Intermetallic Morphology Research Articles

Joint morphology, characteristics, and role of intermetallics in AA7075 friction stir lap joints with copper interlayer

Joint morphology, characteristics, and role of intermetallics in AA7075 friction stir lap joints with copper interlayer

Machinability of recycled aluminum alloys

The use of recycled (hereafter called secondary) aluminum alloys is increasing more and more in light of sustainability as the energy needed for their production is much lower than in the case of the primary alloys and they may have mechanical properties comparable to those of the latter. Most of the secondary aluminum alloys are used to fabricate parts through casting processes, which may need further machining operations to get the part’s final shape. While the mechanical properties of the secondary aluminum alloys have been comprehensively addressed in the literature and correlated to the different intermetallic particles that characterize their microstructure, the same is not true when addressing machinability. In this framework, the paper investigates the machinability of one primary and two secondary aluminum alloys in terms of cutting forces and surface finish after turning trials carried out at fixed cutting parameters. A detailed characterization of the alloys’ microstructure was carried out making use of both optical and scanning electron microscopy to identify the size, morphology, and distribution of intermetallics. The highest cutting force was registered when machining the primary alloy, being characterized by the highest specific cutting energy. The surface damage in terms of tearings induced by cutting was comparable between the primary and secondary alloys. In contrast, the different roughness features that characterize the machined surfaces of the considered alloys can be partly ascribed to the different intermetallics they present. Nevertheless, the surface topography analysis results must be interpreted based on the specific application.

Post-heat treatment of laser powder bed fusion fabricated Al–La–Mg–Mn alloy: On intermetallic morphology control and strength-ductility balance

In this study, a newly developed Al–La–Mg–Mn alloy was subjected to solution heat treatment for the first time. During the solution heat treatment at 500 ℃, the continuous network Al11La3 intermetallics were transformed into a discontinuous granular form. Ultrafine intermetallics (∼0.20 µm) were achieved via multi-point spheroidization with the aid of endogenous nanoprecipitates. Of note, the intragranular intermetallics were inert to coarsening, while the intergranular intermetallics grew gradually to ∼0.58 µm within 6 h. Compared to the as-printed alloy (UTS ∼588 MPa, Elongation ∼6.4%), the solution-treated alloy at 500 ℃ for 1 h (UTS ∼477 MPa, Elongation ∼14.3%) demonstrated a remarkable improvement in ductility, with a 123% increase, despite a modest decrease of 19% in strength, thereby balancing the strength-ductility. The limited decline in strength is primarily attributed to the high-content solute Mg atoms and thermally stable intragranular intermetallics. Interestingly, the mechanical properties were independent of the holding times at 500 ℃, despite the coarsening of intergranular intermetallics. This is mainly due to the optimization of intermetallics’ scale distribution, which tailors and equalizes the stress distribution between the grain boundary and interior, as well as between the melt pool boundary and interior during deformation. These findings provide the first insight into the effect of post-heat treatment on the intermetallic evolution and mechanical properties of LPBF Al–(La, Ce) alloys.

Identification of intermetallics in the laser-welded joint of rare-earth magnesium alloy and the corresponding strengthening mechanisms

Morphology and distribution of intermetallic compounds in the heat-affected zone (HAZ) are easily altered during welding thermal cycles, contributing to the softening of the HAZ. Finding solutions to mitigate the softening and enhance the strength of the joint is currently at the forefront of Mg alloy processing and manufacturing. In this work, 0.7 mm thick Ce-containing Mg alloy sheets were successfully joined using fiber laser welding. The microstructure of the joint was observed employing scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Accordingly, the phase compositions in both HAZ and fusion zone (FZ) were determined. Subsequently, the mechanical properties of the joint were evaluated through tensile-shear and hardness tests. The results revealed that the HAZ experienced softening, retaining 76.6% of the strength of the base metal in the tensile-shear test. Interestingly, intermetallics composed of conventional alloying elements within the HAZ transformed into coral-like structures during the welding thermal cycles, whereas the morphology and distribution of intermetallics containing rare-earth (RE) elements barely changed. As a result, the joint exhibited cracks along the boundaries of the coral-like precipitates during tensile loading. In comparison, both intermetallics consisting of conventional elements and those containing RE elements were fragmented into micron-sized fine particles in the FZ, leading to an increase in the strength of the FZ. Ultimately, the mechanisms responsible for the softening of the HAZ and the strengthening of the FZ were elucidated, presenting a promising welding solution for Mg alloys.

Quantifying the effects of grain refiners (AlTiB and Y) on microstructure and properties in W319 alloys

For aluminum alloys, grain refinement is the most efficient method to improve both strength and ductility. However, this rule may not apply for the recycled Al due to the large amount of intermetallics. In this paper, both secondary dendritic arm spacing (SDAS) and intermetallics have been quantified as a function of grain refinements including traditional AlTiB and most recent Y refiners. Using U-net CNN machine learning algorithm, both Fe-rich intermetallics and eutectic Si have been successfully segmented from optical and SEM/EDS images. Different from traditional refiners, (AlTiB + Y) not only strengthens the refining ability but also reduces the percentage of harmful needle-like Fe-rich intermetallics and transforms flaky Si particles into fibrous morphology. Harvested from the comprehensive grain refiners, SDAS was reduced by 38.9%, the average equivalent diameter of eutectic Si was reduced by 37.9%, and Fe-rich intermetallics content was significantly reduced. In particular, the microstructure improved by AlTiB+0.05Y (0.6 wt.% AlTiB + 0.3 wt.%Y) resulted in an increase in elongation of the refined alloy to 3.9 ± 0.4%, which is 63.3% higher compared to the base alloy, while the UTS remained at the original 260 MPa. This paper offers a promising refining technology for the recycling and casting of Al-Si alloys. • Hybrid addition of Al-Ti-B and rare earth element Y can achieve improved refining effect. • The morphology of eutectic Si particles and Fe-rich intermetallics can be modified by new grain refiners. • Quantitative information of particle distribution can be obtained by U-net CNN machine learning algorithm • Transforming flaky Si particles into fibrous eutectics and reducing harmful β-Fe size significantly improve the properties. • Combined with thermodynamics, Fe-rich intermetallics reduction mechanism is revealed.

Variety of microstructure and mechanical properties along the laser impact welded interface

Laser impact welding (LIW) is an attractive technique for thin film dissimilar materials solid-state welding. To understand the mechanism of laser impact welding, the wave character of wavy interfaces, the morphology of intermetallics, the diffusion phenomenon and the hardness along the welding direction were investigated with Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometer (EDS), and nanoindentation, respectively. There is a microstructure and mechanical property gradient not only along the welding direction, but also perpendicular to it. The variety of both microstructure and mechanical properties is the combined result of impact pressure and impact angle. The gradient of the nanoindentation demonstrated the variety of the strengthening mechanism nearby the weld interface. The study provides valuable knowledge for welding process understanding and control.

Aluminum alloys with natural ratio of alloying elements manufactured by powder metallurgy

Due to the lack of mineral resources, unconventional raw materials have to be considered. In this work, the use of reduced deep-sea nodules from Clarion-Clippertion Fracture Zone of Pacific Ocean was tested as the natural mixture of alloying elements for aluminum alloys. The nodules were reduced by aluminothermic process without the extraction of individual elements. As the technology for the manufacture of the alloys, the powder metallurgy comprising mechanical alloying and spark plasma sintering was applied. The produced Al-Mn based alloys are characterized by uncommon spherical morphology of Al-Mn intermetallics, which leads to a significant ductility even in the case of high total amount of additives (up to 20 wt%). The wear resistance of the alloys strongly increases with growing amount of the reduced deep-sea nodules.

Formation of intermetallic phases in unrefined and refined AA6082 Al alloys investigated by using SEM-based ultramicrotomy tomography

• Microstructure of AA6082 Al alloys was studied using SEM serial ultramicrotomy tomography. • The formation sequence of intermetallic phases is related to the coarsening of α-Al dendrites. • TiB 2 particles act as nuclei for the formation of β, α intermetallics and Mg 2 Si. • The grain refinement processing causes the severe interconnectivity of α-intermetallics. • Melt shearing is better to uniformly distribute and refine intermetallics than grain refiner. The spatial arrangement, distribution and morphology of Fe-bearing intermetallics in AA6082 alloys depends on the manufacturing process of the alloy and thus influences the macroscopic properties. Here, the microstructure of a near industrial scale casting AA6082 Al alloy fabricated by: (a) direct chill casting, (b) Al-5Ti-1B grain refiner addition and (c) intensive melt shearing has been investigated by three-dimensional visualization using SEM-based serial ultramicrotomy tomography. The formation sequence of phases in AA6082 alloys is generally categorized into four stages: formation of α-Al grains, Fe-bearing intermetallics, Mg 2 Si phase, and eutectic rosettes. Results of three-dimensional visualization of the microstructure indicated that TiB 2 particles not only could nucleate Fe-bearing β-intermetallics, but also could provide substrate for the formation of Fe-bearing α-intermetallics and Mg 2 Si. A further deep analysis reveals that the essential condition for the formation of secondary phases such as Fe-bearing intermetallics and Mg 2 Si phase is the build-up of a supersaturated solute front at the α-Al solid-liquid interface irrespective of the specific nucleation site. In addition, the results indicate that grain refinement processing causes the severe interconnectivity of Fe-bearing α-intermetallics. However, the intensive melt shearing is a better manufacturing process because the intermetallics are more evenly distributed and refined than with the addition of the grain refiner, thereby improving the properties of the alloy.

Mechanical Properties and Microstructural Evolution of TiNi-Based Intermetallic Alloy with Nb Addition.

TiNi intermetallic alloys were prepared with 2, 4 and 6 at.% niobium (Nb) addition. The mechanical properties and microstructures of the alloys were investigated under both static (1 × 10−1 to 1 × 10−3 s−1) and dynamic (4 × 103 to 6 × 103 s−1) loading conditions. The intermetallic alloy structures and surface morphologies of the alloys were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. In addition, the fracture morphologies were observed by optical microscopy (OM). It was shown that the addition of 2 to 4 at.% Nb increased the strength of the TiNi alloy. However, as the level of Nb addition was further increased to 6 at.%, a significant reduction in strength occurred. For a constant Nb addition, the plastic flow stress and strain rate sensitivity increased with increasing strain rate under both loading conditions (static and dynamic). The XRD and SEM results showed that the original surface morphologies were composed primarily of dendritic structures and fine β-Nb + TiNi eutectic systems. Moreover, the OM results showed that the alloys underwent a transition from a brittle fracture mode to a ductile fracture mode as the level of Nb addition increased.

Effects of Si and Fe additions on mechanical properties of as-cast Al–5Cu alloys

The effects of different Fe and Si additions on the microstructure and mechanical properties of as-cast Al–5Cu alloys were studied. The results show that the evolution order of the Fe-rich intermetallic morphology is needle-like β-Fe then Chinese-script Fe-rich intermetallic, and the area size of Chinese-script Fe-rich intermetallic increased from 66 to 1058 μm2 with the increase of Fe addition. It was found that the combined additions of Fe and Si can change the morphology and size of Fe-rich intermetallic, and thus improving mechanical properties. However, the excessive Si addition would cause Si particles to precipitate, and the morphology of Fe-rich intermetallic had a tendency to needled-like. The alloy's solidification path of second phases is α-Fe + Al m Fe→β-Fe→binary eutectic (Al2Cu + Si) in Al–5Cu–XSi–XFe alloys.

Morphology, thermal stability, electronic structure and mechanical properties of α-AlFeMnSi phases with varying Mn/Fe atomic ratios: Experimental studies and DFT calculations

Separation of Fe-containing intermetallics is an effective method to decrease Fe content in Al-Si alloys. The three-dimensional (3D) morphology and stability of Fe-containing intermetallics are important parameters affecting their separation efficiency. Combined with experiments and first-principles calculations, the variation of 3D morphology, stability, electronic structure and mechanical properties of α-AlFeMnSi are studied with the evolution of Fe/Mn atomic ratio. The experimental results show that the 3D morphology of α-AlFeMnSi transforms from Chinese character to regular polyhedron with the increment of Mn/Fe atomic ratio and all α-AlFeMnSi phases have cubic crystal structure. The calculated mixing enthalpies indicate that all α-AlFeMnSi phases are stable in thermodynamics due to the negative values. The electronic structure and Mulliken population calculations show that the variation of Mn/Fe ratio results in the overall change of chemical bondings of Si-Mn, Si-Fe, Al-Mn and Al-Fe, which leading to the evolution of stability and mechanical properties of α-AlFeMnSi phases. The calculated results of mechanical properties demonstrate that the Young’s modulus and hardness of α-AlFeMnSi phases are improved with the increment of Mn/Fe atomic ratio, which is also validated by nano-indentation measurements. These results provide the componential guidance about designing easy-separable Fe-containing intermetallics and try to explain physical mechanism of property evolution of α-AlFeMnSi phase.

Understanding crystal structure and morphology evolution of Fe, Mn, Cr-containing phases in Al-Si cast alloy

As a common detrimental element in Al-Si cast alloy, Fe can be effectively removed by Fe-containing intermetallic separation in the alloy melt. However, the thermodynamic formation mechanism and growth morphology evolution of high separable Fe-containing intermetallic have not been investigated and clarified in detail until now. In this study, thermodynamic calculation, three-dimensional (3D) growth morphology and crystal structure were characterized to investigate the effect of Mn and Cr on morphology evolution, crystal structure and precipitation temperature of Fe-containing intermetallics. It is demonstrated that the addition of Mn and Cr can increase the precipitation temperatures of Fe-containing intermetallics in melt, and also result in the variation of crystal structures from monoclinic γ-AlFeSi to cubic α-AlFeMnCrSi phases. Moreover, the addition of Mn and Cr significantly changes the morphology of Fe-containing intermetallics from plate-like to cubic shapes. α-AlFeMnCrSi phase precipitated at high temperature exhibits the same structural basis as α-Al9Mn2Si2. The Fe-containing phase of cubic structure provides higher thermo stability which is in favor of its sufficient precipitation and separation.

Microstructure, tensile and bending behaviour of the as-cast AM50 alloy modified with different antimony and copper additions

The tensile and bending behaviours of the as-cast AM50 alloy were studied after adding different amounts of Sb/Cu (x = 0, 0.5, 1, 3 and 5 wt-%). The optimum levels of the Ultimate Tensile Strength (UTS) and tensile elongation (El%) values were found by adding 0.5 wt-% Sb and 0.5 wt-% Cu. The Yield Strength (YS) and hardness values improved with further additions of these elements. The tensile and hardness properties were investigated to be associated with microstructural features. With Sb additions, the new Mg3Sb2 intermetallics were formed. Moreover, by adding different Cu additions, three Cu-containing intermetallics were scrutinised by using the X-ray diffraction (XRD), thermodynamic calculations and also scanning electron microscopy (SEM). The formation of the new intermetallics could change the morphology of the β-Mg17Al12 phase and suppress its creation. Moreover, the grain refinement effects were different for each alloying element. Therefore, the grain refinement and the presence of new intermetallics had prominent impacts on the tensile and bending properties of the AM50 alloy. While grain refinement had a more positive effect on the UTS values, the YS and bending properties were chiefly controlled by morphology and size of intermetallics.

Morphology and Nucleation of Intermetallic Phases in Casting Al–Mg–Si Alloys

The changes in the structure of Al–Mg–Si casting alloys after additional alloying are observed. In order to predict and to explain with greater confidence, the phase transformations in the studied alloys equilibrium phase diagrams were calculated using Thermo-Calc software. The results of the Thermo-Calc are in good agreement with the microstructure analysis. Morphology and chemical composition of intermetallic phases were investigated by scanning electron microscopy, energy-dispersive x-ray analysis, and electron probe microanalysis on polished and deep etched microsections. Several intermetallic phases with Fe and Mn, phases that contain Zn and Cu, and Al3Ti crystals were described. It was found that the addition of Mn changes the morphology of Fe-containing intermetallics that improve strength and ductility of the alloys; crystals Al3Ti can act as nucleating particles of α-Al dendrites; Cu and Zn lead to the formation of several fusible eutectic phases. To better understand the influence of the different intermetallics, the mechanical properties (Brinell hardness, ultimate tensile strength, yield strength, elongation) were measured. Alloying with Cu and Zn exhibits the best values of hardness and strength (up to 85 HB and UTC = 251 MPa), while the highest ductility was achieved in the alloy with composition Al–5.5Mg–2.5Si–0.6Mn (6.7%). Alloys possess the lowest properties (both strength and ductility) with the highest concentrations of Fe and Si (UTS up to 173 MPa with elongation 3.4%).

Solidification thermal parameters, microstructural evolution and dendritic growth in an AlCuNi alloy

ABSTRACT Unsteady-state upward solidification experiment was carried out with the Al-3Cu-3Ni (wt.%) alloy. Temperature profiles were obtained through a water-cooled directional solidification device. Analysis on the resulting as-cast microstructure and its adaptation to thermal solidification parameter, such as growth and cooling rates and solidification local time, VL, TR and tSL, respectively, was carried out. The results showed that the resulting microstructure is constituted by an Al-rich primary phase (Al α ), characterised by a dendritic network, and by second phases that take place within the interdendritic regions, forming a eutectic mixture composed by the β-Al3Ni + θ-Al2Cu intermetallic compounds (IMCs), whose size and morphology have been affected by VL and TR. The length of the microstructural scale of the Al α phase was measured by the secondary dendritic spacing (λ 2). The experimental mathematical expression of λ 2 obtained in this work was compared with mathematical models proposed in the literature that associate tSL-λ 2 for non-equilibrium and equilibrium solidification conditions.

Effect of Ce Addition and Heat Treatment on Microstructure Evolution and Tensile Properties of Industrial A357 Cast Alloy

The effects of adding different Ce contents (0–0.32 wt.%) on the microstructure, mechanical properties, and fracture morphology of industrial A357 cast alloy in as-cast and T6 heat treatment were studied. The main purpose of this study is to improve the microstructure stability and tensile properties of industrial A357 cast alloy. The microstructural analyses indicate that the addition of Ce causes refinement of the α-Al primary phase for the reason that the formation of intermetallic compounds containing (AlSiCeMg) elements enriches the front of the solid–liquid interface, which causes an increase in constitutional undercooling. Simultaneously, the addition of Ce also affected the characteristics of eutectic Si particles, which make its morphology change from acicular structures into fragmented and spheroidized. This is mainly due to the formation of Ce-rich precipitates during solidification, which increase the constitutional undercooling and suppress the nucleation of the eutectic Si particles, resulting in the change of eutectic Si characteristics. Moreover, the needle-like morphology of a Fe-containing intermetallic is transformed into α(AlSiFeCe) phase containing rare earth Ce when part of the Ce atoms entered β(Al5FeSi) phase compounds. The tensile properties of the modified alloys were improved in the as-cast and T6 heat treatment as a consequence of simultaneous refinement of both secondary dendrite arm spacing and grains and the improvement of eutectic Si particles and Fe-containing intermetallic morphology. The fracture surface of the modified alloy has more dimples than the unmodified alloy, which indicates that the main fracture pattern of the modified alloy is dimple fracture caused by the crack of eutectic Si particles. The optimal percentage of Ce in industrial A357 cast alloy was determined to be 0.16 wt.% according to the change of microstructures structure and mechanical properties. These experimental results provide a new basis for adding rare earth Ce to improve the performance of parts in the actual production of industrial A357 cast alloy.

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.

An experimental investigation on the influence of rotational speed on microstructure and mechanical properties of friction stir welded dissimilar Al-Cu joints

The present study focuses on understanding the effect of variation of rotational speed on microstructure and mechanical properties of friction stir welded joints. AA 6061-T6 and pure copper 3 mm plates were welded at 800 rpm, 1,000 rpm, 1,500 rpm and 1,800 rpm using a constant traverse speed of 60 mm/min. Alterations on forces and torque, weld morphology, macrostructure, microstructure and formation of intermetallics were observed. A detailed electron dispersive spectrum (EDS) and X-ray diffraction (XRD) analysis showed the formation of Al2O3 and CuO along with Al2Cu and Al4Cu9 at higher rotational speed in the nugget zone (NZ). These intermetallics and oxides resulted in an inhomogeneous distribution of hardness at 1,800 rpm but due to presence of micro-defects the values were comparatively lower. Joint efficiency of 77% is achieved at 1,000 rpm which may be due to strengthening effect by the intermetallics. The joint strength deteriorated at 1,500 rpm and 1,800 rpm due to the presence of oxides making the joints brittle.

Segregation and removal of transition metal impurities during the directional solidification refining of silicon with Al-Si solvent

Segregation behavior of transition metal impurities (Fe and Ti) during the Al–Si solvent refining of silicon was investigated. A directional solidification with induction heating was employed to agglomerate Si crystals and simultaneously to enhance elimination of impurities. The inductively coupled plasma optical emission spectrometry (ICP-OES) results show that the concentration of transition metal impurities increased gradually from bottom to top of the sample during solidification. The morphology of iron-containing intermetallics has undergone a transformation from plate β-Al5FeSi to script α-Al8Fe2Si and needle-like Al–Fe–Si, while that of titanium-containing intermetallics maintained as plate TiSi2. The acid leaching results demonstrate that an aqua regia was effective to remove the iron-containing phases embedded in Si enrichment zone, while a hydrofluoric acid (HF) should be more efficient to eliminate titanium-containing phases. Therefore, it suggests that a combination of aqua regia and HF could be employed to efficiently remove transition metal impurities during Al–Si solvent refining.

Effect of Aging Temperature on the Microstructure and Properties of Economical Duplex Stainless Steel.

In this study, NSSC 2120 economical duplex stainless steel was prepared and the effects of aging temperatures on its intermetallic phase morphology, tensile strength, elongation, corrosion resistance, and antimicrobial properties were investigated. The results revealed that after aging the sample at 650 °C, it exhibits better pitting corrosion resistance and higher tensile strength. Upon increasing the aging temperature up to 750 °C, the pitting corrosion resistance and tensile strength of the samples were decreased due to the precipitation of the ε-Cu phase in the matrix. Moreover, with the further increase in the aging temperature to 850 °C, oxides containing Mn and Cr (CrMn1.5O4) and sulfides (MnS) precipitated from the samples, further decreasing their pitting corrosion resistance and tensile strength. Upon aging the samples at 950 °C, no second phase was observed and the corrosion resistance was less than that of the sample after aging at 650 °C, but the tensile strength was greater than that of the sample after aging at 650 °C. Antibacterial test results revealed that the sample after aging at 750 °C exhibited a good antibacterial effect due to the precipitation of the rod-shaped ε-Cu phase.