A Computer Aided System for Simulating Weld Metal Solidification Crack

A computer-aided system for simulating weld solidification crack has been developed by which a welding engineer can carry out the welding solidification crack simulation on the basis of a commercial finite element analysis software package. its main functions include calculating the heat generations of the moving arc. mesh generation, calculating stress-strain distributions with element rebirth technique.

The effect of solidification behaviour of austenitic stainless steel on the solidification cracking susceptibility

On the basis of quantitative evaluation of susceptibility to solidification cracking with Trans-Varestraint-Test, the microstructures of two stainless steels of 316L and alloy 800H with different Creq/Nieq ratios during solidification process were analyzed with several methods. It is concluded that the susceptibility to solidification cracking of 316L-stainless steel is much lower than that of alloy 800H due to different solidification behaviors of the weld metal of the two materials. The weld metal of alloy 800H solidifies in the form of primary austenite whose boundaries are straight and smooth and easily wetted by low melting-point liquid phases, which increases the susceptibility to solidification cracking; while the 316L weld metal solidifies into primary austenite/ferrite. Owing to a series of dynamic microstructure changes during solidification such as peritectic reactions, migration of austenitic boundaries and nailing of δ-ferrite to the boundaries, the grains become finer, the orientations of columnar grains get disordered and the boundaries are curved and complex. Also high temperature δ-ferrite exists, segregation of impurities at boundaries decreases and the boundaries are hard to be wetted by liquid films, which reduces the cracking susceptibility.

Three Dimensional Numerical Simulation for the Driving Force of Weld Solidification Cracking

The double ellipsoidal model of heat source is used to analyze the thermal distributions with a three dimensional finite element method (FEM). In the mechanical model, solidification effects are treated by a dynamic element rebirth scheme. The driving force is obtained in the cracking susceptible temperature range. Moreover, this paper presents the effect of solidification shrinkage, external restraint, weld start locations and material properties on the driving force. The comparison between the simulated driving force and the experimental measurements of the material resistance predicts the susceptibility of weld metal solidification cracking.

EFFECT OF INTERMITTENT HIGH FREQUENCY MAGNETIC FIELD ON INITIAL SOLIDIFICATION IN CONTINUOUS CASTING

Contacting state between molten metal and a mold and initial solidification process of continuously cast metals can be controlled by imposing an intermittent magnetic field.In this study, effect of the intermittent magnetic field on the initial solidification ofcontinuoasly cast metals was investigated by measuring the temperature distribution in the melt pool and the initial solidification starting position of shells. It was found that under the condition with intermittent magnetic field, the melt near meniscns is in slow cooling state, the initial solidification starting position descends, initial solidification shell thickens and the liquid-solid interface becomes smooth.

Temperature distribution in front of the liquid-solid interface in the undercooled pure melt influenced by a transverse far field flow

The directional solidification in the undercooled pure melt influenced by a transverse far field flow was studied by using the multiple scale method. The result shows that in the boundary layer near the liquid-solid interface, when affected by a transverse far field flow, the temperature distribution in the direction of crystal growth presents an oscillatory and decay front in the side of liquid phase. The crucial distinguishing feature of a temperature pattern due to the transverse convection is the additional periodic modulation of the pattern in the growth direction. The wave number and eigenvalue that satisfy the Mullins-Sekerka dispersion relation are suppressed by the transverse far field flow.

Melt nucleating and the three-dimension steady model of the temperature fluctuation with convection

A three-dimensional steady model of temperature fluctuation with melt convection is studied. It is proved that there exists a unique and stable solution in the model and the solution is expressed in a Fourier series form. It theoretically confirms the mechanism of melt nucleating: as long as the convection with transverse directions exists, the melt temperature on the front of the solid-liquid interface would be not only periodical along the direction which is perpendicular to the direction of crystal growth, but also oscillatory and exponential decay along the direction of crystal growth; this oscillatory property, i.e. temperature fluctuation, leads to local supercooling, accelerates local temperature fluctuation and then results in a large number of nuclei.

Laser Multi-layer Cladding Experiment on the DD3 Single Crystal Using FGH95 Powder:Investigation on the Microstructure of Single Crystal Cladding Layer

Laser multi\|layer cladding experiments were performed on the substrate of DD3 single crystal with FGH95 powder as cladding material.The solidification microstructure in the sample was investigated.It was found that the solidification microstructure was greatly influenced by the crystallography orientation of the substrate and the local solidification conditions.When the angle between the preferred orientation of the single crystal and the direction of heat flow in the cladding layer is less than 30°,single crystal cladding layers were acquired.Otherwise the crystallography orientation of the cladding layer will deviate from the orientation of the substrate and the microstructure with polycrystalline appears.Meanwhile,even when the experiments were performed on the same preferred crystal surface,the solidification microstructures will be different distinctly resulting from the variation of the local solidification conditions.The secondary arms were degenerated and the primary arm spacing was about 10\|20μm.Further investigation shows that the phases of the cladding layer are mainly made up ofγ,γ′,the flower\|likeγ/γ′eutectic and carbide.The morphology ofγ′was cubical and the size is less than 0.1μm.

AlN precipitation during steel solidification using CA model

Based on the principles of metal solidification and cellular automaton(CA),as well as AlN precipitation thermodynamics and kinetics,a CA model of interdendritic AlN precipitation was established by coupling a large-size mesh describing the dendrite growth of Fe-C-Al-N alloys and a small-size mesh representing AlN precipitation based on transient chemical equilibrium.The results of single dendrite growth stimulated by this model were compared with the Lipton-Glicksman-Kurz solution to verify the correctness of the matrix dendrite growth simulation.The AlN morphology and dimensions obtained from the CA model simulations are following the experimental results.The presence of equiaxed dendrite in the computational domain results in a significant coarsening of the columnar dendrite and a uniform solute distribution around them,and the AlN solid phase fraction decreases.Simulations of AlN precipitation at different wetting angles were also performed,and it was found that the solid phase fraction of AlN decreased with the increase in wetting angle.Thus,it is confirmed that the established model is an effective method to simulate interdendritic AlN precipitation.

Microstructural Evolution and Performance of In-situ Ag-Ni Composite after Solidification under Electromagnetic Stirring and Deformation

The effect of electromagnetic stirring（EMS）on microstructure and performance of Ag-8 mass%Ni composite was investigated under both solidified and deformed conditions.Without EMS,the Ag matrix formed short,thick dendrites in the ingot;whereas with EMS,dendrites were long and slim.Ni phase mainly formed particles or ribbons,distributed along boundaries between dendrite arms.Cold drawing of the solidified Ag-Ni ingots,both with and without EMS,produced high strength in-situ metal-matrix composite（MMC）consisting of Ag matrix reinforced by Ni ribbons.EMS improved the ductility of the composite,consequently enhancing its drawability and strength.EMS also increased the electrical conductivity in both solidified ingots and deformed in-situ composite wires.In both cases,hardness and tensile strength remained high.A model based on a combination of the modified linear rule of mixtures and the Hall-Petch relationship was used to rationalize the tensile strength and hardness with respect to its fabrication parameters and the microstructure of Ag-Ni in-situ composite.

Centerline Segregation in Continuous Casting Billets

Centerline segregation is of practical significance since it affects the material properties. Center- line segregation in continuous casting billets was studied by solving the fluid flow, solidification, and solute transport equations from the initially liquid steel to the completely solid state using the finite difference method with the SIMPLER algorithm. The results show that the centerline segregation is induced by both the fluid flow in the mushy zone and the accumulation of solute-rich liquid near the solidification front. The species concentration in the center of the strand rises quickly in the mushy region to a maximum at the end of solidification. The most serious segregation occurs along the billet centerline.

Numerical Modeling of Dendrite Growth in Al Alloys

Dendritic grains are the most often observed microstructure in metals and alloys. In the past decade, more and more attention has been paid to the modeling and simulation of dendritic microstructures. This paper describes a modified diffusion-limited aggregation model to simulate the complex shape of the dendrite grains during metal solidification. The fractal model was used to simulate equiaxed dendrite growth. The fractal dimensions of simulated Al alloy structures range from 1.63-1.88 which compares well with the experimentally-measured fractal dimension of 1.85; therefore, the model accurately predicts not only the dendritic structure morphology, but also the fractal dimension of the dendrite structure formed during solidification.

Structural evolution of Al–8%Si hypoeutectic alloy by ultrasonic processing

High intensity power ultrasound was respectively introduced into three different solidification stages of Al–8%Si hypoeutectic alloy, including the fully liquid state before nucleation, the nucleation and growth process of primary α（Al） phase and L →（Al） ＋（Si） eutectic transformation period. It is found that both the primary α（Al） phase and（Al ＋ Si） eutectic structure were refined by different degrees with various growth morphologies depending on the ultrasonic treatment stage. Based on the experimental results,the cavitation-induced nucleation due to the high undercooling caused by the collapse of tiny cavities was proposed as the major reason for refining the primary α（Al） phase. Meanwhile, obvious eutectic morphological change was observed only when ultrasound was directly introduced in the eutectic transformation stage, in which typical divorced eutectics and（Al ＋ Si） eutectic cells with symmetrical flower shape were formed at the top of the alloy sample. The introduction of ultrasound in each solidification stage also improves the yield strength of Al–8% Si alloy to a diverse extent.