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.

Structures and macrosegregation of a 2024 aluminum alloy fabricated by direct chill casting with double cooling field

Central region coarse grains and centerline segregation are common defects in aluminum ingots fabricated by direct chill(DC)casting.A double cooling field was introduced into the DC casting process to reduce these defects,whereby the external cooling was supplied by the mold and water jets,and intercooling was achieved by inserting a rod of the same alloy into the molten pool along the central axis of the ingot.Rather than forming a good metallurgical interface during solid-liquid compound casting,in the present work,the purpose of inserting the rod is to enforce internal cooling and consequently decrease the sump depth.Moreover,the insertion provides more nucleation sites with the unmoltenα-Al particles.The structure and the macrosegregation of 2024 Al alloy ingots prepared by DC casting with and without the inserts were investigated.Results show that when the inserting position is 50 mm above the upper edge of the graphite ring,significant grain refinement in the central region of the ingot and a reduced centerline segregation are achieved.

Efects of low frequency electromagnetic ield on surface quality, microstructure and hot-tearing tendency of semi-continuous casting ZK60 magnesium alloy billets

The low frequency electromagnetic field was applied during direct chill(DC) semi-continuous casting of the ZK60 magnesium alloy billets. Effects of low frequency electromagnetic field on surface quality, microstructure and hot-tearing tendency of Φ500 mm ZK60 magnesium alloy billets were investigated. The results showed that with the application of the low frequency electromagnetic field, the surface quality of the ZK60 magnesium alloy billets is markedly improved and the depth of cold fold is decreased. The microstructure of the billets is also significantly refined. Besides, the distribution of the grain size is relatively uniform from the billet surface towards its center, where the average grain size is 42 μm at surface and 50 μm at center. It also shows that the hot-tearing tendency of DC semi-continuous casting ZK60 magnesium alloy billets is significantly reduced under low frequency electromagnetic field.

Prediction of hot tearing susceptibility of direct chill casting of AA6111 alloys via finite element simulations

To predict hot tearing susceptibility(HTS)during solidification and improve the quality of Al alloy castings,constitutive equations for AA6111 alloys were developed using a direct finite element(FE)method.A hot tearing model was established for direct chill(DC)casting of industrial AA6111 alloys via coupling FE model and hot tearing criterion.By applying this model to real manufacture processes,the effects of casting speed,bottom cooling,secondary cooling,and geometric variations on the HTS were revealed.The results show that the HTS of the billet increases as the speed and billet radius increase,while it reduces as the interfacial heat transfer coefficient at the bottom or secondary water-cooling rate increases.This model shows the capabilities of incorporating maximum pore fraction in simulating hot tearing initiation,which will have a significant impact on optimizing casting conditions and chemistry for minimizing HTS and thus controlling the casting quality.

Effect of harmonic magnetic field and pulse magnetic field on microstructure of high purity Cu during electromagnetic direct chill casting

The effects of two types of magnetic fields,namely harmonic magnetic field(HMF)and pulse magnetic field(PMF)on magnetic flux density,Lorentz force,temperature field,and microstructure of high purity Cu were studied by numerical simulation and experiment during electromagnetic direct chill casting.The magnetic field is induced by a magnetic generation system including an electromagnetic control system and a cylindrical crystallizer of 300 mm in diameter equipped with excitation coils.A comprehensive mathematical model for high purity Cu electromagnetic casting was established in finite element method.The distributions of magnetic flux density and Lorentz force generated by the two magnetic fields were acquired by simulation and experimental measurement.The microstructure of billets produced by HMF and PMF casting was compared.Results show that the magnetic flux density and penetrability of PMF are significantly higher than those of HMF,due to its faster variation in transient current and higher peak value of magnetic flux density.In addition,PMF drives a stronger Lorentz force and deeper penetration depth than HMF does,because HMF creates higher eddy current and reverse electromagnetic field which weakens the original electromagnetic field.The microstructure of a billet by HMF is composed of columnar structure regions and central fine grain regions.By contrast,the billet by PMF has a uniform microstructure which is characterized by ultra-refined and uniform grains because PMF drives a strong dual convection,which increases the uniformity of the temperature field,enhances the impact of the liquid flow on the edge of the liquid pool and reduces the curvature radius of liquid pool.Eventually,PMF shows a good prospect for industrialization.

Microstructure and mechanical properties of Mg-Nd-Zn-Zr billet prepared by direct chill casting

High-quality Mg-Nd-Zn-Zr magnesium alloy billets with diameter of 200 mm were successfully prepared by direct chill(DC)casting.The results show that microstructures of the as-cast billet prepared by DC casting are mainly composed of equiaxed a-Mg and Mg 12 Nd eutectic compound distributing along the grain boundaries.The average grain size decreases along the radius of the billet.And the alloying elements of Nd and Zn distribute homogeneously across the large billet.The optimum process parameters for DC casting of the Mg-Nd-Zn-Zr magnesium alloy billet with diameter of 200 mm have been experimentally determined as follows:casting temperature 710℃ and casting speed 80 mm/min.

Effect of low frequency electromagnetic field on microstructures and macrosegregation of horizontal direct chill casting aluminum alloy

The influences of low frequency electromagnetic field on cast surface, microstructures and macrosegregation in horizontal direct chill(HDC) casting process were investigated experimentally. The cast surfaces, microstructures and macrosegregation of the ingots manufactured by conventional HDC and low frequency electromagnetic HDC casting were compared. The results show that low frequency electromagnetic field significantly improves the surface quality, refines the microstructures and reduces macrosegregation. Further more, increasing electromagnetic intensity or decreasing frequency is beneficial to the improvement. In the range of ampere-turns and frequency employed in the experiments, the optimum ampere-turns is found to be 10 000 A·turn and the frequency to be 30 Hz.

Effects of low-frequency magnetic field on grain boundary segregation in horizontal direct chill casting of 2024 aluminum alloy

Effects of low frequency electromagnetic field on grain boundary segregation in horizontal direct chill (HDC) casting process was investigated experimentally. The grain boundary segregation and microstructures of the ingots, which manufactured by conventional HDC casting and low frequency electromagnetic HDC casting were compared. Results show that low frequency electromagnetic field significantly refines the microstructures and reduces grain boundary segregation. Decreasing electromagnetic frequency or increasing electromagnetic intensity has great effects in reducing grain boundary segregation. Meanwhile, the governing mechanisms were discussed.

Effect of Low Frequency Electromagnetic Field on Macrosegregation of Horizontal Direct Chill Casting Aluminum Alloys

The horizontal direct chill (HDC) casting process is a well-established production route for aluminum alloy ingot but the ingot may suffer from macrosegregation sometimes. In order to control the defect, a low frequency electromagnetic field has been applied in HDC casting process and the relevant influence has been studied. The results show that application of low frequency electromagnetic field can reduce macrosegregation in HDC casting process; and two main parameters of electromagnetic field density and frequency, have great influences on the solution distribution along the diameter of ingot. Moreover, the mechanisms of reduction of macrosegregation by electromagnetic field have been discussed.

Thermal-stress Simulation of Direct-chill Casting of AZ31 Magnesium Alloy Billets

Two-dimensional （2D） transient coupled finite element model was developed to compute the temperature and stress field in cast billets, so as to predict the defects of the I-type billets made from AZ31 magnesium alloy and find the causes and solutions for surface cracks and shrinkages during direct-chill （DC） casting process. Method of equivalent specific heat was used in the heat conduction equation. The boundary and initial conditions used for primary and secondary cooling were elucidated on the basis of the heat transfer during the solidification of the billet. The temperature and the thermal-stress fields were simulated with the thermal-structural coupled module of ANSYS software. The influences of casting parameters on the distributions of temperature and stress were studied, which helped optimize the parameters （at pouring temperature of 680 ℃, casting speed of 2 min/s, heat-transfer coefficient of the second cooling equals to 5 000 W/m^2.℃^-1）. The simulation results of thermal stress and strain fields reveal the formation mechanism of some casting defects, which is favourable for optimizing the casting parameters and obtain high quality billets. Some measures of controlling processes were taken to prevent the defects for direct-chill casting billets.

EFFECT OF LOW-FREQUENCY ELECTROMAGNETIC FIELD ON THE AS-CASTING MICROSTRUCTURES AND MECHANICAL PROPERTIES OF HDC 2024 ALUMINUM ALLOY

The influences of the low frequency electromagnetic field on the horizontal direct chill casting process were investigated experimentally. Ingots of 2024 aluminum alloy with a cross size of 40 mm× 200 mm were produced by the conventional horizontal chill casting process and low frequency electromagnetic horizontal chill casting processre- spectively. The as-cast structures and the mechanical property of the ingots were examined. The results showed that the low frequency electromagnetic field could sub- stantially refine the microstructures and pronouncedly reduce the macrosegregation in the horizontal direct chill casting process. Moreover, the surface quality of the ingot was prominently improved by the low frequency electromagnetic field. The fracture strength and elongation percentage of the ingot was increased with the low frequency electromagnetic field.

Effect of withdrawal rate on microstructure of a hypoeutectic Al-Si alloy by low superheat DC casting

To obtain the billet with homogeneous and spheroidized primary grains is the key step in the semi- solid forming process. Among the semi-solid billet preparation methods, the process of low-superheat direct chill （DC） casting is simpler and more effective. In this study, the billets of AISi7Mg alloy were prepared by low- superheat DC casting at various withdrawal rates. The effect of withdrawal rate on the surface quality of the billets was investigated, and the evolution mechanism of the microstructural morphology in the billets was analyzed. The results show that the periodic corrugations and a large quantity of fine shrinkage pits appear in the billet surface when the withdrawal rate is 100 mm.min-1, and the above defects in the billet surface can be eliminated completely when the withdrawal rate is above 150 mm.min-1. But when the withdrawal rate is too fast （250 mm.min-1）, the primary a grains, except the ones in the billet center, have not enough time for ripening due to the high silidification rate, and will solidify as the dendrite structure. When the withdrawal rate is between 150 - 200 mm.min-1, the dendritic growth of the primary a grains is effectively inhibited, and a billet of AISi7Mg alloy with a smooth surface and homogeneous, fine, non-dendritic grains can be obtained.

Investigation of Out-Phase Electromagnetic Casting of Aluminum Alloy

The influences of out-phase electromagnetic field on the as-cast structure of horizontal direct chill casting aluminum alloy ingot were experimental by and numerical by studied. The results of numerical analysis show that the interaction of the out-phase electromagnetic field and the melt can generate an electromagnetically induced forced flow in the melt, which, in turn, changes flow pattern and temperature field in the mold. The as-cast structure of the ingot can be greatly improved by the changes of flow pattern and temperature field. The results of experimental analysis show that with application of out-phase electromagnetic field, temperature distribution in the melt is more uniform, grain morphology changes from columnar grains to equiaxed grains and grain size decreases.

Effect of casting speed on surface quality of horizontal direct chill casting 7075 aluminum alloy ingot

7075 aluminum alloy ingot with the diameter izontal direct chill casting in different casting of 100 mm has been produced by horspeed. The effect of the casting speed on the ingot surface and subsurface layer was studied by surface observation and subsurface structure analysis. It was found that increasing the casting speed results in the adding of segregation knots in the ingot surface. The thickness of the dendrite microstrueture layer in the subsurface reduces with increasing the casting speed. And the elements of Zn, Cu and Mg enrich in the coarse dendrite microstructure layer of the ingot.

航空用高品质7050铝合金大规格扁锭熔铸技术研究

7050铝合金具有密度低、强度高、热加工性能优异等优点,在航空航天结构件上应用广泛。但在7050铝合金大规格铸锭半连续铸造过程中,存在热裂、成形困难、严重显微疏松和氧化夹杂等问题。本研究开发了大规格扁锭半连续铸造过程中合金熔体冶金质量控制技术,系统考察了精炼工艺、晶粒细化技术对大规格扁锭品质的影响规律,确定了大规格扁锭铸造工艺。结果表明,优化工艺制度能严控熔体内氢和夹杂物含量,采用Al-Ti-C晶粒细化工艺制度将晶粒度控制在1~1.5级,高质量的熔体质量缓解了合金凝固收缩应变,极高比例地解决了扁锭开裂问题。最终,形成了一套适用于大规格7050合金扁锭的熔铸工艺制度。

AZ31镁合金圆锭连铸过程温度场的数值模拟

通过有限差分法,利用Visual C++6.0建立直接水冷法(DC)AZ31镁合金连铸过程的数学模型,该模型可预测铸锭以及底模的温度分布。通过对铸造过程物理现象的理论研究,确定一冷区、二冷区以及铸锭与底模之间的边界条件。通过与文献实测温度的比较,证明该模型可以用来模拟实际铸造过程。考察不同铸造条件对铸锭温度场分布的影响,为优化镁合金直冷连铸的工艺参数提供了依据。

电磁铸造对Al-Zn-Mg-Cu-Zr合金微观组织及晶内固溶度的影响

分别采用电磁铸造和普通连铸技术制备Al-Zn-Mg-Cu-Zr合金,研究电磁场对合金显微组织、合金元素晶内固溶度的影响。结果表明:采用电磁铸造技术制备的Al-Zn-Mg-Cu-Zr合金具有良好的表面质量和内部组织;电磁场显著提高合金元素在晶内的固溶度,减小了微观偏析程度。相对于普通连铸,电磁铸造法制备的Al-Zn-Mg-Cu-Zr合金的晶粒平均尺寸从68μm减小到53μm,Zn、Mg和Cu元素的晶内相对溶质固溶度分别由56%、62%和25%提高到79%、74%和44%。

低频电磁水平半连续铸造中磁场的分布及其对宏观偏析的影响

采用数值模拟的方法研究了低频电磁水平半连续铸造7075铝合金中不同强度及频率的磁场在熔体中的分布,并采用试验的方法研究了不同条件的磁场对铸锭宏观偏析的影响。将使用传统水平半连续铸造生产出的铸锭与低频电磁水平半连续铸造生产的铸锭进行了比较。研究结果表明:在传统水平半连续铸造工艺条件下,铸锭中易出现较严重的负偏析及重力偏析;然而,在低频电磁水平半连续铸造过程中,低频电磁场的存在明显削弱了铸锭中的宏观偏析。此外,磁场在熔体中的分布情况将直接影响其作用效果,提高磁场强度及适当降低磁场频率均有利于对宏观偏析的改善。

低频电磁场对水平半连续铸造7075铝合金的影响

采用实验的方法研究了低频电磁场对水平半连续铸造铝合金的表面质量及铸态组织的影响·将使用传统水平半连续铸造生产出的铸锭与低频电磁水平半连续铸造生产的铸锭进行了比较·研究结果表明,低频电磁场可有效地减少铸锭的表面缺陷并显著改善铸锭的微观组织·此外,提高磁场的强度或适当降低磁场的频率均有利于对铸锭质量的改善·实验所采用的安匝数及频率范围中,最优的条件分别为10000,30Hz·

EMS-DC法制备的半固态ZL101A铝合金的组织与性能

研究了EMS DC法工艺参数对半固态ZL1 0 1A合金组织的影响 ,阐明了浇注温度 (中间包温度 )、拉坯速度、冷却速度与合金组织形貌的关系。研究表明 :在 0 .0 8～ 0 .0 9T的磁感应强度作用下 ,当铝合金在固液两相区的冷却速度小于 0 .3℃ /s时 ,即能得到较理想的非树枝晶组织。在优化各工艺参数的基础上 ,形成了半固态ZL1 0 1A合金的水平半连续铸造中试生产技术 ,其连铸坯铸态和热处理态强塑性综合指标分别为 430 .4和 497.4。