Optimizing the Heat Treatment Process of Cast Aluminium Alloys

The influence of heat treatment process on the microstructure and mechanical properties, and the distortion of a low-pressure die-cast AlSi7Mg0.3 alloy is reported. This work is aimed at optimizing the whole heat treatment process, including solution heat treatment, quenching and artificial ageing, of A356 alloy wheels, produced by low pressure die casting. In particular, the optimization process is focused on reducing time and temperature of solution and ageing heat treatments, and on selecting the right quenching medium, reducing the distortion of A356 alloy 17-in and 18-in wheels and obtaining the required mechanical properties. The time and temperature of solution treatment are normally chosen to dissolve coarse \u3b2-Mg2Si phase coming from the solidification process, especially where the solidification rate is slow as in the thickest zones of wheel, e.g. the hub and the spoke regions. Generally, the time required is about 45 minutes at the temperature of 540\ub0C, which is the typical solution temperature used for A356-type alloy. More time is however required at this temperature to change the morphology of eutectic silicon particles. This is aimed to improve the mechanical properties, especially ductility and fatigue properties, of wheels after further artificial ageing. The solutionising temperature is here selected to reduce the time of the heat treatment and to avoid any type of incipient melting of the material within an industrial tunnel furnace. The quenching is the most critical step in the sequence of heat-treating operations. The objective of quenching is to preserve the solid solution formed at the solution heat-treating temperature, by rapidly cooling to some lower temperature, usually near the room temperature. The most rapid quench rate, giving the best mechanical properties, can also cause unacceptable amounts of distortion or cracking in components. This is particularly true for aluminium wheels where the different thickness throughout the casting can produce distortion higher than 2-3 mm, critics for subsequent machining operations. The optimization process is here focused on the quenching rate, which is varied by changing the temperature of the quenchant, in order to reduce the wheels\u2019 distortion and to guarantee the appropriated supersaturation level of atoms for subsequent ageing treatment. Therefore, the temperature of forced water, used as quenchant, has been varied in the range of 50 to 95\ub0C, and the distortion level and the hardness of the wheels systematically measured. A temperature of 95\ub0C is observed to be the optimum solution, but useless from an industrial point of view. The extreme vapour produced by the boiling water can compromise the automatism for handling of wheels and delay too much the cooling of the wheels after solutioning. Therefore, the best compromise between distortion and mechanical properties and productivity has been revealed to be at a temperature of 75\ub0C. The time and temperature of ageing are considered in the present work in order to reach an underageing temper of the material, which is typical for the manufacture of wheels to improve impact and fatigue properties. The influence of the different painting processes, generally carried out at a temperature of 180\uf7190\ub0C for several minutes after ageing and subsequent machining operations, have to be considered to determine the final mechanical properties. These temperatures are typical temperature for ageing treatment of cast aluminium alloys, such as the A356-type alloys. Therefore, the present study takes into account the painting temperatures and times to optimize the ageing treatment of the wheels, reaching the desired mechanical properties. This study develop an optimization approach of the whole heat treatment process of cast aluminium alloy wheels. The work evidences the typical problems and targets of wheels\u2019 producers, suggesting an integrated approach to improve the productivity and the quality of castings. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring during heat treatment; while hardness and tensile testing measurements have been carried out to monitor the evolution of the mechanical properties after each step of T6 heat treatment

A New Criterion to Optimize ORC Design Performance using Efficiency Correlations for Axial and Radial Turbines

Several studies on Organic Rankine Cycles (ORCs) in the literature search for the optimal cycle parameters and working fluids that maximize the net power output. Only few studies carry out a preliminary turbine design to calculate an accurate value of turbine efficiency, but this is done only after the cycle thermodynamic optimization is performed assuming a fixed and somewhat arbitrary value of turbine efficiency. Instead, a new design optimization procedure of ORCs is proposed here which embeds correlations for the design efficiency of both axial and radial turbines. The correlations are obtained from published data in the literature and use the volumetric expansion ratio (VR) and the size parameter (VH) as performance predictors. While been applied to a selected number of working fluids and single stage turbines, the procedure has a general validity being the correlations applicable to any fluid and turbine type. Results show how the turbine efficiency, and in turn the optimum cycle parameters, are influenced by the fluid properties through the turbine VH and VR values, highlighting that the procedure for working fluid selection cannot ignore the real turbine behaviour. So, the optimum design that is obtained is expected to give a behaviour much closer to reality

Solar-aided precombustion CO2 capture in natural gas combined cycles

The integration of solar energy in natural gas combined cycles has recently received much attention in the global efforts to reduce CO2 emissions. Optimum integration options have been proposed in the literature which enhance the conversion of solar thermal energy into electricity compared to solar-only power plants. The so-called \u201cIntegrated Solar Combined Cycles\u201d (ISCCs) may embed the solar heat input in the bottoming steam cycle as well as in the topping gas turbine. Another route for the efficient abatement of CO2 in natural gas combined cycles consists in the integration of hydrogen/syngas production technologies such as reforming and water-gas shift reactions. The heat required for the endothermic steam reforming reaction can be provided by burning part of the hydrogen-rich syngas or alternatively by the partial oxidation of methane and oxygen within an autothermal reformer. After CO2 removal the resulting syngas mostly containing hydrogen is burned in the gas turbine to produce electricity or may be further processed to high-purity hydrogen. In this work the integration of solar energy in natural gas combined cycles with precombustion CO2 capture is evaluated. These plants include additional high temperature heat sinks compared to a plant without CO2 capture that could be conveniently fed by solar thermal energy. Different layouts are proposed and analyzed in the search for the optimum integration. The achievable solar share and thermodynamic and environmental performance is compared against those achievable in ISCCs without CO2 capture

Analysis of Superimposed Elementary Thermodynamic Cycles: from the Brayton-Joule to Advanced Mixed (Auto-Combined) Cycles

none2The need for efficiency improvement in energy conversion systems leads to a stricter functional integration among system components. This results in structures of increasing complexity, the high performance of which are often difficult to be understood easily. To make the comprehension of these structures easier, a new approach is followed in this paper, consisting in their representation as partial or total superimposition of elementary thermodynamic cycles. Although system performance cannot, in general, be evaluated as the sum of the performance of the separate thermodynamic cycles, this kind of representation and analysis can be of great help in understanding directions of development followed in the literature for the construction of advanced energy systems, and could suggest new potential directions of work. The evolution from the simple Brayton-Joule cycle to the so called “mixed” cycles, in which heat at the turbine discharge is exploited using internal heat sinks only without using a separate bottoming section, is used to demonstrate the potentiality of the approach. Mixed cycles are named here "auto-combined cycles” to highlight the combination of different (gas and steam) cycles within the same system components.noneLAZZARETTO A.; MANENTE GLazzaretto, Andrea; Manente, Giovann

Compressibility factor as evaluation parameter of expansion processes in Organic Rankine Cycles

In the conversion of low temperature heat sources into electricity using an Organic Rankine Cycle system the working fluid selection is a major design choice to maximize the overall performance. The placement of the power cycle on a T-s diagram depends on the fluid critical temperature. Several studies have shown that the power output can be maximized by using fluids with critical temperatures similar or lower than the inlet temperature of the heat source, which allow a better temperature profile match between the heat source and the working fluid. However, the choice of a fluid having a specific critical temperature also influences the fluid properties in the expansion process over a given temperature interval, as shown by the generalized compressibility chart. The aim of this paper is providing a better insight into the results of optimized ORCs through the analysis of the compressibility factor in the expansion process. To this purpose the real enthalpy change in the expansion process is regarded as two separate terms associated with temperature and pressure drops, respectively. Starting from the analysis of different expansion processes in optimized cycles a correlation is obtained between the compressibility factor and the ratio between real enthalpy change and the enthalpy change term associated with temperature drop. Thus, the ratio between the former and the latter can be directly evaluated from the simple knowledge of pressure and temperature values along the expansion process and the observation of the compressibility chart

A criterion to optimize ORC design performance taking into account real turbine efficiencies

Several studies on Organic Rankine Cycles (ORCs) in the literature search for the optimal cycle parameters and working fluids that maximize the net power output. Only few of these studies evaluate the real turbine efficiency, but this is done only after a thermodynamic optimization is performed assuming a fixed and somewhat arbitrary efficiency value. Instead, a new design optimization procedure of the ORCs is proposed here which embeds a correlation for the design turbine efficiency. The correlation was obtained from published data in the literature and uses the volumetric expansion ratio and the size parameter as predictors of the turbine efficiency. The procedure has a general validity being the correlation applicable to any fluid. Results show how the selection of the working fluid influences the achievable turbine efficiency and how the optimal maximum pressure of the cycle deviates from that obtained in a thermodynamic optimization in which the turbine efficiency is fixed

Analysis of superimposed elementary thermodynamic cycles: from the Brayton-Joule to advanced mixed (auto-combined) cycles

The need for efficiency improvement in energy conversion systems leads to a stricter functional integration among system components. This results in structures of increasing complexity, the high performance of which are often difficult to be understood easily. To make the comprehension of these structures easier, a new approach is followed in this paper, consisting in their representation as partial or total superimposition of elementary thermodynamic cycles. Although system performance cannot, in general, be evaluated as the sum of the performance of the separate thermodynamic cycles, this kind of representation and analysis can be of great help in understanding directions of development followed in the literature for the construction of advanced energy systems, and could suggest new potential directions of work. The evolution from the simple Brayton-Joule cycle to the so called \u201cmixed\u201d cycles, in which heat at the turbine discharge is exploited using internal heat sinks only without using a separate bottoming section, is used to demonstrate the potentiality of the approach. Mixed cycles are named here "auto-combined cycles\u201d to highlight the combination of different (gas and steam) cycles within the same system components

Improved layouts and performance of single- and double-flash steam geothermal plants generated by the Heatsep method

Single- and double-flash steam power plants are commonly used in the utilization of high enthalpy liquid-dominated geothermal resources. In these plants, the expansion line in the wet steam region results in significant penalties of turbine isentropic efficiency and power output. Accordingly, the \u201cself-superheating\u201d and \u201cinterstage heating\u201d plant modifications have been recently proposed in the literature, where the saturated steam at turbine inlet is superheated by using the heat of the geothermal liquid, which is cooled before the flashing process. In this study, the aforementioned and additional new flash steam plant layouts are generated by using a systematic method, called Heatsep, for the optimum design of energy systems. All the thermal connections between consecutive basic plant components are \u201ccut\u201d to let these temperatures vary and in turn generate additional hot and cold streams, which are combined to enhance the overall performance of the system. It is demonstrated that the single-flash plant with self-superheating is simply obtained by cutting two out of five thermal links. In the double-flash plant, the higher number of components allows for a higher number of thermal cuts and heat integration options. Unlike the existing literature, the maximum power output is not constrained by a predefined heat transfer network. The optimization results show that the maximum power output of the novel single- and double-flash steam plants exceeds by 5.5\u20139.2% and 3.9\u20137.7% the maximum attainable by the corresponding traditional plants without internal heat integration

High efficiency power generation from biomass sources using externally fired supercritical CO2 Brayton cycles

In the small to medium power range the main technologies for the conversion of biomass sources into electricity are based either on internal combustion engines or Organic Rankine cycles. Relatively low electric efficiencies are obtained in both cases due to thermodynamic losses in the conversion of biomass into syngas and to the heat transfer between combustion gases and working fluid, respectively. Higher efficiencies can be obtained using the supercritical closed CO2 Brayton cycles, the applications of which are restricted in the literature to nuclear power plants and more recently to concentrating solar power plants. The cascaded configuration of two supercritical CO2 cycles enables to overcome the intrinsic limitation of the single cycle in the effective utilization of the whole heat available from the heat source. The aim of this paper is to evaluate whether this power plant configuration could be a good alternative option in the conversion of biomass sources into electricity, which was never explored in the literature up to now. The focus is on the search of the thermodynamic operating parameters which maximize power output. Results of the optimization procedure show that a total heat recovery efficiency in the range 30-34% can be achieved, which is approximately 5%-points higher than that of the existing biomass power plants in the small to medium power range