Cooling control for castings by adopting skeletal sand mold design

The cooling control of the melt during the casting process is of great significance. A comprehensive closed-loop cooling control of castings by adopting a skeletal sand mold design was proposed. The skeletal sand mold consisting of an adaptive shell, functional cavities and a support was designed and created based on the finite difference meshes of a casting. It was applied to a round wall test casting. Two kinds of skeletal sand molds, one with lattice support and the other with enforcing ribs for this casting were designed and printed out by the 3 D printing(3 DP) method. Aluminum alloy A356 was cast by using these two sand molds. The first mold was cooled by natural convection, the other one by water spray cooling. Two sound castings were obtained. The sand mold temperature, cooling curves, microstructures, mechanical properties, residual stress and deformation were measured, compared and discussed. Water spray cooling hastened the cooling rate by 62%, increased the content of Mg and Cu in the α-Al matrix, improved the mechanical properties, and altered the surface residual stress state.     

Cooling rate and carbon content effect on the fraction of secondary phases precipitate in as-cast microstructure of ASTM F75 alloy

Simple test castings were used to study the effect of cooling rate and carbon content in as-cast microstructure of alloy ASTM F75, Co–26 wt.% Cr–5.7 wt.% Mo. Alloys with four C content (0.45, 0.33, 0.36 and 0.25 wt.%) were poured into investment ceramic molds. In order to obtain different cooling rates, the castings were constituted of three axisymmetrical cylinders of different diameters (12, 16 and 24 mm). Cooling curves were obtained from each cylinder and the fraction of secondary phases in as-cast microstructure was measured by image analysis. Average cooling rates of 100, 60 and 20 °C/min, were obtained in the 12, 16 and 24 mm diameter cylinders respectively, at the first solidification step occurring in the temperature range from 1390 to 1350 °C. A significant effect of this cooling rate range on the fraction of secondary phase was not observed.It was observed that the fraction of blocky carbides increased proportionally as the C content increased, whereas the amount of the eutectoid constituent showed a significant increase only in the sample containing 0.45 wt.%, as compared with the samples of other carbon contents. It was also found that the initial solidification undercooling affects the temperature of sigma phase precipitation with which the solidification of the alloy is completed.     

Investigation on heat-transfer-coefficient between aluminum alloy and organic/inorganic sand mold based on inverse method

A kind of cylinder sand mold was designed to investigate the heat-transfer-coefficients(HTCs) between aluminum alloy and organic/inorganic binder bonded sand mold during the solidification processes. Temperature during the solidification process was recorded and input into the simulation software. The inverse model of MAGMA was used to calculate the HTC based on the actual temperature. Results show that the temperature of the inorganic sand mold increased faster than the organic sand mold; while the temperature of the casting part with the inorganic sand mold decreased faster. The optimal HTCs between Al and the organic/inorganic sand mold are confirmed to be 300 to 700 and 1000 to 1800 W·m~(-2)·K~(-1), respectively, along with the change of solid-liquid phase line. The simulated temperature curves show the same trend as the measured ones. The maximum deviation between the two temperature curves are 17.32 °C and 18.77 °C for castings by inorganic and organic sand molds.     

Linear contraction of ductile iron castings

The effect of casting design on the linear contraction of ductile iron castings produced in clay- and silicate-bonded sand moulds has been studied. The designs included free and restrained square bar castings.

Free contraction of very thin sections of ductile iron in sand moulds was found to approach 1.35%. As sections increased to 25 mm thick, the contraction decreased linearly to 1.08% in silicate-bonded sand moulds, but to 0.90% in clay-bonded sand moulds.

For test pieces with end flanges which stimulated various degrees of constraint as would be experienced in a shaped casting, the contraction was found to be sensitive to the third power of the relative cooling time of the flange and the cast section. In conditions of high constraint, contraction could fall as low as 0.55%.

The data from this study represent a first attempt to provide a designer and toolmaker with realistic contraction allowances for shaped castings.     

微观组织对A201铝合金铸件机械性能之研究

系统改变A201铝合金平板铸件的长度、厚度及冒口大小,探讨微观组织中微孔隙量及树枝状晶胞尺寸(DCS)对A201铸件抗拉强度及伸长率的影响程度,进而于铸造实务工作时,可为冒口设计及金属凝固之参考,为研究的目的。砂型的种类有三种,分别是100%石英砂的A类石英砂、50%石英砂及50%铬砂的B类、及100%铬砂的C类。实验结果显示,A201铝合金平板铸件的机械性质同时受空孔量及DCS之影响,当微孔隙量增加及DCS变大时,均会降低铸件的抗拉强度及伸长率,其中微孔隙量影响为主要的因素。     

Optimization of Mold Yield in MultiCavity Sand Castings

The productivity of ductile iron foundries engaging in mass production of castings for the automobile and other engineering sectors depends on the number of cavities per mold. A denser packing of cavities, however, results in slower heat transfer from adjacent cavities, leading to delayed solidification, possible shrinkage defects, and lower mechanical properties. In this article, we propose a methodology to optimize mold yield by selecting the correct combination of the mold box size and the number of cavities based on solidification time and mold temperature. Simulation studies were carried out by modeling solid and hollow cube castings with different values of cavity-wall gap and finding the minimum value of the gap beyond which there is no change in casting solidification time. Then double-cavity molds were modeled with different values of cavity-cavity gap, and simulated to find the minimum value of gap. The simulation results were verified by melting and pouring ductile iron in green sand molds instrumented with thermocouples, and recording the temperature in mold at predetermined locations. The proposed approach can be employed to generate a technological database of minimum gaps for various combinations of part geometry, metal and process, which will be very useful to optimize the mold cavity layouts.     

Chill zone copper with the strength of stainless steel and tailorable color

We report here on 90 at.% copper 2–4 mm thick castings with fracture strengths up to 1.9 MPa due to the formation of a thick, scratch-resistant nanocrystalline chill-zone next to the copper mold contact surfaces. It is found that the unusually hard nanocrystalline surface layers of thicknesses in the range of 200–300 μm form when the melt can be undercooled: when the alloy composition is such that the contact surface of the copper mold with the liquid alloy does not serve as a preferred site for crystal nucleation, supercooling of the melt in the chill-zone is possible. These castings are the hardest coppers ever reported to date in macroscopic specimens. They have mechanical strength superior to those of many stainless steels as well as to Cu–Be alloys and of the order of those of Cu-based bulk metallic glasses (BMGs) of similar dimensions. In addition, and unlike in copper-based BMGs, the color and luster can be tailored by elemental additions to vary from copper-like to gold-like.     

Improvement of surface characteristics for long life of metal molds by large-area EB irradiation

This paper describes the applicability of large-area EB irradiation method to a new surface modification for metal molds. The previous papers clarified that the surface roughness on wide area uniformly decreased to less than 1 μm Rz in a few minutes by the large-area EB irradiation. Therefore, the large-area EB irradiation method has high practicability as an efficient surface finishing process for metal molds, and then the EB irradiation equipment has already been introduced into the market for practical use. In this method called EB polishing, the surface material melts and a very thin resolidified layer is formed on the surface. In this paper, the surface structure of EB polished metal mold steel SKD11 was first observed by TEM, and also component analysis was carried out by EDX. Then it was found out that the crystal grain size became small and chromium carbide is uniformly rearranged in the layer. Next, using heat conduction analysis model considering the electron penetration on the surface, the temperature distribution was calculated. The results showed that the surface temperature rapidly raised over its melting point until the end of EB irradiation, and cooled down rapidly. As the important practical surface characteristics for long life of metal mold, resistance to corrosion, water repellency, and the releasability of molded resin from metal mold surface were evaluated. The corrosion resistance and the water repellency could be improved by EB polishing, since the surface structure changes and Cr content distributed uniformly on the surface. Also, the releasability of molded resin from the surface became better.     

Effect of iron content on the formation of β-Al5FeSi and porosity in Al–Si eutectic alloys

A preliminary investigation has been carried to evaluate the influence of Fe on Sr-modified and unmodified eutectic Al–Si alloys in as-cast and heat treatment conditions. The castings were produced in zircon-coated steel permanent mold and were solutionized at 500 °C for 8 h and followed by artificial aging at 155 °C for 5 h, i.e., T6-temper. The microstructure changes in the β-Al₅FeSi particle morphology were analyzed. The results indicate that dendrite arm spacing is strongly related to the cooling rate rather than the chemical composition, increasing the iron content leads to increase porosity and hardness either in the as-cast condition or after T6-temper. The Sr-modified alloys have higher hardness than unmodified at all Fe-added values. The precipitated long branched β-platelets result in the formation of large shrinkage cavities due to the inability of liquid metal to feed the space between them during solidification.     

Research progress on refractory composition and deformability of shell molds for TiAl alloy castings

At present, most TiAl components are produced by an investment casting process. Environmental and economic pressures have, however, resulted in a need for the industry to improve the current casting quality, reduce manufacturing costs and explore new markets for the process. Currently, the main problems for investment casting of TiAl alloys are cracks, porosities, and surface defects. To solve these problems, many studies have been conducted around the world, and it is found that casting defects can be reduced by improving composition and properties of the shell molds. It is important to make a summary for the related research progress for quality improvement of TiAl castings. So, the development on refractory composition of shell molds for TiAl alloy investment castings was reviewed, and research progress on deformability of shell mold for TiAl alloy castings both at home and abroad in recent years was introduced. The existing methods for deformability characterization and methods for improving the deformability of shell molds were summarized and discussed. The updated advancement in numerical simulation of TiAl alloy investment casting was presented, showing the necessity for considering the deformability of shell mold during simulation. Finally, possible research points for future studies on deformability of shell mold for TiAl alloy investment casting were proposed.     

1种新的模拟铝合金铸态组织的随机性方法

采用1种新的随机性模拟方法并与宏观传热过程相耦合，对铝合金的微观组织进行了模拟研究。计算中采用了简化的枝晶形状。建立了简化的枝晶形状的物理与数学模型，并提出了1种形状函数来描述晶粒的外部轮廓。基于简化的晶粒形状，采用坐标变换技术来描述过冷液相中晶粒的生长过程及其对周围节点的捕获过程。连续形核模型被用来处理异质形核现象，在生长模型中则考虑了枝晶尖端生长动力学和择优生长方向。开发了等轴晶生长的模拟程序，并进行了二维计算。进行了浇注金属型和砂型试样的模拟验证实验。结果表明，对两种不同的工艺，所得到的晶粒组织不同，金属型铸造时得到的晶粒尺寸较小，砂型铸造的较大。模拟结果与金相观察结果相符。     

Numerical simulation and process optimization for producing large-sized castings

3-D velocity and temperature fields of mold filling and solidification processes of large-sized castings were calculated, and the efficiency and accuracy of numerical calculation were studied. The mold filling and solidification processes of large-sized stainless steel, iron and aluminum alloy castings were simulated by using of new scheme; their casting processes were optimized, and then applied to produce castings.     

An alternative approach in ceramic shell investment casting of AZ91D magnesium alloy: In situ melting technique

In this research, the possibility of ceramic shell investment casting of a magnesium alloy using in situ melting technique was explored. AZ91D granules were charged into shell investment mould and in situ melted under various processing parameters including heating temperature, flux application, shell mould thickness and permeability. Scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction techniques were used to characterise the cast samples. Thermal analysis was employed to further investigate the effect of mould thickness on the solidification behaviour of the metal. It was found that mixing flux with the granules not only reduced the temperature at which melting can be achieved, but it also contributed to produce castings with acceptable surface quality. The use of thinner mould provided higher solidification rate, which is believed to favour in situ melting of the granules. It enabled melting of the granules at 650 °C, which in turn helped to suppress the mould–metal reaction and produce castings with good surface quality. Shell mould permeability showed no influence on suppressing the mould–metal reaction at 650 °C.     

呋喃树脂砂生产铸铁件临界吃砂量的模拟试验研究

为确定呋喃树脂砂的合理吃砂量,研究了以硅砂为原砂制备的呋喃树脂砂在100～500℃内的溃散性,确定了型砂全部溃散时的最低温度为350℃；然后,运用ProCAST软件模拟了不同壁厚的圆筒形铸铁件的凝固过程及铸型和型芯的温度场,并研究了圆筒形铸铁件壁厚(模数)与呋喃树脂砂临界吃砂量的关系,最终得到了两者之间的回归方程.     

Alloy design concepts for refined gamma titanium aluminide based alloys

The influence of the Al content and the addition of further alloying elements on the cast microstructure of γ(TiAl) + α₂(Ti₃Al) alloys has been examined. The results show that particularly fine and homogeneous microstructures without strong segregation can be obtained for certain alloy compositions solidifying through the β phase. This behavior can be attributed to the avoidance of peritectic solidification and to the alloying influence on the kinetics of the β ⇒ α transformation following solidification. The experimental findings were used to propose a design concept for γ-TiAl + α₂-Ti₃Al alloys. This concept aims at the production of high-quality castings as well as at ingot material for wrought processing routes because the chemically homogeneous and fine-grained microstructures would be a good precondition for improved workability.     

含氮树脂砂型（芯）—钢液界面气体分解产物的测定

砂型（芯）－金属界面气氛地钢铁铸铁气孔缺陷有很大影响。本文采用四种含氮呋喃树脂砂浇注铸钢液，对砂型（芯）＿钢液界面的气体分解产物浓度进行了测定。     

航空发动机铝合金缸体的快速砂铸工艺研究

以结合了光固化成型技术与砂型铸造的快速砂铸技术为基础,针对某小型航空发动机中的复杂部件——缸体,分别进行了铸造工艺设计、铸型及砂芯模具的CAD设计、树脂件模具的制作及填砂制芯和浇注试验的工作,同时提出了铸件精度控制及浇注缺陷控制两大问题并着手解决。最后针对浇注所得铸件进行关键尺寸测量及精度分析,证明了通过该工艺方法制造出的铸件,与传统铸造工艺相比,其精度达到较高水平。     

Development of micro-plasma transferred arc (μ-PTA) wire deposition process for additive layer manufacturing applications

Micro-plasma transferred arc (μ-PTA) deposition process has potential to meet requirements of the meso-sized fabrication and repair of the high value components. This paper reports on the development of μ-PTA as cost effective and energy efficient alternative process for small sized deposition with an overall objective to repair and/or remanufacture the defective dies and molds. An experimental setup was developed to deposit 300 μm diameter wire of AISI P20 tool steel on the substrate of the same material which is one of the most commonly used materials for making the dies and molds used for various applications. Two stage experiments were conducted to indentify the important process parameters generating regular and smooth single bead geometry. The process was further explored for highest possible deposition rate for fabrication of straight walls through multi-layer deposition. The μ-PTA deposition process was found to be capable of fabricating straight walls having total wall width of 2.45 mm and effective wall width of 2.11 mm. The deposition efficiency was found to be 87% for the maximum deposition rate of 42 g/h. The microscopic examination and micro-hardness measurements revealed that the deposited wall is free from cracks, porosity, and inclusions. This study confirms the capability of μ-PTA for ALM in comparison to the existing high energy deposition processes used for meso-scale fabrication and repair applications of the dies and molds. This work confirms that μ-PTA wire deposition process offers the advantages of the laser based processes at much lower cost and more energy efficiency thus making it potential alternative process for repair and remanufacturing of the defective dies and molds. Use of finer wire can further reduce the deposition size enabling μ-PTA wire deposition process to fabricate the miniaturized parts.     

Filling modes of polymer during submicron and nano-fabrication near glass transition temperature

In this study, we used a glass mold coated with TiN layer to fabricate submicron and nano-gratings on a PMMA (polymethylmethacrylate) film. The cavities on the mold, with sizes varying from 71 nm to 980 nm, was etched by ion beam. The deformation and filling modes of polymer during fabrication process were studied. Dual-peak deformations, which were considered as the characteristic filling modes of “viscous-dominant” polymer, were observed. Because our fabrication experiments were conducted near the glass transition temperature (T_g) of PMMA at which the polymer was “elastic–plastic-dominant”, the appearance of the dual-peak filling mode meant solid-state polymer might exhibit some characters of fluidic polymer at submicron and nano-scale. In addition, we presented a simple and effective mold release process at the end of this paper, which could reduce defects during molding release process.     

Computer simulation for centrifugal mold filling of precision titanium castings

Computer simulation codes were developed based on a proposed mathematical model for centrifugal mold filling processes and previous computer software for 3D mold filling and solidification of castings (CASM-3D for Windows). Sample simulations were implemented for mold filling processes of precision titanium castings under gravity and different centrifugal casting techniques. The computation results show that the alloy melt has a much stronger mold filling ability for thin section castings under a centrifugal force field than that only under the gravity. A "return back" mold filling manner is showed to be a reasonable technique for centrifugal casting processes, especially for thin section precision castings.