Principles and practice of low pressure-expendable pattern casting process for magnesium alloys

A newly developed low-pressure expendable pattern casting (LP-EPC) process was introduced and its basic principles or effect factors were further analyzed. According to theoretical calculation and experimental results, the major casting parameters that are of great and critical importance on the process include pressure and flux of filling gas, decomposition characteristic and density of foam pattern, thickness and permeability of coating, pouring temperature, vacuum degree and their combination. Most of casting defects can be effectively avoided by choosing the suitable parameters. The success achieved in pouring motor housing and exhaust manifold castings demonstrates the advantages of LP-EPC process in the production of high-complicated castings with high dimension accuracy.

Application of digital pattern-less molding technology to produce art casting

Compared with the conventional casting process, digital pattern-less casting technology has many advantages such as good machining accuracy, a short processing cycle, and low production cost. It is a new rapid manufacturing technology for castings, integrated with CAD/CAM, casting, CNC machining and many other advanced technologies. With this digital casting technology, no pattern is needed for making molds; it is precise, f lexible, and green. Usually, art castings have complex structures and are made in small batches or even made in a single-piece, especially for large-sized art castings. So it has the shortcomings of high cost, low eff iciency and long time for making a pattern to produce art castings with the conventional casting processes. However, the digital pattern-less casting technology can be applied to fabricate art castings, since it can greatly shorten the manufacturing cycle and lower the production cost, thus having a very good prospect. In this study, based on the digital pattern-less casting technology, a plaque casting with artistic Chinese characters(a Chinese poem) was designed and manufactured, and the production process was demonstrated in detail.

Research on cracking mechanism of the thin shell mould in expendable pattern shell casting during pattern removal process

Aiming at the cracking phenomenon of the thin shell mould in the expendable pattern shell casting during the pattern removing process, some systemic researches are presented.The influence of the pattern removing method and temperature on the pattern removing were investigated.The shell mould cracking mechanism was analyzed by using thermo-gravimetric analysis (TGA), and combining the temperature field and the volume change of the expanded polystyrene (EPS) foam pattern being tested.The results indicated that the shell mould was not easily cracked when the pattern removing process was carried out with the furnace being heated little by little because of the shell slowly shrinking with dehydration and shell strength gradually increasing.The shell mould was soon destroyed when it was set directly into the furnace at above 400 oC because of the thin shell mould rapidly shrinking and the foam pattern hindering.However, the shell mould had no cracking when it had been preheated for a long time even if the furnace temperature was above 400 oC and the shell was put into the furnace directly.Moreover, when the shell mould was directly set into the furnace at lower temperatures, 250 to 300 ℃, the shell would shrink slowly and the foam pattern would stay at the maximum expansion stage temperature of 100 to 110 ℃ for a long time; and the shell mould would experience an expansion force from the foam pattern for a long time.The expansion force is related to the pattern removing temperature, holding time, foam pattern thickness and density.Therefore, the foam pattern with higher density could make the shell crack.

Diagnosis parameters of mold filling pattern for optimization of a casting system

For optimal design of a gating system,the setting of diagnosis parameters is very important.In this study,the permanent mold casting process was selected because most of the other casting processes have more complicated factors that influence the mold filling pattern compared to the permanent mold casting process,such as the surface roughness of mold,gas generation from the mold wash and binder of sand mold,and the gas permeability through a sand mold,etc.Two diagnosis parameters(flow rate difference and arrival time difference) of molten metal flow pattern in the numerical simulation are suggested for design of an optimum casting system with a permanent mold.The results show that the arrival time difference can be used as one important diagnosis parameter of the complexity of the runner system and its usefulness has been verified via making aluminum parts using permanent mold casting(Fig.9).

Effect of process parameters on density of magnesium alloy parts by low-pressure expendable pattern casting

The combination of magnesium alloys with the low-pressure expendable pattern casting(LP-EPC) process would bright future for application of magnesium alloys. The researches are focused on the effect of process parameters on the internal casting quality of magnesium alloy parts. AZ91D magnesium alloy castings were produced for different combinations of the LP-EPC process parameters. Specifically,pouring temperature,vacuum,filling velocity and coupling action of these factors were manipulated to observe their effect on the casting porosity and density distribution. The results indicate that the pouring temperature with LP-EPC process is lower than it in gravity casting. The selected process parameters,such as vacuum,filling velocity and coupled modes of them,must ensure melt metal flowing front profile exhibiting smooth and convex shape. The optimal process parameters for the castings are pouring temperature 983-1 023 K,vacuum 0.02-0.03 MPa,filling velocity 60-95 mm/s,and simultaneous filling with sucking.

Microstructure and mechanical properties of AZ91D magnesium alloy by expendable pattern shell casting with different mechanical vibration amplitudes and pouring temperatures

To refine the microstructure and improve the mechanical properties of AZ91 D alloy by expendable pattern shell casting(EPSC),the mechanical vibration method was applied in the solidification process of the alloy.The effects of amplitude and pouring temperature on microstructure and mechanical properties of AZ91 D magnesium alloy were studied.The results indicated that the mechanical vibration remarkably improved the sizes,morphologies and distributions of the primaryα-Mg phase andβ-Mg17 Al12 phase,and the densification and tensile properties of the AZ91 D alloy.With an increase in amplitude,the microstructures were gradually refined,resulting in a continuous increase in mechanical properties of the AZ91 D alloy.While,with the increase of pouring temperature,the microstructures were continuously coarsened,leading to an obvious decrease of the mechanical properties.The tensile strength and yield strength of the AZ91 D alloy with a vibration amplitude of 1.0 mm and a pouring temperature of 730℃were 60%and 38%higher than those of the alloy without vibration,respectively.

Mg alloy surface alloying layer fabricated through evaporative pattern casting technology

The influencing factors of surface alloying layer by evaporative pattern casting technology were investigated.A certain thickness alloying layer was formed on the surface of Mg-alloy matrix when the pouring temperature was 780°C with different vacuum degree and alloying powder size.The surface layer microstructure,micro area composition of the new phase formed on the matrix and the composition characteristics on the surface layer were examined by SEM and element scanning.The results show that the content of aluminum increases greatly on the surface layer.The micro-hardness of alloyed layer has a more obvious increase compared with that of the matrix.The size of alloying element and the vacuum degree are the key factors influencing the alloying layer,with the increase of element powder size from 0.074 to 0.15 mm and vacuum degree from 0.04 to 0.06 MPa,the surface alloying effect becomes better.

An investigation on bonding interface microstructure of ceramic coating prepared on AZ91D by evaporated pattern casting technique

PbO-ZnO-Na2O ceramic coating was fabricated on the AZ91D Mg-alloy substrate surface by using of evaporated pattern casting(EPC) process.The ceramic coating was characterized through scanning electron microscopy(SEM) observation,energy dispersive X-ray spectrometer(EDS) and so on.The research was emphasized on the formation process of ceramic coating and the interface bonding conditions between ceramic coating and the substrate.Results show that the glass powder(PbO-ZnO-Na2O) melts when contacts with the high temperature liquid metal,and solidifies on the surface of the substrate with the decrease of temperature.Therefore,the ceramic coating was successfully prepared with the formation of the bonding interface with the substrate.Beside the influence of coating layer thickness,the vacuum level was also investigated.Further analysis indicates that oxide inclusions and decomposition products of foam pattern had a significant effect on the bonding interface.To obtain a good bonding interface between the ceramic coating and the substrate,the metal liquid oxidation and inclusions must be decreased and the decomposition products of foam pattern should be exhausted from the EPC coating completely.

Rapid casting technology based on selective laser sintering

Selective laser sintering(SLS),as a kind of additive manufacturing technology,which uses a laser beam to scan and heat powder material layer by layer to form parts(models),is widely used in the field of casting,mainly for preparing casting coated sand cores,investment casting patterns,etc.The SLS technique facilitates rapid casting and shortens the casting production periods by eliminating mold preparation.In this study,we reached conclusions for the basic principles and characteristics of SLS methods,and focused on the research status,key technology and development trend of SLS in the fields of forming coated sand-casting molds and investment casting patterns.

Investigation of carbon contamination in lost foam castings of low carbon steel

Lost foam casting(LFC) process is a special casting method in which polymeric foam patterns with refractory coatings are utilized as a mould component. In this work, four types of foam: expandable polyethylene(EPE), expandable polypropylene(EPP) and expandable polystyrene(EPS) foams with two different densities were employed as pattern materials. LFC and conventional green sand mould casting methods were used to cast a low carbon steel, A216 Grade WCB. Both casting processes were carried out at 1,580 °C. Chemical analysis results showed that the carbon contamination level was high and was influenced by pattern type. Metallographic investigations revealed a significant increase in the percentage of pearlite phase in all LFC samples. Densities of manufactured samples were calculated in order to evaluate porosity of the products. It was determined that the densities of the LFC samples were lower than the green sand mould cast reference sample(RS). Vickers hardness tests were also carried out and increments in hardness values with increased carbon content was observed.

Study of the Interface between Steel Insert and Aluminum Casting in EPC

为钢的有效表面处理方法与艾尔一起插入 composited 在钢之间的过程和结合的接口插入的由可耗尽的模式扔的碱金属(EPC ) 和艾尔碱金属被调查。钢上的 Zn plating 插入，这被发现在改进在钢之间的结合的性质上是有效的插入，在 EPC process.Zn 的艾尔碱金属被认为支持扩散层的形成。但是 Zn 的几乎没有内容在是由艾尔，组成的钢 insert.A 扩散层上的 plated 的边界被观察 Si 和 Fe 在 insert/alloy 接口被形成，它的坚硬比钢高是插入当然的事艾尔碱金属。这层结果是 Al-Si-Fe 系统的金属间化合的混合物。更高的流温度支持了 Fe 的散开进艾尔合金，因此在金属间化合的层的 Fe 内容在更高倒温度增加了。接近钢的层由于应用压力消失了。

Analysis of three-dimensional vortexes below the free surface in a continuous casting mold

To study fluctuations of the free surface of liquid steel in the mold,two different models with the same casting conditions but different thicknesses were employed to analyze the hydrodynamic behavior at the top of the mold.The first model was a standard thickness slab,and the second had a thickness three times wider.It is found with the second model that above the plane formed by the steel jets,it is possible to observe four three-dimensional vortexes that interact with the submerged entry nozzle(SEN)and mold walls.By using a biphasic model to simulate the interface between the liquid and air inside the mold,the flow asymmetry and the fluctuations of the free surface can be clearly observed.

Control of liquid column height in electromagnetic casting with fuzzy neural network model

The control of suitable and stable height of liquid column is the crucial point to operate the electromagnetic casting(EMC) process and to obtain ingots with desirable shape and dimensional accuracy. But due to the complicated interact parameters and special circumstances, the measure and control of liquid column are quite difficult. A fuzzy neural network was used to help control the liquid column by predicting its height on line. The results show that the stabilization of the height of liquid column and surface quality of the ingot are remarkably improved by using the neural network based control system.

Study on accuracy of casting flow simulation

In the field of casting flow simulation, the application of body-fitted coordinate(BFC) has not been widely used due to the difficulty and low efficiency of grid generation, despite the availability of good quality analysis results. Cartesian coordinates, on the other hand, have been used predominantly in casting process simulations because of their relatively easy and fast grid generation. However, Cartesian grid systems cannot obtain accurate results because they cannot express the geometries properly. In this study, Cut Cell method was applied to solve this problem. The three-dimensional incompressible viscous governing equation was analyzed using a function defined for the volume and area of the casting in the cutting cell. Using the Cut Cell method, accurate flow analysis results were also obtained in the Cartesian grid systems. The tests of simple shape and the applications of actual casting product have been tried with Cut Cell method.

CAST工艺的强化脱氮研究

根据李家沱污水处理厂的实际运行经验以及对近7个月的运行数据的分析，得出适用于城市污水处理厂CAST工艺脱氮的运行工况和运行参数值。结果显示，运行工况、污泥回流比和碳氮比是影响CAST工艺脱氮效果的主要因素，通过合理调整运行工况可减弱低进水碳氮比和高氮负荷带来的不利影响；在污泥龄为15d、MISS为3500-4500mg／L、回流比为30％时，脱氮率可达到70％以上，出水TN稳定达标。

Fabrication of agarose hydrogel with patterned silver nanowiresfor motion sensor

In this work, a facile strategy is proposed to construct stretchable electronics based on agarose hydrogels. The hot agarose solution is casted onto a template with patterned Ag nanowires, endowing agarose hydrogel with patterned conductive surface. After further heating treatment, Ag nanowires can be embedded into the agarose hydrogel, which improves the stability of Ag pattern and has no obvious e ffect on the conductivity of hydrogels. The agarose hydrogel with patterned Ag nanowires is certi fied to be an e ffective stretchable electrode to record the motion of joints, which has great potential applications in the field of wearable devices.

搅拌叶片表面原位合成TiC颗粒增强高锰钢复合层的制备及性能

采用燃烧合成反应铸渗结合真空消失模铸造工艺,在ZG120Mn13Cr2高锰钢搅拌叶片表面制备原位合成TiC颗粒增强高锰钢复合层,研究了该复合层的显微组织、硬度以及在湿砂磨损条件下的耐磨性能,并与BTMCr20高铬铸铁和ZG120Mn13Cr2高锰钢的硬度和耐磨性能进行对比,测试了复合层叶片在稳定土条件下的现场使用寿命。结果表明:复合层与高锰钢基体呈冶金结合,其组织由原位合成的细小TiC颗粒、外加的粗大TiC颗粒和高锰钢基体相构成,原位合成的TiC颗粒呈圆球或近圆球状均匀分布在基体相中;复合层的平均硬度为58 HRC,高于高锰钢低于高铬铸铁,磨损质量损失略高于高铬铸铁但远低于高锰钢。复合层叶片的现场使用寿命为1700 h,为高锰钢叶片的2倍以上,略高于高铬铸铁叶片。

基于SLS的叶轮精密铸造工艺及性能

为提升叶轮零件的铸造质量,将3D打印模样与石膏型精密铸造相结合,对模样的成型位置及浸蜡工艺进行分析,利用选区激光烧结技术(SLS)制得高质量叶轮模样,基于顺序凝固和排气优良设计顶注式横浇道–内浇道式浇注系统,经ProCAST软件模拟验证设计方案的合理性,最后通过真空加压浇注成功得到合格铸件,并分析T5热处理对铸件力学性能的影响。结果表明:SLS可与石膏型铸造良好结合,3D打印模样的力学性能取决于受力面是否为打印面,浸蜡可明显提升模样的力学性能;铸件经T5热处理后,其抗拉强度和布氏硬度分别从184 MPa和57 HB上升至330 MPa和109 HB,共晶硅的熔断和钝化是力学性能提升的主要原因。

大尺寸燃气轮机叶片蜡模注射成型与尺寸精度控制

应用蜡模注射成型数值模拟方法,分析了单晶涡轮叶片蜡模充填过程、体积收缩率与翘曲变形和熔接痕;建立了注蜡工艺参数与翘曲变形的响应面模型,并使用多岛遗传算法获得全局最优的工艺参数。工艺参数对翘曲变形的影响最大的是保压压力,其次是注射温度和保压时间,叶片蜡模的最优注蜡工艺参数为保压压力13.66 bar,注射温度69.96℃,保压时间155.73 s,经试验验证,叶片蜡模尺寸精度达到CT4级别。

不同加工步骤对不同牙预备体设计冠适合性偏差的影响

目的:评估3D打印蜡型铸造技术中计算机辅助设计(CAD)、3D打印蜡型以及熔模铸造步骤对5种牙预备体设计钴铬(CoCr)冠适合性偏差的影响。方法:Imageware软件构建5种牙预备体数据并设计得到相应牙冠数据,将牙冠数据导入3D打印机中成型得到蜡冠,光学扫描后再将蜡冠铸造成CoCr冠并完成光学扫描。最后将扫描得到的蜡冠及铸造冠凹面数据分别与牙预备体数据“最佳拟合配准”获得颜色偏差图和均方根误差(RMS)。用SPSS 25.0软件对数据进行统计分析(n=10,α=0.05)。结果:经CAD、3D打印蜡型或熔模铸造步骤后,5个设计组牙冠的颜色偏差图均不相同。CAD步骤中,5个设计组牙冠的RMS均值范围为27.99~31.01μm(H=37.392,P=0.000);3D打印蜡型步骤后,5个设计组RMS均值范围为19.93~29.15μm(H=23.106,P=0.000);熔模铸造步骤后,5个设计组牙冠的RMS均值范围为27.64~31.43μm(H=8.165,P=0.086);3D打印蜡型和熔模铸造步骤本身的RMS均值范围分别为-11.08~0.42μm和-1.52~11.52μm,5个设计组在3D打印蜡型(H=26.568,P=0.000)以及熔模铸造(H=25.033,P=0.000)步骤中的差异均极为显著,但羽状组在3D打印蜡型(0.42±6.20)μm和熔模铸造(-1.52±6.89)μm步骤中的RMS均值比较无统计学差异(Z=-0.529,P=0.597)。结论:CAD、3D打印蜡型或熔模铸造步骤对CoCr冠适合性偏差的影响因预备体设计而存在差异;羽状设计受3D打印蜡型或熔模铸造的影响较小;各组冠的适合性偏差远小于临床观察到的适合性差异。