压铸镁合金缺陷带的组织特征及形成机理

研究了压铸镁合金缺陷带的组织形貌及分布特征,建立缺陷带与压室预结晶(ESCs)、气缩孔及压铸工艺参数之间的对应关系,在此基础上探讨了缺陷带的形成及演化机理。结果表明,压铸镁合金截面凝固组织以缺陷带为界可划分为3个有明显组织特征差异的区域。随着压铸低速速度提高,镁合金凝固组织中ESCs含量逐渐降低,而缺陷带组织愈加明显并向压铸件中心靠拢,缺陷带宽度减小,其内部孔洞更加聚集;高速速度越大,压铸镁合金凝固组织中ESCs越分散,缺陷带向压铸件中心靠拢,压铸在无高速速度情况下,镁合金凝固组织横截面出现双缺陷带现象。压铸充型过程中金属液流的形态决定了缺陷带的分布位置和发展趋势,在高速金属液的剧烈冲刷及增压压力的作用下,靠近金属液流外轮廓的晶粒发生破碎或转动,在晶粒间形成大于剩余金属液体积的间隙,随着凝固的进行,形成沿液流轮廓分布孔洞聚集的缺陷带组织。     

镁合金AZ91D真空压铸件的气孔分布

总结了镁合金真空压铸的优点,并进行了镁合金AZ91D的真空压铸,比较了真空压铸件和普通压铸件不同位置的气孔分布情况,发现真空压铸在降低镁合金压铸件气孔率方面有很大的作用;同时观察了两种压铸件热处理后表面气泡的分布情况,镁合金真空压铸大大降低压铸件中的气孔含量.     

基于数值模拟的镁合金普通压铸和真空压铸件性能分析

通过Flow-3D软件,对镁合金散热器普通压铸及真空压铸进行模拟仿真,对比充型过程的缺陷和卷气结果,并进行产品试制。通过分析普通压铸和真空压铸件的微观组织和力学性能,发现真空压铸能够获得充型完整、外观优良的镁合金压铸件,并且其抗拉强度和伸长率较普通压铸件分别提高14.1%和42.1%。     

基于分形理论及数理统计的压铸件内部孔洞缺陷定量分析

对分布不规则的压铸件内部孔洞采用分形理论与数理统计的原理进行了分析.根据铸件切面的孔洞金相照片,采用计算机图像处理方法得到了反映孔洞缺陷分布均匀性的分形维数、孔洞面积的百分比、孔洞半径分布方差、孔洞的平均半径4个定量的参量.利用这4个定量参数可以综合的反映铸件内孔洞位置的分布状态和孔洞大小及分布状态.通过不同压铸工艺镁合金铸件内部气孔的分析,对计算和分析结果进行了验证.     

高真空压铸技术在汽车变速箱壳体中的应用

以变速箱壳体压铸生产中的缺陷为研究对象,采用高真空压铸技术,改善了铸件的内部气孔状态和力学性能。对比发现,使用高真空压铸工艺后,压铸件的油道漏气率大幅降低,合格率达到97%,降低了成本。高真空压铸件的抗拉强度和伸长率分别比普通真空铸件提高了约17.4%和37.1%。     

真空压铸对LA42镁合金热导率和力学性能的影响

以Mg-4La-2Al-0.3Mn(LA42)合金为研究对象，利用OM、XCT等方法，结合复合材料的导热模型、强化模型，研究了真空压铸工艺对合金热导率和力学性能的影响。结果表明，真空压铸对LA42合金的热导率影响不大，归因于压铸件的孔隙率低，其相比固溶原子对热导率的影响，可以忽略不计，但屈服强度和伸长率相比常规压铸件分别提高了5.8%和25.7%。屈服强度的提升主要归因于真空压铸条件下预结晶组织的减少和细小等轴晶分数的提高，伸长率的提升主要归因于真空压铸条件下孔洞缺陷和预结晶组织分数的降低。     

镁合金冷室压铸组织中压室预结晶组织的研究

镁合金压铸过程中压室预结晶对压铸件最终凝固组织及使用性能具有重要的影响。针对冷室压铸工艺,论述了压铸镁合金凝固组织中压室预结晶组织的研究所取得的成果,包括压室预结晶的组织特征,压室预结晶现象的试验及模拟验证,压铸件不同位置及工艺参数下压室预结晶的形态、分布和含量,以及压室预结晶组织对镁合金压铸件力学性能及断裂失效的影响机制。最后分析了现有冷室压铸镁合金压室预结晶研究所存在的不足并对未来的发展方向进行了展望。     

不同快压射速度对镁合金真空压铸件成形质量的影响

以AZ91D镁合金为研究对象,采用自主设计的真空抽气系统进行3组真空压铸试验,研究不同快压射速度对真空压铸件力学性能及组织的影响。结果表明,该真空抽气系统制得的压铸件外形完整,力学性能优于普通压铸件。当快压射速度为3 m/s时,压铸件表面容易形成冷隔和流痕且内部组织存在多处缩松、缩孔;当快压射速度为5 m/s时,压铸件则容易出现飞边且内部组织存在多处缩孔;当快压射速度为4 m/s时,压铸件外形完整且内部只有少许缩孔,此时散热片的抗拉强度为226 MPa,伸长率为5.4%。     

考虑压室预结晶的镁合金压铸组织实验及模拟研究

针对镁合金压铸件表层晶粒细小均匀而心部晶粒较粗大且不均匀的特点,开展一系列压铸实验,研究镁合金压铸工艺对压室预结晶和压铸件最终凝固组织(特别是对晶粒大小、压室预结晶组织的含量及分布形态)的影响.采用数值模拟方法,以压铸实验统计数据为基础,建立了考虑压室预结晶的镁合金压铸形核模型,以此模拟更接近实际的压铸组织.应用所建立的形核及生长模型,模拟了不同压铸工艺下AM50镁合金阶梯压铸件的凝固组织,模拟结果与实验结果吻合较好.     

真空压铸工艺参数对AM50镁合金力学性能的影响规律

采用阶梯试验模具及AM50合金,进行了系统的真空压铸试验,实测了不同厚度的阶梯试样在不同工艺条件下的密度及力学性能,研究了高真空压铸工艺参数对AM50镁合金力学性能的影响规律.结果表明,随着型腔真空压力的降低,铸件密度、抗拉强度和伸长率均随之提高;铸造压力对力学性能的影响在真空压铸和常规压铸中遵循基本相同的规律,即增大铸造压力可以使铸件的致密程度、抗拉强度、屈服强度和伸长率得到提高;随着高速速度的增大,薄壁铸件的抗拉强度、屈服强度和伸长率均表现出明显的增加,这一点与常规压铸的规律相反.结合高真空和高速工艺,可以使薄壁铸件的抗拉强度和伸长率得到较为明显的提升.     

Effects of Post-weld Heat Treatment on Microstructure,Mechanical Properties and the Role of Weld Reinforcement in 2219 Aluminum Alloy TIG-Welded Joints

In as-welded state, each region of 2219 aluminum alloy TIG-welded joint shows diff erent microstructure and microhardness due to the diff erent welding heat cycles and the resulting evolution of second phases. After the post-weld heat treatment, both the amount and the size of the eutectic structure or θ phases decreased. Correspondingly, both the Cu content in α-Al matrix and the microhardness increased to a similar level in each region of the joint, and the tensile strength of the entire joint was greatly improved. Post-weld heat treatment played the role of solid solution strengthening and aging strengthening. After the post-weld heat treatment, the weld performance became similar to other regions, but weld reinforcements lost their reinforcing eff ect on the weld and their existence was more of an adverse eff ect. The joint without weld reinforcements after the post-weld heat treatment had the optimal tensile properties, and the specimens randomly crack in the weld zone.     

The First Phase of the SRIF Industrialized Base in ＂West Area＂ Has Been Put into Operation

After nearly two years＇ tense construction, the first phase of industrialized base of Shenyang Research Institute of Foundry （SRIF）, located at the Tiexi Casting and Forging Industrial Park in the west of Tiexi District, has now been completed and formally put into operation.     

Thermodynamics Study on the Decomposition of Chromite with KOH

Institute of Process Engineering, Chinese Academy of Sciences, China, has proposed a method for oxidative leaching of chromite with potassium hydroxide. Understanding the mechanism of chromite decomposition, especially in the potassium hydroxide fusion, is important for the optimization of the operating parameters of the oxidative leaching process. A traditional thermodynamic method is proposed and the thermal decomposition and the reaction decomposition during the oxidative leaching of chromite with KOH and oxygen is discussed, which suggests that chromite is mainly destroyed by reactions with KOH and oxygen. Meanwhile, equilibrium of the main reactions of the above process was calculated at different temperatures and oxygen partial pressures. The stable zones of productions, namely, K2CrO4 and Fe2O3, increase with the decrease of temperature, which indicates that higher temperature is not beneficial to thermodynamic reactions. In addition, a comparison of the general alkali methods is carried out, and it is concluded that the KOH leaching process is thermodynamically superior to the conventional chromate production process.     

Influence of Heat Treatment on Damping Behaviour of the Magnesium Wrought Alloy AZ61

The effect of isochronal heat treatments for 1h on variation of damping, hardness and microstructural change of the magnesium wrought alloy AZ61 was investigated. Damping and hardness behaviour could be attributed to the evolution of precipitation process. The influence of precipitation on damping behaviour was explained in the framework of the dislocation string model of Granato and Lücke.     

Effect of Rare Earth Doping on Impedance,Modulus and Conductivity Properties of Multiferroic Composites:0.5(BiLa_xFe_(1-x)O_3)–0.5(PbTiO_3)

The Lanthanum-doped bismuth ferrite–lead titanate compositions of 0.5(Bi LaxFe1-xO3)–0.5(Pb Ti O3)(x = 0.05,0.10,0.15,0.20)(BLxF1-x-PT) were prepared by mixed oxide method.Structural characterization was performed by X-ray diffraction and shows a tetragonal structure at room temperature.The lattice parameter c/a ratio decreases with increasing of La(x = 0.05–0.20) concentration of the composites.The effect of charge carrier/ion hopping mechanism,conductivity,relaxation process and impedance parameters was studied using an impedance analyzer in a wide frequency range(102–106Hz) at different temperatures.The nature of Nyquist plot confirms the presence of bulk effects only,and non-Debye type of relaxation processes occurs in the composites.The electrical modulus exhibits an important role of the hopping mechanism in the electrical transport process of the materials.The ac conductivity and dc conductivity of the materials were studied,and the activation energy found to be 0.81,0.77,0.76 and 0.74 e V for all compositions of x = 0.05–0.20 at different temperatures(200–300 °C).     

Using Finite Element and Contour Method to Evaluate Residual Stress in Thick Ti-6Al-4V Alloy Welded by Electron Beam Welding

This work was to reveal the residual stress profile in electron beam welded Ti-6Al-4V alloy plates(50 mm thick) by using finite element and contour measurement methods.A three-dimensional finite element model of 50-mmthick titanium component was proposed,in which a column–cone combined heat source model was used to simulate the temperature field and a thermo-elastic–plastic model to analyze residual stress in a weld joint based on ABAQUS software.Considering the uncertainty of welding simulation,the computation was calibrated by experimental data of contour measurement method.Both test and simulated results show that residual stresses on the surface and inside the weld zone are significantly different and present a narrow and large gradient feature in the weld joint.The peak tensile stress exceeds the yield strength of base materials inside weld,which are distinctly different from residual stress of the thin Ti-6Al-4V alloy plates presented in references before.     

Microstructures and Tensile Properties of Ultrafine-Grained Ni–(1–3.5) wt% SiC_(NP) Composites Prepared by a Powder Metallurgy Route

Silicon carbide nanoparticle-reinforced nickel-based composites(Ni–Si CNP),with a Si CNPcontent ranged from1 to 3.5 wt%,were prepared using mechanical alloying and spark plasma sintering.In addition,unreinforced pure nickel samples were also prepared for comparative purposes.To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–Si CNPcomposites transmission electron microscopy(TEM) was used,while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests.TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample,whereas,for the Ni–Si CNPcomposites,the presence of nano-dispersed Si CNPand twinning crystals was observed.These homogeneously distributed Si CNPwere found located either within the matrix,between twins or on grain boundaries.For the Ni–Si CNPcomposites,coerced coarsening of the Si CNPassembly occurred with increasing Si CNPcontent.Furthermore,the grain sizes of the Ni–Si CNPcomposites were much finer than that of the unreinforced pure nickel,which was considered to be due to the composite ball milling process.In all cases,the Ni–Si CNPcomposites showed higher strengths and hardness values than the unreinforced pure nickel,likely due to a combination of dispersion strengthening(Orowan effects) and particle strengthening(Hall–Petch effects).For the Ni–Si CNPcomposites,the strength increased initially and then decreased as a function of Si CNPcontent,whereas their elongation percentages decreased linearly.Compared to all materials tested,the Ni–Si CNPcomposite containing 1.5% Si C was found more superior considering both their strength and plastic properties.     

Directional Growth of Tin Crystals Controlled by Combined Solute Concentration Gradient Field and Static Magnetic Field

A new method was introduced to achieve directional growth of Sn crystals. Microstructures in liquid(Pb)/liquid(Sn) diffusion couples were investigated under various static magnetic fields. Results show that the β-Sn crystals mainly reveal an irregular dendritic morphology without or with a relatively low static magnetic field(B0.3 T). When the magnetic field is increased to 0.5 T, the β-Sn dendrites close to the final stage of growth begin to show some directional character. With a further increase in the magnetic field to a higher level(0.8–5 T), the β-Sn dendrites have an enhanced directional growth character, but the dendrites show a certain deflection. As the magnetic field is increased to 12 T, the directional growth of the β-Sn dendrites in the center of the couple is severely destroyed. The mechanism of the directional growth of the β-Sn crystals and the deflection of the β-Sn crystals with the application of static magnetic field was tentatively discussed.     

Quantitative Analysis of the Crystallographic Orientation Relationship Between the Martensite and Austenite in Quenching–Partitioning–Tempering Steels

The orientation relationships(ORs)between the martensite and the retained austenite in low-and medium-carbon steels after quenching–partitioning–tempering process were studied in this work.The ORs in the studied steels are identified by selected-area electron diffraction(SAED)as either K–S or N–W ORs.Meanwhile,the ORs were also studied based on numerical fitting of electron backscatter diffraction data method suggested by Miyamoto.The simulated K–S and N–W ORs in the low-index directions generally do not well coincide with the experimental pole figure,which may be attributed to both the orientation spread from the ideal variant orientations and high symmetry of the low-index directions.However,the simulated results coincide well with experimental pole figures in the high-index directions{123}_(bcc).A modified method with simplicity based on Miyamoto’s work was proposed.The results indicate that the ORs determined by modified method are similar to those determined by Miyamoto’method,that is,the OR is near K–S OR for the low-carbon Q–P–T steel,and with the increase of carbon content,the OR is closer to N–W OR in medium-carbon Q–P–T steel.     

Growth Kinetics of Single Inclusion Particle in Molten Melts

On the basis of the single-particle framework, a new theory on inclusion growth in metallurgical melts is developed to study the kinetics of inclusion growth on account of reaction and collision. The studies show that the early growth of inclusion depends on reaction growth and Brawnian motion collision, and where the former is decisive, the late growth depends on turbulence collision and Stokes＇ collision, and where the former is dominant; collision growth is very quick during the smelting process, lessened in the refining process, but nearly negligible in the continuous casting process.