Cellular Dendritic Growth Research Articles

Experimental Comparison of Laser Cladding and Powder Plasma Transferred Arc Welding Methods for Depositing Wear-Resistant NiSiB + 60% WC Composite on a Structural-Steel Substrate.

A Ni-based powder composed of NiSiB + 60% WC was deposited onto a structural-steel substrate using two methods: laser cladding (LC) and plasma powder transferred arc welding (PPTAW). The resulting surface layers were analyzed and compared. Both methods resulted in the precipitation of secondary WC phases in the solidified matrix, but the PPTAW clad exhibited a dendritic microstructure. The microhardness of the clads prepared by both methods was similar, but the PPTAW clad showed higher resistance to abrasive wear compared to the LC clad. The thickness of the transition zone (TZ) was thin for both methods, with a coarse-grain heat-affected zone (CGHAZ) and peninsula-like macrosegregations observed in clads from both methods. The PPTAW clad showed a unique cellular-dendritic growth solidification (CDGS) and a type-II boundary at the TZ attributed to its thermal cycles. While both methods resulted in metallurgical bonding of the clad to the substrate, the LC method exhibited a lower dilution coefficient. The LC method also resulted in a larger HAZ with higher hardness compared to the HAZ of the PPTAW clad. The findings of this study indicate that both methods are promising for antiwear applications due to their wear-resistant properties and metallurgical bonding to the substrate. The PPTAW clad may be particularly useful in applications that require higher resistance to abrasive wear, while the LC method may be advantageous in applications that require lower dilution and larger HAZ.

Computational simulation of thermo-mechanical field and fluid flow and their effect on the solidification process in TWIP steel welds

The primary austenitic solidification mode of Twinning Induced Plasticity (TWIP) steel weld joints is one of the most important issues limiting its weldability. In order to gain knowledge on this topic, a comprehensive study was carried out through numerical simulation of diffusive-convective phenomena of fluid flow and heat transfer of the melted material in the weld pool of TWIP steel weld joints. Weldments were performed using different heat inputs and constraint conditions. The predictions of convective motion, thermal history and cooling rates estimated by finite element (FE) and finite volume (FV) simulations were correlated with chemical species segregation (Mn, C, Al and Si) and the solidification mode in the fusion zone. The microstructural characterization of weld joints and the numerical results provided a full analysis of the weld pool solidification process. Results revealed a relationship between both the dendrite size and Mn segregation with the estimated velocity field of liquid material in the melted zone. Moreover, the presence of δ-ferrite was experimentally detected in some weld joints and associated with the convective movement of melted material and chemical segregation. The cellular dendritic growth mode found in some weld joints contributed to the formation of δ-ferrite and promoted the transition of the solidification mode from austenitic to ferritic-austenitic. As well δ-ferrite allowed to mitigate hot-cracking and interrupted the Mn-enriched liquid film migration towards the heat-affected zone, avoiding the formation of liquation cracks. The presence of δ-ferrite phase maintained a high mechanical strength providing improved elongation regarding the welded material mechanical properties.

Influence of Surfacing Fe-Based Alloy Layers on Wire Arc Additive Manufactured Ni-Based Superalloys Material on Its Microstructure and Wear Properties.

Wire arc additively manufactured (WAAM) Ni-based materials have good properties but are costly and hard to cut, leading to difficulties in machining after welding and wasting the materials. To overcome these shortcomings, this work proposes a method of surfacing Fe-based alloy layers on WAAM Ni-based material. The effect of this method on the microstructure and wear properties of WAAM Ni-based materials is discussed. In this work, a Fe-based alloy (JX103) was welded as the last layers of the WAAM Ni-based superalloy (JX201) material. The hardness, microstructure, and wear behavior of the material with different residual Fe-based materials were tested and analyzed. Our results indicate that the surface hardness was smoothly increased from HV350 to HV400 by overlaying Fe-based alloy layers. Microstructure analysis shows that γ-Fe gradually disappears, and the carbide form changes from WAAM Ni-based superalloys to Fe-based alloys. In the fusion boundary, the occurrence of cellular dendritic growth, a type -Ⅱ boundary, and low dilution indicate good crack resistance and good connection performance between these two materials. The wear test showed that the wear resistance of JX201 was decreased by changing the last layer to JX103. However, as the residual thickness of JX103 decreased, the influence gradually reduced. Meanwhile, the wear mechanism changed from severe abrasive and adhesive wear to light abrasive wear. When the thickness is less than 0.5 mm, the wear weight per minute is at the same level as the sample without JX103.

Microstructural evolution and numerical simulation of laser-welded Ti2AlNb joints under different heat inputs

The influence of heat input on the microstructural evolution of laser-welded Ti2AlNb joints was investigated in this study. The thermal cycles during welding process were analyzed by numerical simulation. In the heat affected zone (HAZ), the amount of α2 and O phases decreased with laser power increasing. During the heating period, α2 → B2 and O → B2 transformations occurred, but the decomposition of the B2 phase into α2 and O phases was suppressed during the cooling period. The heat transfer in the HAZ generated more equiaxed B2 grains, fewer LAGBs and a weaker {001} $$ $$ texture due to recovery, recrystallization and grain growth. The phase composition of the fusion zone remained single with only the B2 phase with the increase in heat input, but the mode of grain growth transformed from cellular growth into cellular dendritic growth. A finite element model was established to simulate the thermal cycles during the welding process. Higher heat input induced higher peak temperature, leading to higher temperatures in the HAZ for longer periods of time, which was beneficial for the α2 → B2 and O → B2 transformations. The calculated cooling rates in both the HAZ and in the fusion zone were faster than the critical cooling rate for B2 → α2 and B2 → O transformations.

Effect of Welding Parameters on TIG Welding of Inconel 718 to AISI 4140 Steel

The combination of Inconel 718 with AISI 4140 steel joint is in continuous demand for various applications. The difference in the physical and metallurgical properties between the two materials lead to difficulty in joining with conventional fusion welding methods. TIG welding is a suitable welding method to join such materials without any major issues. Therefore, the welding parameter effects on mechanical properties were studied experimentally and using the response surface methodology model. The microstructural characteristics and fracture surfaces of the welds were analyzed using the optical and scanning electron microscope. The reactions between the filler metal and substrates were evaluated by the electron probe microanalyzer for the presence of secondary phases. The formation of equiaxed and cellular dendritic growth of microstructure was observed from the welds. The fractography analysis investigated the ductile mode of failure after lap shear and fatigue tests.

Morphology and competitive growth during the development of the parallel lamellar structure by self-seeding in directionally solidified Ti-50Al-4Nb alloy

Bridgman-type directional solidification was conducted for the parallel lamellar structure of the TiAl-Nb alloys by self-seeding technique. The growth morphology and the competitive growth were investigated during the growth of the non-preferred crystals. The primary α phase and the final α2/γ lamellar structure are obtained at the selected growth rates. The non-preferred crystals with the parallel lamellar always have the asymmetry morphology with the tilting primary arms and the developed second arms at the front-lateral direction, which can affect the non-preferred growth advantage greatly. For the converging growth process the competitive growth takes on the struggle of the primary arms, while the second arms for the diverging growth, in which the parallel crystals always have the advantage to growth out and eliminate the misorientated grain formation at the crucible sidewall. Generally the parallel columnar crystals with the parallel lamellar structure can be achieved during the cellular-dendritic growth at the proper growth rates by self-seeding technique.

Continued growth controlling of the non-preferred primary phase for the parallel lamellar structure in directionally solidified Ti–50Al–4Nb alloy

Bridgman-type directional solidification experiments were conducted for Ti–50Al–4Nb alloy. The continued growth of the non-preferred primary phase with the effect of the growth rate was investigated for the parallel lamellar structure. The planar–cellular–dendritic growth of primary α phase is observed with increasing growth rate and α2/γ lamellar structure is obtained finally. The growth of the non-preferred crystals is blocked by the nucleated preferred crystals at low growth rates (<3μm/s) due to the external particles, as well as that at high growth rates (⩾15μm/s) due to the tilting crystals and the high undercooling. While the continued growth of the non-preferred crystals is achieved at proper middle growth rates, which always takes on a slope form owning the developed lateral dendrites. Finally the parallel columnar crystals with the parallel lamellar structure are obtained at selected growth rates (5–7μm/s) by controlling the continued growth of the preliminary non-preferred crystals at the initial interface.

Simulation of cellular-dendritic solidification structures of binary alloys in three-dimensional growth using a multiparticle diffusion-limited aggregation model

Abstract Solidification in binary alloy melts was simulated with two different three-dimensional models, that differ in the way the role of the solid/liquid interface energy on the solidification is expressed: either surface rearrangement or a surface curvature-dependent attachment probability was applied. In the melt, the simultaneous diffusion of all diffusing particles (atoms) was taken into account. Cellular–dendritic growth modes, as observed in practice, could be well simulated. The surface rearrangement model is essential for exhibiting details (as facetting) and the effect of the next-nearest neighbour interaction.

Influence of Crystallographic and Heat Flow Orientations on Growth of Cellular Dendrites.

A numerical program has been developed in order to calculate the time-dependent thermal gradient and isotherm velocity within a solidifying pool obtained by stationary laser treatment. The heat diffusion equation is solved for an axisymmetric geometry with an enthalpy formulation using a standard FDM implicit scheme. The program has been used to investigate to which extent the cellular dendritic growth orientation is affected by the direction and amplitude of the thermal gradient.Experiments were carried out on Al-1 mass%Si dendritic single crystals oriented by Laue back reflection technique and cut along precise crystallographic orientations prior to laser irradiation and on Al-3 mass%Cu eutectic polycrystalline alloys. Deviation of cellular dendritic growth direction from crystallographic and heat flux directions are reported. In addition, the comparison of the microstructural spacings with theoretical predictions outlines the influence of crystallography on the microstructural development during stationary laser treatment.

二相ステンレス鋼の溶接割れ感受性に関する研究 （第４報） 溶接凝固時におけるミクロ偏析の数値解析法による検討

A two dimensional solidification modelling of cellular dendritic growth was proposed in order to study the weld solidification phenomena of stainless steels. The modelling was made based on the quenched solidification microstructures. Fraction solid with temperature and segregations of the solutes within the cellular dendrite were calculated by means of finite difference method. Main conclusions obtained are as follows:(1) In this modelling, fraction solid was increased very rapidly just below liquidus temperature, and thus the temperature where the beginning of solid bridge was formed was very close to the liquidus temperature. Therefore, the temperature range where the residual liquid was confined by the sllid bridges was very long.(2) These behaviors mentioned in (I) were fitted well with the actual solidification microstructures revealed by liquid-tin quenching. On the other hand, the parabolic growth model where solid-liquid interface advances in proportion to square root of time did not fit with the actual microstructure.(3) Distributions of Ni and Cr in cellular dendrite calculated corresponded well to the EPMA results.(4) It was concluded that the two dimensional modelling proposed in this reoprt was very useful to understand the weld solidification phenomena.

二相ステンレス鋼の溶接割れ感受性に関する研究 （第５報） 溶接凝固脆性温度領域に関する数値解析法による検討

Two dimensional modelling of the cellular dendritic growth during weld solidification was applied to the prediction of Solidification Brittleness Temperature Range (BTR) of several kinds of stainless steels. The calculation of BTR was done based on the observation that the lower temperature limit of BTR corresponded to the temperature at which residual liquid enriched with impurities solidified completely. This treatment gave a good agreement between the calculated and the experimental BTR for ferritic and fully austenitic stainless steel by paying attention to mesh size in the model. Moreover, the treatment was also useful to rank harmful elements in crack susceptibility. Then the modelling was applied to the prediction of the reason why duplex stainless steel had a relatively large BTR. As the results of this calculation, it was judged that nitrogen was very harmful element. This prediction was confirmed experimentally by making the tentative deposited metals containing low nitrogen.Therefore, the calculation proposed was considered to be very useful to predict or analyze BTR for materials which solidified as single phase.

Kinetics of Explosive Crystallization Phenomena in Amorphous Silicon and Crystal Structure of the Layers Formed

ABSTRACTSystematic relations are shown between the parameters controlling explosive crystallization in silicon (temperature, width of a liquid zone, and gradient of melt undercooling), the kinetics of the crystallization process, the predominance of preferential growth directions, and the crystal structure observed. Three typical regions of crystallization are found: (a) cellular growth of a laminated crystalline layer controlled by the formation of high densities of twins at a nearly plane liquid/solid interface, (b) cellular-dendritic growth of crystal lamellae characterized by branching of a curved interface, and (c) formation of a diffuse “slush zone” due to random nucleation in a-Si and grain growth in the liquid zone.

Microsegregation in cellular dendritic growth in binary alloys of Al-Cu

Microsegregation in binary alloys of Al-2%wtCu in a large range of cooling rates using the unidirectional growth technique was studied. The spacing of substructures was determined and the value of the minimum composition in the center of the dendritic cells applying metallographic techniques and electronic microprobe was measured. The results obtained confirmed that the tendency of the experimental points can be explained with a good accuracy by the theoretical models of Solari and Biloni and of Trivedi.

Microsegregation in cellular and cellular dendritic growth

On the basis of Flemings' microsegregation model and the work done by Burden and Hunt describing cell tip composition and undercooling as a function of the growth variables, a general microsegregation equation is proposed. The particular cases consider different conditions: (i) cellular solidification; (ii) columnar growth; (iii) solidification at high speed and (iv) processes where the solidification rate changes during the process, as in welding.