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).

Critical cooling rate on carbide precipitation during quenching of austenitic manganese steel

Critical cooling rate to avoid carbide precipitation during quenching of austenitic manganese steel was investigated by means of optical microscopy,image analyzer and numerical analysis.An efficient heat treatment analysis program including temperature-dependent material properties was developed for the prediction of cooling rate and probability of carbide precipitation during quenching by finite difference method.Time-dependent heat transfer coefficient was adopted to achieve more precise results.Area ratio of carbide precipitation was measured by image analyzer to determine the critical point of carbide precipitation.Temperature-dependent critical cooling rate at that point was calculated by the developed numerical program.Finally,the probability of carbide precipitation on the whole area of specimen can be predicted by the proposed numerical program and the numerical result of a specimen was compared with the experimental result.

Effects of displacement boundary conditions on thermal deformation in thermal stress problems

Most computational structural engineers are paying more attention to applying loads rather than to DBCs (Displacement Boundary Conditions) because most static stable mechanical structures are working under already prescribed displacement boundary conditions. In all of the computational analysis of solving a system of algebraic equations, such as FEM (Finite Element Method), three translational and three rotational degrees of freedom (DOF) should be constrained (by applying DBCs) before solving the system of algebraic equation in order to prevent rigid body motions of the analysis results (singular problem). However, it is very difficult for an inexperienced engineer or designer to apply proper DBCs in the case of thermal stress analysis where no prescribed DBCs or constraints exist, for example in water quenching for heat treatment. Moreover, improper DBCs cause incorrect solutions in thermal stress analysis, such as stress concentration or unreasonable deformation phases. To avoid these problems, we studied a technique which performs the thermal stress analysis without any DBCs; and then removes rigid body motions from the deformation results in a post process step as the need arises. The proposed technique makes it easy to apply DBCs and prevent the error caused by improper DBCs. We proved it was mathematically possible to solve a system of algebraic equations without a step of applying DBCs. We also compared the analysis results with those of a

Measurement of heat treatment induced residual stresses by using ESPI combined with hole-drilling method

In this study, residual stresses in heat treated specimen were measured by using ESPI (Electronic Speckle-Pattern Interferometry) combined with the hole-drilling method.The specimen, made of SUS 304 austenitic stainless steel, was quenched and water cooled to room temperature.Numerical simulation using a hybrid FDM/FEM package was also carried out to simulate the heat treatment process.As a result, the thermal stress fields were obtained from both the experiment and the numerical simulation.By comparision of stress fields, results from the experimental method and numerical simulation well agreed to each other, therefore, it is proved that the presented experimental method is applicable and reliable for heat treatment induced residual stress measurement.

A comparative study of interface reconstruction algorithms in molten metal flow

In the present research,two numerical schemes for improving the accuracy of the solution in the flow simulation of molten metal were applied.One method is the Piecewise Linear Interface Calculation(PLIC) method and the other is the Donor-Acceptor(D-A) method.To verify the module of the interface reconstruction algorithms,simple problems were tested.After these validations,the accuracy and efficiency of these two methods were compared by simulating various real products.On the numerical simulation of free surface flow,it is possible for the PLIC method to track very accurately the interface between phases.The PLIC method,however,has the weak point in that a lot of computational time is required,though it shows the more accurate interface reconstruction.The Donor-Acceptor method has enough effectiveness in the macro-observation of a mold filling sequence though it shows inferior accuracy.Therefore,for the problems that need the accurate solution,PLIC is more appropriate than D-A.More accuracy may cause less efficiency in numerical analysis.Which method between D-A method and PLIC method should be chosen depends on the product.

Virtual Mold Technique in Thermal Stress Analysis during Casting Process

It is important to analyse the casting product and the mold at the same time considering thermal contraction of the casting and thermal expansion of the mold. The analysis considering contact of the casting and the mold induces the precise prediction of stress distribution and the defect such as hot tearing. But it is difficult to generate FEM mesh for the interface of the casting and the mold. Moreover the mesh for the mold domain spends lots of computational time and memory for the analysis due to a number of meshes. Consequently we proposed the virtual mold technique which only uses mesh of the casting part for thermal stress analysis in casting process. The spring bar element in virtual mold technique is used to consider the contact of the casting and the mold. In general, a volume of the mold is much bigger than that of casting part, so the proposed technique decreases the number of mesh and saves the computational memory and time greatly. In this study, the proposed technique was verified by the comparison with the traditional contact technique on a specimen. And the proposed technique gave satisfactory results.

Prediction of mechanical properties of Al alloys with change of cooling rate

The solidification process significantly affects the mechanical properties and there are lots of factors that affect the solidification process.Much progress has been made in the research on the effect of solidification on mechanical properties.Among them,the PF(Phase Field) model and CA(Cellular Automata) model are widely used as simulation methods which can predict nucleation and its growth,and the size and morphology of the grains during solidification.Although they can give accurate calculation results,it needs too much computational memory and calculation time.So it is difficult to apply the simulation to the real production process.In this study,a more practical simulation approach which can predict the mechanical properties of real aluminum alloys is proposed,by identifying through experiment the relationship between cooling rate and SDAS(Secondary Dendrite Arm Spacing) and mechanical properties.The experimentally measured values and the values predicted by simulation have relatively small differences and the mechanical properties of a variety of Al alloys are expected to be predicted before casting through use of the simulation.