Melting Point Depressant Research Articles

The reliability study of selected Sn–Zn based lead-free solders on Au/Ni–P/Cu substrate

Since both Ag and In are important melting point depressants in Sn–Zn based solders, a series Sn–Zn based solders with various amounts of Ag and In additions was studied in the experiment. The melting behavior of solder alloys, wetting characteristics, coefficients of thermal expansion, microstructural evolution and long-term reliability of the selected Sn–Zn based solder on Au/Ni–P metallized copper substrate were examined. Based on the experimental result, there is little change in the melting range of Sn–Zn based solder alloys by minor addition of Ag. On the contrary, the melting point of Sn–Zn based alloys can be effectively decreased by In additions. However, the difference between solidus and liquidus temperature is broadened as the increment of In into Sn–Zn based solders. 76Sn–9Zn–15In has the lowest liquidus temperature among all alloys, and it can effectively bond the Au/Ni–P metallized copper substrate. The microstructure of 76Sn–9Zn–15In alloy soldered at 200 °C for 20 min is primarily comprised of Sn–In γ phase and needle-like ZnO 2. Since there is no flux usage during soldering, zinc oxide cannot be avoided even the process performed under 2×10 −2 mbar vacuum environment. It is also noted that there is no interfacial reaction layer between 76Sn–9Zn–15In and Au/Ni–P metallized copper substrate after soldering. However, there is a reaction layer between 76Sn–9Zn–15In and substrate as the soldered specimen aged at 90 °C for 168 h. Its chemical composition is close to Zn-rich γ phase (NiZn 3) alloyed with minor Sn, In, Cu and P. For the specimen further aged at 90 °C for 336 h, there are cracks along the interface between solder alloy and electroless Ni–P layer. The oxidation of the interfacial Zn-rich γ phase plays an important role in deterioration of the bonding between 76Sn–9Zn–15In and Au/Ni–P metallized copper substrate.

High-Temperature Brazing of Superalloys and Stainless Steels with Novel Ductile Ni-Hf-Based Filler Metals

Nickel-based filler metals are generally used for brazing superalloys and high-alloyed steels to give joints with high-temperature resistance and corrosion resistance. The use of hafnium as a melting point depressant for nickel based filler metals instead of the conventional metalloids represents a metallurgical approach to produce ductile nickel brazing alloys with moderate brazing temperatures. Here the authors show that especially alloys of the system Ni-Hf-Cr possess suitable properties in terms of flowing and wetting behaviour on stainless steel and inconel 600. These new Ni-based filler metals are rollable and possesses high tolerances in terms of brazing gaps.

A wettability study of Cu/Sn/Ti active braze alloys on alumina

Active brazing is one of the ideal ways to make metal/ceramic joints. The active braze alloy contains active element(s), such as: Ti, Zr, Cr... etc., reacting and wetting the ceramic surfaces during brazing. Therefore, a strong chemical bonding can be formed after brazing. Cu base active braze alloys are alternatives among active braze alloys. With the aid of additional melting point depressant, Sn, in Cu-Ti alloys, the intermetallic phase in the active braze can be changed. However, its ability to braze structural ceramics, e.g. alumina, needs further study. The purpose of this research is concentrated on the wettability study of the Cu/Sn/Ti alloy on polycrystalline alumina. Based on the experimental results, the minimum Ti content is 6 wt pct in order to effectively wet alumina. Volume fraction of the intermetallic phase in the braze will be greatly increased if the Ti content in the alloy is increased to 12 wt pct. According to sessile drop test results, 70Cu-21Sn-9Ti demonstrates the best wetting ability on alumina. Meanwhile, the Sn content in Cu/Sn/Ti alloy should be less than 21 wt pct in order to maintain proper wettability of the braze. In addition, Cu/Sn/Ti alloys have both lower wetting angle on alumina and lower thermal expansion coefficients than commercial Ticusil® braze.

Brazing of Inconel X-750 to stainless steel 304 using induction process

The proposed paper discusses results of the joining of Inconel X-750 to stainless steel 304 by induction brazing. The experimental brazing was carried out at 1423 K using a nickel-based brazing BNi-2 (Ni–7Cr–3Fe–3.2B–4.5Si–0.06C wt.%) filler metal. By means of induction heating, joining process can be completed within seconds in argon ambient. The effects of joining time on the microstructures of joint and base materials, elemental distribution within the brazed joint, and the joint shear strengths were investigated. Experimental results show that BNi-2 containing boron and silicon melting point depressants can achieve good wetting and spreading between Inconel X-750 and stainless steel 304. Joint shear strengths of as high as 483 MPa were obtained when processed at approximately 1423 K for 180 s. The width of the braze-affected zone increases with increasing joining time but it does not cause any significant reduction in the joint strength. Inconel X-750 and stainless steel 304 exhibited no noticeable change in their microstructures due to the rapid heating cycle of induction processing.

Liquid infiltrated powder interlayer bonding: a process for large gap joining

The liquid infiltrated powder interlayer bonding (LIPIB) process is proposed to make large gap, transient liquid phase joints. Some basic aspects of the LIPIB process are discussed. In the LIPIB process, a joint with high mechanical properties requires a low volume fraction of porosity in the powder interlayer and low concentrations of melting point depressants in the joint. To achieve the fully infiltrated large gap joint, it is essential to maintain an open pore structure at the initial stage of infiltration, while allowing complete elimination of pores behind the infiltration front. Approximate analytical solutions are applied to describe dissolution of capillaries and isothermal diffusional solidification during infiltration. Calculations based on the Cu–Ag binary system show that infiltration kinetics are strongly affected by the geometry of the capillaries. When the effective capillary size is smaller than the critical effective capillary radius at a certain infiltration temperature, the isothermal solidification time will be less than the time required for full infiltration, leading to an incompletely infiltrated powder interlayer. Full infiltration of powder compacts requires tight control of the rearrangement rate of the particles such that a relatively large capillary radius can be maintained to prevent diffusional solidification before full infiltration. Meanwhile, the rearrangement should be able to close the pores behind the infiltration front. Small amounts of chromium or nickel powder addition into spherical copper powders were found to affect the rearrangement significantly, whereas little effect was observed for irregular copper powders. Copper–copper square joints with clearance up to 22 mm have been successfully made.

Microstructural evolution during transient liquid-phase bonding in a Ni-base superalloy/sapphire fiber composite

Transient-liquid-phase (TLP) bonding was used to fabricate a Haynes 230 Ni-base superalloy/sapphire fiber composite for high-temperature applications. Boron was used as a melting-point depressant for the Ni, to aid superalloy infiltration of the fibers. Preliminary study of the composite indicated an incomplete TLP bonding cycle. Therefore, microstructural and microchemical analyses were carried out to determine the TLP bonding mechanism. It was found that the TLP process did not occur under local thermodynamic equilibrium conditions at the solid/liquid interfaces, contrary to the primary assumption of conventional models, so a modified model for TLP bonding is proposed. The main differences between the proposed and the conventional models are: (a) the concentration of the melting-point depressant increases with time during isothermal solidification, (b) extensive boride segregation at γ grain boundaries and boride precipitation occurs within γ grains adjacent to the interlayer in the initial composite assembly, (c) because of the relatively high boron concentration in the interlayer, the TLP bonding cycle was incomplete, resulting in residual-liquid borides. To achieve ideal TLP bonding, four modifications are recommended: (i) use less boron, (ii) use finer sapphire fibers, (iii) create smaller initial grain sizes in the γ matrix and (iv) increase the homogenization time.

Isothermal solidification kinetics of diffusion brazing

Diffusion brazing (DB) is a process that produces interface-free joints that approach the bulk properties of the material that is to be joined. Solid-state diffusion of the melting point depressant (MPD) element into the joint metal causes the solid/liquid (S/L) interface to advance until the joint is solidified. The time required to complete this isothermal solidification stage was modeled using a moving boundary analysis. Precision measurements of the interlayer width as a function of time were made on the copper-silver system. The bonding apparatus consisted of a suspended load feedback system that prevented leakage of joint metal due to extrusion or surface wetting, thus preserving mass balance. The interlayer width vs bonding time measurements obtained were found to be within a factor of 4 of the theoretical model. The difference was ascribed to the difficulty in accurately measuring the actual liquid width and loss of silver due to vaporization. Large spherical protrusions grow during bonding, which further roughens the interface. However, experimental and predicted concentration profiles of silver were found to be in complete agreement. As the liquid remaining in the grain boundary grooves is diffused away, porosity can develop due to volume changes. By holding the joint at temperature for extended periods, complete solidification followed by homogenization will occur. Selecting the appropriate bonding temperature to achieve a specified maximum concentration of braze metal at the joint is dependent on both the isothermal solidification and homogenization kinetics.

Transition liquid phase bonding of a Hastelloy X and the bond strength at 1173 K

Transition liquid phase (TLP) bonding has been conducted on a Hastelloy X using amorphous foil filler metals. The inserted fillers were made by rapid solidification of a Ni-15% Cr alloy containing 3 to 5 mass% boron as a melting point depressant. The microstructural change with isothermal bonding time from 2.4 to 38.4 ks was examined in the bond regions by optical microscopy and electron probe microanalysis (EPMA). An assessment of the bond strength was made by stress rupture tests performed at 1173 K. It was found that rupture strain and rupture life increased with increasing bonding time. A bond efficiency greater than 80% was achieved when the bonding time was 38.4 ks.

Microstructure and bonding behavior of a new Hf-bearing interlayer alloy for single crystal nickel-base superalloy

Single crystal nickel-base superalloys have been widely used in aero-engine blades, but a limited factor is that this kind of alloy is very sensitive to hot cracking during fusion welding due to high content of {gamma}{prime}-former such as Al, Ti. Transient liquid-phase(TLP) bonding has been developed to join the superalloys susceptible to hot cracking. It is very important to design a suitable interlayer alloy which does not contain any deleterious phases and has a melting point lower than that of the base metal. At present, the interlayer alloys which generally used for superalloys are of the Ni-Cr-Si-B system. However, addition of B and Si should be avoided because these elements are harmful impurity elements in single crystal superalloys. Hf is a potential melting point depressant in the interlayer alloys which has not been evaluated up to date. The goal of this investigation is to characterize the microstructure of a Hf-bearing interlayer alloy and determine the effect of Hf on the solidification of bonding layer during TLP processes for single crystal superalloys.

Transient liquid-phase bonding of ferritic oxide dispersion strengthened superalloy MA957 using a conventional nickel braze and a novel iron-base foil

It has been shown that an iron foil based on the Fe-B-Si system is a suitable material for use as a high-temperature interlayer for transient liquid-phase (TLP) bonding of ferritic oxide dispersion strengthened alloys. TLP bonding produced ferritic joints, free from intermetallic precipitates and identical in composition to that of the parent metal. In contrast, however, TLP bonding using the nickel-base foil, Bni1a, resulted in an austenitic bond region stabilized by the high nickel concentration. Furthermore, the retention of the melting point depressants, particularly silicon, at the centre of the joints resulted in the bond solidifying with the formation of silicide-boride precipitates both at the bond centre and at the braze/parent metal interface. High-temperature heat treatments failed to remove the γ-Fe phase and the precipitates, and their presence detrimentally affected the mechanical properties of the joint. The formation of intermetallic precipitates at the braze centre has been attributed to the initial high concentration of chromium present in the Bni1a brazing foil. Preliminary mechanical tests showed that bond strengths of joints made using the iron-base foil were superior to those obtained using the commercial nickel braze. When the iron-base foil was used, bond strengths both at room temperature and at the service temperature (700 °C) were near parent metal strengths. However, at room temperature, failure was observed to occur away from the bond interface, whilst at elevated temperatures, joints failed along the bond interface; this was attributed to the effects of melt-back phenomenon characteristic in TLP bonding.

Powder Metallurgy Repair of Turbine Components

An advanced powder metallurgy repair process called Liburdi Powder Metallurgy (LPM) has been developed for the repair, overlay or joining of nickel and cobalt-based high-temperature alloys. This process involves mechanical cleaning, followed by the application and consolidation of a filler metal powder, which has substantially the same composition as the base metal, and produces joints with mechanical properties similar to those of the parent material. While previously activated braze or “wide-gap” repair processes have been limited to clearances of approximately 1 mm, the LPM technique has the ability to bridge larger gaps of over 5 mm. In addition, the LPM joints contain significantly lower concentrations of melting point depressants such as silicon and boron than conventional wide-gap repair techniques and exhibit superior microstructural features. The characteristics and typical applications of the LPM process for blade and vane repairs are highlighted and the results of laboratory and engine tests are discussed.

Microstructural investigations of a nickel-based repair coating processed by liquid phase diffusion sintering

Modern stationary gas turbines are designed for the best possible efficiencies and outputs. However, as temperatures and stresses are increased, the design and manufacture of cooled blading in gas turbines becomes complex. This results in higher costs and longer delivery times for cast components and it becomes worthwhile to repair damaged components. The microstructure of samples processed by liquid phase diffusion sintering of pre-alloyed atomized and mixed powders is investigated by electron microprobe analysis, X-ray diffraction and optical microscopy involving colour metallography. The melting and remelting behaviour is analysed by differential thermal analysis. Materials obtained from powders based on the cast superalloys NIM-75, IN-939 and IN-738LC and the braze materials based on the systems Ni-Cr-B, Ni-Cr-Fe-Si-B are shown to consist of a dispersion of borides and/or silicides in the base metal matrix and a more or less eutectic grain boundary precipitation depending on the thermal cycle and the composition. The porosity exhibited does not exceed that of the cast base materials IN-738LC and IN-939. The thermal fatigue proved to be excellent. The corrosion resistance was only sufficient in blends with a high chromium content as boride precipitation caused chromium depletion. Liquid phase diffusion sintering meets in many cases the requirements of repairing cracks and erosion damage by filling and sintering with a braze mix consisting essentially of a superalloy base material and a braze component containing a melting point depressant.

Water as a melting point depressant for acrylic polymers

Water is known to strongly depress the melting point of polyacrylonitrile (PAN) and acrylic copolymers. A scanning calorimetric technique has been developed that utilizes this melting point depression in probing the structure of acrylic fibers. In this report the melting and crystallization of PAN and acrylonitrile-vinyl acetate copolymers is studied as a function of water content. Addition of water continually depresses the polymer melting point until a critical water level is reached where the molten polymer separates from the water and no further reduction in the melting point is observed. Both the minimum melting point and the critical amount of water required for phase separation decreases as the level of vinyl acetate comonomer is increased. The latter relationships are examined in terms of the acrylic polymer morphology and the possibility that the water molecules become associated with the nitrile group during the melting process.