Donghong Wang

Numerical Simulation of the Wax Injection Process for Investment Casting

The dimensions of the wax pattern overwhelmingly determine the final quality of an investment casting component, therefore its very important to know the deformation and local dimension changing of the wax pattern, and as a result, learn how to modify the tooling design. As the most promising way with the advantages in cost and period, numerical simulation learns what exactly happens inside the tooling when the wax is injected, and how the pattern will change with the cooling and demolding. This article achieved some material properties by some measures used for application in polymer study, and then constructed a thin-walled 3D model of the wax injection process. The predicted results, like filling time, shrinkage, etc., were compared with those of experimental data. A qualitative agreement between them with an acceptable difference in shrinkage was reported, and the optimization of location of the injection gates was discussed as well.

Cavity Pressure and Dimensional Accuracy Analysis of Wax Patterns for Investment Casting

The dimension error of an investment casting always begins at the injection of the wax pattern, and the knowledge on this stage will help to decide the proper pattern shrinkage allowances, and finally understand the dimensional variations of the casting. This study investigated the influence of the process parameters on the cavity pressure and dimensional variations of the wax patterns with different thicknesses by both simulative and experimental methods. A commercial simulation code, three-dimensional (3D) laser scanner, and the pressure sensor were used. Simulated and measured cavity pressure and diameters were compared and found to be matched very well. It was found that the holding time and packing pressure affect the thickness stability of the wax pattern greatly. However, changes in injection temperature did not affect the magnitude of cavity pressure and contraction greatly. The cavity pressure is more sensitive to packing pressure. There was no notable effect on the shrinkage of the wax pattern when holding time greater than the gates sealing time. The shrinkage of the thickness increases with the increase of the thickness. The work presented here is significant to model the deformation of wax pattern and understand the effect of the process parameters and geometry variables on the wax pattern shrinkage allowances.

Formation Mechanism of Spherical TiC in Ni-Ti-C System during Combustion Synthesis

The formation mechanism of TiC particles in a Ni-Ti-C system were revealed by using differential thermal analysis (DTA), XRD, and SEM to identify the reaction products in different temperature ranges. The results indicated that the synthesis mechanism of TiC in Ni-Ti-C system was complex; several reactions were involved in the combustion synthesis of TiC-Ni composite. The Ni-Ti intermediate phases play important roles during the formation of TiC. Moreover, the influence of heating rate on the size range of TiC was also discussed.

Experimental and Numerical Analysis on Core Deflection during Wax Injection

As the large thin-walled and hollow investment casting is widely used, control of core deflection during wax injection becomes increasingly important. Core deflection during wax injection can cause wall thickness to exceed tolerance in final cast part. This paper presents the numerical and experimental study on the core deflection of a thin-walled hollow wax pattern. The predicted deflection displacements were compared with the experimental data to find good agreement. The mechanism of core deflection was found through 3D numerical analysis. The packing pressure differentials in packing phase are the major cause for core deflection during wax injection, which can be reduced by low packing pressures. The results show that more fixed core-pins can also reduce the thickness variation in the two sides of the thin-walled wax pattern. In this study, the core deflection during wax injection was successfully modeled, which will significantly help to understand the core deflection behavior in complex geometries during injection molding.

Prediction of Cavitation Depth in an Al-Cu Alloy Melt with Bubble Characteristics Based on Synchrotron X-ray Radiography

The size of cavitation region is a key parameter to estimate the metallurgical effect of ultrasonic melt treatment (UST) on preferential structure refinement. We present a simple numerical model to predict the characteristic length of the cavitation region, termed cavitation depth, in a metal melt. The model is based on wave propagation with acoustic attenuation caused by cavitation bubbles which are dependent on bubble characteristics and ultrasonic intensity. In situ synchrotron X-ray imaging of cavitation bubbles has been made to quantitatively measure the size of cavitation region and volume fraction and size distribution of cavitation bubbles in an Al-Cu melt. The results show that cavitation bubbles maintain a log-normal size distribution, and the volume fraction of cavitation bubbles obeys a tanh function with the applied ultrasonic intensity. Using the experimental values of bubble characteristics as input, the predicted cavitation depth agrees well with observations except for a slight deviation at higher acoustic intensities. Further analysis shows that the increase of bubble volume and bubble size both leads to higher attenuation by cavitation bubbles, and hence, smaller cavitation depth. The current model offers a guideline to implement UST, especially for structural refinement.

In Situ Observation of the Zr Poisoning Effect in Al Alloys Inoculated by Al-Ti-B

In situ synchrotron X-ray radiography observations of the Zr-poisoning phenomenon of an Al-20 wt pct Zn alloy inoculated by Al-5Ti-1B were carried out. The effects of Zr addition on heterogeneous nucleation, grain growth, and final grain size were quantitatively studied and further analyzed using the interdependence model. The experimental results show that the undercooling needed for nucleation increases and the nucleation rate decreases at the early stage of solidification with Zr addition, resulting in fast grain growth, a high solidification rate and increased severity of solute segregation in the solid–liquid coexistence regions. At the same time, the poisoning is a progressive process that is enhanced with the increasing Zr content, holding temperature, and holding time. The nucleation-free zone of the Zr-containing sample, either measured from the radiographs or calculated by the interdependency theory, is larger than that of the Zr-free sample. Our analysis shows that both the increase in the nucleation-free zone and the average interparticle spacing of the most potent available nucleation particles contribute to the increase of the grain size caused by Zr poisoning.