirement of MIL-STD-883K, and larger than conventional Pb-based solder. Formation of intermetallic compounds are thought to strengthen the interface and matrix. Remelting temperature shifted from eutectic temperature of Sn–Bi to approximately 262 °C, allowing the application of modules at higher operating temperature than the processing temperature.

Enhancement of optical performance of the light emitting diode packages with advanced thermal design of die-attaching layers

To guarantee long lifetime of light emitting diodes (LEDs), thermal reliability of LED packages should be secured with suitable die-attaching materials. Die-attaching materials are important for the interconnection between optical devices and substrates, as they not only provide electrical transmission but also the emission of thermal budget generated under the operation of the devices. In this study, the joints with different die-attaching materials were closely investigated in conjunction with the optical performance of LED packages. Thermal reliability of the joints was quantitatively analyzed by estimating the thermal resistances of the metallic and intermetallic layers incorporating the joints. The experimental results revealed that insertion of nano-Ag paste between eutectic Au-Sn and Ag finish significantly improved heat emission by effectively suppressing thermal resistance of eutectic Au-Sn layer. However, to ensure long-term reliability, complete removal of numerous nano-voids among Ag nanoparticles should be accomplished to prevent accumulation of thermal budget.

Structural evolution of Ag-Cu nano-alloys confined between AlN nano-layers upon fast heating.

The structural evolution of a Ag-Cu/AlN nano-multilayer (NML), as prepared by magnetron-sputtering on a α-Al2O3 substrate, was monitored during fast heating by real-time in situ XRD analysis (at the synchrotron), as well as by ex situ microstructural analysis using SEM, XPS and in-house XRD. The as-deposited NML is constituted of alternating nano-layers (thickness ≈ 10 nm) of a chemically inert AlN barrier and a eutectic Ag-Cu(40at%) nano-alloy. The nano-alloy in the as-deposited state is composed of a fcc matrix of Ag nano-grains (≈6 nm), which are supersaturated by Cu, and some smaller embedded Cu rich nano-grains (≈4 nm). Heating up to 265 °C activates segregation of Cu out of the supersaturated Ag nano-grains phase, thus initiating phase separation. At T > 265 °C, the phase-separated Cu metal partially migrates to the top NML surface, thereby relaxing thermally-accumulated compressive stresses in the confined alloy nano-layers and facilitating grain coarsening of (still confined) phase-separated nano-crystallites. Further heating and annealing up to 420 °C results in complete phase separation, forming extended Ag and Cu domains with well-defined coherent Ag/AlN interfaces. The observed outflow of Cu well below the eutectic melting point of the bulk Ag-Cu alloy might provide new pathways for designing low-temperature nano-structured brazing materials.

Effect of Ti content in Ag–Cu–Ti activated filler metal on dissimilar joint formation of sialon and WC–Co alloy by laser brazing

Laser brazing was carried out for dissimilar joining of sialon and a WC–Co alloy. Eutectic type Ag–Cu alloys as filler metals with different Ti content ranging from 0 to 2·8 mass-% were used to investigate the effects of Ti on the interface structure and strength of the joint. The filler metal sheet was sandwiched between a sialon block and a WC–Co alloy plate, and a laser beam was irradiated selectively on the WC–Co alloy plate. The brazed joint was obtained using the filler metal containing >0·3 mass-%Ti. TiN, Ti5Si3, and Cu4Ti layers were formed at the interface of sialon and brazed metal as compound layers. The shear strength of the brazed joint increased with increasing Ti content in the filler metal in the range 0·3–1·7 mass-%, reaching a maximum value of 106 MPa. However, the strength decreased when the Ti content became higher than 1·7 mass-%.

Subsurface microstructure evolution and deformation mechanism of Ag–Cu eutectic alloy after dry sliding wear

Dry sliding wear behavior of nanostructured eutectic Ag–Cu with grain size ~114nm was investigated using pin-on-disc testing. The subsurface microstructure and texture evolution were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nano-beam diffraction, and high angle annular dark field. During sliding wear, plastic deformation transforms the subsurface material into hierarchical microstructures. At depths of 1–15μm below the sliding surface, a two phase (Ag-rich and Cu-rich) nano-lamellar structure is observed, where the layer thickness decreases from 98nm at a depth of 15μm to 11nm at a depth of 1μm. Right below the sliding surface, where an equivalent strain of 7.1 was estimated, wear induced plastic deformation drives the subsurface material into a non-equilibrium super-saturated solid solution phase, with ~9nm equiaxed nano-grains. These refined microstructures led to significant work-hardening in the subsurface material, as revealed by nanoindentation testing. Finally, the microstructure evolution and the underlying deformation mechanism of Ag–Cu are discussed on the basis of these characterizations.

Microstructural characteristics of a nanoeutectic Ag–Cu alloy processed by surface mechanical attrition treatment

A nanostructured eutectic Ag–Cu alloy with alternate Ag and Cu lamellae was processed by surface mechanical attrition treatment (SMAT). The orientation/phase distributions of the eutectic Ag–Cu before and after SMAT were investigated using transmission electron microscopy combined with automated crystal orientation mapping. After SMAT, the top surface layer was transformed into a composite with isolated Cu phase dispersed in Ag. The interphase boundaries in the top surface layer were changed from the original cube-on-cube and hetero-twin orientation relationships into general large-angle boundaries accordingly.

Characteristics of dissimilar laser-brazed joints of isotropic graphite to WC–Co alloy

Abstract The effect of Ti serving as an activator in a eutectic Ag–Cu alloy filler metal in dissimilar laser-brazed joints of isotropic graphite and a WC–Co alloy on the joint strength and the interface structure of the joint is investigated in this study. To evaluate the joint characteristics, the Ti content in the filler metal was increased from 0 to 2.8 mass%. The laser brazing was carried out by irradiating a laser beam selectively on the WC–Co alloy plate in Ar atmosphere. The threshold content of Ti required to join isotropic graphite to WC–Co alloy was 0.4 mass%. The shear strength at the brazed joint increased rapidly with increasing Ti content up to 1.7 mass%, and a higher Ti content was found to be likely to saturate the shear strength to a constant value of about 14 MPa. The isotropic graphite blocks also fractured at this content. The concentration of Ti observed at the interface between isotropic graphite and the filler metal indicates the formation of an intermetallic layer of TiC.

Infrared brazing of Ti–6Al–4V using two silver-based braze alloys

Infrared brazing of Ti–6Al–4V using two silver-based alloys is evaluated in the study. For the 72Ag–28Cu brazed specimen, Ag-rich matrix, eutectic Ag–Cu and Cu–Ti interfacial reaction layer(s) are observed in the experiment. In contrast, both Ag-rich matrix and interfacial titanium aluminides, TiAl or Ti 3Al, are found in the 95Ag–5Al brazed joint. In general, the shear strength of 72Ag–28Cu brazed joint is much higher than that of 95Ag–5Al brazed specimen. Additionally, the use of infrared brazing with lower brazing temperature and/or less time can significantly decrease both dissolution of the substrate into molten braze as well as excessive growth of the interfacial reaction layer(s) in the joint. Therefore, infrared brazing has the potential to be applied in industry.

Growth mechanism of bulk Ag 3Sn intermetallic compounds in Sn–Ag solder during solidification

Thermal analysis was used to clarify the growth mechanism of bulk Ag 3Sn IMCs in Sn–Ag lead-free solders and compared with Pandat software calculations and theoretical calculations. In slowly-cooled Sn–Ag lead-free solders, bulk Ag 3Sn IMCs formed when Ag 3Sn crystal nuclei formed at the onset of the eutectic reaction. The fractions of bulk Ag 3Sn IMCs in hypereutectic Sn–4.4 mol%Ag solders, measured by thermal analysis, are larger than those predicted by the equilibrium phase diagram and software calculations. The reasons for this could be attributed to eutectic Ag 3Sn phases clinging to the primary Ag 3Sn crystals during the eutectic reaction with their matching crystalline orientation relationship. The driving force for the growth of the faceted bulk Ag 3Sn IMCs phase is proportional to the degree of undercooling achieved, which fits with the prediction of classical solidification theory. The result explains the formation and rapid growth of bulk Ag 3Sn IMCs in slowly-cooled eutectic Sn–Ag solder with a minimal degree of undercooling during solidification.

Crystallization, morphology and distribution of Ag 3Sn in Sn–Ag–Cu alloys and their influence on the vibration fracture properties

Since Ag 3Sn phase plays a decided role in the performance of Sn–Ag–Cu solders, this study varied the solidification rate and Ag content to clarify the formation, morphology and distribution of Ag 3Sn in Sn–Ag–Cu alloys, as well as their influence on the tensile properties and vibration fracture resistance under different vibration conditions. Results show that a less packed distribution of Ag 3Sn in the eutectic regions and a larger area of β-Sn dendrites contributed to a higher damping capacity and thus a greater vibration life under constant force conditions. Acicular eutectic Ag 3Sn and large primary Ag 3Sn plates accelerated crack growth and thus degraded the ductility and vibration fracture resistance particularly when the damping effect was removed.

Evolution of Structure Unidirectionally Solidified Sn–Ag<SUB>3</SUB>Sn Eutectic Alloy

Sn-Ag 3 Sn eutectic alloy is known as an attractive candidate to replace Sn-Pb soldering alloy. Since the solidified structure directly relates to the mechanical properties of joints, understanding of solidified structure of this alloy system is very important. Therefore, in order to understand the revolution of solidified structure, unidirectional solidification experiments have been carried out using eutectic Sn-Ag 3 Sn alloy. When the growth velocity is larger than the critical value, Sn-dendrites solidifies as a primary phase. This is attributed to the skewed coupled zone in this alloy system. In this case. the volume fraction of the primary Sn increases with increasing growth velocity and approaches a constant value. Interdendritic eutectic is composed with fibrous Ag 3 Sn and Sn matrix. On the other hand, when the growth velocity becomes lower, a eutectic structure appears without any primary phase. In this case, a piece of eutectic Ag 3 Sn becomes large and changes from fibrous to plate-like with decreasing growth velocity. This morphological change may be due to the anisotropy of surface tension of eutectic Ag 3 Sn.

The microstructural observation of brazing Ti–6Al–4V and TZM using the BAg-8 braze alloy

Microstructural evolution of the brazed Ti–6Al–4V and TZM joints using the BAg-8 braze alloy have been evaluated in this study. According to the dynamic sessile drop test, the molten braze alloy can effectively wet Ti–6Al–4V at 850 °C. In contrast, the TZM substrate cannot be effectively wetted by the molten braze below 900 °C. The microstructure of an infrared brazed specimen (800 °C, 60 s) mainly consists of eutectic Ag–Cu, with a reaction layer (Ti 3Cu 4) approximately 2 μm thick. The interfacial reaction is more pronounced in a specimen furnace brazed at 800 °C for 600 s. Both TiCu and Ti 3Cu 4 are observed at the interface with the thickness of about 10 μm. In specimens furnace brazed at 850 °C, the reaction layer is mainly comprised of TiCu. The application of the infrared brazing cannot completely prevent the formation of interfacial reaction layer(s), but it provides an effective way to inhibit the growth of intermetallics at the interface between the braze alloy and substrate. Additionally, the dissolution of the substrates into the molten braze alloy during infrared brazing is significantly decreased due to its rapid thermal cycle.

Thermodynamic properties of Ti in Ag–Cu–Ti alloys

The titanium activities at dilution in Ag–Cu brazing alloys have been measured at 1273 K by an oxygen sensor. The equilibrium oxide phase formed by the reaction of Ti in the eutectic Ag–Cu melts with Al 2O 3 crucible was TiO (solid). It was found that the Ti activity and activity coefficient in the eutectic Ag–Cu melts increase with the increment of Ti concentration and show a negative deviation from the ideal solution behavior at 1273 K. The Ti activity coefficient at infinite dilution is about 0.076 relative to pure solid Ti. Besides, the activity coefficient of Ti decreases with the increment of Cu concentration and increases with the increment of Ag concentration in Ag–Cu melts containing 1 wt.% Ti at 1273 K. Experimental results demonstrate that Sn decreases the activity coefficient of Ti in the Ag–Cu melts, probably due to its higher Ti solubility than Ag at the same temperature.

A new intermetallic phase in AgBi alloy produced by pressure-temperature action

The simple eutectic AgBi alloy system exhibits under pressure-temperature action an intermediate phase of composition Ag 3Bi. The crystal structure of the new phase was investigated in metastable state under ambient conditions by X-ray powder diffraction. An orthorhombic (distorted hexagonal close-packed) structure of an ordered type was derived with lattice parameters a = 3.089 A ̊ , b = 4.847 A ̊ , c = 5.892 A ̊ and space group Pmm2, similar to the corresponding phase in the AgSb system. The new phase decomposes into the eutectic two-phase state Ag + Bi by annealing. The atomic ratios are analyzed as Hume-Rothery criterion for formation of intermediate phases.

The Spreading Kinetics of Ag–28Cu(L) on Nickel(S): Part II. Area of Spread on Surfaces Plated with Electrolytic Ni

Furnace brazing is commonly used in the electronic industry to attach I/O pins, lid-seal rings, and heat spreaders to cofired multilayer ceramic packages. Despite the widespread industry usage of electrolytic and electroless Ni coatings to render base metal surfaces wettable by braze fillers, there is no fundamental treatment of “brazeability” in the published literature that can be used by the materials technologist to design coatings for a given application. In this study, dynamic hot stage microscopy is used to establish the parameters that control the spreading of eutectic Ag–Cu braze on surfaces plated with electrolytic Ni. The effects of plating thickness, substrate type, plating annealing, and the braze thermal cycle are considered. A braze spreading mechanism developed for polycrystalline Ni foil in Part I of this study is linked to Ni-plated surfaces through consideration of differences in microstructure. Eventual extension of this improved understanding to surfaces plated with electroless Ni–B and Ni–P deposits will result in shortened product design time and improved manufacturing process control.

Viscosity of AgSb alloys

Temperature dependent measurements of the viscosity for alloys of AgSb were carried out. For all alloys, except eutectic Ag 59Sb 41 and antimony, the viscosity obeys the Arrhenius law. For the eutectic Ag 59Sb 41 and antimony the temperature dependencies of the viscosity deviates slightly from the Arrhenius relation. Undercooling for antimony, silver, eutectic Ag 59Sb 41 and Ag 30.1Sb 69.9 alloy was observed.

Rapid quenching and mechanical alloying of eutectic Ag Cu Sn alloys

AgCuSn alloys containing 30÷34wt%Cu and 10÷20wt%Sn, which correspond to pseudobinary eutectics were rapidly quenched using melt spinning method. The α Ag, α Cu fee solid solutions, β (Cu 17Sn 3 bcc orederd and δ (Cu 41Sn 11 precipitates were identified in thin ribbons. The amount of β phase in ribbons significantly increased with an increasing tin content. The solubility of all elements in the metastable solid solution was established. It was found that β phase contained predominantly copper and 28%Sn and 10%Ag. The following crystallographic relationship between α and β phases was found: (011)α ∥ (001)β and [001]β and [100]α ∥ [100]β. Mechanical alloying of Ag, Cu and Sn powders of the same composition as the ribbons was performed in a planetary ball mill. The amounts of α Ag and α Cu decreased and tin disappeared completely with increasing milling time; additionally Ag 3Sn 1 Cu 6Sn 5 and Cu 10Sn 3 phases were identified after prolonged milling time.

Beryllium-metals joints for application in the plasma-facing components

The results of the technological development for Be joining with other metals for high heat flux application are presented. The different types of joining technology — high temperature brazing by using the different brazing alloys and solid state diffusion bonding are compared. The results of diffusion bonding technology development for BeCu and Be-dispersion strengthened copper alloys joinings are presented. It was shown that for the joining of Be with austenitic stainless steels, the vacuum high temperature brazing using the ternary brazing alloy of AgCuMe system is more preferable than common eutectic AgCu alloy. The high temperature brazing technology for joining BeBe using the Al brazing alloys was also analyzed. The problems of nondestructive examination of Be joints, the data on mechanical properties, microhardness testing and results of microstructural examination of Be joint are presented.