Effect of Thermal Behavior on the Grain Morphology and Dimension of 80mm-Thick Ti6Al4V Plates Joined by Laser Melting Deposition

Individually fabrication forged parts and then joining them together through Laser Melting Deposition (LMD) is a viable way for manufacturing large components. For investigating the effect of the grain morphology of LMD joint, two 80mm-thick Ti6Al4V plates are successfully manufactured using three different scanning speeds (10, 15, and 20 mm/s). It is essential for understanding the thermal behaviour of melt pool during LMD to improve process quality. This study focuses on the energy density of heat source and the direction of heat flux, analyzing the effect of thermal behavior on the grain morphology and dimension of deposition area, equiaxed crystal zone (EQZ) and the substrate. The macrostructure is evaluated in the different thermal condition and scanning speeds. An extremely fine equiaxed crystal was observed near the joint boundary with a high temperature gradient and cooling rate. The curve epitaxial growth of fine columnar crystal rather than along straight lines is induced by the direction of heat conduction near the joint boundary. However, the orientation angle of epitaxial growth of the coarse columnar crystal is the same as previous deposition layer at the center of deposition area. Given the effect of high heat accumulation and low temperature gradient during LMD, the dimension of columnar crystal is coarsen significantly at the center of deposition area.

Numerical Simulation of the Effect of Wall-Equiaxed Crystal Density on the Number of Columnar Crystals and the Thickness of an Equiaxed Crystal Layer for Al-4.7%Cu Alloy Ingot Based on 3D LBM-CA Method

In this paper, the lattice Boltzmann–cellular automata (LBM-CA) model with dynamic and static grids was used to study the growth of three-dimensional (3D) multidendrites under directional solidification with random preferred angles. In the static grid, the temperature field, flow field, and solute field during solidification were calculated by the LBM method, and in the dynamic grid, each dendrite evolution was calculated based on the CA method at its preferential crystallographic orientation. The coupling of LBM and CA was made by interpolation of the correlation quantities between the two sets of grids. The effects of wall-equiaxed crystal density on the number of columnar crystals and the thickness of the equiaxed crystal layer were studied by this model. The results showed that the density of the wall-equiaxed crystal has little effect on the number of columnar crystals and the thickness of the equiaxed crystal layer. When other conditions were the same, the lower the undercooling, the fewer the columnar crystals, and the thicker the equiaxed layer. In addition, the smaller the heat transfer coefficient, the lower the number of columnar grains, and the smaller the thickness of equiaxed grains.

Quantitative Correlation and Control Strategy for Element Content Fluctuation along Casting Direction in Central Area of Continuous Casting Billet

The statistical correlation was applied to analyze the specific and quantitative correlation relationship between the solidification structure and central segregation along the casting direction in carbon steel billet. On this basis, the segregation formation mechanism of the solute element and related control strategy were investigated. It is found that the equiaxed crystal zone fluctuation along the casting direction determines the fluctuation degree of central segregation. At the same time, the central segregation at a certain position is mostly affected by the equiaxed crystal zone width at the hysteretic position. Moreover, the casting speed can influence the columnar to equiaxed transition (CET) fluctuation along the casting direction by affecting the flow of molten steel in the billet. Overall, the segregation mechanism of solute elements along the casting direction can be summarized into two aspects: First, with the growth of columnar crystals in the initial stage, the segregated solutes are continuously enriched and distributed in the equiaxed crystal zone after CET. The fluctuation of the equiaxed crystal zone will affect the distribution of the enriched solute in the billet and cause the fluctuation of the central segregation. Second, due to the solidification shrinkage at the end of solidification, the solute-enriched liquid phase at the hysteretic position is pumped to the solidification endpoint and forms the central V-shaped segregation. Meanwhile, the stable solidification structure (columnar crystal length or equiaxed crystal zone width) along the casting direction and control measures preceded equiaxed crystal zone formation are beneficial to reduce the central V-shaped segregation.

The Interface Microstructures and Mechanical Properties of Laser Additive Repaired Inconel 625 Alloy

The microstructure and micro-mechanics around the repaired interface, and the tensile properties of laser additive repaired (LARed) Inconel 625 alloy were investigated. The results showed that the microstructure around the repaired interface was divided into three zones: the substrate zone (SZ), the heat-affected zone (HAZ), and the repaired zone (RZ). The microstructure of the SZ had a typical equiaxed crystal structure, displaying simultaneously precipitated block-shaped MC-type carbides (NbC, TiC), with bimodal sizes of approximately 10 μm and 0.5 μm and an irregularly shaped flocculent Laves phase. Recrystallization occurred in the HAZ, and led to significant grain growth; a portion of the second phase dissolved in the original grain boundaries. In the RZ, there was a columnar crystal structure, and the size increased with increasing deposition thickness. Moreover, the microstructure between the layer interface and layer interior was quite different, presenting an overlapping transition zone (OTZ), in which the dendritic structure coarsened and more Laves phase were precipitated, compared to in the layer interior. The hardness and tensile properties of the LARed samples were equivalent to those of the wrought substrate, which indicates that laser additive repairing (LAR) is a reliable repair solution for damaged and mis-machined components comprising Inconel 625 alloy.

Effect of Mg–Ti Treatment on Nucleation Mechanism of TiN Inclusions and Ferrite

Large sizes of columnar crystals and TiN particles have a great influence on the surface quality of ferritic stainless steel. In the present paper, this study proposed to obtain fine-grained equiaxed structures by Mg–Ti treatment. Through the experiment happened in resistance furnace with argon protection, the refining effect of Mg–Ti addition on the microstructure and TiN particles were investigated, and the refinement mechanism was discussed from interface coherence theory. It was found that due to adding Mg and Ti into molten ferritic stainless steel, the equiaxed crystal ratio increased from 37% to 50%, and the size of TiN particles reduced at the same time. The lattice matching characteristics of MgAl2O4/TiN and TiN/δ-Fe were investigated by FIB-HRTEM. According to Bramfitt’s equation, the lattice misfit for (400)MgAl2O4∥(200)TiN and (200)TiN∥(110)δ-Fe was 5.02% and 4.41%, respectively, which were all belong to the effective nucleation range. It could be considered that MgO and MgAl2O4 formed in the molten steel promoted TiN nucleation easier to precipitate out with large quantities in the liquid phase. The TiN particles with more uniform distribution significantly enhanced the heterogenous nucleation of ferritic phase during initial solidification process base on the good lattice fitting condition. Finally the equiaxed crystal ratio of δ-Fe phase increased dramatically.

Effect of growth modes on electrical and thermal transport of thermoelectric ZnO:Al films

Electrical and thermal transport controlled by growth mode can be used to optimize thermoelectric properties of ZnO:Al films, which was adjusted by the re-evaporation of Zn and Al via substrate temperatures. The growth modes include equiaxed crystal, columnar crystal and coexistence of both crystals. In the ZnO:Al film, equiaxed crystals improve the carrier mobility and reduce the lattice thermal conductivity. Thus, the carrier mobility and thermal conductivity are tuned by the ratio of equiaxed crystals to columnar crystals. The carrier mobility is dependent on the growth-mode-related defects of oxygen vacancies, zinc interstitials and the substitutional dopant of Al. Improved thermoelectric properties with a power factor of 198.45 µW m−1 K−2 at 510 K were achieved. This study presents a film with the structure of an equiaxed-crystal buffer layer to enhance its thermoelectric properties.

The Significance of Central Segregation of Continuously Cast Billet on Banded Microstructure and Mechanical Properties of Section Steel

The solidification structure and segregation of continuously cast billets produced by different continuous casting processes are investigated to elucidate their effect on segregated bands in hot-rolled section steel. It suggested that segregated spots are mainly observed in the equiaxed crystal zone of a billet. The solidification structure is directly related to superheating and the intensities of secondary cooling. To a certain extent, the ratio of the columnar crystal increases with the increase of superheating and secondary cooling. Moreover, the number of spot segregations decreases with the decrease of the equiaxed crystal ratio. After hot rolling, the segregation spots are deformed to form segregated bands in steels. The severe segregation of Mn in segregated bands corresponds with that in the segregation spots. The elongation ratio and low temperature toughness deteriorate significantly by a high fraction of degenerate pearlite caused by central segregation. With a decrease of central segregation, the total elongation is increased by 10% and the ductile–brittle transition temperature (DBTT) is also reduced from −10 to −40 °C. According to the experimental results, columnar crystal in billets is preferred to effectively reduce the degree of central segregation and further improve low temperature toughness and the elongation ratio.