Mostafa Payandeh

Influence of Microstructural Inhomogeneity on Fracture Behaviour in SSM-HPDC Al-Si-Cu-Fe Component with Low Si Content

In the current paper, a low-Si containing aluminium alloy (1.4-2.2% Si) was used to fabricate a complex shape telecom component using Semi-Solid High-Pressure Die Cast (SSM-HPDC), process. Microstructure and fracture characteristics were investigated. The cast material exhibited microstructural inhomogeneity, in particular macrosegregation in the form of liquid surface segregation bands in addition to sub-surface pore bands and gross centre porosity. Tensile specimen were taken from the cast components. Elongation and microstructural inhomogeneity were investigated and correlated. Fracture surfaces of the tensile specimen were examined under scanning electron microscope (SEM). The study showed that both near surface liquid segregation bands and subsurface porosity strongly affected the fracture behaviour. Dominant for loss of ductility were gross centre porosity. This centre porosity was found to be a combination of trapped gas and insufficient, irregular feeding patterns.

Effect of Superheat on Melting Rate of EEM of Al Alloys during Stirring Using the RheoMetal Process

The RheoMetal process (previously called the Rapid S- and RSF- process) is a novel method to produce cost effective, high quality, semisolid slurries for component casting. The RheoMetal process uses an Enthalpy Exchange Material (EEM) as cooling agent to absorb heat and produce a slurry. Critical process parameters to create a slurry by robust melting of the EEM are alloy content, stirring speed, EEM to melt ratio, EEM temperature, EEM microstructural characteristics and melt superheat. In this paper, the melting sequence and melting rate of the EEM was studied experimentally. The effect of EEM composition, as well as superheat, on evolution of shape and dimension of the EEM during stirring was investigated. Initial material freezing onto the EEM was observed, followed by a stationary phase with subsequent gradual melting of the EEM. It was shown that the characteristics of freeze-on layer were strongly correlated to melt superheat, EEM temperature, as well as material composition, hence also has significant influence on the melting sequence.

Influence of microstructure and heat treatment on thermal conductivity of rheocast and liquid die cast Al-6Si-2Cu-Zn alloy

Thermal conductivity of a rheocast component made from Stenal Rheo1 (Al-6Si-2Cu-Zn) alloy was investigated in as-cast, T5 and T6 conditions. The thermal conductivity measurement in different locations showed variation of this property throughout the rheocast component. The results of microstructural investigation revealed that the ratio of solute-lean α1-Al particles formed during slurry preparation to fine solute-rich α2-Al particles formed during secondary solidification had significant influence on thermal conductivity. The reduced amount of solutes in the α1-Al particles was determined as the root cause of higher thermal conductivity. A linear relation between the fraction of precipitates and the increase in thermal conductivity was obtained and silicon in solid solution is shown to have a dominant influence. As silicon was precipitated during the heat treatment, thermal conductivity increased. For an optimal combination of thermal and mechanical properties, it is therefore important to use an ageing temperature above the temperature of Si precipitation.

Influence of Process Parameters on Surface Appearance and Roughness of a Low Si Containing Al-Alloy, in Semisolid Casting

The visual appearance and surface roughness were experimentally investigated in industrial scale for a low silicon containing aluminium alloy cast in semisolid state integrated with HPDC machine. A visual comparative technique and surface roughness in the form of the Rq value were used to evaluate the surface appearance and the surface roughness respectively. The results were investigated statistically to find significant models. It was shown that high quality appearance, following a comparative scale, was possible using high die temperature and high injection speed. It was also found that improving the surface roughness will deteriorate the visual appearance.

Effect of Material Inhomogeneity on Thermal Performance of a Rheocast Aluminum Heatsink for Electronics Cooling

The relation between microstructural inhomogeneity and thermal conductivity of a rheocast component manufactured from two different aluminum alloys was investigated. The formation of two different primary α-Al particles was observed and related to multistage solidification process during slurry preparation and die cavity filling process. The microstructural inhomogeneity of the component was quantified as the fraction of α1-Al particles in the primary Al phase. A high fraction of coarse solute-lean α1-Al particles in the primary Al phase caused a higher thermal conductivity of the component in the near-to-gate region. A variation in thermal conductivity through the rheocast component of 10% was discovered. The effect of an inhomogeneous temperature-dependent thermal conductivity on the thermal performance of a large rheocast heatsink for electronics cooling in an operation environment was studied by means of simulation. Design guidelines were developed to account for the thermal performance of heatsinks with inhomogeneous thermal conductivity, as caused by the rheocasting process. Under the modeling assumptions, the simulation results showed over 2.5% improvement in heatsink thermal resistance when the higher conductivity near-to-gate region was located at the top of the heatsink. Assuming homogeneous thermo-physical properties in a rheocast heatsink may lead to greater than 3.5% error in the estimation of maximum thermal resistance of the heatsink. The variation in thermal conductivity within a large rheocast heatsink was found to be important for obtaining of a robust component design.

The Portevin-Le Châtelier Effect in a Rheocast Al-Si-Cu Alloy

The plastic instability phenomenon Portevin-Le Châtelier effect (PLC) was investigated in a Rheocast Al-Si-Cu alloy. The effect of strain rate under as-cast, solution treated and water quenched as well as artificially aged conditions was studied. Tensile tests were conducted at room temperature at different strain rates ranging from 10–5 to 10–1 (1/s). The nature and type of PLC effect were identified and analyzed. The results showed that the PLC effect changed with strain rate. Heat treatment of the material, widened the range of strain rates exhibiting PLC as well as changed the nature of the PLC effect.