Tahir I. Khan

Liquid Phase Bonding of 316L Stainless Steel to AZ31 Magnesium Alloy

Abstract The excellent corrosion resistance, formability and strength make stainless steels versatile for diverse applications. However, its high specific strength and good crashworthiness make it suitable for transportation and aerospace industry. On the other hand, the need to reduce the weight of vehicle and aerospace components has created renewed interest in the use of magnesium alloys. Due to their differences in physical and metallurgical properties, bonding of the 316L steel and AZ31 magnesium alloy using conventional fusion welding methods encountered many limitations. Therefore, the use of liquid phase forming interlayers is required to overcome the differences in their properties, eliminates the need for a high bonding pressure to achieve intimate contact between the bonded surfaces. Both Cu and Ni interlayers successively formed a eutectic phase with magnesium. The formation of intermetallics and Mg diffusion caused the eutectic phase to isothermally solidify with increasing bonding time. The formation of ternary intermetallic phases (γ1 and B2) impaired the bond shear strength particularly at the end of the isothermal solidification stage where no eutectic phase was observed. However, the joints showed a higher shear strength value of 57 MPa when bonding with Cu interlayer at 530°C for 30 min compared to 32 MPa when Ni interlayer was used at 510°C for 20 min.

Transient liquid phase (TLP) brazing of Mg–AZ31 and Ti–6Al–4V using Ni and Cu sandwich foils

Abstract Transient liquid phase (TLP) brazing of Mg–AZ31 alloy and Ti–6Al–4V alloy was performed using double Ni and Cu sandwich foils. Two configurations were tested; first, Mg–AZ31/Cu–Ni/Ti–6Al–4V and second, Mg–AZ31/Ni–Cu/Ti–6Al–4V. The effect of set-up configuration of the foils on microstructural developments, mechanical properties and mechanism of joint formation was examined. The results showed that different reaction layers formed inside the joint region depending on the configuration chosen. The formation of ϵ phase (Mg), ρ (CuMg2), δ (Mg2Ni) and Mg3AlNi2 was observed in both configurations. Maximum shear strength obtained was 57 MPa for Mg–AZ31/Ni–Cu/Ti–6Al–4V configuration and in both configurations, the increase in bonding time resulted in a decrease in joint strength to 13 MPa. The mechanism of joint formation includes three stages; solid state diffusion, dissolution and widening of the joint, and isothermal solidification.

Transient liquid phase diffusion bonding of oxide dispersion strengthened nickel alloy MA758

AbstractTransient liquid phase diffusion bonding has been used to join an oxide dispersion strengthened (ODS) nickel alloy (MA758) using an amorphous metal interlayer with a Ni–Cr–B–Si composition. A microstructural study was undertaken to investigate the effect of parent metal grain size on the joint microstructure after isothermal solidification. The ODS alloy was bonded both in fine grain and recrystallised conditions at 1100°C for various hold times. The work shows that the final joint grain size is independent of the parent alloy grain structure and the bonding time. However, when the alloy is bonded in the recrystallised condition and given a post-bond heat treatment at 1360°C, the joint grain size increases and a continuous parent alloy microstructure across the joint region is achieved. If MA758 is bonded in the fine grain condition and then subjected to a recrystallising heat treatment at 1360°C, the grains at the joint appear to increase in size with increasing bonding time. The joint grains are...

Reactive brazing of ceria to an ODS ferritic stainless steel

This research study shows that a ceria ceramic can be bonded to an ODS ferritic stainless steel (MA956) by reactive brazing using a Ag68-Cu27.5-Ti4.5 interlayer. The ability to join these materials provides an alternative to the current ceramic interconnects used in the development of solid oxide fuel cells. Initial results show that the ceramic-metal bonds survived the bonding process irrespective of the degree of porosity within the ceria ceramic. Metallographic analyses indicate that a reaction zone formed along the ceria/braze interface, which was not only titanium rich, but also consisted of a mixture of copper oxides. When the ceramic-metal bonds were exposed to high bonding temperatures or when subjected to thermal cycling at 700°C, this reaction layer increased in thickness and had a detrimental affect on the mechanical strength of the final joints.

Development of Wear Resistant Nano Dispersed Composite Coating by Electrodeposition

This study concerns the development of wear resistant coatings of Ni-Al2O3 composite on steel substrates by electrodeposition. Each of the coating experiments was performed in an electrolytic bath, containing a nano-sized dispersion of Al2O3 particles in nickel sulfate and boric acid solution. Composition of the coating mixture was systematically varied with respect to the contents of the dispersed particles, while the amount of the dissolved nickel sulfate, and boric acid and the applied current were kept constant during the experimental measurements. The coated substrates were characterized for their morphology, Vickers hardness, and scratch resistance properties. It was observed that hardness and scratch resistance of the coated substrates increased with an increase in the Al2O3 content in the coating. It was noted that hardness of the composite coating decreased after heat treatment at 400oC in air atmosphere.

Effect of bonding temperature on transient liquid phase bonding behavior of a Ni-based oxide dispersion-strengthened superalloy

The effect of joining temperature on the transient liquid phase (TLP) bonding of MA758 superalloy was investigated. The TLP bonds were made at temperatures of 1100 and 1200 °C. Analysis was undertaken to determine the changes within the joint microstructure. The bonding temperature affected the extent of parent metal dissolution, the time for isothermal solidification, and the attainment of microstructural continuity across the joint region. Bonding at 1100 °C did not result in extensive parent metal dissolution, and subsequent shear testing showed failure through the center of the joint. However, bonding at 1200 °C increased parent metal dissolution resulting in significant agglomeration of Y2O3 particles at the joint interface. Failure was observed along the joint interface in regions depleted of strengthening particles. Bonding at a higher temperature reduced the time for isothermal solidification but also reduced the strain energy of the oxide dispersion-strengthened alloy so that grain growth across the joint region could not be achieved.

Effect of Ni–Al2O3 nanocomposite coating thickness on transient liquid phase bonding of Al 6061 MMC

Abstract Transient liquid phase diffusion bonding of Al 6061 containing 15 vol.-% alumina particles was carried out as a function of interlayer thickness. Electrodeposited Ni coatings containing 18 vol.-% nanosized alumina particles with thickness ranging from 1 to 13 μm was used as the interlayer. Joint formation was attributed to the solid state diffusion of Ni into the Al 6061 metal matrix base metal followed by eutectic formation, base metal dissolution and isothermal solidification at the joint interface. Examination of the joint region using scanning electron microscopy, wavelength dispersive spectroscopy and X-ray diffraction showed the formation of intermetallic phases such as AlFe3Si, Ni17Al3·9Si5·1O48 and Ni3Si within the joint zone. The results indicated that a maximum joint strength of 92% of the base metal shear strength can be obtained if an 11 μm thick Ni–Al2O3 nanocomposite coating is used as the interlayer.

Diffusion Bonding of Austenitic Stainless Steel 316L to a Magnesium Alloy

Dissimilar metal combinations are often necessary when manufacturing a component in order to meet particular functional and engineering requirements or protect against environmental degradation. Stainless steels are used in a diverse range of applications due to their excellent corrosion resistance, formability and strength. The 316L stainless steel also shows good crashworthiness due to its high strain rate sensitivity which makes it suitable for the transportation industry. The joining together of the 316L steel and AZ31 magnesium alloy cannot be achieved using conventional fusion welding methods and therefore, diffusion bonding using interlayers was used to overcome the differences in their physical properties. The results show that Cu and Ni interlayers form a eutectic with the magnesium which enhances wettability and bond formation through isothermal solidification. The effect of hold time on the microstructural developments across the joint region was studied at a bonding temperature of 530oC and 510oC for the Cu and Ni interlayers respectively using a bonding pressure of 0.2 MPa. This preliminary investigation shows that by increasing the bonding time from 5 to 60 minutes results in a Cu-Mg and Ni-Mg eutectic phase structure forming along the bond interface. By holding the joint at the bonding temperature for 15 minutes initiates isothermal solidification of the joint and this was confirmed by DSC analysis. However, the movement of the solid/liquid interface on solidification pushes intermetallic phases into the center of the bond during the solidification stage. The intermetallics increase the hardness value of the bond interface and lower final bond strengths.

High Temperature Oxidation Behaviour of Nickel Based Nanostructured Composite Coatings

The electrodeposition of nanostructured composite coatings involves the co-deposition of nanosized oxide particles such as TiO2, Al2O3 and Y2O3 into a corrosion resistant metal matrix such as nickel to improve the high temperature oxidation and erosion resistance of nickel coatings. The technique has several advantages over other methods for producing nanostructured composite coatings such as thermal metal spraying. Some of the main advantages are lower cost for equipment setup and lower material cost and the ease with which the process can be controlled. Although electrodeposited nanostructured coatings are being developed for various aerospace and marine applications, they have not yet been considered for protecting surfaces of components and piping that is used in technologies for the oil sands industry such as the In-Situ Combustion (ISC) process. The challenge with in-situ combustion oil production is that the combination of high temperature combustion gases and the presence of moving sand particles create an extremely severe environment in which high oxidation and erosion rates are expected. As a result there is a need to develop function specific coatings that can withstand both high temperatures and erosive environments in the oil sands industry. This paper presents results of high temperature oxidation behaviour of nickel coatings containing two types of nanosized oxide dispersions (TiO2 and Al2O3). High temperature oxidation tests were conducted in dry air for 500°C and 700°C. The oxidized specimens were examined by metallographic surface analysis and surface composition techniques such as Scanning Electron Microscopy (SEM), Wavelength Dispersive X-Ray Spectroscopy (WDS). The effects of nanosized oxide particles on high temperature oxidation behavior of nickel coatings have been studied. The results show an improvement in the high temperature oxidation resistance of nickel coatings dispersed with Al2O3 and TiO2.

Strain Hardening of Heat Treated Nanostructured WC-17Co Coatings and Their Sliding Wear Behavior

Grain size, their distribution and geometry are important to study the dislocation behavior and grain boundary sliding of ceramic reinforced metallic composite materials. Grain size reduction has been shown to lead to significant improvements of the wear resistance in nanostructured materials. As the grain size decreases from polycrystalline to nanocrystalline range, abrasive wear resistance increases considerably from the increased hardness and volume loss following Archard’s law of wear. Further, the heat treatment effect on the content of the metallic binder in a ceramic-metallic (cermet) material is thought to increase the hardness with decreasing crystalline size, thereby improving the sliding wear behavior of materials. In this study, the high velocity oxy-fuel (HVOF) thermal spraying of nanostructured WC-17Co coatings with engineered ‘duplex outer coating’ is conducted. The microhardness and sliding wear studies of the coatings and their heat-treated counterparts are performed. The nanostructured coatings showed a significant increase in the microhardness and wear resistance when compared with those of the conventional microstructured coatings of the same composition. It is believed that the improved performance is related with the work hardening as well as dispersion hardening of the nanostructured grains in the deposited coatings.Copyright