Microstructure Of Columnar Grains Research Articles

Anisotropic microstructure and tensile property of laser powder bed fusion fabricated Al–Mn–Mg–Sc–Zr alloy built at different layer thickness

Laser powder bed fusion (LPBF) fabricated Al–Mn–Mg-Sc-Zr alloy generally possesses a bi-modal microstructure of columnar grains and equiaxed grains. Such an anisotropic microstructure would introduce varied tensile performance in different orientations. To date, few study on the microstructure and tensile property anisotropy have been reported for LPBF fabricated Al–Mn–Mg-Sc-Zr alloys built at different layer thicknesses, which limits the exploration in mechanical property optimization. In this work, the microstructure anisotropy and room-temperature tensile properties of LPBF produced and peak-aged Al–Mn–Mg-Sc-Zr alloys built at 30 and 60 μm layer thicknesses were systematically investigated by scanning electron microscope, transmission electron microscope, and tensile testing. A higher layer thickness of 60 μm resulted in coarser grains with the precipitates size remained similarly compared to the 30 μm specimens. This led to a reduced yield strength in 60 μm specimens (492–509 MPa) comparing to those in 30 μm specimens at 502 MPa–510 MPa. In addition, the existence of columnar grains within melt pools contributed to a larger effective slip length in the vertical direction than that in the horizontal orientation. Such difference in effective slip length in the loading direction contributed to a lower strength in vertical orientations. For ductility, the slightly higher defect level in 60 μm specimens (0.58%) resulted in reduced elongations of 3%–6% compared to those of 30 μm specimens (0.10%). The ductility anisotropy was attributed to the preferential distribution of gas pores and keyhole defects at melt pool boundaries.

Repairability and effectiveness in direct energy deposition of 316L stainless steel grooves: A comparative study on varying laser strategy

Direct energy deposition (DED) has gained attention in the field of metal repair because of its ability to integrate additively deposited material with preexisting components. Having the beneficial aspects of depositing right material, DED is evaluated the best option for repair deposition. Machining grooves in damaged components is the common first to repair, which is necessary to remove cracked or damaged material and provide suitable geometry for deposition. Preprocessing as grooving is commonly applied to prevent the propagation cracks or tears. However, challenges such as dimensional error and crack induced by residual stress emerge owning to complex interactions between the DED parameters and groove geometry during repair deposition. This study investigates effective deposition strategies for repairing 316L stainless steel grooves with wall-angles of 90° and 135° by exploring the relationship between the repair integrity and parameters such as groove angles, laser energy input, and powder supply, all with reference to the underlying mechanism of metallurgical bonding in angled areas. Based thereon, a modified effective repair strategy is proposed, using enhanced dual laser power to address bonding issues in the angled areas. Samples repaired using this method exhibit a defect-free groove–deposit interface and a continuous columnar-grained microstructure with abrupt changes in grain size between the repaired area and groove. This microstructural heterogeneity results in an exceptional combination of strength and ductility when compared to the base material and samples repaired using a single laser strategy. The findings of this study provide insights into the optimization of DED strategies for achieving robust metallurgical bonding within grooves, thereby advancing technological maturity in the field of metal repair.

Microstructure evolution and property of high manganese steel coatings by laser shock assisted laser wire cladding

Field-assisted laser cladding (FALC) overcomes the limitations of laser cladding through the inherent advantages brought about by the use of various auxiliary energy fields (AEFs). Laser wire cladding (LWC) is limited by the common problems of laser cladding but has a promising future due to the extremely high material utilization rate. In this study, a laser shock assisted laser wire cladding (LSALWC) manufacturing process was proposed. In situ laser shock was introduced into the LWC to obtain finer grains structure and improve product performance. LSALWC does not change the composition of the coating, does not require the use of pretreatment and post-treatment, and does not reduce the efficiency of production. Using high manganese steel (HMS) as the model alloy, LSALWC was employed to achieve the expectation of reducing the proportion of columnar grains, refine the microstructure of columnar grains, and transform columnar grains into equiaxed grains. Compared with the sample (LC) manufactured by LWC, the performance of the sample (LSA) manufactured by LSALWC has been improved, the hardness increased from 285 ± 5 HV0.5 to 303 ± 6 HV0.5, the volume wear rate decreased from 2.1 × 10−4 mm3/N·m to 1.7 × 10−4 mm3/N·m, the primary dendrite spacing of the coating was reduced by 11.5%, and the heterogeneity of coating was more obvious. The cladding process under laser shock was studied using a high-speed camera, and the influence of laser shock on the coating was explained. The mechanism of the performance improvement of the LSA sample was explained. This study showed the disturbance of the molten pool was enhanced by laser shock, and the dendrites at the bottom were deflected or even broken. The broken dendrites were dispersed with the liquid flow and became new nucleation sites. Laser shock increased the supercooling during solidification by reducing the temperature gradient in the molten pool, and created an environment conducive to grain nucleation and growth. This process mitigates various issues in the casting process of HMS and provides a novel method for regulating the grain and microstructure of laser cladding. This research delves into the coupling between the AEF and the dynamic behavior of the molten pool, culminating in a model depicting the evolution of laser cladding solidification under the influence of the AEF, which will provide a new perspective for the research of FALC.

Anisotropic tensile creep behavior in laser powder bed fusion manufactured Al–Mn–Mg–Sc–Zr alloy

The recently emerged Al–Mn–Mg–Sc–Zr alloys were regarded as high-strength Al alloys suitable for high-temperature applications. However, the influence of laser powder bed fusion (LPBF) fabricated anisotropic microstructure on tensile creep performance has not been explored. In this work, we reported an anisotropic tensile creep behavior, which is superior in the vertical specimens than the horizontal counterparts. The tensile creep anisotropy was attributed to the bi-modal microstructure of columnar grains within melt pool and equiaxed grains at melt pool boundary, the “fish-scale” melt pool morphology and the resulted tortuous crack path in the vertical orientation.

Manufacturing high strength Ti alloy with in-situ Cu alloying via directed energy deposition and evaluation of material properties

In recent years, researchers have increasingly focused on alloy design in additive manufacturing (AM) to adjust the microstructure and improve the mechanical properties. The microstructure of columnar grains, strong texture, and chemical inhomogeneity in designed alloy exhibit anisotropic properties, which negatively affect the mechanical properties of the materials. One such material affected by this problem is titanium (Ti), which is known for its exceptional mechanical properties. Unlike other alloys, titanium lacks grain refiners that can ameliorate the columnar structure. Consequently, numerous studies are currently underway to refine the Ti grain through the addition of solute elements. In this study, Ti6Al4V with Cu element was deposited using the directed energy deposition (DED) process to investigate the feasibility of Ti6Al4V-xCu and analyze the effect of Cu addition. Ti6Al4V with Cu addition was successfully deposited and exhibited evenly distributed composition. In addition, finer grains were more created than the deposited Ti6Al4V by converting columnar to equiaxed grain due to the high cooling rate facilitated by high restriction factor Q of Cu. The deposited Ti6Al4V–4Cu of mechanical properties exhibited a 19 % increase in ultimate tensile strength, a 19 % increase in hardness at the top, and reduced elongation compared to the deposited Ti6Al4V, owing to the precipitation of Ti2Cu, grain refinement, and solid solution of Cu. However, although the hardness of the deposited Ti6Al4V–7Cu exhibited a 36 % increase in hardness at the top, Ti2Cu precipitate was generated significantly, which adversely exhibited a 26 % decrease in ultimate tensile strength compared to the deposited Ti6Al4V.

Giant elastocaloric effect covering a wide temperature region in a directionally solidified Ni50Mn30Ti20 alloy

High-performance elastocaloric materials coupled with giant elastocaloric effect and broad working temperature range are highly demanded for solid-state refrigeration applications. Here, we demonstrate giant elastocaloric response covering a wide temperature region in a directionally solidified Ni50Mn30Ti20 alloy with the columnar-grained microstructure and <001>A preferred orientation. Owing to the effective reduction in the transformation temperatures (∼200 K) through composition tuning, large superelastic response with the recovery strain of 9% is obtained from 213 K to 333 K, giving rise to the maximum adiabatic temperature change as high as −31.3 K upon unloading. Moreover, giant |ΔTad| values higher than 20 K have been well achieved within a wide temperature window of 90 K ranging from 223 K to 313 K.

Effect of travel speed on the microstructure and mechanical properties of laser beam welded nitronic-50 stainless steel joints

In this study, fiber laser beam welding was used to join 3 mm-thick nitronic-50 stainless steel. The effect of travel speed on the mechanical, microstructural, and corrosion properties of the weld joints was studied. The experimental findings reveal that travel speed significantly affected the size of bead geometry and the fusion boundary of the weld joints. The microstructure of columnar grains transformed into equiaxed dendritic grains at the center of the weld nugget due to fast cooling at higher travel speeds. The increase in ferrite content was due to the incomplete transformation of ferrite to austenite at higher travel speeds, which led to higher strength and microhardness in the weld joints. Similarly, the corrosion studies demonstrate that the weld joints made at higher travel speeds have superior corrosion resistance due to a higher fraction of equiaxed dendritic grains and smaller dendrites with less inter-dendritic arm spacing.

Anisotropic tensile behavior of laser powder-bed fusion made Ti5553 parts

Directional heating during metal additive manufacturing promotes the growth of a columnar-grained microstructure along the building direction with a preferred crystal orientation, which results in anisotropy of properties. In this work, the anisotropy of laser powder bed fused Ti-5553 metastable β titanium alloy components are investigated. Samples printed in the normal (vertically printed), and parallel (horizontally printed) orientations to the building direction were subjected to quasi-static uniaxial tensile tests. In comparison, samples printed parallel to the building direction exhibit a significantly higher ultimate strength of 846 ± 6 MPa, whereas the samples printed normal to the building direction reached 780 ± 10 MPa. Fracture initiation and propagation were investigated using interrupted tensile testing. The fracture path of partially fractured specimens reveals the development of slip bands oriented at a 45 ± 5° angle with respect to the loading direction, which promoted fracture by coalescence of aligned pores. Electron backscatter diffraction results indicate that the grain boundaries act as favorable nucleation sites for fracture initiation, particularly when they align perpendicular to the loading direction. Conversely, fracture in samples printed parallel to the building direction was predominantly transgranular, which is expected to be a major contributing factor to the higher strength observed.

Laser Powder Bed Fusion Processing of Fe-Mn-Al-Ni Shape Memory Alloy—On the Effect of Elevated Platform Temperatures

In order to overcome constraints related to crack formation during additive processing (laser powder bed fusion, L-BPF) of Fe-Mn-Al-Ni, the potential of high-temperature L-PBF processing was investigated in the present study. The effect of the process parameters on crack formation, grain structure, and phase distribution in the as-built condition, as well as in the course of cyclic heat treatment was examined by microstructural analysis. Optimized processing parameters were applied to fabricate cylindrical samples featuring a crack-free and columnar grained microstructure. In the course of cyclic heat treatment, abnormal grain growth (AGG) sets in, eventually promoting the evolution of a bamboo like microstructure. Testing under tensile load revealed a well-defined stress plateau and reversible strains of up to 4%.

A generalised hot cracking criterion for nickel-based superalloys additively manufactured by electron beam melting

Abstract The influences of microstructure and solidification parameters on the hot cracking behaviour of an additively manufactured Ni-based superalloy via selective electron beam melting (SEBM) have been investigated. Severe intergranular hot cracking is found to occur at grain boundaries (GBs) in the SEBM-built superalloy. A generalised cracking criterion that considers various process parameters along with the GB inclination is developed on the basis of a prevailing hot cracking criterion for columnar-grained microstructure. A parametric evaluation of influencing factors is quantitatively conducted to predict the optimal conditions that could prevent hot cracking while maintaining the desirable fine microstructure. With the aid of numerical simulations, it is determined that while micro-segregation occurs at all types of GBs, its occurrence at the divergent ones tend to promote cracking. Using the results of this work, hot cracking in additively manufactured Ni-based superalloy single crystals can be potentially mitigated.

Additive Manufacturing of Co-Ni-Ga High-Temperature Shape Memory Alloy: Processability and Phase Transformation Behavior

Co-Ni-Ga high-temperature shape memory alloy is additively processed by selective laser melting for the first time. Reversible martensitic transformation of the as-built material is proven by differential scanning calorimetry. Microstructural analysis reveals a columnar-grained microstructure resulting from epitaxial solidification. Columnar-grained microstructures are characterized by a very low degree of constraints being beneficial for superior functional performance in numerous shape memory alloys. However, process-induced crack formation remains a challenge towards robust realization of adequate conditions showing good mechanical properties.

Columnar grain growth in non-oriented electrical steels via plastic deformation of an initial columnar-grained solidification microstructure

Abstract The growth behavior of columnar grains in non-oriented electrical steels is studied via the plastic deformation of an initial columnar-grained solidification microstructure and annealing, based on changes in grain orientation and grain orientation spread (GOS) value. The proportion of high-strain grains increased with increasing plastic deformation. Moreover, slender {1 0 0}-textured columnar grains with low GOS values are obtained from the anisotropy of the strain energy induced by the temperature gradient. Besides, relatively coarse {1 0 0}- and {1 1 0}-textured columnar grains are obtained with a higher GOS value, resulting mainly from the migration of grain boundaries along the rolling plane induced by the anisotropy of the surface energy. The development of columnar-grained microstructures is an efficient method for improving the magnetic behavior of the final product.

On the dwell-fatigue crack propagation behavior of a high strength superalloy manufactured by electron beam melting

To demonstrate the reliability of additively manufactured superalloys for critical turbine engine components, dynamic tests simulating in-service condition are required. The present study aims to study the dwell-fatigue crack propagation behaviors of IN718 manufactured via electron beam melting (EBM). The textured and columnar-grained microstructure of EBM IN718 shows anisotropic dwell-fatigue cracking resistance when loading axis is aligned parallel and perpendicular to the columnar grains. High and low angle grain boundaries interact differently with the dwell-fatigue cracking path. The effect of different heat treatments on the cracking behavior is also discussed. The dwell-fatigue crack propagation rate of EBM IN718 is compared with forged IN718 under both dwell-fatigue test condition and pure fatigue test condition. The superiority of dwell-fatigue cracking resistance of EBM IN718 to forged IN718 is shown and discussed.

Influence of thermal post treatments on microstructure and oxidation behavior of EB-PBF manufactured Alloy 718

The effect of thermal post treatments consisting of heat treatment (HT), hot isostatic pressing (HIP), and combined HIP-HT on microstructure and oxidation behavior of Alloy 718 manufactured by electron beam powder bed fusion (EB-PBF) technique was investigated. Oxidation of the as-built and post-treated specimens was performed in ambient air at 650, 750, and 850 °C for up to 168 h. Directional columnar-grained microstructure, pores and fine Nb-rich carbides were observed in the as-built specimen. The HT specimen presented the columnar microstructure, plate-like δ phase at grain boundaries, and pores. The dominant grain crystallographic orientation was changed from 〈001〉 in the as-built specimen to 〈101〉 after HT. No grain boundary δ phase was observed in the HIPed specimen, but recrystallization occurred in both the HIP and HIP-HT specimens due to a rapid cooling after HIPing motivating the nucleation of fine grains with limited time to grow. After oxidation exposure at 650 and 750 °C for 168 h, no big difference between weight changes of the as-built and post-treated specimens was noted, whereas at 850 °C, the combined HIP-HT specimen showed the most promising corrosion resistance with the least weight change. At 850 °C, a protective scale of Cr2O3 rich in Cr, Ti, and Ni as well as an internal oxide (branched structure of alumina) developed in all the specimens, while, only a protective Cr2O3 scale was found at 650 and 750 °C. The HIP-HT specimen at 850 °C developed an oxide scale, which was denser and more adherent in comparison to the oxide scales formed on the other three specimens, associated with its limited defect distribution and more homogenized microstructure. Moreover, the δ phase formed close to the surface of the exposed specimens during the oxidation exposure at 850 °C most probably led to nucleation and growth of the oxide scale.

Microstructure and superelasticity control by rolling and heat treatment in columnar-grained Cu-Al-Mn shape memory alloy

The effects of rolling and heat treatment on the microstructure and superelasticity of columnar-grained Cu71Al18Mn11 shape memory alloy were investigated in this paper. Two different rolling strategies were adopted: (i) multipass high-temperature rolling (HR); (ii) one-pass HR followed by several-pass cold rolling (HR+nCR). For the first rolling strategy, the results showed that columnar-grained microstructure was reserved after one-pass HR at 800°C with rolling reduction of above 80%, and recrystallization would occur if more HR processes were applied. The superelastic strain could reach 5.9% in multipass HR sample through microstructure control by annealing at 800°C. For the second rolling strategy, after the first pass HR with the reduction of 80% and annealing at 550°C, the alloy could be cold rolled at room temperature with total reduction of 50–70%. The columnar-grained microstructure still existed in the cold-rolled alloy which consisted of two phases (i.e. β1+α). After recrystallization annealing, the HR+nCR alloy tend to form <011> texture along the rolling direction, which was helpful to obtain high superelasticity. Finally, the grain growth heat treatment was used to further improve the superelasticity of the cold-rolled alloy. After 2–3 times abnormal grain growth heat treatment, the grains of the alloy could grow up from several hundred micrometers to more than one centimeter in diameter; they still had strong <011> texture along the rolling direction, which enabled the superelastic strain of as high as about 7%.

Ti–6Al–4V triply periodic minimal surface structures for bone implants fabricated via selective laser melting

Triply periodic minimal surface (TPMS) structures have already been shown to be a versatile source of biomorphic scaffold designs. Therefore, in this work, Ti–6Al–4V Gyroid and Diamond TPMS lattices having an interconnected high porosity of 80–95% and pore sizes in the range of 560–1600μm and 480–1450μm respectively were manufactured by selective laser melting (SLM) for bone implants. The manufacturability, microstructure and mechanical properties of the Ti–6Al–4V TPMS lattices were evaluated. Comparison between 3D micro-CT reconstructed models and original CAD models of the Ti–6Al–4V TPMS lattices shows excellent reproduction of the designs. The as-built Ti–6Al–4V struts exhibit the microstructure of columnar grains filled with very fine and orthogonally oriented α′ martensitic laths with the width of 100–300nm and have the microhardness of 4.01±0.34GPa. After heat treatment at 680°C for 4h, the α′ martensite was converted to a mixture of α and β, in which the α phase being the dominant fraction is present as fine laths with the width of 500–800nm and separated by a small amount of narrow, interphase regions of dark β phase. Also, the microhardness is decreased to 3.71±0.35GPa due to the coarsening of the microstructure. The 80–95% porosity TPMS lattices exhibit a comparable porosity with trabecular bone, and the modulus is in the range of 0.12–1.25GPa and thus can be adjusted to the modulus of trabecular bone. At the same range of porosity of 5–10%, the moduli of cortical bone and of the Ti–6Al–4V TPMS lattices are in a similar range. Therefore, the modulus and porosity of Ti–6Al–4V TPMS lattices can be tailored to the levels of human bones and thus reduce or avoid “stress shielding” and increase longevity of implants. Due to the biomorphic designs, and high interconnected porosity and stiffness comparable to human bones, SLM-made Ti–6Al–4V TPMS lattices can be a promising material for load bearing bone implants.

Improved Processing Technique for Preparation of Non-Oriented Electrical Steels with Low Coercivity

Non-Oriented Electrical Steels with Low Coercivity I. Petryshynetsa,∗, F. Kovac, J. Marcin, I. ’korvanek Institute of Materials Research SAS, Watsonova 47, 040 01 Ko2ice, Slovakia Institute of Experimental Physics SAS, Watsonova 47, 040 01 Ko2ice, Slovakia In order to improve soft magnetic properties of vacuum degassed NO steels, an adjusted temper rolling process for development of particular textures {100} was used. The main idea here relies on a deformation-induced grain growth, which promotes preferable formation of the grains with desired orientation. Two vacuum degassed NO steels were chosen as an experimental material. In both cases, a coarse or columnar grained microstructure, with pronounced intensity of cube and Goss texture components, was achieved during a continuous nal annealing. The obtained microstructure leads to a signi cant decrease of coercivity, measured in DC magnetic eld. The coercivity of steel with silicon content 2.4 wt.% decreased from 42 A/m to 17 A/m. Even more remarkable improvement of the soft magnetic properties was observed for the steel with Si 0.6 wt.%, where the coercivity value dropped from 68 A/m to 12.7 A/m.

Interfaces of c-axis oriented ZnO thin films on MgO (001) substrates

c-axis oriented ZnO thin films are epitaxially grown on MgO (001) substrates by pulsed laser deposition. The orientation relations between the films and the substrates and the atomic details at the interfaces are investigated by means of conventional and aberration-corrected transmission electron microscopy. The ZnO thin films show a microstructure of columnar grains that follow the two types of in-plane orientation relations with the MgO substrates: [11 2¯ 0]ZnO//[1 1¯ 0]MgO and [1 1¯ 00]ZnO//[1 1¯ 0]MgO. These columnar grains grow with either an oxygen atom plane or a zinc atom plane as the starting plane on the charge-neutral MgO (001) surfaces, forming locally polar down or polar up interface, respectively. The relative positions of different atoms including oxygen at the interface are determined.

Liquid-solid interface control of BFe10-1-1 cupronickel alloy tubes during HCCM horizontal continuous casting and its effect on the microstructure and properties

Based on horizontal continuous casting with a heating-cooling combined mold (HCCM) technology, this article investigated the effects of processing parameters on the liquid-solid interface (LSI) position and the influence of LSI position on the surface quality, microstructure, texture, and mechanical properties of a BFe10-1-1 tube (ϕ50 mm × 5 mm). HCCM efficiently improves the temperature gradient in front of the LSI. Through controlling the LSI position, the radial columnar-grained microstructure that is commonly generated by cooling mold casting can be eliminated, and the axial columnar-grained microstructure can be obtained. Under the condition of 1250°C melting and holding temperature, 1200–1250°C mold heating temperature, 50–80 mm/min mean drawing speed, and 500–700 L/h cooling water flow rate, the LSI position is located at the middle of the transition zone or near the entrance of the cooling section, and the as-cast tube not only has a strong axial columnar-grained microstructure $$(\{ hkl\} , \{ hkl\} )$$ due to strong axial heating conduction during solidification but also has smooth internal and external surfaces without cracks, scratches, and other macroscopic defects due to short solidified shell length and short contact length between the tube and the mold at high temperature. The elongation and tensile strength of the tube are 46.0%–47.2% and 210–221 MPa, respectively, which can be directly used for the subsequent cold-large-strain processing.

Seed-layer effect on the microstructure and magnetic properties of Co/Pd multilayers

We have investigated Co/Pd multilayers deposited on either Ta or indium tin oxide (ITO) seed layers as a potential perpendicular recording media. We have examined the microstructural evolution of the films deposited on the two different types of seed layers and related it to the magnetic properties of the films. Ta underlayer produces a strong 〈111〉 fiber texture in the multilayer while ITO produces randomly oriented grains. Transmission electron microscopy reveals a microstructure of columnar grains separated by less dense material at the boundaries for the multilayers with an ITO underlayer. However, the less dense material is absent when using a Ta underlayer. The films exhibited strong perpendicular anisotropy and a higher coercivity of ∼6800 Oe and squareness of ∼0.99 are obtained for the films deposited on an ITO seed layer. The differences in the value of coercivity and squareness in the films can be correlated with the differences in the evolution of microstructures for different seed layers.