High silicon electrical steel (Fe-6.5%Si alloy, mass fraction) has excellent soft magnetic properties. However, the alloy is very brittle at room temperature and quite hard to be fabricated into cold-rolled sheet by conventional rolling process due to the existence of ordered phases. In recent years, high silicon electrical steel sheet has been successfully obtained though rolling process, and many studies have focused on the recrystallization of the alloy sheet in order to optimize the magnetic properties. Furthermore, it is necessary to further investigate the plasticity of recrystallized high silicon electrical steel sheet for improving the subsequent plastic deformation, such as coiling, uncoiling, blanking and secondary cold-rolling. In this study, effects of recrystallization on the microstructure, ordering, mechanical properties and cold-rolling workability of cold-rolled high silicon electrical steel samples were investigated by using SEM, TEM, EBSD, bending test, tensile test and cold-rolling. The results show that when the cold-rolled samples were recrystallized at 800~1200 ℃ for 1 h followed by furnace-cooling, the plasticity of the sample is sharply decreased, which is proved by the decrease of bending angles from about 150° to 50° and the occurrence of serious edge cracks after secondary cold-rolling. The plasticity of the recrystallized sample is significantly improved by increasing both the cooling temperature and cooling rate during the recrystallization. When the cold-rolled samples were recrystallized at 1000 ℃ for 1 h followed by oil-quenching from 900 ℃, the bending angles are increased to about 175°, the average elongation to failure are increased from 0.2% of furnace-cooling sample to 5.2%, and the secondary cold-rolling edge cracks are suppressed effectively. The plasticity improvement can be attributed to the refinement of ordered domain during recrystallization annealing with high cooling temperature and high cooling rate. For instance, the size of ordered domain in the sample by oil-quenching at 600 ℃ or below is about 5 μm, while the sizes in the samples by oil-quenching from 700 ℃ and 900 ℃ are reduced to less than 50 nm and 25 nm, respectively.
In recent years, the development of aerospace industry puts forward a higher requirement for the high strength, high toughness and good weldability of steel materials. Cr12Ni4Mo2Co14 steel is a new kind of ultrahigh strength steel, which is usually used as structure materials under complex stress/strain conditions. In this work, the stress corrosion cracking behavior of Cr12Ni4Mo2Co14 steel in acid environment was studied by slow-strain rate test (SSRT) and SEM. The results showed that the stress corrosion cracking (SCC) susceptibility of Cr12Ni4Mo2Co14 steel was enhanced by chlorine ions remarkably. The critical value of chlorine ion concentration was about 0.15%, above which, severe SCC occurred induced by chlorine ions, and the SCC cracks started from pits. The SCC mechanism of Cr12Ni4Mo2Co14 steel in acid solution containing chlorine ions can be clarified by “slide-film breaking” model. The hydrogen-charged samples of Cr12Ni4Mo2Co14 steel were sensitive to hydrogen induced cracking, and second cracks occurred on the fracture surface. The critical value of hydrogen-charged current density is 10 mA/cm2 for 30 min in solutions with pH=5, above which, the losses of strength and plasticity of Cr12Ni4Mo2Co14 steel reached to the maximum. The effects of chlorine ions and hydrogen had a complex interaction which cannot be added up directly.
The liquid-liquid phase separation has been used recently to design two-glassy-phase alloys with desirable mechanical, magnetic and thermal properties. The occurrence of the liquid-liquid phase separation in the Zr-Ce binary immiscible alloys can lead to the formation of two coexistent crystalline Zr-rich and Ce-rich phases after complete solidification. In this work, a new quaternary complex alloy system (ZraCeb)(1-x)(CocCud)x was designed on the basis of the addition of metastable Co-Cu immiscible alloys in the stable Zr-Ce immiscible alloys. Distribution ratio of Co and Cu in two coexistent liquids was calculated. The mechanisms of phase formation and microstructure evolution were investigated using OM, SEM, EDS, XRD and DTA. The results show that a single-phase homogeneous melt of (ZraCeb)(1-x)(CocCud)x alloys takes place the liquid-liquid phase separation during cooling through the miscibility gap. The metal elements Co and Cu are mainly concentrated in the Zr-rich and Ce-rich liquids, respectively, which results in the formation of the two coexistent Zr-Co-rich and Ce-Cu-rich liquids. It was found that the coexistent Zr-Co-rich and Ce-Cu-rich liquids undergo liquid-to-glass transition and thus form dual glassy phases under the rapid quenching, respectively. The effects of the atomic ratio of Co and Cu, the addition amount and the cooling rate on the formation of the glassy phases have been discussed in detail by combining the experimental investigation with the thermodynamic analysis. A strategy for synthesizing liquid-phase-separated metallic glasses on the basis of suitably designed immiscible alloys has been proposed. Two-glassy-phase alloys can be obtained by rapidly quenching alloy melt in which the atomic ratio of Co and Cu and the addition amount are 4∶1 and 38.5%, respectively.
Aluminum alloy and steel thin sheets have been mostly used in the automotive industry to get a lightweight car body. Nowadays several studies are focused on the joining of aluminum alloy to steel by new welding methods especially by laser welding. In this work dual-beam fiber laser keyhole welding was introduced to joining of 1.5 mm-thick aluminum alloys to 1.8 mm-thick 304 stainless steels in an overlap joint configure. The influences of different laser focusing positions on the weld appearance, interface microstructures and tensile mechanical resistance of the welded joints were studied. As a result, the good weld appearance of the aluminum alloy to stainless steel joints were obtained by dual-beam fiber laser keyhole welding process without any filler materials. The thickness of the intermetallic compound layer of the joint interface is comparatively thin when the laser beam with low energy is focusing on the front. The nano-hardness testing results show that the average hardness of intermetallic compound layer is 9.61 GPa, which is significantly higher than that of the parent stainless steel of 4.12 GPa and aluminum alloy of 1.09 GPa. The fracture of the welded joints occurs on the aluminum alloy/stainless steel interface layer. The highest mechanical resistance of 131 N/mm can be obtained by the low energy laser beam focused on the front.
Friction stir welding (FSW) is a new solid-state joining method which offers several advantages compared with conventional welding methods, including better mechanical properties, lower residual stress and reduced occurrence of defects. It has already been used for joining Al alloys in the aerospace and automotive industries. In spite of the advantages, FSW also has drawbacks, such as the risk of root flaws in single-side welds. Using a bobbin tool instead is a promising way to solve this problem since the root region is avoided. Compared with standard (single-side) FSW techniques, the bobbin tool FSW has an extra shoulder attached to the tip of the probe, namely the lower shoulder. This setup makes BTFSW capable of joining closed profiles like hollow extrusions. Furthermore, root flaws, such as lack of penetration, which occasionally occurred in standard FSWtechiques, can be completely avoided. In this work, 6061-T6 aluminum alloy was welded by using bobbin tool friction stir weld (BTFSW). The influence of BTFSW on the microstructure development and hardness distribution in the weldment has been investigated. The corrosion behaviors of the base metal and weld nugget in 3.5%NaCl (mass fraction) solution were investigated using SEM, XRD and electrochemical measurements. The results showed that the weld surface of 6061-T6 welded by BTFSW is of good quality. No welding defect was detected in the joints. Three microstructural zones, i.e., nugget zone, thermo-mechanically affected zone, and heat affected zone were discernible. The microstructural analysis indicates that the weld nugget region exhibited fine and equiaxed grain structure with an average grain size of ~8 μm, indicating the occurrence of dynamic recrystallization due to severe plastic deformation and thermal exposure. The thermo-mechanically affected zone underwent plastic deformation and recrystallization occured in this zone due to deformation strain and thermal input. The low hardness zone, determined by constructing the hardness distribution profile on cross-section of joint, located at thermo-mechanically affected zone of advancing side. Although 6061-T6 alloys are readily weldable, they suffered from severe softening in the heat affected zone because of the dissolution of Mg2Si precipitates during the weld thermal cycle. BTFSW can improve the corrosion resistance of 6061-T6 aluminum alloy in 3.5%NaCl solution. The corrosion behavior results showed that both anodic dissolution and pitting were observed after the immersion test due to the inhomogeneous microstructure of 6061-T6 aluminum alloy. The corrosion products mainly composed of Al(OH)3 and Al2O3. Furthermore, the corrosion process and mechanism were also discussed.
The prospect of joining titanium and aluminum components into structures is desirable for a wide range of aerospace and automobile industry applications. One of the problems related with the joining processes for dissimilar metals such as Ti and Al is the formation of residual stress in the bonded joint, which has significant effect on the joint mechanical properties. In this work, joining of a titanium alloy to an aluminum alloy by ultrasonic assisted brazing using a Zn-Al filler metal was investigated. The microstructures of the titanium/aluminum brazed joints were determined by OM, SEM and TEM. The local tensile deformation characteristics of the brazed joints were also examined using the digital image correlation (DIC) methodology by mapping the local strain distribution during in situ tensile tests. The results showed that the Ti7Al5Si12 phase and the TiAl3 phase were formed at the titanium/brazing seam interface. The brazing seam was primarily composed of a Zn-rich phase and a Zn-24.14%Al (mass fraction) eutectoid structure. At the aluminum/brazing seam interface, no interfacial reaction layer was observed and the primary phase Zn-Al dendrites nucleated at the aluminum base metal and grew into the inside of the bonding region. A diffusion layer was formed in the aluminum base metal. It was found that the tensile deformation of the brazed joints was highly heterogeneous, which led to the deflection of the crack during propagating in the joint. The fracture initiated at the Zn-rich phases, where contained the highest stress concentration due to their low elastic modulus, and propagated in the Zn-rich phases or through the interface between Zn-rich phase and Zn-Al eutectoid structure.
In recent years, the surface nanocrystallization (SNC) technology has received extensive attentions in the field of metal material. The shot peening and surface mechanical rolling processing technology can form the gradient nanostructured (GNS) layer on the surface of metal. The material surface roughness is large generally. Therefore, the problem how to form the thick, smooth, flawless GNS layer is need to solve urgently. By means of the hybrid surface nanocrystallization (HSNC) method of both supersonic fine particles bombarding (SFPB) and surface mechanical rolling treatment (SMRT), a gradient nanostructured surface layer was formed on 2A14 aluminum alloy plate. The electrochemical corrosion behavior of the HSNC sample at the air of room temperature and low temperature liquid nitrogen was compared with that of the original sample in aqueous solution of 3.5%NaCl. The results showed that grain size increases from about 30 nm at the surface layer gradually to coarse grain size of the matrix when the sample was processed by HSNC. The total thickness of the plastic deformation layer is about 130 μm. The surface roughness Ra is about 0.6 μm with the surface microcrack disappeared. Compared to the original sample, the pitting corrosion resistance of the SFPB samples was not improved and the pitting corrosion resistance of the HSNC samples was improved. The self-corrosion potential and pitting corrosion potential increase respectively from -1.01228 and -0.29666 V in the original sample to -0.67445 and 0.026760 V at the air room temperature of the HSNC sample. The pitting corrosion resistance of the HSNC sample at the air of room temperature was the biggest. The analysis showed that the surface GNS grain, significant increase of the nanocrystal boundaries, the introduction of compressive residual stress and the decrease of surface roughness were beneficial to improve the pitting corrosion resistance.
Generally, wear is one of the main failure mechanisms for mold steel. The heavy financial loss will often occur if molds are out of service due to their hard manufacturing process and high cost of metal materials. Therefore, mold repairing is urgent and critical if they fail to function. Ni-matrix wear resistant composited layers reinforced by TiC generated from in situ plasma spray welding NiCrBSi+Ti powders were prepared. The analysis instruments of OM, SEM, XRD and EDS were used to study the microstructural characterization, phase identification and chemical compositions of the layers. And the microhardness and wear resistance were tested using Vickers hardness tester and abrasion tester, respectively. The investigations demonstrated that the layers were mainly composed of basic phases (γ-Ni+β1-Ni3Si) with eutectic features and hypereutectoid (α-Fe+FeNi3) structures, in which hard phases M7C3 and M23C6 were embedded in the matrix. The phase CrB was distributed uniformly in the layers. One part of TiC generated from in situ reaction acted as the nucleation of the chromium compounds precipitates M7C3 and M23C6. The other part of TiC was also distributed in the base with the fine particles (<1 μm) and even bigger size (>1 μm). Ti percentage rising, the microstructures of plasma spray welding layers were refined and the phase M23C6 increased while M7C3 decreased. When the Ti addition reached 6%, the layers had better performance with microhardness of 800 HV0.5. The wear mass loss of layers was 14.5 mg, which were more than 2 times of NiCrBSi layer.
The application of steel in acidic media faces a big challenge due to the corrosion problem. Quaternary Ni-W-Cu-P alloy act as a potential coating material applied to acidic media because of its superior corrosion resistance. However, mechanism of deposition and corrosion of Ni-W-Cu-P coating plated on the surface of steel component is rare in the previous studies. In this work, the Ni-W-Cu-P coatings were deposited onto carbon steel 65Mn substrates via electroless plating. The anti-corrosion properties of the coatings in room and warm acidic solution (20%H2SO4) were evaluated by dipping and electrochemical test, respectively. Their deposition mechanism, composition and structure were investigated using SEM, EDS and XRD, respectively. The results show that the Ni-W-Cu-P coating is composed of spherical and block particles in the early stage of electroless plating, which are gradually transformed into spherical and strip cellular structure with the increasing electroless plating time. With prolonging electroless plating time, the Ni and W contents in the Ni-W-Cu-P coatings increase logarithmically and lineally, respectively. However, the Cu content decreases logarithmically, the P content reaches the maximum value after electroless plating for 60 min and then gradually decreases. The Ni-W-Cu-P coating is amorphous when it is annealed at low temperature, upon increasing the annealing temperature to over 400 ℃, it gradually transforms from amorphous to crystalline. The thermal stability of Ni-W-Cu-P coating can be significantly improved by co-depositing tungsten and copper element. Corrosion resistance of the amorphous coating annealed at 400 ℃ is better than that of amorphous coating as-plated and nanometer crystalline coating annealed at 500 ℃ in both room and warm acid solution. As-plated coatings and those annealed at 400 ℃ are found to corrode selectively, while pitting is observed to be the main corrosion mechanism of coatings annealed at 500 ℃. With increasing the corrosion time, the corrosion rates and corrosion current densities of the Ni-W-Cu-P coatings increase, however, their impedance values decrease.
High velocity oxygen fuel (HVOF) sprayed WC-Co coating has been widely used in the surface protection of components for excellent corrosion resistance and wear resistance. However, with the increasing deteriorated service environment, higher comprehensive properties of WC-Co coating are required. Addition of rare earth elements into WC-Co powder is expected to be an effective way. In this work, the micro WC-12Co, nano modified WC-12Co and CeO2 modified WC-12Co coatings were prepared by HVOF on the Q345 steel substrate. The microstructure, corrosion morphology and phase structure of coatings were observed by SEM and XRD, and the micro-hardness is measured. The corrosion behavior of the coatings in 1 mol/L H2SO4 solution was investigated by polarization test and immersion corrosion test. The results show that the addition of nano-sized CeO2 in the WC-12Co coating not only purifies the grain boundary and increases the micro hardness, but also significantly reduces the porosity of the coating, which can effectively decrease the occurrence of local corrosion. Meanwhile, the addition of nano CeO2 can make the electrode potential of coatings shift positively, reduce the corrosion current density and passivation current density, and then improve the corrosion resistance of the coating. The corrosion mechanism of nano CeO2 modified WC-12Co coating is local corrosion which induced by the pore. Co bonding phase at the pore is constantly being corroded, causing WC particles to lose the support function and to fall off, which promotes the corrosion of the coating, so that the pores are enlarged to form corrosion pits. For the micro WC-12Co coating and nano modified WC-12Co coating, not only the outermost layer of the Co bonding phase is corroded, but also serious local corrosion occurred in the pores.
Austenite-to-ferrite transformation in modern steels is a key metallurgical phenomenon as it can be exploited to produce microstructures that are closely associated with significant improvement of their properties. Both experimental and theoretical studies of this transformation have received much attention. In particular, in recent years, considerable efforts have been directed to the development of numerical models for adequate quantitative descriptions of the nucleation and growth of ferrite grains as well as the overall transformation kinetics. In this work, a modified multi-phase field model has been developed to simulate the isothermal γ-α transformation in a Fe-C alloy. This model takes both the effect of a finite interface mobility and a finite diffusivity into account, which hence enables a clear description of the mixed-mode nature of the transformation. In contrast to the diffusion-controlled phase transformation model, the carbon concentration in front of the moving γ-α interface is found to be non-equilibrium under this circumstance. In order to study the microstructural behavior and kinetics over the entire temperature range of the phase transformation, three different isothermal transformation processes have been imulated. The simulation results indicate that the nucleation density of ferrite increases with decreasing the temperature, which thus leads to a larger volume fraction of ferrite. However, the heterogeneous distribution of carbon in the untransformed austenite is intensified. The final microstructural product of the transformation at low temperature of 1010 K consists of fine residual austenite islands surrounded by fine polygonal ferrite. The simulation results also indicate that the transformation mode from austenite to ferrite varies from essentially diffusion-controlled at high temperature towards interface-controlled at low temperature.
Fe-based amorphous alloys are well known for their good magnetic properties including ultrahigh saturation magnetization, low coercive force, high magnetic permeability and low core loss. But these alloys were only prepared into ribbon form in early times due to their insufficient glass-forming abilities (GFAs). The present work focuses on the design of Fe-B-Si-Nb bulk metallic glasses with good soft magnetic properties and high glass-forming ability. Glass formation in Fe-B system is first considered with cluster-plus-glue-atom model. A basic composition formula [B-B2Fe8]Fe is proposed as the framework for multi-component alloy design. Considering the structural stability of the model glass, Si and Nb are introduced to the [B-B2Fe8] cluster to replace the center B and shell Fe atoms, from which a series of Fe-B-Si-Nb alloys with composition formulas [Si-B2Fe8-xNbx]Fe (x=0.1~1.2) are derived. Copper mold casting experiments revealed that bulk glass alloys with a critical diameter (dc) exceeding 1.0 mm are readily obtained with the Nb content range of x=0.2~1.2, the largest dc (about 2.5 mm) appears in the vicinity of x=0.4~0.5. Considering the local packing efficiency of Fe-B-Si-Nb glass model structure, another series alloy compositions, namely, [(Si1-yBy)-B2Fe8-xNbx]Fe is reached by increasing Nb and decreasing Si simultaneously in [Si-B2Fe7.6Nb0.4]Fe basal glass alloys. The experimental results show that bulk glass alloys with dc=2.5 mm are available over a wide range of compositions from (x=0.5, y=0.05) to (x=0.9, y=0.25). Excellent magnetic softness with high saturation magnetizations (Bs=1.14~1.46 T) and low coercive forces (Hc=1.6~6.7 A/m) is found in the [Si-B2Fe8-xNbx]Fe (x=0.2~0.6) series glass alloys. A high fracture strength of 4220 MPa with a plasticity of 0.5% is observed in the [(Si0.95B0.05)-B2Fe7.5Nb0.5]Fe bulk glass alloy.
Plasma arc welding (PAW) is an important joining technology for plates with medium thickness because of the heat source characteristics, however, most models of PAW neglect the vaporization of metal. An axisymmetrical unified PAW model was developed by taking into account the influence of Fe vapor behavior from the molten pool surface as an anode in this work. The simulation region includes tungsten cathode, plasma arc, weld pool, keyhole and their self-consistence coupling using one set conservation equations. A viscosity approximation is used to express the diffusion coefficient in terms of the viscosities of iron vapor. The main physical properties of Ar plasma are set as function of temperature and mass fraction of Fe vapor and are updated every iterate step to reflect the influence of Fe vapor in real time. The process of keyhole formation in stationary plasma arc welding is simulated under welding currents of 150, 170 and 190 A. The transient production, diffusion and concentration in the plasma arc of Fe vapor were presented. The effects of Fe vapor on the plasma arc behavior and formation of weld pool and keyhole are studied. It was shown that the evaporation rate of Fe was greatly dependent on the temperature of the weld pool. Most Fe evaporates from the top part of the keyhole surface and little from the keyhole bottom. The diffusion of Fe vapor is accelerated in the radial direction and is prevented in the axial direction due to the effect of plasma jets flow and at last it tends to be confined to the fringe of the plasma arc closed to the anode. The mixing of Fe vapor in the plasma results in the increase of radiation losses and the decrease of current density of the arc plasma in the fringe, but it had insignificant influence on the arc center. The heat flux from the plasma arc to the anode is also affected by Fe vapor due to its influence on the plasma arc properties. It is found that the calculation result of the width of the molten pool becomes more accurate to consider the effect of Fe vapor.
Cu-Cr-Zr alloy usually applys to the complex environment at high temperature. The mechanical behaviors of alloy are different from the condition of normal temperature. At high temperature, grains and precipitates of ultra-fine grain Cu-Cr-Zr alloy become coarse and it would affect the hot deformation behavior of alloy. To solve the thermal stability of the ultra-fine grain materials, the grain growth mechanism and the driving force of ultra-fine grain materials must be studied, as well as trace elements on the thermal stability mechanism. Tensile properties, microstructure of fracture and fracture mechanism of ultra-fine grain (UFG) Cu-Cr-Zr alloy made by two different treatment methods were studied at the temperature range of room temperature to 600 ℃. The results show that the ultimate tensile strength (UTS) of alloys decreases with increasing temperature. The UTS and elongation of No.1 alloys are about 577.17 MPa and 14.6% at room temperature, respectively. And No.1 alloy start to occur dynamic recrystallization and UTS decreases fast at 300 ℃. The UTS of No.1 alloy are only 59.12 MPa at 600 ℃. The UTS and elongation of No.2 alloy are about 636.71 MPa and 12.1% at room temperature, respectively. The pinning effect by precipitation on grain boundary in the No.2 alloy begins to weaken at 400 ℃. The UTS of No.2 alloy decreases fast and are only 65.20 MPa at 600 ℃. Compared to No.1 alloy, No.2 alloy have better room temperature property and thermal stability. The elongation of all alloys increases with increasing temperature and show superplasticity on elevated temperature. The high temperature tensile fracture morphologies are an intense and deep dimple pattern. The high temperature fracture mechanism is ductile fracture by gathered microporous.
The importance of proper melt flow in continuous casting tundish for production of clean steel was well recognized, more in-depth research melt flow by physical models can promote tundish metallurgy effect and improve the quality of liquid steel. Based on the requirements of Re number and Fr number similar on the same time in the continuous casting water simulation, a synthetical hydraulic simulation platform of continuous casting was established. Flow characteristics with velocity field, vorticity field, RTD in extra thick slab tundish were got by the platform using NI image processing and PIV. The results showed that flow in the tundish had larger turbulent eddies, which formed a similar "funnel" vortex structure in inlet section of the tundish. Flow brought two big circulations in the exit section because of the channel and had a great impact on the side walls. RTD with noncontact measurement was coincident with the results of numerical simulation, longer residence time and larger dead volume showed the necessity of heating for the tundish.
