Formation Of Brittle Intermetallic Phases 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.

Influence of High-Pressure Die Casting Process Parameters on the Compound Strength of Hybrid Components with Undercut Sheet Metal

Reducing the weight of parts through lightweight designs impacts CO2 emissions, especially in the automotive and transportation sectors, which have significant fuel and electric energy consumption. Using multi-material design approaches, specific material properties can be combined to achieve effective lightweighting. Commonly used metals include aluminum, which is known for its high specific strength, and steel, which is valued for its strength and structural integrity. However, joining aluminum and steel presents challenges given their different thermophysical properties and the potential formation of brittle intermetallic phases, making common joining techniques like fusion welding unsuitable. In this study, a hybrid casting process for the production of a complex workpiece from dissimilar materials was investigated. Aluminum was die-cast around a steel sheet insert. Surface structures with undercuts on the steel sheet were applied through modified cold rolling, allowing molten aluminum to flow into the channels and interlock during solidification. It was found that elevated temperatures of the melt and tool were beneficial for the interlocking, resulting in a 30% increase in compound strength. Furthermore, a reduction in both the piston position at the changeover point, between the pre-filling and cavity filling, and the melt velocity at the gate reduced the compound’s strength by 41% and 30%. Up- and downstream processes did not show any significant influence on the conducted experiments. Based on this, two main detrimental effects were observed: pre-solidification of the aluminum melt and gas entrapment.

Study on Laser Overlap Welding of Titanium/Aluminum Dissimilar Metals Based on Niobium Microalloying

Brittle intermetallic compounds, formed during the welding process of titanium/aluminum (Ti/Al), lead to a significant reduction in joint mechanical properties. The purpose of this study is to mitigate the formation of brittle phases during the laser welding of dissimilar Ti/Al metals, thereby enhancing the mechanical properties of the joints. In this investigation, an innovative approach is adopted, utilizing Nb foil as an interlayer to effectively minimize the formation of brittle intermetallic phases during dissimilar welding. A comprehensive analysis of the microstructure of the transition layer was conducted using material characterization methods, including scanning electron microscope equipped with an energy dispersive X-ray spectrometer. The mechanical performance of the welded joints was assessed using tensile testing. The results indicate that the effective welding width and joint penetration depth at the joint interface were reduced in Ti/Al dissimilar metals when Nb was added as an intermediate layer, under the same welding process parameters, when compared to unalloyed weld seams. Furthermore, the utilization of a 0.05 mm Nb foil as the intermediate layer results in a significant 25% increase in the average shear strength compared to the other condition, with the average shear strength of the joint reaching its peak value at 192 N/mm. The unalloyed Ti/Al weld joint usually fractured along the melting zone, displaying complete brittle fracture characteristics. After Nb microalloying, the joint typically fractures along the transition zone and interface, exhibiting both cleavage and ductile fracture characteristics, indicating the combination of a brittle and toughness fracture. This study provides experimental evidence and new insights for welding Ti/Al composite structures, with significant theoretical and practical applications.

Titanium alloys developed on the basis of the addition of cheap strong eutectoid β-stabilisers

Historically, titanium alloys have been developed disregarding the addition of eutectoid β-stabilisers as they generally lead to the formation of brittle intermetallic phases upon solidification of the alloy. However, such phenomenon can be prevented using powder metallurgy. Thus, this study considered the concurrent addition of cheap strong eutectoid β-stabilisers, namely Mn and Fe, for the development of new ternary Ti–Mn–Fe alloys, reducing the intrinsic cost of Ti alloys. It is found that the progressive addition of Mn and Fe in equal concentration enhances the densification of Ti during sintering, leading to lower amount of residual porosity, the transformation of the microstructure from purely lamellar to metastable, and the associated refinement of the microstructural features (grains and lamellae), as well as the stabilisation of a greater amount of β phase, and the formation of the metastable α″ phase. Such microstructural changes result in the strengthening (higher yield and ultimate tensile strength and hardness) and embrittlement of the alloy by changing the fundamental strain hardening mechanism of the ternary Ti–Mn–Fe alloys.

Fatigue analysis of a progressive secondary A357 alloy with higher iron content in a chloride environment

This paper investigates the fatigue behavior of secondary A357.0-T6 alloy with higher Fe content as a result of the usage of recycled metals. Commercial secondary Al-alloys contain Fe, often as an undesirable impurity negatively affecting plasticity, tensile strength, and so on due to the formation of brittle intermetallic phases. A common unwanted intermetallic phase is β-Al5FeSi in form of brittle platelets. This plate-like morphology acts as a stress concentrator, leading to a decrease in the strength, ductility, and dynamic fracture toughness of the castings. Moreover, promoting casting defects such as porosity. In addition to structural and mechanical properties, the corrosive environment also influences fatigue lifetime. The effect of corrosion damage on fatigue failure is a problem for engineers in the aerospace, automotive, and gas or oil industries. The corrosion reactions or their products can interact with the fatigue process accelerating crack initiation or propagation. Therefore, the aim of this study is to investigate chemical-mechanical interactions during the simultaneous application of cyclic stress and exposure to a corrosive environment.

New approach for multi-material design: Combination of laser beam and electromagnetic melt pool displacement by induced Lorentz forces

Multimaterial structures are a promising solution to reduce vehicle weight and save fuel or electric energy in automotive design. However, thermal joining of steel and aluminum alloys is a challenge to overcome due to different material properties and the formation of brittle intermetallic phases. In this study, a new joining approach for producing overlap line-shaped joints is presented. The lower joining partner (EN AW 5754) is melted by a laser beam, and this melt is displaced into a line-shaped cavity of the upper joining partner (1.0330) by induced Lorentz forces. The melt solidifies in the cavity to a material and form-fitting joint. This approach needs no auxiliary joining elements or filler materials. Previous investigation to produce spot-shaped joints by using this approach showed that quality and reproducibility were limited by known melt pool dynamics of aluminum alloys (keyhole collapses). For line-shaped joints, the melt displacement can take place behind the keyhole. This allows the displacement process to be spatially uncoupled from the influence of keyhole collapses. The study shows that this improved the process stability and the quality of the joint. The created line-shaped joints were microstructurally characterized by transversal sections. Intermetallic phases were identified by electron backscatter diffraction and EDX analysis. The detected intermetallic phases consist of a 5–6 μm compact phase seam of Al5.6Fe2 and a needle-shaped phase of Al13Fe4. Tensile shear tests were carried out to quantify the load capacity. It was possible to create a joint with a load capacity of about 2 kN.

Effect of Ta addition on the sinterability of spark plasma sintered W-Ni-Fe alloy

The purpose of this research is to investigate and characterize the W-Ni-Fe-Ta compositions by solid-state sintering using the Spark Plasma Sintering (SPS) technique at a relatively lower temperature. The sinterability of W-Ni-Fe ternary tungsten heavy alloy (WHA) with the addition of Tantalum in predetermined compositions was sintered using SPS at 1100°C under 30 MPa pressure. The effect of Ta addition on WHA densification, microstructure, and mechanical properties was studied. Ni was found to act as an activator in the sintering of tungsten. SEM and energy dispersive spectroscopy revealed an even distribution of Ta in the W and Ni-Fe matrix. The data of hardness and ultimate tensile strength were explained using grain size, contiguity, and the formation of brittle intermetallic phases. W-7Ni-3Fe-5Ta alloy composition has the highest ultimate tensile strength of 870 MPa, 189.2 HV0.5kgf hardness, and 83.42% relative sintered density. During the tensile test, W grain cleavage fracture, W-W decohesion, matrix tearing, and W matrix intergranular types of fracture surface were observed.

Deformation and Stresses During Alkali Metal Alloying/Dealloying of Sn-Based Electrodes

Abstract Enhancement of energy density and safety aspects of Li-ion cells necessitate the usage of “alloying reaction”-based anode materials in lieu of the presently used intercalation-based graphitic carbon. This becomes even more important for the upcoming Na-ion battery system since graphitic carbon does not intercalate sufficient Na-ions to qualify as an anode material. Among the potential “alloying reaction” based anode materials for Li-ion batteries and beyond (viz., Na-ion, K-ion battery systems), Si and Sn have received the major focus; with the inherently ductile nature of Sn (as against the brittleness of Si) and the considerably better stability in the context of electrochemical Na-/K-storage, of late, tilting the balance somewhat in favor of Sn. Nevertheless, similar to Si and most other “alloying reaction”-based anode materials, Sn also undergoes volume expansion/contraction and phase transformations during alkali metal-ion insertion/removal. These cause stress-induced cracking, pulverization, delamination from current collector, accrued polarization and, thus, fairly rapid capacity fade upon electrochemical cycling. Unlike Si, the aforementioned loss in mechanical integrity is believed to be primarily caused by some of the deleterious first-order phase transformations and concomitant formation of brittle intermetallic phases during the alloying/de-alloying process. Against this backdrop, this review article focuses on aspects related to deformation, stress development and associated failure mechanisms of Sn-based electrodes for alkali-metal ion batteries; eventually establishing correlations between phase assemblage/transformation, stress development, mechanical integrity, electrode composition/architecture and electrochemical behavior.

Effects of lap configuration on mechanical properties and composition distribution of three-layer overlapped laser weldments on Al/Cu dissimilar materials

Battery cells are connected via bus-bars to meet performance requirements, such as power and capacity, and multiple layers of dissimilar materials functioning as anodes, cathodes, or bus-bars are overlapped and welded together. In laser welding, the formation of brittle intermetallic phases in the weld joint is inevitable and, in turn, deteriorates the mechanical properties. To obtain the desirable joint performance, appropriate welding parameters to avoid intermetallic phase formations and joint designs to release stress concentrations must be obtained. This study investigates the effects of lap configurations and process parameters on the tensile-shear load, T-peel load, and composition distribution when multi-layered joints of dissimilar materials are produced by laser welding. Two layers of 0.4 mm Al sheets were welded with a single 0.2 mm Cu sheet, which was emulated using electric vehicle battery interconnects. The results show that the penetration depth varied in accordance with the lap configuration even under the same heat input condition. The lap configuration and welding parameters influenced the composition distribution of the welds, as they altered the solidification rate, number of Cu/Al contact interfaces, and location of the high-density material. The failure load of the T-peel specimens was lower than that of the tensile-shear specimens except for the Cu − Al − Al lap configuration. The T-peel load of the Cu − Al − Al lap configuration was similar to that of the tensile-shear load. When the stress-concentrated joint was homogeneous, it was more robust.

Flexible Powder Production for Additive Manufacturing of Refractory Metal-Based Alloys

The quality and properties of metal powders are essential for powder metallurgical (PM) processes in general and for additive manufacturing (AM) processing routes in particular. Thus, a variety of atomization technologies were established meeting the multiple needs of the different processing technologies. However, the production of refractory metal alloy powder remains challenging due to their high liquidus temperatures (>2000 °C), the formation of brittle intermetallic phases, as well as the reactivity with and sensitivity to interstitials of the constituting elements. In this contribution, powders made of Mo-20Si-52.8-Ti (at.%) were produced by a novel ultrasonic atomization (UA) process at laboratory-scale using an industrial electrode induction gas atomization (EIGA) process with a modified electrode concept for the first time. UA allows flexibility in alloy composition due to the arc melting-based principle, while the EIGA electrode is PM manufactured from elemental powders to provide similar flexibility on a larger scale. The powders resulting from these two processes were compared with respect to size distribution, sphericity, microstructure and phase constitution, chemical composition, and interstitial impurity content. In addition, several powder batches were produced with the UA process in order to assess the process reliability and stability. The properties, quality, and quantities of UA powders perfectly meet the requests for alloy development for powder bed fusion AM, while the modified EIGA process allows the upscaling of the alloy powder quantities.

Feasibility, processing properties and thermal behavior of Ni nanopastes produced by ultrasound-enhanced dispersing

Nickel nanoparticles are materials that can be used for a novel type of joining, so called nanojoining, as an alternative to conventional brazing. The physical characteristics of these nano-sized particles lead to a lower melting temperature compared to nickel bulk material and even lower than typical brazes, without the need for melting point depressants. Such elements like boron or silicon cause issues due to formation of brittle intermetallic phases. In order to be able to use Ni nanoparticles in practical applications, they have to be in forms such as suspensions, e.g pastes. In the present work, therefore, a series of Ni nanopastes is prepared and investigated with emphasis on the solvent systems. These suspensions consist of Ni nanoparticles (mean diameter 20 / 90 / 180 / 300 nm) and organic compounds. The examinations show the feasibility of producing such nanopastes using ultrasound-enhanced dispersing, as well as margins for adjusting their processing properties. In addition, first results of the thermal behavior of the Ni nanopastes, measured by simultaneous DSC/TG, are shown.

A novel L12-strengthened multicomponent Co-rich high-entropy alloy with both high γ′-solvus temperature and superior high-temperature strength

A L12-strengthened multicomponent Co-rich high-entropy alloy with superior microstructural stability was developed in this study. High-density cuboidal γ′ particles were found to embed in the matrix for the alloy subjected to isothermal aging at temperatures ranging from 800 to 1100 °C without the formation of brittle intermetallic phases neither at grain boundaries nor grain interior after long-term thermal exposure. Atom probe analyses indicated that Ta and Nb elements preferentially partitioned to the γ′ intermetallic phase, which is found to be substantially important for stabilizing the L12 ordered crystalline structure. More importantly, the present alloy exhibited a good combination of both high γ′-solvus temperature (1125°C) and low mass density (8.28 g cm−3), together with superb high-temperature tensile strength (755 MPa at 800 °C and 664 MPa at 900 °C). The present findings offer an essential paradigm for designing advanced L12-strengthened high-temperature alloys with broader temperature capabilities based on Co.

Numerical analysis of intermetallic compounds formed during laser welding of Aluminum-Magnesium dissimilar couple

This paper investigates the laser welding process of dissimilar Aluminum-Magnesium materials. A three-dimensional transient model has been developed using the 'Ansys-fluent' software to study the transport phenomenon in the weld bead. The results show the formation of a heterogeneous mixture in the molten pool. The transport phenomenon is strongly influenced by convection, thermal and mass diffusivities during laser beam welding. Microhardness tests show that the mechanical properties of welded joint are affected by the formation of brittle intermetallic phases in the Al-Mg transition zone, which is the main cause of weld cracking. The formation of intermetallic phases in the melting zone was highlighted. The layer thicknesses of intermetallic compounds, comprising Al3Mg2 and Al12Mg17 phases were determined numerically by studying the distribution of chemical elements in the weld pool. The results of the numerical simulation are in good agreement with the corresponding experimental results.

Modeling of chemical composition in the melt pool during laser welding of Aluminum / Magnesium alloys

Joining dissimilar metals is very difficult due to the formation of brittle intermetallic phases which may be detrimental to mechanical properties. The present work aims to investigate the transport phenomenon in the weld bead and to understand the materials mixing during laser welding process of dissimilar Aluminum-Magnesium alloys. A three-dimensional transient model based on fluid flow, heat and mass transfer has been developed to predict the formation of the weld and to study numerically and experimentally the diffusion of alloying elements in the melted zone. SEM analysis of chemical composition has been realized to map elements distribution in the melted zone. The results of simulation show the formation of a heterogeneous mixture in the melt pool. The elements distribution map and the presence of brittle intermetallic phases in the fusion zone were analysed. The formation of intermetallic compounds, comprising Al3Mg2 and Al12Mg17 phases were predicted by studying the chemical elements distribution in the weld pool. A good tendency between experimental and numerical results is noticed for the weld.

Investigation of surface roughness effects on microstructural and mechanical properties of diffusion bonding between dissimilar AZ91-D magnesium and AA6061 aluminum alloys

The effects of surface roughness were studied on microstructural and mechanical properties of solid-state diffusion bonding between two dissimilar alloys of magnesium AZ91-D and aluminum 6061 by means of hot pressing. Although applied pressure, temperature, and duration of diffusion bonding are known as the most effective parameters, surface roughness can alter the mechanical and microstructural properties remarkably, which is usually neglected. To investigate the effect of surface roughness between 0.06 and 0.15 μm, the width of the diffusion bond in the interfacial transition zone (ITZ) and the presence of intermetallic phases were analyzed. Moreover, microhardness, shear strength, and fracture surfaces were evaluated. The results exhibit that in the applied roughness range, the bond width and the microhardness of the joints improved by increasing the surface roughness of the both metals; however, the shear strength decreased. It may be attributed to more disruption of the oxide films on the metal surfaces in the rougher samples and also increase in the effective interface boundary during the process. Consequently, there would be more chance for aluminum-magnesium contacts, resulting in more diffusion, further formation of brittle intermetallic phases (such as Al12Mg17 and Al3Mg2), and therefore more hardness and less shear strength.

Study on mechanical and metallurgical properties of fiber laser welded Nb-1% Zr-0.1% C alloy

Laser welding of Nb-1% Zr-0.1% C was attempted in butt-welding configuration using top and bottom sided inert gas shielding. The precautionary measure during welding was to limit the reactivity of niobium alloy in ambient atmosphere. The ranges of input parameters, that is, laser power (P), welding speed (V) and beam diameter (D) for full penetration welding were attempted by carrying out bead-on-plate (BOP) experiments. The selection of the combination of process parameters was such that the formed weld area could be minimized without hampering the depth of penetration. Bead-geometry, hardness and tensile strength were quantified to study the influence of input process parameters during laser welding. Base metal had an average hardness of 108 VHN and the average hardness of fusion zone (FZ) was found to lie between 278 and 546 VHN. The steeper increment in microhardness value of the FZ could be due to the grain refinement, dissolution of precipitates and formation of brittle intermetallic phases of carbide and oxides, which were evident by the result of energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) phase analysis. The weld joint that failed in the weld zone exhibited the brittle failure, and ductile mode was achieved in the joint, where failure occurs at base metal. The range of elongation in laser welded joints varied in between 1.97 and 5.73 mm. The reduction of tensile strength and ductility of the joints could be due to marginal enhancement of microhardness and increment of brittle phase density in fusion zone, as were evident from XRD phase analysis. The main focus of the present work was intended towards the establishment of laser welding as an alternative technique for fabrication of reactive niobium alloy in ambient atmosphere.

Comparing the weldability of AA6013-T4 aluminium alloy on DP600 dual-phase steel with laser welding and resistance spot welding

The welding of high-strength aluminium alloys on AHSS with laser beam welding (LBW) and resistance spot welding (ERW) is challenging due to the formation of brittle intermetallic phases. The weldability of a lap dissimilar joint between an AA6013-T4 aluminium alloy and a DP600 dual-phase steel with respect to microstructure and mechanical behaviour was investigated in this study. Microstructural analysis with optical microscopy shows that the ERW process did not show any significant dilution among the base materials. However, dilution and crack formation occurred in the fusion zone (FZ) during LBW. Martensite formed in the DP 600 dual-phase steel and liquation formed in the AA6013-T4 aluminium alloy within the heat-affected zone (HAZ) with both processes. X-ray diffraction and energy-dispersive X-ray spectroscopy analyses showed the formation of Fe3Al intermetallic phases at the interface between the FZ and HAZ with both welding processes. Tensile test results showed no differences between the strength limits for both processes with an average value of 68 MPa. However, the shear deformation was 62% higher in samples welded using ERW. This study shows that dissimilar joint welding of AA6013-T4 aluminium alloy to DP600 dual-phase steel is possible, despite the formation of the Fe3Al intermetallic phase, and that the ERW process is the most appropriate.

Microstructure and mechanical properties of dissimilar Ti/Nb/Cu/steel laser joints

The absence of intermetallic phases in Fe/Cu, Cu/Nb and Nb/Ti binary systems opens the possibility to obtain reliable joints between titanium alloys and steels by using a multimaterial copper/niobium insert. Continuous Yb:YAG laser welding of 1 mm thick titanium and 316L stainless steel plates through niobium/copper multimaterial insert was performed. The use of a 100 µm laser beam allowed producing isolated molten zones and thus completely avoiding the formation of brittle intermetallic phases, according to SEM and XRD analysis. The effect of energy per unit length applied to the niobium/copper welds on the mixing process and mechanical properties of the joints was investigated. A ductile fracture systematically occurred in the unmelted copper zone of the joints at maximal average UTS of 250 MPa.

Modeling of laser welding of plates made of dissimilar metals with the use of a composite insert

This paper deals with computer simulations of laser welding of plates made of dissimilar metals using a composite intermediate insert. They are performed with the previously developed 3D numerical model implemented with the collocations and least squares method. The use of the composite insert when welding dissimilar alloys prevents the formation of brittle intermetallic phases that adversely affect the strength of the joint. Based on the proposed mathematical model, the features of butt-welding of titanium and stainless steel plates and aluminum and titanium plates are investigated. The rational thermal conditions of laser welding and the geometric parameters of the welds are determined.

Prospects of Production and Application of Hybrid Structures and Composites Made of Titanium Alloys and Nitinol: Overview

Titanium-based alloys are widely used in various industries due to their combination of high mechanical properties and low density. These properties are most effectively used when fabricating aircraft components and medical implants. Shape-memory alloys based on titanium nickelide (nitinol) are promising materials for the fabrication of superelastic medical implants and tools, as well as thermomechanical elements in aerospace technology. The joint application of these materials as elements of hybrid structures or composites is promising for the development of products with a unique set of properties such as high mechanical properties, superelasticity, and damping capacity; increased wear resistance; and thermal shape memory. The basic properties of alloys based on titanium nickelide and the most widely used VT6 titanium alloy (Ti–6Al–4V) are analyzed. It is shown that the combination of functional properties of nitinol and structural properties of titanium alloys in an integrated structure makes it possible to fabricate various products, especially for aerospace and medical industries. To fabricate such structures, various welding (mainly laser and diffusion welding) and soldering methods are currently investigated, and the best prospects are associated with the use of intermediate layers that eliminate the formation of brittle intermetallic phases in permanent joints.