1,721,198 research outputs found
Constitutive and stochastic models to predict the effect of casting defect on the mechanical properties of high pressure die cast AlSi9Cu3(Fe) alloys
No full textThe effect of casting defects on mechanical properties of a high-pressure die-cast AlSi9Cu3(Fe) alloy is reported. A series of U-shaped structural components are cast using a combination of injection parameters and pouring temperatures in order to generate different types and amount of casting defects throughout the casting. It has been found that castings contain defects, primarily pores and oxides, and that the presence and distribution of these defects are highly sensitive to the process conditions. Moreover, significant variations of the defect distribution have, however, been found in castings produced under the same conditions, indicating the stochastic nature of defects in die castings. The tensile properties are affected by the amount and distribution of defects and are determined by the defect area fraction. The influence of casting defects on mechanical properties are investigated through a theoretical verification based on constitutive and stochastic models. The analytical approach, based on the Ghosh constitutive model of tension instability, correctly indicates the trends of the experimental results, while the Weibull statistics evidences how the scale parameter and the Weibull modulus are strongly affected by the casting conditions. An integrated stochastic-analytical approach is then proposed and it appears to be applicable to describe the tensile properties in terms of fractographic defects and cumulative failure probability Pi
Correlation between processing and quality of aluminium alloy castings (Correlazione tra parametri di processo di fonderia e qualità dei prodotti)
Full text linkThe influence of processing and process parameters plays a key role for the Aluminium foundry and transport industries as it affects the quality and soundness of the cast products. Particularly, the choice of a process chain in Aluminium foundry, otherwise of process parameters, influences the reject rates, hence casting costs, the process yield and the production rate. The process chain in Aluminium foundry is a complex sequence of processes and the final casting quality depends on many parameters. Several aspects of this subject are still not fully understood. The motivation of the research presented in this doctoral thesis work was, therefore, to fill this gap in knowledge. The study has aimed at understanding the influence of various process and process parameters of foundry on the quality of aluminium alloy castings and, in particular, Al-Si based castings.
A literature review and a sufficient background of previously reported results on the influence of processing and process parameters on the quality of aluminium alloy castings, physical fundamentals as well as industrial challenges, motivation and goals were carried out.
Special attention in Aluminium process chain has been given to:
The modification of aluminium-silicon cast alloys: before casting aluminium alloys, the molten metal can be treated in order to improve the microstructure and properties of alloys by addition of small quantities of certain "modifying" elements.
The pouring of molten metal into the mould: this is one of the critical steps in foundry technology, since the behaviour of the liquid and its subsequent solidification and cooling determine whether the cast shape will be properly formed, internally sound and free from defects.
The chill casting processes, such as gravity, low-pressure and high-pressure die casting processes: the essential feature of chill casting is the use of permanent metal moulds, into which the molten alloy is either poured directly or injected under pressure, giving rise to the separate processes of gravity and low/high pressure die casting. Permanent moulds offer obvious advantages in terms of simplicity of production for large quantities of parts, but are subject to limitations yet to be discussed.
The heat treating process applied to high-pressure die castings: conventional die castings are utilised to produce many products but unfortunately the presence of porosity limits the application. In addition to porosity, the microstructure inherent with conventional die casting could not meet the mechanical requirements needed for many applications. Subsequent heat treating, which can positively alter the microstructure, is rarely possible due to defects that emerge during thermal processing, such as blistering
High-Pressure Die-Cast AlSi9Cu3(Fe) Alloys: Models for Casting Defects and Mechanical Properties
No full textThe effect of casting defects on the tensile properties of high-pressure die-cast AlSi9Cu3(Fe) alloys is reported. A series of U-shaped structural components has been die-cast using a combination of injection parameters and pouring temperatures in order to produce different types and amount of casting defects throughout the casting. The results reveal how the die-castings contain defects, primarily pores and oxides, and their presence and distribution are highly sensitive to the process conditions. The tensile properties are affected by the amount and distribution of defects and can be determined by the defect area fraction. The influence of casting defects on the tensile properties are investigated through a theoretical verification based both on constitutive and stochastic models. The analytical approach, based on the Ghosh constitutive model of tension instability, correctly indicates the trends of the experimental results, while the Weibull statistics evidences how the scale parameter and the Weibull modulus are strongly affected by the casting conditions
Microstructure, defects and properties in aluminium alloys casting: a review
No full textMicrostructure, defects and final mechanical behaviour in castings are strongly correlated and inter-dependent. With specific reference to Aluminium alloys cast products, the present paper is aimed at describing
- firstly the more frequently encountered defects in castings,
- then the typical microstructure that can be achieved
- and finally how defects and microstructure are affecting the mechanical properties of such castings.
The need of an integrated knowledge about these aspects is outlined, in view of an efficient and optimised design of cast components manufactured using Aluminium alloys
Fluidità delle leghe leggere da fonderia Parte II – Le leghe Al-Si
No full textLa fluidità è una proprietà fisica che caratterizza un materiale allo stato liquido; questa proprietà influenza la dinamica di riempimento degli stampi, da cui dipende direttamente la qualità dei getti finali realizzati. Nonostante l’importanza che questo aspetto riveste in fonderia, le conoscenze su questo fronte sono ancora molto limitate. Pochi dati sono disponibili e, a volte, tra loro in contrapposizione; inoltre, sono state condotte indagini approfondite e dettagliate solo un numero limitato di leghe. In questo lavoro si ripercorre un’analisi della letteratura scientifica soffermandosi sulle variabili che influenzano maggiormente la fluidità di una lega e ponendo particolare attenzione ai dati disponibili su leghe Al-Si da fonderia, molto diffuse nel settore automobilistico e aeronautico
Microstructural investigation on aluminium castings for automotive applications
No full textIn the diesel engine cylinder head, especially to resist to high combustion pressure, it has been demanded to develop high strength materials. The traditional cast iron cylinder head meets most of the requirements. However, cast iron has the disadvantage of being heavy and having a low thermal conductivity. To improve the fuel efficiency and the drivability of vehicles, light weighted cylinder heads made of aluminium alloys have been developed. In recent production, Al-Si-Cu-Mg cast alloys have been used so far because of their good performance of castability and mechanical strength with age hardening.
This work analysed the microstructural properties of the mentioned cast aluminium alloys on two real complex-shaped castings like diesel cylinder heads, as cast and T6 heat treated, experimentally produced by Teksid Aluminum. The secondary dendrite arm spacings (SDAS) and the size and morphology of eutectic silicon were considered with this study under quantitative aspects. It is well known how finer SDAS, obtained with high solidification conditions, result in more uniform microstructure and create the condition for more closely distributed and finer, smaller particles, like eutectic silicon and intermetallics. In this study the cast aluminium alloy (A354) adopted to cast cylinder heads underwent a heat treatment (T6) in order to obtain an optimum combination of strength and ductility. The mechanical properties of this alloy are generally dependent by the followings conditions:
distribution and size of the precipitates, β-Mg2Si and θ-Al2Cu,
structure and size of eutectic silicon particles.
Therefore, it is evident how the mechanical properties of the casting are severely influenced by the combined effect of SDAS and eutectic silicon
Modelling the effect of casting defects on tensile properties of high-pressure die-cast AlSi9Cu3(Fe) alloys
No full textThe effect of casting defects on mechanical properties of a high-pressure die-cast AlSi9Cu3(Fe) alloy is reported. A series of U-shaped structural components are cast using a combination of injection parameters and pouring temperatures in order to generate different types and amount of casting defects throughout the casting. It has been found that castings contain defects, primarily pores and oxides, and that the presence and distribution of these defects are highly sensitive to the process conditions. The tensile properties are affected by the amount and distribution of defects and are determined by the defect area fraction. The influence of casting defects on mechanical properties are investigated through a theoretical verification based on constitutive and stochastic models. The analytical approach, based on the Ghosh constitutive model of tension instability, correctly indicates the trends of the experimental results, while the Weibull statistics evidences how the scale parameter and the Weibull modulus are strongly affected by the casting conditions
Fluidity of aluminium die castings alloy
No full textThe aim of the present work was to investigate the fluidity of four different high pressure die cast Al-Si alloys at different pouring temperatures. A vacuum fluidity test apparatus was employed to measure fluidity. The analysed alloys showed different flow sensitivities to casting temperatures. Furthermore, it is showed that among the considered alloying elements, magnesium and silicon affected the fluidity of the melt. One alloy was then contaminated with 50% scrap addition, increasing the amount of oxide inclusions. The fluidity of the contaminated melt has then been measured and compared with the fluidity of the clean melt. The results show that the fluidity of the alloy with scrap addition is lower than that of the clean melt. Further the fluidity linearly increases at increasing temperatures within the range between 580 and 680 degrees C until it reaches a plateau at the highest pouring temperatures
The Role of Bismuth as Trace Element on the Solidification Path and Microstructure of Na-Modified AlSi7Mg Alloys
Full text linkThe effects of Bi content as a trace element on the microstructure and the solidification path of A356.2 alloy have been investigated. The alloys containing different Bi levels (0, 20, and 200 ppm) have been modified by sodium. The experimental alloys have been thermally analyzed by using the two-thermocouple method. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes occurring at different Bi and Na concentrations. The results indicate how the presence of Bi as a trace element affects the eutectic structure. Upon increasing the Bi level, the nucleation and growth temperatures of eutectic Si raise, and the eutectic Si particles appear coarser in Na-modified alloys. The EBSD analyses show that the crystallographic orientation between eutectic Al and surrounding primary Al dendrites becomes identical in Na-modified Bi-containing alloy. Furthermore, an irregular Bi + (Mg,Na)(3)Bi-2 eutectic is formed prior to the precipitation of the eutectic Si, thus reducing the efficiency of Na addition to fully modify the eutectic Si
Influence of tin and bismuth on machinability of lead free 6000 series aluminium alloys
No full textThis paper presents the microstructural characteristics of an extruded AA6012A-T6 (AlMgSiBiSn) alloy and the microstructural changes occurring during turning operations, analysing the mechanism involved in chip breaking. An experimental investigation has been conducted to determine the effects of different cutting speed and feed rate on the machinability of the alloy. The machinability of the AA6012A-T6 alloy, where Pb is substituted by Bi and Sn, has then been compared to the standard AA6012-T6 (AlMgSiPb) and AA6082-T6 (AlSiMg) alloys. The results indicate that the extensive plastic deformation induces a preferred orientation of the grain structure and secondary phases along the shear plane, and a local increase in the alloy temperature. Low melting point compounds, such as the Sn- and Bi-bearings particles, transform into a soft or liquid state, change their initial compact shape to assume a needle-like morphology. The β-Mg2Si and α-Al(FeMn)Si particles are not influenced by the working temperature and keep the initial shape. The AA6082-T6 alloy shows a very poor machinability, with long and continuous strips, while the AA6012A-T6 alloy reveals a good chip formation with small and discontinuous C-shaped chips, similar to the AA6012-T6 (AlMgSiPb) alloy. In particular, a feed rate higher than 0.2 mm/rev provides short and suitable chips, independently of cutting speed
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