Intermetallic FeAl Research Articles

Self-lubricating laser claddings for friction control during press hardening of Al-Si-coated boron steel

In recent years, the use of Al-Si-coated ultra-high strength steels has grown rapidly in hot stamping applications in order to protect the workpiece from detrimental mechanisms such as scale formation or decarburisation affecting the quality of the finished product. However, the formation of Al-Fe intermetallics due to diffusion at high temperatures can lead to unstable friction, increased wear and damage both the tool and the workpiece surfaces.In the present study, self-lubricating coatings incorporating silver and molybdenum disulfide have been prepared by means of laser cladding, aiming at controlling friction and wear in hot stamping. The friction and wear characteristics of these coatings have been investigated at high temperatures against Al-Si-treated boron steel using two different test configurations featuring open and closed tribosystems respectively. A significant reduction in friction as well as decreased material transfer from the Al-Si-coated workpiece to the tool surface have been observed for the self-lubricating claddings. The closed configuration tribological tests have been found to underestimate friction and wear of the tool/workpiece system, and may be less suited for the lab-scale simulation of hot metal forming compared to an open test configuration. These findings are considered relevant as most of the published results in the available literature have been obtained by using closed configurations tests.

Fabrication and Characterization of Highly Porous FeAl‐Based Intermetallics by Thermal Explosion Reaction

Porous FeAl‐based intermetallics with different nominal compositions ranging from Fe–40 at% Al to Fe–60 at% Al are prepared by a novel process of thermal explosion (TE) mode. The results show that the Al content significantly affects the combustion behavior of the specimens, the ignition temperature of the Fe–Al intermetallics varies from 641 to 633 °C and the combustion temperature from 978 to 1179 °C. The porous materials exhibit uniform pore structures with porosities and average pore sizes of 52–61% and 20–25 µm, respectively. The TE reaction is the dominant pore formation mechanism regardless of the alloy composition. However, differences in the porosity and average pore size are observed depending on the Al content. The compressive strength of porous Fe–Al intermetallics is in the range of 23–34 MPa, duly applied as filters. Additionally, a surface alumina layer is formed at the early stage and both of the oxidation process and the sulfidation process follows the familiar parabolic rate law in the given atmosphere, exhibiting excellent resistance to oxidation and sulfidation. These results suggest that the porous Fe–Al intermetallics are promising materials for applications in harsh environments with a high‐temperature sulfide‐bearing atmosphere, such as in the coal chemical industry.

Structural and magnetization crossover in electrodeposited FeAl2O4 - effect of in situ oxidation.

Amongst other spinels, iron aluminium oxide (FeAl2O4) exhibits exceptional chemical and physical properties. However, magnetic properties of FeAl2O4 still need further investigation. DC electrodeposition is used to deposit intermetallic Fe3Al thin films. Oxidation time is varied from 0 to 20 min with a constant metallic layer deposition time of 10 min. Electrodeposited iron aluminium oxide thin films are annealed in the presence of 500 Oe applied magnetic field (MF) at 300 °C in a vacuum. Mixed structural phases, i.e. FeAl2O4 & χ-Al2O3, are observed at 5 min, 10 min and 15 min oxidation time. Whereas phase pure FeAl2O4 is observed at 20 min oxidation time. Magnetization loops show ferromagnetic behavior of iron aluminum oxide thin films with anisotropic nature for in-plane and out-plane configurations. The law of approach to saturation magnetization (LAS) is used to calculate magnetocrystalline anisotropy. Phase purity at 20 min oxidation time results in high saturation magnetization of 29.5 emu cm−3 with a low value anistopy constant of 1.28 × 108 erg cm−3. Easy axis of magnetization is shifted from perpendicular to parallel at an oxidation time of 20 min. Results show that in situ oxidation of thin films for 20 min leads to structural and magnetization crossover from impure to phase pure FeAl2O4 thin films with a high value of magnetization.

Effects of solid loading on pore structure and properties of porous FeAl intermetallics by gel casting

Solid loading as a key parameter in gel casting influence both rheological behavior and stability of slurry. It also has strong impacts on microstructure and final properties of porous FeAl intermetallics. In this study, a designed non-aqueous gel casting has been developed for fabrication of porous FeAl intermetallics. The SEM analysis, bubble point method and compression test were used to investigate pore structure and properties of porous FeAl intermetallics. Results reveal that higher solid loading will improve the uniformity of pore structure and lead a decrease in maximum pore size. Gas permeability decreases but compressive strength increases with increasing solid loading. The quantitative relationship between maximum pore size (dm) and solid loading (φ) is dm = 10.52 − 0.15φ (35 ≤ φ ≤ 50); the relationship between gas permeability (K) and solid loading (φ) is K = 13.99 − 0.22φ (35 ≤ φ ≤ 50). Further, the distinctive phase transformation and pore formation mechanism are the main reasons for tiny volume expansion and highly open porosity of final products, respectively. Altogether, this work provides a strategy to fabricate porous FeAl intermetallics with adjustable pore structure and properties by controlling the solid loading, suggesting that the non-aqueous gel casting is a promising candidate for the preparation of porous FeAl intermetallics.

On the reactive wetting of a medium-Mn advanced high-strength steel during continuous galvanizing

The effects of process atmosphere oxygen partial pressure (pO2) and Sn-addition on the reactive wetting of a prototype medium manganese third generation advanced high strength steel by a 0.20 wt% Al (dissolved) zinc bath was determined through galvanizing simulations. External MnO layers with thicknesses ranging from 19 nm to 45 nm were formed after annealing for 120 s at 690 °C under a variety of experimental process atmospheres. Despite the presence of surface oxides prior to immersion, an integral zinc coating and good coating adherence was obtained on all steel substrates, with the exception of the reference steel annealed under the lowest pO2−50 °C dew point process atmosphere, which exhibited poor reactive wetting. Excellent reactive wetting was demonstrated by the Sn-containing steels annealed under the −30 °C and +5 °C atmospheres. It was determined that the Al uptake as well as the population of the Fe-Al intermetallics at the coating/steel interface increased as a result of annealing under the higher pO2 atmospheres and adding Sn to the steel. The significant improvement in reactive wetting of the Sn-added steel was attributed to the decreased surface enrichment of Mn and the discrete distribution of the fine external oxide particles formed on the surface of these steel prior to dipping; which allowed for progression of reactive wetting through mechanisms such as the aluminothermic reduction of MnO, oxide bridging and oxide lift-off by the molten galvanizing bath.

Effect of Welding Parameters on the Corrosion Behavior of Dissimilar Alloy Welds of T6 AA6061 Al-Galvanized Mild Steel

Partial replacement of steel by Aluminium (Al) alloys is a promising approach adopted by automotive sector to improve fuel efficiency without compromising on the strength. However, this results in the generation of dissimilar welds of Al alloys and steel. Understanding the corrosion behavior of Al alloys–steel welds as a function of welding parameters is critical in the successful application of such alloys. The present study reports the intergranular corrosion behavior of T6 heat-treated AA6061 Al alloy-galvanized mild steel lap joints, welded by metal inert gas welding–brazing technique as a function of different welding parameters, viz. weld speed (4-6 m/min) and wire feed rate (0.8-0.9 m/min), following ASTM G 67-04. Weld characterization was performed by field emission scanning electron microscopy (FESEM), x-ray diffraction and nano-hardness measurement. Results indicate heavy dissolution and metal loss at the interface. A high volume fraction of Al-Fe intermetallics was precipitated at the weld interfaces, resulting in high hardness and higher localized corrosion. The thickness of Al-Fe intermetallic layer increased with increasing wire feed rate and lower weld speed which enhanced the severity of galvanic and intergranular corrosion.

Role of interfacial reaction on the mechanical performance of Al/steel dissimilar refill friction stir spot welds

ABSTRACTAl/steel welds were successfully fabricated by refill friction stir spot welding. The welding parameters were optimised based on the clamping ring temperature and weld strength. 85.7% of welds achieve a strength which exceeds the American Welding Society requirement when the clamping ring temperature ranges from 230 to 265°C. Cracks are formed under the pin and sleeve in the Al substrate at the Al/steel interface, which are associated with the tool sleeve plunging period and attributed to the α-Al + Al–Zn eutectic structure. The interdiffusion between Al and Zn at the steel surface produced an Al–Zn eutectic structure layer at the Al/steel interface, while part of the zinc coating materials is squeezed out of the sleeve periphery, leading to a brazing effect which contributes to weld strength. Nanoscale discontinuous Fe4Al13 and FeAl intermetallics form as a layer localised at the Al–Zn layer/steel substrate interface.

Effect of dew point on the reactive wetting of a C-2Mn-1.3Si (wt%) advanced high strength steel during continuous galvanizing

The effect of oxygen partial pressure on the reactive wetting of a Fe-0.1C-2Mn-1.3Si (wt%) advanced high strength steel during continuous galvanizing was determined. The steel was intercritically annealed at 1093 K (820 °C) for 120 s in a N2–5 vol% H2 process atmosphere, where the oxygen potential was controlled by varying the process atmosphere dew points at 223 K (−50 °C), 243 K (−30 °C) and 278 K (+5 °C). All samples were immersed in a 0.2 wt% dissolved Al continuous galvanizing bath at 733 K (460 °C) for 4 s. For the 243 K (−30 °C) and 278 K (+5 °C) dew point atmospheres, a well-developed Fe-Al intermetallic layer, indicative of good reactive wetting, was observed due to the presence of either thin or plate/nodule-like oxides on the steel surface after annealing. This external oxide morphology facilitated contact between the Zn-alloy bath and the substrate via a variety of mechanisms, including interfacial infiltration through cracks due to thermal coefficient of expansion mismatch between the external oxide and substrate and aluminothermic reduction, which resulted in Fe dissolution from the substrate and consequent formation of the observed Fe-Al intermetallics. On the other hand, the poor reactive wetting observed for the samples annealed under the 223 K (−50 °C) dew point atmosphere was due to the formation of a thick, compact oxide layer on the steel surface, which provided an effective barrier between the substrate and Zn bath, preventing contact between the two phases and the required Fe dissolution from the substrate surface.

Nano-mechanical properties of Fe-Mn-Al-C lightweight steels

High Al Low-density steels could have a transformative effect on the light-weighting of steel structures for transportation. They can achieve the desired properties with the minimum amount of Ni, and thus are of great interest from an economic perspective. In this study, the mechanical properties of two duplex low-density steels, Fe-15Mn-10Al-0.8C-5Ni and Fe-15Mn-10Al-0.8 C (wt.%) were investigated through nano-indentation and simulation through utilization of ab-initio formalisms in Density Functional Theory (DFT) in order to establish the hardness resulting from two critical structural features (κ-carbides and B2 intermetallic) as a function of annealing temperature (500–1050 °C) and the addition of Ni. In the Ni-free sample, the calculated elastic properties of κ-carbides were compared with those of the B2 intermetallic Fe3Al−L12 and the role of Mn in the κ structure and its elastic properties were studied. The Ni-containing samples were found to have a higher hardness due to the B2 phase composition being NiAl rather than FeAl, with Ni-Al bonds reported to be stronger than the Fe-Al bonds. In both samples, at temperatures of 900 °C and above, the ferrite phase contained nano-sized discs of B2 phase, wherein the Ni-containing samples exhibited higher hardness, attributed again to the stronger Ni-Al bonds in the B2 phase. At 700 °C and below, the nano-sized B2 discs were replaced by micrometre sized needles of κ in the Ni-free sample resulting in a lowering of the hardness. In the Ni-containing sample, the entire α phase was replaced by B2 stringers, which had a lower hardness than the Ni-Al nano-discs due to a lower Ni content in B2 stringer bands formed at 700 °C and below. In addition, the hardness of needle-like κ-carbides formed in α phase was found to be a function of Mn content. Although it was impossible to measure the hardness of cuboid κ particles in γ phase because of their nano-size, the hardness value of composite phases, e.g. γ + κ was measured and reported. All the hardness values were compared and rationalized by bonding energy between different atoms.

Fabrication of porous FeAl-based intermetallics via thermal explosion

Abstract Porous FeAl-based intermetallics were fabricated by thermal explosion (TE) from Fe and Al powders. The effects of sintering temperature on phase constitution, pore structure and oxidation resistance of porous Fe-Al intermetallics were systematically investigated. Porous Fe-Al materials with high open porosity (65%) are synthesized via a low-energy consumption method of TE at a temperature of 636 °C and FeAl intermetallic is evolved as dominant phase in sintered materials at 1000 °C. The porous materials are composed of interconnected skeleton, large pores among skeleton and small pores in the interior of skeleton. The interstitial pores in green powder compacts are the important source of large pores of porous Fe-Al intermetallics, and the in-situ pores from the melting and flowing of aluminum powders are also significant to the formation of large pores. Small pores are from the precipitation of Fe-Al intermetallics particles. In addition, the porous specimens exhibit high resistance to oxidation at 650 °C in air.

Effect of in-situ oxidation on structure and ferromagnetic properties of Fe3Al and FeAl2O4 thin films prepared by electrodeposition

Magnetic oxide thin films are potentially important in the coming era in data storage and spintronic devices. But the effects of oxygen positional parameter on different properties of oxide thin films are not completely understood and scarcely reported. Using DC electro-deposition, FeAl2O4 thin films were deposited on copper substrates by in-situ oxidation of intermetallic Fe3Al at 2 V. Deposition time for each intermetallic film was a 30 min while oxidation time was varied from 0 min to 30 min with the interval of 5 min. All the thin films were annealed for 1 h at 300 °C in the 500 Oe magnetic field. XRD patterns confirmed the formation of spinel structure for the FeAl2O4 thin films with no preferred orientation. Variations in lattice parameters and crystallite size might be due to surface activation energy and in cooperation of oxygen atoms at the tetrahedral sites for charge compensation. Cation distribution and lowest stacking fault probability confirmed the formation of defect free pure phase of FeAl2O4 for the thin film with the 30 min oxidation. Tolerance factor found to very close to unity representing the formation of defect free thin films. Magnetic results showed that all the thin films carried the ferromagnetic behavior and FeAl2O4 thin film with the 30 min oxidation carried maximum saturation magnetization.

Ultrahigh-strength friction stir spot welds of aluminium alloy obtained by Fe3O4 nanoparticles

ABSTRACTThe present research was aimed at simultaneous increase in strength and fracture energy of friction stir spot weld of Al-5083 alloy via reinforcing by Fe3O4 nanoparticles. Compared to non-reinforced welds, failure load and fracture energy of reinforced welds increased up to ∼115 and 560%, respectively. These findings were discussed in the light of grain refinement as well as considerable increase in hardness of fracture location due to the presence of sub-micron and nano reinforcing particles (RPs). Fe3O4 nanoparticles were transformed to Al–Fe intermetallics due to their small size and heat generated during the welding process. The stir zone of reinforced welds consisted of three different grain structures among which the finest one was related to the composite structure containing well-distributed RPs in the aluminium matrix.

Hot-dip Aluminizing of Low Carbon Steel in Al & Al-5wt % Cr Baths

Hot dip aluminizing of low carbon steel is carried out in pure aluminium bath and Al-5wt% Cr bath. The coating is characterized by scanning electron microscopy and chemical composition of the coating is analysed by EDS (energy dispersive spectroscopy) attached to SEM. The coating consists of three regions, viz., outer aluminium topcoat, intermediate Fe-Al intermetallics layer and the base alloy. The intermetallics layer consists of FeAl3 and Fe2Al5 phases. Fe2Al5 is the major phase in the intermetallics layer. The growth kinetics of intermetallics layer is parabolic in nature implying that it is diffusion controlled. Addition of chromium forms Al7Cr dispersed intermetallics phases in the aluminium topcoat. Addition of chromium has no influence on the morphology of the intermetallics layer. Scratch resistance of the coating is carried out to evaluate the scratch hardness of the coating. Chromium addition improves scratch resistance of the coating.

A Study of Susceptibility and Evaluation of Causes of Cracks Formation in Braze-Weld Filler Metal in Lap Joints Aluminum – Carbon Steel Made with Use of CMT Method and High Power Diode Laser

Abstract In this article results of studies on cracks formation susceptibility in braze-welded joints of thin aluminum sheets and double-sided zinc galvanized steel sheets for car body parts made by laser brazing with high power diode laser ROFIN DL 020 and CMT MIG-brazing, with filler material in form of powder and wire accordingly, were presented. Optimal welding parameters were determined by visual acceptance criteria. On joints made with optimal parameters further examinations were carried. Results of macro- and microscopic metallographic examinations, structural roentgenography, EDS microanalysis and hardness tests were presented. Causes of brittle intermetallic Fe-Al phases formation in Al-matrix filler metal in dissimilar aluminum – zinc plated carbon steel joints were pointed.

Influence of Plastic Deformation During Extrusion Process on Heat Resistance Alloys Fe40Al

AbstractThe article presents the results of studies on the effects wrought on the corrosion resistance of the alloy matrix phase inter-metallic FeAl. Researches were carried out on the Fe40Al5Cr0.2TiB alloy and involved the oxidation of the samples after the crystallization after plastic deformation made by extrusion. The tests were performed in an oven in air at 1100°C for 100, 300 and 500 h. Determined to change the mass of the samples after corrosion research setting kinetics of corrosion processes, as well as an analysis of the microstructure of the alloy after the crystallization and after forming. The structure was examined using light microscopy and scanning electron microscopy and X-ray microanalysis with EDS chemical composition of the corrosion products. The test results revealed that plastic deformation during extrusion of intermetallic alloy led to structural changes, the effect of which was to improve the heat resistance at a temperature of 1100°C.

Microstructure evolution in refill friction stir spot weld of a dissimilar Al–Mg alloy to Zn-coated steel

ABSTRACTIn the present study, dissimilar welds of an Al–Mg–Mn alloy and a Zn-coated high-strength low-alloy steel were welded by refill friction stir spot welding. The maximum shear load recorded was approximately 7.8 kN, obtained from the weld produced with a 1600 rev min−1 tool rotational speed. Microstructural analyses showed the formation of a solid–liquid structure of an Al solid solution in Mg–Al-rich Zn liquid, which gives rise to the formation of Zn-rich Al region and microfissuring in some regions during welding. Exposure of steel surface to Mg–Al-rich Zn liquid led to the formation of Fe2Al5 and Fe4Al13 intermetallics. The presence of defective Zn-rich Al regions and Fe–Al intermetallics at the faying surface affects the weld strength.

DTA Research on Interfacial Contact in Fe–Al Powder Mixture

The interfacial contact in exothermically reacting mixture of Fe and Al powders (or Al-alloy) designed for the formation of intermetallic Fe3Al powder is studied by differential thermal analysis (DTA). The effect of alloying elements such as Mg and Ti on this process is analyzed. The behavior of the Fe–Al-based powder produced by heating under DTA conditions is compared with that of the powder with the same composition produced by mechanochemical synthesis.

Effect of number of passes on the corrosion behavior of Fe/Al surface composites produced by plasma spraying and friction stir processing

Friction stir processing (FSP) was applied on the surface of the low carbon steel substrate and the steels which coated by Al–30wt.% Fe with different (1–4) passes. Metallographic examinations and electron backscatter diffraction (EBSD) were used to analyze the microstructure of the stir zones (SZs). The influence of the number of FSP passes on electrochemical properties of the SZs in steels and the surface composites in 3.5wt.% NaCl solution was analyzed via electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. The Fe-Al intermetallics were produced in-situ on the surface of the composite layers during FSP. Widmänstatten ferrite plates were formed in the SZs of both groups of samples. The corrosion resistance of the FSPed steels was improved by increasing the number of passes due to the increment of grain size in SZs. Among the surface composites, the single-pass FSPed surface exhibited the highest polarization resistance and charge transfer resistance. This was attributed to the presence of elemental aluminum and Al-rich compounds at the surface.

High-Temperature Corrosion of Aluminized Diffusion-Coating for Fe-Base Alloy in N<sub>2</sub>/H<sub>2</sub>O/H<sub>2</sub>S-Mixed Gas

The pack-cementation is one of economical, efficient coating processes for Fe-base alloys. It can provide good protection against high-temperature oxidation and corrosion. In this study, the high-temperature corrosion behavior of the aluminized diffusion-coating on low-carbon T20 steel (Fe-2.0Cr-0.5Mo-0.8Mn-0.3S in at.%) was studied at 800 °C in N2/H2O/H2S-mixed gas. The aluminized coating consisted of Fe3Al. The aluminized T20 steel after corrosion at 800 °C for 10~100h in N2/H2O/H2S-mixed gases, the scale formed on the Fe3Al coating consisted primarily of α-Al2O3, Al2S3, FeAl2O4 and FeO, with relatively slow scaling rates. The Fe3Al intermetallics has reasonable corrosion-and oxidation-resistance, because it can form a protective alumina scale. Without the aluminized diffusion-coating, T20 steel corroded fast with serious scale failure. At the surface, coarse FeS grains with cracks formed. Since FeS has a very high concentration of cation vacancies, it grew fast through the outward diffusion of Fe2+ ions.

Influence of heating parameters on properties of the Al-Si coating applied to hot stamping

The Al-Si coating of ultra-high strength steel has been applied to hot stamping more and more widely, owing to solving the problem of oxidation and decarburization. However, the evolution of Al-Si coating during the heating process was rarely studied in the previous study. The tests about the influence of heating parameters, such as heating temperature, heating rates and dwell time, on properties of the Al-Si coating were carried out on the Gleeble-3500 thermal simulator. The properties of the Al-Si coating, for instance, volume fraction of FeAl intermetallics, α-Fe layer as well as porosity and 3D surface topography, were explored in the study. Results showed that more and more Kirkendall voids and cracks appeared in the Al-Si coating when the heating temperature exceeded 600°C. The heating rates almost had no influence on properties of the Al-Si coating when the temperature was equal to or lower than 500°C. The volume fraction of FeAl intermetallics in the coating with dwell time from 3 s to 8 min at 930°C was 0, 6.19%, 17.03% and 20.65%, separately. The volume fraction of the α-Fe layer in the coating changed from zero to 31.52% with the prolonged dwell time. The porosity of the coating ranged from 0.51% to 4.98% with the extension of dwell time. The unsmooth degree of the surface of the coating rose gradually with the increasing of heating rates and the extension of dwell time. The 3D surface topography of the coating was determined by the comprehensive effect of atoms diffusion, new formed phases, surface tension and the degree of oxidation of the coating surface. Experiments indicated that rapid heating was not suitable for the coating when the temperature exceeded 500°C. Experiments also demonstrated that enough dwell time was essential to obtain the superior properties of the coating.