High Power Laser Powder Deposition Repair of AISI 4340 Steel in Aircraft Applications

High power laser powder deposition (LPD) has been used to investigate the potential of repairing damaged aero-grade high strength steel.Metallurgical analysis was performed to analyze the integrity of the clad layer.A 4kW fiber laser was used to deposit two separate alloys (AISI 4340 and AISI 420 stainless steel) on an AISI 4340 steel substrate and metallurgical analysis was performed to analyze the integrity of the clad layer.No microcracks was observed on the clads,but porosity and high dilution was observed on most clads.However,microstructural analysis showed a crack and porosity free clad layer with low dilution can be achieved for some laser conditions.     

laser cladding of Ni-Al bronze on Al alloy AA333

The present study investigates the effects of laser-processing parameters, such as laser power, traverse speed, powder-feed rate, and flow rate and species of assisting gas, and material prop-erties, such as substrate surface condition, on laser cladding of Ni-Al bronze on Al alloy AA333. The proper processing parameters were determined experimentally and are discussed in terms of their effects on laser-clad quality and microstructure as observed using optical and scanning electron microscopes. Despite a large difference in melting points between the cladding material, Ni-AI bronze (MP = 1063 °C), and the substrate, Al-alloy AA333 (MP = 577 °C), clads of thickness from 1.2 to 2.5 mm that are crack-free and had good fusion were achieved. The substrate surface condition and the flow rate and species of assisting gas were found to be important for clad formation. A sandpaper-polished substrate absorbs less energy at the molten pool front and facilities reducing dilution. A large flow rate of assisting gas, such as helium, also has an effect on reducing dilution. A laser-generated molten-pool model was developed to explain the preceding experimental results. Y. LIU formerly Research Associate, Center for Laser Aided Materials Processing, Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign.     

Numerical Simulation of Brazing Aluminium Alloys with Al–Si Alloys

Joining parts using low-melting temperature alloys has long been used for manufacturing complex components such as heat exchangers made of aluminium alloys. Investigations of the process have shown that core/clad interaction during heating and brazing can lead to a significant decrease in the amount of liquid available for joint formation. This study presents a transient one-dimensional model for the process that takes into account the diffusion of silicon and the movement of the core/clad interface, with the model equations being implemented in the finite element software COMSOL Multiphysics; the results are compared to literature experimental data. Silicon profiles in the core are well described, while there appears a significant difference between predicted and experimental values of remaining clads which suggest a strong effect of silicon diffusion and liquid penetration at core grain boundaries.     

Determination of the chemical diffusion of oxygen in liquid iron oxide at 1615 °c

The chemical diffusion of oxygen in liquid iron oxide has been studied by the oxidation of a melt in a long capillary at 1615 °C. When pure oxygen was used as the oxidizing agent, the surface composition of the slag was found to be in close agreement with the expected gas-slag equilibrium, suggesting that diffusion is the controlling step. This was not the case when air, 5 pct oxygen in argon or pure CO₂ was used to oxidize the slag. The deviation of the surface composition from the expected equilibrium was in accordance with a mechanism of mixed control by both the gas-slag reaction and diffusion in the bulk. The average value of the chemical diffusivity of oxygen (or iron) in liquid iron oxide with Fe²⁺/Fe _T between 0.25 and 0.77 was established to be 3(±1) × 10^-7 m²/s. This value is one to two orders of magnitude higher than those from earlier studies. There seems to be a reasonable correlation between the chemical and the ionic self-diffusivities through the Darken equation. A quantitative analysis in this respect and on the role of electron hole migration depends on the availability of data on the ionic conductivity and the tracer diffusivities. Formerly Postgraduate Student     

Phase transformations at steel/IN626 clad interfaces

The microstructures of 4130 and 2.25Cr-1Mo steels clad to nickel base IN625 by welding and HIPing were examined by Analytical Electron Microscopy (AEM) and Secondary Ion Mass Spectroscopy (SIMS) to determine the interfacial microstructural characteristics which could affect their mechanical properties and corrosion resistance. The interface microstructures of the clads produced by the two methods were considerably different. The clad produced by welding was characterized by a low density of carbide precipitates confined to a very narrow region (∼1 μm) at the interface of ferrite and austenite. In addition, a thin region of untempered martensite was present at the interface which could affect its resistance to hydrogen embrittlement as well as other mechanical properties. The interface of the HIP clad composite contained several regions of distinct microstructural characteristics with widely varying densities of carbide precipitates. Relative to the clad produced by welding, extensive precipitation was observed both in the steel and in the IN625 at the interface, separated by a region free from precipitation. The extent of precipitation at the interface regions appears to be controlled essentially by the extent of carbon transport across the interface. The article describes the detailed analysis of the interface characteristics, and models are proposed to explain the microstructural evolution at the interface of the HIP and weld clad composites.     

Microstructural Evolution of Inconel 625 and Inconel 686CPT Weld Metal for Clad Carbon Steel Linepipe Joints: A Comparator Study

Microstructural evolution of Inconel 625 and Inconel 686CPT filler metals, used for the fusion welding of clad carbon steel linepipe, has been investigated and compared. The effects of iron dilution from the linepipe parent material on the elemental segregation potential of the filler metal chemistry have been considered. The results obtained provide significant evidence to support the view that, in Inconel 686CPT weld metal, the segregation of tungsten is a function of the level of iron dilution from the parent material. The data presented indicate that the incoherent phase precipitated in the Inconel 686CPT weld metal has a morphology that is dependent on tungsten enrichment and, therefore, iron dilution. Furthermore, in the same weld metal, a continuous network of finer precipitates was observed. The Charpy impact toughness of each filler metal was evaluated, and the results highlighted the superior impact toughness of the Inconel 625 weld metal over that of Inconel 686CPT.     

Dilution control in gas-tungsten-arc welds involving superaustenitic stainless steels and nickel-based alloys

Fusion welds were prepared between a superaustenitic stainless steel, (the AL-6XN alloy) and two Ni-based filler metals (IN625 and IN622) using the gas-tungsten-arc welding (GTAW) process. Fusionzone compositions over the full range of dilution levels (0 to 100 pct) were produced by varying the independent welding parameters of arc power and volumetric filler-metal feed rate. Microstructural characterization of the welds was conducted via light optical microscopy, with quantitative chemical information obtained through electron-probe microanalysis (EPMA). The dilution level of each weld was determined from the EPMA data as well as through geometric measurements of the weld cross-sectional areas. The dilution level was observed to decrease with increasing filler-metal feed rate and decreasing arc power. These effects are quantitatively interpreted based on a previously proposed processing model. The model is used to demonstrate that, in terms of welding parameters, the dilution level can be correlated exclusively to the ratio of the volumetric filler-metal feed rate (V _fm) to arc power (VI), i.e., the individual values of V _fm and VI are not important in controlling the dilution and resultant weld-metal composition. Good agreement is obtained between experimental and calculated dilution values using the model. It is also demonstrated that the melting enthalpies of the filler metal and substrate have only a minor influence on dilution at dilution levels in the range from 40 to 100 pct. This knowledge facilitates estimates of dilution levels in this range when the substrate and fillermetal thermal properties are not accurately known. The results presented from this study provide guidelines for controlling the weld-metal composition in these fusion-zone combinations.     

Combined processing of metallurgical slurry and gas-purification dust

Information is provided on the combined hydrometallurgical processing of slurry from blast-furnace gas treatment and metallurgical dust. The proposed method includes hydrochloric-acid leaching, reduction (Fe³⁺ → Fe²⁺), crystallization of FeCl₂ · 4H₂O, decomposition of chlorides to obtain oxides Fe _x O _y , and reduction of iron oxides to obtain thread crystals of PZh-1 powder iron. Approximate calculations show that the cost of such PZh-1 powder may be 15000–18000 rub/t.     

Laser cladding of quasi-crystal-forming Al-Cu-Fe-Bi on an Al-Si alloy substrate

We report here the results of an investigation aimed at producing coatings containing phases closely related to the quasi-crystalline phase with dispersions of soft Bi particles using an Al-Cu-Fe-Bi elemental powder mixture on Al-10.5 at. pct Si substrates. A two-step process of cladding followed by remelting is used to fine-tune the alloying, phase distribution, and microstructure. A powder mix of Al₆₄Cu_22.3Fe_11.7Bi₂ has been used to form the clads. The basic reason for choosing Bi lies in the fact that it is immiscible with each of the constituent elements. Therefore, it is expected that Bi will solidify in the form of dispersoids during the rapid solidification. A detailed microstructural analysis has been carried out by using the backscattered imaging mode in a scanning electron microscope (SEM) and transmission electron microscope (TEM). The microstructural features are described in terms of layers of different phases. Contrary to our expectation, the quasi-crystalline phase could not form on the Al-Si substrate. The bottom of the clad and remelted layers shows the regrowth of aluminum. The formation of phases such as blocky hexagonal Al-Fe-Si and a ternary eutectic (Al + CuAl₂ + Si) have been found in this layer. The middle layer shows the formation of long plate-shaped Al₁₃Fe₄ along with hexagonal Al-Fe-Si phase growing at the periphery of the former. The formation of metastable Al-Al₆Fe eutectic has also been found in this layer. The top layer, in the case of the as-clad track, shows the presence of plate-shaped Al₁₃Fe₄ along with a 1/1 cubic rational approximant of a quasi-crystal. The top layer of the remelted track shows the presence of a significant amount of a 1/1 cubic rational approximant. In addition, the as-clad and remelted microstructures show a fine-scale dispersion of Bi particles of different sizes formed during monotectic solidification. The remelting is found to have a strong effect on the size and distribution of Bi particles. The dry-sliding wear properties of the samples show the improvement of wear properties for Bi-containing clads. The best tribological properties are observed in the as-clad state, and remelting deteriorates the wear properties. The low coefficient of friction of the as-clad and remelted track is due to the presence of approximant phases. There is evidence of severe subsurface deformation during the wear process leading to cracking of hard phases and a change in the size and shape of soft Bi particles. Using these observations, we have rationalized possible wear mechanisms in the Bi-containing surface-alloyed layers.     

Direct-Chill Co-Casting of AA3003/AA4045 Aluminum Ingots via Fusion™ Technology

Laboratory-scale experiments were conducted to cast AA3003/AA4045 clad ingots via Fusion? Technology, a novel process developed by Novelis Inc. for the production of aluminum clad materials such as brazing sheet. Experimental results were used to validate a steady-state thermofluids model of the Fusion? Technology co-casting process. The numerical model was able to accurately predict the temperature field within the AA3003/AA4045 clad ingot as well as the shape of the AA3003 liquid sump. The model was also used to quantify the temperature, fraction solid, and velocity fields in a clad ingot cast with an asymmetrical molten metal-feeding system. Feeding of core and clad molten metals at opposite corners of the mold was found to reduce the risks of hot spots and liquid metal breakthrough from the core sump to the clad side of the Fusion? Technology mold. The use of a diffuser for the AA3003 core molten metal and of a vertical feeding tube for the AA4045 clad produced different flow patterns and liquid sump shapes on either side of the mold. The quality of the metallurgical bond at the core/clad interface appeared good near the clad inlet and at the ingot centerline, but poor near the edges of the ingot. SEM–EDS analysis of the chemical composition across the interface showed that a 1 to 20-μm-deep penetration of silicon from the AA4045 clad into the AA3003 core had occurred at visually acceptable interfaces, whereas silicon diffusion across poor interfaces was very limited. A study of the model-predicted fraction solid history at different points along the interface indicated that reheating of the AA3003 core is not required to form a visually acceptable metallurgical bond. However, a sufficient amount of interaction time between the solid AA3003 core shell and the silicon-rich AA4045 clad liquid is required to chemically dissolve the surface of the core and form a good metallurgical bond. An approximate dissolution depth of 750 to 1000 μm was observed along the visually good interface. Partial dissolution of the Mn-rich AA3003 core led to the formation of Al(Mn,Fe)Si intermetallic particles in the AA4045 clad and an increased manganese concentration near the core/clad interface.     

Activities in liquid Fe-V-O and Fe-B-O alloys

The activities in liquid Fe-V-0 and Fe-B-O alloys have been determined using the following galvanic cells Cr-Cr₂O₃(s) | ZrO₂(CaO) | Fe-V-O (l, saturated with oxide) Cr-Cr₂O₃(s) | ThO₂(Y₂O₃) | Fe-V-O (l, saturated with oxide) Cr-Cr₂O₃(s) | ZrO₂(CaO) | Fe-B-O (l, B₂O₃ saturated with Al₂O₃) The solubility of oxygen in Fe-V alloys at 1600°C decreases with increasing vanadium content to a minimum of about 180 ppm at 3 wt pct V, and then increases to over 4000 ppm at 36.3 wt pct V. Vanadium was found to decrease the activity coefficient of oxygen and the value of the interaction coefficient e_o ^V at infinite dilution of vanadium is -0.14. The activity of vanadium was calculated from the measured electromotive force, and log γ_v was found to be represented well by the quadratic formalism for N_v < 0.4: log γ_V = -0.70N ² _Fe -0.30 At 1550°C boron decreases the solubility of oxygen down to about 80 ppm at 0.67 wt pct B in Fe-B melts in equilibrium with B₂O₃ saturated with A1₂O₃ (N_{Al 2} O₃ = 0.087). The boron deoxidation product, ’K′ = (wt pct B)²(wt pct 0)³ at infinite dilution of boron is 4.4 × 10_-9 and 1.5 × 10^-8 at 1550° and 1600°C, respectively. Boron decreases the activity coefficient of oxygen in liquid iron, and the value of the interaction coefficient e_o ^B is -2.6 at infinite dilution of boron. The activity coefficient of boron at infinite dilution (γ^° _B) is 0.083 at 1550°C relative to solid boron.     

A Numerical Study of the Direct-Chill Co-Casting of Aluminum Ingots via Fusion™ Technology

For the last 70 years, direct-chill (DC) casting has been the mainstay of the aluminum industry for the production of monolithic sheet and extruded products. Traditionally, clad aluminum sheet products have been made from separate core and clad DC cast ingots by an expensive roll-bonding process; however, in 2005, Novelis unveiled an innovative variant of the DC casting process called the Fusion? Technology process that allows the production of multialloy ingots that can be rolled directly into laminated or clad sheet products. Of paramount importance for the successful commercialization of this new technology is a scientific and quantitative understanding of the Fusion? casting process that will facilitate process optimization and aid in the future development of casting methodology for different alloy combinations and ingot and clad dimensions. In the current study, a numerical steady-state thermofluids model of the Fusion? Technology casting process was developed and used to simulate the casting of rectangular bimetallic ingots made from the typical brazing sheet combination of AA3003 core clad with an AA4045 aluminum alloy. The analysis is followed by a parametric study of the process. The influence of casting speed and chill-bar height on the steady-state thermal field within the ingot is investigated. According to the criteria developed with the thermofluids model, the AA3003/AA4045 combination of aluminum alloys can be cast successfully with casting speeds up to 2.4 mm s^?1. The quality of the metallurgical bond between the core and the clad is decreased for low casting speeds and chill-bar heights >35 mm. These results can be used as a guideline for improving the productivity of the Fusion? Technology process.     

Properties of electrolyte solutions relevant to high concentration chloride leaching. III. Solubility of pertinent solids and iron(III)/iron(II) redox potential measured in concentrated magnesium chloride solutions

Results of solubility measurements of nickel chloride, manganese chloride, iron(II) chloride, hematite and akaganeite in aqueous solutions of MgCl₂ (0.5–3.5 mol L^− 1) at temperatures of 60 and 90 °C are reported. Solubilities of metal(II) chlorides decrease almost linearly with MgCl₂ concentration due to the common ion effect. Nickel chloride and iron(II) chloride solubilities are very similar, while manganese chloride is about 30% more soluble.Hematite is more stable (i.e. less soluble) than akaganeite under all conditions investigated in this study, while ferrihydrite is considerably less stable. In other words, there is no change in the relative stabilities of these phases effected by the presence of high magnesium chloride concentrations. The solubility of all of these phases decreases with temperature and, for each temperature, the solubility constants increase linearly with the MgCl₂ concentration. The present results allow the prediction of the iron concentration as a function of the H⁺ and MgCl₂ molality at equilibrium with hematite or akaganeite.The Fe(III)/Fe(II) redox behaviour has been characterized in concentrated aqueous solutions of MgCl₂ (1.5–3.5 mol L^− 1) at a temperature of 25 °C. Standard redox potentials are ca. 100 mV lower than at infinite dilution and change linearly by only 13 mV in the range 2–4 mol L^− 1 MgCl₂.     

Hot corrosion behavior of Cr-modified NiAl coatings on 310 stainless steel produced by a gas tungsten arc cladding process

The microstructure and hot corrosion behavior of Ni/Al–Cr composite claddings produced by gas tungsten arc welding (GTAW) on a 310 stainless steel substrate were studied. The phase analyses and the microstructure of the cladding layers were evaluated using optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy. The influence of Cr addition (0, 5, 10 at% Cr) on the microstructure and hot corrosion behavior of the NiAl coatings was assessed. The cyclic hot corrosion behavior of the base metal and different claddings was investigated at 900°C and in static air, with a 2–3?mg/cm² Na₂SO₄–10%NaCl (wt%). It was found that a dendritic microstructure was formed on the clad surfaces. The results of the XRD analyses indicated that a NiAl phase was synthesized in situ during GTAW cladding and the presence of Cr reduced the intensity of diffraction peaks of NiAl. Hot corrosion experiments also revealed that the addition of Cr had a crucial influence on the hot corrosion behavior of NiAl coatings. It was found that the larger the amount of Cr, the superior the resistance of the coatings to hot corrosion. This improvement was attributed to the formation of Al₂O₃ as a protective oxide layer, as evidenced by XRD patterns. However, the iron containing phases produced as a result of interactions with the substrate were found to be a detrimental factor influencing the corrosion properties of different cladded layers.     

The dependence of the lower yield strength in iron and steel on grain size and temperature

Experimental results obtained from the literature on the dependence of the lower yield strength on grain size and temperature in α-Fe at temperatures between 90° and 373°K are shown to substantiate the validity of the relation, σ _aF = σ _i +V _c/NθKl ²)^1/m . Values ofm, the dislocation velocity-stress exponent, and its temperature dependence show reasonable agreement with results obtained from stress relaxation, double strain-rate cyclic and premacroyield tests in polycrystalline iron and Ti-stabilized iron after homogeneous and inhomogeneous deformation. An apparent activation enthalpy of 0.73 ev was determined and an enthalpy of 0.86 ev was computed which agree well with values reported for homogeneous deformation in α-Fe. The results obtained indicate that the dislocation mechanisms controlling inhomogeneous and homogeneous deformation in α-Fe at low temperatures are the same. As temperature is lowered, the marked increase in lower yield strength in α-Fe is related to a strong decrease in θ, the average dislocation velocity at unit effective stress.     

Aqueous slurry erosion in some cobalt base superalloys

An orifice erosion test was used to study the influence of metallurgical variables in a series of cobalt base superalloys on their resistance to erosion by aqueous slurries of SiO₂. A slurry of 30 pct solids by weight of 4.5 μm silica was used to erode a variety of microstructures obtained by compositional and processing control. The time de-pendence of the pressure drop across an orifice (P) was found to follow the relationP = Kt^-@#@ n wheret is time (min), andK andn are constants. The erosion exponent,n, varied from 0.052 to 0.148 and was found to be dependent upon carbon content and processing variables.     

The chemical diffusivity of oxygen in liquid iron oxide and a calcium ferrite

Measurements have been made of the chemical diffusion coefficient of oxygen in liquid iron oxide at temperatures from 1673 to 1888 K and in a calcium ferrite (Fe/Ca = 2.57) at temperatures from 1573 to 1873 K. A gravimetric method was used to measure the oxygen uptake during the oxidation of the melts by oxygen or CO₂-CO mixtures. The rate was shown to be controlled by mass transfer in the liquid melt. The chemical diffusivity of oxygen in liquid iron oxide at oxygen potential between air and oxygen was found to be 4.2±0.3 × 10⁻³ cm²/s at 1888 K. That in iron oxide at oxidation state close to iron saturation was established to be given by the empirical expression log D=−6220/T + 1.12 for temperatures between 1673 and 1773 K. For the calcium ferrite (Fe/Ca=2.57) at oxygen potential between air and oxygen, the diffusivity of oxygen was found to be given by log D=−1760/T−1.31 for temperatures between 1673 and 1873 K. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

INFLUENCE OF IRON POWDER TYPE ON THE PROPERTIES OF SINTERED IRON AT VARIOUS DENSITY LEVELS

Abstract

The properties of various commercial and experimental iron powder types and of compacts made from them in the density range 6·8–7·87 kg/dm³ by single-pressing, double-pressing, and hot-forging techniques have been determined. It was shown that the ductility in all cases was more adversely affected than the tensile strength by the presence of porosity. However, it was also shown that at any particular density level or with a given processing schedule the mechanical properties varied widely, depending on the iron powder used. On the basis of the mechanical-property results, the powder types to be preferred at different density levels are indicated.     

Distribution of lead between copper and matte and the activity of PbS in copper-saturated mattes

The distribution coefficients of lead between copper and matte phases have been measured at 1405 and 1505 K and correlated with lead and iron concentrations in metallic copper and matte. Experiments covered the ranges of 0.009 to 1.8 and 0 to 11 wt pct lead and iron in matte, respectively. Distribution coefficients of lead between copper and iron-free matte, at infinite dilution of lead, have been determined to be 3.60 and 4.06 at 1505 and 1405 K, respectively. From the measured values of distribution coefficients, the Raoultian activity coefficients of PbS in copper-saturated matte have been calculated. At the temperatures and matte compositions investigated, activities of PbS deviate negatively from Temkin's ideality. At infinite dilution, the following equation correlating the Raoultian activity coefficients of PbS, , with matte composition and temperature has been obtained: In , whereN _FeS , is the mole fraction of FeS in the Cu₂S-FeS-PbS matte. The present values of γ _PbS ^o must be used exclusively in conjunction with the following Gibbs free energy of formation: Pb(l) + 1/2S₂(g) = PbS(l); ΔGℴ = −26730 + 12.20T (cal/mol). M. NAGAMORI, Formerly Associate Professor of Metallurgy at The University of Utah,     

Effect of the bubble size and chemical reactions on slag foaming

Slag foams have been investigated with smaller bubbles than those used in the previous studies.^[5,6,7] The bubbles were generated by argon gas injection with the nozzle of multiple small orifices and by the slag/metal interfacial reaction of FeO in the slag with carbon in the liquid iron. The foam stability in terms of the foam index for a bath-smelting type of slag (CaO-SiO₂-Al₂O₃-FeO) was determined for different bubble sizes. The average diameter of bubbles in the foam was measured by an X-ray video technique. When the foam was generated by the slag/metal interfacial reaction at 1450 °C, it was found that the average bubble diameter varied from less than 1 to more than 5 mm as a function of the sulfur activity in the carbon-saturated liquid iron. The foam index was found to be inversely proportional to the average bubble diameter. A general correlation is obtained by dimensional analysis in order to predict the foam index from the physical properties of the liquid slag and the average size of the gas bubbles in the foam. Formerly Graduate Student and Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University.