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.

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.

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.

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

The effective surface treatment method for steel insert composited with Al base metal by expendable pattern casting （EPC） process and the bonding interface between steel insert and Al base metal were investigated.It was found that Zn plating on steel insert was effective on improving the bonding property between steel insert and Al base metal in EPC process.Zn is thought to promote the formation of diffusion layer.But almost none content of Zn was observed in the boundary which had been plated on the steel insert.A diffusion layer consisting of Al,Si and Fe was formed at the insert/alloy interface and its hardness was higher than the steel insert as matter of course Al base metal.This layer turned out to be intermetallic compounds of Al-Si-Fe system.Higher pouring temperature promoted the diffusion of Fe into Al alloy,so Fe content in intermetallic layers increased at higher pouring temperature.The layer nearest to steel disappeared due to applied pressure.

V-EPC铸渗制备自生TiC颗粒增强钢基表面复合材料

为了优化自生TiC颗粒增强表面复合材料的工艺参数,利用真空实型铸渗方法制备了自生TiC颗粒增强钢基表面复合材料,重点研究了预制块钢粉含量对制备复合材料的表面形貌、过渡层组织、复合层组织及复合过程的影响规律。结果表明,随着预制块中钢粉含量的增加,得到的复合材料复合层的表面质量先提高后下降,复合层中TiC颗粒的分布趋于不均匀。自生TiC颗粒增强钢基表面复合材料的形成是钢液渗透和自蔓延反应两者共同作用的结果。

V-EPC法制备SiC颗粒增强铸铁基表面复合材料

以HT150为基体,采用消失模铸渗工艺(V-EPC)制备出了不同粒径尺寸的SiC颗粒增强铸铁基表面复合材料。结果表明,SiC粒子尺寸为560μm时,制备的铸铁基复合材料表面复合良好,且表面平整,SiC粒子分布均匀,复合层厚度为3 mm左右。随着SiC粒子尺寸的减小,SiC粒子的铸渗效果明显变差,到180μm时,已无法得到复合层。

V-EPC法铸渗工艺的实验研究

对影响铸渗质量的主要因素进行了分析 ,采用正交实验对 1 6Mn钢表面进行了 Ni基合金 V- EPC法铸渗处理 ,从中确定出合理的工艺 ,为生产实践提供了依据。

V-EPC法铸渗过程的热力学及动力学研究

对铸渗表面合金化过程的热力学和动力学进行了分析 ,导出了铸渗速度和深度的公式 ;并在不同气氛条件下 ,对镍基粉末在钢表面及石墨表面的润湿角进行了测定 ;指出提高浇注温度和增加真空度是增加铸渗深度的 2种有效措施。

预置法对V-EPC铸渗制备复合材料组织和质量的影响

通过涂覆预制块法和压制预制块法两种预置体的预置方法,利用V-EPC铸渗工艺制备了WC增强铁基表面复合材料,着重研究了两种预置法对复合材料的表面质量和组织的影响。结果表明,压制法制备的复合材料表面质量较涂覆法好,复合层厚度均为3 mm左右,表面较平整,复合位置对铸渗件表面质量影响不大,且随着WC颗粒体积分数的增加,复合材料铸件的铸渗层表面质量呈下降趋势。对显微组织的研究表明,两种方法制备的复合材料过渡层都较平缓,在压制法制备的复合材料靠近复合层处的过渡层组织中没有石墨出现。压制法制备的复合材料复合层中WC的分布较涂覆法均匀,WC颗粒体积分数较高,WC的大小不一,一些WC颗粒产生裂纹、破碎和搭接现象,随着WC颗粒体积分数的增加,WC颗粒将变均匀。

EPC－V法铸铁件皱皮缺陷的研究

描述了ＥＰＣ－Ｖ法在浇注期间ＥＰＳ的热变化及铸铁件皱皮缺陷形成的过程；研究了ＥＰＣ－Ｖ法工艺因素对铸铁件皱皮缺陷的影响，提出了减轻和消除皱皮缺陷的方法和措施。生产实践证明，采用ＥＰＣ－Ｖ法工艺可以大批量生产合格的精密铸铁件。

V-EPC铸渗工艺的试验研究

通过对V EPC铸渗工艺的试验研究 ,得出了影响渗层品质的主要工艺参数 ,并对其进行优化 ;根据优化方案进行实验。研究结果表明 ,在 14 2 0℃、粒径 0 .40 0mmSiC和 0 .0 4MPa真空度的情况下能得到

V-EPC法铸渗表面合金化研究

采用V -EPC法对 4 5号钢表面进行了Ni基合金的铸渗处理 ,探讨了铸渗表面质量及合金层的显微组织特征 ,结果表明 ,当铸渗工艺设计合理时 ,Ni基合金粉能很好地熔于基体材料 ,合金层与基体材料结合良好 ,未见宏观及微观裂纹。

V-EPC铸渗铁基复合材料的研究

通过对V -EPC铸渗工艺的试验研究 ,得出了影响铁基复合材料渗层质量的主要工艺参数 ,并对其进行优化 ,根据优化方案进行实验。研究结果表明在 142 0℃ ,45 # SiC和 0 .0 4MPa的真空度的情况下能得到 4mm厚的良好渗层。

Structures and Properties of Iron Matrix Composites with Tungsten Carbide Particle by EPC-V Process

In this paper the Expendable Pattern Casting with dry sand Vacuum(EPC-V) process is used to manufacture iron matrix composites with tungsten carbide particle.Microstructures of the composites layers were analyzed.The abrasive wear resistance of the composites layers were tested and compared with that of high chromium cast iron.The results show that the iron matrix composites with tungsten carbide particle have high hardness.The abrasive wear resistance of composites with tungsten carbide particle is higher than that of high chromium cast iron.The properties of the matrix materials have been improved remarkably.

预制层中合金种类对V-EPC制备表层复合材料组织和界面的影响

以高铬钢为基材,WC颗粒为增强颗粒,通过在预制层中分别加入钢粉和高碳铬铁粉来调节预制层的成分,利用真空实型铸渗法(V-EPC)制备WC颗粒增强钢基表层复合材料,分析预制层中合金种类对制备表层复合材料界面的影响。研究结果表明:预制层中合金粉末的粒度对复合层中WC颗粒和基体的界面结合产生影响,在金属液的作用下,400目的钢粉比160~200目的高碳铬铁粉更易熔化,有利于WC颗粒与基体界面的形成。预制层中合金粉末为钢粉的试样,过渡层不明显,存在显微裂纹;高碳铬铁粉的试样,复合层和基材之间过渡平缓,没有孔洞和微裂纹。

镁合金负压消失模铸造充型过程的研究

镁合金负压消失模铸造充型过程的直接观察实验的结果表明 ,不抽真空时 ,金属前沿呈微凸形平缓地向前推进 ;抽真空时 ,因在模样分解产物的排除方向上存在一正的压力梯度 ,金属液优先沿模样壁充填型腔使流动形态呈不规则的凹形形态。提高真空度 ,不规则的流动形态的凹度更大且充型时间明显缩短。浇注方式对流动形态和充型时间的影响不大 ,与顶注和侧注相比 ,底注时充型过程最为平稳 ,且其充型时间最长。根据试验结果 。

镁合金真空低压消失模铸造新技术

介绍了一种新的铸造技术——真空低压消失模铸造的工作原理。建立了其在充型过程中的物理模型与数学模型,分析了其铸造工艺的特点及影响因素,以及在铸造高精度、复杂镁合金铸件方面的优势。作为压力铸造、低压铸造的补充,真空低压消失模铸造新技术在铸造高质量和高精度的镁合金铸件中具有广泛的应用前景,另外还可用于铸造高精度、复杂的铝合金铸件。展示了用该新技术铸造成形的复杂的电机壳体、排气管等镁合金铸件。

面向21世纪的消失模铸造技术

对国内外消失模铸造的现状进行了评估；对其发展前景进行了预测；对我国消失模铸造的进一步发展提出了建议。

镁合金真空低压消失模铸造的技术特征与实践

介绍了镁合金在真空低压消失模铸造下的技术特征。研究表明真空低压消失模铸造的流动性受充型气体的流量与压力、浇注温度、涂层厚度、泡沫密度、真空度等多方面的因素影响;在低充型速度和低真空度的条件下,液态镁合金在真空低压消失模铸造过程中的充型形貌均呈现以内浇道为中心的拱形层状推进流动,如充型速度加快金属液流动前沿拱形形貌更加突出,而真空度增加会出现明显的“附壁效应”;与重力消失模铸造比较,真空低压消失模铸造镁合金铸件凝固更呈现“同时凝固”特征;由于快速充型、压力下凝固,镁合金真空低压消失模铸造零件的铸态抗拉强度(σb=180.8MPa)、屈服强度(σ0.2=113.2MPa)、伸长率(δ=4.4%),高于重力消失模铸造、树脂砂空型铸造的铸态性能,达到了金属型铸造的铸态性能,经热处理达到了压铸镁合金的性能范围。浇注实践表明,真空低压消失模铸造对液态镁合金,具有良好的抗氧化保护能力、优良的浇注充型性能和力学性能,可铸造出高精度的、薄壁复杂的镁合金消失模铸件,是一种极有潜力和优势的镁(铝)合金精密铸造技术。