Formation Of Brittle Intermetallic Phases Research Articles

Microstructure, residual stresses and mechanical properties of diffusion bonded tungsten–steel joint using a V/Cu composite barrier interlayer

Diffusion bonding is a preferred method to join W and steel for divertor applications. To minimize the residual stress induced by the large mismatch of thermal expansion coefficients and to inhibit the formation of brittle intermetallic phases, a V/Cu composite barrier interlayer was designed and examined to produce a joint between W and steel. The diffusion bonding was carried out at 1050°C for 1h under a 10MPa pressure in vacuum. Metallographic analysis revealed excellent bonding at all of the joining interfaces. Neither intermetallic compounds nor other brittle phases were found in the bonded region. Nano-indentation test across the joint interfaces demonstrated the effect of solid solution strengthening in the diffusion zone. The strength of the joint was as high as 402MPa and the failure occurred predominantly at the W substrate near the W/V interface due to the residual stress concentration.

Laser welding of Ti–6Al–4V to Nitinol

Formation of brittle intermetallic phases in addition to different thermal expansion coefficients associated with dissimilar welding leads to the formation of transverse cracks in weld metal and eventually restricts widespread applications of dissimilar joints. Therefore, joining technology should be expanded in field of dissimilar welding in order to solve its difficulties. In the present study, an experimental work with pulsed Nd:YAG laser was performed for dissimilar welding of Ti–6Al–4V and Nitinol. Autogenous welding of these two alloys resulted in joints with poor strength and ductility due to the formation of transverse cracks in the weld metal. Therefore, the chemical composition of the weld metal has to be modified in order to reduce the formation of brittle phases and eliminate subsequent cracking. In this work, this was done by insertion of a copper interlayer with a thickness of 75μm between the base metals. The results indicated that insertion of copper interlayer has a great influence on the reduction of the amount of Ti2Ni brittle intermetallic phase, elimination of transverse cracks through the weld metal and eventually improvement of mechanical properties of the joints. Insertion of copper interlayer was very useful since it altered the cracked autogenous joint to a joint which could withstand a tensile stress of 300MPa.

CuTiNiZrV Amorphous Alloy Foils for Vacuum Brazing of TiAl Alloy to 40Cr Steel

Vacuum brazing of TiAl alloy to 40Cr steel sheets was conducted with newly developed CuTiNiZrV amorphous foils. It was found that a diffusion layer, filler metal and reaction layer existed in the brazed seam. The diffusion layer in the joint brazed with Cu43.75Ti37.5Ni6.25Zr6.25V6.25 (at.%) foil was flat and thin, containing Ti19Al6 and Ti2Cu intermetallic compounds; however, the diffusion layer brazed with Cu37.5Ti25Ni12.5Zr12.5V12.5 foil was uneven with bulges, consisting of essentially Ti-based solute solution. The foil with 12.5 at.% V showed inferior spreadability compared to that with 6.25 at.% V at brazing temperature. However, fracture happened along the diffusion layer with 6.25 at.% V foil due to the formation of brittle intermetallic phases, but the joints brazed with 12.5 at.% V foil failed through the TiAl substrate. These results show that designing amorphous alloy with less Ti and more V for brazing TiAl alloy to steel is appropriate.

Characterization of Explosion-Bonded Ti-Alloy/Steel Plate with Ni Interlayer

A three-layered explosion-welded (EXW) plate consisting of Ti-alloy/Ni/steel was investigated. Both the Ti/Ni and Ni/steel interfaces displayed a wavy shape typical of the EXW process, which reflects the occurrence of massive mass transfer during bonding. New phase formation was not observed at the Ni/steel interface, while, according to the SEM, XRD, and nanoindentation testing, the formation of brittle intermetallic phases took place at the Ti-alloy/Ni interface. These intermetallics serve as the weakest part of the joint and determine a brittle mode of fracture under tensile stress. The tensile strength of the joint is about 300 MPa, which is determined by intermetallic phases formed at the Ti/Ni interface.

Beneficial Interface Geometry for Laser Joining of NiTi to Stainless Steel Wires

Joining the dissimilar metal pair of NiTi to stainless steel is of great interest for implantable medical applications. Formation of brittle intermetallic phases requires that the joining processes used for this dissimilar pair limits the amount of over-melting and mixing along the interface. Thus, because of its ability to precisely control heat input, laser joining is a preferred method. This study explores a method of using a cup and cone interfacial geometry, with no filler material, to increase the tensile strength of the joint. Not only does the cup and cone geometry increase the surface area of the interface, but it also introduces a shear stress component, which is shown to be beneficial to tensile strength of the wire as well. The fracture strength for various cone apex angles and laser powers is determined. Compositional profiles of the interfaces are analyzed. A numerical model is used for explanation of the processing parameters.

The Preparation of Metallic Joinings to Optimise their Topography

Abstract Modern light structures increasingly require different alloys or metals to be joined with one another in the form of composite materials [1]. As a result of their different melting temperatures or of the formation of brittle inter-metallic phases, many combinations of metals cannot be produced using thermal methods. One alternative, however, is the use of solid state welding such as explosive or magnetic impulse welding [2; 3]. The examination and analysis of the bonding interfaces is therefore of central importance in the evaluation of the quality of the bond produced. Scanning electron microscopy together with electron probe micro­analysis of the characteristic X-rays emitted using EDX and WDX techniques can reveal information regarding the distribution of the metals present within the bonding interface [4], the presence of inter-metallic phases [5] and of areas of inadequate bonding. For an accurate evaluation of the characteristic X-rays produced, however, the surface of the specimen must be guaranteed to be both flat and smooth in order to prevent the effects of shadowing and deflection of the primary beam which might otherwise cause erroneous results in the elements present and their distribution [6]. A specimen surface which is not perfectly flat also makes it difficult to focus the electron beam. The simultaneous flat and smooth preparation of metallic composites which have very different strength and hardness properties is very difficult due to the different rates of removal of material during grinding and polishing [7; 8]. In view of this, the following methods of preparation used to produce flat and smooth specimen surfaces on two types of metallic joinings are presented.

Effect of heat treatment on interface microstructure and bond strength in explosively welded Ti/304L stainless steel clad

The effects of heat treatment on the microstructure and bond strength at the interface of explosively welded titanium/304L stainless steel clad have been investigated. The microstructure of the clad interface were examined using optical and scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) techniques. At 700°C, the formation of intermetallic phases σ and Fe2Ti besides β-Ti were confirmed, while in samples, heat treated at 800°C and 900°C, other intermetallic phases such as λ and FeTi, NiTi, NiTi2 phases were detected in addition to σ and Fe2Ti phases. The shear test results show that the shear strengths of heat treated samples are overall significantly lower than that of stress relieved samples. This could be due to the formation of brittle intermetallic phases at the interface. Despite of formation of the intermetallic phase at 700°C, the shear strength of the sample is still more than the minimum standard amount of 137·9 MPa. It is also observed that the higher the heat treatment temperature, the lower the interface shear strength, which can be explained by the fact that the volume fraction of intermetallics increases with increasing temperature. The shear strength values of heat treated samples are lower than those of diffusion bonded clads. This could be attributed to the fact that the total widths of intermetallic layers achieved in heat treated samples are larger than those of for diffusion bonded ones.

Joining of AA1100–16 vol.-%B4C metal matrix composite using laser welding and friction stir welding

This work investigated the weldability of AA1100–16B4C (vol.-%) metal matrix composite using laser welding and friction stir welding (FSW). The microstructure and mechanical properties of joints produced by two welding techniques were compared. For laser welding, a large amount of brittle needle-like phases, together with some residues of B4C particles were observed in the fusion zone of the joint. The use of Ti filler can improve the microstructure and increase the joint efficiency from 63 to 75%. On the other hand, the FSW, being a solid state welding technique, was interesting as it minimised the formation of brittle intermetallic phases. In addition, the breakage of B4C particles and the grain refinement of the matrix were observed in the weld zone. The FSW joints show superior mechanical properties than the laser welded joints. Up to 100% joint efficiency can be achieved by FSW for annealed AA1100–16B4C materials.Ce travail a examiné la soudabilité du composite à matrice métallique AA1100-16B4C (% en vol) en utilisant le soudage au laser et le soudage par friction-malaxage (FSW). On a comparé la microstructure et les propriétés mécaniques des joints produits par les deux techniques de soudage. Pour le soudage au laser, on a observé une grande quantité de phases fragiles en forme d'aiguille ainsi que des résidus de particules de B4C, dans la zone de fusion du joint. L'utilisation de métal d'apport de Ti peut améliorer la microstructure et augmenter l'efficacité du joint de 63 à 75%. D'un autre côté, le FSW, une technique de soudage à l'état solide, était intéressant puisqu'il minimisait la formation de phases intermétalliques fragiles. De plus, on a observé, dans la zone de soudure, la rupture des particules de B4C et l'affinement de grain de la matrice. Les joints du FSW montraient des propriétés mécaniques supérieures aux joints soudés au laser. On peut obtenir une efficacité du joint jusqu'à 100% par FSW pour les matériaux recuits d'AA1100-16B4C.

Laser welding of Mg alloy MgAl3Zn1 (AZ31) to Al alloy AlMg3 (AA5754) using ZnAl filler material

Welding of magnesium with aluminium alloys is challenging due to the formation of brittle intermetallic phases. In this study laser welding of an Mg alloy with an Al alloy, using a Nd∶YAG laser with a two focus optics and ZnAl filler material, was investigated. Mechanical tests, as well as microstructure investigations by light optical microscopy, scanning electron microscopy and energy dispersive X-ray microanalysis were carried out in order to characterise the hybrid joint. The strength of the joints is significantly affected by the aluminium content in the filler wire. Strengths of the joint were comparable to those obtained by other joining techniques such as diffusion or adhesive bonding using welding speeds of 1·75 m min−1.

Factors affecting the properties of Friction Stir Welds between aluminum and magnesium alloys

Abstract Extruded Al–0.5Mg–0.3Si (6063) aluminum and rolled Mg–3Al–1Zn (AZ31B) magnesium alloy sheets were joined by Friction Stir Welding. The dissimilar metal weld joints exhibit tortuous interfaces. The nugget grain size on both the Al and Mg sides monotonically increase as the tool rotational speed increases. The midplane microhardness traverses show fluctuating hardness peaks due to the presence of different microstructural phases in the nugget zone. The maximum tensile strength of the dissimilar weld joint is 68% of the 6063-T5 base metal with a maximum elongation of 1%. The low ductility is attributed to the formation of brittle intermetallic phases at the Al–Mg interface in the weld joint. Transverse tensile test results are correlated with several interface features: (1) actual interface length, (2) extent of interpenetration between the aluminum and magnesium base metals, (3) maximum intermetallic layer thickness, and (4) area fraction of micro-void coalescence on the tensile fracture surfaces. Results indicate that maximizing the extent of interpenetration and hence promoting mechanical interlocking between the metallic phases, is the key to attaining reasonable transverse strength in Al–Mg dissimilar metal welds. The welding process control variables that promote higher mechanical interlocking of the weld joints are discussed. The process response variables (welding torque, power, x-axis force and the nugget grain size) are presented for a range of welding parameters.

Optimization of Aluminium-to-Magnesium Ultrasonic Spot Welding

The ability to join dissimilar materials in the automotive industry will result in more efficient multimaterial structures. However, welding of aluminium (Al) to magnesium (Mg) alloys is problematic because of the rapid formation of brittle intermetallic phases at the weld interface. Ultrasonic welding (USW) is a solid-state joining technology that may offer a potential solution, but USW of Al to Mg is currently not well understood. Here, we have investigated the effect of process variables and energy input on joint formation between Al-6111 and Mg-AZ31 alloys, and we report on the optimum welding conditions, heat generation, and the formation of a significant intermetallic reaction layer. Furthermore, the factors influencing the interface reaction rate and the advantages of precoating the Mg with Al are discussed.

Transport phenomena in conduction mode laser beam welding of Fe–Al dissimilar couple with Ta diffusion barrier

Fusion welding of dissimilar metals always encounters immense difficulties due to the formation of brittle intermetallic phases that make the joint unsuitable for structural applications. Using a barrier layer that wets well with either component but does not undergo melting itself and prevents intermixing and undesirable phase formation could be a possible remedy. In order to explore this hypothesis, we have made an attempt to numerically simulate and experimentally validate the heat and mass transport phenomena in the fusion zone concerning laser assisted welding of Fe–Al dissimilar couples with 0.2 mm Ta foil as the diffusion barrier butted between Fe and Al. The results suggest that the Fe-side undergoes greater degree of melting resulting in an asymmetric thermal and velocity fields. Furthermore, partial melting of Ta seems unavoidable but proves benign as this leads to good bonding between Fe–Ta and Ta–Al while precluding formation of Fe–Al alloy/intermetallic phases.

Influence of cerium on hot workability of 00Cr25Ni7Mo4N super duplex stainless steel

The effect of Ce on hot workability of 00Cr25Ni7Mo4N steel melted in vacuum induction furnace was studied by Gleeble thermal simulation machine and scanning electron microscopy (SEM). The results showed that ductility of the steel with Ce addition was increased significantly because of increasing content of austenite, segregation of Ce at grain boundaries and modification of inclusion. The optimum range of Ce content in the steel was 0.030 wt.%–0.047 wt.%, and the optimum value was about 0.047 wt.%. The effect of rare earth on the improvement of hot workability would decrease after adding 0.062 wt.% Ce because of generation of bigger aggregated inclusions and formation of brittle intermetallic phases (Ce-Fe-Cr-Mn-Mo). The temperature of steady plastic deformation of the steel with Ce addition was reduced to 1000 °C at a strain rate of 10/s and 1050 °C at a strain rate of 50/s, respectively.

Microstructure and Mechanical Properties of Dissimilar Joints of AZ31 MG Alloy to Aluminium Alloys

In attempts to improve the performance of dissimilar joints between AZ31 Mg alloy and different Al alloys, solid state joining processes such as Magnetic Pulse Welding (MPW) and Friction Stir Welding (FSW) were applied for minimizing the formation of brittle intermetallic phases. MPW process has been concentrated mainly on round section tube to tube and tube to bar welds. Mg alloy AZ31 has been successfully welded to pure Al A1070 as well as to Al alloy A3003. Tensile test clearly showed the MPW welds were stronger than the weaker of the base metal so failure occurred in aluminum base metal. While FSW process for the dissimilar joint between AZ31B/A6061 alloys with a thickness of 2mm revealed optimum weldability under the conditions of travel speed of 0.8mm/sec and tool rotation speed of 850rpm. For the sound dissimilar joint, the maximum tensile strength of 179 MPa, which was about 80 % of the Mg base metal tensile strength, has been obtained.

Cladding of aluminum substrates with iron based wear resistant materials using controlled short arc technology

For lightweight constructions aluminum alloys are established since many years. Seal face and inlays, made of wear resistant materials, are commonly used to reach a partial wear protection for aluminum components. Problems concerning this approach are the necessary space and the endurance strength of the inlay-part joint. An alternative for the replacement of the inlays by cladding using innovative and easy to handle short arc technology will be discussed in this paper. The presented technology offers the potential to control the energy input into the substrate and so the formation of brittle intermetallic phases which occur in the aluminum–steel interface. The usage of wear resistant iron based feedstock materials enables further applications due to advantageous physical and mechanical properties. In the experiments the aluminum alloy AlSi8Cu3 was cladded with stainless steel and a nanocrystalline solidifying wear resistant iron based alloy using the coldArc equipment. Due to the low heat input, intermetallic phases could be minimized using stainless steel as cladding material as well as nanocrystalline solidifying iron based material. Further investigations in the resulting interface section, which did not exceed 10 µm, had been carried out by using SE-microscopy and energy-dispersive X-ray analysis.

Joining of cemented carbides to steel by laser beam welding

AbstractWelding of dissimilar materials such as steel and cemented carbides (hardmetals, cermets) is particularly challenging e.g. because mismatches in their thermal expansion coefficients and thermal conductivities result in residual stress formation and because of the formation of brittle intermetallic phases. Laser beam welding of cemented carbides to steel appears as an attractive complementary technique to conventional brazing processes due to its high precision, high process speed, low heat input and the option of welding without filler.Here a laser welding process including pre‐heat treatment and post‐heat treatment was applied successfully to joining as‐sintered and nitrided hardmetals and cermets to low alloyed steel. The microstructure and mechanical properties of the welds are investigated by microscopy, X‐ray diffraction, microhardness measurements, and bending tests. The results reveal that the three‐step laser beam welding process produced crack‐free and non‐porous joints. Nitridation of the cemented carbides results in a significant reduction of the amount of brittle intermetallic phases. The mechanical properties of the joints are competitive to those of the conventional brazed steel‐cemented carbide joints.

Laser brazing of a steel/aluminium assembly with hot filler wire (88% Al, 12% Si)

The main problem in joining steel to aluminium by a thermal process (e.g. TIG, MIG) is the formation of brittle intermetallic phases, that causes the mechanical performance of the assembly to be poor. A laser beam, however, can localize the fusion and inhibit the formation of these phases. This article presents a feasibility study for steel/aluminium assemblies with filler wire of 88% Al and 12% Si. Analysis of the resultant joints may show the causes of fracture. An experimental design study is conducted, to see how the mechanical performance of such joints could be improved. This method establishes what the best operational limits are and shows how different operational conditions influence the various ways in which fractures occur.

Compaction of amorphous aluminum alloy powder by direct extrusion and equal channel angular extrusion

Al 89Gd 7Ni 3Fe 1 alloy powder produced by gas atomization was consolidated using equal channel angular extrusion (ECAE) and direct extrusion. The powder particle size was below 30 μm and the as-received powder was ∼70–80% amorphous. ECAE compaction of loose powder was conducted at 200 °C. The microstructure of the compacted material was studied after different levels of ECAE deformation, which allowed analysis of the mechanism of particle interactions, deformation and bonding during different stages of consolidation. Direct extrusion compaction was conducted at 210, 250, 400, and 450 °C. After compaction at 210 °C, the material was about 70% amorphous and it had 98–99% of the theoretical density. A fully dense alloy with a nanocrystalline structure (grain size ∼700 nm) was produced after extrusion at 450 °C. Extrusion at 250 and 400 °C led to poor consolidation of the powder due to formation of brittle intermetallic phases with volume fraction of about 40%.

Corrosion of High Temperature Alloys by Molten Glass

In the glass industry, the lifetime of many metallic parts in contact with molten glass is controlled mainly by high temperature corrosion which, as for aqueous solutions is governed by oxidation-reduction reactions: reduction of glass components counterbalances the oxidation of metallic components. Depending on the glass composition, O 2 , Fe III , Si IV or other oxidants can be reduced, leading to various forms of degradation. If the corrosion potential is high enough, passivation can be observed all the more than diffusion of oxidants is accelerated. If the potential is low, active corrosion is induced. In the case where reduction of glass components leads to the formation of metals (Si, Pb, Mo...), formation of brittle intermetallic phases can be observed. The results obtained on Fe, Co, Ni, Cr as well as synthetic and commercial alloys, lead to a general description of the corrosion of refractory alloys in molten glass.

Internal friction study of decomposition kinetics of SAF 2507 type duplex stainless steel

During the last decade, super duplex stainless steels (DSSs) with increased nitrogen content have been an object of intensive studies. Present work is devoted to the study of the peculiarities of {delta}-ferrite decomposition in SAF 2507 type duplex steel, and redistribution of nitrogen between ferrite and austenite phases in a wide temperature range by means of internal fraction (IF). Unlike local methods of electron microscopy or engineering methods of hardness or impact toughness testing, which give basically information on the formation of brittle intermetallic phases, the internal friction technique allows to study the state of solid solution and kinetics of changes in the relative amounts of ferrite and austenite phases during thermal treatment.