Columnar Crystal Zone Research Articles

Study on corrosion characteristics and mechanism of laser powder bed fusion of Mg–Zn–Zr alloy

Laser Powder Bed Fusion (LPBF) presents novel opportunities for the processing and manufacturing of magnesium (Mg) alloys. However, Mg alloys consistently face challenges with poor corrosion resistance. Consequently, it becomes imperative to delve into the corrosion behavior and mechanisms of Mg alloys formed through LPBF. This paper investigated the electrochemical corrosion behavior, the distribution of corrosion products, and the corrosion mechanism of LPBF ZK60 Mg alloy. The results show that the corrosion current density (Icorr) value of LPBF ZK60 Mg alloy is 3.76 μA∙cm−2. The hydrogen evolution in Hank's solution steadily increases with immersion time and reaches a stable state after 24 h. The corrosion rates calculated based on hydrogen evolution and weight loss are 2.1 mm/y and 2.4 mm/y, respectively. The corrosion resistance of LPBF ZK60 Mg alloy is predominantly affected by three factors: uneven microstructure, internal crack defects, and precipitate phases. The melt pool boundary is identified as a corrosion-vulnerable zone, which extends into the columnar crystal zone after corrosion. Crevice corrosion occurs at microcracks, and the precipitate phases within the grains and matrix induces galvanic corrosion. These findings suggest that the corrosion resistance of LPBF ZK60 Mg alloy may be regulated through targeted measures, such as eliminating cracks, achieving microstructure homogenization, and controlling the precipitate phases (including quantity, composition, and distribution).

Simulation and Study of Influencing Factors on the Solidification Microstructure of Hazelett Continuous Casting Slabs Using CAFE Model.

The Hazelett continuous casting and rolling process represents a leading-edge production method for cold-rolled aluminum sheet and strip billets in the world. Its solidification microstructure significantly influences the quality of billets produced for cold rolling of aluminum sheets and strips. In this study, employing the CAFE (Cellular Automaton-Finite Element) method, we developed a coupled computational model to simulate the solidification microstructure in the Hazelett continuous casting process. We investigated the impact of nucleation parameters, casting temperature, and continuous casting speed on the microstructural evolution of the continuous casting billet. Through integrated metallographic analyses, we aimed to elucidate the controlling mechanisms underlying the Hazelett continuous casting process and its resultant microstructure. The results demonstrate that the equiaxed rate of grains increases with an increase in nucleation density, and the grain size decreases under constant cooling strength. With other nucleation parameters held constant, the grain size decreases as undercooling increases, and the columnar crystal zone expands. The nucleation density of the Hazelett continuous casting aluminum alloy has been determined to range between 1011 m-3 and 1013 m-3, and the undercooling ranges between 1 °C and 2.5 °C. The solidified grain structure can be controlled between 35 μm and 72 μm. The grain size of the continuous casting billet increases with an increase in pouring temperature and decreases as the casting speed increases. Elevating the pouring temperature positively impacts the fraction of high-angle grain boundaries and promotes the dendritic to equiaxed grain transition. Moreover, there exists potential for further optimization of continuous casting process parameters.

Texture Intensity in Grain-Oriented Steel in the Main Stages of the Production Cycle

Grain-oriented electrical steel (GOES) has been used for many years for application in transformed cores due to its excellent magnetic properties. Magnetic properties are strongly influenced by obtaining a texture with a certain orientation (110) [001] for BCC structure. This is related to the easy direction of magnetization [001]. So far, the main research has been focused on obtaining a strong texture in the last stages of the process. The aim of the present study was to additionally trace textural changes for a slab after the continuous casting (CC) process and for a sheet after the hot rolling process. The scope of such an analysis has not been conducted before. With regard to the state after continuous casting (CC), the texture was related to measurements of the anisotropy of Barkhausen magnetic noises and the macrostructure of the slab. Based on the X-ray diffraction examinations that compared the texture intensity calculated from the texture coefficient of the slab, the hot rolled steel and the final product of grain-oriented electrical steel contained 3.1% of Si. The studies performed with the material taken from three different production steps showed high differences in the values of textural intensity indicating the occurrence of a crystallization texture, especially in the area of the columnar crystal zone; textural weakness after the hot rolling process and high texturing in the final product for textural components corresponding to the desired Goss texture.

Differences in hardness and microstructure of laser cladding M2 coatings

The M2 coatings on medium carbon steel were prepared using a laser cladding process, and the microstructure and microhardness of the coatings were studied. The results show that the microhardness of the coating shows an increasing trend from the coating top to the bonding interface. The microhardness is 694 HV1 at the surface of the coating, which reaches 836 HV1 in the bottom regions of the coating. The wear rate of the coating is 0.32 mm3/N·m, which shows lower wear rate and better wear resistance. The coating microstructure is also different, and the coating consists of overlapping and non overlapping regions. The microstructure of overlapping region is a dendritic solidified crystalline structure encapsulated on α - Fe phase primary crystals with a dispersed carbide. The microstructure of non overlapping zone is composed of columnar crystal zone, basket crystal zone, and dendritic zone from the bottom of the coating to the surface, respectively, with the carbides distributed along the grain boundaries in the form of a network. Near the coating interface, needle-like high-carbon martensite forms in the coating. The carbides in the coating are relatively coarse.

Study of Phase Evolution Behavior of Ti6Al4V/Inconel 718 by Pulsed Laser Melting Deposition

In this study, a pulsed laser was used as the heat source for the additive work. The Ti6Al4V/Inconel 718 alloy wire was deposited on the substrate by melting using a pulsed laser. Using the above method, single-layer and double-layer samples were printed. The sample material printed in this way is highly utilized. Compared to the complicated pre-preparation work of metal powder pre-mixing, this printing method is simple to prepare and only requires changing the wire feeding speed. The study of this paper provides a theoretical guide for the subsequent fusion deposition of heterogeneous wire materials. The samples were analyzed after molding using SEM, EDS and XRD to characterize the microstructure of the samples. The samples can be divided into three zones depending on the microstructure, the bottom columnar crystal zone, the middle mixed phase zone, and the bottom equiaxed crystal zone. From the bottom to the top of the sample, the phase microstructure changes as γ + Laves → α + β + Ti2Ni + TiNi + Ni3Ti → α + β. The hardness data show that the highest value in the transition zone is 951.4 HV. The hardness of the top part is second only to the transition zone due to a large number of equiaxed crystals. The bottom region is dominated by columnar crystals and is the softest of the three regions with the lowest hardness value of 701.4 HV.

The Influence Mechanism of HSLA Weld Metal with Different Composition on High‐Temperature Strength

In order to study the influence mechanism of HSLA weld metal with different composition on high‐temperature strength, four kinds of HSLA weld metal are designed by using low‐carbon Si–Mn and Si–Mn–Ni alloy system. Different samples are prepared by simple process of normalizing and high‐temperature tempering. The experimental results show that the weld metal is mainly composed of polygonal ferrite, quasipolygonal ferrite, pearlite, and granular bainite. Ni can change the microstructure shape of reheating zone and inhibit the formation of granular bainite. When the tempering temperature is lower than 580 °C, the strength increases. After tempering at 580 °C, the tensile strength and yield strength reach the peak value. When the tempering temperature exceeds 580 °C, the strength begins to decrease. The microhardness of the two alloy systems decreases gradually from columnar crystal zone to recrystallization zone. The impact absorption energy of the four samples is greater than 200 J at 600 °C. Under different temperature conditions, the content of Ni in all second‐phase particles is less than 10%, the content of Mn is lower, and the content of Si is higher.

Energy reconstruction and metallurgical characteristics in 316L NG-LWFW with assisted wire wobbling by numerical and experimental analysis

A novel, new approach of narrow gap laser wire feeding welding (NG-LWFW) with electromagnetic assisted wire wobbling is proposed to solve incomplete sidewall fusion and improve solidification morphology. The energy distribution and fusion zone formation during multi-pass NG-LWFW process are clarified. The melt pool is driven into sidewall position and the melted metal accumulates towards that side groove, leading to the increase of sidewall penetration with concave bead profile. The simulation results show that the wire wobbling obviously reduces peak temperature of melt pool and increases sidewall temperature. The heat affected zone (HAZ) width reduced, and a finer and diversity weld microstructure is presented. As multi-pass welding proceeds, the former weld is heat-treated by the latter weld, leading to the re-melting of columnar grains and increasing diversity of interlayer grains. The columnar crystal zone is refining and a narrower dendrite spacing of δ-ferrite phase is found with deposited filling layers. The micro-hardness of weld centerline shows an overall increase trend and the impact energy increases with denser dimples from the bottom layer to upper layer.

Effect of welding heat input on microstructure and properties of TC4 titanium alloy ultra-narrow gap welded joint by laser welding with filler wire

A 10 mm narrow gap TC4 titanium alloy welded joint by laser welding with filler wire was obtained by different welding heat inputs. The microstructures of the welded joint were analyzed by OM, SEM, XRD and EBSD. The mechanical properties of the welded joint were analyzed by microhardness test and tensile test. The results show that with the increase of laser welding heat input, the average grain size and width of the equiaxed crystal zone in the center of the weld increases, the grain angle in the columnar crystal zone gradually tends to be perpendicular to the center of the weld, and the width of the heat affected zone (HAZ) increases without obvious grain coarsening. The HAZ of the welded joint is softened, and its microhardness value is lower than that of the weld and the base metal. The tensile strength of the welded joint is slightly higher than those of the base metal, but the elongation after fracture is less than that of the base metal, and it decreases with the increase of laser welding heat input.

Исследование структуры и свойств сплава AlSi10Mg, полученного методом селективного лазерного сплавления

The paper investigates the structure and properties of samples made of ASP-25 AlSi10Mg, a Russian powder designed to replace expensive additive manufacturing powders of European origin featuring the same chemical composition. We detected that the particle size in the ASP-25 AlSi10Mg powder varies in the range of 7 to 50 μm, the average particle size being 23 μm for the standard deviation of 9.15 and dispersion of 83.7. On the surface of powder particles, we observed smaller satellite particles, individual aggregates, and particles of pure aluminium. We detected the following at the transition boundary between adjacent tracks: a columnar crystal zone and a heat-affected zone consisting of three layers of large, medium and small grains generated as a result of varied cooling conditions. These grains display different silicon lattice thicknesses along their boundaries. We detected no critical size pores (over 15 μm) or burning in the heat-affected zone. The fact that microhardness increases towards the sample edges and is non-monotonic over the transverse section is due to a range of factors acting simultaneously to create non-uniform temperature and force fields that cause differences in conditions of structure formation. Fractography studies of fractures in the AlSi10Mg alloy showed that the nature of failure varies along the sample depth. The central part of the sample, which is subjected to the highest thermal effects, shows clear signs of viscous failure along the main cracks developing along the boundaries of construction layers. We showed that the AlSi10Mg alloy is more resistant to pitting corrosion and general corrosion than the AK9сh (AK9ч) alloy

Brazing diamond grits onto AA7075 aluminium alloy substrate with Ag–Cu–Ti filler alloy by laser heating

The brazing of diamond is a promising way to fabricate grinding wheels for efficient machining and precision grinding. This work investigated the feasibility of bonding diamond grits onto Aluminium Alloy 7075 (AA7075) substrate with a Ag–Cu–Ti filler alloy via laser fusion brazing. The interfacial microstructures and the strength of the brazed diamond joints were studied. The cross-section of the brazed diamond joint consists of a molten filler alloy layer, a molten pool, a heat effect zone, a columnar crystal zone and an equiaxed crystal zone. Within the interface of the filler alloy/substrate metal, microstructures observed possibly were Ag(s.s), Al(s.s), TixAl, Al2Cu and Mg intermetallic compounds. A layer of TiC with a thickness of about 30–50 nm was found at the bonding interface of the diamond/filler alloy. The averaged peak shear force of the brazed joints was found to be approximately 39.8 N. The abrasion grinding test indicated that the diamond/AA7075 brazed joint was adequate for grinding. However, the pulled-off of grit was found to be the primary failure of this type of brazed joint. This work broadened the brazing diamond technique and the range of applications of brazed diamond wheels for efficient grinding.

A Study on the 2060-T8/2099-T83 Aluminum-Lithium Alloys T-Joints Welded by Double-Sided Laser Beam Welding

The mechanical properties of 2060 and 2099 T-joints welded by double-sided laser beam welding were analyzed in this experiment. Various mechanical tests were performed to determine the transverse tensile properties of the region perpendicular to the weld, the tensile properties of the skin and stringer, the compressive properties of the region parallel to the weld seam, and the fatigue properties perpendicular to the weld seam. The T-joint was found to consist of the base metal, partial melting zone, equiaxed zone (EQZ), columnar crystal zone, and cellular dendritic zone from the base metal to the center of the weld seam. The EQZ was proved to be the weak area during mechanical properties tests of the T-joint. Moreover, the welding penetration had a great influence on the transverse tensile properties and weak areas were located in the EQZ near the lower fusion line. The EQZ near the upper fusion line was the weak area in the process of compression. In addition, severe porosities and cracks greatly influenced the mechanical properties of the joint. The fatigue property was closely related to the deformation angle; the fatigue source was located at the weld toe, and the crack propagation was along the base metal.

Effect of Ultrasonic Flexural Vibration on Solidification Structure and Mechanical Properties of Large-Size 35CrMoV Cast Ingot

In order to improve the performances of large-size 35CrMoV cast ingot, ultrasonic flexural vibration was guided into 35CrMoV steel melt through L-shaped ultrasonic waveguide rod during the solidification, and the effects of ultrasonic flexural vibration on macrostructure, microstructure, and mechanical properties of large-size 35CrMoV cast ingot were investigated. It is found that the columnar crystal zone has disappeared and the ingot is composed of the equiaxed crystals present in the ultrasonic ingot. The size of grains treated by ultrasonic are significantly smaller than conventional ingot. The distribution of ferrite in matrix structure is also more uniform than conventional ingot. The tensile strength is increased by 3.14%∼17.12%, and the elongation is increased by 39.13%∼287.50% compared with the conventional ingot at different positions.

Solidification Structure and Segregation of High Magnetic Induction Grain-Oriented Silicon Steel

The solidification structure and centerline segregation of high magnetic induction grain-oriented silicon steel slabs were studied to describe the characteristics of solidification and compare the degree of centerline segregation of continuously cast slabs. Industrial experiments were conducted to investigate the solidification behavior of slabs by secondary cooling segment electromagnetic stirring. Three typical slabs were produced by S-EMS with current intensities of 0, 200, and 350 A. Molten steel cast at a low stirring intensity (0 A) resulted in a coarse structure relative to those cast at higher stirring intensities (200 and 350 A). The centerline segregation of carbon and silicon markedly increased with increases in S-EMS current intensity. Composition distribution by electron probe microanalysis identified segregation spots as the sources of centerline segregation. Experimental results indicate that to optimize the centerline segregation of grain-oriented silicon steel slabs, the columnar crystal zone should be enlarged and the equiaxed crystal zone be reduced.

Analysis of droplet transfer, weld formation and microstructure in Al-Cu alloy bead welding joint with pulsed ultrasonic-GMAW method

The ultrasonic intensity was changed in different ultrasonic radiator heights (URH). Compared with conventional GMAW, the pulsed ultrasonic assisted gas metal arc welding (PU-GMAW) was an effective and newly-developed aluminum alloy welding method, because of its rapid droplet transfer and deep weld penetration. In PU-GMAW, the penetration and width of weld was increased due to the increase of the arc energy. When the URH was 20 mm and 22 mm, the droplet transfer mode of projected transfer changed to the mixed transition, which included projected transfer and short-circuiting transfer. The hardness of welded joint increased with the effect of pulsed ultrasonic, and the columnar crystal zone width of weld seam decreased. Ultrasonic radiation force was a determinant factor in arc compression and droplet transfer acceleration. The microstructure improvement in the PU-GMAW was caused by ultrasonic vibration and ultrasonic cavitation.

Production of hot-rolled sheet of specified quality by a new slab-casting technology

New methods of producing continuous-cast slab cannot be based on equilibrium crystallization of the steel at low speed. If melt is supplied to the wall layers in the mold through a pressurized submerged nozzle with eccentric outputs, the solidification rate of the steel in casting may be significantly increased. The results show that the new technology ensures improved quality in casting slabs of large cross section. When using the experimental nozzle, the width of the columnar-crystal zone is reduced and the disoriented-crystal zone is enlarged, while less axial segregation is observed. In casting by the new technology, heat transfer in the mold is increased by 10–12%. Impact tests reveal characteristics of the metal that are not evident in other tests. Structure formation of the metal in the continuous finishing group of a continuous broad-strip mill and subsequent strip cooling are studied by physical modeling. Tapered low-carbon steel samples taken from slab cooled on the intermediate roller conveyer of the broad-strip mill are rolled in one and two passes on a tworoller high-speed laboratory mill with a bypass unit. The rolling processes are geometrically and kinematically similar. After a fixed holding time in air, the rolled samples are set on one edge by means of a special device for partial quenching in coolant solution. Gradations of reduction are produced over the length of the sample, and gradations of cooling rate are produced over the width. The results of physical modeling illustrate the structure formation in low-carbon steel on rolling and accelerated strip cooling on a continuous broad-strip mill. Using these results and the Hall–Petch equation, specified structure and yield point of the final hotrolled sheet may be ensured. By eliminating the additional slab heating before rolling and improving the product quality, considerable economic benefit may be ensured.

Effect of Casting Temperature on Continuously-Cast Bearing Steel GCr15 Billet Structure Formation

Modeling is conducted for solidification and formation of a cast structure of continuously-cast blooms of steel GCr15 (Russian analog is steel ShKh15). A finite element method is used in order to model the formation of the crystal structure of continuously-cast blooms. Solidification is modelled for a bloom of steel GCr15 470 × 350 mm in cross section with different values of steel overheating above the liquidus line. Results of modeling are used for simulating continuously-cast billet macrostructure. It is shown that with a reduction in casting temperature the extent of the columnar crystal zone is reduced gradually within the structure, the extent of the skin and equiaxed zones is expanded, grain density increases gradually, and the maximum grain area and radius are significantly reduced. There are no small size fine dendritic crystals within the center of the equiaxed grain zone.

Influence of nonmetallic inclusions in rail steel on the high-temperature plasticity

The influence of nonmetallic inclusions in Э76Ф rail steel on the high-temperature plasticity is considered. In terms of the shear strain, a plasticity maximum is observed in all three zones of the continuouscast billet: the crust, the columnar-crystal zone, and the central zone of the billet. The high-temperature torsion of samples heated to 950–1250°C with 10-min holding is studied for continuous-cast billet of electrosmelted rail steel. Oxides and silicates are present in the crust; oxides and oxysilicates are found in the columnar-crystal zone; and sulfides, oxides, silicates, and alumosilicates are found in the central zone of the billet. The concentration of inclusions that impair the plasticity is greatest in the central zone of the billet.

Effect of the Al–Zr–Y master alloy composition on the modifying effect in the Al–4% Cu alloy

The results of modifying the Al–4% Cu alloy with test Al–1.04% Zr–0.70% Y and Al–1.23% Zr–0.39% Y master alloys are reported; the Zr-to-Y atomic-percentage ratio in the alloys is 1.41 and 3.08, respectively. The effect of small amounts of Zr and Y (from 0.1 to 0.3%) added in the form of test ternary master alloys of different compositions and binary Al–Zr and Al–Y master alloys on the grain refinement in the Al–4% Cu alloy has been studied. The structure of the initial alloy is characterized by pronounced directional solidification of the α phase. As the Zr + Y content increases, the columnar-crystal zone decreases and the equiaxed-crystal zone increases; at a (Zr + Y) content of 0.326%, only equiaxed crystals ~200 μm in size are present in an ingot. When Zr and Y are added with binary master alloys, the macrostructure of the modified Al–4% Cu alloys indicates that columnar crystals grow until their contact at the center of the ingot, and their growth is independent of the amount of added Zr and Y.

Effects of Grain Refiner and Cooling Rate on Solidification Structure of H62 Brass

In this paper, effects of different grain refiners on solidification structure of H62 brass under different cooling rates were investigate by macro corrosion, SEM and EDX analyses. Results of experiments indicate that the columnar crystal zone of H62 brass is completely eliminated and the grain size is reduced remarkably by 0.01%B. It has been found that the columnar grain zone is also eliminated by 0.1Ti, 0.1Zr, 0.1Ti+0.1Zr and 0.1Ti+0.1Zr+0.01B additions, respectively, but their grain refining effects are not as good as 0.01B. The grain size of H62 ingot with 0.1Ti addition prepared by sand mold casting is coarser than prepared by permanent mould casting. The microstructures of as cast H62 alloy are composed of α dendrite phases and rodlike β' phases which enrich on grain boundary to form an array structure.

Reducing the central porosity of continuous-cast billet by modification of the solidification process

The quality of continuous-cast billet is considered. Means of improving billet solidification are developed, with the pulsed injection of inert gas into the metal in the mold and shear-enhanced mild compression of the billet in the secondary-cooling zone of the continuous-casting machine. These measures are found to shrink the columnar-crystal zone and reduce the central porosity.