Equiaxed Grain Research Articles

Process Parameter Optimization of Directed Energy Deposited QT17‐4+ Steel

AbstractThe feasibility of using argon‐atomized QT 17‐4+ stainless steel powder for directed energy deposition (DED) additive manufacturing is studied. The QT 17‐4+ steel is a novel martensitic steel designed based on the compositional modification of the standard 17‐4 precipitation‐hardened (PH) stainless steel. This modification aims to achieve better mechanical properties of as‐deposited components compared to the heat‐treated wrought 17‐4PH steel. In this study, QT 17‐4+ steel powder is used for DED, for the first time. The influence of laser power, laser scan speed, powder feed rate, and hatch overlap on the density is studied. The central composite design is used to determine the experimental matrix of these factors. The response surface methodology is used to obtain the empirical statistical prediction model. Both columnar and equiaxed parent austenite grain structures are observed. X‐ray diffraction analyses reveal a decrease in the percentage of retained austenite from 19% in the powder to 5% after DED. The microhardness of the DED processed sample in the as‐deposited state is slightly higher than that of wrought 17‐4PH steel either solution‐annealed or H900‐aged. A higher 0.2% yield strength, a lower ultimate tensile strength, and lower elongation are observed for the vertically printed test sample, when compared to the horizontal one.

The nucleation and growth of fine austenitic grain at interface of the L415QS/N08825 bimetallic composite pipe by explosive welding

The nucleation and growth of fine austenitic grain at interface of L415QS/N08825 bimetallic composite pipe achieved by explosive welding is systematically studied. Results show that the L415QS/N08825 interface have heat affected zone (HAZ) that is similar with GH2132 superalloy (Fe-25Ni-15Cr). Two kinds of austenitic grains: columnar grains and ultra-fine equiaxed grain with large angle grain boundaries were identified at HAZ that nucleated from both L415QS and N08825 base alloy in the melts zone, and the growth of the nucleus is based on the size of melts zone. Room temperature nanoindetation test revealed that these fine equiaxed and columnar austenitic grains exhibit enhanced hardness compared with those of the base metals.

Generalized stability criterion for columnar to equiaxed grain transition during solidification upon vertical twin roll casting

To promote the columnar to equiaxed grain transition (CET) to obtain uniform equiaxed grain, most of the available theoretical works mainly focus on the growth but ignore the nucleation. Regarding it, a systematical framework integrating nucleation catalyzed by refiners with dendritic growth, was constructed, where, a new conception of generalized stability (GS), considering evolving thermodynamic driving force (ΔG) and kinetic energy barrier (Q) upon solidification of VTC, was adopted to study the CET position, the grain radius and the mechanical properties. By adding the refiners, especially the Al–5Ti–1B refiner, the larger thickness of 5–6 mm for equiaxed grain and finer grain radius of 19 μm are exhibited by increasing the ΔG for nucleation and the GS for growth, possessing simultaneously improved strength and plasticity. Following a criterion of high ΔG–high GS, the refiner diameter is optimized to be 0.3 μm.

Investigating post-processing impact on fatigue performance of LPBF Ti6Al4V with heat treatment, high pressure heat treatment, and dry electropolishing strategies

This study investigates the impact of post-processing techniques on the fatigue behaviour of specimens in Ti6Al4V made by laser powder bed fusion (LPBF) for relevant applications, such as motorsport. Despite the LPBF advantages in terms of complex design and weight reduction, challenges such as reduced fatigue performance persist. Therefore, the application of post-processing treatments can play a critical role. In addition to traditional sandblasting (SB), three post-processing conditions were investigated to enhance mechanical properties under quasi-static and fatigue loading conditions: heat treatment (HT) and the innovative High Pressure Heat Treatment (HPHT) along with dry electropolishing (EP) treatment. The results indicate that both the HT and HPHT treatments produced a macrostructure with equiaxial grains composed of fine α + β lamellae. Mechanical analyses demonstrated that the HPHT-SB-EP condition exhibits properties comparable to those of forged Ti6Al4V used in the automotive field, with a good balance between strength and ductility. The EP treatment effectively reduced surface roughness and inherent surface irregularities caused by the LPBF process. The best fatigue results were achieved with the HPHT-SB-EP condition, despite data scattering. An increase in fatigue resistance was observed by using SB at a higher pressure.

Selective laser melting of Hastelloy-X alloy and cerium oxide reinforced Hastelloy-X composite: Microstructural examination and corrosion behavior

The study investigated the microstructures and corrosion behaviors of Hastelloy X (HX) and HX reinforced by cerium oxide particles (HX-C) produced by selective laser melting (SLM) and compared to the wrought equivalent (HX-W). The aim was to eliminate hot cracking and investigate the HX's corrosion behavior in the presence of cerium oxide particles. The HX samples showed fine cellular and columnar solidification structures with uniform-size sub-cells and severe microcracking. Incorporating cerium oxide particles (1 wt%) resulted in finer and relatively equiaxed grains without microcracking. The HX-W alloy contained a micron-sized carbide (Mo-rich M6C type) within the matrix. The HX-C sample displayed higher porosities compared to the HX sample (1.8 % against 0.3 %). Potentiodynamic polarization and electrochemical impedance spectroscopy tests were conducted in a standard NaCl solution at temperatures of 25, 50, and 70 °C to assess the corrosion behavior of Hastelloy samples. The HX-C matrix showed superior corrosion resistance compared to the HX-W and HX samples due to its finer grain structure and stable passive layer formation. Cerium oxide particles were found to be efficient in decreasing hot cracking and improving corrosion resistance.

Thermal Cycling Performance of Heterogeneous Solder Ball Joints with Sn-3.0Ag-0.5Cu and Sn-0.75Cu(-Ni,Bi) Solder Ball Composition for BGA Package

Sn-3.0Ag-0.5Cu (SAC305) is one of the most commonly used Pb-free solder composition nowadays. A minor alloying element addition can improve mechanical properties and thermal reliability of the solder joint. Minor amount of Bithmuth (Bi) or Nickel (Ni) addition can improve mechanical properties of solder by solid solution strengthening, or ensure the structural stability of the intermetalllic compounds. Also, low melting temperature solder with Bithmuth can reduce the cost of soldering process and keep components from deterioration. In this study, reliability of heterogeneous and homogeneous solder joint in the ball-grid-array (BGA) package solder joint were investigated. Using Sn-3.0Ag-0.5Cu (SAC305) solder paste and two kinds of solder balls, we have investigated which combination of solder joint improves the thermo-mechanical reliability. SAC305 and SCN (Sn-0.75Cu-Ni,Bi) balls were used for homogeneous and hetrogeneous joints. Solder balls were attached in BGA package. To examine the solder joint reliability, thermal cycling test was conducted after reflow soldering. Shear strength measurement and cross-sectional analysis of the BGA package solder joints were carried out before and after 2000 cycles of the test. The shear strength of homogeneous solder joint decreased to 68.2% and heterogeneous decreased to 43.5%. The cross-sectional analysis using scanning electorn microscopy (SEM) and electorn back-scattered diffraction (EBSD) showed the (Ni,Cu)<sub>6</sub>Sn<sub>5</sub> and Cu<sub>6</sub>Sn<sub>5</sub> IMCs which were generated in the solder joint, and the crack propagated longer in the homogeneous solder joint of SAC305 after the thermal cycling test. IPFM showed the grain refinement effect of the SCN solder joint after thermal cycling. The minor addition of Bi had played a role as dislocation pinning site and became to equiaxied grain, so that the lifetime of heterogeneous SCN solder joint was superior than that of the SAC305.

Achieving high strength-ductility combination of Al–Mn–Mg-Sc-Zr alloy subjected by laser powder bed fusion and suitable post heat treatment

Post heat treatment is crucial for optimizing the performance of additive manufacturing components, yet it is still challenging to synergistically improve the strength and ductility through simple heat treatment. The present work investigated the microstructure evolution and mechanical properties of laser powder bed fusion (L-PBF) additive manufactured Al–Mn–Mg-Sc-Zr alloy after aging at 300 °C for different durations. All the as-built and aged samples exhibited a typical bi-modal microstructure consisting of coarse columnar and fine equiaxed grain regions. The as-built samples contained a number of primary Al6Mn precipitates distributed along grain boundaries; meanwhile, Mg solutes segregation was visible at grain boundaries, forming “Mg walls”. After aging for 4 h, the Al6Mn and a small content of Al3Sc were precipitated. Two types of Mg-rich precipitates evolved along grain boundaries simultaneously, increasing ultimate tensile strength from 401 MPa to 501 MPa, while the ductility was significantly reduced from 19.6 % to 4.6 %. When increased to 6 h, the Mg-rich precipitates disappeared, and the density of Al6Mn and Al3Sc precipitates reached its peak concurrently, providing a dual-precipitate strengthening effect. As a result, an excellent strength-ductility balance with an ultimate tensile strength of 528 MPa and ductility of 14.5 % was obtained. The Al6Mn precipitates became coarsening with increasing to 7 h, resulting in lower ductility. The strategy of modulating elemental distribution and precipitated phases by simple heat treatment is expected to be further developed for high strength and ductility Al alloys prepared by L-PBF.

Research on microstructure and mechanical properties of Ti–6Al–4V ELI fabricated by hybrid directed energy deposition with in-situ rolling and annealing

To achieve superior damage tolerance and fatigue properties, it is desirable for the microstructure of Ti-6Al-4V ELI (Extra Low Interstitial) titanium alloy to consist of equiaxed grains with a homogeneous lamellar structure. However, traditional forging and single additive manufacturing techniques require extreme processes such as quasi β heat treatment and hot isostatic pressing. In this paper, a hybrid directed energy deposition method that integrates in-situ rolling with simple annealing is proposed to simultaneously achieve these microstructures. The results indicate that the initial microstructures produced by hybrid directed energy deposition consist of fine equiaxed prior-β grains, inconspicuous interpass bright bands, randomly oriented lamellar structure and numerous small substructures at the TEM scale. Due to the plastic deformation caused by in-situ rolling, the primary β grains underwent significant refinement with a reduction in grain diameter from over 2000 μm to 113.8 μm. Besides, the unique microstructures and thickness of α laths are influenced by coarsening and dissolution mechanisms that are controlled by diffusion during phase transformation. Furthermore, a uniform microstructure without bright bands and excellent comprehensive properties can be achieved through heating at 880 °C for 2 h and natural cooling. These findings validate that hybrid directed energy deposition coupled with simple annealing present a novel and cost-effective approach for the direct manufacturing of uniform titanium alloy forgings.

Development of Low Anisotropic Commercially Pure Titanium Foils by Asymmetric Rolling and Annealing

This work studies the evolution of microstructure, texture, and anisotropy of pure titanium foils by symmetric rolling (SR) and asymmetric rolling (ASR) and subsequent annealing. The as‐received material shows equiaxed alpha grains and strong transverse texture (S‐T texture), which leads to severe anisotropy. The index of plane anisotropy (IPA) of yield stress (YS) and ultimate tensile stress (UTS) of the as‐received material is 15.2% and 7.5%, respectively. With the increase of deformation, the S‐T texture is mainly transformed into a mixed texture of transverse texture (T texture) and rolling direction texture (R texture), which balances the start‐up of the slip system when loading in the rolling direction (RD) and transverse direction (TD), so that the mechanical stress along RD and TD are close to each other and the anisotropy is improved. Especially for the samples prepared by ASR and annealing, the appearance of more R texture or basal texture (B texture) at the same deformation makes them show lower anisotropy. Therefore, after the same annealing treatment, the IPA of titanium foil with a thickness of 0.2 mm prepared by ASR is the smallest, which is 6.6% and 0.03%, respectively, while the IPA prepared by SR is 10.5% and 2.5%.

Influence of Macroscale Dimension on the Electrocrystallization of Cu Pad and Redistributed Layer in Advanced Packaging

Copper with equiaxed grains (EG Cu), copper with columnar nanotwinned grains (C‐NT Cu), and copper with mixed equiaxed and columnar grains (MG Cu) are electroplated on pads and redistributed layers (RDLs) with different dimensions. When the diameters of pads increase from 40 to 105 μm, the flatness of pads decreases from 34.5% to 24.0%. The flatness gradually deteriorates from 16.5% to 34.4% with the increase of RDL width from 8 to 20 μm. In contrast, the flatness of C‐NT Cu and MG Cu fluctuates between 1% and 5%, showing a weak correlation on the dimension. In terms of microstructure, the EG Cu and MG Cu has a weak correlation with size change, and the grain orientation shows a randomness. However, the (111) texture component of C‐NT Cu differs greatly with different dimensions, especially in transition layer. The (111) texture component changes from 9.6% to 38.1% for ϕ 40–105 μm pad and from 14.3% to 18.9% for 8–20 μm‐wide RDL. The study reveals the significant size effect on the deposited profile and microstructure of electrodeposited copper materials, which should give insights of the materials design in advanced packaging technology.

Cryogenic rolling impacts on microstructures and properties of a novel Ni–W–Co–Ta medium-heavy alloy

A high-strength Ni–W–Co–Ta medium-heavy alloy (MHA) was prepared through the melting–casting–forging followed by cryogenic rolling process. Detailed microstructure and mechanical property characterizations were conducted to unveil the influence of cyrorolling on Ni–W–Co–Ta MHA. The results revealed that initial equiaxial grains were elongated along the rolling direction firstly and then transformed to fibrous texture with increase in accumulated deformation, in which a large number of slip bands were generated to coordinate the intensive plastic deformation. A sharp increase in dislocation density significantly promoted the dislocation interactions, which, in turn, refined the grains down to nanometer scale. The strength and hardness increased significantly with the increase of the deformation, whereas the elongation decreased sharply. The fracture morphology changed from a typical ductile mode for the undeformed sample gradually to quasi-cleavage and ductile mixed mode for 90% deformed material.

Achieving ultrastrong-tough CrAlN coatings with low friction coefficient by Y incorporating

Despite having otherwise outstanding hardness and oxidation resistance, CrAlN-based hard coatings are limited by poor fracture toughness and high friction. Although incorporating alloying elements such as C and Magnéli phase-forming elements like V, W, Nb, etc, offer one opportunity for additional toughness and low friction, it requires significant and undesirable compromises to high-temperature performance. Therefore, this work explores an alternative, facile route to achieve ultrastrong-tough CrAlN-based coatings with low friction by incorporating Y. The results indicated that the doped Y elements primarily dissolved in the CrAlN crystalline lattice. Increasing the Y content from 0 to 2 at.% resulted in a change in preferred orientation from (111) to (200), accompanied by a crystal transition from typical columnar grains to unusual equiaxed grains. Notably, when the Y content was about 1.2 at.%, the coating exhibited the maximum toughness of 9.65 MPa·m1/2, almost 2 times larger than that of pure CrAlN coating. Additionally, this coating demonstrated a high hardness of 36.7 GPa and a reduced coefficient of friction (COF) of 0.32. These improvements were attributed to the synergistic contribution of the ultrastrong-tough capability and the formation of Cr2O3 lubricating phases, facilitated by the small amount of Y doping.

Semi-Solid Slurries for Rheocasting of Hypoeutectic Al-Si-X Alloys Produced by Self-Stirring in Serpentine Channels

Nowadays it is common to see the production of complex and critical automotive and aeronautical components reduced in weight for energy efficiency using light alloys with improved microstructural and mechanical properties. The casting processes involved in this trend are strong; in this study, an optimized design of a vertical serpentine channel and a novel design of a horizontal serpentine channel to produce semi-solid slurry (S2S) with thixotropic behavior by self-stirring for rheocasting of A380 and A356 alloys are tested. Simultaneously, chilling during solidification, flow development, and shearing on the alloys to improve the performance of solid fractions and self-stirring at high shear rate are applied. The effects of these conditions on the modification of the morphology transition of the α(Al) phase from dendrite to equiaxed grain are discussed. The results suggest the ability of the mentioned processes to promote the morphological transition of the primary solid due to the produced equiaxed grains of α(Al) phase having sizes between 25–50 µm from A380 alloy processed by vertical self-stirring. On the other hand, the treatment of the A356 alloy using the new horizontal serpentine channel produces equiaxed grains with an average size of 39 µm. Unexpected Si crystals, trapped in the α(Al) phase using both methods with both alloys, are detected. The applied operation parameters were aided by gravity-pouring close to the liquidus temperature, and the obtained microstructural results show the ability for S2S to form alongside thixotropic behavior and non-dendritic solidification by mean of self-stirring in the serpentine channels, suggesting the potential for further experiments under die-casting conditions.

Underwater friction stir welding of ultrafine-grained laminated composites produced through accumulative roll bonding process

In this work, ultrafine-grained (UFG) plates of AA2024/AA5083 laminated composite with alternating layers of heat-treatable aluminum AA2024 and strain-hardening aluminum AA5083 were fabricated through accumulative roll bonding (ARB) technique and were subsequently joined using underwater friction stir welding (UFSW). Friction stir welding (FSW) is a solid-state welding method which is suitable for the joining of similar and dissimilar UFG alloys of AA2024 and AA5083, which are mainly used in aerospace and aviation industries. Welding variables, such as pin profile, rotational speed, welding speed, and tilt angle were optimized using the Taguchi optimization technique. Different zones of UFSWed joints including stir zone (SZ), thermo-mechanically affected zone (TMAZ), and heat affected zone (HAZ) were investigated through OM, TEM, and SEM. The results showed that the tapered cylindrical pin, 1000 rpm rotational speed, 110 mm.min−1 welding speed, and 2.5° tilt angle were the optimal welding variables in terms of tensile strength. The microstructural results showed the formation of nano-size, recrystallized equiaxed grains with a size of 500 nm (the same size as that in the UFG base metal) in the optimal sample. The hardness in the weldment of the optimal UFSW sample decreased slightly, so that the efficiency of 85% was presented and the minimum hardness location (MHL) disappeared. The ultimate tensile strength (UTS) of 425 MPa and 64% efficiency were observed in the optimal conditions. Due to the presence of very fine grains and large dislocations, and the uniform distribution of precipitates in the underwater state, high strength and elongation were obtained.

Magnetocaloric properties of bulk Fe2AlB2 synthesized by reactive hot isostatic pressing

This study presents a new single-step synthesis of Fe2AlB2 at 1150°C for 4 hours using hot isostatic pressing (HIPing). X-ray diffraction confirmed the material as single-phase, while metallographic etching revealed predominant equiaxial Fe2AlB2 grains (∼25 μm) and minor unreacted boron particles and Al-rich zones at grain boundaries. Magnetocaloric investigations demonstrated an impressive isothermal entropy change of 2.9 J/kg.K at 2 T and 6.8 J/kg.K at 5 T, with corresponding adiabatic temperature changes of 1.1 K and 2.8 K at 2 and 5 T, respectively. The HIPed Fe2AlB2 performance matches that of studies without post-treatments or doping. Fe2AlB2, composed of light and earth-abundant elements, can be synthesized in a single step using HIPing, making it an attractive option for magnetic refrigeration with improved energy efficiency and simplified manufacturing.

Wire arc additive manufacturing of the Al-5.55Cu alloy: Insight the effect of intrinsic heat treatment on microstructure and mechanical properties

Al-5.55Cu alloy single wall sample were fabricated by cold metal transfer wire arc additive manufacturing (CMT-WAAM) technique, and the microstructure characteristics and evolution mechanism of the different deposition regions were analyzed. Under the influence of intrinsic heat treatment and remelting process caused by layer-by-layer deposition, the top region exhibits dendritic characteristics, while the middle and bottom are characterized by a non-uniform banding distribution of fine equiaxial grains alternating with columnar grains. According to the non-equilibrium solidification theory and the related nucleation mechanism, the formation of equiaxed grains in the inter-layer region is related to the elements such as Ti and Zr contained in Al-5.55Cu alloy, which can be used as nucleation particles. In addition, the morphology and quantity of the θ'-Al2Cu phase inside the grain change with the increase of deposition height due to the thermal cycling effect. After heat treatment, the tensile strength of the deposited sample is greatly improved and remains isotropic. The ultimate tensile strength of the heat-treated sample is 508.0 ± 18.9 MPa in the horizontal direction and 503.8 ± 6.2 MPa in the vertical direction.

Creep behavior and creep-oxidation interaction of selective electron beam melted Ti–48Al–2Cr–2Nb alloy at different temperatures

In this work, creep behavior and creep-oxidation interaction behavior of Ti–48Al–2Cr–2Nb alloy fabricated by selective electron beam melting at different temperatures were investigated in detail. This alloy exhibits a near-γ microstructure composed of coarse/fine mixed equiaxed grains. Creep test results show that creep behavior of this alloy is sensitive to temperature. This is mainly manifested in that creep life reduces significantly by 84.8 % as temperature increases from 800 °C to 850 °C. During creep, discontinuous dynamic recrystallization characterized by grain nucleation and growth and continuous dynamic recrystallization characterized by strain gradient and orientation accumulation occur simultaneously. The formation of mechanical twins plays an important role in stimulating dynamic recrystallization. Meanwhile, creep deformation and fracture mechanisms of this alloy at different temperatures were also discussed. On the other hand, oxidation test results reveal that the applied tensile stress has a positive effect on the oxidation behavior of this alloy: the growth of TiO2 oxide is inhibited and the oxidation rate is slowed down. However, under creep-oxidation interaction, the intergranular cracking of the oxide film occurs along with the necking of the alloy matrix during creep.

Effect of heat treatment on the microstructure and superelasticity of NiTi alloy via selective laser melting

In this study, a Ni50.4Ti49.6 shape memory alloy (SMA) was fabricated by selective laser melting (SLM), and it is found that heat treatments were effective to change the phase transformation behavior, microstructure, and superelasticity under compression for SLM Ni50.4Ti49.6. Detailed results based on EBSD demonstrated that, for SLM Ni50.4Ti49.6 during solid solution and aging (450 °C) treatment, the ultrafine cellular and columnar grains (∼16.5 μm) in as-fabricated Ni50.4Ti49.6 became equiaxed grains (∼70.2 μm), and the frequency of low-angle grain boundaries (26.7%→13.5%) and kernel average misorientation (0.37 → 0.21) decreased. The SEM and TEM analyses show that Ti-rich Ti2Ni/Ti4Ni2Ox and Ni-rich Ni4Ti3 precipitate phases were formed in the matrix simultaneously. After solid solution treatment, the size and distribution of Ti2Ni/Ti4Ni2Ox almost unchanged with aging temperature increasing from 350 °C to 550 °C, however, the size of Ni4Ti3 in aging-treated NiTi samples increased with aging temperature obviously, and it is interesting to find that the distribution of Ni4Ti3 became uneven and dislocation networks were formed in the matrix when the aging treatment reached up to 550 °C, which originated from the internal stress generated by supersaturated Ni atoms. Furthermore, the results of superelasticity for aging-treated NiTi demonstrated that the characteristics of Ni4Ti3 precipitate phase were powerful to influence the process of superelasticity decay/fatigue instead of determining the ultimately recovery strain after 10 compressive cycles. The aging-treated NiTi with evenly-distributed, high-density, and relatively-small Ni4Ti3 precipitate phase shows minimum recovery strain at 1st cycle compression and best superelasticity stability among three aging-treated NiTi, which was attributed to strong obstruction effect of these Ni4Ti3 precipitate for the occurrence of stress-induced martensitic transformation process and formation of dislocations. In contrast, the aging-treated NiTi with unevenly-distributed and low-density Ni4Ti3 precipitate shows maximum recovery strain at 1st cycle compression and worst superelasticity stability. These results in this work can bring some meaningful insights into tailoring the microstructure and superelasticity of SLM NiTi SMAs.

Microstructural comparison of hot isostatically pressed Mo[sbnd]10Nb alloys before and after hot rolling

In this work, hot isostatic pressing (HIP) and HIP followed by hot rolling (HIP+HR) were employed to process high-performance Mo10Nb alloys, respectively. The relative density, O content, oxide types, grain size and shape, and texture type of HIP-processed Mo10Nb alloys before and after HR were systematically investigated. The results indicate that the relative densities of Mo10Nb alloys prepared by the two methods are both over 98%. The oxygen content of HIP-processed and (HIP+HR)-processed Mo10Nb alloys is 638 ± 38 ppm and 2297 ± 152 ppm, respectively. X-ray photoelectron spectrometry (XPS) confirms that the main oxides in HIP-processed Mo10Nb alloys are MoO2, MoO3, NbO and Nb2O5 while those in (HIP+HR)-processed Mo10Nb alloys are stable MoO3 and Nb2O5 resulting from the oxygen contamination during HR. More importantly, HR crushes large Nb-rich phase particles and elongates equiaxed grain in HIP-processed Mo10Nb alloys. The high temperature during HR leads to the moderate growth of grains. Noteworthy, two weak textures along {001}〈110〉 and {111}<110 > orientations were found in HIP-processed Mo10Nb alloys after HR, which could be caused by the incomplete development of deformation textures and recrystallization textures. This work helps us to clarify the microstructure difference of HIP-processed Mo10Nb alloys before and after HR, which can provide some valuable science and technology guidance for fabrication of refractory alloys by powder metallurgy and the following plastic deformation.

Изучение влияния содержания гафния и эрбия на формирование микроструктуры при литье алюминиевого сплава 1590 в медный кокиль

Introduction. High-magnesium aluminum alloys are widely used in the automotive, building and aerospace industries due to its low specific gravity and high strength. The characteristics of such alloys can be improved by small additions of scandium and zirconium. However, scandium is very expensive, so in new generation alloys its amount is tended to be reduced. In the recently developed 1590 aluminum alloy, this was achieved by addition of erbium and hafnium. The objective of the paper is to study the effect of erbium and hafnium concentrations on the modification of the cast structure in 1590 aluminum alloy at high solidification rates. Research Methods. The paper investigates the microstructure, chemical composition and size of intermetallic compounds in specimens from ten alloy 1590 modifications with different hafnium and erbium contents cast into a copper chill mold with a solidification rate of 10 °C/sec. The grain structure was studied using an optical microscope. The chemical composition and size of the intermetallic phases were studied using a Tescan Vega 3 scanning electron microscope. Results and discussion. It is established that as the amount of hafnium and erbium increases, the cast structure is modified. In general, grain refinement with the addition of hafnium and erbium can be explained by a higher degree of supercooling between the solid and liquid phases. At a hafnium content of 0.16 %, the dendritic structure begins to transform into an equiaxed grain structure. This transformation can be explained by the appearance of primary intermetallic compounds of the Al3Sc type in the liquid phase. Such intermetallic compounds are identified at a concentration of erbium and hafnium equal to 0.16 %. Moreover, in all alloys eutectic intermetallic compounds are identified that contained manganese and iron and had no effect on the cast structure. Comparison with previously obtained results on the grain size of specimens cast into a steel mold shows that with higher solidification rate, the structure modification in 1590 alloy is getting less efficient. This is explained by an increase in the concentration of transition elements in the solid solution, primarily scandium, necessary for the formation of primary intermetallic particles.