Effects of cooling rates on microporosity in DC casting Al-Li alloy

During the direct chill(DC)casting process,primary cooling from the mold and bottom block,and secondary cooling from the waterjets produce a concave solid shell.The depth of this liquid pocket and mushy zone not only depends on the solidification range of the alloy but also the boundary conditions such as cooling rates.Al-Li alloys solidify in a long solidification range increasing the susceptibility of porosity nucleation in the semi-solid region.In this study,the effects of cooling rate on the porosity formation were quantified for the large ingot casting using X-ray computed tomography(XCT).By characterizing pore size distributions at four different cooling conditions,the correlation between the mechanical properties at both room and high temperatures and the microstructure features was identified.The constitutive equations were constructed.It is found that increasing the cooling rate reduces the grain size,increases the number density of micropores,and minimizes the number of large pores,thereby improving the mechanical performance.Therefore,long mushy zones and deep liquid pockets in Al-Li alloys can be effectively controlled by controlling the boundary conditions of the DC casting solidification process,thereby obtaining castings with excellent mechanical properties.

Microstructure and Hot Tearing Sensitivity Simulation and Parameters Optimization for the Centrifugal Casting of Al-Cu Alloy

Four typical theories on the formation of thermal tears:strength,liquid film,intergranular bridging,and solidifica-tion shrinkage compensation theories.From these theories,a number of criteria have been derived for predicting the formation of thermal cracks,such as the stress-based Niyama,Clyne,and RDG(Rapaz-Dreiser-Grimaud)criteria.In this paper,a mathematical model of horizontal centrifugal casting was established,and numerical simulation analysis was conducted for the centrifugal casting process of cylindrical Al-Cu alloy castings to investigate the effect of the centrifugal casting process conditions on the microstructure and hot tearing sensitivity of alloy castings by using the modified RDG hot tearing criterion.Results show that increasing the centrifugal rotation and pouring speeds can refine the microstructure of the alloy but increasing the pouring and mold preheating temperatures can lead to an increase in grain size.The grain size gradually transitions from fine grain on the outer layer to coarse grain on the inner layer.Meanwhile,combined with the modified RDG hot tearing criterion,the overall distribution of the castings’hot tearing sensitivity was analyzed.The analysis results indicate that the porosity in the middle region of the casting was large,and hot tearing defects were prone to occur.The hot tearing tendency on the inner side of the casting was greater than that on the outer side.The effects of centrifugal rotation speed,pouring temperature,and preheating temperature on the thermal sensitivity of Al-Cu alloy castings are summarized in this paper.This study revealed that the tendency of alloy hot cracking decreases with the increase of the centrifugal speed,and the maximum porosity of castings decreases first and then increases with the pouring temperature.As the preheating temperature increases,the overall maximum porosity of castings shows a decreasing trend.

I-DCGAN and TOPSIS-IFP:A simulation generation model for radiographic flaw detection images in light alloy castings and an algorithm for quality evaluation of generated images

The intelligent detection technology driven by X-ray images and deep learning represents the forefront of advanced techniques and development trends in flaw detection and automated evaluation of light alloy castings.However,the efficacy of deep learning models hinges upon a substantial abundance of flaw samples.The existing research on X-ray image augmentation for flaw detection suffers from shortcomings such as poor diversity of flaw samples and low reliability of quality evaluation.To this end,a novel approach was put forward,which involves the creation of the Interpolation-Deep Convolutional Generative Adversarial Network(I-DCGAN)for flaw detection image generation and a comprehensive evaluation algorithm named TOPSIS-IFP.I-DCGAN enables the generation of high-resolution,diverse simulated images with multiple appearances,achieving an improvement in sample diversity and quality while maintaining a relatively lower computational complexity.TOPSIS-IFP facilitates multi-dimensional quality evaluation,including aspects such as diversity,authenticity,image distribution difference,and image distortion degree.The results indicate that the X-ray radiographic images of magnesium and aluminum alloy castings achieve optimal performance when trained up to the 800th and 600th epochs,respectively.The TOPSIS-IFP value reaches 78.7%and 73.8%similarity to the ideal solution,respectively.Compared to single index evaluation,the TOPSIS-IFP algorithm achieves higher-quality simulated images at the optimal training epoch.This approach successfully mitigates the issue of unreliable quality associated with single index evaluation.The image generation and comprehensive quality evaluation method developed in this paper provides a novel approach for image augmentation in flaw recognition,holding significant importance for enhancing the robustness of subsequent flaw recognition networks.

Predict the evolution of mechanical property of Al-Li alloys in a marine environment

The ocean is one of the essential fields of national defense in the future,and more and more attention is paid to the lightweight research of Marine equipment and materials.This study it is to develop a Machine learning(ML)-based prediction method to study the evolution of the mechanical properties of Al-Li alloys in the marine environment.We obtained the mechanical properties of Al-Li alloy samples under uniaxial tensile deformation at different exposure times through Marine exposure experiments.We obtained the strain evolution by digital image correlation(DIC).The strain field images are voxelized using 2D-Convolutional Neural Networks(CNN)autoencoders as input data for Long Short-Term Memory(LSTM)neural networks.Then,the output data of LSTM neural networks combined with corrosion features were input into the Back Propagation(BP)neural network to predict the mechanical properties of Al-Li alloys.The main conclusions are as follows:1.The variation law of mechanical properties of2297-T8 in the Marine atmosphere is revealed.With the increase in outdoor exposure test time,the tensile elastic model of 2297-T8 changes slowly,within 10%,and the tensile yield stress changes significantly,with a maximum attenuation of 23.6%.2.The prediction model can predict the strain evolution and mechanical response simultaneously with an error of less than 5%.3.This study shows that a CNN/LSTM system based on machine learning can be built to capture the corrosion characteristics of Marine exposure experiments.The results show that the relationship between corrosion characteristics and mechanical response can be predicted without considering the microstructure evolution of metal materials.

Experimental observations on the nonproportional multiaxial ratchetting of cast AZ91 magnesium alloy at room temperature

The nonproportional multiaxial ratchetting of cast AZ91 magnesium (Mg) alloy was examined by performing a sequence of axial-torsional cyclic tests controlled by stress with various loading paths at room temperature (RT).The evolutionary characteristics and path dependence of multiaxial ratchetting were discussed.Results illustrate that the cast AZ91 Mg alloy exhibits considerable nonproportional additional softening during cyclic loading with multiple nonproportional multiaxial loading paths;multiaxial ratchetting presents strong path dependence,and axial ratchetting strains are larger under nonproportional loading paths than under uniaxial and proportional45°linear loading paths;multiaxial ratchetting becomes increasingly pronounced as the applied stress amplitude and axial mean stress increase.Moreover,stress-strain curves show a convex and symmetrical shape in axial/torsional directions.Multiaxial ratchetting exhibits quasi-shakedown after certain loading cycles.The abundant experimental data obtained in this work can be used to develop a cyclic plasticity model of cast Mg alloys.

Microstructure and mechanical properties of Co-28Cr-6Mo-0.22C investment castings by current solution treatment

This study examined the impact of current solution treatment on the microstructure and mechanical properties of the Co-28Cr-6Mo-0.22C alloy investment castings.The findings reveal that the current solution treatment significantly promotes the dissolution of carbides at a lower temperature.The optimal conditions for solution treatment are determined as a solution temperature of 1,125°C and a holding time of 5.0 min.Under these parameters,the size and volume fraction of precipitated phases in the investment castings are measured as6.2μm and 1.1vol.%.The yield strength,ultimate tensile strength,and total elongation of the Co-28Cr-6Mo-0.22C investment castings are 535 MPa,760 MPa,and 12.6%,respectively.These values exceed those obtained with the conventional solution treatment at 1,200°C for 4.0 h.The findings suggest a phase transformation of M_(23)C_(6)→σ+C following the current solution treatment at 1,125°C for 5.0 min.In comparison,the traditional solution treatment at 1,200°C for 4.0 h leads to the formation of M_(23)C_(6)and M_(6)C carbides.It is noteworthy that the non-thermal effect of the current during the solution treatment modifies the free energy of both the matrix and precipitation phase.This modification lowers the phase transition temperature of the M_(23)C_(6)→σ+C reaction,thereby facilitating the dissolution of carbides.As a result,the current solution treatment approach achieves carbide dissolution at a lower temperature and within a significantly shorter time when compared to the traditional solution treatment methods.

Effect of slow shot speed on externally solidified crystal,porosity and tensile property in a newly developed high-pressure die-cast Al-Si alloy

The effect of slow shot speed on externally solidified crystal(ESC),porosity and tensile property in a newly developed high-pressure die-cast Al-Si alloy was investigated by optical microscopy(OM),scanning electron microscopy(SEM)and laboratory computed tomography(CT).Results showed that the newly developed AlSi9MnMoV alloy exhibited improved mechanical properties when compared to the AlSi10MnMg alloy.The AlSi9MnMoV alloy,which was designed with trace multicomponent additions,displays a notable grain refining effect in comparison to the AlSi10MnMg alloy.Refining elements Ti,Zr,V,Nb,B promote heterogeneous nucleation and reduce the grain size of primaryα-Al.At a lower slow shot speed,the large ESCs are easier to form and gather,developing into the dendrite net and net-shrinkage.With an increase in slow shot speed,the size and number of ESCs and porosities significantly reduce.In addition,the distribution of ESCs is more dispersed and the net-shrinkage disappears.The tensile property is greatly improved by adopting a higher slow shot speed.The ultimate tensile strength is enhanced from 260.31 MPa to 290.31 MPa(increased by 11.52%),and the elongation is enhanced from 3.72%to 6.34%(increased by 70.52%).

A comparative study on Sn macrosegregation behavior of ternary Al-Sn-Cu alloys prepared by gravity casting and squeeze casting

A comprehensive study on Sn macrosegregation behavior in ternary Al-Sn-Cu alloys was carried out by comparative analysis between gravity casting and squeeze casting samples.The microstructure and Sn distribution of the castings were characterized by metallography,scanning electron microscopy(SEM),energy-dispersive X-ray(EDX)spectroscopy,and a direct reading spectrometer.Results show that there are obvious differences in Sn morphology between gravity casting and squeeze casting alloys.Under squeeze casting condition,the grain size of the casting is smaller and the distribution ofβ(Sn)is uniform.This effectively reduces the segregation of triangular grain boundary as well as the segregation of Sn.The segregation types of Sn in gravity casting and squeeze casting samples are obviously different.The upper surfaces of gravity casting samples show severe negative segregation,while all the lower surfaces have positive segregation.Compared with gravity casting,squeeze casting solidifies under isostatic pressure.Due to the direct contact between the upper surface of the casting and the mold,the casting solidifies faster under higher undercooling degree and pressure.Consequently,the uniform distribution of Sn reduces the segregation phenomenon on the surface of the casting.

Effects of magnesium and copper additions on tensile properties of Al-Si-Cr die casting alloy under as-cast and T5 conditions

Aluminum high pressure die casting(HPDC)technology has evolved in the past decades,enabling stronger and larger one-piece casting with significant part consolidation.It also offers a higher design freedom for more mass-efficient thin-walled body structures.For body structures that require excellent ductility and fracture toughness to be joined with steel sheet via self-piercing riveting(for instance,shock towers and hinge pillars,etc.),a costly T7 heat treatment comprising a solution heat treatment at elevated temperatures(450℃-500℃)followed by an over-ageing heat treatment is needed to optimize microstructure for meeting product requirement.To enable cost-efficient mass production of HPDC body structures,it is important to eliminate the expensive T7 heat treatment without sacrificing mechanical properties.Optimizing die cast alloy chemistry is a potential solution to improve fracture toughness and ductility of the HPDC components.The present study intends to tailor the Mg and Cu additions for a new Al-Si-Cr type die casting alloy(registered as A379 with The Aluminum Association,USA)to achieve the desired tensile properties without using T7 heat treatment.It was found that Cu addition should be avoided,as it is not effective in enhancing strength while degrades tensile ductility.Mg addition is very effective in improving strength and has minor impact on tensile ductility.The investigated Al-Si-Cr alloy with a nominal composition of Al-8.5wt.%Si-0.3wt.%Cr-0.2wt.%Fe shows comparable tensile properties with the T7 treated AlSi10MnMg alloy which is currently used for manufacturing shock towers and hinge pillars.

Mechanical Evaluation of AZ80 Magnesium Alloy in Cast Wrought Form

Wrought magnesium alloy AZ80 with a thick section of 20 mm was prepared by squeeze casting (SC) and permanent steel mold casting (PSMC). The porosity measurements of the SC and PSMC depicted that SC AZ80 had a pore content of 0.52%, which was 77% lower than 2.21% of PSMC AZ80 counterpart. The YS, UTS, ef, E and strengthening rate of cast AZ80 were determined by mechanical pulling. The engineering stress versus strain bended lines showed that SC AZ80 had a YS of 84.7 MPa, a UTS of 168.2 MPa, 5.1% in ef, and 25.1 GPa in modulus. But, the YS, UTS and ef of the PSMC AZ80 specimen were only 71.6 MPa, 109.0 MPa, 1.9% and 21.9 GPa. The findings of the mechanical pulling evidently depicted that the YS, UTS, ef and E of SC AZ80 were 18%, 54%, 174% and 15% higher than PSMC counterpart. The computed resilience and toughness suggested that the SC AZ80 exhibited greater resistance to tensile loads during elastic deformation and possessed higher capacity to absorb energy during plastic deformation compared to the PSMC AZ80. At the beginning of permanent change, the strengthening rate of SC AZ80 was 10,341 MPa, which was 9% greater than 9489 MPa of PSMC AZ80. The high mechanical characteristics of SC AZ80 should be primarily attributed to its low porosity level. .

Recent advances on grain refinement of magnesium rare-earth alloys during the whole casting processes:A review

High-performance cast magnesium rare-earth(Mg-RE) alloys are one of the most important materials among all developed Mg alloy families, and have shown great potential in military and weapons, aerospace and aviation, orthopedic implants, etc. Controlling grain size and distribution of it is key to the promising mechanical performance of Mg-RE alloy casting components. During the casting of a real component, nearly every procedure in the fabrication process will influence the grain refinement effect. The procedure may include and may not be limited to the chemical inoculations, possibly applied physical fields, the interfere between grain refiner and purifications, and the casting techniques with different processing parameters. This paper reviews the recent advances and proposed future developments in these categories on grain refinement of cast Mg-RE alloys. The review will provide insights for the future design of grain refinement techniques,the choosing of processing parameters, and coping strategies for the failure of coarsening for cast Mg-RE components with high quality and good performance.

Solidification microstructure of Ti-43Al alloy by twin-roll strip casting

As a near-net-shape technology,the twin-roll strip casting(TRC)process can be considered to apply to the fabrication of TiAl alloy sheets.However,the control of the grain distribution is very important in strip casting because the mechanical properties of strips are directly determined by the solidification microstructure.A three-dimensional(3D)cellular automation finite-element(CAFE)model based on ProCAST software was established to simulate the solidification microstructure of Ti-43Al alloy.Then,the influence of casting temperature and the maximum nucleation density(nmax)on the solidification microstructure was investigated in detail.The simulation results provide a good explanation and prediction for the solidification microstructure in the molten pool before leaving the kissing point.Experimental and simulated microstructure show the common texture<001>orientation in the columnar grains zone.Finally,the microstructure evolution of the Ti-43Al alloy was analyzed and the solidification phase transformation path during the TSC process was determined,i.e.,L→L+β→β→β+α→α+γ+β/B2 phase under a faster cooling rate and L→L+β→β→β+α→γ+lamellar(α_(2)+γ)+β/B2 phase under a slower cooling rate.

Sc对铸造Al-Li合金组织及力学性能的影响

铸造Al-Li合金可用于成型复杂薄壁构件,在航空航天、国防军工等领域具有广阔的应用前景。细化铸造Al-Li合金组织,提高合金强塑性对其实际应用具有重要意义。本文通过金属型铸造制备不同Sc含量的Al-2Li-2Cu-0.5Mg-xSc合金,分析了Sc含量对合金组织及力学性能的影响规律。结果表明,随着Sc含量逐渐增加,合金晶粒由树枝晶向近等轴晶转变,晶粒尺寸逐渐减小。δ'(Al_(3)Li)相倾向于以Al_(3)Sc相为形核核心,形成Al_(3)(Li, Sc)复合粒子,这些粒子在变形过程中不易被位错切过,减少了Al_(3)Li粒子引起的共面滑移。当添加0.3%Sc(质量分数)时,合金具有较好的综合力学性能,经过175℃/32 h的峰值时效处理后,屈服强度、抗拉强度和伸长率分别可以达到381.3 MPa、449.5 MPa和5.8%。

Characterization of localized corrosion pathways in 2195-T8 Al-Li alloys exposed to acidic solution

The corrosion properties of aluminum-lithium(Al-Li) alloys, which are potential materials used to construct for tanks of liquid rockets or missiles, are essential for safe propellant storage and transport. In order to manifest the corrosion resistance of the 2195 Al-Li alloy in practical propellant tanks filled with N2O4, the alloy was soaked in 30% nitric acid solution, an accelerating corrosion environment, to test its corrosion behavior. Scanning electron microscopy(SEM) and transmission electron microscopy(TEM)were used to characterize microstructure and corrosion morphology of the alloy. Focused ion beam(FIB),combined with SEM, was used to demonstrate localized corrosion features and the propagation of corrosion pathways beneath the alloy surface. It was found that the corrosion network was formed with most intergranular corrosion and sparse intragranular corrosion. Additionally, the distribution and number of intermetallic particles influenced the localized corrosion degree and the direction of corrosion pathways. Aggregated particles made corrosion pathways continuously and caused more severe corrosion. The results from this work were valid and useful to corrosion prevention and protection for storage safety on propellant tanks in N_(2)O_(4).

Experimental investigation on uniaxial cyclic plasticity of cast AZ91 magnesium alloy

The uniaxial cyclic plasticity of cast AZ91 magnesium(Mg) alloy was investigated by conducting a series of cyclic straining and stressing tests at room temperature, and a unique cyclic plasticity(especially for ratchetting) and its physical nature were revealed. The experimental results demonstrate that the cast AZ91 Mg alloy behaviors tension-compression symmetry, because the dislocation slipping and twinning occur during both the tensile and compressive deformations;although the cast AZ91 alloy presents a certain pseudo-elastic behavior during unloading due to the detwinning, there is no obvious S-shaped asymmetric hysteresis loop like that of wrought Mg alloy in the cyclic tensile-compressive tests, and an obvious cyclic hardening is observed;moreover, the ratchetting of the cast AZ91 alloy presented in the cyclic stressing tests depends remarkably on the prescribed mean stress and stress amplitude, but slightly changes with the stress rate, and the evolution of responding peak/valley strain greatly differs from that of wrought Mg alloys and stainless steels. This work provides rich experimental data for establishing the constitutive model of cast Mg alloys.

Effect of Electromagnetic Frequency on Microstructures of Continuous Casting Aluminum Alloys

The relationship between electromagnetic frequency and microstructures of continuous casting aluminum alloys was studied. 7075 aluminum alloy ingot of 100 mm in diameter was produced by electromagnetic continuous casting process, the microstructures of as-cast ingot was examined by scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS). The results showed that electromagnetic frequency greatly influenced segregation and microstructures of as-cast ingot, and product quality can be guaranteed by the application of a proper frequency. Electromagnetic frequency plays a significant role in solute redistribution; low frequency is more efficient for promoting solution of alloying elements.

Microstructure and mechanical properties of NZ30K alloy by semi-continuous direct chill and sand mould casting processes

The Mg-3.0Nd-0.2Zn-0.4Zr (NZ30K) alloys were prepared by direct-chill casting (DCC) and sand mould casting (SMC) processes,respectively and their microstructures and mechanical properties were investigated.The results indicate that casting method plays a remarkable influence on the microstructure and mechanical properties of as-cast NZ30K alloy.The grain size increases from 35-40μm in the billets made by the DCC to about 100-120μm in the billets by the SMC.The aggregation of Mg12Nd usually found at the triple joints of grain boundaries in the billets prepared by SMC while is not observable from the billets by DCC.The tensile strengths and elongations of the billets are 195.2 MPa and 15.5% by DCC,and 162.5 MPa and 3.2% by SMC,respectively.The tensile strength of the alloy by DCC is remarkably enhanced by T6 heat treatment,which reached 308.5 MPa.Fracture surfaces of NZ30K alloy have been characterized as intergranular fracture by SMC and quasi-cleavage fracture by DCC,respectively.

The Role of Particles in Fatigue Crack Propagation of Aluminum Matrix Composites and Casting Aluminum Alloys

Fatigue crack propagation （FCP） behaviors were studied to understand the role of SiC particles in 10 wt pct SiCp/A2024 composites and Si particles in casting aluminum alloy A356. The results show that a few particles appeared on the fracture surfaces in SiCp/Al composites even at high △K region, which indicates that cracks propagated predominantly within the matrix avoiding SiC particles due to the high strength of the particles and the strong particle/matrix interface. In casting aluminum alloy, Si particle debonding was more prominent.Compared with SiCp/Al composite, the casting aluminum alloy exhibited lower FCP rates, but had a slight steeper slope in the Paris region. Crack deflection and branching were found to be more remarkable in the casting aluminum alloy than that in the SiCp/Al composites, which may be contributed to higher FCP resistance in casting aluminum alloy.

Study on Multiple Electromagnetic Continuous Casting of Aluminum Alloy

To obtain semi-solid Al alloy billet with high quality, an investigation was carried out by imposing a multiple magnetic field from the outside of a copper mold in the continuous casting. AISi6Mg2 alloy designed for semi-solid metal （SSM） processing was continuously cast through a submerged entry nozzle under various conditions. Effects of multiple magnetic field on meniscus motion, temperature distribution and billet quality were examined. The experimental results showed that meniscus disturbance caused by electromagnetic stirring could be controlled effectively and the surface quality of semi-solid AI alloy billet was improved greatly, and an uniformly fine, globular microstructure across the transverse section of the billet was achieved by optimizing the distribution of multiple magnetic field.

Microstructure and mechanical properties of BFe10 cupronickel alloy tubes fabricated by a horizontal continuous casting with heating-cooling combined mold technology

A new horizontal continuous casting method with heating-cooling combined mold （HCCM） technology was explored for fabri- cating high-quality thin-wall cupronickel alloy tubes used for heat exchange pipes. The microstructure and mechanical properties of BFe 10 cupronickel alloy tubes fabricated by HCCM and traditional continuous casting （cooling mold casting） were comparatively investigated. The results show that the tube fabricated by HCCM has smooth internal and external surfaces without any defects, and its internal and external surface roughnesses are 0.64 μm and 0.85 μm, respectively. The tube could be used for subsequent cold processing without other treatments such as surface planning, milling and acid-washing. This indicates that HCCM can effectively reduce the process flow and improve the pro- duction efficiency of a BFel0 cupronickel alloy tube. The tube has columnar grains along its axial direction with a major casting texture of {012}〈 621 〉. Compared with cooling mold casting （6 = 36.5%）, HCCM can improve elongation （3 = 46.3%） by 10% with a slight loss of strength, which indicates that HCCM remarkably improves the cold extension performance of a BFe 10 cupronickel alloy tube.