X.P. Niu’s research while affiliated with Singapore Institute of Manufacturing Technology (SIMTech) and other places

The die casting of thin-wall magnesium components free of voids and having complete filling, resulting in high strength, can only be achieved under optimum cavity pressure. The pressure peaks occurring in the cavity are an important criterion for the consistent quality of these parts. The results from the measurement and monitoring of the cavity pressure can achieve minimum scrap and ensure constant quality.

Vertical semi-solid casting process was developed for the fabrication of small precision and high stiffness computer actuator using an MMC material for hard disk drive applications. Slug heating temperature, injection speed and applied pressure were found to be key parameters for successful vertical semi-solid casting. The MMC material showed good flow ability with laminar behavior during the vertical casting process.

A well-designed runner and gating system is very important to secure good quality die castings through providing a homogenous mould filling pattern. Numerical simulation is a cost-effective tool in the design of runner and gating systems to visibly analyse the mould filling process. A thin-walled magnesium telecommunication part was selected to be hot chamber die cast and a numerical simulation technique was applied for the optimisation of the runner and gating. Two types of runner and gating systems were designed and analysed. A preliminary design with a split gating system led to a swirling filling pattern and insufficient central flow, which prematurely closed the edges and left the last filled areas falling into the inner portion of the part. It resulted in a high possibility of air entrapment in the casting and the design was not proper for the part. The design was improved by using a continuous gating system and a bigger size runner. The gate area was increased and the gating speed was slightly reduced. Numerical simulation showed that the new design provided a homogenous mould filling pattern with the last filled areas being located at the upper edge of the part, where overflows and vents were conveniently attached. For the study, die inserts for both designs were fabricated. A series of casting experiments were conducted. The short shot filling tests proved that the simulation results matched the actual casting results very well. Good quality thin-walled telecommunication parts with sound microstructure were produced based on this optimised runner and gating system.

High pressure die castings usually contain gas porosity due mainly to the entrapment of air or gas in the melt during the very high speed injection of the molten metal into the cavity. In this paper, the advantages of using an evacuated die cavity during mould filling were evaluated. ASTM standard die casting tensile specimens of three Al alloys, Al–5%Si, Al–8%Si and Al–18%Si, were cast on a Buhler shot control die casting machine equipped with a Fondarex vacuum assist system. The effect of vacuum assistance on the porosity distribution and mechanical properties of the produced castings were studied in detail. Selected specimens were also subjected to a T6 tempering treatment to evaluate the formation of surface blisters and effects on the mechanical properties.It was found that the volume of gas porosity and the pore sizes in the castings were significantly reduced by using vacuum assistance during die casting. As a result, the density and the mechanical properties, particularly the tensile strength and ductility, were improved markedly. An optimum injection speed was also identified for producing high performance castings. After heat treatment, vacuum assisted die cast parts showed much less surface blistering when compared to conventional die cast parts, demonstrating that vacuum assisted die castings are promising for heat treatment at elevated temperatures to improve the mechanical properties.

In this study, an attempt was made to demonstrate the use of a near liquidus cast Al-Mg based alloy for semi-solid thixocasting. A component of a fishing reel was both conventional high pressure die-cast from a liquid melt and thixocast from a liquidus cast billet using the same mold. Components were examined using both optical and electron microscopy and tensile samples were taken from the runner of a cast part. Parts were compared against samples that were cast using conventional high-pressure die-casting. The results indicate that liquidus casting is a viable alternative for producing thixocast billets.

Free flowing of liquid metal through the gate during die filling and solidification is critical to the final product quality in aluminium high pressure die casting. Early close-off of the gate due to freezing detrimentally affects the pressure transmission from the injection system to the casting as it solidifies, resulting in poor soundness of castings due to improper feeding of the shrinkage. In this paper, piezoelectric cavity pressure sensors were employed to study the gate freezing behaviour in aluminium high pressure die casting. Metal pressure at the runner, gate and cavity was directly measured during casting. From the on-line measured metal pressure profile, a method was established for analysing the gate freezing time. The effect of process variables on the gate freezing time, such as intensification pressure and gate speed, as well as their relation to the mechanical properties of the produced castings, was studied in detail. It was found that the gate freezing time could be determined by the duration from the start of the cavity fill to the drop in pressure measured at a location just after the gate. The application of higher gate velocity and pressure assisted in keeping the gate open longer. As a result, sound castings were produced and mechanical properties were improved. The results indicate that the method developed in this work can be used to control and optimise the die casting process for the fabrication of high internal integrity components.

In this study, the important role of atomized flow (jet flow) in high pressure die casting manufacture is cited. A series of experimental studies with a water analogue were conducted. It was found that the critical gate velocity for full atomization of water under the conditions of this study was about 19.6 m/s. A formula was used to estimate the critical gate velocity of liquid aluminium by correlating the difficulty of water to atomize with that of liquid aluminum via surface tension and other parameters. As a result, liquid aluminum was found to be about 3.33 times more difficult to atomize than water. Hence, it was estimated that liquid aluminum might be fully atomized at a gate velocity higher than 65 m/ s. Furthermore, based on some reports in die casting practice, it is suggested that atomization in high pressure die casting might not be always regarded as a consistent problem, but might provide some solutions for extremely thin wall die casting manufacturing to explore new markets in the electronics and communications industries.