Formation Of Brittle Intermetallics Research Articles

Dissimilar friction stir welding and post-weld heat treatment of Ti-6Al-4V and AA7075 producing joints of unprecedented strength

Dissimilar welding of Ti and Al alloys is challenging due to the potential formation of brittle intermetallics, which can compromise weld strength. Friction stir welding (FSW) is an advanced joining method with the potential to drastically reduce or prevent the formation of intermetallic phases because melting is avoided. However, in previous studies, dissimilar joints of Ti-6Al-4V and EN AW-7075 with satisfactory strength could not be achieved. Thus, we investigated friction stir welding of these two materials in a butt configuration, and analyzed the effects of post-weld heat treatments on the mechanical properties. Tensile strengths of 441 MPa in the as-welded condition and 505 MPa after heat treatment were achieved, significantly surpassing strength values reported in earlier studies. Microscopic investigations showed no evidence of formation of brittle intermetallic phases, and ductile fracture was observed. Our results show that FSW is a very promising candidate for future aerospace applications requiring dissimilar joining of Ti and Al alloys.

The effect of vacuum brazing time on the microstructure and mechanical properties of Ti-6Al-4 V to 316 L dissimilar joint using BNi-2 filler metal

Due to the distinct differences in physical and metallurgical properties, it is hard to join titanium alloys to stainless steels directly via conventional welding processes. In this research, the grade 5 titanium alloy was subjected to brazing with austenitic stainless steel through the vacuum furnace, and the process was conducted above the β transition temperature (BTT). In order to reduce brittle intermetallic formation, a nickel base filler metal (Ni-6.6Cr-4.5Si-3B) was employed under a vacuum condition, and the effect of time on the microstructure and mechanical properties of the joints was investigated. The brazing temperature led to the thickening of the interface to 400 µm, toward the titanium alloy, and the FeTi formed adjacent to the diffusion affected zone (DAZ). Moreover, the isothermal solidified zone (ISZ) widened during the whole process, which, led to higher dissolution of the titanium base metal in the ISZ. Because of the widening of the interfaces, there were no Ni-rich compounds or residual filler metal in this region. SEM result showed that TiB strengthening borides, which were distributed after 45 min, and this evolution caused an increase in the shear strength of the joint up to 67 MPa. Fractography analyses and XRD results of the fractured surfaces revealed that the Ti2Ni phase, which formed in the athermaly solidified zone (ASZ) during a two-step solidification reaction, caused the brittle fracture in all joints.

Achieving ultra-high stiffness by solidifying and precipitating micro-compounds in HCP/BCC dual matrix of a new Mg-Li alloy

One of the major obstacles restricting the wide applications of Mg alloys is their low Young's modulus at ∼45 GPa, which is uncompetitive to Al alloy over 60 GPa. To resolve this issue, we proposed a new strategy of designing ultra-light Mg-Li alloys by employing the microscale compounds in both HCP and BCC grains to achieve high Young’s modulus. HCP or BCC alone cannot achieve a high modulus due to the segregation of solutes at the grain boundaries and the formation of brittle intermetallics in the interdendritic region. We designed chemistry that is so perfect to arrest the excess solutes inside the BCC matrix during solidification and engulf pre-solidified rare earth particles by HCP primary grains which have such a lean chemical composition that has almost no segregation. During solution heat treatment, the dissolution of solutes pushes the BCC boundary towards HCP grains by Ostwald ripening (BCC structures are interconnected but the HCP structures are isolated). During low-temperature aging heat treatment, nucleation of HCP structure on the rare earth particles inside the BCC grain is found, degrading the strength. Therefore, a novel Mg-Li-Al-Zn-Y-Mn-Gd (LAZWMV) alloy with a very high modulus was designed which reaches Young's modulus as high as 61.4 GPa. Micro-compounds (AlLi and Al-X) with average equivalent diameters of 3.76 µm and 6.55 µm were quantified in 3D using X-ray computed tomography and nanoindentation tests for establishing the microstructure-modulus correlation. The influence of Li addition on Mg-Mg was calculated by DFT calculations and the mechanism of modulus improvements has been proposed.

Metallurgical aspect of a joint made of Inconel 718 and CuCrZr brazed with Ag-Cu eutectic filler

The realization of bimetallic structures is one of the major challenges of manufacturing industry, to this aim different technologies are employed, among these vacuum brazing presents some advantages: this technology, in fact, is relatively easy to use and the relative low temperature and low process time employed, avoid the formation of brittle intermetallics and thermally affected zones. An application of vacuum brazing technology is the realization of regenerative thrust chamber for rocket engines where cooling channels of CuCrZr are joined to Inconel 718 combustion chamber. In this work the critical metallurgical aspects occurring during brazing of Inconel 718 and CuCrZr using eutectic Ag-Cu filler are described in detail. These aspects include the formation of two distinct metallurgical zones inside the filler and an interdiffusion zone with Kirkendall porosity at the interface Inconel 718-Brazing Alloy. The analysis of fracture surface after tensile tests indicates that the interdiffusion zone is the weak point of the joint.

Supersaturated solid solution enhanced biodegradable Zn-Mn alloys prepared by mechanical alloying and selective laser melting

Zn-Mn alloys have been recognized as promising biodegradable metals due to moderate degradation and good osteogenesis. However, the alloying content of Mn has been severely limited by the low solid solubility (0.8 wt%) in conventional preparation process to avoid the excessive formation of brittle intermetallics. In this work, supersaturated solid solution was achieved in Zn-Mn alloys via mechanical alloying (MA) and selective laser melting (SLM). During MA process, the mixed Zn and Mn powders experienced repeated welding and fracturing, which resulted in refined grains and abundant crystal defects that stored extra energy. This could reduce the energy barrier of interatomic diffusion and induce the rapid diffusion of Mn atoms, leading to supersaturated solid solution with ∼3.6 wt% Mn homogeneously dissolved in Zn lattices. Moreover, the rapid solidification process of SLM effectively maintained the metastable supersaturated structure. In addition, the Zn-15Mn alloys prepared by MA and SLM were composed of fine equiaxed grains (∼4.34 µm) and high content of high-angle grain boundaries (54.8%). Owing to solution strengthening and grain refinement, the Zn-15Mn alloy prepared by MA and SLM exhibited a compressive yield strength of 178 MPa which was 2.8 times and 1.6 times those of pure Zn and Zn-15Mn alloy without MA process, respectively, as well as a more ductile fracture. Meanwhile, the Zn-Mn alloys exhibited accelerated degradation and comparable cytocompatibility to pure Zn. These findings indicated that the combined process of MA and SLM may be an efficient strategy for the preparation of alloys characterized by supersaturated solid solution and refined grain structure.

Influence of brass interlayer and water cooling on microstructure, mechanical and corrosion behaviour of friction stir welded AA6061-T6 alloy

This study investigated the effect of brass interlayer and water cooling on the mechanical properties and corrosion behaviour of friction stir welded AA6061 alloy. Although grain refinement was observed in the stir zone (SZ) of FSW without interlayer, strength loss occurred due to thermal softening. The insertion of a brass interlayer resulted in more grain refinement which could be attributed to brass particles acting as a nucleation site for further recrystallization. The weld strength with interlayer improved due to the formation of strengthening intermetallic compounds (IMCs) such as Al2Cu and Al4Cu9. However, the strength recovery is not attainable in a heat-affected zone (HAZ). Moreover, hard and brittle IMCs formed in SZ of weld with interlayer resulted in premature failure. Therefore, water cooling was used to enhance the hardness in HAZ. Very fine grains in SZ with a high fraction of high angle grain boundaries were obtained with water cooling FSW with interlayer, while HAZ softening was also reduced. Moreover, the water-cooling FSW with interlayer inhibited the formation of brittle intermetallics and decreased volume fraction of IMCs attributed to faster cooling rate. Thereby the strength of the overall weld joint was enhanced. The corrosion test was performed in 3.5% NaCl solution. The corrosion findings stated that the decrease in the number of intermetallics formed low galvanic coupling with aluminium matrix resulted in better corrosion resistance and minimum corrosion rate for water-cooled FSW than natural cooling.

Dissimilar P-TIG welding between Inconel 718 and commercially pure Titanium using niobium interlayer

The formation of brittle intermetallics is a challenge for the direct joining of commercially pure titanium (CpTi) and Inconel 718 (IN718) alloy. In the present work, dissimilar weldments using the P-TIG (Pulsed-Tungsten Inert Gas) welding technique were produced, with elemental Niobium (Nb) as an interlayer. The insertion of Nb acted as a barrier and suppressed the interdiffusion of Ti and Ni across the interlayer resulting in no TixNiy intermetallic compounds (IMCs) at the CpTi/IN718 dissimilar joint. Titanium (Ti) and Nb on CpTi side joined by solid solution (Ti, Nb) strengthening mechanism. Nickel (Ni) joined with Nb at IN718 side via eutectic reaction, where a diffused layer comprising of NbNi3, and Nb7Ni6 IMCs was found. Owing to the formation of brittle IMCs (NbNi3, Nb7Ni6), the diffused layer at Nb/IN718 interface exhibited the microhardness of ∼782 HV, which is ∼64% higher than the Nb/CpTi interface and considered highest in comparison to the other regions of the joint. The maximum tensile strength of the weldment was 150 MPa, which is significantly less than CpTi (360 MPa) and IN718 (850 MPa) base alloys possibly due to the presence of cracks in the diffused layer and the welded joint.

Controlling intermetallics formation at V-4Cr-4Ti/316SS laser-welded joint

In a fusion reactor, the connection between the blanket and peripheral components is closely related to heat transfer. Therefore, the sound joining of the structural materials of the blanket and peripheral components, V-4Cr-4Ti and 316 stainless steel (316SS), is of great importance. However, the properties of these joints are restricted by brittle intermetallics. The effect of a Cu interlayer on the formation of brittle intermetallics was studied. After welding, no significant defects (e.g., pores, macrocracks or microcracks) were observed in the welded joints, which showed good weldability. According to the microstructural analysis, it was verified that there were a large number of Cu-rich phases, Fe-rich phases, and a small amount of the σ phase which comprised Fe and Cr in the weld. In the hardness test, the welded joint was divided into hardened and softened zones. The average hardness of the softened zone was approximately 160 HV, while the average hardness of the hardened zone was approximately 292 HV, which was due to the hardening effect caused by a small amount of the σ phase. According to the tensile test results, the introduction of the Cu interlayer improved the strength and elongation of the weld. The ultimate tensile strength and maximum elongation of the weld were 469 ± 3 Mpa and 9.5 ± 0.2%, respectively. The tensile fracture morphology showed that the welded joint was ductile. The correlation between the microstructure and mechanical properties is discussed.

Mitigation of cracks in laser welding of titanium and stainless steel by in-situ nickel interlayer deposition

• Fusion joining of SS-Ti by directed energy deposition of Ni interlayer demonstrated. • Ni interlayer restricted brittle intermetallic formation of Fe and Ti. • Ni between SS and Ti formed joint with graded thermal expansion coefficient. • Thus, Ni interlayer eliminated longitudinal and transverse cracks in SS-Ti joint. • Directed energy deposition of Ni interlayer improved the weld strength. In-situ laser deposition of Ni powder as an interlayer has been exploited to join dissimilar metals, titanium and stainless steel. Direct fusion joining of these materials often suffers from longitudinal cracks due to the formation of hard and brittle intermetallics of Ti and Fe, and also transverse cracks because of large difference in their thermal expansion coefficients. The Ni-interlayer mitigated the longitudinal cracks by acting as an effective barrier to elemental diffusion, thus restricting the growth of brittle intermetallics. As the thermal expansion coefficient of Ni lies between that of SS and Ti, this acted as a functionally graded layer to reduce the thermal residual stresses, thus preventing the formation of transverse cracks across the joint. Maximum ultimate tensile strength of the weld joints obtained was 375 MPa. Results were analyzed by performing various microstructural characterizations using scanning electron microscopy, x-ray diffraction, and energy-dispersive X-ray spectroscopy. The novelty of this method is its easy adaptability in three dimensions contour welding any dissimilar metals using an interlayer of a compatible material.

Influence of Complete Tool-Offset Position on Intrinsic Microstructure and Its Effect on Fatigue Life of AA2024 Alloy-Copper Joint Made by Friction Stir Welding

Abstract Tool offset position is the key parameter to consider during the welding of dissimilar metals by friction stir welding technique. It helps to control the formation of brittle intermetallics and volume defects. Even tool offset position can leave defects at the interface when it exceeds a certain limits, and it has been reported as two-thirds of the bottom radius of the tool. In this work, the effect of the maximum tool deviation (complete offset) on intrinsic microstructure and fatigue life of AA 2024-copper weld was studied. For this, an experiment was carried out with complete tool offset position. The results highlighted that the stir zone was formed mainly by aluminum and the total volume of copper transferred into the stir zone was measured as 5.6 % of the total volume of the stir zone. This kind of structure gives excellent fatigue life to the weld compared to the regular microstructure formed during friction stir welding.

INFLUENCE OF HIGH-VELOCITY IMPACT PARAMETERS ON THE STRUCTURE AND PROPERTIES OF THE VT1-0 + AMG5 JOINT

The paper presents the results of studying the joints of VT1-0 titanium with the aluminum-magnesium alloy AMg5, obtained by explosion welding in different modes of impact. The analysis of microstructures showed that, depending on the conditions of collision, the boundary of the joint zone can have both a wave and a waveless profile. Areas of melted metal are formed even with minimal energy input, but until a certain point they do not significantly affect the strength of the joint. At relatively low values of the energy W, titanium weakly dissolves in aluminum and is present in the melted areas in the form of crushed particles. A further increase in the modes of explosion welding leads to an increase in the degree of dissolution of titanium in melts, which is accompanied by the appearance of cracks. The results of the performed mechanical tests showed that the values of the peel strength of the layers are noticeably higher and reach an equal-strength joint in comparison with the method of welding through an intermediate layer of pure aluminum. In the investigated range of welding modes, the strength of the joint increases in the energy range W = 0,85...1,1 MJ/m, and at W = 1,1 MJ/m it reaches a maximum. With a further increase in the modes of explosion welding, the strength of the joint decreases, which is associated with a change in the structure of the melted regions with the formation of brittle intermetallics in them.

Welding of high-entropy alloys and compositionally complex alloys\u2014an overview

High-entropy alloys (HEAs) and compositionally complex alloys (CCAs) represent new classes of materials containing five or more alloying elements (concentration of each element ranging from 5 to 35 at. %). In the present study, HEAs are defined as single-phase solid solutions; CCAs contain at least two phases. The alloy concept of HEAs/CCAs is fundamentally different from most conventional alloys and promises interesting properties for industrial applications (e.g., to overcome the strength-ductility trade-off). To date, little attention has been paid to the weldability of HEAs/CCAs encompassing effects on the welding metallurgy. It remains open whether welding of HEAs/CCAs may lead to the formation of brittle intermetallics and promote elemental segregation at crystalline defects. The effect on the weld joint properties (strength, corrosion resistance) must be investigated. The weld metal and heat-affected zone in conventional alloys are characterized by non-equilibrium microstructural evolutions that most probably occur in HEAs/CCAs. The corresponding weldability has not yet been studied in detail in the literature, and the existing information is not documented in a comprehensive way. Therefore, this study summarizes the most important results on the welding of HEAs/CCAs and their weld joint properties, classified by HEA/CCA type (focused on CoCrFeMnNi and AlxCoCrCuyFeNi system) and welding process.

Development of a Novel Ni-Based Multi-principal Element Alloy Filler Metal, Using an Alternative Melting Point Depressant

Brazing is a crucial joining technology in industries where nickel-superalloy components must be joined. Nickel-based brazing filler metals are extensively employed, possessing excellent mechanical properties, corrosion resistance, and retained strength at elevated temperatures. To function as a filler metal, the alloy melting point must be reduced to below that of the materials being joined, but the addition of melting point depressants (MPDs) such as boron, silicon, and phosphorus can, however, lead to the formation of brittle intermetallics, potentially compromising the joint performance. In the present work, a novel multi-principal element brazing alloy (in the style of a high entropy alloy), utilizing Ge as an alternative MPD along with a reduced B addition, is investigated. The design process considered binary phase diagrams and predictions based on Thermo-Calc software and empirical thermodynamic parameters. The alloy was used to vacuum braze nickel-superalloy Inconel-718, and microstructural and mechanical investigations are reported. The maximum shear strength achieved was 297 MPa with a brazing temperature of 1100 °C and 60-minute hold time, with isothermal solidification completed. Shear strength was only slightly reduced with increased joint width. Assessments are made of the ability to accurately predict properties of multi-principle element alloys using Thermo-Calc software and empirical thermodynamic parameters.

Friction stir engineering for fabrication of ultra-refined CuNiMgZn alloys

The extrapolation of grain size to nanoscale and simultaneous alloying of Cu with Mg, Ni and Zn was reached via friction stir engineering (FSE). Plastic shear strain during deformation-induced bimodality in the FCC structure and promoted degree of alloying. Average grain size reduced from ~76 μm to less than 2 μm, accompanying microhardness enhancement without formation of brittle intermetallics. Elemental and morphological heterogeneity was attained throughout the stirred region. Gradients of composition and grain structure in the stirred region are reported which can further the design of metamaterials.

High strength impact welding of NiTi and stainless steel wires

This work demonstrates the use of a solid-state microwelding process to create very high strength welds between superelastic NiTi and themselves as well as stainless steel (SS) wires. Vaporizing foil actuator welding (VFAW) uses the pressure from a vaporized metal foil to impact weld the wires by driving them into one another. The small scale nature of this explosive-like process allows its use in factories. Advanced techniques were used to examine the structures and properties of the welds that were created. The NiTi/SS welds had the typical wavy interface of ideal impact welds, with no thermally induced defects such as chemical segregation near the interface or brittle intermetallic formation. These latter two are significant issues in traditional fusion based methods. Microhardness tests show that the NiTi/NiTi and NiTi/SS weld interfaces were locally strengthened compared to the softening of other welding processes. Lap shear tests show that NiTi/SS welds made by VFAW present much higher joint efficiency (near 100%) compared to less than 60% for other traditional joining technologies. Tensile cycling of both the NiTi/NiTi and the NiTi/SS welds shows similar stabilization of the pseudoelastic curves as the NiTi base metal. The irrecoverable strains of the NiTi/NiTi welds and NiTi/SS welds after cycling were comparable to that in the NiTi base metal. The unparalleled performance of these high-strength solid-state similar and dissimilar welds of nitinol is a solution to the increasing demands of applications in various industrial fields including the medical, aerospace and electronic sectors.

Microstructural evolution and its effect on joint strength during laser welding of dual phase steel to aluminium alloy

Abstract Conduction laser welding of dual phase steel (DP 600) to aluminium alloy (AA 6061) was produced in lap fashion using a high power diode laser. The effect of laser interaction parameters such as power density and interaction time on the quality of the joint was studied. The joint properties such as shear strength and fractural strength of the joint were correlated with the microstructural variation along the weld zone. During welding of steel to aluminium, the formation of Fe-Al intermetallics plays a vital role in determining the shear strength of the joint. Microstructural structural analysis was done along the interface of the weld-aluminum alloy to determine the intermetallic phases (Fe2Al5 and FeAl3). Another significant phenomenon studied was the softening of the heat-affected zone along the weld-steel interface. The softening phenomenon of heat affected zone is important in determining the formability of sheets for post-welding operations such as forming. Mechanical testing such as microhardness and tensile testing was done to analyze the quality of the joint. The microhardness test showed different hardness values along the weld interface zone and the heat-affected zone. The increase in hardness in the interface zone of the weld-aluminum zone was due to the formation of brittle intermetallics. Whereas the reduction in microhardness in the subcritical heat affect zone of the weld-steel side was due to the softening phenomenon, owing to the transition of martensite to tempered martensite during welding. It was observed that a maximum shear strength of 231 N/mm was obtained when the thickness of intermetallics was in the range of 8−11 μm. The experimental results also showed that the best quality welds were obtained under the conditions of power density and interaction time such as 1.98 kW/mm2, 0.15 s and 2.26 kW/mm2, 0.187 s, respectively.

RETRACTED: Microstructural variation at interface during Fiber laser joining of NiTi/Ti6Al4V and effect of mechanical strength

Abstract Welding of nitinol (NiTi) shape memory alloy (SMA) and titanium-based alloy (Ti6Al4V) finds potential application in aerospace and medical industries. However, joining of these materials poses challenging issues because material properties of each material interacts to give rise to hybrid system performance. In the present study, copper was used as an intermediate layer to restrict the formation of brittle intermetallics during welding of NiTi with Ti6Al4V using laser welding. The formation of Cu4Ti3 and CuNi2Ti complex phases attempts to minimize the formation of brittle phases like Ti2Ni. It was observed that the maximum welding strength of 353 MPa could be achieved at 800 W laser power and 17 mm/s scan speed. Physical analysis of weldments indicates the presence of pores at nano-levels (

Influences of shape of the new interfaces and morphology of the intermetallics on mechanical properties of aluminum AA2024–pure copper joints by friction stir welding

Friction stir welding (FSW) joints are inevitably accompanied with the change in shape of the abutting surfaces due to the tool movement. Further, the formation of brittle intermetallics (during dissimilar welding) is also unavoidable. Therefore, the current work is focused to study the role of the shape of the new interface in properties of the joints and to control the morphology of intermetallics subsequently. FSW experiments were carried out between aluminum AA2024 and pure copper for different tool offset positions (on either side of the interface), plate positions (copper in advancing or retreating side), and tool geometry (plain and threaded). The microstructure observation showed noticeable changes in the shape of the new interface with respect to the selected parameters. Tensile tests highlighted that the weld which has continuous and smooth new interface gave better properties than the welds which had an irregular discontinuous interface. Also, this kind of interfaces had formed with a continuous thin intermetallic layer, and it was a desirable morphology of the intermetallics. The weld with this thin intermetallic layer (metallurgical bonding) alone has the maximum strength, whereas the welds that also had fine steps at the interface (mechanical bonding from interlocking effect) have maximum ductility. The mechanism behind this was explained in detail with the help of material flow behavior study for the selected conditions. This concluded that to achieve better properties, the shape of the new interface and the bonding along this interface are mainly important to be considered.

Benefits of interfacial regulation with interlayers in laser welding Ti6Al4V/316L steel

Laser welding of Ti6Al4V and 316L stainless steel is performed using single Cu interlayer and composite Cu-Nb interlayer. The microstructural characterization and mechanical properties of Ti6Al4V/316L steel laser welded joints at various laser offsets were investigated. Proper laser offset distance to 316L steel side ensured weld integrity and restrained mass production of intermetallic compounds for single Cu interlayer. Energy dispersive spectroscopy showed that Cu solid solution and discontinuous Ti-Fe and Ti-Cu intermetallics were formed due to the dissolution and diffusion of atoms. The joint had a lower tensile strength and fractured along Ti6Al4V/Cu weld interface with obvious cleavage steps and tongue pattern. The inserting of Nb interlayer with high-melting point was conducive to prevent atomic diffusion and suppress the formation of brittle intermetallics. Similarly, the laser energy distribution was crucial to the weld formation and metallurgical reaction in fusion zone. The Cu sheet and 316L steel was mainly connected by heat conduction. The weld fusion zone consisted of (Cu, Nb) and (Ti, Nb) solid solution with no intermetallics. Compared to the welded joints with single Cu interlayer, the microhardness of fusion zone was reduced and tensile strength was improved because no brittle IMCs were formed with composite Cu-Nb interlayer. The maximum tensile strength reached 215 MPa and ductile fracture mode occurred with composite Cu-Nb interlayer, which was approximately 160% greater than that found with single Cu interlayer.

Zn-Sn Interdigitated Eutectic Alloy Anodes with High Volumetric Capacity for Lithium-Ion Batteries

Summary Zn-Sn interdigitated eutectic alloy (IdEA) foils are highly promising anodes for lithium-ion batteries. Synthesized via severe plastic deformation, a two-phase alloy is converted into a nanostructured foil composed of tin domains distributed throughout an electrically conductive zinc matrix. In addition to providing mechanical integrity, uncycled capacity in the zinc matrix eliminates the lithium-plating risk. These monolithic foil anodes offer an electrode-level volumetric capacity approximately double that of graphite (797 mA h cm−3), with a highly promising formation loss of