Influence of EPS bead fusion on pattern degradation and casting formation in the lost foam process

The effects of bead fusion in the pattern on foam degradation and on casting formation have been studied. Injection molded ASTM D638 tensile specimens have been used to develop a microscopic technique to quantify the extent of bead fusion in the pattern. The tensile properties of the polymer have been correlated with the measured degree of bead fusion. A variety of experiments have been conducted to highlight the effects of fusion on foam degradation. The flow behavior of a molten aluminum alloy has been studied in patterns with various levels of fusion. The results indicate that the degree of fusion has a significant effect on bead collapse and viscous residue formation in the polymer. The mold fill times generally increase with increasing bead fusion in the pattern. The temperature at the metal front drops more rapidly as the bead fusion increases.     

Bead fusion in polystyrene foams

A quantitative technique has been developed to measure the extent of fusion between expanded beads in molded polystyrene foams. Experiments were conducted with ASTM D638 tensile test specimens that were molded under conditions to produce various levels of bead fusion in the foam. The tensile properties of the foam for various levels of bead fusion were measured according to ASTM D638 standards. The fracture surfaces of the tensile specimens were analysed by quantitative image analysis techniques to measure the degree of bead fusion in the sample. This technique was then used on a commercial pattern to map the variations in bead fusion at numerous locations in the molded part. The results indicate that there is a good correlation between the tensile properties and the measured bead fusion. Significant variations in bead fusion may be present in a single molded part.     

Transport Phenomena in the Production and Use of Expanded Polystyrene Patterns in Lost Foam Casting

Experiments were carried out to study the effect of process and design variables on defect formation in the lost foam casting process. Emphasis was placed on the production of expanded polystyrene foam patterns and the use of these patterns to create desired castings. Spatial pattern density variation caused by varying the fill and canister pressure for molds with varying distance between injector and opposing wall was measured. The heat transfer coefficient and gas pressure in the gap between the metal front and the foam pattern were also measured. The study found a close correlation between the production of a uniform pattern and the production of a defect-free casting.     

Microstructure Characteristics of the Eutectics of Die Cast AM60B Magnesium Alloy

Under the cold-chamber high pressure die casting (HPDC) process, samples were produced with AM60B magnesium alloy to investigate the microstructure characteristics of the eutectics, especially focusing on the constitution, morphology and distribution of the eutectics over cross section of the castings. Attentions were also paid to study the effect of heat treatment on the eutectics in the die castings. Based on experimental analysis using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), it was determined that fully divorced eutectics consisting of α-Mg and β-Mg17Al12 appeared at the grain boundary of the primary α-Mg in the as-cast microstructure. Islands and networks of β-Mg17Al12 phase were observed in the central region of the castings, while the β-Mg17Al12 phase revealed a more dispersed and granular morphology on the surface layer. The two phases ratio β/α in the central region of the castings was approximately 10%, which was higher than that on the surface layer. Besides, the defect bands contained a higher percentage of the eutectics than the adjacent regions. After aging treatment (T6), only α-Mg phase was detected by XRD in the AM60B magnesium alloy, though a small amount of precipitated β-Mg17Al12 phase was observed at the grain boundary. In contrast to the microstructure of die cast AZ91D magnesium alloy under the same T6 heat treatment, no discontinuous precipitation of the β-Mg17Al12 phase was observed in AM60B magnesium alloy die castings.     

Influence of oxide film defects generated in filling on mechanical strength of aluminium alloy castings

Abstract

The influence of oxide film defects generated from the filling process on the mechanical strength of aluminium alloy castings has been investigated. Using numerical simulation and experimental validation, the investigation aims to reveal the relationships among the liquid aluminium flow behaviour in the filling, the likely oxide film defect distribution caused by surface turbulence and the final mechanical strength of the castings. CFD modelling was used to investigate the liquid metal flow behaviour and the likely oxide film defect distribution in the filling at different ingate velocities. In particular, a numerical algorithm - Oxide Film Entrainment Tracking (OFET, 2-D) has been proposed and developed for predicting such oxide film defect distribution in the liquid aluminium throughout the filling. Also, light microscopic and SEM techniques were used to identify the microstructures of oxide film casting defects. The Weibull statistics method was employed to quantify the effects of oxide film distributions on the mechanical strength and reliability of the acquired aluminium alloy castings for different runner systems. It was found from the numerical simulations that the ingate velocities acquired using different runner systems have significant influence on the distribution of oxide film defects generated by surface turbulence in the filling process, which results in the disparities of the final mechanical strength of the castings. The results of the mechanical property test and the SEM micro-structural analysis of the castings are consistent with the numerical simulations.     

Strontium modification of aluminium alloy castings in the expendable pattern casting process

The influence of strontium modification on microstructure, pore characteristics and tensile properties of Al-Si-Cu castings produced by the expendable pattern casting (EPC) process has been investigated. Test castings have been produced at strontium concentrations varying from 0.0004%–0.03%. The castings have been analysed by microscopic and image analysis techniques. The results indicate that optimum strontium concentrations are between 0.01% and 0.015%. A coarsening of silicon particles is observed at strontium concentrations greater than 0.015%–0.02%. The addition of strontium to the melt also refines the iron- and copper-bearing phases in the casting. The amount of bulk porosity, average pore size and tensile properties of modified EPC castings are comparable to green sand castings. EPC castings are prone to surface porosity. Strontium modification has a significant effect on the pore distributions in EPC castings. The overall porosity and the maximum pore size increase with strontium modification.     

热轧板冷弯开裂分析

采用扫描电子显微镜和光学显微镜观察了热轧板在冷弯过程中开裂试样的断口形貌并进行了成分分析。发现裂纹内存在氧化铁且呈断续分布，裂纹两侧基体内存在着内氧化颗粒且其显微组织也有差异。分析表明，热轧板冷弯开裂是由热轧折叠缺陷造成的，但不影响整批材料的性能。实际使用中只要切除缺陷部位，即可正常使用。     

镶铸管铸造工艺铸件无损检测分析

目的 研究镶铸工艺成形钛合金铸件镶铸部位冶金缺陷形成原因及无损检测表征,为镶铸工艺铸件的无损检验判定提供依据,并为此类产品冶金质量评价提供依据,建立质量评价体系标准。方法 通过对采用镶铸钛管工艺成形铸件的镶铸部位进行X射线检测及微观形貌、成分检测,结合铸造成形工艺的数值模拟过程进行对比分析。结果 预埋钛管表面在铸造工艺过程中出现表面氧化,浇注过程中,在镶铸部位存在多股金属液充型交汇,镶铸管部位结构融合效果存在差异,存在未融合现象。结论 采用镶铸工艺制备的铸件在无损检测方面存在不同于传统的冶金评判特征,镶铸部位的未融合缺陷可以通过X射线检测发现,在X光底片显示为沿钛管界面的黑线。     

石墨铸型材料对钛合金内环铸件冶金质量的影响研究

目的研究不同石墨铸型材料对钛合金内环铸件冶金质量的影响。方法试验选取颗粒直径分别2,0.8,0.04 mm的3种典型石墨材料,制备内环石墨铸型,采用离心浇注,然后对3种不同石墨材质研制的ZTC4钛合金铸件进行表面目视检验、荧光和X光检测。结果 2,0.8,0.04 mm这3种颗粒直径的石墨铸型材质研制的铸件表面外观和荧光质量依次提高,其中荧光缺陷平均数量依次为21.4,13.9,2.2个/件;铸件内部缺陷数量均小于1.5个/件,夹杂均小于0.7个/件。结论不同颗粒度直径的石墨铸型对铸件的外观和荧光质量会产生显著的影响,而对铸件内部冶金质量影响较小。     

Effect of Process Parameters on Porosity in Aluminum Lost Foam Process

1.IntroductionLost foam casting process is still a new technology,and is gaining confidence among manufacturers[1].It isexpected to dramatically improve the competitiveness ofthe foundry industry.Many advantages like eliminatingmachining steps,making complex casting without coresand reducing environmental loads can be offered by thisprocess,because molding binder is not added into themold of the lost foam process.Aluminum alloy castings are widely used in the auto-mobile and aerospace industri…     

消失模EPS模型材料表征及性能的研究

为了探究消失模EPS模型的表面粗糙度影响因素、应力应变特征以及消失模EPS模型材料的热解规律和内部胞腔结构形态,达到降低消失模铸件的表面粗糙度,科学制定消失模工艺过程的控制参数,选择适合消失模工艺的合金材料,控制铸件内碳化物残留量,提高消失模铸件的表面质量和力学性能.本文采用激光粒度分析仪对用于消失模EPS产品模型成形...     

窄间隙P-GMAW多信息融合侧壁熔合状态识别研究

窄间隙焊缝坡口间距小且焊道较深,摆动中心与焊缝中心偏差较大时,坡口两侧侧壁受热不良,易发生未熔合缺陷。为了及时了解窄间隙侧壁熔合情况,掌握侧壁内部焊接质量,本文提出了一种基于BP神经网络和D-S证据理论的多信息融合方法,预测侧壁熔合状态。对窄间隙焊接未熔合缺陷产生机制进行了分析,研究发现焊接电弧信号和熔池变化与侧壁成形质量存在密切关系,为此进行了一系列偏差实验,建立了电弧电信号和电弧熔池图像信号的实时采集系统,采用批量特征提取算法,提取了与侧壁熔合状态密切关联的峰值电流、峰值电压、电弧弧长、熔池长宽比、熔池面积和熔池周长等特征参量。采用BP算法训练神经网络,在此基础上通过D-S证据理论进行决策级融合。实验结果表明,该模型识别率可达96.667%,避免了神经网络识别时的误诊,获得了比单一传感信息更好的预测结果,提高了熔合状态识别的准确度和可靠度。     

MODELING THE SOLIDIFICATION OF FOUNDRY CASTINGS

A comprehensive technique is described for performing the simulation of the solidification of foundry castings. A finite element method (FEM) mesh is produced for the pattern of the part, using geometric modeling techniques. Results are presented to demonstrate selection rules for the mesh size. The size of the heat transfer problem is reduced by replacing the FEM mesh in the mold material by appropriate boundary conditions on the surface of the casting. The boundary conditions are found automatically by a program which relates local surface curvature on the part to a pre-calculated library of solutions. Methods for ensuring that all of the latent heat of fusion is included are presented, and comparisons are made between different solution methods.     

Mechanical properties of conventionally cast,directionally solidified,and single-crystal superalloys

Abstract

In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal (‘monocrystal’) castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and low-cycle fatigue properties depend on the stress direction relative to the crystallographic directions of the material for both directionally solidified and single-crystal castings. For single crystals, individual alloys show variable dependences of properties on the crystallographic directions. Directionally solidified materials show advantages in thin sections and are less sensitive to the effects of impurities compared to conventional castings.

MST/329     

Effect of nodule count on austenitising and austempering kinetics of ductile iron castings and mechanical properties of thin walled iron castings

Abstract

In the present work, the potential for producing thin walled ductile iron castings with an ausferritic matrix is presented. Experimentally, thin walled iron castings of 2 mm in thickness were obtained and characterised by a nodule count of 1992 mm^?2. In addition, a reference casting was produced with a 25 mm thick wall and a nodule count of 330 mm^?2. Austenitising was carried out at 920°C, whereas austempering was implemented in the 300–400°C temperature range. The austenitising and austempering transformation rates were determined by dilatometry, and the results were confirmed by microstructural analyses. It was found that in thin walled castings, the austenitising and austempering times were reduced by either one-half or one-third of the ones corresponding to the reference casting. The exhibited mechanical properties of the thin walled castings were also determined as a function of austempering time and temperature. It was found that austempering at 300°C for 1200 s leads to thin walled castings with a tensile strength of 1500 MPa. Accordingly, from this work, it is plausible to produce high strength thin walled castings that satisfy all the ASTM 897M grades of ausferritic ductile iron through proper heat treating.     

Characteristics of defect bands and their formation mechanisms in A356 wheel fabricated by horizontal squeeze casting

Based on the opinion that a decreasing solid fraction gradient in the cross-section develops during the intensification stage, the shear stress in the solidifying alloy was analysed, and a mechanism for dilatant band formation was discussed. That is, when the shear stress at one position of the stable grain network reaches a critical value, the network collapses and the band forms. The characteristics of defect bands at different locations of an A356 wheel fabricated by horizontal squeeze casting were then studied. Three types of defect bands, i.e. skin related band, dilatant band and separate band, were observed. Combining the process and conditions of defect band formation, several types of defect bands in the casting were discussed. Furthermore, the influence of the structural characteristics of castings on the defect bands, including thickness, geometry and location, was summarised.     

采用新型喷砂磨料改善高温合金铸件表面质量

本文简述了使用一种新型磨料球形砂（ARSAND）替代传统喷砂磨料，对一些对表面质量要求较高并采用荧光检验的高精密铸件进行喷砂的方法。该方法可显著消除经荧光检测出现的弥散点状表面缺陷，有效降低了铸件返修率、报废率。     

OPTIMIZATION OF WELD BEAD DIMENSIONS IN GTAW OF ALUMINUM-MAGNESIUM ALLOY

The Gas Tungsten Arc Welding (GTAW) process is frequently used in welding of aluminum alloys, because of its possible heat input control. This control can be utilized through a good selection of the process variables, which in turn results in optimizing the bead dimensions. The object of this investigation was to study the effect of TIG process parameters on weld bead dimensions. Suitable combinations of tungsten electrode parameters and process variables can lead to optimum GTAW bead dimensions. With alternative current (AC) polarity, a weld bead may be formed between two 3-mm thick pieces of 5005 aluminum-magnesium alloy sheets. The effect of electrode diameter, vertex angle, and the welding current and speed on the bead dimensions were investigated. Results revealed that the rate of increase of bead width with current increase is greater than that produced by decreasing travel speed, and means that the bead width can be controlled more efficiently by welding current rather than by welding speed. For example, bead width can be reduced by half by increasing the welding travel speed three times, whereas it can be doubled when the current has is doubled. In contrast, bead depth is found to be more sensitive to welding speed rather than to the welding current. One of the important results of the present investigation was that the average heat-affected zone width decreased as the welding current and/or speed increased. On the other hand, it was found that the influences of electrode diameter and apex angle on the bead width were similar to their effects on the arc size. The bead width was found to decrease with an increase in the electrode diameter to a certain extent, and increase slightly with an increase of the apex angle.     

Numerical simulation of the casting process of titanium removable partial denture frameworks

The objective of this work was to study the filling incompleteness and porosity defects in titanium removal partial denture frameworks by means of numerical simulation. Two frameworks, one for lower jaw and one for upper jaw, were chosen according to dentists recommendation to be simulated. Geometry of the frameworks were laser-digitized and converted into a simulation software (MAGMASOFT). Both mold filling and solidification of the castings with different sprue designs (e.g. tree, ball, and runner-bar) were numerically calculated. The shrinkage porosity was quantitatively predicted by a feeding criterion, the potential filling defect and gas pore sensitivity were estimated based on the filling and solidification results. A satisfactory sprue design with process parameters was finally recommended for real casting trials (four replica for each frameworks). All the frameworks were successfully cast. Through X-ray radiographic inspections it was found that all the castings were acceptably sound except for only one case in which gas bubbles were detected in the grasp region of the frame. It is concluded that numerical simulation aids to achieve understanding of the casting process and defect formation in titanium frameworks, hence to minimize the risk of producing defect casting by improving the sprue design and process parameters.     

Surface bead formation of a mild steel by pulsed current TIG arc

The influence of pulse frequency on the surface bead formation and the fusion boundary microstructure of a mild steel was investigated using a pulsed current TIG arc within low and middle frequency regions. The appearance of the beads formed on the surface shows a distinct ripple shape. However, in the middle frequency region and at faster travel speeds, humped beads are formed. The number of ripples on the surface bead increases and decreases with increasing pulsed frequency and travel speed, respectively. The bead width and depth both increase with increasing pulse frequency and travel speed. In particular, a large decrease in the melt penetration is observed at middle frequency, which is probably due to a lowering of arc gouging and a discontinuity of the bead. The minimum reduction in grain size is obtained at pulse frequencies of 2.5, 5.0, 50 and 100 Hz, which is an equiaxed structure in the fusion zone, and consists of acicular ferrite and fine pearlite in a heat-affected zone. The best bead formation occurred at pulse frequencies of 2.5 and 5.0 Hz, and a travel speed of 280 mm min^–1.