The behavior of zinc flow in the zinc bath plays an important role in hot galvanizing process, which has an important influence on the temperature distribution, the composition of zinc coat, the control of air knife, and so on, thus affecting the surface quality of zinc products (surface oxidation, rake slag). However, due to the high temperature, strong activity, opacification of the zinc bath and harsh, complex industrial environment, it is difficult to directly measure the flow behavior of zinc in the zinc bath through conventional methods. In this work, based on the principle of electromagnetic induction, Lorentz force velocimetry (LFV) method was used to measure and analyze the velocity of zinc flow in the bath during the galvanizing process for the first time. The LFV has the characteristics and advantages of non-contact, online and continuous measurement, and can realize the real-time quantitative measurement of molten metal flow by reasonable design and ingenious implementation. The key parameters of LFV, such as the gap between device and molten zinc, penetration depth and geometry of the applied model, were discussed through numerical analysis, the LFV device suitable for the characteristics of zinc plating process was designed, and the in-plant measurement was carried out. The results show that the fluctuation range of zinc flow velocity in the zinc bath is almost 0.13~0.20 m/s, which is within typical range referenced in previous studies. In addition, the flow behavior and flow field characteristics of zinc liquid were analyzed, and these discussions reflect the capacity of zinc slag or ash in the zinc flow at the monitoring position. The work promoted in this study revealed that this LFV method can measure the surface velocity of zinc liquid in real time, on-line and quantitatively, which provides a new way for the velocity monitoring of high temperature liquid metal in metallurgical industry.
The low-cycle fatigue (LCF) behavior of 1100 MPa grade tempered high strength steel under symmetrical strain control conditions was studied at the strain amplitude ranges of 0.4%~1.2% in this work. The LCF properties of quenching and tempering high strength steel were examined by means of OM, SEM and TEM. The microstructure changes, fracture morphology, crack propagation characteristics and inclusion morphology were studied in detail. The results show that the cyclic hardening and cyclic softening depend on strain amplitude. At the low strain amplitude of 0.4%, rapid cyclic hardening occurs in initial 10 cyc, and then the stress remains almost unchanged until the sample breaks. While at the strain amplitude ranges of 0.5%~1.2%, the cyclic hardening reaches a peak at the first few cycles, followed by the remarkable cyclic softening until the sample fails. The cyclic softening is mainly due to the recovery of some martensite lath under low-cycle fatigue loading and the decrease of dislocation density in the laths. 1100 MPa grade high-strength steel is found to obey LCF Manson-Coffin relationship. The high-strength steel has excellent LCF performance for two main reasons, which is related with the shape and size of inclusions. One is that the shape of the inclusion is nearly circular, and the diameter is 2~5 μm, which is lower than the critical dimension of the inclusions causing fatigue crack initiation. The crack is initiated on the surface of the sample. This increases fatigue crack initiation life. The other one is that the original austenite grain boundary, the martensite packet/block boundary and the inclusions or cavities can induce the crack deflection, reducing the crack propagation rate and increasing fatigue crack propagation life.
There are different crystal structures and stacking fault energies (SFEs) for two phases of duplex stainless steel (DSS), and the Mn substitution for Ni also can change SFE of two phases and cause austenite stability variation during high temperature deformation. Thus, the thermal deformation behavior of DSS with Mn addition become more complex compared with that of single phase steel during high temperature tensile process. In this work, the high temperature tensile behavior of 23%Cr low nickel type DSS with different Mn contents (6.26%~14.13%, mass fraction) has been studied in the temperature of 300~1050 ℃ at strain rate of 0.01 s-1 by using a thermal simulation machine. The results showed that the austenite phases mainly accommodate tensile deformation stress, and the volume fraction of them increased with increasing Mn contents, which is beneficial to enhance the thermoplasticity, and has little effect on the tensile strength. With more Mn addition, the reduction of area increases when deformed in the temperature of 550~1050 ℃, but decreases at lower temperature of 300 ℃. The value of crack sensitive point increased slightly when stretched at lower temperature (450 ℃, 750 ℃) with more Mn addition, and optimum plastic temperature zones are in the range of 500~650 ℃ and 850~1050 ℃. The effect of Mn addition on work hardening rate is slight when deformed at 300 ℃, while high Mn addition is favorable for dynamic recrystallization occurence at lower strain when deformed at higher temperature of 1050 ℃. The tensile deformation microstructure of different Mn addition samples are mainly dependent on the austenite dislocations evolution. As the Mn content attained 14.13%, a large number of dislocation cells with high density and small size formed in austenite phase, which is contributed to grains refinement and improves thermoplasticity.
The addition of Al and Ni to ferritic heat resistant steel can improve its high temperature strength and high temperature oxidation resistance. Al and Ni in steel have some effects on the thermal deformation behavior of steel. But there is less research on it. In this work, a new type of 1Cr9Al(1~3)Ni(1~ 7)WVNbB high aluminum ferrite heat-resistant steel was prepared by adding Al element to T92 steel and adjusting Ni content properly. Gleeble-3800 thermal simulation test machine was used to conduct isothermal and constant speed thermal compression experiment with 60% deformation at 950~1150 ℃ and 0.1~10 s-1 strain rates, respectively. The effects of Al and Ni additions on the hot deformation behavior, peak stress and activation energy of hot deformation of the steel were studied. The rheological stress expression containing Zener-Hollomon parameters were obtained by fitting, and the constitutive equation of the test steel was established. The results show that the addition of Al and the increase of Al content significantly reduces rheological stress and peak stress under thermal compression, which greatly reduces the difficulty of test steel processing. Compared with T92 steel, the thermal deformation activation energies of four sample groups increase by 38.136%, 19.188%, 28.003%, and 11.915%, respectively. The peak stress calculated by the constitutive equation is in good agreement with the experimental data. The results show that the constitutive equation has high accuracy. The relationship between peak flow stress, deformation temperature and strain rate can be expressed well.
Directional solidification (DS) has been widely used to produce aero-engine and gas turbine blades of nickel-based superalloys. The preferred crystallographic orientation of nickel-based superalloys is [001], so the [001] columnar-grain structure can form after DS. Due to the low Young's modulus and the elimination of transverse grain boundaries, the [001] columnar-grain structure has beneficial mechanical behavior. The competitive grain growth dominates the production of columnar grains. There are two views about competitive grain growth, which are consistent for diverging grains but not consistent for converging grains. In the case of convergence of the first view, the grain boundary (GB) was parallel to the favorably aligned dendrites, which indicates that the favorably aligned grain cannot be eliminated. For converging grains of the second view, not only the favorably aligned dendrites could block unfavorably aligned ones, but also the unfavorably aligned dendrites could block favorably aligned ones. Thus, the converging grain boundary moved from unfavorably aligned grain to favorably aligned grain. Finally, the favorably aligned grain may be eliminated. The study about the two views was carried out in the case of the same secondary orientation but did not taken into account the secondary orientation. Up to now, the literatures about the effect of secondary dendrite orientation on competitive growth is rarely and their views contradict with each other. In this work, the bi-crystal and ter-crystal plates with different secondary orientations were produced to study the influence of secondary orientation on competitive grain growth. For the bi-crystal with the same primary orientation, as the secondary GB angle increased, the GB was nearly at the middle of the plate sample, which indicated that the competitive grain growth was weak and could be neglected. For the ter-crystal with different primary orientations, not the secondary orientation but the primary orientation could obviously affect competitive grain growth. In the case of converging grains, the change of secondary dendrite orientation had no effect on the competitive growth behavior and grain growth rate; the favorably and unfavorably aligned dendrites could block each other, which disagreed with Walton-Chalmers model and in good agreement with the results of Zhou. In the case of diverging grains, the result agreed with Walton-Chalmers model and Zhou's result.
β solidifying γ-TiAl alloys are being considered for high-temperature application in the aerospace and automotive industries as high efficiency materials which can withstand temperatures up to 800 ℃ and owns attractively thermal and mechanical properties. Through thermos-mechanical process can obtain excellent alloy properties, such as high strength and better elongation. But it will cause anisotropy. Ti-43.5Al-4Nb-1Mo-0.1B alloy rectangular bar was prepared by isothermal hot canned extrusion process. The OM, SEM, XRD, TEM and tensile methods were used to study the microstructure and tensile properties of the rectangular rods in different states and locations. The results show that the extruded structure of the rectangular rods is relatively uniform and there is no significant difference in the microstructure at different locations. The extrusion deformation makes the orientation of the lamellar uniform, tending to be parallel to the extrusion direction; γ phase in the grain boundary exists in the three forms of graininess, bulk and strip; the β phase is shredded during extrusion and is elongated in a parallel extrusion direction. Under the TEM observation, lamellar at the edge of the bar was completely shredded, and lamellar at the core position was elongated after lamellar was broken. A large number of ω0 phases are generated in the β0 phase, and the phase relationship of the two follows: [111ˉ]β0//[0001]ω0, {110}β0//{21ˉ1ˉ0}ω0. The tensile strength reaches 1000 MPa or more and elongations are about 0.5% of the rectangular bar at room temperature; the yield strength is above 400 MPa at 800 ℃, which exhibits remarkable plasticity. After the ageing treatment of the hot extruded alloy, a large amount of lens-shape γ phase is formed in the β0 phase, and the ageing treatment improves the high temperature tensile properties of the alloy, but the ω0 phase can not be eliminated.
Alloy GH4975 is a newly developed hard-deformed Ni-based superalloy which can keep high performance at elevated temperatures. And it is expected to be applied above 850 ℃. The as-cast microstructure, hot deformation of as-cast alloy, and the microstructural evolution during homogenization of alloy GH4975 were investigated utilizing a combination of FESEM, EBSD and extractive phase analysis. The results show that the γ′ phase, primary MC carbide and eutectic phase are the main precipitates in the as-cast alloy. Alloying elements Ti, Nb and W exhibit severe microsegregation during solidification. Cracking phenomenon can be observed in the hot-deformed samples of as-cast alloy due to the incoordination deformation between matrix and the MC carbide, primary coarse γ′ phase and eutectic phase. Microsegregation of alloying elements is eliminated after heat treated at 1180 ℃ for 50 h. Furthermore, besides of the redissolution of eutectic phase, the morphologies and size of MC carbide also evolved during homogenization process. Thermoplasticity and deformability can be improved obviously after homogenization due to improvement of the coordinated deformation capacity of MC carbide and strengthening phase.
In order to reduce the weight of car body, Al-Mg-Si-Cu alloys have been widely studied and used to produce outer body panels of automobiles due to their favorable high-strength-to-weight ratio, recyclability and good formability. Moreover, the strength of Al-Mg-Si-Cu series alloys can be enhanced by bake hardening treatment. However, their formability and final strengths still need to be further improved compared to steels, which are the major obstacles to wide-scale application of Al alloys in the automotive fields. In this work, the effect of ageing routes on the precipitation behavior of Al-Mg-Si-Cu-Zn alloy with high Mg/Si ratio was systematically studied by DSC, TEM, tensile and hardness tests. The results show that the precipitation activation energies of the β″ phase formed in the non-isothermal ageing processes of the as-quenched and pre-aged alloys are 80.1 kJ/mol and 64.5 kJ/mol, respectively; both the different age hardening rates and microstructure evolution behaviors can be also observed during their isothermal ageing processes, such as, the age hardening rate of as-quenched alloy is much higher, but the peak hardness and strength values of the alloy treated by the two routes were basically the same. However, the elongation and strain hardening rate of the pre-aged alloy in the peak ageing state, and the hardness reduction rate in the over-ageing state are all much higher. In addition, a large number of complex solute clusters can be formed during pre-ageing, which will further grow in high temperature isothermal ageing process, but the sizes of precipitates formed in the under-ageing, peak-ageing and over-ageing states all show a multi-scale characteristics; in comparison, this feature cannot be found in the precipitates formed in the as-quenched alloy aged in the different conditions. The ageing routes cannot change the precipitation sequence of the alloy, but give a significant influence on the nucleation and growth rates of precipitates. As a consequence, a schematic diagram of nucleation and growth process of precipitates in the alloy with ageing routes is proposed.
Ultra-high strength Al-Zn-Mg-Cu alloy is a promising lightweight structural material, and there is still room to improve its mechanical property. As a typical precipitation strengthening material, controlling the size and distribution of precipitate is an effective way to enhance the mechanical properties of the ultra-high strength Al-Zn-Mg-Cu alloy. The influence of pre-deformation on the microstructures and properties of the ultra-high strength Al-Zn-Mg-Cu alloy after ageing treatment was studied by TEM, XRD, SEM, DSC and tensile tests. The microstructures and the tensile mechanical properties of Al-Zn-Mg-Cu alloy without pre-deformation and with 3% and 4% pre-deformation were compared. It is found that the pre-deformation can promote the ageing precipitation rate and enhance the precipitate density in the Al-Zn-Mg-Cu alloy, and the pre-deformation ratio of 3% can promote the dispersion of the precipitate phase in the grain interiors, but the pre-deformation ratio of 4% may result in coarsening of precipitate. The size of precipitate along the grain boundaries and the width of precipitation free zones decreased in the pre-deformation treated ultra-high strength Al-Zn-Mg-Cu alloys. The tensile strength and yield strength of the pre-deformation treated ultra-high strength Al-Zn-Mg-Cu alloys increased, and the elongation also increased slightly, in which the tensile strength and elongation of 3% pre-deformation alloy combined with 80 ℃ for 12 h and 120 ℃ for 8 h ageing were (813±4) MPa and 10.10%±0.77%, respectively. The results show that the dislocations produced by pre-deformation may provide more heterogeneous nucleation sites for the precipitate formation, and improve the precipitate's distribution.
WC-Co cemented carbides, consisted of hard phase WC and ductile phase γ-phase, are usually prepared by a powder metallurgy and liquid phase sintering methodology. Due to the combined properties of high hardness and toughness, cemented carbides have high wear resistance and are widely used as machining, cutting, drilling, mining and forming tools. When the grain size of WC phase in WC-Co alloy is reduced to submicron, the hardness, toughness and strength of the material can be improved. TaC was considered as an effective additive of WC-Co based tools, for it made a great contribution to the enhancement of mechanical properties of WC-Co alloy. In this work, the effect of TaC on the microstructure and mechanical properties of WC-TiC-TaC-Co cemented carbide were investigated by means of SEM, EDS, three-point bending apparatus and hardness tester. The results show that WC-TiC-TaC-Co cemented carbide is mainly composed of three phases: WC phase, (W, Ti, Ta)C phase and γ phase. With the increase of TaC content from 4.6% (mass fraciton) to 7.3%, the proportion of WC grains with the size of less than 0.5 μm increases; the proportion of (W, Ti, Ta)C grains with the size of larger than 1 μm increases, and the (W, Ti, Ta)C grains begin to aggregate; the density of the alloy first decreases then increases and decreases, and the variation tendencies of hardness and fracture toughness are consistent with the density; the transverse rupture strength of the alloy first increases and then decreases. WC-TiC-TaC-Co cemented carbide with 6.3% TaC shows the best mechanical properties: the hardness, fracture toughness and transverse rupture strength are 1749 HV30, 10.2 MPa·m1/2and 2247 MPa, respectively.
The (Ti, W)C1-x composite coatings were prepared on the surface of Ti-6Al-4V (TC4) alloy by high energy electron beam cladding technology using WC-10Co powder. The microstructure and phase composition of the composite coatings under different cladding currents were analyzed by SEM, EPMA and XRD, and the formation mechanism of each phase was discussed in detail. The microhardness and friction property of the composite coatings were analyzed by microhardness tester and ball-disk friction test equipment, and the friction mechanism of the composite coatings under different cladding currents was discussed. The results show that the WC powders in the three composite coatings were completely dissolved. The coating consists of α-Ti, β-Ti, dendritic and block (Ti, W)C1-x, and a small amount of W. The thickness of the coatings ranges from 400 to 600 μm, and the adhesion between the coatings and the substrate was good. Compared with the substrate, the average hardness and wear resistance of the composite coatings increased by 2~3 times and decreased with the increase of cladding current. The surface microhardness was up to 860 HV at the cladding current of 12 mA. In addition, the friction mechanism was abrasive wear at 12 mA and it became severer at 15 mA; at the cladding current of 18 mA, a little fatigue wear was also proved.
The W-Cu alloy has been widely applied in metallurgy, electronics, military and other fields because of its good arc-resistance, anti-welding, heat and electricity conducting etc. In the recent years, attention to the immiscible W-Cu alloy has been shifted to the problem of stabilizing the W/Cu interface by alloying. However, there are still research lacks of the mechanisms of diffusion, segregation of alloying elements in this alloy. It, obviously, will limit the further optimizing design for the W-Cu alloy. This work is focused on the first-principle study of the electronic structure of W/Cu interfaces. Calculations showed that the same alloying elements in W-Cu system may have significant differences in grain boundary segregation and interface segregation behavior, and related micromechanism was revealed. It was demonstrated that the relationship of the segregation energies of Sc, Ti, Y and In into W/Cu interfaces and grain boundaries of pure W and Cu were related to their stability. The correlation between segregation energy and interface stability was also disclosed by the first-principle interface calculation for W-Sc and W-Y systems. Further, combined with the solute segregation calculations for the W/Cu interfaces, W grain boundaries, Cu grain boundaries and the formation energy for the Cu solid solution, the criterion for solute optimizing selection for the W-Cu system was proposed. According to which, Y was selected as the candidate alloying element to stabilize the W/Cu interface. This work proposed a more universal method for the optimal alloying element selection and may provide a new design method for the development of high-performance W-Cu alloy.
The stability of the magnetohydrodynamics (MHD) of aluminum reduction cell is determined by the bath-metal two-phase flow field. So, konwing how to optimize the metal flow field and restrain the bath/metal interface deformation is the key to maintain the stable and efficient operation of cell. Many previous works on the bath-metal flow field are based on the static electromagnetic force stirring the melt, however, it should be have some deviation from the actual cell state. A three dimensional bath-metal two-phase quasi-steady flow model (based on transient electromagnetic force) for full 500 kA aluminum reduction cell was built by means of numerical simulation in this work, and validated by metal velocity and bath/metal interface deformation measurement in industrial cells. The effects of 60% increase of local cathode current on melt flow distribution and interface deformation were simulated and evaluated according to abnormal 6 cases in realistic electrolytic process. It was found that the increase of local cathode current has little effects on the general pattern of flow field and interface deformation in cell, but the amplitude of local metal velocity and interface deformation would be changed in certain extent. The increase of local cathode current in A2~A3 could decrease the interface height in middle cell of downstream side (side B), with anode cathode distance (ACD) increasing by 3.0%. But the other 5 cases could deteriorate the low ACD zone further in side B, especially the increase of local cathode current in A10A11, with average ACD decreasing by 4.6% in B12~B20. The solution is to cut cathode flexes partially in abnormal position to decrease the effect on the bath-metal two-phase flow. According to the evaluation results, it is found that the uneven distribution of cathode current may be helpful to decrease the interface deformation and improve the MHD stability of cell. Based on this finding, the bath-metal two-phase flow field was changed by increasing the proportion of cathode current at the two ends of cell, the middle part of cell and side A and side B respectively, and then was analyzed in this work. The simulation results show that it is beneficial to restrain the interface deformation by increasing the cathode current at both ends of cell properly, and it is also helpful to solve the cooling problem at cell ends. In particular, when the cathode currents at A1~A4 and A21~A24 increase by 28%, the distribution trend of melt flow field remains unchanged basically, and the maximum of metal velocity under A19~A20 increases by 10%, and the maximum of interface height decreases by 2.4 mm, and the average of ACD under B7~B18 increases by 9.5%. It provides a valuable reference for optimizing the busbar design and improving the cell MHD stability.
