B.H. Hu’s research while affiliated with Singapore Institute of Technology and other places

The paper reports the results of three projects concerned with the processing of nanostructured magnesium alloy, polymer/clay nanocomposites and nano ceramic powder. The challenges and approaches taken to process the material are highlighted and discussed. For processing of bulk magnesium alloy, equal channel angular extrusion (ECAE) of ultra fine and nanostructured magnesium based on the principle of simple shearing was conducted. Three types of dies for ECAE of magnesium were designed and numerically analysed. Numerical simulation using ANSYS/LS-DYNA was conducted to evaluate the material effective strain after numerous passes of ECAE. A grain size of 100μm can be transformed to nanostructure after 16 passes of ECAE. For processing polymer/clay nanocomposites, isotactic polypropylene (PP)/clay nanocomposites were fabricated through melting compounding of PP with organically modified clay. To enhance the compatibility, maleated PP (mPP) with low maleic anhydride content was used as compatibilizer, The resulted PP/clay and PP/mPP/clay nanocomposites show a much-improved tensile modulus. The addition of clay was found to enhance the impact strengths of PP/clay nanocomposites. For processing of nano ceramic powder, work has been centered on using powder injection moulding (PIM) technique to produce complex components with nano-structures. PIM feedstocks with 8 to 15 nm powder sizes are injection-molded into green parts, and after debinding and sintering, a grain size of about 30 nm were obtained. However, nano cracks have been encountered and remain as a challenge for the PIM of nano ceramic powder.

Semi-solid metal forming is a process for producing high integrity metallic components for various industries. Electromagnetic stirring (EMS) is a typical process for fabricating raw materials used for semi-solid forming. EMS has mainly been used for aluminium alloy, but it is not as developed for magnesium materials. A laboratory-scaled semi-automated electromagnetic stirring apparatus to fabricate magnesium alloys for semi-solid forming was developed. An AZ91 magnesium alloy was selected in this investigation. An optimum process parameter was found to have an electrical frequency of 15 Hz and a melt temperature of 610 °C. The microstructures obtained were equiaxed dendritic grains. The EMS samples were heated and the microstructure was changed to discrete, rosette, oval or spherical shaped particles. Under an optimum heating condition, the microstrucrure resulted in a homogeneous spherical fine grain structure. The average grain size was about 100μm. This indicated that the AZ91HP alloy prepared by this apparatus was suitable for semi-solid forming.

Magnesium is known to have poor formability at room temperature, mainly attributed to its hexagonal lattice structure. This paper presents the findings generated from development of warm forming to produce thin-walled magnesium components. Finite element analysis was conducted to evaluate the formability of the material and the simulated results were compared to the tensile results and product formability. Tensile tests were used to verify the formability of the sheet metal at difference temperatures. The process parameters considered in the feasibility study were forming temperature, in the range of 28°C to 300°C and magnesium sheet (AZ31B-H24) thickness, in the range of 0.4 mm to 1 mm. Magnesium hand phone covers of a thickness down to 0.4 mm have been successfully produced. The feasible forming temperature was found to be between 200°C and 250°C. Metallograhic examination shows evident of recrystallisation when the magnesium sheets are formed at evaluated temperature.

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.

This paper highlights the advantages of warm forming technique to produce thin-walled magnesium parts. The process parameters considered in the feasibility study were forming temperature, in the range of 28°C to 300°C and magnesium sheet (AZ31B-H24) thickness, in the range of 0.4 mm to 1 mm. Magnesium hand phone covers of a thickness down to 0.4 mm have been successfully produced. It was found that the optimum forming temperature is between 225°C and 275°C. Metallograhic examination shows a sound microstructure of the magnesium sheets formed at 250°C.

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.