Giulio Timelli

The Effects of Microstructure Heterogeneities and Casting Defects on the Mechanical Properties of High-Pressure Die-Cast AlSi9Cu3(Fe) Alloys

Detailed investigations of the salient microstructural features and casting defects of the high-pressure die-cast (HPDC) AlSi9Cu3(Fe) alloy are reported. These characteristics are addressed to the mechanical properties and reliability of separate HPDC tensile bars. Metallographic and image analysis techniques have been used to quantitatively examine the microstructural changes throughout the tensile specimen. The results indicate that the die-cast microstructure consists of several microstructural heterogeneities such as positive eutectic segregation bands, externally solidified crystals (ESCs), cold flakes, primary Fe-rich intermetallics (sludge), and porosities. In addition, it results that sludge particles, gas porosity, as well as ESCs, and cold flakes are concentrated toward the casting center while low porosity and fine-grained structure is observed on the surface layer of the castings bars. The local variation of the hardness along the cross section as well as the change of tensile test results as a function of gage diameter of the tensile bars seem to be ascribed to the change of porosity content, eutectic fraction, and amount of sludge. Further, this behavior reflects upon the reliability of the die-cast alloy, as evidenced by the Weibull statistics.

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.

Influence of Sludge Particles on the Tensile Properties of Die-Cast Secondary Aluminum Alloys

The effects of sludge intermetallic particles on the mechanical properties of a secondary AlSi9Cu3(Fe) die-casting alloy have been studied. Different alloys have been produced by systematically varying the Fe, Mn, and Cr contents within the composition tolerance limits of the standard EN AC-46000 alloy. The microstructure shows primary α-Alx(Fe,Mn,Cr)ySiz sludge particles, with polyhedral and star-like morphologies, although the presence of primary β-Al5FeSi phase is also observed at the highest Fe:Mn ratio. The volume fraction of primary compounds increases as the Fe, Mn, and Cr contents increase and this can be accurately predicts from the Sludge Factor by a linear relationship. The sludge amount seems to not influence the size and the content of porosity in the die-cast material. Furthermore, the sludge factor is not a reliable parameter to describe the mechanical properties of the die-cast AlSi9Cu3(Fe) alloy, because this value does not consider the mutual interaction between the elements. In the analyzed range of composition, the design of experiment methodology and the analysis of variance have been used in order to develop a semi-empirical model that accurately predicts the mechanical properties of the die-cast AlSi9Cu3(Fe) alloys as function of Fe, Mn, and Cr concentrations.

Fluidity of aluminium die castings alloy

Abstract The 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°C until it reaches a plateau at the highest pouring temperatures.

Grain refinement of gravity die cast secondary AlSi7Cu3Mg alloys for automotive cylinder heads

Abstract The effects of AlTi5B1 grain refinement and cooling rate on the microstructure and mechanical properties of a secondary AlSi7Cu3Mg alloy were reported. Metallographic and image analysis techniques have been used to quantitatively examine the macrostructural and microstructural changes occurring with the addition of grain-refining agent at different cooling rates by using a step casting die. The results indicate that the addition of AlTi5B1 produces a fine and uniform grain structure throughout the casting and this effect is more pronounced in the slowly solidified regions. Increasing the cooling rate, lower amount of grain refiner is necessary to produce a uniform grain size throughout the casting. On the other hand, the initial contents of Ti and B, present as impurity elements in the supplied secondary alloy, are not sufficient to produce an effective grain refinement. The results from the step casting geometry were applied to investigate a gasoline 16V cylinder head, which was produced by gravity semi-permanent mould technology. The grain refinement improves the plastic behaviour of the alloy and increases the reliability of the casting, as evidenced by the Weibull statistics.

Influence of Melt Superheat, Sr Modifier, and Al-5Ti-1B Grain Refiner on Microstructural Evolution of Secondary Al-Si-Cu Alloys

AbstractThe role of impurity elements and melt superheat on the efficiency of Sr modification, grain refinement with Al-Ti-B and the precipitation behavior of intermetallic phases in a secondary Al-7Si-3Cu-0.3Mg alloy were investigated. Metallographic and thermal analysis techniques were used to quantitatively examine the macro- and microstructural changes occurring with modifier and grain refiner additions at various pouring temperatures. The results indicate how the Sr modification and grain refinement with Al-Ti-B can be effective enough despite the presence of impurity elements in the material and the variation of pouring temperature. A slight poisonous effect of impurities, in particular, Zr and V, in the grain refinement efficiency can be eventually induced due to their action in promoting the formation of primary AlSiTi compounds. Moreover, grain refiner addition exerted a pronounced influence on the precipitation sequence of Fe-rich phases. The TiB2 particles appeared to promote the formation of Al5FeSi during solidification by acting as a favorable nucleation site.

Effects of chromium and bismuth on secondary aluminium foundry alloys

Abstract The effects of impurities such as chromium and bismuth on the microstructure and mechanical properties of an AlSi9Cu3(Fe) secondary die casting alloy are investigated. Metallographic and image analysis techniques have been used to examine the microstructural changes. The results indicate that the area fraction and the size of brittle α-Alx(Fe,Mn,Cr)ySiz intermetallic compounds increase by increasing the Cr content, and this affects the ultimate tensile strength and the ductility of the alloy at higher Cr level. On the other hand, the Bi addition seems to not produce significant changes in the microstructure and in the average mechanical properties of the secondary AlSi9Cu3(Fe) alloy. However, the Weibull statistics show that the addition of Bi leads to less reliable castings, which may be associated with an increase in oxide film defects.

Influence of Grain Refiner Addition on the Precipitation of Fe-Rich Phases in Secondary AlSi7Cu3Mg Alloys

The effect of grain refiner addition on the precipitation of Fe-rich compounds in secondary AlSi7Cu3Mg has been studied. Thermal and metallographic analysis techniques have been used to examine the formation of Fe-bearing intermetallics. The results show that grain refinement with AlTi5B1 can alter the precipitation sequence of Fe-rich phases. In non-grain refined alloy and grain-refined alloy with AlTi10, two different Fe-rich phases, α-Al15(Fe,Mn)3Si2 and π-Al8Mg3FeSi6, form, whereas, with AlTi5B1 addition, the β-Al5FeSi phase also tends to crystallize, nucleating on TiB2 particles. It has also been demonstrated how at high cooling rates, the β-Al5FeSi-involved reactions occur more preferentially, when compared to α-Al15(Fe,Mn)3Si2 phase. This crystallization behaviour is explained in terms of phase diagram relationship and the nucleation kinetics of competing α-Al15(Fe,Mn)3Si2 and β-Al5FeSi phases.

Optimization of a Permanent Step Mold Design for Mg Alloy Castings

The design of a permanent Step mold for the evaluation of the mechanical properties of light alloys has been reviewed. An optimized Step die with a different runner and gating systems is proposed to minimize the amount of casting defects. Numerical simulations have been performed to study the filling and solidification behavior of an AM60B alloy to predict the turbulence of the melt and the microshrinkage formation. The results reveal how a correct design of the trap in the runners prevents the backwave of molten metal, which could eventually reverse out and enter the die cavity. The tapered runner in the optimized die configuration gently leads the molten metal to the ingate, avoiding turbulence and producing a balanced die cavity filling. The connection between the runner system and the die cavity by means of a fan ingate produces a laminar filling in contrast with a finger-type ingate. Solidification defects such as shrinkage-induced microporosity, numerically predicted through a dimensionless version of the Niyama criterion, are considerably reduced in the optimized permanent Step mold.