Elena Fiorese

New Classification of Defects and Imperfections for Aluminum Alloy Castings

In recent years, aluminum alloys have become more and more relevant because of their low density, coupled with good mechanical and corrosion properties. Different processes are available for the production of aluminum alloy components, such as rolling, extrusion, and powder metallurgy, but a significant role is played by foundry processes. Defects and imperfections are physiologically generated by different casting techniques as a result of the process stages, alloy properties and die or mold design.In the present work, a multi-level classification of structural defects and imperfections in Al alloy castings is proposed. The first level distinguishes defects on the basis of their location (internal, external, or geometrical), the second level distinguishes on the basis of their metallurgical origin, while the third level refers to the specific type of defect, because the same metallurgical phenomenon may generate various defects.

Influence of Injection Parameters on the Porosity and Tensile Properties of High-Pressure Die Cast Al-Si Alloys: A Review

Aluminum-Silicon alloys are the most extensively used Al foundry alloys and are widely used in high-pressure die casting (HPDC) of automotive components. Several process parameters need to be controlled during HPDC in order to obtain sound and reliable castings. Among the different process variables, the determination and control of the injection parameters, such as the gate velocity and intensification pressure (IP), remain a key requirement throughout the HPDC process. This work critically reviews the effects of the injection parameters on the porosity and tensile properties of the die castings. The results of the literature review are summarized and optimal values for the gate velocity and IP are suggested.

Improving die casting processes through optimization of lubrication

Lubrication plays a significant role among the processes that affect aesthetic quality in die casting. Nevertheless, comprehensive investigations are still missing in literature. In this paper, the effect of lubrication is investigated and optimized, by focusing on its influence on the die temperature. Indeed, since the surface quality of castings is related to the interaction between melt and die, lubrication allows ejection of castings without generating defects and cooling of the die, by preserving its surface. Hence, the die temperature should be accurately selected and spraying should be improved to achieve the optimal temperature. In the experimentation proposed, the lubricant release agent percentage and the lubrication duration have been varied to trade-off between some conflicting effects, while making the die temperature approach the ‘Nukiyama point’. At the same time, reduction of the cycle time has been obtained. The results corroborate the approach, since defects are reduced after optimization.

Plunger Kinematic Parameters Affecting Quality of High-Pressure Die-Cast Aluminum Alloys

The selection of the optimal process parameters in high-pressure die casting has been long recognized as a complex problem due to the involvement of a large number of interconnected variables. Among these variables, the effect of the plunger motion has been proved to play a prominent role, even if a thorough and exhaustive study is still missing in the literature. To overcome this gap, this work aims at identifying the most relevant plunger kinematic parameters and estimates their correlation with the casting quality, by means of a statistically significant sample manufactured with different plunger motion profiles. In particular, slow and fast shot velocities and switching position between two stages have been varied randomly in accordance with design of experiment methodology. The quality has been assessed through the static mechanical properties and porosity percentage. As a further proof, the percentage of oxides has been estimated on the fracture surfaces. These measurements have been correlated to novel parameters, representing the mechanical energy and the inertial force related to the plunger motion, that have been extracted from the time-history of the displacement curves. The application of statistical methods demonstrates that these novel parameters accurately explain and predict the overall quality of castings.

Simultaneous Effect of Plunger Motion Profile, Pressure, and Temperature on the Quality of High-Pressure Die-Cast Aluminum Alloys

High-pressure die casting has been used widely to manufacture a large variety of products with high dimensional accuracy and productivity. Although this process has a considerably lower cycle time than the other metal forming processes, it is not yet optimized, due to the complexity of the process and the number of parameters to be controlled. Hence, the identification of the parameters affecting quality of castings is the current challenge toward efficient and effective production. In their previous work, the authors proposed and validated some novel kinematic parameters of the plunger, which explain and forecast both the static mechanical properties and the internal quality of castings. The present work extends such an approach by including two other meaningful parameters, which describe the effect of upset pressure and temperature on the final outcome. These parameters are here formulated and have been validated by means of a statistically significant sample manufactured with different plunger motion profiles, upset pressures, and temperatures of the melt and die. The quality of the castings was assessed through static mechanical properties and density measurements. As further proof, internal defects were analyzed on the fracture surfaces of some meaningful castings.

Evaluating the Tensile Properties of Aluminum Foundry Alloys through Reference Castings—A Review

The tensile properties of an alloy can be exploited if detrimental defects and imperfections of the casting are minimized and the microstructural characteristics are optimized through several strategies that involve die design, process management and metal treatments. This paper presents an analysis and comparison of the salient characteristics of the reference dies proposed in the literature, both in the field of pressure and gravity die-casting. The specimens produced with these reference dies, called separately poured specimens, are effective tools for the evaluation and comparison of the tensile and physical behaviors of Al-Si casting alloys. Some of the findings of the present paper have been recently developed in the frame of the European StaCast project whose results are complemented here with some more recent outcomes and a comprehensive analysis and discussion.

Correlation between process, microstructure and properties in high pressure die casting aluminium-silicon alloys

Abstract Nowadays automotive industry and, in general, transportation one increasingly needs light components in order to reduce total weights and therefore limit harmful emissions and fuel consumption. The die casting process, on one hand, is a versatile and highly productive process but, on the other, the elevated amount of defects found in the castings sometimes compromises the characteristics of the final product. This paper presents the results of the analyses carried out using a Reference Die, referred to as horse shoe-shaped die, specifically designed to generate as many kinds of defects as possible, at different levels of severity. A Design of Experiments method has been applied for analysing the influence of the main process parameters on the casting quality and an innovative sensor network installed on the machine allowed a continuous control of the process itself. Visual inspection, X-ray investigations and micrograph analyses have been carried out in order to assess the casting quality. Correlations between process parameters and casting quality have been deduced analysing the data collected through the equipment installed on the die casting machine: in-cavity sensor network, plunger speed and displacement sensors. Results show the effectiveness of the horse shoe-shaped casting in highlighting the correlations between process and quality and the strong influence of second phase plunger velocity, temperature and pressure on defect formation.

Correlations between Defect Content, Mechanical Properties and Fractographic Investigation of AlSi9Cu3(Fe) Alloy Reference Castings

High Pressure Die Casting (HPDC) is a foundry process particularly suitable for high production rates and applied in several industrial fields, but the amount of scrap, caused by defects or incomplete filling, is sometimes very high. Thus it is important to know which are the main causes of defect formation and their effects on microstructure and mechanical properties. This paper presents, within the European MUSIC project, the qualitative and quantitative results of a study conducted on AlSi9Cu3(Fe) alloy castings, referred to as Horse-shoe Reference Castings, specifically designed to generate different kinds of defects with different severity levels. The work focuses on the correlations obtained between the casting mechanical properties, their defect content in terms of porosity and oxide films and the process parameters adopted, mainly second phase plunger velocity and intensification pressure. The three point bending test was carried out on the four specimens obtained from the two appendixes of the casting. The fracture surfaces were studied by scanning electron microscopy (SEM) and optical microscopy (OM) highlighting that the defect content is clearly correlated to the mechanical properties and the process parameter settings.

Overview of the Correlations between Process Parameters, Microstructure and Mechanical Properties of Reference Castings and Industrial Demonstrators

Among the Aluminum casting processes, High Pressure Die Casting (HPDC) is an efficient, versatile and economic way for producing large number of components. Nevertheless, because of the elevated amount of rejected castings, it is important to know which are the main causes of defect formation and their effects on microstructure and mechanical properties. This paper presents, within the European MUSIC project, an overview of the preliminary correlations obtained studying both castings with defect generator geometry, referred to as Horse-shoe Reference Castings, and industrial demonstrators, referred to as Gear Box Housing. The deduced correlations between static mechanical properties and casting defects highlighted interesting trends in both cases.

Reference Dies for the Evaluation of Tensile Properties of Gravity Cast Al-Si Alloys: An Overview

Aluminium-Silicon (Al-Si) alloys are the most extensively used Al foundry alloys and are widely used in gravity die-casting (GDC) of automotive components. Reference dies are used to characterize the tensile properties of the castings. Among the various die configurations, the ASTM B-108 (also called the Stahl Mold), and the AA Step Mold are most popular in the foundries. Several modifications to the standard dies have been suggested in the scientific literature to obtain better mechanical properties in minimizing casting defects. This work reviews the scientific literature on the effect of different reference dies on the porosity and tensile properties of the Al alloy castings.