The physical properties of polymer quench bath directly affect the cooling rate of a quenched part. These properties include the type of quenchant, its temperature, concentration, and agitation level. These parameters must be controlled to optimize the quenching process in terms of alloy microstructure, properties and performance. Statistically designed experiments have been performed to investigate the effects of the process parameters (i.e. polymer concentration and agitation) on the heat transfer behavior of cast aluminum alloy A356 in aqueous solution of Aqua-Quench 260 using the CHTE quenching-agitation system. The experiments were designed using Taguchi technique and the experimental results were analyzed with Analysis of Variance (ANOVA) based on the average cooling rate. It is found that average cooling rate dramatically decreases with the increase in polymer concentration. Agitation only enhances the average cooling rate at low and medium concentration levels. From ANOVA analysis, the process parameter that affects the variation of average cooling rate most is the polymer concentration, its percentage contribution is 97%. The effects from agitation and the interaction between polymer concentration and tank agitation are insignificant. The mechanical properties of age-hardenable Al-Si-Mg alloys depend on the rate at which the alloy is cooled after the solutionizing heat treatment. A model based on the transformation kinetics is needed for the design engineer to quantify the effects of quenching rates on the as-aged properties. Quench Factor analysis was developed by Staley to describe the relationship between the cooling rate and the mechanical properties of an age-hardenable alloy. This method has been previously used to successfully predict yield strength, hardness of wrought aluminum alloys. However, the Quench Factor data for aluminum castings are still rare in the literature. In this study, the Jominy End Quench method was used to experimentally collect the time-temperature and hardness data as the inputs for Quench Factor modeling. Multiple linear regression analysis was performed on the experimental data to estimate the kinetic parameters during quenching. Time-Temperature-Property curves of cast aluminum alloy A356 were generated using the estimated kinetic parameters. Experimental verification was performed on a L5 lost foam cast engine head. The predicted hardness agreed well with that experimentally measured.

Conventional casting alloys Al-7Si-Mg A356/7 contain between 6.5 and 7.5% Si, together with 0.25-0.7% Mg and are used for critical castings in the automotive and aerospace industries. These alloys are also the most popular alloys used for semi-solid metal (SSM) forming due to good castability and fluidity imparted by the large volumes of the Al-Si eutectic. Despite their industrial importance, there is a lack of detailed research work revealing precipitate micro- and nanostructural evolution during aging of these alloys compared with the Al-Mg-Si 6000 series wrought alloys. This study characterises the heat treatment response of SSM-processed Al-7Si-Mg alloys in comparison with conventionally liquid cast alloys (investment casting and gravity die casting). It is shown that, provided that the maximum quantity of the alloy’s Mg is placed into solid solution during solution treatment, and that the alloy’s Fe content is within specification, the response to age hardening of Al-7Si-Mg alloys is independent of the processing technique used. The nanostructural evolution of Al- 7Si-Mg alloys after artificial aging with and without natural pre-aging has been characterized using transmission electron microscopy and atom probe tomography and correlated with hardness and mechanical tensile properties. The number densities and Mg:Si ratios of solute clusters, GP zones and β"-needles were determined. The heat treatment response of SSM-processed casting alloys A356/7 alloys are also compared with SSM-processed Al-Mg-Si 6000 series wrought alloys, with the advantage of having similar globular microstructures. The high Si-content of the casting alloys compared to the wrought alloys offers several advantages, including a faster artificial aging response (shorter T6 aging cycles), higher strength for comparable Mg contents and less sensitivity to prior natural aging on peak strength. Finally, an age-hardening model was developed for the Al-7Si-Mg alloys, including a method of incorporating the effects of changes in Mg-content on the aging curves. / Thesis (PhD(Eng))--University of Pretoria, 2011. / Materials Science and Metallurgical Engineering / unrestricted

Due to the increasing use of cyclically loaded cast aluminum components in automotive and aerospace applications, fatigue and fatigue crack growth characteristics of aluminum castings are of great interest. Despite the extensive research efforts dedicated to this topic, a fundamental, mechanistic understanding of these alloys' behavior when subjected to dynamic loading is still lacking. This fundamental research investigated the mechanisms active at the microstructure level during dynamic loading and failure of conventionally cast and SSM Al-Si-Mg alloys. Five model alloys were cast to isolate the individual contribution of constituent phases on fatigue resistance. The major constituent phases, alpha-Al dendrites, Al/Si eutectic phase, and Mg-Si strengthening precipitates were mechanistically investigated to relate microstructure to near-threshold crack growth (Delta Kth) and crack propagation regimes (Regions II and III) for alloys of different Si composition/morphology, grain size, secondary dendrite arm spacing, heat treatment. A procedure to evaluate the actual fracture toughness from fatigue crack growth data was successfully developed based on a complex Elastic-Plastic-Fracture-Mechanics (EPFM/J-integral) approach. Residual stress-microstructure interactions, commonly overlooked by researches in the field, were also comprehensively defined and accounted for both experimentally and mathematically, and future revisions of ASTM E647 are expected.