Direct Observation of Filling Process and Porosity Prediction in High Pressure Die Casting

A Novel Approach to Visualize Liquid Aluminum Flow to Advance Casting Science

Turbulent filling of molten metal in sand-casting leads to bi-films, porosity and oxide inclusions which results in poor mechanical properties and high scrap rate of sand castings. Hence, it is critical to understand the metal flow in sand-molds, i.e., casting hydrodynamics to eliminate casting defects. While multiple numerical methods have been applied to simulate this phenomenon for decades, harsh foundry environments and expensive x-ray equipment have limited experimental approaches to accurately visualize metal flow in sand molds. In this paper, a novel approach to solve this challenge is proposed using Succinonitrile (SCN) as a more accurate metal analog in place of water. SCN has a long history in solidification research due to its BCC (Body-Centered-Cubic) crystal structure and dendrite-like solidification (melting temperature ~60 °C) like molten aluminum. However, this is the first reported study on applying SCN through novel casting hydrodynamics to accurately visualize melt flow for casting studies. This paper used numerical simulations and experiments using both water and SCN to identify the critical dimensionless numbers to perform accurate metal flow analog testing. Froude's number and wall roughness were identified as critical variables. Experimental results show that SCN flow testing was more accurate in recreating the flow profile of molten aluminum, thus validating its utility as a metal analog for metal flow research. Findings from this study can be used in future metal flow analysis such as: runner, in-gate and integrated filling-feeding-solidification studies.

Influence of Process Parameters on Filling and Feeding Capacity during High-Pressure Die-Casting Process

The quality of high-pressure die casting (HPDC) is largely determined by the process parameters. The objective is to study the influence of process parameters on melt filling and feeding capacity. The research is carried out by means of numerical simulations and orthogonal experiments. The results show that the influence of process parameters is different. The piston velocity of the fast stage has the most obvious impact. As the piston velocity of the fast stage increases, the filling capacity and feeding capacity also increase. The melt injection temperature should not be too low; otherwise, it would be easy to cause gas porosity. The piston velocity of the slow stage should not be too high; otherwise, it would cause air entrainment. The influence of the pressure maintaining time is not obvious due to the ingate solidifying prematurely. The recommended process parameters are a melt injection temperature of 660–680 °C, a piston velocity of 0.1–0.2 m/s in the slow stage, and a piston velocity 2.4–3.2 m/s in the fast stage. This also suggests that appropriate technology should be adopted to delay the solidification time of the ingate in order to realize the feeding effect of pressurization.

Research and Evaluation of the Influence of the Construction of the Gate and the Influence of the Piston Velocity on the Distribution of Gases into the Volume of the Casting

Distribution of gasses to the cast volume and volume of pores can be maintained within the acceptable limits by means of correct setting of technological parameters of casting and by selection of suitable structure and gating system arrangement. The main idea of this paper solves the issue of suitability of die casting adjustment—i.e., change of technological parameters or change of structural solution of the gating system—with regards to inner soundness of casts produced in die casting process. Parameters which were compared included height of a gate and velocity of a piston. The melt velocity in the gate was used as a correlating factor between the gate height and piston velocity. The evaluated parameter was gas entrapment in the cast at the end of the filling phase of die casting cycle and at the same time percentage of porosity in the samples taken from the main runner. On the basis of the performed experiments it was proved that the change of technological parameters, particularly of pressing velocity of the piston, directly influences distribution of gasses to the cast volume.

Possibility of Utilization of Gate Geometry to Modify the Mechanical and Structural Properties of Castings on the Al-Si Basis

The mechanical properties of die castings correlate with the inner structure of a casting, distribution of the eutectic phases, and with the content and distribution of porous cavities in the casting volume. This submitted paper deals with the issue of the possibility to influence the basic parameters affecting the quality of castings through structural modifications of the gating system. The structural parameter under assessment is the gate height. In the case of the diverse height of the gate, five sets of castings were produced. The individual sets of castings were subjected to examination of selected mechanical properties, i.e., of permanent deformation and surface hardness. At the same time, the individual sets of castings were subjected to metallographic examination of the eutectic structure of the casting. It was proved that the gate height influences the aforementioned properties of the castings and significantly affects the ratio of the eutectic phases in the volume of the casting. The conclusion describes the mutual correlation between the gate structure, the mechanical properties of the casting, and its structural composition.

Effect of Flow State of Pure Aluminum and A380 Alloy on Porosity of High Pressure Die Castings

Air entrapment defects prevent the heat treatment from improving the mechanical properties of die castings, which limits the die casting of high-performance components. The flow pattern of the filling process is complicated and experimental analysis is difficult in thin-walled complex die castings. In this study, we constructed a shock absorption tower to observe in real-time the filling process of pure aluminum and A380 aluminum alloy at different fast injection speeds. The degree of breakup of pure aluminum was larger than that of A380 during the filling process, which caused the porosity of pure aluminum to be greater than that of the A380 at each observation position. Re-Oh diagrams explained the difference in porosity between the two metals. The porosity in different regions was closely related to the flow state of aluminum liquid. In addition to porosity measurements, we specifically analyzed the relationship between the porosity of the flowback zone, the final filling zone, and the near-tail zone of cylinder. At the same injection velocity, the porosity at flowback zone was greater than that at the final filling position, the porosity at final filling position was larger than that at the near-tail zone of cylinder, and the final filling position changed as the injection velocity changed.