Radiation heat transfer model for complex superalloy turbine blade in directional solidification process based on finite element method

For the sake of a more accurate shell boundary and calculation of radiation heat transfer in the Directional Solidification(DS) process, a radiation heat transfer model based on the Finite Element Method(FEM)is developed in this study. Key technologies, such as distinguishing boundaries automatically, local matrix and lumped heat capacity matrix, are also stated. In order to analyze the effect of withdrawing rate on DS process,the solidification processes of a complex superalloy turbine blade in the High Rate Solidification(HRS) process with different withdrawing rates are simulated; and by comparing the simulation results, it is found that the most suitable withdrawing rate is determined to be 5.0 mm·min^(-1). Finally, the accuracy and reliability of the radiation heat transfer model are verified, because of the accordance of simulation results with practical process.

Modeling of Heat Transfer and Solidification of Composite Roll

Modeling of heat transfer and solidification of composite roll was established and used to predict the thermal history and solidification process of roll during spray forming. Evolution of temperature field of the preform and cooling rate in the growing deposit during spray deposition and post-deposition were numerically simulated.

Directional solidification and physical properties measurements of the zinc-aluminum eutectic alloy

Zn-5wt% Al eutectic alloy was directionally solidified with different growth rates （5.32-250.0μm/s） at a constant temperature gradient of 8.50 K/mm using a Bridgman-type growth apparatus.The values of eutectic spacing were measured from transverse sections of the samples.The dependences of the eutectic spacing and undercooling on growth rate are determined as λ=9.21V-0.53 and ΔT=0.0245V0.53,respectively.The results obtained in this work were compared with the Jackson-Hunt eutectic theory and the similar experimental results in the literature.Microhardness of directionally solidified samples was also measured by using a microhardness test device.The dependency of the microhardness on growth rate is found as Hv=115.64V0.13.Afterwards,the electrical resistivity （r） of the casting alloy changes from 40×10-9 to 108×10-9 Ω·m with the temperature rising in the range of 300-630 K.The enthalpy of fusion （ΔH） and specific heat （Cp） for the Zn-Al eutectic alloy are calculated to be 113.37 J/g and 0.309 J/（g·K）,respectively by means of differential scanning calorimetry （DSC） from heating trace during the transformation from liquid to solid.

Grain Size Distribution and Interfacial Heat Transfer Coefficient during Solidification of Magnesium Alloys Using High Pressure Die Casting Process

The objective of this study is to predict grain size and heat transfer coefficient at the metal-die interface during high pressure die casting process and solidification of the magnesium alloy AM60. Multiple runs of the commercial casting simulation package, ProCASTTM, were used to model the mold filling and solidification events employing a range of interfacial heat transfer coefficient values. The simulation results were used to estimate the centerline cooling curve at various locations through the casting. The centerline cooling curves, together with the die temperature and the thermodynamic properties of the alloy, were then used as inputs to compute the solution to the Stefan problem of a moving phase boundary, thereby providing the through-thickness cooling curves at each chosen location of the casting, Finally, the local cooling rate was used to calculate the resulting grain size via previously established relationships. The effects of die temperature, filling time and heat transfer coefficient on the grain structure in skin region and core region were quantitatively characterized. It was observed that the grain size of skin region strongly depends on above three factors whereas the grain size of core region shows dependence on the interracial heat transfer coefficient and thickness of the samples. The grain size distribution from surface to center was estimated from the relationship between grain size and the predicted cooling rate. The prediction of grain size matches well with experimental results. A comparison of the predicted and experimentally determined grain size profiles enables the determination of the apparent interracial heat transfer coefficient for different locations.

Numerical study of heat transfer of gas-atomized Fe-6.5 %Si （mass and solidification behavior fraction） droplets

During spray atomization process, the heat transfer and solidification of droplets play very important roles for the deposition quality. Due to the difficulties of experimental approach, a numerical model is developed, which integrates liquid undercooling, nucleation recalescence and post-re- calescence growth to present the full solidification process of Fe-6.5%Si （mass fraction） droplet. The droplet velocity, temperature, cooling rate as well as solid fraction profiles are simulated for droplets with different sizes to demonstrate the critical role of the size effect during the solidification process of droplets. The relationship between the simulated cooling rate and the experimentally obtained secondary dendrite arm spacing is in excellent agreement with the well-established formula. The pre-constant and exponent values lie in the range of various rapid solidified Fe-based alloys reported, which indicates the validity of the numerical model.

Study on inner defects of continuously-cast blooms and soft reduction process

At the initial operation stage of the continuous bloom caster at Baosteel, a lot of central porosity and cracks occurred in blooms, especially in the blooms of high-pressure boiler steel. In this study, a model of heat transfer and solidification was set up to analyze the process of continuous bloom casting. Based on the model, the distributions of temperature field and shell thickness were obtained, the mushy zone was defined, and thereby the technology of soft reduction for blooms was developed. After several tests were conducted, a reasonable test plan was developed, which led to the determination of the optimal reduction zone and gauge reduction. Now, central porosity and cracks in blooms are almost avoided and the inner quality has been obviously improved.

A study of microstructure and properties of cast Fe-10Cr-1.5B alloy

In the present study, the microstructure and mechanical properties of cast Fe-10Cr-1.5B（FCB） alloy after different heat treatments were studied. The results showed that the as-cast microstructure of FCB alloy consists of α-Fe, M（M=Cr, Fe, Mn）2（B, C） and M（M=Cr, Fe, Mn）7（C, B）3 type borocarbides, and small amounts of pearlite and austenite. After oil quenching treatment, metal matrix transformed into the martensite from the mixture of martensite, pearlite and austenite. There are many M（M=Cr,Fe,Mn）23（C,B）6 type borocarbide precipitates in the metal matrix, and eutectic borocarbide appears with an apparent disconnection and isolated phenomenon. When the quenching temperature reaches 1,050℃, the hardness of FCB alloy is the highest, but the change of quenching temperature has no obvious effect on impact toughness of FCB alloy. After tempering, the eutectic microstructure of FCB alloy appears with a ＂two links＂ trend. With the increase of tempering temperature, the hardness of FCB alloy decreases gradually and impact toughness increases gradually. Cast FCB alloy oil-quenched from 1,050℃ and tempered from 200℃ has excellent combined properties; its hardness and impact toughness are 61.5 HRC and 8.8 J·cm^-2 respectively.

Generalized constructal optimization for the secondary cooling process of slab continuous casting based on entransy theory

Based on constructal theory and entransy theory,a generalized constructal optimization of a solidification heat transfer process of slab continuous casting for a specified total water flow rate in the secondary cooling zone was carried out.A complex function was taken as the optimization objective to perform the casting.The complex function was composed of the functions of the entransy dissipation and surface temperature gradient of the slab.The optimal water distribution at the sections of the secondary cooling zone were obtained.The effects of the total water flow rate in the secondary cooling zone,casting speed,superheat and water distribution on the generalized constructal optimizations of the secondary cooling process were analyzed.The results show that on comparing the optimization results obtained based on the optimal water distributions of the 8 sections in the secondary cooling zone with those based on the initial ones,the complex function and the functions of the entransy dissipation and surface temperature gradient after optimization decreased by 43.25%,5.90%and 80.60%,respectively.The quality and energy storage of the slab had obviously improved in this case.The complex function,composed of the functions of the entransy dissipation and surface temperature gradient of the slab,was a compromise between the internal and surface temperature gradients of the slab.Essentially,it is also the compromise between energy storage and quality of the slab.The"generalized constructal optimization"based on the minimum complex function can provide an optimal alternative scheme from the point of view of improving energy storage and quality for the parameter design and dynamic operation of the solidification heat transfer process of slab continuous casting.

Multiphase Flow and Thermo-Mechanical Behaviors of Solidifying Shell in Continuous Casting Mold

The metallurgical phenomena occurring in the continuous casting mold have a significant influence on the performance and the quality of steel product.The multiphase flow phenomena of molten steel,steel/slag interface and gas bubbles in the slab continuous casting mold were described by numerical simulation,and the effect of electromagnetic brake（EMBR） and argon gas blowing on the process were investigated.The relationship between wavy fluctuation height near meniscus and the level fluctuation index F,which reflects the situation of mold flux entrapment,was clarified.Moreover,based on a microsegregation model of solute elements in mushy zone with δ/γ transformation and a thermo-mechanical coupling finite element model of shell solidification,the thermal and mechanical behaviors of solidifying shell including the dynamic distribution laws of air gap and mold flux,temperature and stress of shell in slab continuous casting mold were described.